Wear behaviour of a steel weld-joint

Wear behaviour of a steel weld-joint

Wear 260 (2006) 1285–1294 Wear behaviour of a steel weld-joint S.N. Krishnan 1 , V. Toppo 2 , A. Basak, K.K. Ray ∗ Department of Metallurgical and Ma...

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Wear 260 (2006) 1285–1294

Wear behaviour of a steel weld-joint S.N. Krishnan 1 , V. Toppo 2 , A. Basak, K.K. Ray ∗ Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India Received 5 January 2005; received in revised form 16 August 2005; accepted 23 August 2005 Available online 6 October 2005

Abstract A series of experiments has been carried out to examine the dry sliding wear behaviour of the different regimes of a single pass steel weld-joint vis-`a-vis their microstructures and micro-hardness characteristics. Wear tests have been performed on two types of specimen configurations: first, on limited regions of the weld-joint exhibiting a specific microstructure, and second, continuously from the weld metal to the base metal via the heat affected zone (HAZ). The former type of experiments was found capable in delineating the wear characteristics of the base metal, weld metal and the different regimes of the HAZ. The wear behaviour of the varied regimes of the weld-joint, specifically that of the different sub-zones of the HAZ, has been explained using their microstructures, initial micro-hardness, work hardening during wear and their inherent residual stresses. This is a first report on the sequential wear behaviour of a weld-joint. © 2005 Elsevier B.V. All rights reserved. Keywords: Wear; Weld-joint; Heat affected zone; Steel; Coefficient of wear resistance

1. Introduction A weld-joint comprises of base metal, heat affected zone (HAZ) and weld metal. The HAZ of mild steel weldments, which has width of approximately a few millimeters, is commonly categorized into four sub-zones each exhibiting different microstructures [1,2]. These sub-zones are known as sub critical (SCHAZ), inter critical (ICHAZ), fine grain (FGHAZ) and coarse grain (CGHAZ) heat affected zones in the order of their location from the base metal to the weld metal [2]. The role of microstructure on the mechanical properties of the sub-zones is well known [3,4]. For example, the fracture toughness of the CGHAZ has been reported to be significantly lower compared to its adjacent FGHAZ for weld-joints fabricated from low carbon–manganese steels [5,6]. However, any systematic investigation related to the wear behaviour of this gradient microstructure is lacking, which comprises the primary content of this investigation. That materials with different hardness values will result in different wear behaviour, originates from the early report of

Archard [7], indicating that wear rate is inversely proportional to the hardness of a material. Since the microstructural constituents govern the hardness of a material, it is a natural expectation that a gradient microstructure like that in a weld-joint should lead to a variation in wear rate across its profile. This expectation is further substantiated by the report of Wang et al. [8], who emphasized that microstructure plays a more significant role than original hardness of a material. On the other hand, it is also known that the wear behaviour is a system response rather than the response of the characteristics of a specific material [9]. So the influence of the microstructure of a material on its wear rate would also depend to some extent on the selection of the system. An attempt is made here to examine whether the wear behaviour of the different sub-zones of a weld-joint can be distinguished under identical test conditions. The inherent aim of this investigation is, however, to understand the wear behaviour of gradient microstructures. 2. Experimental procedure

∗ Corresponding author. Tel.: +91 3222 283278; fax: +91 3222 282280/255303. E-mail address: [email protected] (K.K. Ray). 1 Currently with Directorate of Naval Design, New Delhi, India. 2 On study leave from National Institute of Foundry and Forge Technology, Ranchi, India.

0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.08.007

2.1. Material, processing and microstructures The base metal selected for fabricating the weld-joint is a low carbon–manganese steel, used for manufacturing hull structures of ships. The chemical composition of the selected

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Table 1 Chemical composition of the base metal (in weight percentage)

Table 3 Some pertinent details of the pin-on-disc wear machine used in this study

Elements

Amount

Machine particulars

Details

C Mn Si S P Cr Al Ni, Mo, V Fe

0.14 1.10 0.32 0.009 0.018 0.02 0.045 Trace Bal.

Disc material and hardness Disc size Track diameter Range of sliding speed Wear measurement range Maximum normal load

EN 31steel, hardness: Rc 58–62 160 mm diameter and 8 mm thick 10–140 mm 0.26–10 m/s ±2 mm 196.1 N

2.2. Hardness and tensile tests The microhardness measurements were carried out on polished T-S and L-S planes of a few specimens using a digital microhardness tester (LECO DM 400). These tests were carried out at a load of 0.3 kg for indentation duration of 10 s using a Vickers pyramid indenter. Microhardness profiles were determined by taking hardness readings at 500 ␮m intervals along several lines perpendicular to the weld-joint (T direction) with the distance between each line being 500 ␮m. Tensile properties of the base and the weld-joint along the T direction were obtained as per ASTM standard 8M-03 [10]. Flat sub-size tensile specimens of 6 mm width, 3 mm thickness and 25 mm gauge length were tested at a nominal strain rate of 6.67 × 10−4 s−1 using a 5 kN capacity screw driven Shimadzu (model: AG 5000G) universal testing machine at room temperature.

Fig. 1. Schematic diagram of a weld plate indicating the various types of test specimens. The T-S plane of specimen coupon-1 was used for determining hardness profile along the weld joint, L-S plane of specimen coupons 2 and 3 were used for wear studies and both T-S and L-S planes were used for microstructural studies.

steel is shown in Table 1. The weld-joint was fabricated by single pass butt-welding from 10 mm thick sample blanks of approximately 150 mm × 400 mm size using metal active gas (MAG) welding process. The important welding parameters adopted are summarized in Table 2. The fabricated weld-joints were not subjected to any post-weld heat treatment (PWHT). Specimen blanks for microstructure, hardness and wear studies were cut from the weld-joint as illustrated in Fig. 1. The length, width and thickness of the plate are referred here as L, T and S, respectively for convenience of further discussion. Microstructural studies were carried out on the TS and L-S planes. The microstructures were prepared with standard metallographic practice, followed by etching in 2% Nital.

2.3. Wear tests Wear tests were carried out on a pin-on-disc machine (model: Ducom TR 20) following ASTM standard G 99-03 [11]. Some pertinent details of this machine are summarized in Table 3. The pins for the wear tests were cut from the weld plates as shown in Fig. 1. Wear pins of both round and rectangular cross sections were fabricated. The cylindrical pins were approximately 4.6 mm in diameter whereas the rectangular pins were 4.2 × 4.0 mm in cross section; both types of pins were approximately 30 mm in length. The L-S plane of the weld-joint was selected for the wear test, so that the gradient microstructure is subjected to wear. To understand the wear behaviour of the different regimes of the weld-joint vis-`a-vis that of the base metal, wear studies have been carried out using pins of rectangular cross section, which are easy to fabricate from the weld-joints and in which the microstructures on the transverse sections can be examined conveniently. But ASTM standard [11] suggests that cylindrical pins should be used for pin-on-disk type wear tests. So a series of

Table 2 Some pertinent details of the single pass welding process Parameters

Description

Electrode

1.2 mm diameter wire electrode, AWS code: A5.18-ER 70 S-6 composition: C = 0.09, Mn = 1.55, Si = 0.8, S and P = 0.02, Cu = 0.2, and Fe-bal. (all in weight percentage) Composition: C = 0.1, Mn = 1.6, Si = 0.9, S and P = 0.02, Cu = 0.2 and Fe-bal. (all in weight percentage) Single V groove with an included angle of 60◦ and root gap of 6–8 mm Current: 195–205 A, voltage: 27.3 V, speed: 120 mm/min shielding gas: CO2 , backing strip: ceramic

Weld metal Nature of weld-joint Welding parameters

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Fig. 3. Typical macrostructure of the single pass weld-joint. Fig. 2. Typical microstructure of the investigated steel.

experiments have been carried out on pins (made of base metal) having both round and rectangular cross sections for examining the suitability of the latter ones. The cross sectional areas of both the cylindrical and the rectangular pins were kept constant, so that the contact pressures (CP) are equal and wear volume loss for both types of pins is the same for identical height loss. All wear tests were carried out at the sliding speed (SS ) of 1 m s−1 . Four different sets of wear tests were carried out. These are: (a) wear of cylindrical and rectangular pins of base metal at applied normal loads (NL ) of 19.6, 29.4, 49, 58.8, 68.6 and 78.4 N (corresponding CP were 1.2, 1.8, 2.9, 3.5, 4.1, and 4.7 MPa, respectively); (b) wear of rectangular pins whose surface exhibits only weld metal microstructure; (c) wear of rectangular pins whose surface exhibits only one of the different regimes of HAZ, and (d) wear of rectangular pins whose surface initially exhibits microstructure of the base metal but while the test is continued, the surface microstructure sequentially changes to those of HAZs and finally to that of the weld metal. Tests (b) and (c) were performed for short duration so that each test remains confined only to the microstructure of interest, whereas test (d) was done for a long duration so that each microstructure present in the weld-joint is sequentially subjected to wear. Tests (b), (c) and (d) were done only at NL = 58.8 N (CP = 3.5 MPa). All these tests were carried out at room temperature under dry sliding condition. The average initial surface roughness of the disc was 0.35 ␮m and that of the pins was better than 0.25 ␮m. The microhardness of each pin before and after the wear tests were determined. In addition, the worn out surfaces of the specimens and the fractured surfaces of the tensile specimens were studied using a scanning electron microscope (model JEOL 5800). 3. Results and discussions 3.1. Microstructural analysis A typical representative microstructure of the base metal (on the T-S plane of a specimen) is shown in Fig. 2. The microstructure exhibits ferrite and pearlite in the banded form. Banding has been observed also on the L-S plane. The presence of banding

is due to the combined effect of high manganese [12] and prior hot rolling of the steel [13]. The macrostructure of the single pass weld-joint on the T-S plane distinctly revealed the base metal, HAZ and the solidified weld metal in sequence as depicted in Fig. 3. Representative microstructures for the different regimes of the weld-joint except that for the base metal (Fig. 2), are depicted in Fig. 4. The microstructures in Fig. 4(a)–(d) correspond to inter critical, fine grain and coarse grain HAZ and the weld zone, respectively. The salient microstructural features of these zones are: (a) mixture of ferrite and pearlite with distorted banding (Fig. 4a) in the ICHAZ; (b) well-distributed ferrite and pearlite (Fig. 4b) in the FGHAZ; (c) a mixture of predominantly Widmanstatten ferrite and martensite (Fig. 4c) with traces of pearlite in the CGHAZ; and (d) columnar ferrite grains (Fig. 4d) in the solidified weld metal. However, the weld metal also exhibits planar, cellular and cellular dendritic structures at alternate locations. The observed microstructures in the HAZ and that in the weld metal are in line with several earlier reports for steels of similar composition [2,6,14]. Approximate estimates of the width of each regime of HAZ are shown in Table 4. 3.2. Microhardness variation across the weld-joint The microhardness values (HV 0.3) of the base metal, HAZ and the solidified weld metal were observed to be in the ranges 159–173, 155–241 and 161–218, respectively. The microhardness profile across the weld-joint is shown in Fig. 5(a). Each point in Fig. 5(a) represents an average of two/three individual hardness-readings falling within a specific sub-zone at a specific depth of the weldment as indicted by the legends in the figure. The average distance of these points from an arbitrary reference indicates their abscissa. The results in Fig. 5(a) infer that: (a) Table 4 Width of the different regimes of the heat affected zone Parameters widtha —left

Mean side (mm) Mean widtha —right side (mm) a

ICHAZ

FGHAZ

CGHAZ

HAZ

0.83 1.18

1.83 1.67

2.8 1.13

5.5 3.98

Three measurements were made on each region at different heights in the HAZ and the mean values are reported.

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Fig. 4. Microstructures at different regions of a single pass weld-joint: (a) distorted banded ferrite–pearlite aggregate (ICHAZ); (b) well-distributed ferrite and pearlite (FGHAZ); (c) Widmanstatten ferrite and martensite (CGHAZ); (d) columnar ferrite grains in weld zone.

each regime of HAZ has different hardness values, which fluctuate along the depth of the weldment, and (b) the magnitude of hardness of a regime increases from ICHAZ to CGHAZ. The highest hardness of CGHAZ is due to: (a) its close proximity to the fusion line, which causes this zone to experience the fastest cooling rate [2], which in turn, leads to the formation of harder microstructural constituents like martensite or bainite together with Widmanstatten ferrite [1], and (b) the presence of high residual stresses. The variation of microhardness along the solidified weld pool is shown in Fig. 5(b). The weld metal appears to indicate the highest hardness at its centerline, where it solidifies last. The average hardness values (HV 0.3) of the base metal, ICHAZ, FGHAZ, CGHAZ and that of the weld metal can be ascribed as 166, 171, 176, 203 and 184, respectively. The hardness of the weld metal is higher than that of the base

metal though it contains lower carbon (Table 2) than the latter. Inhomogeneity of microstructure and considerable residual stresses [2] in the weld metal are considered to lead to this observation. 3.3. Tensile properties of base metal and weld-joint Tensile properties of the base metal and the weld-joint were determined from their engineering stress–strain diagrams. The yield strength (YS), tensile strength (TS) and the percentage elongation (%EL) of the base metal are found to be 329 MPa, 464 MPa and 47.2%, respectively, and those for the weld-joint are obtained as 334 MPa, 479 MPa and 29.6%, respectively. The weld-joint thus possesses almost similar YS, marginally higher TS and significantly lower %EL compared to the base metal. Tensile specimens with weld-joint at the center of the

Fig. 5. Variation of microhardness: (a) at different sub-zones in a weld-joint for S = 0.5 to 4.5 mm and (b) within the solidified weld region for S = 0.5 to 3 mm from the bottom surface of the weld-joint. S indicates distance along thickness direction of the weld joint.

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for both types of pins. The average values of wear rate and frictional force were estimated for sliding distance (SD ) ranging between 500 and 1000 m. The variations of wear rate and frictional force with normal load for cylindrical and rectangular pins are shown in Fig. 7(a) and (b), respectively. The results in these figures assist to infer: (a) For cylindrical pins, the wear rate remains almost constant for loads upto 29.4 N (CP = 1.8 MPa), then sharply increases in-between 29.4 and 49 N (CP = 1.8–2.9 MPa), followed by a gradual fall upto 68.6 N (CP = 4.1 MPa) load, beyond which it remains almost invariant with further increase in NL . For rectangular specimens, the wear rate increases up to NL = 58.8 N (CP = 3.5 MPa), beyond which its variation with NL is similar to that for cylindrical specimens. (b) At lower NL , the wear rate for rectangular pins is higher than that of cylindrical specimens, but at higher NL it is similar to that of the latter. (c) The frictional force increases with increasing NL for both types of pins almost in a similar manner.

Fig. 6. Fracture surface after tension test of: (a) base metal, and (b) weld metal specimen.

gauge length were observed to fail at the weld metal. On S.E.M. examination, the fractured surface of the base metal indicated mixed cleavage and dimples, whereas that of the weld metal revealed primarily dimples as shown in Fig. 6. But the size of the dimples in the weld metal was found to be smaller than that of the base metal. Higher void growth in the base metal is considered responsible for its higher %EL compared to that of the weld metal. 3.4. Wear behaviour of base metal The wear tests on base metal specimens have been done using a sliding speed (SS ) of 1 m s−1 under different normal loads (NL )

The nature of variation in wear rate with NL , as shown in Fig. 7(a), reveals that transition from mild to severe wear (commonly termed as T1 transition) occurs between NL of 29.4 and 49 N (CP = 1.8–2.9 MPa) for both types of specimens. Typical worn out surfaces of specimens made from the base metal and tested at two different contact pressures are shown in Fig. 8. The difference in the morphology of the worn out surfaces tested at the normal loads of 19.6 N (CP = 1.2 MPa) and 58.8 N (CP = 3.5 MPa), representative of mild and severe wear regimes, respectively, are obvious. The worn out surface at low load is significantly smoother compared to the other, but it exhibits less metallic lustre. The higher wear rate of rectangular specimens at lower NL can be attributed to higher inhomogeneity in stress distribution on their contact surfaces compared to that of the cylindrical specimens. But the influence of such inhomogeneous stress distribution on wear rate is significant only during mild wear. In summary, it can be inferred that: (i) specimens with rectangular cross section can be used for wear studies in the severe wear regime for comparative assessment of different materials/microstructures, and (ii) the severe wear regime for the

Fig. 7. Variation of: (a) average wear rate, and (b) average frictional force with normal load for cylindrical and rectangular pins of base metal.

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Fig. 8. Typical wear surfaces generated at: (a) 19.6 N (CP = 1.2 MPa), and at (b) 58.8 N (CP = 3.5 MPa). (a) and (b) exhibit smooth oxidised surface and rough surface with deep tearing marks, respectively, indicating mild and severe wear regimes.

investigated steel at the selected sliding speed appears to start at NL ≥ 49 N (CP = 2.9 MPa). 3.5. Wear behaviour of weld metal Wear tests on the weld metal have been done using rectangular pins (surface of which exhibits only weld metal) at operating test parameters identical to those used for the base metal but only at NL = 58.8 N (CP = 3.5 MPa). The wear behaviour of the weld metal is compared with that of the base metal in Fig. 9. The results in Fig. 9(a) indicates that the amount of volume loss of weld metal increases continuously with increasing sliding distance (SD ) and its wear rate is higher than that of the base metal. Since the base metal has been found to exhibit severe wear at NL = 58.8 N (CP = 3.5 MPa), it is considered here that the wear rate of the weld metal also falls within the severe wear regime at this load. The average wear rate of the weld metal (11.3 × 10−3 mm3 /m) is about 1.4 times higher than that (7.66 × 10−3 mm3 /m) of the base metal. But its average frictional force (33.8 N) is marginally higher compared to that (32.18 N) of the base metal. The wear rates and the average frictional forces have been calculated for the data corresponding to sliding distance between 500 and 1000 m. The weld metal is observed to have higher wear rate inspite of its higher hardness than the base metal, contradicting Archard’s proposition [7]. This can be attributed to several possibilities like: (a) carbon content of weld metal is lower than that of the

Fig. 9. Wear behaviour of weld metal vis-`a-vis that of the base metal: (a) wear volume; (b) frictional force; (c) CWR.

base metal; (b) weld metal possesses tensile residual stresses [1] unlike that in the base metal far away from the weld-joint, and (c) microstructures of the base and the weld metals are different (as discussed in Section 3.1). Wang et al. [8] have suggested that difference in wear resistance for various microstructures depends on their stability with respect to frictional heating, resistance to plastic deformation, and resistance to initiation and propagation of microcracks in sliding. Lower carbon content of steel can reduce the wear resistance but it also reduces hardness; however, it has been observed that the hardness of the weld metal is higher than that of the base metal. The residual stress in the weld metal has been determined (by biaxial stress analysis) using X-ray diffractometer (model: Philips PW 1710), and this was found to

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be tensile in nature, with the magnitudes of the normal and shear stress components as 233.3 and −135.2 MPa, respectively. Thus a combination of the reasons (b) and (c) appears responsible for the observed higher wear rate of the weld metal. Several authors have pointed out that the frictional force is a response of a tribo-system [9]. The combined influence of frictional force and wear volume loss can be used to understand the effect of different microstructures on wear behaviour. A parameter, termed as coefficient of wear resistance (CWR), can be used to differentiate wear behaviour of different microstructures. This is expressed as: CWR =

FF × SD , V

(1)

where FF is the frictional force in N. SD = (2 πrt ) (Nt/60), the sliding distance in m, where rt is the wear track radius in m, N the revolutions/min of the disc, and t the time in s. V = π r2 h, wear volume loss in mm3 , where r is the radius of test pin in mm and h the height loss of the pin in mm. The expression for CWR as given by Eq. (1) can also be rewritten as: CWR =

FF FF = , V/SD WR

(2)

where WR is wear rate = (V/SD ). The values of CWR calculated by Eq. (2) give marginally different values when compared with those obtained by Eq. (1). The difference originates due to direct (V/SD ) and indirect calculations (using slope of the wear volume versus SD ) of wear rate by the two equations. In this report, all subsequent calculation of CWR has been made using Eq. (1). A typical plot indicating the variations of CWR with respect to SD for the base and the weld metal is illustrated in Fig. 9(c). It may be observed that CWR appears to stabilize for SD > 500 m and the average values of CWR for SD between 500 and 1000 m for the weld and the base metal are 2.55 and 4.4 N ␮m/␮m3 , respectively. The magnitude of CWR for the base metal is approximately 1.7 times larger than that of the weld metal. It may thus be inferred that CWR is an interesting parameter to compare wear behaviour of the base metal with that of the weld metal. Additionally, the applicability of the CWR was examined by a comparison of its magnitude for cylindrical and rectangular pins of base metal for a range of applied normal loads (NL ) used for the wear tests. It can be observed in Fig. 10 that in severe wear regime, both types of pins exhibit similar values of CWR. Thus the use of rectangular pins to differentiate wear behaviour of different microstructures in the severe wear regime can be justified. This observation is in line with that of Wang’s [8] proposition that different microstructures are better differentiated only in the severe wear regime.

Fig. 10. Variation of CWR of cylindrical and square pins of base metal with normal load.

is encountered (these are termed here as “continuous” experiments). Wear pins used for these two types of experiments are illustrated schematically in Fig. 11. The depth of each regime of HAZ was measured on the side surface of the rectangular pins by prior revelation of the microstructure using standard polishing and etching techniques. The continuous experiments were performed with five or six interruptions because of the limitation of recording a maximum of 2 mm of height loss in the wear machine in each run. The data collected at all the stages in the continuous experiments were combined to obtain the total wear behaviour of the weld-joint. The microstructures of the pin surfaces were examined and their hardness values were determined prior to all wear tests. All these tests were performed with operating parameters similar to those used for the weld metal, i.e. SS = 1 m s−1 , SD = 1500 m and NL = 58.8 N (CP = 3.5 MPa). The wear volume loss, frictional force and coefficient of wear resistance (CWR) for different regimes of the HAZ obtained from isolated experiments and that of the base metal are plot-

3.6. Wear behvaiour of HAZ Wear behaviour of HAZ was studied by performing two types of experiments: (a) experiments started and completed within a specific microstructure of HAZ (these are termed here as “isolated” experiments), and (b) experiments started on a surface exhibiting weld metal and continued till the base metal structure

Fig. 11. Schematic layout of microstructure on T-S plane of weld-joint from which rectangular wear pins fabricated for: (a) isolated, and (b) continuous wear tests.

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ted against SD in Fig. 12. The following inferences can be drawn from the results in Fig. 12: (a) all the regimes of HAZ exhibit severe wear, (b) CGHAZ exhibits the highest wear rate (12.3 × 10−3 mm3 /m), whereas, the ICHAZ exhibits the lowest wear rate (8.84 × 10−3 mm3 /m), (c) it is difficult to distinguish the different sub-zones in terms of frictional force during wear because of the associated high magnitude of scatter; apparently the highest and the lowest frictional forces are generated by the CGHAZ and ICHAZ, respectively, and (d) CWR distinguishes the sub-zones of HAZ; the average values of CWR for ICHAZ, FGHAZ and CGHAZ are 4.4, 3.04 and 2.32 N ␮m/␮m3 , respectively. It may be mentioned at this stage that wear rate and CWR values have been calculated for SD between 500 and 1000 m. The different wear rates that the various sub-zones of the weld-joint exhibit can be understood on the basis of their microstructures and their response to frictional heating. Frictional heating causes a change in hardness of a microstructure and this phenomenon is related to “microstructural thermal stability” as suggested by Wang et al. [8]. The hardness of a contact surface during wear is often termed as dynamic hardness. A pearlite phase can retain its hardness upto a temperature of 700 ◦ C, while the hardness of martensite starts to decrease at about 200 ◦ C [8]. The hardness of martensite-containing microstructure becomes less than that of ferrite–pearlite structures at a temperature of 650 ◦ C. The martensite present in the CGHAZ softens during wear and exhibits higher wear rate, whereas the lowest wear rate exhibited by the base metal is possibly due to its banded pearlite. The ICHAZ with distorted bands of ferrite and pearlite indicates similar wear rate and CWR to those of the base metal, whereas the FGHAZ, containing the well-distributed ferrite and pearlite, possesses an intermediate value of wear rate and CWR between those of CGHAZ and ICHAZ. Thus it can be concluded that the nature of distribution of ferrite and pearlite in FGHAZ, ICHAZ and base metal, and the martensite content in CGHAZ lead to different wear behaviour of the sub-zones of a weld-joint. Interestingly for the investigated steel, the CGHAZ with the highest room temperature hardness yields the highest wear rate. The salient observations from the continuous wear tests (Fig. 13) are: (a) the wear rates of all the sub-zones of the weld-joint appear to be similar except for ICHAZ showing a marginally low wear rate, and (b) the transition from one subzone to the other (weld to CGHAZ and so on) gets apparently reflected in the variation of CWR. A comparison of the wear

Fig. 12. A comparative assessment of the wear results of CGHAZ, FGHAZ, ICHAZ, and base metal obtained from isolated wear tests: (a) wear volume loss; (b) frictional force; and (c) CWR against sliding distance.

Fig. 13. Wear behaviour of the different sub-zones of HAZ: (a) wear rate, (b) CWR. The arrows in (b) indicate restart of the various stages of the continuous test.

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and the weld metal and minimum for the CGHAZ. There is a sudden jump in the percentage increase in hardness from CGHAZ to the weld metal. The base metal which shows the highest workhardening (indexed through increase in hardness) is observed to exhibit the lowest wear rate while the CGHAZ which shows the least work-hardening exhibits the highest wear rate. As the wear rate of base metal and that of the different sub-zones of HAZ can be related to the dynamic hardness of their respective microstructures, it is suggested that the observed nature of workhardening can be considered as index for dynamic hardness. However, the same explanation cannot be extended to the weld metal, which shows high work-hardening as well as relatively high wear rate compared to that of the base metal (Fig. 9). The characteristic wear behaviour of the weld metal can plausibly be attributed to the fact that it generally possesses tensile residual stress, unlike compressive residual stress in HAZ [1] or no resid-

Fig. 14. Effect of work-hardening on the different regimes of the weld-joint: (a) microhardness before and after wear, and (b) percentage increase in microhardness. The error bands indicated in (a) refer to the standard deviations of the mean values.

rates obtained from isolated and continuous wear tests indicates that different regimes of the HAZ can be distinguished only by carrying out “isolated” wear tests. The inability of the continuous wear tests to demarcate the various regimes of the HAZ can be attributed to the combined effect of work hardening and the degree of microstructural instability [8] of a layer by its adjacent upper layer. The lower wear rate of ICHAZ in the continuous experiment is in agreement with its wear behaviour in “isolated” experiments and is probably less influenced by the wear characteristics of its adjacent upper layer in FGHAZ. 3.7. Work-hardening of wear surface Any metallic surface subjected to wear gets work hardened [15], and thus comparing the hardness of a surface of interest before and after wear one can assess the degree of work-hardening. The degree of work-hardening of the different sub-zones of the weld-joint has been examined for the specimens subjected to isolated wear tests. The microhardness values for the different regimes of the weld-joint before and after the wear test are shown in Fig. 14(a) and the percentage change in hardness due to wear is depicted in Fig. 14(b). These results indicate that the percentage increase in hardness is maximum for the base

Fig. 15. Variation of average values of: (a) wear rate; (b) frictional force; (c) CWR across the single pass weld-joint. The error bands indicated in the figures refer to the standard deviations of the mean values.

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ual stress in base metal, apart from its different composition and microstructure. 3.8. Some generalization about wear behaviour of the weld-joint The discussions on the wear behaviour of the base metal, weld metal and the HAZ in Sections 3.4–3.6 are based on typical results. The experiments on isolated microstructures have been repeated and the average values of wear rate, frictional force and the coefficient of wear resistance (CWR) have been estimated for each sub-zone. The average values of these wear parameters for the different regimes of the weld-joint are depicted in Fig. 15. The estimated wear parameters do demarcate subtle variation between the wear behaviour of the various sub-zones of HAZ, but their associated standard deviation values appear to suppress the clarity. It may be inferred, however, that the base metal and the ICHAZ indicates highest value of CWR, in contrast to the lowest value of CWR exhibited by the weld metal and the CGHAZ. Finally, it may be concluded that CWR brings out the characteristic wear behaviour of the different sub-zones of a weld-joint in a distinct manner. 4. Conclusions 1. The sub-zones of a weld-joint exhibiting different microstructures indicate subtle difference in the wear rates and frictional forces though these exhibit prominent variation in hardness values. 2. The product of frictional force, sliding distance and inverse of wear rate has been suggested as a parameter to characterize wear behaviour of gradient microstructures. Using this parameter the differences in the wear behaviour amongst the base metal, weld metal and the varied regimes of a HAZ have been brought forward in a distinct manner. 3. The transition from one microstructure to another can be revealed using ‘continuous’ wear tests of a HAZ, with the help of the variation of CWR. 4. The wear behaviour of the different regimes of a HAZ (possessing varied microstructure) is found different when wear tests are carried out continuously from the weld metal to

the base metal and when these are done on isolated regions exhibiting only a specific sub-zone. This difference originates due to the varying effect of work hardening and microstructural thermal stability. 5. The difference between the hardness values of a sub-zone before and after a wear test (under identical conditions) depends on its nature. This has been attributed to the characteristic work hardening behaviour of each sub-zone. References [1] J.F. Lancaster, Metallurgy of Welding, Chapman & Hall, London, 1993, p. 187. [2] K. Easterling, Introduction to Physical Metallurgy of Welding, Butterworths Heinemann, Oxford, 1993, p. 126. [3] C. Liu, S.D. Bhole, Fracture behaviour in a pressure vessel steel weld, Mater. Design 23 (2002) 371–376. [4] J.H. Kim, Y.J. Oh, I. Hwang, D.J. Kim, J.T. Kim, Fracture behaviour of heat-affected zone in low alloy steels, J. Nucl. Mater. 299 (2001) 132–139. [5] A. Seshu Kumar, Studies on simulated HAZ condition in a structural steel, Ph.D thesis, IIT Kharagpur, 1999, p. 183. [6] R.E. Dolby, Factors controlling HAZ and weld metal toughness in C–Mn steels, in: G.G. Garrett, D.L. Marriott (Eds.), Engineering Application of Fracture Analysis, Pergamon Press, 1979, pp. 117–134. [7] J.F. Archard, Contact and rubbing of flat surfaces, J. Appl. Phys. 24 (8) (1953) 981–988. [8] Y. Wang, T. Lei, J. Liu, Tribo-metallographic behaviour of high carbon steel in dry sliding II. Microstructure and wear, Wear 231 (1999) 12–19. [9] K. Kato, Wear in relation to friction—a review, Wear 241 (2000) 151–157. [10] ASTM standard E 8M-03, Standard test method for tension testing of metallic materials (Metric), ASTM Annual Book of Standards, vol. 03.01, West Conshohocken, PA, 2003. [11] ASTM standard G 99-03, Standard test method for wear testing with a pin-on-disc apparatus, ASTM Annual Book of Standards, vol. 03.02, West Conshohocken, PA, 2003. [12] R. Ramanathan, R.P. Foley, Effect of prior microstructure on austenite decomposition and associated distortion, Report, Illinois Institute of Technology, Chicago, Illinois, USA, August 2001, p. 6. [13] D. Chae, D.A. Koss, A.L. Wilson, P.R. Howell, The effect of microstructural banding on failure initiation of HY-100 steel, Met. Mater. Trans. A 31A (2000) 995–1005. [14] S.N. Krishnan, A. Basak, K.K. Ray, Wear characteristics of a multipass weld-joint of a plane carbon steel, Met. Mater. Process. 16 (2004) 375–384. [15] H. Goto, Y. Amamoto, Effects of varying load on wear resistance of carbon steel under unlubricated conditions, Wear 254 (2003) 1256–1266.