bur failure.pdf

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Premature failure analysis of forged cold back-up roll in a continuous

tandem mill

Hamid Reza Bakhsheshi Rad1,2, Ahmad Monshi2, Hassan Jafari1, 3, Mohd Hasbullah Idris1,

Mohammed Rafiq Abdul Kadir1

1Materials Engineering Dept., Faculty of Mechanical Engineering, Universiti Teknologi

Malaysia, Skudai 81310, Johor, Malaysia2Materials Engineering Dept., Islamic Azad University, Najafabad branch, Isfahan, Iran

3Materials Engineering Dept., Faculty of Mechanical Engineering, Shahid Rajaee Teacher

Training University, Tehran 16785-136, Iran

Cooresponding author:

Hamid Reza Bakhsheshi Rad

[email protected]

[email protected]

Add: No. 801, U8A, Kolej Perdana, UTM, Skudai 81310, Johor, Malaysia

Tel.: 0060-147382258

Fax.: 0060-75534610


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Abstract

In this paper, premature failure of a forged back-up roll from a continuous tandem mill was

investigated. Microstructural evolutions of the spalled specimen and surface of the roll were

characterized by optical microscopy, X-ray diffraction, scanning electron microscopy andferritscopy, while hardness value of the specimen was measured by Vickers hardness testing.

The results revealed that the presence of pore and MnS inclusion with spherical and oval

morphologies were the main contributing factors responsible for the poor life of the back-up

roll. In addition, metal pick up and subsequently strip welding on the surface of the work roll

were found as the major causes of failure in work roll which led to spalling occurrence in the

back-up roll. Furthermore, relatively high percentage of retained austenite, say 9%, in outer

surface of the back-up roll contributed spalling due to conversion of this meta-stable phase to

martensite and creation of volume expansion on the outer surface through work hardeningduring mill campaign.

Keywords:A. Ferrous metals and alloys; F. Microstructure; H. Failure analysis

1. Introduction

In rolling mill operation a four roll high stand tandem mills including two work rolls and two

back-up rolls are used to reduce force and power of work roll as well as increase the accuracy

and thickness uniformity of thin sheets. Back-up rolls are the main trait of hot and cold roll

mills which decrease unintended bending and support the work rolls, enabling them to endure

higher loads without failing [1]. Work roll reduces the thickness of strip by plastic

deformation which creates by high compressive stress via the rolls. Generally, steel material

used for back-up roll is refined in electric arc furnace followed by vacuum degassing. The

produced ingot is then forged and subsequently differential heat treatment is performed in

order for the material to withstand the campaign in milling. Repeated loading under bending

and compressive stresses, severe friction and wear under corrosive environments at high

temperatures are some conditions that back-up rolls should endure during mill campaign

[2,3].

The main reason for premature failure of the forged back-up roll can be the combined effects

of mechanical and metallurgical factors. Mechanical factors include rolling parameter

misalignment, uneven roll surface, lubrication, bearing, rolling speed seizure, insufficient

stock removal during grinding and the experience of operators [4,5]. Metallurgical factors


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comprise the presence of nonmetallic inclusions, localized overloading, casting defects,

temperature gradients due to insufficient cooling and phase transformations [5,6]. It was

observed [6] that spalling, cracking, metal pick up and subsequently strip welding are three

critical factors responsible for the poor service life of back-up and work rolls during milling

operation.Spalling can be classified into two types. The first type is surface initiated spalling, which is

identified by fatigue path accompanied arrest marks, originate from thermally crack or

mechanically indentation, and subsequently fatigue path propagates circumferentially

opposite to the direction of roll rotation. The second one, subsurface initiated spalling, which

is recognized by the presence of a concentric fatigue pattern (fish eye) on the fracture surface

with arrest marks in the form of oval pattern, originates from a material defect and propagates

in different directions away from the initiation site usually within a single plane of

propagation [3]. In the present study the premature failure of a forged back-up cold roll usedfor continuous tandem cold strip rolls was investigated. The main factors affecting the

premature failure of the forged back-up roll were determined and analyzed [3,7].

2. Experimental procedure

A spalled sample from a forged back-up roll used in a 5 stand 4 Hi tandem mill was

investigated. The chemical composition and detailed specifications of the forged back-up and

work cold rolls are given in Table 1 and 2, respectively. The sample was washed thoroughly

with running distilled water, rinsed and ultrasonically degreased with acetone and dried.

Afterwards, it underwent microstructural and fractographic examinations. To prevent

converting back up roll being scraped and any phase transformation during cutting as well as

to obtain reliable results several methods were investigated for sampling; consequently a ring

(Fig.1a) measuring 1365 mm in diameter, 15 mm in thickness and 100 mm in depth was

carefully removed during 24 hours from a tandem mill back up roll by heavy duty lathe

machine (INNSE 62"). Several samples (Fig.1b) were then cut from the ring for further

microstructural and microhardness experiments.

Table 1

Table 2

Figure 1


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Microstructure of the samples prepared from the working surface and spalling specimen was

characterized via optical (OM) and scanning electron microscopes (SEM) equipped with

energy dispersive spectrometry (EDS), while microhardness value of the samples was

measured by Vickers hardness testing method using 10 Kg force. For microstructural

examination of phases, the specimens were etched in Vilella's reagent (1 g C 6H3N3O7, 5 mLHF, 95 mL CH3CH2OH) and Berahas reagent (3 g K2S2O5, 10 g Na2S2O3, 100 mL H2O).

Fischer MP3 ferritscope was employed to measure the amount of ferromagnetic -

martensite phase of the samples. X-Ray diffraction (XRD) analysis was carried out on the

samples to measure retained austenite content by measuring the integrated intensities of

(111)and (110) diffractions using equations (1) [8]:

V= 1.4l/ (I+ 1.4l) (1)

where V is the volume fractions of austenite and Iand Iare the integrated intensities of

(111)and (110) peaks, respectively.

3. Results and discussion

3.1. Hardness profile measurement

Hardness profile, Fig. 2, was measured at every 5 mm distance from the working surface of

back-up roll. As can be observed, hardness value of the samples decreased from 607 to 489

HV with increasing the distance from the working surface. However, there was a fluctuation

in hardness value between 607 to 553 HV until the 17 mm distance from the surface of the

back-up roll. The result revealed that the hardness value of the roll was almost close to the

normal, whilst the main reason of fluctuated behavior in hardness profile may attributed to

the softening some parts of working surface [3,4]. Fig 3 shows the microstructure of the

softening regions indicating a large amount of precipitated carbide caused by superficial

tempering due to localized heating.

Figure 2

Figure 3


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3.2. Microstructure

Fig. 4 shows optical microstructures of the back-up roll material as a function of depth from

the roll surface. Figs. 4a and b represent the microstructures of the samples cut from 5 mm

and 15 mm distances from the surface of the back-up roll showing high percentage oftempered martensite, approximately 87% and 67% volume fraction, retained austenite,

around 9.5% and 9.2% volume fraction, respectively, and chromium carbide. Due to the

presence of medium amount of carbon and alloying elements in chemical composition of the

steel, retained austenite usually coexists with tempered martensite in microstructure after

continuous cooling process [3,7]. However, the microstructure of the sample selected from 15

mm distance from the surface exhibited small amount of non-tempered martensite and

bainite, owing to a decrease in cooling rate with an increase in distance from the surface of

the roll. The reason for transforming austenite to bainite is accessibility of sufficient time forcarbon to have long range diffusion in the microstructure. The microstructure of the samples

in 55 mm and 70 mm depths from the roll surface comprised low percentage of volume

fractions of martensite, spheroidised pearlite, retained austenite and a considerable amount of

undissolved carbide, as seen in Figs. 4c and d. At the inner layer, precipitation of undissolved

carbide occurred in the microstructure due to sufficient heat and time during continuous

cooling of the material.

Figure 4

3.2.1. Retained austenite

The content of retained austenite from surface to depth of the back-up roll was determined by

the XRD analysis. Fig. 5 exhibits the variation of retained austenite content versus depth from

surface of the back-up roll. The content of retained austenite was measured as relatively low

as 5.3% at inner layer, while it was almost high in the outer layer, about 9.5%. The optical

microstructure evolutions of the specimens in 5 and 75 mm distance from the surface of the

roll are shown in Figs. 6a and b, respectively. It can be seen that the content of retained

austenite (light area) in outer layer is higher than inner layer of the roll. As a matter of fact, in

differential hardening method, the surface barrel of back-up roll was subjected to

austenitization process by back-up roll differential heat treatment furnace. Subsequently the

surface barrel was heated to about 900C for a few hours which lead to a decrease in

temperature gradient from the outer layer towards the inner layer. This will result in the outer


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layer having a higher temperature compared to the inner layer causing an increase in the

solubility of chromium carbide in the outer layer. This will increase the concentration of

alloying elements in the solution which leads to martensite start temperature (MS) decreased

significantly [9]. The martensite finish temperature (Mf) will subsequently fell to under room

temperature which brought about a large amount of non-transformed austenite microstructureat the outer layer [10,11]. During mill campaign, this meta-stable phase can transform into

martensite through work hardening which creates external pressure due to volume expansion

and causes spalling.

Figure 5

Figure 6

3.2.2. Carbide characterization

Fig. 7 shows SEM microstructures of the back-up roll material as a function of depth from

the roll surface in the region away from spalled sample. It was revealed that all samples

comprised carbides with different morphologies, sizes, volume fractions and distribution in

the matrix of tempered martensite. Figs. 7 a and b indicate the microstructure of the sample in

5 mm depth from the surface of the back-up roll. As can be seen high volume fraction of fine

spherical carbide homogeneously has distributed in tempered martensite matrix. Thismicrostructure increases wear resistance of the steel and subsequently improves service life

of the back-up roll. Figs. 7c and d represent the microstructure of the sample selected from a

depth of 20 mm from the surface of the roll. It can be seen that significant volume fraction of

irregular shaped carbide compared to the previous sample has distributed uniformly in the

microstructure. Figs. 7e and f exhibit the microstructure of the inner layer (40 mm distance

from the surface) of the roll encompassing coarse needle shaped carbide with lower volume

fraction and heterogeneously distributed in the matrix. It is believed that the needle shape of

the carbide acts as the center for crack initiation and propagation. Moreover, intersectionpoint of the coarse needle shaped carbide can be an impact in accelerating crack initiation and

propagation. Therefore, it causes poor service life of the back-up roll.

Figure 7


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EDS analyses and quantitative microprobe of the carbides are shown in Fig. 8. Based on the

quantitative microprobe, it can be concluded that the stoichiometry of the carbides in all

samples were M7C3due to the atomic percentage ratio of carbon to other elements is almost

equal to 3:7 [12,13]. The EDS analysis of carbide in outer layer of the back-up roll (5 mmdistance from the surface) include 8.57% C, 87.22% Fe, 2.08% Cr, 0.53%Mo and 0.61%Mn.

The atomic percentage of carbon was about 30.30%, whilst the entire atom percentage of the

other associate elements was 70.20% which indicated that the stoichiometry of the carbide

was (Fe,Mn,Mo,Cr)7C3(Fig. 8a). In the middle and inner layers, 20 and 40 mm distance from

the surface respectively, the (Fe,Mo,Cr)7C3 carbide contain around 8.48% C, 87.07% Fe,

4.01% Cr and 0.44%Mo (Fig. 8b and c).

Figure 8

3.2.3. Non-metallic inclusion

Fig. 9 shows the presence of inclusion in the samples of 10 and 15 mm distance from the

surface of the back-up roll. The EDS analysis of working surface exhibited that the

stoichiometry of inclusion was manganese sulfide (MnS) with globular and oval

morphologies, as given in Figs. 9a and b. The inclusion contain 40.83% S and 59.17% Mn

while, the atomic percentage of it was 54.12% and 45.88% for S and Mn, respectively

(Fig. 9d). Generally, the inclusion can be originated from steel making process and it is

believed that during plastic deformation inclusion can act as a critical location for

concentration of dislocation and stress, so it causes crack to initiate in inclusion/matrix

interface [14, 15] subsequently it propagates and spalling occur in the outer layer. Stress

concentration is created in the interface as a result of two main factors, namely the difference

in thermal expansion coefficients of inclusion and matrix and the concentrations of

dislocation due to work hardening.

Fig. 9c also indicates the presence of pore in the 12 mm distance from the surface of the

back-up roll. The pore distributed randomly near the spalling surface during the rolling

operation which might be the location for crack initiation. From this position sub-surface

crack initiated and propagated within a single plane in all directions away from the initiating

site [4,6]. Since the roll is continuously dressed up during milling process, the shear and

residual stresses produced a high accumulated stress. After initiation, the crack propagated

circumferentially until the strength of the surrounding material was reduced to such a degree

that large spalling occurred (Fig. 11).


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Figure 9

3.3. Metal wrapping and strip welding

Figs. 10a and b show the metal wrapping and strip welding occurred on the surface of the

work roll during mill campaign, respectively. The strip welding located at the surface of the

work roll measured as 250 mm length and 130 mm width. Metal wrapping is the most

important factor for failure of back-up and work rolls which is attributed to improper

operating techniques. Generally the initial operating temperature of the back-up roll in

continuous tandem mill is between 90 to 110 C. However, it decreases significantly to about

55 C by continuous cooling liquid (spray) of water and oil during the milling campaign. But

when metal wrapping occurs in work roll/ back-up roll contact zone due to the fluctuation in

thickness of strip within the roll bite, compressive stress increases locally and subsequentlycauses a significant increase in roll surface temperature [16]. This condition leads to rupture,

welding and even melting of the local region of the strip on the surface of work roll [6]. Fig.

10c demonstrates a typically round localized indentation (roll mark) formed on the surface of

the back-up roll having a maximum diameter of approximately 75 mm and a depth of 3 mm.

The reason for happening this defect can be attributed to the strip welding occurred on the

surface of the work roll which caused severe plastic deformation on the surface of the back-

up roll. Afterwards, with continuing milling service, surface cracks initiated from the

localized indentation area and propagated radially and circumferentially until spallingoccurred.

Figure 10

3.4. Spalling

Fig. 11a indicates overload and bruise areas as well as roll marks on the surface of spalling

sample of the back-up roll. The hardness of the bruise area was measured as 564 HV showing

around 27 HV lower than that of the base material, owing to an increase in local temperature

beyond the tempering temperature of the roll material during mill campaign. The mainsources of bruising in this spalled sample were metal wrapping (strip welding) and

fluctuation in thickness of strip which can be prevented by controlling operating techniques.

Figure 11


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Fig 11a indicates roll marks on the spalled sample as well. This kind of indentation can create

surface cracks by acting as stress concentration factors. The entire roll surface experiences a

high tensile and compressive stresses cycle during each roll revolution. Therefore, the

concentration of stress at any roll marks could develop a surface crack. Fig. 11b shows the

fatigue path with beach mark on the back side of the spalling sample which consists of twoparts, namely shiny (rubbed) and dark (oxidized) areas. As the drawn lines for the

fractography show, fatigue path initiated and propagated circumferentially as result of

overload on the surface of the spalled sample, as indicated in Fig. 11a.

Figs. 12a and b show fatigue cracks which probably initiated from inclusion or pore defects

on the sub-surface of the back-up roll where stress concentrated. As can be clearly seen in

Fig. 12c, the presence of dimples at the fracture surface of this spalling sample is an evidence

for the ductile fracture mechanism which can be attributed to the existence of inclusions and

pore in the structure. It is reported that [17] inclusions and pore can produce dimple onfracture surface due to creation of the sites for crack nucleation.

Figure 12

4. Conclusions

(1) The presence of spherical and oval MnS inclusions and pores can be acted as the centers

for crack initiation at the inclusion or pore/matrix interfaces resulted in crack propagation,

subsequently spalling occurred. Modifying steel making process by new methods such as

electroslag remelting (ESR) or ladle refine furnace with VOD function (LFV) is a

recommendation for minimizing the defects.

(2) The other reason for premature failure of the back-up roll was spalling which created by

metal wrapping and strip welding in work roll/ back-up roll contact zone. Wrapping and strip

welding can be reduced by controlling operating techniques and thickness of product being

rolled.

(3) Relatively high percentage of retained austenite was found one of the main reasons caused

poor service life of the roll during mill campaign. This meta-stable phase converted to non-

tempered martensite through work hardening and created volume expansion led to external

pressure and spalling occurrence. Triple temper or sub-zero treatment are recommended as

appropriate remedies to reduce the content of retained austenite.


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(4) Heterogeneously distributed coarse needle shaped carbide in the inner layer of the back-

up roll with (Fe,Mo,Cr)7C3stoichiometry can be considered as another crack initiation source

and poor service life of the roll.

References

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Res 2006; 19: 66-71.

[2] Fractography, vol. 12, ASM Handbook, ASM Int, OH; 1992.

[3] Pantazopoulos G, Vazdirvanidis A. Fractographic, Metallographic Study of Spalling

Failure of Steel Straightener Rolls. J Fail Anal and Preven 2008; 8: 509514.

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forged cold work rolls. Eng Fail Anal 2008; 14: 401-410.

[5] Li H, Jiang Z, Tieu K.A, Sun W. Analysis of premature failure of work rolls in a coldstrip plant. Wear 2007; 263: 14421446.

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[11] Akhbarizadeh A, Golozar M.A, Shafeie A, Kholghy M. Effects of austenizing time on

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[12] Michaud P, Delagnes D, Lamesle P, Mathon M.H, Levaillant C. The effect of the

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chromium martensitic steels. Acta Materialia 2007; 55: 48774889.


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[13]Hoseiny H, Klement U, Sotskovszki P, Andersson J. Comparison of the microstructures

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during cold working process. Mater Design 2010; 31: 26252630.[15]Hashimoto K, Fujimatsu T, Tsunekag N, Hiraoka K, Kida K. Study of rolling contact

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[16] Gaspard C, Ballani J, Batazzi D, Adams T. Use of HSS rolls to skip the chrome plating

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Captions

Tables

Table 1. Chemical composition of the 3Cr-MO steel (wt.%).

Table 2. Detailed specifications of the investigated back-up and work rolls.

Figures

Fig. 1. (a) The ring cut from the tandem mill back up cold roll and (b) samples cut from the

sector of the ring.

Fig. 2. Hardness profile of back-up roll.

Fig. 3.SEM micrographs of softening region at (a) 5mm (b) 7mm from the roll surface.

Fig. 4. Microstructures of material of back-up roll as a function of depth from the roll surface;

(a) 5, (b) 15, (c) 55, and (d) 70 mm.

Fig. 5. Content of retained austenite as a function of depth from the surface of back-up roll.

Fig. 6. Optical microscopic image of retained austenite (light area) as a function of depth

from the surface of back-up roll; (a) 5 and (b) 75 mm.

Fig. 7. SEM microstructures of material of back-up roll as a function of depth from the roll

surface; (a) 5, (b) 20, and (c) 40 mm.

Fig. 8. EDS analyses of the carbides in the microstructures of back-up roll material as a

function of depth from the roll surface; (a) 5, (b) 20, and (c) 40 mm.


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Fig. 9. SEM micrographs of samples from different distance departing from surface of back-

up roll; (a) 10, (b) 15, (c) 12 mm, and d) EDS microanalysis of inclusions in back-up roll.

Fig. 10. (a) metal warpping, (b) strip welding, and (c) localized indentation on the surface of

the rolls.

Fig. 11. (a) overload area, roll marks and bruise area on the surface of spalling sample and (b)fatigue propagation accompanied beach marks on the back side of spalling sample.

Fig. 12. SEM fractograph of spalling sample revealing; (a) fatigue cracks initiate of inclusion

of sub-surface of roll, (b) crack propagate radially, and (c) fracture origin with existence of

dimples.


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Table 1. Chemical composition of the 3Cr-MO steel (wt.%).

C Si Mn P S Cr Mo Ni Sn As Cu Al

0.68 0.31 0.77 0.014 0.008 3.15 0.69 0.07 0.005 0.009 0.045 0.003


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Table 2. Detailed specifications of the investigated back-up and work rolls.

Back-up roll size (mm) 48221525

Back-up scrap size (mm) 48221365

Back-up roll weight (Kg) 36310

Back-up scrap weight (Kg) 31680Back-up surface hardness (HV) 551-587

Depth of hardened layer (mm) 160

Work roll size (mm) 3765585

Work roll scrap size (mm) 3765510

Work roll weight (Kg) 4865

Work roll scrap weight (Kg) 4008

Work roll surface hardness (HV) 874-952

Depth of hardened layer (mm) 75

No. of stands 5

Annual production (ton) 1,500,000

Maximum speed (m/min) 1150

Entry strip thickness (mm) 2-4

Exit strip thickness (mm) 0.18-3

Coil width (mm) 650-1500


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Research highlight

Metal wrapping and strip welding in work/back-up rolls contact zone caused spalling

MnSinclusion and pore initiated crack which propagated in milling led to spalling

Retained austenite conversion to -martensite accelerated spalling failure

Needle shaped carbide, (Fe,Mo,Cr)7C3, my cause poor service life of back-up roll


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