applying thermal diffusion galvanization on wood screws1089629/... · 2017-04-10 · degree project...

DEGREE PROJECT IN TECHNOLOGY,FIRST CYCLE, 15 CREDITSSTOCKHOLM, SWEDEN 2016

Applying Thermal Diffusion Galvanization on Wood ScrewsEffects on Corrosion Resistance and Mechanical Properties

ANTE VALLIENBENJAMIN SOLEMPHILIP WERNSTEDT

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

i

Abstract

Today, fastening articles such as screws and nails are treated with different surface coatings to

withstand corrosion. The Swedish distributor ESSVE® Produkter AB uses a nano coating called

CorrSeal™ for high corrosion protection of their screws. Thermal diffusion galvanization (TDG) is a

more environmentally friendly method that the company seeks to use as replacement for the current

treatment. This process of zinc diffusion is carried out at around 400 °C for several hours. The aim of

the project is to investigate the possibility to surface treat a wood screw using TDG. The elevated

temperature is suspected to decrease the hardness of the hardened screw. Therefore, a hardened and

tempered screw without surface treatment is sent to a TDG facility. Industrial furnaces are used for

similar heat treatments of screws with different hardenings. Both processes are analyzed by evaluating

the results of hardness, bending, and microscopy. No immediate correlation between the TDG process

and heat treatment in the industrial furnaces is found. Results show that the tested screws softened

to a higher degree in the TDG process compared to treatment in the industrial furnaces. The

mechanical properties of the tested screws, after the TDG process, are not acceptable. The zinc layer

thickness on the screws is uneven yet believed to meet the required demands on corrosion resistance.

Results also show that incorporating the TDG process in the tempering step is essential to meet the

demands on hardness. Additionally, changing the composition of the material can lead to higher

resistance against softening at the elevated temperatures. Further research is however needed to

present a screw with sufficient corrosion resistance from the TDG process that will meet the demands

on hardness and bending.

ii

Table of Content

1 Introduction.......................................................................................................................... 1

2 Literature.............................................................................................................................. 3

2.1 Sherardizing ............................................................................................................................. 3

2.2 TDG Process by DiSTeK ............................................................................................................ 3

2.3 Hardening ................................................................................................................................ 4

2.3.1 Surface Hardening ........................................................................................................... 4

2.3.2 Quenching ....................................................................................................................... 5

2.3.3 Tempering ....................................................................................................................... 5

2.4 Diffusion .................................................................................................................................. 5

3 Experimental ........................................................................................................................ 7

3.1 Sample Specification ............................................................................................................... 7

3.2 Bending Test .......................................................................................................................... 10

3.3 Light Optical Microscopy ....................................................................................................... 12

3.4 Scanning Electron Microscope – Energy-Dispersive X-ray Spectroscopy .............................. 12

3.5 Hardness ................................................................................................................................ 14

3.6 Corrosion Resistance ............................................................................................................. 15

4 Results ................................................................................................................................ 16

4.1 Bending Test .......................................................................................................................... 16

4.2 LOM ....................................................................................................................................... 18

4.2.1 CorrSealREF ................................................................................................................... 18

4.2.2 Untreated ...................................................................................................................... 19

4.2.3 TDG-test ......................................................................................................................... 20

4.2.4 TDG-nail ......................................................................................................................... 23

4.3 SEM-EDS ................................................................................................................................ 24

4.4 Hardness ................................................................................................................................ 26

4.5 Corrosion Resistance ............................................................................................................. 28

5 Discussion ........................................................................................................................... 29

6 Conclusions and Recommendations ..................................................................................... 35

7 Acknowledgements ............................................................................................................. 36

8 References .......................................................................................................................... 37

1

1 Introduction

Sherardizing is a process that aims to increase the corrosion resistance in steel and iron articles [1].

The method utilizes solid state thermal diffusion to apply a layer of zinc to the surface of the article

and the basic mechanics are similar to that of case hardening of steel [1]. Sherardizing was developed

in the early years of 1900 and has been commercially used for more than 80 years [1] [2]. The process

is best suited for smaller, compact articles, like nails, bolts and nuts and has the advantage of applying

a uniform coating even to more complex shapes [3]. In the literature the procedure of sherardizing is

often compared to other galvanizing methods such as hot dip galvanizing (HDG), electro-plating and/or

zinc spraying [1] [3] [4] [5].

Today a refined method of sherardizing is applied to nails, intended for outdoor usage, and distributed

by ESSVE® Produkter AB (ESSVE) [6]. Being one of the leading distributors of fastening solutions in the

Nordic region, ESSVE aims to be in the front line of technology and environmental friendly solutions

[7]. The refined method is patented by DiSTeK Group™ (DiSTeK) and differs slightly from the

sherardizing process mentioned above. The thermal diffusion galvanization (TDG) of the nails has been

used due to its many advantages over other galvanization methods. According to ESSVE, this

technology provides good corrosion resistance to a lower price than that of previous methods [8].

Furthermore, the production provides a higher level of safety for the plant workers as opposed to that

of the commonly used hot dip galvanizing [8]. Studies have shown that the operations associated with

hot dip galvanizing and electroplating give rise to gaseous and particulate emissions as well as

numerous hazardous wastes [5] [9].

Within the wide range of non-stainless steel articles, intended for outdoor environments, ESSVE has a

collection of screws with high corrosion resistance. At present time these screws are surface treated

using a nano coating technology called CorrSeal™ (CorrSeal) that meets a C4-standard [8] [10]. This

level of corrosion resistance is the highest ESSVE uses for non stainless steel articles [11]. According to

ISO 11997-1 Method B, the requirement for C4-standard is to withstand a specified test cycle in salt

spray chamber for at least 11.5 weeks [12]. Due to previously mentioned advantages, ESSVE seeks to

replace this surface treatment with the technology developed by DiSTeK. The majority of the all the

screws produced by ESSVE are hardened in contrast to nails [8]. This provides improved mechanical

properties such as enhanced hardness and increased resistance to bending. The screw that ESSVE

currently aims to improve (wood screw, 5.0 x 50 mm, countersunk head, art.no 117113) is made of the

low alloy steel AISI 1022, see Table 1. The manufacturer for this item is called KWANTEX® Research Inc.

(Kwantex). The screw is required to have a surface hardness of 450-800 HV, a core hardness of 320-

450 HV and a bending angle before break greater than 15 degrees [8]. Seeing as the technique

patented by DiSTeK is conducted at temperatures around 400°C for several hours, the microstructure

of the hardened steel may be altered leading to softening and/or embrittlement [1] [13] [14].

2

Table 1. The chemical composition of AISI 1022 acquired from AZo materials [15]. According to another source, there is silicon (0.1-0.4 wt%) present as well and a slightly higher amount of manganese [16].

Element Content (wt%)

Carbon, C 0.17-0.23

Iron, Fe 98.68-99.13

Manganese, Mn 0.70-1.0

Phosphorous, P ≤ 0.040

Sulfur, S ≤ 0.050

The aim of this study is to investigate the possibility to apply a surface treatment to screws that is

cheaper, more environmentally friendly, less toxic, and more efficient than the current methods.

The goal is to present a specific hardening profile for a wood scew (ESSVE art. no. 117113) that can be

combined with the TDG process to serve the aim and still maintain the required mechanical and

chemical properties. Furthermore, another goal is to ascertain if the changes in mechanical properties,

by the method of sherardizing by DiSTeK, can be approximated in a standard industrial furnace for

easing further investigations in the subject.

A hypothesis is that applying TDG on screws will be associated with a few problems. According to

Kwantex, the screws are case hardened using quenching and tempering. This hardening process gives

rise to a dual-phase steel containing a balanced amount of martensite and ferrite to provide the steel

with high hardness and sufficient toughness [17]. Seeing as martensite is an oversaturated solution of

carbon, the carbon locked in the structure will diffuse and form cementite and ferrite given the chance.

This would cause the steel to soften or become brittle [17] [18]. If enough martensite is allowed to

transform, the screw will become unusable due to the extensive change in mechanical properties.

Considering that TDG is conducted at high temperatures for quite a long time, this will allow the carbon

to diffuse, causing the screw to become too soft.

3

2 Literature

In order to understand the mechanics behind the problem, the most essential subjects are reviewed

and presented in this section.

2.1 Sherardizing This chapter will discuss the sherardizing process as described by Swedish Standard SS-EN ISO 14713-

3:2009 [19]. Sherardizing is to be distinguished from the method developed by DiSTeK. The

sherardizing process involves several steps in order to achieve optimal results.

Impurities on the surface of the specimen, such as rust or grease, are not bridgeable by the sherardizing

process. Because of this, it is of great importance that the article is thoroughly cleaned before it is

treated. There are mainly three kinds of pre-cleaning methods; shot blasting, degreasing in organic

solvent, or hydrochloric acid pickling. The kind of method used is dependent on the surface

contamination and the type of component [1] [19] [20].

After the article has been cleaned to an acceptable level, it is loaded into a container together with a

zinc powder mixture. An inert filler, often silica sand, is added with the purpose of distributing the zinc

evenly as well as preventing damage to the article [1] [20]. The container is closed, air tightened and

placed in a furnace at temperatures ranging from 380-450 °C [1] [19] [20] [21]. The method can also

be carried out at temperatures around 300 °C at the cost of longer processing time [19]. The vessel is

not fully loaded in order to allow even distribution of heat and materials by rotating in the furnace at

approximately five rounds per minute. Zinc vapor is generated by sublimation due to the increased

temperature in combination with the presence of catalysts [1] [22]. The vapor comes in direct contact

with the surface of the steel and diffusion occurs. The time that the batch is kept in the furnace differs

depending on specimen size and the required corrosion resistance [22]. The corrosion resistance is

proportional to the thickness of the zinc layer and the desired thickness of the layer depends on the

application of the article [19] [20]. When sufficient time has passed, the container is removed from the

furnace and cooled before it is opened [20] [22]. The articles, zinc powder, and the inert filler are

separated. While the filler is recycled, the residual zinc power is discarded [1].

Succeeding the sherardizing process, a passivation layer is often applied. This layer minimizes the risk

of increased local formation of zinc oxide, also known as wet storage stains or white rust. Furthermore,

the passivation layer helps to maintain the corrosion resistance [1] [3] [19].

2.2 TDG Process by DiSTeK The TDG process developed by DiSTeK is identical to that of the standardized sherardizing method in

almost every aspect. However, there are some significant differences as mentioned in chapter 1

Introduction [5] [22] [23]. According to DiSTeK, the pre-cleaning step is not required in most cases [24].

TDG uses a patented zinc powder mixture where the diffusion mechanism may differ slightly [5] [22]

[23]. However, company secrets make it impossible to accurately describe the exact differences.

DiSTeK also claims that this method is more environmentally friendly than the standardized

sherardizing process [24]. The passivation layer, used to maintain corrosion resistance after TDG, is

developed and patented by DiSTeK [25].

It is worth noting that DiSTeK-Scandinavia has expressed their doubts about the accuracy of the

temperature control in the furnaces of the process [23]. In many cases, they believe that the actual

temperature may be higher than the set temperature. This comes as a result of uncertain methods of

temperature readings in the cylinders.

4

DiSTeK claims to follow several standards, for example ASTM A 1059A/1059M – 08 [26]. This standard

specifies many relevant process details for the actual workmanship when applying TDG to a steel item.

It also specifies the sampling for tests, number of tests, sample preparation and test methods.

Regarding the surface layer, it is stated that the coating is mainly an iron-zinc alloy containing up to 10

% iron.

2.3 Hardening The wood screws from ESSVE are manufactured by Kwantex. Mail correspondence with Kwantex

provides a general process chain as given in Figure 1 [27]. This information shows that the screws are

treated with a type of surface hardening called carburizing. However, due to corporate secrets, more

detailed information regarding the hardening process is unobtainable. This chapter will thus describe

the basic processes involved in hardening a low alloy steel product.

Bath in water Quenching (carburizing)

Rapid cooling in oil

Bath in water

Tempering (for ductility) Final finished products

Figure 1. Hardening process of screws as described by Kwantex [27]. Chronological order from left to right.

2.3.1 Surface Hardening Almost all product types are hardened differently. In many applications, such as gears, shafts, and

bolts, there is a need for a hard surface yet a though core. For this purpose, there are several useful

methods available. One method is to treat the surface by flame hardening or induction hardening

which includes heating the surface to the austenitic state followed by rapid cooling [28] [29]. Another

method is to change the composition of the outermost material by case hardening, which is commonly

applied on plain carbon and low alloy steels to increase the hardenability of the surface [28] [30]. This

is often achieved by carburizing where carbon is present in a controlled atmosphere and diffuses into

the steel at elevated temperatures. Another case hardening method, called carbonitriding, uses

nitrogen in addition to carbon. The two methods changes the composition of the surface and the

hardening level increases. The steel is then rapidly cooled to form martensite, mainly in the areas with

a higher content of carbon, resulting in the desired hard surface and softer core.

Both case hardening methods are controlled by holding temperature, atmospheric composition, time,

and cooling rate. Carbonitriding can be slightly more expensive than carburizing due to the addition of

ammoniac. However, the shorter times and lower temperatures may compensate this [31].

Carburizing is often used on steels with a carbon content less than 0.2 % [28] [29] [30]. The carbon

diffusion is performed at austenitic state at temperatures around 870-980 °C [32]. After the treatment,

the carbon content of the surface is around 0.7-0.8 %, at a depth of 2-6 mm. This results in a

composition gradient to create a hard martensitic case and a soft but tough core after the article is

quenched [29] [32].

Carbonitriding is commonly performed at slightly lower temperatures and for shorter periods, which

results in a reduced hardening depth, generally between 0.1 and 0.75 mm [33]. It is therefore often

applied to smaller articles. The atmosphere in the carbonitriding furnace contains both carbon and

nitrogen, the latter by an addition of ammoniac. The nitrogen enhances the hardenability of the steel

by increasing the stability of austenite, and lowering the critical quenching speed [28] [33]. Following

both processes, a tempering cycle is applied to reduce the brittleness [29] [33].

5

2.3.2 Quenching Different kinds of quenching may be performed, the two main types being water quenching and

quenching in oil. Water is the fastest quenching method, which makes it the only option when

hardening plain, low carbon steels, which have a high demand on cooling rate [17]. However, a

disadvantage with water quenching is that the martensite is formed so rapidly it gives rise to high inner

tensions and potential cracks in the material. That is why oil is often the preferred method, and why

nitrogen is added to lower the critical quenching speed in carbonitriding [17].

Thelning [34] shows that the core hardness is affected by the quenching temperature after the

carburizing process. Two major results were found when steel bars with different diameters made

from an SS 1370 steel, with the composition of 0.16 % C, 0.35 % Si, and 0.84 % Mn, were carburized at

925 °C followed by quenching in water from three different temperatures. All specimens with

diameters from 10 to 50 mm were carburized to around 1 mm depth. The most important factor was

the diameter, with a difference of 200 HV in core hardness between the 10 mm and 50 mm rod when

cooled to 830 °C and then quenched. However, the results also show that the core hardness was higher

when directly quenched from 925 °C instead of being hardened from 830 °C or 770 °C.

2.3.3 Tempering Hardened articles often need an improved ductility due to the hard yet brittle martensitic structure

formed during quenching. This is achieved by tempering, i.e. holding the articles at an elevated

temperature for a sufficient amount of time [17]. The increased temperature allows the carbon to

diffuse, leaving the metastable martensite to form cementite and ferrite. Seeing as ferrite is more

ductile than martensite, the steel will become tougher as a result. This increase will be matched by a

decrease in hardness and strength, which makes it important to balance these changes to get a

material with optimal mechanical properties. The extent to which tempering effects will occur depends

on the time of the process as well as the tempering temperature, though the latter generally will have

the highest impact [17] [35]. As a result, tempering is conducted at many different temperatures,

depending on the desired properties. However, it is important that the temperature is held below the

eutectoid temperature, as the steel will otherwise be austenitized [17] [18] [35].

In some cases, the steel is used in, or otherwise exposed to, elevated temperatures after hardening

and tempering. The tempering will then proceed, continuing to soften the material. To prevent this,

carbide forming alloys, such as W, Mo, V, and Cr may be added. These alloys will help the steel form

hard carbides that can increase the hardness over time and partially postpone the hardness decrease

caused by the degrading of martensite [18]. A similar effect can be obtained by the addition of silicon.

Because silicon is not soluble in cementite, it will assemble along the surface of the cementite particles,

preventing further growth. Slowing the growth of cementite will in turn slow the decomposition of the

martensite [18] [36] [37]. Adding alloys of this sort makes it possible to maintain the martensitic lattice

at tempering temperatures up to 450 °C, whereas it will disappear already at 300 °C in a plain carbon

steel [36]. An addition of silicon, to above 0.5 wt%, is often used for this purpose [37] [38].

2.4 Diffusion Understanding diffusion is essential in order to understand the processes involved in TDG. Below, a

short introduction to the basic mechanics of diffusion are presented.

Diffusion is a process for mass transport driven by concentration differences. It can be described as

atoms moving randomly in a phase due to thermal fluctuations. A net flux of mass will be created if

the atoms are not evenly distributed throughout the structure [39].

6

Solid state diffusion can take place in two different ways; interstitial or substitutional diffusion.

Substitutional diffusion is also called vacancy diffusion. The first kind will be dominating if the diffusing

atoms are much smaller than those of the matrix. The smaller atoms can then pass between the spaces

of the larger ones, allowing relatively fast transportation [39] [40]. The latter will occur if the diffusing

atoms and the atoms of the matrix are close to the same size. This diffusion process requires the

presence of vacancies, as the only way for the atoms to move is by jumping into an empty space in the

lattice. Because of this, substitutional diffusion is much slower than interstitial [39] [40].

There are several factors influencing the diffusion speed apart from the type of diffusion. Temperature

will to a high degree affect the diffusion speed. A heightened temperature will increase thermal

fluctuations, allowing the atoms to move more freely within the structure. The diffusion rate can vary

several orders of magnitude with only a few hundred degrees of temperature difference [39].

TDG uses diffusion to move zinc atoms into the steel, creating a zinc-iron alloy that provides the desired

corrosion resistance [5]. Zinc and iron atoms are of approximately the same size, and zinc will therefore

diffuse substitutionally in an iron matrix [41]. Carbon, however, is a very small atom compared to iron,

and will therefore use interstitial diffusion [41]. Temperatures during the TDG process allows

measurable diffusion to take place over a reasonable amount of time for both carbon and zinc [5].

7

3 Experimental

A series of experiments were carried out to investigate the hypothesis as well as work towards the goal

of the study. Screws were provided by ESSVE in order to perform various treatments including the TDG

process, hardening, and tempering. The mechanical properties, constituent phases, and corrosion

resistance of the screws were then evaluated. Arithemtic mean values were calculated and is

henceforth referred to as mean value. The standard deviations were calculated using Equation 1. All

samples, a set of screws treated in the same way, were named in accordance to their respective

treatment and are henceforth written in italic.

Equation 1. Equation used in this study for calculating standard deviation. 𝒙 is the arithmetic mean value, 𝒙𝒊 is the observed value and 𝒏 is the number of values.

3.1 Sample Specification An unhardened and non-corrosion protected version of the wood screw mentioned in chapter 1

Introduction was named Unhardened. A decking screw without surface treatment was given the

designation Untreated and can be seen in Figure 2. The two samples were treated accordingly.

Figure 2. The screws of Untreated.

Sample Untreated was sent to Promet S.A. (Promet) in Poland. Mail correspondence with Promet

states that the screws underwent the TDG process with the times and temperatures as presented in

Table 2 [42]. The combination of these times and temperatures was given the notation Ap. The

temperatures that are presented in Table 2 were the final temperatures at the given times. After 190

minutes, the air-tightened vessel containing the screws was cooled in room temperature. The surface

treated screw was named TDG-test.

√∑ (𝑥𝑖 − �̅�)2𝑛𝑖=1

𝑛

8

Table 2. Times and temperatures for the TDG process for TDG-test. The combination of these times and temperatures is called Ap. RT is room temperature.

Ap

Time (min) Temperature (°C)

0 RT

80 320

130 350

190 350

In order to perform the heat treatments, an external, Sweden based company called Bodycote

Värmebehandling AB (Bodycote) was consulted. Industrial furnaces were used for hardening and

tempering a series of screws as well as replicating the circumstances in the TDG process of Promet.

After analysis, three different tempering series were constructed (Table 3) with varying temperatures,

temperature gradients, and total times. Table 4 lists all the samples in the study.

The tests examining mechanical properties, chemical composition, and participating phases were

carried out at the laboratory of KTH Department of Materials Science and Engineering (KTH MSE),

Bodycote and ESSVE’s laboratory. CorrSealREF, TDG-nail, and HDG-nail are part of ESSVE’s assortment

and were used as reference samples when needed. Specifications of the reference samples are listed

in Table 4. Figure 3 shows these samples along with TDG-test, Untreated, and Unhardened.

Figure 3. Two of each reference sample along with TDG-test, Untreated, and Unhardened. Designations are shown below each sample.

BC in the name of some of the samples discussed below refers to Bodycote. In order to investigate

maximum hardness achievable in the core of Unhardened, the sample was austenitized followed by

quenching in water. The maximum hardened sample was referred to as BC:max with its hardening H0.

For BC:CN1 and BC:CN2, Unhardened was case hardened in two different ways using carbonitriding.

Based on the properties of Untreated, Bodycote used their standard program for 0.1 mm case depth

for one of the carbonitriding processes. The process was performed at 820 °C with a carbon content

of 0.94 %. This hardening process is further on called CN1. The second program was performed with

9

the same conditions but held 15 % longer time at the elevated temperature to reach a greater case

depth. The second hardening process is further on called CN2.

For BC1–BC4 and BC7–BC8 as well as BC10–BC11, the samples BC:CN1 and BC:CN2 were tempered in

different temperatures and for varying times as described in Table 3 and Table 4 combined. For BC5,

Untreated was tempered using the same times and temperatures as TDG-test (Table 2). For BC6 and

BC9, Untreated was tempered in accordance to Table 3 and Table 4. C-test1 and C-test2 were the

sample Untreated with the CorrSeal corrosion protection.

Some of the samples lacked information regarding their hardening and tempering process and were

given the designations Hardened1, Hardened2, Unknown1, and Unknown2, as seen in Table 4.

Table 3. Times and temperatures for the tempering at Bodycote, called A1, A2, and A3. RT is room temperature.

A1 A2 A3

Time (min) Temperature (°C) Time (min) Temperature(°C) Time (min) Temperature(°C)

0 RT 0 RT 0 RT

30 320 30 300 30 320

80 350 80 320 80 340

140 350 130 350 130 370

- - 190 350 190 370

Table 4. Sample specification. Information is absent regarding the hardening and tempering process for samples marked with Hardened1, Hardened2, Unknown1, and Unknown2. H0 is the hardening process designed to find maximum hardness. Ap is the combination of time and temperature described in Table 2 and the combinations for A1-A3 is described in Table 3. CN1 and CN2 are two different carbonitriding processes.

Designation Type Nominal diameter [mm]

Nominal length [mm]

Corrosion protection

Hardening Tempering/ Annealing

Unhardened Wood screw

5.0 50 None None None

CorrSealREF Wood screw

5.0 90 CorrSeal Hardened1 Unknown1

Untreated Decking screw

4.8 55 None Hardened2 Unknown2

TDG-nail Collated nail

3.1 90 TDG None None

TDG-test Decking screw

4.8 55 TDG Hardened2 Unknown2

HDG-nail Nail 3.4 100 HDG None None

BC:Max Wood screw

5.0 50 None H0 None

BC:CN1 Wood screw

5.0 50 None CN1 None

BC:CN2 Wood screw

5.0 50 None CN2 None

BC1 Wood screw

5.0 50 None CN1 Ap

10

BC2 Wood screw

5.0 50 None CN1 A1

BC3 Wood screw

5.0 50 None CN2 Ap

BC4 Wood screw

5.0 50 None CN2 A1

BC5 Decking screw

4.8 55 None Hardened2 Unknown2+Ap

BC6 Decking screw

4.8 55 None Hardened2 Unknown2+A2

BC7 Wood screw

5.0 50 None CN1 A2

BC8 Wood screw

5.0 50 None CN2 A2

BC9 Decking screw

4.8 55 None Hardened2 Unknown2+A3

BC10 Wood screw

5.0 50 None CN1 A3

BC11 Wood screw

5.0 50 None CN2 A3

C-test1 Decking screw

4.8 55 CorrSeal Hardened2 Unknown2+A2

C-test2 Decking screw

4.8 55 CorrSeal Hardened2 Unknown2+A3

3.2 Bending Test ESSVE demands a bending angle of 15° on their screws before detectable crack initiation. Therefore,

several mechanical bending tests were conducted on Unhardened, Untreated, TDG-test, BC:max,

BC:CN1, BC:CN2, C-test1, C-test2, and BC1–BC11. See sample specification in Table 4 above. The

bending tests were carried out accordingly.

Each screw was carefully loaded into the bending machine (depicted in Figure 4 and Figure 5) in order

to ensure repeatable experimental conditions. All screws were placed with the round of the head as

tightly as possible to the fixed roll. After analyzing the first set of results, all samples except for CN1,

CN2, Unhardened, C-test1, and C-test2, were tested in two different ways.

11

Figure 4. The tilted bending machine where the screws were tested.

Figure 5. A schematic view of the bending machine in Figure 4 with the loading piston (1), load cell (2) and the holder for the screw (3).

To get repeatable results, each tested screw of every sample was bent to the same angle by turning

the loading piston a total of 720 degrees, thus applying a force to the middle of the screw by pulling

the black holder. The test was named two rotations test. The applied load was measured by the

machine’s s-beam load cell (Vetek VZ-101BH 3000 kg) connected to the loading piston and the peak

value of each test was noted.

12

For the screws that underwent two tests, each sample was bent to 15°, estimated by the experimenter.

The test was named 15° test. After analyzing the first set of results, Untreated was instead bent to the

breaking point in the second test.

Apart from BC6, BC9, and Unhardened, each test was conducted on 10 randomly chosen specimens.

BC6 and BC9 were tested on ten specimens in the two rotations test and five specimens in the 15° test.

Unhardened had a sample size of five specimens.

3.3 Light Optical Microscopy Light Optical Microscopy (LOM) was used to get an understanding of the microstructural changes

taking place in the steel during the TDG process. The samples were observed across the whole cross

section with pictures taken on the edges and the center.

Samples for analysis using LOM were prepared using standardized methods of enclosing into Bakelite.

Screws were cut above the threads and a few millimeters below the head in order to exclude any

effects of deformation caused in the manufacturing chain. The nails were cut in a similar way. A

solution of alcohol and nitric acid (4 % Nital) was used for etching the samples sufficient amount of

time depending on the corrosion protection and hardening. Each sample was observed in a microscope

of model Olympus PMG 3 and pictures of the microstructure were taken in the range of 20x-100x

magnification. Samples Unhardened, CorrSealREF, Untreated, TDG-nail and TDG-test, were observed

using LOM.

Surface coating thickness and carburizing depth of the two samples TDG-test and Untreated were

examined using LOM. Measurements were taken using the build-in measuring tool of the program

Leica QWin V3. The coating thickness of TDG-test was evaluated at 50x magnification at three

randomized points of five individual specimens. The measurements were taken perpendicular to the

curved surface. A qualitative examination of the coating of TDG-test and TDG-nail was conducted at

5x-50x magnification.

3.4 Scanning Electron Microscope – Energy-Dispersive X-ray Spectroscopy A Scanning Electron Microscope – Energy-Dispersive X-ray Spectroscopy (SEM-EDS) was conducted to

get high resolution images over the cross section of TDG-test, as well as provide information about the

composition of the steel and zinc layer of samples TDG-test, Untreated and Unhardened. Preparation

for SEM was carried out in the same way as the preparation for LOM, only with a different kind of

Bakelite. Hitachi S-3700N was used to analyze the samples. Chemical composition and layer thickness

were examined with a magnification up to 2000x.

The composition was examined over certain areas in each sample, called spectrums. The center of

sample TDG-test was called spectrum 1. Spectrum 2 was placed in the zinc layer and spectrum 3 in the

steel close to it, as seen in Figure 8. The center of Unhardened was examined over spectrum 4, and the

center of Untreated over spectrum 5.

A linear composition gradient measurement was performed along a line in the cross section of sample

TDG-test in Area1 (Figure 6) and Area2 (Figure 7).

13

Figure 6. An overview of the first tested area in the TDG-test. The yellow arrow indicates the line of measurement.

Figure 7. The second area for the linear composition measurements in TDG-test.

14

Figure 8. The markings in yellow are the areas where the analysis was conducted. The brighter area to the left is the zinc-iron layer. The picture is taken at 1000x magnification on an unetched screw from TDG-test.

3.5 Hardness ESSVE have certain demands of hardness for their screws. Thus, a standardized Vickers hardness (HV)

test was used to evaluate the hardness of the surface and the hardness of the core of samples TDG-

test and Untreated. Only the hardness of the core was measured for sample Unhardened. A load of 50

grams (HV0.5) and 200 grams (HV2) was applied using the microhardness tester MXT-α1 for the zinc

layer and the base material respectively. Four points of impact in the core and four points of impact in

the surface were measured on nine different specimens of TDG-test and five different specimens of

Untreated. Sample Unhardened was tested with four points of impact in the core on three different

specimens and close to the edge on one specimen. The HV mean value and the standard deviation

were calculated for each specimen. The diagonals of the imprints were measured, and the hardness

calculated, using the build in measuring tool of the program Leica QWin V3 at 50x magnification. The

imprints were placed at least 2.5 times the diagonal length of the resulting indentation from the edge

of the specimen and at least 2.5 times the diagonal length from each other as seen in Figure 9. The

distance from the edge was 80-90 μm.

15

Figure 9. A picture from the hardness test of TDG-test shows the four indentations for the surface test.

All samples that underwent any treatment at Bodycote, as well as the TDG-test and Untreated, were

evaluated at Bodycote’s facility in Västerås, Sweden. That evaluation included hardness

measurements. These measurements were conducted using 100 grams and 30 grams, referred to as

HV1 and HV0.3 respectively. To evaluate the surface hardness, one or two values were measured on

both sides of the determined case depth and then extrapolated to give the hardness at the surface

[43].

3.6 Corrosion Resistance The screw ESSVE seeks to replace meet a demand of C4-standard on corrosion resistance and TDG-test

were therefore be tested to see if the same demands were met.

The corrosion resistance of TDG-test was examined in LOM by measuring the thickness of the surface

coating in accordance with methods stated above in chapter 3.3 Light Optical Microscopy. The results

were compared with values from SP reports regarding TDG applied on nails, such as the TDG-nail [12]

[44].

The corrosion resistance of samples TDG-test, Unhardened, Untreated, a CorrSeal treated decking

screw (same corrosion resistance as CorrSealREF however of a different dimension), HDG-nail as well

as an electroplated drywall screw was also tested in a homemade corrosion chamber, kept at room

temperature. The screws were put alongside each other in a plastic bottle and the bottle was filled

with salt water (15 ml cooking salt in 3 dl tap water). After 16 days the salt water was substituted. The

new mixture contained 45 ml cooking salt in 4.5 dl tap water.

16

4 Results

The results of the Bending test, LOM, SEM-EDS, hardness and corrosion resistance are presented

below.

4.1 Bending Test One of the requirements for the screws delivered by ESSVE is that the screws must withstand a bending

of 15° without major cracks. In this chapter, a summary of the results from the bending tests is

presented. For a clarification of the samples, see Table 4 at page 9. The full tables with results are

available in Appendix: Bending.

Unhardened showed that before the hardening process, the steel is ductile enough to

withstand the maximum bending allowed by the testing equipment. This is without showing

any signs of cracks on the surface.

Untreated showed higher peak results both in the two rotations test as well as in the crack

test. The crack test showed that all the specimens broke or lost load bearing capacity with a

resulting angle at 15° or more. Sometimes the values turned out to be up to around 30° at the

breaking point. The two rotations test gave peak results at 100 kg or higher for eight out of ten

screws. The two specimens with the lowest results failed just before completing two turns on

the piston and the approximate bending angle at this point was 15°.

TDG-test showed lower peak values than Untreated in the two rotations test. The bent screws

were free from any cracks except for smaller fractures on the surface.

BC1 did not display any cracks in the two rotations test. The resulting angle of bending was

estimated to around 5° for all specimens. For the 15° test, six out of ten specimens maintained

load bearing capacity and the resulting angle was around 15°. The other four lost the load

bearing at close to 15°.

BC2, had a bending angle in the two rotations test that was approximately halfway to 15° with

no detected cracks. Five out of ten screws did break in the 15° test. Eight specimens were

estimated to have a final bending angle of 15°. One screw that was fractured had a slightly

higher angle and one intact specimen was bent to slightly less than 15°.

BC3 did not display any cracks in the two rotations test. The resulting angle was around 10-

12°. In the 15° test, 50% of the specimens lost their load bearing. Two of the screws that broke

had brittle fractures and for one of these, the resulting angle became greater than 15°. For the

other screws the estimated angle was around 15° except for one that was slightly lower.

BC4 did not display any cracks from the two rotations test and the angle were the same as for

BC3. Only four specimens made it close to 15° in the 15° test without failure. Four of the other

screws had complete brittle fractures and the other two lost weight bearing at under or close

to 15°.

BC5 did not display any cracks from the two rotations test and all specimens were bent to

almost 15°. In the second test BC5 showed no indications of cracks at 15° and most screws

showed a ductile fracture first at a very high angle.

17

BC6 were free from cracks in the two rotations test with a resulting angle of between 8-12°.

None of the five screws that underwent the second test showed any severe cracks and all

specimens were bent to 15°.

BC7 reached a bending angle of around 7-8° in the two rotations test and showed no signs of

crack initiations. Eight screws were bent to 15° without any cracks in the 15° test and two

screws had brittle partial fractures and bending angles over 15°.

BC8 displayed the same results in the two rotations test as BC7. In the 15° test almost every

specimen reached an angle slightly below 15°. Four specimens had a drop in load bearing, with

two of them being more distinct.

BC9 were bent close to 15° in the two rotations test and without any cracks. The five specimens

bent in the 15° test all had a resulting angle at 15°. One of the screws showed a tiny surface

crack.

BC10 reached a small bending angle but showed no cracks in the two rotations test. In the

second test, all screws were bent to 15°, resulting in small cracks in the surface of two of the

specimens.

BC11 showed the same results as BC10 in the two rotations test. However, six screws displayed

small surface cracks and one screw had a bigger crack in the 15° test where all specimens

reached a bending angle of 15°. Two of these lost the load bearing capacity. The other three

all got to 15° without any cracks.

C-test1 reached a bending angle of around 8-12° and showed no cracks in the two rotations

test.

C-test2 reached a bending angle slightly below 15° and no cracks were observed in the two

rotations test.

BC:CN1 had brittle fractures in nine out of ten specimens and a mixed brittle and ductile

fracture in the last one. All failures occurred between one, and one and a quarter of a rotation

of the piston.

All specimens of BC:CN2 displayed complete brittle fractures after one rotation.

All the mean values for the peak loads of all tested samples, rounded to three significant figures, are

presented below; see Table 5 for the two rotations test and Table 6 for the 15° test.

Table 5. Mean peak values and standard deviation, rounded to three significant figures, for the two rotations bending test.

Sample Test Mean Peak Value (kg) Standard deviation (kg)

Unhardened 2 rotations 41.8 2.04

TDG-test 2 rotations 69.9 4.32

Untreated 2 rotations 102 3.10

BC1 2 rotations 82.7 1.85






18






C-test1 2 rotations 91.1 7.42

C-test2 2 rotations 84.1 3.11

BC:CN1 2 rotations 75.0 4.75

BC:CN2 2 rotations 66.2 2.14

Table 6. Mean peak values and standard deviation, rounded to three significant figures, for the 15° bending test.

Sample Test Mean Peak Value (kg) Standard deviation (kg)

TDG-test 15 deg. 74.0 3.69

Untreated 15 deg. (To crack) 103 6.26

BC1 15 deg. 85.7 2.45

BC2 15 deg. 85.0 1.84

BC3 15 deg. 85.5 3.41

BC4 15 deg. 85.6 2.84

BC5 15 deg. 93.3 2.57

BC6 15 deg. 90.4 3.93

BC7 15 deg. 84.9 3.33

BC8 15 deg. 84.4 2.62

BC9 15 deg. 85.4 4.32

BC10 15 deg. 82.9 3.45

BC11 15 deg. 82.3 1.79

4.2 LOM The LOM of the CorrSealREF, TDG-test, Untreated, and TDG-nail gave indications of the constituent

phases and microstructures of the samples. It also gave an idea of the case hardening depth, as well

as how the surface treatments differed between the samples.

4.2.1 CorrSealREF The edge of the CorrSealREF showed a sharp gradient in the materials microstructure at around 90-

100 μm from the edge. Some kind of surface treatment could be seen around the edges of the sample.

Small grains were present throughout the sample, except in the darker areas closest to the surface.

This can be observed in Figure 10. As seen in Figure 11, the grain boundaries were not visible

throughout the observed area.

19

Figure 10. Surface of the CorrSealREF at 20x magnification. The edge of the screw is visible in the right part of the picture, with the Bakelite at the right edge of the figure.

Figure 11. Picture at 50x magnification, around 100 μm from the edge. The center of the screw is to the left.

4.2.2 Untreated A gradient in the materials microstructure was observed at approximately 100 µm from the edge of

the sample, see Figure 12. Grains were present in the major part of the sample, even though they were

much harder to distinguish in the area between the gradient and the surface. As seen in Figure 13 the

grain boundaries were not easily detected.

20

Figure 12. Surface of the Untreated at 20x magnification. The edge of the screw is visible in the right part of the picture, with the Bakelite at the right edge of the figure.

Figure 13. The center of the Untreated at 50x magnification.

4.2.3 TDG-test A layer of zinc, as well as a post-treatment passivation layer was observed at the surface of the screw.

The grain boundaries were distinct and easily observed throughout the sample, except in areas very

close to the zinc layer. This is seen in Figure 14 and Figure 15. The area close to the zinc layer had a

brighter color.

21

Figure 14. Edge of the TDG-test. 20x magnification.

Figure 15. Center of the TDG-test. 50x magnification.

22

The observed surface layer of TDG-test showed varying results in regularity and uniformity. Some

impurities left from the polishing are visible on the sample. Figure 16 to Figure 18 below visualizes the

differences.

Figure 16. The unetched TDG-test at 5x magnification. The core of the screw is to the bottom right of the picture and the zinc coating is visible as the whiter outermost layer.

Figure 17. Another specimen of an unetched TDG-test at 10x magnification. The core of the screw is to the bottom left.

23

Figure 18. 50x magnification of the surface layer of TDG-test. The core of the screw is to the right in the picture. The red arrow indicates the area absent of a zinc layer and the smaller blue arrow points in the direction of a boundary line between the surface layer and the Bakelite.

4.2.4 TDG-nail The LOM of TDG-nail revealed the appearance of the zinc layer for the commercial product. It showed

a thin and even coating with a few pores. See Figure 19 for an overview. Figure 20 and Figure 21 gives

a more detailed view of two etched specimens.

Figure 19. The unetched TDG-nail at 5x magnification with the core to the bottom right corner. The zinc coating is visible as the whiter outermost layer.

24

Figure 20. A micrograph of the etched sample of TDG-nail at 20x magnification. The red arrow points at a hole in the zinc layer where the passivation material is present all the way into the base material.

Figure 21. The etched TDG-nail at 10x magnification. The red arrow points at a thicker part of the passivation layer.

4.3 SEM-EDS The SEM-EDS analysis provided high resolution images over the cross sections of the samples and

implicated the material composition of the screws. The results for TDG-test, Untreated, and

Unhardened are presented in figures and tables in the following section. The composition of each

spectrum (described at page 12) is presented in Table 7.

25

Table 7. Composition of samples Unhardened (spectrum 4), Untreated (spectrum 5), and different areas of TDG-test (spectra 1-3).

Spectrum C (wt%) Mn (wt%) Si (wt%) Zn (wt%) O (wt%) P (wt%) Fe (wt%)

Spectrum 1 - 0.77 0.02 - - - 99.21

Spectrum 2 - - - 85.90 1.25 0.11 12.74

Spectrum 3 - 0.78 0.02 0.14 - - 99.06

Spectrum 4 - 0.73 0.05 - - - 99.22

Spectrum 5 - 0.73 0.08 - - - 99.19

A linear composition gradient of the surface layer in two different areas, Area1 and Area2, of TDG-test,

are presented in Figure 22 and Figure 23 respectively. Area1 showed a post-treatment layer of

approximately 10 µm thickness, consisting of zinc and phosphorous. Closer to the core, a zinc-iron alloy

layer of around 25 µm thickness was observed. Area2 showed no phosphorous post-treatment layer,

but a zinc-iron alloy layer of approximately 35 µm thickness.

Figure 22. The composition for Area1 is plotted along the green line drawn in the micrograph. The micrograph shows the same area from Figure 6 but mirrored.

26

Figure 23. The composition for Area2 in TDG-test is plotted along the green line drawn in the micrograph.

4.4 Hardness ESSVE demands a hardness of 450-800 HV in the surface, and a hardness of 320-450 HV in the core.

Results from the hardness evaluation, performed at KTH MSE, of TDG-test, Untreated, and Unhardened

are presented in Table 8 to Table 11 below. For the complete sheet with test results, see Appendix:

Hardness. All hardness values presented are the mean values of the Vickers tests.

Table 8. Hardness (HV) in the zinc layer, surface, and core of TDG-test.

Sample No. Mean Surface (HV2) Mean Zink (HV0.5) Mean Core (HV2)

TDG-test 1 434 334 251

2 392 352 275

3 337 286 253

4 332 - 242

5 389 - 283

6 371 - 278

7 400 - 303

8 344 - 273

9 357 - 283

Mean 373 324 271

Standard Deviation (all measurements)

32.8 45.5 19.1

27

Table 9. Hardness (HV) in the surface and core of Untreated.

Sample No. Mean Surface (HV2) Mean Core (HV2)

Untreated 1 650 451

2 575 446

3 560 447

4 570 453

Mean 589 449


39.6 5.81

Table 10. Hardness (HV) on the edge and in the core of Unhardened.

Sample No. Mean Surface (HV2) Mean Core (HV2)

Unhardened 1 - 164

2 166 166

3 - 154

Mean 166 161


1.45 6.35

The hardness values for BC:Max, BC:CN1, BC:CN2, BC1-BC11, C-test1, C-test2, Untreated, and TDG-test

were obtained from Bodycote. These values are presented in Table 11.

Table 11. Hardness data recieved from Bodycote. Note that the applied load when testing BC:Max was not obtained in the report from Bodycote.

Sample Core hardness Surface hardness

Untreated (1) (HV1) 432 716

Untreated (2) (HV0.3) 434 617

BC:Max 454 -

BC:CN1 (HV1) 459 908

BC:CN2 (HV1) 407 901

BC1 (HV1) 382 617

BC2 (HV1) 376 637

BC3 (HV1) 372 613

BC4 (HV1) 398 646

BC5 (HV1) 398 518

BC6 (HV1) 382 546

BC7 (HV1) 363 622

BC8 (HV1) 394 625

BC9 (HV1) 361 495

BC10 (HV1) 358 617

BC11 (HV1) 367 619

C-test1 (HV1) 386 578

28

C-test2 (HV1) 374 512

TDG-test (1) (HV1) 252 354-372

TDG-test (2) (HV1) 302 398-428

4.5 Corrosion Resistance The results from the measurements of the thickness of the zinc layer are presented in the Table 12.

Table 12. Measured thickness of the zinc layer in TDG-test.

Sample No. Mean (μm) Max (μm) Min (μm) Standard deviation (μm)

TDG-test 1 45 62 31 13

2 22 29 19 5

3 30 37 25 5

4 34 45 27 8

5 36 37 33 2

Mean 33.3

Standard Deviation (of mean values)

7.54

The homemade salt chamber showed immediate rust attacks on the samples Untreated and

Unhardened, shown in Figure 24. Yet after almost six weeks there were no signs of corrosion on the

CorrSeal treated decking screw or TDG-test. Samples Unhardened, Untreated, and the electroplated

drywall screw were completely covered in rust and spots of red rust were visible on the HDG-nail.

Figure 24. The homemade salt chamber after 5 days. The samples in order from left to right in the picture are: Untreated (3), TDG-test (2), a CorrSeal coated decking screw (1), Unhardened (6), an electroplated drywall screw (5), and HDG-nail (4). There were visible precipitates of rust particles in the water and Unhardened and Untreated had evident layers of corroded material on the surface. Thumb for scale.

29

5 Discussion

The aim of the study was to investigate the possibility to apply a surface treatment to screws that is

cheaper, more environmentally friendly, less toxic, and more efficient that the current methods. The

goal was to ascertain if the changes in mechanical properties by the method of sherardizing by DiSTeK

could be approximated in a standard industrial furnace for easing further investigations in the subject.

Furthermore, another goal was to present a specific hardening profile for a wood screw that could be

combined with the TDG process to serve the aim and still maintain the required mechanical and

chemical properties.

Seeing as ESSVE aims to use the TDG process on a large-scale production, the safety of the plant

workers is important to take into consideration. As mentioned in chapter 1 Introduction, the method

of TDG has been described as more environmental friendly in comparison to hot dip galvanizing or the

nano coating technology that is currently used by the company [8]. Other benefits are the lesser

amount of gaseous and particulate emissions as well as decreased hazardous wastes due to the nature

of the method [5] [9]. When developing a new product it is of great importance to take social and

ethical aspects into consideration. The method of TDG would lower the risks for the plant workers with

decreased exposure to harmful substances. According to ESSVE, the ethical and environmental aspects

are of high priority and one of the primary reason to the change of coating method [45]. It is to be

noted that some amount of waste is created in the TDG process, mainly in the form of unused zinc

dust [5]. However, a process completely free from emissions and hazardous wastes would be a dream

scenario with the available technology.

The focus of the study changed as the project progressed. This came as a result of evaluating the

experiments. To find a hardening profile that could be combined with the TDG process was early on

proven difficult during the timespan of the thesis. Results of the hardening at Bodycote, as seen in

chapter 4.4 Hardness, showed that the screw Untreated was already at, or close to, maximum possible

hardness in the core before tempering. The possibility of increasing the core hardness prior to

tempering, to better withstand the TDG process, was therefore excluded in this study. Seeing as the

hardness of Untreated had already reached its maximum, the focus of the project was altered towards

examining the tempering step and the possibility of incorporating it into the TDG process. If the high

temperatures in the TDG process could be used to temper the screw, it would not need to be tempered

in a separate step after hardening. To avoid unnecessary shipment costs, to save time, and to take

environmental aspects into account, Bodycote was consulted to examine this.

However, the correlation between the TDG process by Promet and the tempering methods used by

Bodycote, needed to be examined. The processes turned out to be uncorrelated in the temperature

span Ap (Table 3). Several tests (A1-A3) were therefore performed to establish a relationship between

the furnaces at Bodycote and the furnace at Promet. An accurate relation could not be established

during this project. However, such a relationship was found probable and further investigations are

needed for it to be determined. The usage of thermocouples as well as extended comparisons between

the TDG process and industrial furnaces are to be considered.

During the course of the project, the methods of the experiments were strongly influenced by three

different sources. The ASTM standard [26] that DiSTeK complies with, the evaluation of TDG on nails

performed by SP mentioned in chapter 3.6 Corrosion Resistance, as well as ESSVE’s demands for

bending and hardness. The tests were not always executed precisely as described in these sources due

to limitations in time and equipment. For some tests, distinguishable conclusions could be drawn.

Other tests showed only trends, and rough estimations were made.

30

As mentioned in chapter 3.2 Bending Test, not all samples had the same amount of screws tested in

each method of bending. The reasons for this were that during the tests of the screws, it became

evident that BC:CN1 and BC:CN2 were too brittle to be bent in any other way than to fracture. It also

proved to be redundant to test Unhardened more than on a few specimens since it was too ductile.

Additional testing on that sample would not have resulted in more relevant information. The sample

size of C-test1 and C-test2 did not allow the samples to be tested in two ways. The same issue was

present for BC6 and BC9 where the sample size was not adequate for 10 specimens in both tests and

therefore only five was used for the second test. Another deviation was the testing of Untreated.

Estimating 15 degrees bending angle without breaking the screw was proven too difficult in a pretest

for maintaining repeatability. This turned out to be a minor issue where the results show that most

samples in fact got to 15 degrees before crack initiation.

It is also of importance to note that two different dimensions and lengths are compared in the bending

tests. The difference between the tested wood screws and decking screws is 0.2 mm in diameter and

5 mm in length, as seen in Table 4. This might affect the load capacity for the screws. Furthermore, to

get data that are more exact, a machine that uses a constant bending rate could be used. However,

since the results obtained when using this method is not very precise, this might be of little use.

Therefore, the experimenter estimated the same rotation speed by hand. This being said, the method

still provided relevant and comparable data.

The results from the primary bending tests are of great interest when evaluating the effects on the

mechanical properties by the surface treatment. These results prove that Unhardened is by far the

most ductile as well as the least load bearing sample, as seen in Table 5. Untreated is the screw with

highest load bearing capabilities and do pass the test of 15 degrees angle without major cracks. This is

to be expected since Untreated is the screw that is already tempered and used today by ESSVE and is

sold with the CorrSeal treatment. The TDG-test was suspected to fall somewhere between these

results and that is confirmed. It passes the required angle of 15 degrees bending without any major

cracks in the material. This implies that the screw has lost some of its resistance to bending but might

still be practically useful in comparison to the unhardened screw. That the screw has lost its resistance

to bending means that it cannot carry the same weight before large deformations takes place. There

is no expressed demand on this matter, hence the screw has to be analyzed in other ways. However,

the bending test gives an idea of the outcome of other evaluations regarding mechanical properties.

Evaluating the results of Untreated and TDG-test against the samples BC1-BC11 gives an idea of how

the screws mechanical behavior has changed in the furnaces at Bodycote compared to in the TDG

process at Promet. Comparing the results from the two rotations test of TDG-test, BC5, BC6, and BC9

shows that the load bearing capabilities were much higher of the screws that went through the furnace

tempering at Bodycote (BC5, BC6, and BC9) compared to the ones that had the surface treatment

(TDG-test). Additionally, results from the hardness tests reveals that TDG-test lost noticeably more of

its hardness that BC5. This is of great interest since BC5 was designed to follow the same times and

temperatures as TDG-test (Table 2).

The results of comparing TDG-test with the samples tempered by Bodycote, complies with the

previously discussed topic of a correlation between the furnaces at Bodycote and the furnace of

Promet. As stated by DiSTeK-Scandinavia, the times and temperatures used by Promet in the process

of TDG-test is a subject of uncertainty [23]. This is supported by results from bending and hardness

tests, in combination with the fact that Bodycote claims to have a margin of ± 5 °C in their furnaces

[46]. Therefore, it is suspected that the temperatures that the screws are exposed to in the TDG

process are most likely higher than stated. Another possibility is that the first heating step in Ap (Table

31

2), RT to 320 °C in 80 minutes, is not actually linear as stated by Promet, but logarithmic increasing

towards 320 °C [42]. The suspicion of the inaccurate temperatures at Promet is supported by test

results of BC6 and BC9. These samples have been treated for a longer time at a higher temperature

than BC5, but still maintain higher load bearing capacity and hardness than TDG-test.

This raises many questions about the accuracy in the TDG process and whether something else in the

process might affect the mechanical properties of the screws, such as pressure, gas composition etc.

The results also indicate that the data from BC1-BC4, BC7-BC8, BC10-BC11, C-test1, and C-test2 cannot

be seen as directly translatable to the screws behavior in a TDG process.

However, what can be seen is that all the screws with the carbonitriding processes behave almost in

the same way when it comes to load bearing in all bending tests. When comparing BC1 and BC2, it is

noted that the shortened tempering time in BC2 does not show any significant changes in the bending

tests. BC2 shows signs to be slightly more brittle which could be expected, but the uncertainty in the

testing is to be taken into consideration.

For the screw with higher case depth and shortest tempering time, BC4, the increased brittleness

compared to BC3 is more noticeable. It is debatable whether it can be accepted or not, in accordance

to the demands by ESSVE. BC4 is slightly too brittle to be accepted, since it does not withstand 15

degrees bending without cracks.

Some samples (BC5-BC6 and BC9) have been exposed to tempering twice, first during the

manufacturing in Taiwan, and later in the heat treatment at Bodycote. It is noted that there is a

difference in load bearing capacity between these samples, and correlating samples tempered only

once by Bodycote (BC7-BC8 and BC10-BC11). Expected results would be that tempering twice should

lead to a lower load bearing capacity. This is however contradicted by the results. Due to the different

dimensions of the screws, as well as other possible factors, comparisons between the two types should

not be made. Samples with the same dimensions correlates and can be directly compared. Only trends

can be studied between samples with different dimensions.

Samples hardened by Kwantex show a different behavior when exposed to increased temperatures

(A3), compared to samples carbonitrided by Bodycote. The decrease in load bearing capacity when

moving from A2 to A3 is smaller for the carbonitrided samples than for samples hardened by Kwantex.

This could indicate that carbonitriding provides a better resistance to tempering softening than

carburization. On the other hand, this might also come as a result of the carburized screws being

tempered twice. Regardless, the carbonitrided screws behave more promising towards serving the aim

of the project.

Analyzing results from the experiments points towards a correlation between the hardness of the

material and the results of the bending tests. The surface hardness of BC6-BC11 exhibit the same

trends as can be seen in the results of the bending tests. Evaluating Table 6 and Table 11 combined

shows that the screws based on Untreated loses both hardening and load required for 15 degrees

bending when subjected to A3 compared to A2 (and A1 as well as Ap). This change is not evident when

comparing the screws based on CN1 and CN2 in neither bending nor hardness. However, the hardness

values need to be critically reviewed due to the methods used for measuring. Nonetheless, this

relationship is interesting and should therefore be examined in full detail to determine if it is

statistically correct or not in later studies.

The values of surface hardness tests, performed at KTH MSE, should not be directly compared to the

measurements at Bodycote. This comes as a result of the criteria of indentation distance for the Vickers

hardness method. In this case, the criteria demand the indentations to be placed at a distance from

32

the edge of the sample of at least 2.5 times the diagonal length of the resulting indentation. This is a

recommended distance given by the technician at KTH MSE [47] for iron based alloys to avoid any

influence of the surrounding material on the results. In this case it came to be close to the observed

case depth. The great difference in the measured values, seen in chapter 4.4 Hardness, might therefore

depend on the gradient of the phases at this distance from the edge. The problem is also present at

the zinc layer, from which it is even more difficult to obtain reliable results due to its thin nature.

The method for measuring the surface hardness can be chosen differently. However, the current

method, using HV0.5 for the zinc layer and HV2 for the steel, was chosen in discussion with the

technician at the KTH MSE [47]. When Bodycote later on were consulted, it turned out that they

measured the surface hardness by a linear extrapolation with two or more points [43]. This will most

likely give a significantly higher value of the surface hardness compared to tests conducted at KTH

MSE. Furthermore, Bodycote performs the hardness test on only one specimen per sample in

accordance to their standard. This is a subject of great uncertainty from a statistic standpoint and must

be considered when drawing conclusions.

The results of the LOM show a difference in microstructure between Untreated and TDG-test, which is

to be expected. In Untreated, no visible grain boundaries are observed, whilst the grain boundaries of

TDG-test are evident. This indicates a precipitation in the grain boundaries during the TDG process.

Assuming that martensite is present in the surface of Untreated, one explanation might be that the

martensite has decayed into cementite. The cementite will form in the old austenite grain boundaries,

as they serve as nucleation points.

The LOM results also show signs of a phase gradient in the surface of all samples, though it is most

distinguishable in CorrSealREF. This phase gradient is estimated to be about 100 µm thick, and is

probably caused by the carburizing process as a result of the carbon diffusing into the steel. This theory

is supported by the results from Bodycote where their first test of Untreated showed a case depth of

around 100 µm and a second measurement of one specimen showed a case depth of 70 µm after the

first tempering [31] [43]. The SEM-EDS analysis cannot be used to further investigate this. Due to

carbon being such a small atom, SEM-EDS is not able to measure the content accurately and it will

therefore be approximated to 0 % [47]. How much the results from SEM-EDS and LOM analysis

influence the hardness of the screw is difficult to say. The fact that it has an impact is however a

reasonable statement.

As mentioned in the literature study, adding silicon is a way to decrease the loss of hardening due to

tempering. A very small amount of silicon is detected in all screws examined in the SEM-EDS. Seeing

as the current steel follows a certain standard, a change to another steel type might be needed. A steel

with a higher amount of silicon will most likely help to prevent the tempering softening that is present

during the TDG process. Silicon is a relatively cheap element, and is therefore the preferred option

over more expensive elements such as Cr, W, V, and Mo. A content of 0.5-1.5 % should provide a higher

resistance towards tempering softening [37] [38]. Furthermore, as described in 2.3.2 Quenching,

carburization followed by quenching from a higher temperature might lead to slightly increased core

hardness and could be a subject for future research.

There is a discussion going on at ESSVE regarding the screws that are distributed today. ESSVE believes

that the screws might be unnecessarily hard [48]. This is debated since a lower hardness would tolerate

the screws to be bent in one way and then bent back again. Today, the wood screw examined in this

project would not withstand this without breaking. This would mean that some results from the tests

at Bodycote is still promising and worth taking into consideration for a continued work.

33

Despite the fact that ESSVE might consider to lower their demands regarding hardness, TDG-test still

displays values too far from what could be acceptable. Additionally, there are more noticeable

variations in core hardness values between different specimens of TDG-test in comparison to

Untreated (see Table 8 and Table 9). It is worth discussing if there is a correlation between these

fluctuations and the fluctuations observed in the zinc layer thickness of TDG-test. It is possible that,

during the TDG process, potential temperature differences in the furnace is a common denominator

for the varying core hardness and zinc layer thickness.

For measuring corrosion resistance, ESSVE uses a salt chamber test. However, it was not considered

necessary during the timespan of this project. The only sample in the study to be relevant for testing

is TDG-test. Seeing as that sample does not meet the mechanical requirements, such a time consuming

experiment was discarded. However, assumptions about the outcome of the salt chamber test can be

made based upon the zinc layer thickness. For example, in a report from SP [12] about TDG-nail, a

mean layer thickness of 35.3 µm, calculated from 19 tested samples, met the C4-standard by a

considerable margin. Seeing as TDG-test has a mean value of 33.3 µm, this would indicate that TDG-

test stand a fair chance of meeting the demands of corrosion resistance. In addition, SEM-EDS confirms

that the measured surface layer of TDG-test is actually a zinc iron alloy consisting of around 85 wt%

zinc.

The results of the zinc layer thickness measurements of TDG-test vary to quite a high degree, both

between samples and within the same specimen. In some areas, there are sections where the zinc

layer is practically nonexistent, see Figure 17 for an overview and Figure 18 for higher magnification.

The sections containing no observable zinc are of varying lengths, but no area observed is larger than

a hundred microns. During the circumstances, this might be considered small.

The unevenness of the layer could be due to the cutting of the samples as a preparation for LOM,

where surface treatment might have fallen off. There is evidence of this when cutting CorrSealREF,

where visible sheets of surface paint fell off. However, as seen in Figure 18, LOM analysis of TDG-test

shows that the phosphorus post-treatment coating is still visible in the areas where the zinc layer is

very thin. This fact contradicts the theory of the LOM preparations damaging the surface layer. The

most feasible reason for the uneven layer is therefore that the zinc was applied in a way that did not

allow a higher degree of uniformity.

One reason to why the zinc layer might be uneven is that the screws were spotted with rust when they

were sent to Promet. According to their website [24], DiSTeK claims that no pre-cleaning is necessary

before application of the zinc layer. However, some sources say that articles in most cases do require

some manner of pre-cleaning, though not as thoroughly as in sherardizing [23] [26].

The irregularity in the surface coating does not necessarily mean that the screw will fail to meet the

demands concerning corrosion resistance. Because of the nature of zinc, the surface coating will act

both as a protective layer and as a sacrificial anode. This means that the surrounding zinc atoms can

and will corrode, instead of the iron, even in areas not completely covered by the coating [3]. In

addition, TDG-nail, which is approved for commercial usage, also shows an irregular thickness.

According to SP’s report [12] the 19 different specimens had individual deviations of 0.9-7.2 µm and a

standard deviation of the mean values of the zinc layer thickness of 8.9 µm. The same calculations for

the five tested specimens of TDG-test (Table 12) give individual standard deviations of 2-13 µm and a

standard deviation of the mean values of the zinc thickness at 7.5 µm. A difference can be seen

between the samples, however, it is not very pronounced.

34

The scientific reliability of the homemade salt test is to be considered. The concentrations and

conditions will be easy to replicate, however, the sample size is worth some criticism. Since only one

specimen of each sample is tested the results depends fully on the certainty of the surface treatments.

For the commercialized products, this may be acceptable but the risk of getting a not representative

sample of the TDG-test is higher. The result of this test will therefore be treated with caution when

included in the conclusions.

35

6 Conclusions and Recommendations

The screw as it is delivered today, hardened and tempered, should not be used for the thermal

diffusion galvanization (TDG) process. The TDG treated screws passes the test of bending 15°

before break, but fails in regards to hardness.

Using TDG process as the tempering step is highly recommended for further analysis.

No correlation between the TDG process at Promet and tempering in industrial furnaces is

found. However, it is probable that such a relationship does exist.

The screws surface treated by Promet are most likely exposed to higher temperatures than

the temperatures given for the process. It is recommended to use thermocouples to further

investigate this.

There is a correlation between hardness and bending angle before break. Trends seen in

bending tests are also detected when measuring hardness.

The results points towards that TDG-test will meet the corrosion resistance demands of C4

standard.

Using another steel alloy with higher resistance against tempering softening is an alternative

way for ESSVE to introduce a screw with the TDG treatment to the market. It is recommended

to more closely analyze the effects of adding additional silicon for this purpose.

It will require additional research to apply the TDG process by DiSTeK, on wood screws

distributed by ESSVE, while still maintaining the desired mechanical properties.

36

7 Acknowledgements

We, the authors of this report, would like to thank

Anders Tilliander, KTH Department of Material Science and Engineering,

Christofer Lindberg, ESSVE Produkter AB,

David Thunberger, ESSVE Produkter AB,

Wenli Long, KTH Department of Material Science and Engineering,

Sigve Bø, DiSTeK-Scandinavia,

Anders Lindhe, Bodycote Värmebehandling AB,

Jessika Lindsjö, Tyresö,

Annika Borgenstam, KTH Department of Material Science and Engineering,

for guidance and support during the course of this project.

Ante Vallien, Benjamin Solem, Philip Wernstedt

2016-05-16

Stockholm, Sweden

37

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38

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USA, 2008.

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i materialdesign, Stockholm, Instutionen för Materialvetenskap, Kungliga Tekniska Högskolan,

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http://www.bodycote.com/en/services/heat-treatment/case-hardening-with-subsequent-

hardening-operation/atmospheric-carburising.aspx. [Accessed 07 03 2016].

39

[33] Bodycote plc, “Carbonitriding,” [Online]. Available:

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hardening-operation/carbonitriding.aspx. [Accessed 7 03 2016].

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Heinemann, 1984, pp. 476-478.

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Elsevier Science, 2006, p. 183–208.

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Science and Technology, vol. 15, no. 5, pp. 527-530, 1999.

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Jernkontoret, 2004.

[39] W. Callister, “Chapter 5: Diffusion,” in Materials Sience and Engineering, 7 ed., John Wiley & Sons,

Inc., 2007, pp. 109-130.

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GmbH & Co. KGaA, 2005, pp. 27-44.

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8:th ed., John Wiley & Sons, Inc., 2007, p. 1.

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[43] Hårdhetsprovning - Vickers: Protokoll, Västerås: Bodycote, 2016.

[44] SP Technical Research Institute of Sweden, “Test of nails according to SS-EN 1492:2008,” SP

Technical Research Institute of Sweden, 2011.

[45] D. Thunberger, Interviewee, Divsion Manager Retail, ESSVE Produkter AB. [Interview]. 06 2016.

[46] A. Lindhe, Interviewee, Plant Manager, Bodycote Värmebehandling AB. [Interview]. 28 04 2016.

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[Interview]. 03 2016.

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Edition), Oxford, Butterworth-Heinemann, 1992, pp. 13-1 - 13-119.

I

Appendix: Bending

2 rotations test

Sample No. Kg (at peak) Bending test Crack

Unhardened 1 44 2 rotations No

2 38 2 rotations No

3 42 2 rotations No

4 43 2 rotations No

5 42 2 rotations No

Mean value 41.8

Standard deviation 2.04


TDG-test 1 65 2 rotations No, but small surface fractures.

2 74 2 rotations No, but small surface fractures.









Mean value 69.9

Standard deviation

4.32


Untreated 1 103 2 rotations No

2 104 2 rotations No

3 99 2 rotations Yes. Crack noise and load bearing drop. 15 degrees







10 96 2 rotations Yes. Crack noise and load bearing drop. 15 degrees

Mean value 102

II

Standard deviation

3.10

Sample No. Kg (at peak) Bending test Crack Angle

BC1 1 82 2 rotations no <<15

2 86 2 rotations no <<15









Mean value 82.7 Estimated to around 5°

Standard deviation

1.85












Mean value 84.2 Approximately halfway to 15°

Standard deviation

1.25


BC3 1 84 2 rotations no <15

2 83 2 rotations no <15





III





Mean value 84.0 Estimated to 10-12°

Standard deviation

1.55












Mean value 84.7 Estimated to 10-12°

Standard deviation

2.72


BC5 1 88 2 rotations no ~<15

2 93 2 rotations no ~<15






8 84 2 rotations no ~15



Mean value 89.6 Almost 15°

Standard deviation

5.06




IV









Mean value 88.2 Estimated between 8-12°

Standard deviation

5.15













Standard deviation

1.61













V

Standard deviation

3.22


BC9 1 82 2 rotations no ~<15










Mean value 83.8 Close to 15°

Standard deviation

3.16












Mean value 82.3 Very small angle, max 5°

Standard deviation

1.73








VI





Mean value 82.5 Very small angle, max 5°

Standard deviation

1.80


C-test1 1 89 2 rotations no <15










Mean value 91.1 Estimated to around 8-10°

Standard deviation

7.42


C-test2 1 80 2 rotations no ~<15










Mean value 84.1 Close to 15°

Standard deviation

3.11


BC:CN1 1 80 2 rotations yes Brittle failure around 1.25 rotations

VII

2 72 2 rotations yes Brittle failure around 1 rotation

3 66 2 rotations yes Brittle failure around 1 rotation

4 75 2 rotations yes Partially brittle failure around 1.25 rotations

5 81 2 rotations yes Brittle failure around 1.5 rotations






Mean value 75.0

Standard deviation

4.75


BC:CN2 1 67 2 rotations yes Brittle failure at 1 rotation

2 63 2 rotations yes Brittle failure at 1 rotation









Mean value 66.2

Standard deviation

2.14

VIII

15 degrees test

Sample No. Kg (at peak) Bending test Angle Comment

TDG-test 1 71 15 deg. >15 Larger surface crack

2 72 15 deg. ~>15 Small surface crack

3 77 15 deg. ~<15 Small surface crack

4 80 15 deg. 15 Small surface crack

5 75 15 deg. ~>15

6 69 15 deg. 15 Larger surface crack



9 78 15 deg. 15

10 77 15 deg. 15

Mean value 74.0

Standard deviation

3.69


Untreated 1 101 15 deg. (to crack) 15 Crack. Crack noise and load bearing drop

2 100 15 deg. (to crack) 15 Visible crack but without noise

3 102 15 deg. (to crack) ~>15 Crack. Crack noise and load bearing drop


5 114 15 deg. (to crack) ~30 Crack. Crack noise and load bearing drop

6 96 15 deg. (to crack) 15 Crack. Crack noise and load bearing drop

7 113 15 deg. (to crack) >30 Crack. Crack noise and load bearing drop




Mean value 103

Standard deviation

6.26


BC1 1 81 15 deg. ~15 Partially brittle fracture

2 85 15 deg. ~15 Lost load bearing

3 85 15 deg. ~>15 Auidible, lost load bearing

IX


5 86 15 deg. ~15 No load bearing drop

6 86 15 deg. ~<15 No load bearing drop





Mean value 85.7

Standard deviation

2.45


BC2 1 85 15 deg. ~15 Lost load bearing

2 85 15 deg. ~>15 Audible brittle fracture









Mean value 85.0

Standard deviation

1.84


BC3 1 86 15 deg. ~15 No load bearing drop

2 83 15 deg. ~15 Audible fracture, lost load bearing, less than 2.5 rotat.


4 85 15 deg. ~15 Some drop in load bearing





9 83 15 deg. >15 Audible fracture, lost load bearing, less than 2.5 rotat.


Mean value 85.5

Standard deviation

3.41

X


BC4 1 88 15 deg. --- Complete brittle fracture

2 88 15 deg. --- Complete brittle fracture

3 89 15 deg. <15 Only ~2.1 rotations. Intact


5 90 15 deg. ~15 Intact


7 86 15 deg. ~<15 Intact

8 84 15 deg. ~15 Intact

9 84 15 deg. ~15 Crack

10 83 15 deg. ~15 Complete brittle fracture

Mean value 85.6

Standard deviation

2.84

Sample No. Kg (at peak) Bending test Crack Comment

BC5 1 95 Failure (max 5 rotations)

No Very large angle

2 91 Failure (max 5 rotations)

Yes Ductile failure (~4 rotations)


Yes Ductile failure (~4.5 rotations)






No Very large angle. Small load bearing drop.






Yes Ductile failure (~4.5 rotations)


No Very large angle. Small load bearing drop.

Mean value 93.3

Standard deviation

2.57


BC6 1 87 15 deg. ~15 No crack.

2 96 15 deg. ~15 No crack, small surface crack

XI

3 89 15 deg. ~15 No crack, small surface crack

4 94 15 deg. ~15 No crack.

5 86 15 deg. ~15 No crack.

Mean value 90.4

Standard deviation

3.93


BC7 1 87 15 deg. ~15 No crack.

2 88 15 deg. ~15 No crack.

3 78 15 deg. ~15 No crack.

4 88 15 deg. ~15 No crack.

5 85 15 deg. ~15 No crack.

6 87 15 deg. ~15 No crack.

7 88 15 deg. ~15 No crack.

8 84 15 deg. ~15 No crack.

9 80 15 deg. >15 Partially brittle fracture.

10 84 15 deg. >15 Partially brittle fracture.

Mean value 84.9

Standard deviation

3.33


BC8 1 85 15 deg. ~<15 No crack.

2 83 15 deg. ~15 Crack, with load drop.

3 90 15 deg. ~<15 No crack.

4 80 15 deg. ~<15 Small surface crack, with load drop.

5 84 15 deg. ~<15 Small surface crack, with load drop.

6 82 15 deg. ~<15 No crack.

7 85 15 deg. ~<15 No crack.

8 87 15 deg. ~<15 No crack.

9 85 15 deg. ~<15 No crack.

10 83 15 deg. ~<15 Crack, with load drop.

Mean value 84.4

Standard deviation

2.62


BC9 1 81 15 deg. ~15 Just a small surface crack

2 82 15 deg. ~15 No crack

XII




Mean value 85.4

Standard deviation

4.32


B10 1 85 15 deg. ~15 No crack


3 83 15 deg. ~15 Just a small surface crack. Small load drop


5 81 15 deg. ~15 Just a small surface crack





10 82 15 deg. ~15 No crack

Mean value 82.9

Standard deviation

3.45


B11 1 81 15 deg. ~15 Small surface crack. Load drop

2 82 15 deg. ~15 Small surface crack.





7 82 15 deg. ~15

8 80 15 deg. ~15

9 81 15 deg. ~15

10 85 15 deg. ~>15 Crack. Load drop

Mean value 82.3

Standard deviation

1.79

I

Appendix: Hardness

In the following tables, S stands for surface, Z for zinc, and C for core.

The hardness for the surface and core is measured with 200g (HV2) while the zink layer is measured with 50g (HV0.5).

TDG-test No. S 1 S 2 S 3 S 4 Z 1 Z 2 Z 3 Z 4 C 1 C 2 C 3 C 4 Mean S

Standard deviation S

Mean Z

Standard deviation Z

Mean C

Standard deviation C

Diagonal 1 28.7 29.3 29.4 29.5 12.3 11.4 11.9 11.6 38.9 38.3 38.2 38.4 29.2 0.311 11.8 0.339 38.5 0.269

(μm) 2 31.0 30.6 31.2 30.3 11.6 10.6 12.3 11.6 36.2 37.2 36.8 30.8 0.349 11.5 0.606 36.7 0.411

3 33.7 32.9 32.5 33.7 14.0 11.4 12.4 13.6 38.3 37.9 38.4 38.4 33.2 0.520 12.9 1.02 38.3 0.206

4 33.2 33.5 33.5 33.4 39.6 39.0 38.9 38.9 33.4 0.122 39.1 0.292

5 30.5 31.2 31.0 30.9 35.8 36.3 36.4 36.3 30.9 0.255 36.2 0.235

6 31.0 31.3 32.4 31.7 36.4 36.0 36.5 37.1 31.6 0.524 36.5 0.394

7 30.5 30.7 30.5 30.2 34.5 34.6 36.3 34.7 30.5 0.179 35.0 0.740

8 32.2 32.5 33.4 33.2 36.6 37.0 37.2 36.7 32.8 0.492 36.9 0.238

9 32.1 32.4 32.3 32.2 36.3 35.9 36.7 36.0 32.3 0.112 36.2 0.311

Hardness 1 449 432 430 426 305 356 329 346 246 253 254 251 434 8.62 334 19.3 251 3.13

(HV) 2 386 397 382 403 346 411 307 346 282 269 274 392 8.43 352 37.3 275 5.63

3 327 342 352 327 238 356 301 251 253 258 251 251 337 10.9 286 46.4 253 2.52

4 336 330 331 333 236 244 245 244 332 2.27 242 3.54

5 400 382 385 388 289 282 280 282 389 6.81 283 3.50

6 385 378 355 368 280 286 279 269 371 11.3 278 5.88

7 399 395 400 405 311 309 282 309 400 3.88 303 12.1

8 357 352 332 336 277 270 268 276 344 10.3 273 3.80

9 360 354 356 359 282 288 276 286 357 2.19 283 4.46

II

Untreated No. S 1 S 2 S 3 S 4 C 1 C 2 C 3 C 4 Mean S


Mean C


Diagonal 1 24.6 23.7 23.8 23.5 28.5 28.9 28.5 28.8 23.9 0.418 28.7 0.179

(μm) 2 26.1 25.2 25.3 25.1 29.0 28.9 28.7 28.7 25.4 0.396 28.8 0.130

3 25.7 25.6 25.7 26.0 28.7 29.0 28.6 28.9 25.8 0.150 28.8 0.158

4 25.6 25.1 25.4 26.0 28.5 28.9 28.4 28.6 25.5 0.327 28.6 0.187

Hardness 1 612 662 655 672 457 445 457 446 650 23.0 451 5.57

(HV) 2 544 586 580 590 442 445 449 449 575 18.3 446 2.71

3 561 567 563 547 451 440 453 444 560 7.51 447 5.35

4 567 588 576 551 457 444 459 452.8 570 13.6 453 5.94

Unhardened No. S 1 S 2 S 3 S 4 C 1 C 2 C 3 C 4 Mean S


Mean C


Diagonal 1 47.9 47.0 47.2 47.9 47.5 0.406

(μm) 2 46.9 47.5 47.3 47.4 46.4 47.5 48.1 47.2 47.3 0.228 47.3 0.612

3 48.4 49.8 49.3 48.9 49.1 0.515

Hardness 1 162 168 167 162 164 2.84

(HV) 2 168 164 166 165 172 164 160 167 166 1.45 166 4.26

3 159 150 153 155 154 3.24

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