5

TABLE OF CONTENTS

CHAPTER NO. TITLE PAGE NO.

ABSTRACT 8

LIST OF TABLES 9

LIST OF FIGURES 9

1. LITERATURE REVIEW 10

1.1 INTRODUCTION 10

1.1.1 Definition 10

1.1.2 Composition 10

1.1.3 Classification 11

1.1.3.1 Weathering Steels

1.1.3.2 Control Rolled Steels

1.1.3.3 Pearlite Reduced Steel

1.1.3.4 Acicular Ferrite Steel

1.1.3.5 Dual Phase Steel

1.1.3.6 Micro-Alloyed Steel

1.1.4 Properties 11

1.1.5 Specifications 12

1.1.6 Applications 14

1.2 DEVELOPMENT OF HSLA 15

1.2.1 Mechanism of Alloying 15

1.2.2 Effect of Alloying Elements on Properties of HSLA 16

1.2.2.1 Nitrogen

1.2.2.2 Manganese

1.2.2.3 Silicon

1.2.2.4 Copper

1.2.2.5 Phosphorus

6

1.2.2.6 Chromium

1.2.2.7 Nickel

1.2.2.8 Molybdenum

1.2.2.9 Aluminum

1.2.2.10 Vanadium

1.2.2.11 Titanium

1.2.2.12 Zirconium

1.2.2.13 Boron

1.2.2.14 Calcium

1.3 STRENTHENING MECHANISM OF HSLA 21

1.3.1 Grain Refinement 21

1.3.2 Solid Solution Strengthening 23

1.3.3 Phase Balance Strengthening 25

1.3.4 Precipitation Strengthening 28

1.3.5 Work Hardening 32

1.4 EFFECT OF GRAIN SIZE ON PROPERTIES OF HSLA 33

1.4.1 Measurement of ASTM Grain Size Number 33

1.4.1.1 Comparison Method

1.4.1.2 Grain Counting Method

1.4.1.3 Intercept Method

1.4.2 Importance of Grain Size 34

2. EXPERIMENTATION 38

2.1 SAMPLE PREPARATION 38

2.1.1 Sectioning 38

2.1.2 Mounting 38

2.1.3 Coarse Grinding 39

2.1.4 Medium and Fine Grinding 39

2.1.5 Mechanical Polishing 40

2.1.6 Etching 41

7

2.2 MICROSCOPIC ANALYSIS 42

2.3 HARDNESS TESTING 42

2.4 HEAT TREATMENT 43

2.5 RETESTING 43

3. RESULTS AND DISCUSSION 44

CONCLUSION 45

REFRENCES 46

8

ABSTRACT

Structural steels with a ferrite-pearlite microstructure are widely used in civil, mechanical and

chemical engineering. Increased working loads, reliability requirements, extreme temperature and

chemically aggressive environments lead to an increased demand for high strength, combined with

high toughness, in these steels. The use of welding as a joining method requires good weldability, for

which the carbon content in a steel composition should be decreased. A decrease in carbon content

results in strength decrease, due to a decrease in the amount of the second phase (pearlite).

To overcome this strength drop, additions of titanium, niobium and vanadium microalloying

elements, in amounts no more than 0.15 wt%, have been used to provide precipitation strengthening

and grain size refinement. Thus, during the last thirty years high strength low alloy (HSLA)

structural steels have found widespread use in automotive, construction and pipe-line transportation

industries.

The aim of the present project is to study the influence of steel chemistry on grain size as well as to

establish a study of effect of grain size on the mechanical properties of High Strength Low Alloy

Steels. To bring about the above study, various literatures were surveyed and related information was

gathered. An experiment was also conducted on HSLA steel sample in which the sample was

subjected to various heat treatments to bring about change in its grain size and hardness testing was

done on the heat treated samples so as to establish a relationship between grain size and hardness.

9

LIST OF TABLES

S. No. TABLE NAME Pg. No.

1 Typical composition of HSLA steel 10

2 SAE HSLA Steel Grade composition 12

3 SAE HSLA Steel Grade Mechanical properties 13

4 Experimental value for stress and Ky terms in Hall-Petch equation 22

5 Strengthening coefficient for a no. of solutes 24

6 Increment of yield stress from the micro alloying element

precipitation 31

7 Effect of grain size on properties of HSLA 37

8 Composition of HSLA Sample Under Study 38

9 Results obtained from experimentation 44

LIST OF FIGURE

S. No. FIGURE Pg. No.

1 Fe-Mn Phase Diagram 16

2 Fe-Si Phase Diagram 17

3 Fe-Cr Phase Diagram 18

4 Fe-Ni Phase Diagram 19

5 Fe-Ti Phase Diagram 20

6 Strengthening effects of substitutional solute atoms in iron 24

7 Factors contributing to the strength of C-Mn Steels 25

8 Effect of pearlite on work hardening rate 26

9 Influence of carbon content on strength of plain carbon steel 26

10 Influence of 50% transformation temperature on tensile strength

via formation of different steel structures 28

11 Strengthening contributions of different parameters on yield

strength of hot-rolled 29

12

Addition to strength predicted by Orowan and Ashby-Orowan

equations compared with the observed increments of yield strength

in micro alloyed steels (vertical lines are experimental data)]

30

10

CHAPTER 1: LITERATURE REVIEW

1.1: INTRODUCTION

1.1.1 Definition

High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical

properties or greater resistance to corrosion than carbon steel. They have carbon content between

0.05–0.25% to retain formability and weldability. Other alloying elements include up to 2.0%

manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium,

molybdenum, titanium, calcium, rare earth elements, or zirconium. These steel alloys provide

increased strength-to-weight ratios over conventional low-carbon steels for only a modest price

premium. Because HSLA alloys are stronger, they can be used in thinner sections, making them

particularly attractive for transportation-equipment components where weight reduction is important.

1.1.2 Composition

Typically, HSLA steels are low-carbon steels with up to 1.5% manganese, strengthened by small

additions of elements, such as columbium, copper, vanadium or titanium and sometimes by special

rolling and cooling techniques. Improved-formability HSLA steels contain additions such as

zirconium, calcium, or rare-earth elements for sulfide-inclusion shape control.

Element % Composition

C 0.06 -0.12

Mn 1.4 -1.8

Nb 0.02 -0.05

V 0 - 0.06

Mo 0.2-0.35

Table 1: Typical Composition of HSLA Steels

11

1.1.3 Classification

1.1.3.1 Weathering Steel: Steels which have better corrosion resistance. A common example is

COR-TEN.

1.1.3.2 Control Rolled Steel: Hot rolled steels which have a highly deformed austenite structure

that will transform to a very fine equiaxed ferrite structure upon cooling.

1.1.3.3 Pearlite Reduces Steel: Low carbon content steels which lead to little or no pearlite, but

rather a very fine grain ferrite matrix. It is strengthened by precipitation hardening.

1.1.3.4 Acicular Ferrite: These steels are characterized by a very fine high strength acicular

ferrite structure, a very low carbon content, and good hardenability.

1.1.3.5 Dual Phase Steel: These steels have a ferritic microstructure that contains small,

uniformly distributed sections of martensite. This microstructure gives the steels low yield

strength, high rate of work hardening, and good formability.

1.1.3.6 Micro Alloyed Steel: Steels which contain very small additions of niobium, vanadium,

and/or titanium to obtain a refined grain size and/or precipitation hardening.

1.1.4 Properties

The added elements are intended to alter the microstructure of plain carbon steels, which is usually a

ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite.

This eliminates the toughness-reducing effect of a pearlitic volume fraction, yet maintains and even

increases the material's strength by precipitation strengthening and by refining the grain size, which

in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Its

yield strength can be anywhere between 250-590 MPa (35000-85000 psi).

HSLA steels are also more resistant to rust then most carbon steels. Although the material quickly

becomes covered with surface rust, this is superficial and rust takes a long time to threaten the

integrity of a structure made from the material. HSLA steels usually have densities of around 7800

kg/m3.

12

1.1.5 Specification

According to the Society of Automobile Engineers, the Standard Steel Grade Compositions for

HSLA are as follows:

Table 2: SAE HSLA steel grade compositions

Grade

%

Carbon

(max)

%

Manganese

(max)

%

Phosphorus

(max)

%

Sulfur

(max)

%

Silicon

(max)

Notes

942X 0.21 1.35 0.04 0.05 0.90 Niobium or vanadium treated

945A 0.15 1.00 0.04 0.05 0.90

945C 0.23 1.40 0.04 0.05 0.90

945X 0.22 1.35 0.04 0.05 0.90 Niobium or vanadium treated

950A 0.15 1.30 0.04 0.05 0.90

950B 0.22 1.30 0.04 0.05 0.90

950C 0.25 1.60 0.04 0.05 0.90

950D 0.15 1.00 0.15 0.05 0.90

950X 0.23 1.35 0.04 0.05 0.90 Niobium or vanadium treated

955X 0.25 1.35 0.04 0.05 0.90 Niobium, vanadium, or nitrogen treated

960X 0.26 1.45 0.04 0.05 0.90 Niobium, vanadium, or nitrogen treated

965X 0.26 1.45 0.04 0.05 0.90 Niobium, vanadium, or nitrogen treated

970X 0.26 1.65 0.04 0.05 0.90 Niobium, vanadium, or nitrogen treated

980X 0.26 1.65 0.04 0.05 0.90 Niobium, vanadium, or nitrogen treated

13

Table 3: SAE HSLA steel grade mechanical properties

Grade Form Yield strength

(min) [psi (MPa)]

Ultimate tensile strength

(min) [psi (MPa)]

942X Plates, shapes & bars up to 4 in. 42,000 (290) 60,000 (414)

945A, C

Sheet & strip 45,000 (310) 60,000 (414)

Plates, shapes & bars:

0–0.5 in. 45,000 (310) 65,000 (448)

0.5–1.5 in. 42,000 (290) 62,000 (427)

1.5–3 in. 40,000 (276) 62,000 (427)

945X Sheet, strip, plates, shapes & bars up to 1.5

in. 45,000 (310) 60,000 (414)

950A, B, C, D

Sheet & strip 50,000 (345) 70,000 (483)

Plates, shapes & bars:

0–0.5 in. 50,000 (345) 70,000 (483)

0.5–1.5 in. 45,000 (310) 67,000 (462)

1.5–3 in. 42,000 (290) 63,000 (434)

950X Sheet, strip, plates, shapes & bars up to 1.5

in. 50,000 (345) 65,000 (448)

955X Sheet, strip, plates, shapes & bars up to 1.5

in. 55,000 (379) 70,000 (483)

960X Sheet, strip, plates, shapes & bars up to 1.5

in. 60,000 (414) 75,000 (517)

965X Sheet, strip, plates, shapes & bars up to 0.75

in. 65,000 (448) 80,000 (552)

970X Sheet, strip, plates, shapes & bars up to 0.75

in. 70,000 (483) 85,000 (586)

980X Sheet, strip & plates up to 0.375 in. 80,000 (552) 95,000 (655)

14

1.1.6 Applications

Improved-formability HSLA steels were developed primarily for the automotive industry to replace

low-carbon steel parts with thinner cross-section parts for reduced weight without sacrificing strength

and dent resistance. Typical passenger-car applications include door-intrusion beams, chassis

members, reinforcing and mounting brackets, steering and suspension parts, bumpers, and wheels.

Trucks, construction equipment, off-highway vehicles, mining equipment, and other heavy-duty

vehicles use HSLA sheets or plates for chassis components, buckets, grader blades, and structural

members outside the body. For these applications, sheets or light-gage plates are specified. Structural

forms (alloys from the family of 45,000 to 50,000-psi minimum yield strength HSLA steels) are

specified in applications such as offshore oil and gas rigs, single-pole power-transmission towers,

railroad cars, and ship construction. In equipment such as power cranes, cement mixers, farm

machinery, trucks, trailers, and power-transmission towers, HSLA bar, with minimum yield strengths

ranging from 50,000 to 70,000 psi is used. Forming, drilling, sawing, and other machining operations

on HSLA steels usually require 25 to 30% more power than do structural carbon steels.

15

1.2: DEVELOPMENT OF HIGH STRENGTH LOW ALLOY STEELS

1.2.1 Mechanism of alloying

Chemical compositions for the HSLA steels are specified by ASTM standards. The principal

function of alloying elements in these ferrite-pearlite HSLA steels, other than corrosion resistance, is

strengthening of the ferrite by grain refinement, precipitation strengthening, and solid-solution

strengthening. Solid-solution strengthening is closely related to alloy contents, while grain

refinement and precipitation strengthening depend on the complex effects of alloy design and

thermo-mechanical treatment.

Alloying elements are also selected to influence transformation temperatures so that the

transformation of austenite to ferrite and pearlite occurs at a lower temperature during air cooling.

This lowering of the transformation temperature produces a finer-grain transformation product,

which is a major source of strengthening. At the low carbon levels typical of HSLA steels, elements

such as silicon, copper, nickel, and phosphorus are particularly effective for producing fine pearlite.

Element such as, manganese and chromium, which are present in both the cementite and ferrite, also

strengthen the ferrite by solid-solution strengthening in proportion to the amount, dissolved in the

ferrite.

In the presence of alloying elements, the practical maximum carbon content at which HSLA steels

can be used in the as-cooled condition is approximately 0.20%. Higher levels of carbon tend to form

martensite or bainite in the microstructure of as-rolled steels, although some of the higher-strength

low-alloy steels have carbon contents that approach 0.30%.

The required strength is developed by the combined effect of:

Fine grain size developed during controlled hot roiling and enhanced by micro alloyed elements

(especially niobium)

Precipitation strengthening caused by the presence of vanadium, niobium, and titanium in the

composition.

16

1.2.2 Effect of alloying elements on the properties of HSLA Steels

1.2.2.1 Nitrogen - Additions to high-strength steels containing vanadium are limited to 0.005% and

have become commercially important because such additions enhance precipitation

hardening. The precipitation of vanadium nitride in vanadium-nitrogen steels also improves

grain refinement because it has a lower solubility in austenite than vanadium carbide.

1.2.2.2 Manganese -It is the principal strengthening element in plain carbon high-strength structural

steels. It functions mainly as a mild solid-solution strengthener in ferrite, but it also provides

a marked decrease in the austenite-to-ferrite transformation temperature. In addition,

manganese can enhance the precipitation strengthening of vanadium steels and to a lesser

extent, niobium steels.

[Figure-1: Fe-Mn Phase Diagram]

17

1.2.2.3 Silicon - One of the most important applications of silicon is its use as a deoxidizer in molten

steel. Silicon has a strengthening effect in low-alloy structural steels. In larger amounts, it

increases resistance to scaling at elevated temperatures. Silicon has a significant effect on

yield strength enhancement by solid-solution strengthening and is widely used in HSLA

steels for riveted or bolted structures.

1 .̀

[Figure-2: Fe-Si Phase Diagram]

1.2.2.4 Copper – Cu in levels in excess of 0.50% also increases the strength of both low- and

medium-carbon steels by virtue of ferrite strengthening, which is accompanied by only slight

decreases in ductility. Copper can be retained in solid solution even at the slow rate of

cooling obtained when large sections are normalized, but it is precipitated out when the steel

is reheated to about 510 to 605°C (950 to 1125°F). At about 1% copper, the yield strength is

increased by about 70 to 140 MPa regardless of the effects of other alloying elements. Copper

in amounts up to 0.75% is considered to have only minor adverse effects on notch toughness

or weldability. Copper precipitation hardening gives the steel the ability to be formed

extensively and then precipitation hardened as a complex shape or welded assembly.

18

1.2.2.5 Phosphorous - The atmospheric-corrosion resistance of steel is increased appreciably by the

addition of phosphorus, and when small amounts of copper are present in the steel, the effect

of the phosphorus is greatly enhanced. When both phosphorus and copper are present, there is

a greater beneficial effect on corrosion resistance than the sum of the effects of the individual

elements.

1.2.2.6 Chromium -It is often, added with copper to obtain improved atmospheric-corrosion

resistance.

[Figure-3 Fe-Cr Phase Diagram]

19

1.2.2.7 Nickel -It is often added to copper-bearing steels to minimize hot shortness.

[Figure-4 Fe-Ni Phase Diagram]

1.2.2.8 Molybdenum –Mo in hot-rolled HSLA steels is used primarily to improve hardenability

when transformation products other than ferrite-pearlite are desired. Molybdenum (0.15 to

0.30%) in micro alloyed steels also increases the solubility of niobium in austenite, thereby

enhancing the precipitation of NbC(N) in the ferrite. This increases the precipitation-

strengthening effect of NbC(N).

1.2.2.9 Aluminum – It is widely used as a deoxidizer and was the first element used to control

austenite grain growth during reheating. During controlled rolling, niobium and titanium are

more effective grain refiners than aluminum.

1.2.2.10 Vanadium - It strengthens HSLA steels by both precipitation hardening the ferrite and

refining the ferrite grain size. The precipitation of vanadium carbonitride in ferrite can

develop a significant increase in strength that depends not only on the rolling process used,

but also on the base composition. Carbon contents above 0.13 to 0.15% and manganese

content of 1% or more enhances the precipitation hardening, particularly when the nitrogen

content is at least 0.01%.

20

1.2.2.11 Titanium –It is unique among common alloying elements in that it provides both

precipitation strengthening and sulfide shape control. Small amounts of titanium (<0.025%)

are also useful in limiting austenite grain growth. However, it is useful only in fully killed

(aluminum deoxidized) steels because of its strong deoxidizing effects; the versatility of

titanium is limited because variations in oxygen, nitrogen, and sulfur affect the contribution

of titanium as carbide strengthened.

[Figure-5: Fe-Ti Phase Diagram]

1.2.2.12 Zirconium –It can also be added to killed high-strength low-alloy steels to improve

inclusion characteristics, particularly in the case of sulfide inclusions, for which changes in

inclusion shape improve ductility in transverse bending.

1.2.2.13 Boron – It has no effect on the strength of normal hot-rolled steel but can considerably

improve hardenability when transformation products such as acicular ferrite are desired in

low-carbon hot-rolled plate.

1.2.2.14 Calcium - Treatment with calcium is preferred for sulfide inclusion shape control.

21

1.3: STRENGTHENING MECHANISM OF HSLA

Most HSLA steels are furnished in the as-hot-rolled condition with ferritic - pearlitic microstructure.

The controlled-rolled steels with an acicular ferrite microstructure and the dual-phase steels with

martensite dispersed in a matrix of polygonal ferrite show high strength. These two types of HSLA

steels use the formation of eutectoid structures for strengthening, while the ferritic - pearlitic HSLA

steels generally require strengthening of the ferrite.

Pearlite is generally an undesirable strengthening agent in structural steels because it reduces impact

toughness and requires higher carbon contents. Moreover, yield strength is largely unaffected by a

higher pearlite content.

Strengthening can be achieved by one or more of the following mechanisms: grain refinement, solid

solution strengthening, phase balance strengthening, precipitation strengthening and work hardening.

Of these only grain refinement results in higher toughness; the others reduce it. The precise mix of

mechanisms used will depend on the specific application of the steel along with other properties such

as weldability for construction steels, formability for further metal forming and machinability for

engineering steels.

1.3.1 Grain refinement

The main relationship between average ferrite grain size, d, and the yield stress in steels, σy, is

described by the Hall – Petch equation:

σy = σ0 + kyd-1/2

where σ0 and ky are constants.

The constant σ0, friction stress, represents the stress required to move free dislocations along the slip

planes in the iron BCC crystal, and can be regarded as the yield stress of a single crystal. This stress

is very sensitive to temperature and the chemical composition of the steel.

22

ky is determined from the slope of the line plot between σy and d-1/2

. It is found to be independent of

temperature, chemical composition of the steel grade, and strain rate.

Different authors have reported variable values for σo and ky due to the different steel grades

examined (Table 3). Reported ky values vary by 9%, which is within experimental scatter, but σo

varies by up to 4.5 times, which indicates a strong dependence of σo on steel composition. The

effects of elements in solution on the yield stress σo can be represented as:

where σI is the friction stress of iron, ci is a concentration of ith

solute and ki is the strengthening

coefficient of ith

solute.

Table 4: Experimental values for stress and ky terms in Hall Petch Equation

23

Grain refinement is influenced by the complex effects of alloy design and processing methods. For

example, the various methods of grain refinement used in the three different stages of hot rolling

(that is, reheating, hot rolling, and cooling) include:

The addition of titanium or aluminum to retard austenite grain growth when the steel is

reheated for hot deformation or subsequent heat treatment

The controlled rolling of microalloyed steels to condition the austenite so that it transforms

into fine-grain ferrite

The use of alloy additions and/or faster cooling rates to lower the austenite-to-ferrite

transformation temperature.

1.3.2 Solid solution strengthening

The main interstitial elements in steel are carbon and nitrogen. Free carbon and nitrogen form

interstitial atmospheres around the dislocations, which lead to dislocation immobilization and yield

strength increase. However, the solubility of C and N in α-iron is limited to 0.02 wt% for C at 723 0C

and to 0.1 wt% for N at 590 0C decreasing to < 0.00005 wt% (C) and <0.0001 wt% (N) at 20

0C.

Formation of cementite Fe3C and other alloy carbides and nitrides takes free carbon and nitrogen

from the solution. Hence, the interstitial solute strengthening effect is limited.

Many metallic elements form substitutional solutions with iron. This also leads to an increase in

strength through elastic straining of the iron matrix, which arises from the size mismatch between the

alloying element and iron atom (Hume-Rothery effect). With an increase in solute element content

strength usually increases (Figure 1). In addition to elastic strain, substitutional solute elements also

influence the grain size, amount and interlamellar spacing of pearlite, and the free content of carbon

and nitrogen. Thus the negative influence of Cr on strength (Figure 1) can be explained by its

removal of free N from solution as chromium nitride.

24

Strengthening coefficients for various alloying elements can be found in the literature, Table 4, and

incorporated into the Hall-Petch equation as:

Generally, solid solution strengthening is used with other mechanisms as it is limited by solubility

limits and other property requirements, such as toughness and formability.

Figure 6: Strengthening effects of substitutional solute atoms in iron.

Table 5: Strengthening coefficients for a number of solutes

25

1.3.3 Phase Balance Strengthening

The microstructure of steels often consists of several phases. In plain carbon steels the predominant

phase is ferrite with pearlite being the commonest second phase. Apart from these phases in alloyed

carbon steels, carbides, nitrides, carbo-nitrides and intermetallic compounds of alloying elements

may be present. These phases also influence strength. Increased hardenability and/or cooling may

replace pearlite by bainite or martensite, which provides increased strength.

For C-Mn steels, increase in carbon content leads to an increase in pearlite content, and, at constant

Mn content, an increase in tensile strength (Figure 2), which is due to a faster work hardening rate

(Figure 3). The influence of carbon on the yield stress is not so high (Figure 4). At constant carbon

content, an increase in Mn (γ-stabiliser) content lowers the eutectoid composition of carbon

increasing the pearlite proportion, whilst Mn itself contributes to strength via solid solution

strengthening and grain refinement.

Figure 7: Factors contributing to the strength of C-Mn Steels.

26

In the literature there are several empirical equations derived for various mechanical properties to

extend the Hall-Petch and solid solution equation. An example for ferrite-pearlite steels containing

up to 0.2 wt% C (pearlite content of about 25-30%) is as follows:

Figure 8: Effect of pearlite on work hardening rate

Figure 9. Influence of carbon content on strength of plain carbon steel

27

where σt - ultimate tensile strength,

ITT - impact transition temperature and cMn , cSi , cN ,

cpearlite - weight percent of Mn , Si , free soluble N and pearlite percentage

respectively.

The strength of pearlite is influenced by its interlamellar spacing in a similar way to the

Hall-Petch equation for ferrite structures:

Here σp is the yield stress of pearlite and s is the ferrite path in the pearlite structure. For the two-

phase steels with more than 25% pearlite the Rule of Mixtures can be used to give mechanical

properties as:

where f is the volume fraction of ferrite, p is the pearlite colony size and t is the carbide lamella

thickness.

28

Fast cooling from the austenite field restricts carbon diffusion, which results in bainite and/or

martensite formation instead of pearlite (Figure 5). Strength increase due to phase balance change

depends on carbon content, and the tensile strength may reach 1600 MPa in bainite and to 2000 MPa

in martensite. At the same time increase in carbon content reduces weldability and formability due to

reduced toughness.

[Figure 10: Influence of 50% transformation temperature on tensile strength via formation of

different steel structures]

1.3.4 Precipitation strengthening

Precipitation strengthening occurs from the formation of finely dispersed carbonitrides developed

during heating and cooling. Because precipitation strengthening is generally associated with a

reduction in toughness, grain refinement is often used in conjunction with precipitation strengthening

to improve toughness. Precipitation strengthening is influenced by the type of carbonitride, its grain

size, and, of course, the number of carbonitrides precipitated.

29

Addition of strong carbonitride formers, e.g. Ti, Nb and V, leads to an increase in strength (Figure 6).

Precipitation strengthening, which arises from the dislocation-particle interactions during slip, can be

described by the Orowan equation:

where G is shear modulus, b is Burgers vector and L is the spacing between the particle centers. L is

related to the particle volume fraction (f) and particle diameter (X) by:

which gives:

[Figure 11: Strengthening contributions of different parameters on yield strength of hot-rolled

0.1% C-0.2% Si-1.4% Mn steel containing either 0.04% Nb or 0.04% Nb-0.07% V [71]: Δσ – is

precipitate strengthening.]

30

Assuming a random distribution of particles the addition to the yield stress from precipitation can be

calculated using the Ashby-Orowan equation:

where, X is in μm.

The comparison between the last equation and experimental data for Nb- and V-alloyed steels

showed that, for the particles larger than 5 nm and with 0.003 – 0.0015 volume fraction, the Ashby-

Orowan equation gives a better prediction than the Orowan equation (Figure 7), due to the use of an

effective spacing in the Ashby-Orowan equation not the minimum spacing, as in the Orowan model.

[Figure 12. Addition to strength predicted by Orowan and Ashby-Orowan equations compared with the observed

increments of yield strength in micro alloyed steels (vertical lines are experimental data)]

More complex modifications of the Orawan equation have been developed by Melander for a random

distribution of hard, spherical particles.

31

where ν = Poisson’s ratio,

lr = is average distance between obstacle centers in the glide plane,

Dg = geometrical mean particle diameter.

This equation has been found to give values within 2 – 11 % of experimental data for small (3-5 nm

diameter) VC particles with volume fractions from 0.0033 – 0.0316, but, as the particle size

increases, the Ashby-Orowan equation again becomes more suitable.

In general, all the shown equations indicate, that greater strengthening results from greater volume

fractions of finer particles, which depend on composition and processing. The strength increase for

various microalloyed steels have been reported between 40 and 150 MPa (Table 5).

Table 6: Increment to the yield stress from the microalloying

elements precipitation

32

1.3.5 Work hardening

In the absence of recrystallisation, an increase in dislocation density results in an increase in yield

stress, however toughness and ductility may decrease. An increase in the shear stress can be

represented by:

where, τ* describes the dislocation interactions with the short range obstacles and τi - with the long

range obstacles :

Where V - Activation volume

ΔH0 - Activation enthalpy at τ = 0

k - Boltzmann’s constant,

T -Temperature

l - Length of dislocation line activated

ε - Strain rate

m - Mobile dislocation density

A -Area of glide plane covered by dislocation

Γ -Frequency of vibration of dislocation line length

α -Constant

μ -Shear modulus

ρ -Dislocation density.

33

4: EFFECT OF GRAIN SIZE ON PROPERTIES OF HSLA

1.4.1 Measurement of ASTM Grain Size Number

The microstructural quantity known as the ASTM Micro Grain Size Number, n, is defined by the

following relationship:

N=2n-1

Where N is the number of grains per square inch, measured at a magnification of 100x. The most

common method for measuring the values of n are:

1.4.1.1 Comparison Method:

This is the simplest yet least quantitative method and is described in section 8 of ASTM

E112. The overall appearance of the microstructure is simply compared with a standard set

of micrographs or plates for which the ASTM Grain size number has been determined.

Because the comparison of grain structure may be influenced by the overall type of

microstructure, four standard categories of grain size plates are used for comparison.

Plate 1: Untwined grains flat etch at100X.

Includes grain size numbers 00, 0,,0.5,1,1.5,2,2.5,3,3.5,4,4.5,5,5.5,6,6.5,7,7.5,8,8.5,9,9.5,

and 10

Plate2: Twinned grains flat-etch at 100X.

Includes grain size numbers 1,2,3,4,5,6,7, and 8

Plate 3: Twinned grains contrast-etch at 75X.

Includes nominal grain boundary diameters of 0.2, 0.15, 0.12, 0.09, 0.07, 0.06, 0.05, 0.045,

0.035, 0.025, 0.020, 0.015, 0.010 and 0.005mm.

Plate 4: Austenitic grains in steel at 100X

Includes grain size number 1,2,3,4,5,6,7, and 8.

34

1.4.1.2 Grain Counting Method:

The number of grains per unit is counted directly .The ASTM grain size number is then

determined according to the definition. An example of grain counting method is the

planimetric procedure also known as Jeffries method.

Inscribe a circle (or other shape) of known area, A, on image of magnification, M.

Count the number of grains that are completely within area.

Count the number of grains that are partially within the area.

Divide the result from (c) by 2

Add the result from (d) to the result from (b).

Divide the result from (f) to grains/in @ 100X.

1.4.1.3 Intercept Method:

The number of grain boundary intercepts per unit test line is measured. This is a measure of

grain boundary area per unit volume and is, therefore, related to grain size. An example of

an intercept method is the linear intercept procedure also known as the Heyn Method. The

basic steps to procedure are given here:

On a single field of view randomly place one or more straight lines known as combined

total length L.

Count the total number intercepts, P, between the test lines(s) and the grain boundaries.

Triple junction count as 1.5. If P < 50 use additional lines.

Divide the number of intercepts, P, obtained from (b) by the total length L.

Repeat (a-c) for 2-4 additional fields of view.

Obtained P as the average of the result from (c) for all fields of view.

The ASTM Grain size Number is given by

n= -3.3 + 6.65 log10 (PL)

where PL is given in mm-1

.

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1.4.2 Importance of Grain Size

For applications at or around the normal temperatures, fine –grained steels are preferred over coarse

grained steels as fine grained steels have better impact resistance (toughness) for corresponding

hardness and strength .The coarse grains raise the ductile to brittle transition temperature.

One of the significant effects is that fine grains improve the strength considerably. Modern steel

maker changes the ferrite grain size of mild steel from d-1/2

=2, i.e., d=0.25 mm to d-1/2

=20, i.e.,

d=0.0025mm, and thus, raises yield strength from 100 MNm-2 to 500 MNm-2

. Thus, achieving grain

size in the range of 2-10 microns is extremely worthwhile. This has led to wide spread use of HSLA

(High strength low alloy) steels having (addition of NB=0.05-0.09%) with yield strengths in range

290-550 MPa and tensile strength in the range of 415-700 MPa.

Grain boundaries act as obstacles to dislocation motion. Fine grains have large grain boundary area.

Dislocation moves only a short distance before meeting a grain boundaries and thus increases the

strength. In a large grain, the pile up of dislocation contains large number of dislocations, which in

turn causes higher stress concentration in the nearing grains to cause slip there.

The shear stress, at the head of dislocation piled of is equal to nt, where n= no. of dislocation

involved and t is shear stress on the slip plane. Thus coarser the grain size higher is stress

concentration, easier it is to propagate the yielding process.

Hardenability of coarse grain steel is more as martensite forms easily. Coarse grain austenite

transform to coarse plate martensite, which is undesirable as it more brittle and a bit less hard. The

low carbon martensite (i.e. the lath martensite) has higher yield strength as austenite grain size

decreases. Lathe martensite forms a packet, where the packet is effectively a grain. The packet size

decreases with the decrease of parent austenitic grain size, thus the strength of lathe martensite may

be related with either the packet size or directly with the parent austenitic grain size.

36

There is a direct relationship between austenite grain size and the grain size of the nucleation best

transformation products. That is finer the austenite grain finer is the grain size of the ferrite-cementite

products and vice versa. The annealed or normalize low carbon steel

(predominantly feritic) show increase in strength and toughness with the decrease in grain size of

austenite as it results in smaller ferritic grain size too.

Even the fine-grained hardened and tempered steel has impact toughness several times that of coarse

grained steel of same grade. Probably, the impurity atoms like Sb, P, Sn, As, co segregate more in

coarse grained steel, because of smaller grain boundary area to produce easily intergranular fracture.

In slowly cooled hypo eutectoid steels, free ferrite is able to form continuous network along coarse

grained steels, which leads to lower impact value, lower elastic ratio and lower elongation.

Ms Temperature of Fe-Ni and Fe-Ni-C alloys decreases significantly as austenite grain size

decreases, probably due to higher strength of fine grained austenite, which shows greater shear

resistance to transformation of austenite to martensite.

37

PROPERTY COARSE GRAIN FINE GRAIN

Yield strength Less More

Tensile strength Less More

Hardness Less More

Ductility More Less

Toughness Less More

Resilience Less More

Hardeniability More Less

Depth of carburizing Deep case Shallow case

Machinability after normalizing More Less

Surface finish Less More

Orange peel effect More Less

Corrosion resistance More Less

Creep resistance above equi-cohesive temp. More Less

Creep resistance below equi-cohesive temp. Less More

Retained austenite More Less

Fatigue resistance Less More

Work-hardening exponent Less More

Strain-rate sensitivity Less More

Tendency to quench crack formation More Less

Segregation More Less

Anisotropy of properties More Less

Dispersion of inclusion and micro-porosity Less More

Response to heat treatment Less More

Possibility of soft-spots in hardening Less More

Table 7: Effect of Grain Size on the Properties of HSLA

38

CHAPTER 2: EXPERIMENTATION

Experiments were carried out to find the relation between grain size of sample and mechanical

properties. Hardness testing of the sample was carried out in this regard. The chemical composition

of the sample used is as follows:

Carbon Manganese Sulphur Molybdenum Phosphorus Silicon Nickel

0.147 1.13 0.028 0.002 0.016 0.2 0.005

Chromium Niobium Vanadium Titanium Arsenic Tin Aluminium

0.011 0.035 0.043 0.003 0.002 0.003 0.025

The steps followed in sample testing are as follows:

2.1 SAMPLE PREPRATION

2.1.1 Sectioning

The sample used was available in the form of a small billet of dimension 1.5cm X 1.5cm X 10cm.

For experimental purpose, the sample was cut into pieces of 1.5cm X 1.5cm X 1cm. This was carried

out by cutting the sample on a bench vice with the help of hand hacksaw.

2.1.2 Mounting

Small samples are generally mounted for convenience in handling and to protect the edges of the

specimen being prepared. Cold mounting of the sample was carried out using following supplies:

- ColdMound Kit (Geologists’ Syndicate Pvt. Ltd., Kolkata).

- Plastic mounting ring with open bottom.

- Lubricant oil.

- Glass Plate.

Table 8: Composition of HSLA Sample Under Study

39

The steps involved are as follows:

1. Apply lubricant oil to the inner walls and base of the mounting ring.

2. Place the specimen on the glass plate with the examination surface facedown.

3. Place the mounting ring cylinder on the glass plate so that the sample is at the center of ring base.

4. Mix the appropriate amount of cold Mount (2/1 Powder/Liquid) in a paper cup.

5. Simply pour the epoxy mixture into the mounting ring.

6. After half hour, remove the sample from the mounting ring and proceed with course grinding.

2.1.3 Coarse Grinding

In view of the perfection required in an ideally prepared metallographic sample, it is essential that

each preparation stage be carefully performed. The specimen must:

1. Be free from scratches, stains and others imperfections which tend to mark the surface.

2. Retain non-metallic inclusions.

3. Reveal no evidence of chipping due to brittle intermetallic compounds and phases.

4. Be free from all traces of disturbed metal.

The purpose of the coarse grinding stage is to generate the initial flat surface necessary for the

subsequent grinding and polishing steps. As a result of sectioning and grinding, the material may get

cold worked to a considerable depth with a resultant transition zone of deformed material between

the surface and the undistorted metal. Coarse grinding can be accomplished either wet or dry using

electrically powered grinding belt, but care must be taken to avoid significant heating of the sample.

The final objective is to obtain a flat surface free from all previous tool marks and cold working due

to specimen cutting. An important factor throughout the Coarse Grinding and Fine Grinding Stages is

that the scratches be uniform in size and parallel to each other in any one grinding stage. Proper

grinding involves rotation of the sample by 90o between stages while the grinding angle must be held

constant during the grinding at any one stage.

2.1.4 Medium and Fine Grinding

Medium and fine grinding of metallurgical samples are closely allied with the coarse grinding which

precedes them. Each stage of metallographic sample preparation must be carefully performed; the

40

entire process is designed to produce a scratch free surface by employing a series of successively

finer abrasives.

The idea is to carefully move from one stage to the next where the abrasives become finer at each

successive stage. Movement from one stage to the next should only proceed when all of the scratches

from the preceding stage are completely removed. In general, successive steps are 240, 320, 400 and

600 grit SiC and the grinding rate should steadily decrease from one stage to the next.

For the experimentation process, the medium and fine grinding was carried out in a double disc

polishing machine, the specifications of which are as follows:

Make : Presi, Grenoble, France.

Model : Minitech 265

Max. Speed : 700 rpm

2.1.5 Mechanical Polishing

Polishing involves the use of abrasives, suspended in a water solution, on a cloth-covered electrically

powered wheel. Diamond abrasives provide the best, and most expensive, compounds utilized in

polishing; standard sized aluminum oxide powders are applied for general use purposes. Following

the final 600 grit fine-grinding stage, the sample must be washed and carefully dried before

proceeding to the first polishing stage. At the polishing stages, even hard dust particles in the air

which settles on the polishing cloth can cause unwanted scratching of the specimen. Careful washing

of the specimen and the operator's hands must be carried out prior to each stage of polishing.

Beginning with suspended aluminum oxide particles (suspended in water) on a Nylon-cloth, the final

fine-grinding surface layer resulting from the previous grinding procedure should be completely

removed with a rotation rate of 300 rpm.

The specimen is initially held at one position on the wheel, without rotation, until most of the

previous grinding marks are removed. It can be rotated slowly, counter to the wheel rotation, until

only scratches from the aluminum oxide are visible. During the initial polishing stage, moderate

pressure can be applied to the specimen and the entire stage should generally take 1 or 2 minutes.

41

After proper polishing, specimen should have a mirror-like surface free of scratches. During final

polishing, minimal pressure should be applied and time should be kept to a minimum since the

napped material will conform to the specimen shape under pressure.

The wetness of the cloth used for Final Polishing has a great influence on the end result. If the cloth

is too wet the sample will show pits; if too dry, buffing and/or smearing will result. To determine the

proper wetness, the sample should be removed from the wheel and the time required for the polishing

film to dry (five to eight seconds) should be checked. A thin opaque film indicates that sufficient

abrasive is present. For precision work, extremely fine grades of diamond abrasives are used for the

final polishing sequence.

2.1.6 Etching

Microscopic examination of a properly polished, unetched specimen will reveal only a few structural

features such as inclusions and cracks or other physical imperfections. Etching is used to highlight,

and sometimes identify, microstructural features or phases present. Even in a carefully prepared

sample, a surface layer of disturbed metal, resulting from the final polishing stage, is always present

and must be removed. Etchants are usually dilute acid or dilute alkalis in water, alcohol or some

other solvent. Etching occurs when the acid or base is placed on the specimen surface because of the

difference in rate of attack of the various phases present and their orientation. The etching process is

usually accomplished by merely applying the appropriate solution to the specimen surface for several

seconds to several minutes. Nital, a Nitric Acid - Alcohol mixture, is the etchant commonly utilized

with common irons and steels. Nital is dripped onto the specimen using an eye-dropper or cotton

swab. Ten seconds to one minute is usually sufficient for proper etching depending on sample and

nital concentration. The sample is immediately washed under running water, rinsed with alcohol and

dried in an air blast. Dry off the rinsing alcohol on the specimen with the air blast and then move on

to the microscopic examination stage.

42

2.2 MICROSCOPIC EXAMINATION

Initial microscopic viewing should be done utilizing a stereomicroscope, which reveals a three-

dimensional scanning of the specimen surface. The specimen is placed on the stage of the

microscope so that its surface is perpendicular to the optical axis.

Detailed viewing is done with a metallurgical microscope. A metallurgical microscope has a system

of lenses (objectives and eyepiece) so that different magnifications (25X to 1000X) can be achieved.

The important characteristics of the microscope are: (1) magnification, (2) resolution and (3) flatness

of field. The resultant magnification is the product of the magnifying power of the objective and that

of the ocular.

The grain size of the samples is measured after microscopic analysis by metallurgical microscope.

The grain size is measured using the Image Analyzer software by using the Grain Size Measurement

Option.

The specifications of the metallurgical microscope used are as follows:

Make : Metal Power India Ltd.

Software Used : Image Analyzer

Max. Magnification Obtainable : 1000X

2.3 HARDNESS TESTING

Hardness of the sample was tested by using the Digital Hardness Testing Machine (Vaishshaikha

Electron Devices, Ambala). The indenter used in this regard was a ball indenter with the ball

diameter of 1/16 inch. The initial test force was of 98.01 N that was applied to hold the specimen in

position. A further load of 980.7 N was applied to obtain a hardness in HRB Scale. The tolerance

limit of the said machine is of ± 2.0%.

43

2.4 HEAT TREATMENT

Heat treatment of the samples was done to alter the grain size of the samples. The knowledge about

the type of treatment required to obtain a certain grain size was known by going through a number of

research papers. Although the exact treatment could not be specified, a tentative treatment schedule

was designed for different samples by the project guide. The following heat treatment was carried

out:

All the samples were heated in a muffle furnace to a temperature of 900°C. On reaching the desired

temperature, the furnace was kept on hold at the same temperature for about 30 minutes. Sample 1

was then cooled in the furnace itself. Sample 2 was given a faster cooling rate by cooling in air.

Sample 3 was cooled in mobile oil and sample 4 was cooled in ice cold water.

2.5 RETESTING

The grain size and hardness of the samples were tested again after the heat treatment process and the

results obtained have been discussed.

44

CHAPTER 3: RESULTS AND DISCUSSIONS

The results obtained are as follows:

Table 9: Result Obtained After Treatment

S. No. Type of Treatment Grain Size* Hardness

1 Untreated 13.0 HRB 92.3

2 Furnace Cooled 10.7 HRB 71.7

3 Air Cooled 10.3 HRB 82.1

4 Oil Cooled 5.43 HRB 75.7

5 Water Quenched 3.48 HRB 72.7

*As tested by Image Analyzer Software.

From the literature reviews, it can be concluded that the hardness keeps on increasing as the grain

size decreases. The change in hardness, however, is not only a function of grain size but result of

various factors like composition of sample, microconstituents, etc. Studies have shown that a

moderate cooling rate forms acicular bainite whereas a faster cooling forms a mixed microstructure

with dispersed ferrite. This ferrite may be the reason for decrease in hardness of sample even though

the grain size obtained is small in the last sample.

Again, the mechanical property changes may be a function of the action of alloying elements at

higher temperature. Hence we can say that various factors need to be taken care of to have proper

results for the effect of grain size on the properties of HSLA.

45

CONCLUSION

As mentioned in previous pages, the need of high strength steels with good corrosion and oxidation

resistance has increased in the recent years. This has lead to increase in research work in the field of

high strength low alloy steels. The project report gives a brief idea as to how the alloying elements

influence the microstructure and grain size of high strength low alloy steels. It can be concluded from

the data mentioned in the above pages that the ferrite grain size decreases with an increase in micro

alloying element content, due to Zener drag effect. Second phase (pearlite) content decreases with

carbon content decrease in steel composition.

The above report also gives an idea of the effect of grain size on the mechanical properties of steel. It

is found that the higher alloyed steels show a larger yield stress drop from plate to pipe, due to the

higher Bauschinger effect.

Apart from grain size, the mechanical properties of HSLA also depend on a variety of other factors

like composition of phases, distribution of phases, etc. Strength of HSLA steels is increased by:

increasing the amount of pearlite

increasing the fineness of the grains structure

increasing the amount of hard precipitate.

Further research work in this field can be carried out which will lead to a better idea about the effect

of various manufacturing methods like rolling, forging, casting, etc. on the grain size and mechanical

properties can be known. Also the effect of individual effect of alloying elements during heat

treatment of samples can be known. This will lead to improved properties of HSLA and will result in

increased fields of application of HSLA.

46

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