The effect of laser surface melting on the erosion behaviour of a low alloy steel

Surface and Coatings Technology, 58 (1993) 85—92

85

The effect of laser surface melting on the erosion behaviour of a low alloy steel D. R. K. Raoa, B. Venkataramana, M. K. Asundi’~and G. Sundararajana ~Def~nceMetallurgical Research Laboratory, Kanchanbagh P0, Hvderabad 500 258 (India) bj\~y~( Chemical and Metallurgical Research Laboratory, Bombay (India) (Received September 10, I992~accepted in final form February II. 1993)

Abstract The objective of the present work is to evaluate the resistance ofa laser-surface-melted O.4% C low alloy steel to solid particle erosion. Towards this purpose, steel samples heat treated to two different base Vickers hardness levels, namely HV 350 and I-TV 550, were subsequently laser surface melted up to a depth of about 800 kim, resulting in hardnesses of HV 500 and HV 660 respectively. An air jet erosion tester utilizing SiC particles as erodent was used for comparing the erosion behaviour of laser-treated steels with those of untreated steel and annealed Armco iron (HV 76). Erosion tests were carried out at two impact velocities (46 and 96 m s - I) and three impact angles (30c, 60°and 90°).The results indicate that laser surface hardening does not improve the erosion resistance of a 0.4% C steel and that the erosion rates of the steels are very similar to that of annealed Armco iron.

1. Introduction Solid particle erosion, involving the removal of material from component surfaces by repeated impingement of hard particles travelling at considerable velocities, is an important material degradation problem in thermal power plants [I], gas turbine engines [2], pipelines transporting solids in the form of slurries [3], etc. Over the years, the solid particle erosion (henceforth called erosion) behaviour of a large number of metallic materials has been investigated [4, 5]. Among these. the microstructure—strength (or hardness)—erosion resistance correlation in the case of quenched and tempered steels has been thoroughly investigated by Finnie et a!. [6], Kleis [7], Gulden [8], Foley and Levy [9], McCabe et a!. [10] and Ambrosini and Bahadur [II]. All these investigations have unambiguously shown that the dramatic increase in the hardness of steels achieved by quenching and tempering and the corresponding change in the microstructure have negligible influence on the erosion rate. This comes about because the increase in strength or hardness of the steels can be achieved only at the expense of the strain hardening capability of the material and the above two effects offset each other, leading to a negligible overall effect on erosion rate [12]. Surface modification of steels by laser surface melting also results in a quenched and tempered microstructure. However, the residence time of the steel in the molten state is extremely short and, further, the cooling rates achieved during solidification after laser melting are

extremely large. Such a rapid solidification should result in refined microstructures in the steel and this may have a beneficial effect on the erosion behaviour. No data exist in the literature regarding the erosion behaviour of laser-treated steels. However, the influence of laser surface treatment of cast irons and Ti—6A1—4V alloys on their erosion behaviour has been characterized by Molian and Baldwin [13] and Yerramareddy and Bahadur [14] respectively. Molian and Baldwin [13] observed a substantial improvement in the erosion resistance of cast irons when they were laser surface melted. In contrast. Yerramareddy and Bahadur [14] obtained no improvement in erosion resistance by laser surface melting. The main objective of the present work is to explore the possibility of improving the erosion resistance of steels by laser surface melting. Towards this purpose, a low alloy steel was heat treated to Vickers hardness levels of HV 350 and HV 550 and subsequently laser surface melted to a depth of approximately 0.8 mm. The erosion resistance of laser-surface-melted steels was then evaluated and compared with those of both untreated steels and annealed Armco iron.

2. Experimental details 2.1. Test materials A low alloy steel with a chemical composition very similar to that of AISI 4340 steel (i.e. 0.4C—l.6Ni—l.lCr)

Elsevier Sequoia

86

D. R. K. Rao et a).

Effect of laser surface melting on erosion of low alloy steel

TABLE I. Heat treatment details and sample codes Specimen

Heat treatment

code

Hardness

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550

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350 660 500 76

and Armco iron were used in the present study. The samples used had dimensions of 30 mm x 30 mm and 5 mm thickness. Armco iron samples were used in the annealed condition while the steel samples were quenched and tempered to result in hardness levels of HV 550 and HV 350 respectively. The heat treatment details and the sample codes are provided in Table 1. 2.2. Laser treatment Steel samples of hardness HV 550 (sample code, 200’r) and HV 350 (sample code, 650T) were subsequently subjected to laser surface melting using a 5 kW continuous-wave CO2 laser manufactured by Spectra Physics (USA). The sample surfaces were shot peened before laser treatment to improve the laser power absorptivity. The process conditions used for laser treatment are provided in Table 2. In particular, an overlap of 50% between successive tracks was selected on the basis of an earlier study [15]. As indicated in Table 1 the laser-treated 200T steel samples (called L200T) exhibited a surface hardness of HV 660 (compared with the base value of HV 550) while the laser-treated 650T steel samples (called L650T) had a surface hardness of HV 500 (compared with the base value of HV 350). The lasertreated surface as well as its sectioned surface were examined in optical and scanning electron microscopes to determine the microstructure of the laser-melted zone, In addition, microhardness profiles were obtained on the sectioned surfaces to determine the melt zone size, 2.3. Erosion testing An air jet erosion tester, a schematic view of which is shown in Fig. I, was used for erosion testing of the laserTABLE 2. Laser treatment conditions Laser beam power Scanning speed Beam diameter Overlap of two successive scans Inert gas with laser beam

I

2 kW 37.5cm min 1 mm 50% Argon

SAMPLE HOLDER

_______________________________ Fig. I. A schematic diagram of the air Jet erosion rig.

treated and untreated steel samples and annealed Armco iron. The SiC particles used as the erodent were fed by a particle feeder at a controlled rate into the mixing chamber where they entrained the dry high velocity air coming from the compressor. The particles further accelerated as they moved with the air stream through a tungsten carbide converging nozzle and then finally hit the sample kept fixed on the sample holder. The velocity of the impinging particles, measured using the double-disc method originally proposed by Ruff and Ives [16], was varied by varying the compressor air pressure, while the impact angle was varied by simply varying the orientation of the sample surface with respect to the impinging particle stream. The erosion test conditions utilized in the present study are listed in Table 3. Thus each type of sample (i.e. Armco iron, 200T, 650T, L200T and L650T) was tested for erosion resistance at two impact velocities (46 and 96 m s’~)and three impact angles (30°,60°and 90°;90°represents normal impact). The erosion test was carried out as follows. The sample was first cleaned in acetone using an ultrasonic cleaner, dried and then weighed using an electronic balance having a resolution of 0.01 mgf. The sample was next fixed to the sample holder of the erosion rig and eroded with SiC particles at the predetermined particle TABLE 3. Test conditions for EN24 steels and Armco iron Particle Particle size Particle feed rate Distance between nozzle and sample surface Air flow rate Particle velocity Impact angles

SiC 215 ±35 ~im 3.7 ±0.1 g mm 5 mm 2.25 and 4.75 SCFM 46 m s’ and 96 m s’ 30, 60 and 90

D. R- K - Rao et a!.

Effect of laser surface ,neltin~on erosion of Ion alloy steel

feed rate. impact velocity and impact angle for a period of about 8 mm. The sample was then removed, cleaned in acetone and dried and weighed to determine the weight loss. This weight loss normalized by the mass of the SiC particles causing the weight loss was then defined as the incremental erosion rate. The above procedure was repeated till the incremental erosion rate attained a constant value independent of the mass of the erodent particles or, equivalently, of testing time. This constant value of the incremental erosion rate was defined as the steady state erosion rate E. To characterize the micromechanism of material removal during erosion, the eroded surfaces were examined in a JEOL scanning electron microscope.

3. Results 3.1. Laser-treated steel The surface appearances of laser-treated and untreated steel are compared in Fig. 2(a). The laser-treated surface is composed of parallel grooves or serrations with each one of them representing the laser scan path. In Fig. 2(b), a higher magnification view of the laser-treated steel surface is presented. The presence of dendrites, formed during solidification subsequent to laser surface melting, can be clearly noted. The sectioned views of the laser-treated steel samples

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57

are presented in Fig. 3. Figures 3(a) and 3(b) represent L200T and L650T steel samples respectively. In both these micrographs. the lasing direction is perpendicular to the sectioned surface presented. The overlapping of adjacent laser tracks can also be clearly noted in both the micrographs. The scanning electron micrographs of the sectioned surfaces of L200T and L650T samples are presented in Figs. 4(a) and 4(b) and Figs. 4(c) and 4(d) respectively. The microstructure of the melt zone is represented in Figs. 4(a) and 4(c) while the microstructure of the base steel is given in Figs. 4(b) and 4(d). The base microstructure has the characteristic tempered martensite structure composed of packets of martensite laths (Figs. 4(b) and 4(d)). In contrast, the melt zone exhibits a “cellular” microstructure (Figs. 4(a) and 4(c)). The average cell size is about 18 j.tm and 4 j.tm in the L200T and L650T samples respectively. The observation of “cellular” structure in the melt region (see Fig. 4) of both L200T and L650T samples is also consistent with the observations of other investigators [17] and it is attributable to the solidification rate of the melt pool being high enough to preclude dendritic growth [18]. The cooling rates estimated on the basis of the analysis due to Flemings [19] relating to the dendritic arm spacing or cell size are of the order of iO~K s’ and l0~K s~ in L200T and L650T respectively. The inner regions of the

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Effect of laser surface melting on erosion of low alloy steel

~4PIIi’0I,. ~ Fig. 4. Microstructures of the melt zone and base of laser-surface-melted steel samples: (a) melt zone of L200T; (b) base of L200T~(c) melt zone of L650T; (d) base of L650T.

cells are composed of fine martensite. The presence of this fine martensite is responsible for the high hardness exhibited by the L200T and L650T samples (see Table I). The microhardness—depth profiles obtained on the sectioned surfaces of L200T and L650T samples are presented in Fig. 5. In the L200T sample, the microhardness (Knoop hardness (HK) at 50 gf load) increases from a value of HK 600 ±50 at just below the sample surface to a value of around HK 700 at a depth of 800 tim. Thereafter the microhardness decreases sharply with _______________________

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increasing depth and attains a value around HK 400 at a depth of 1200 l.tm. Beyond the above depth, the HK value rises to the base value of HK 510 ±10. Thus, as shown in Fig. 5, the L200T sample consists of the melt zone up to a depth of 800 J.Lm, and the heat-affected zone (HAZ) for a further depth of 400 (.tm. The L650T sample also consists of a melt zone and HAZ of approximately the same widths as can be observed in Fig. 5. However, the hardness levels in the melt zone are much lower in the L650T samples than in the L200T samples. In contrast, in the HAZ, both the steels exhibit comparable hardness levels in the range HK 400—450.

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3.2. Erosion behaviour In Fig. 6, the variation of incremental erosion rate with cumulative mass of erodent particles is illustrated for Armco iron, 200T, L200T, 6501 and L650T samples. For each sample, the full line is valid for an impact velocity of 46 m s while the broken line represents a velocity of 96 m s The squares and circles (full or open) represent data corresponding, to 30°,and 90° impact angles respectively. In all materials, the incremental erosion rate reaches a constant or steady state value E by the time 50 g of SiC particles have impacted the sample surface. Before attaining a steady state value, the erosion rate either increases from a low value (as in Armco iron) or decreases from a higher value (as in L650T samples; V= 96 m s t) However, a discernible trend cannot be observed. -

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In Fig. 7, the variation of the steady state erosion rate E (estimated from Fig. 6) with impact angle is illustrated for Armco iron and the steels (2001, L200T, 650T and L650T) at impact velocities of 46 m s~ (Figs. 7(a) and 7(b)) and 96 m s’ (Figs. 7(c) and 7(d)). In the case of Armco iron and the untreated steels (i.e. 2001 and 6501), the steady state erosion rate E decreases with increasing impact angle from a maximum value at an impact angle of 30°.Such behaviour, usually termed a ductile erosion response, is observed at both impact velocities (Fig. 7). The laser-treated steels (i.e. L200T and L650T samples) exhibit a ductile response at higher impact velocities (Figs. 7(c) and 7(d)). However, at the lower impact velocity (V=46 m s’; see Figs. 7(a) and 7(b)), laser treatment causes the erosion rate to peak at an impact angle of 60°. Thus the erosion response under such conditions is no longer “ductile”. The morphology of the eroded surfaces is illustrated by the scanning electron micrographs in Figs. 8 and 9. These micrographs represent the surfaces of 2001 (Figs. 8(a) and 8(c)), L200T (Figs. 8(b) and 8(d)), 650T (Figs. 9(a) and 9(c)) and L6SOT (Figs. 9(b) and 9(d)) steel

samples are similar irrespective of whether the samples are untreated or laser treated. In all cases, material removal is clearly by formation of extruded lips around the impinging particles and their subsequent fracture. However, the lip formation is much more extensive at the impact angle of 30° than at 90°(compare Figs. 8(c) and 8(d) with Figs. 8(a) and 8(b) and also Figs. 9(c) and 9(d) with Figs. 9(a) and 9(b)) because the tangential component of the impact velocity greatly aids in the formation of extruded lips. The influence of a sample’s hardness (HV at 10 kgf load) on its erosion rate is illustrated in Fig. 10 for impact angles of 30°,60°and 90°.At each impact angle, the data corresponding to two impact velocities (full circles, 46 m full triangles, 96 m s~)are presented. Figure 10 clearly shows that increasing the hardness of the steels by conventional quenching and tempering or even by laser surface melting has a negligible effect on the erosion rate, In fact, the erosion rates of Armco iron (HV 76) and L200T steel (HV 660) samples are comparable even though the hardness has increased by a factor of 8.

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4.

Discussion

The melt pool formed by laser surface melting undergoes rapid solidification at high cooling rates in the range l0~—106 K s~’ [20]. In steels, such cooling rates result in a fine martensitic structure in the melt

90

D. R. K. Rao et al.

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Effect of laser surface melting on erosion of low alloy steel

I’ig. 8. Scanning electron micrographs of the eroded surfaces in the case of 200T and L200T samples: (a) 200T, 90°,46 m s~ (b) L200T, 90°.46 m 5_I; (c) 200T, 30°,46 m s - (d) L200T, 30 - 46 m ~

1I!P!31m1!tI!i Fig. 9. Scanning electron micrographs of the eroded surfaces in the case of 650T and L650T samples: (a) 650T, 90°,46 m s m 5_i; (c) 650T, 30°,46 m s~(d) L650T, 30 .46 ms’~.

region with a higher hardness than the conventional martensite formed by water or oil quenching and tempering [20]. In the present case, as noted earlier the cooling rates are estimated to be of the order of I K s and

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D. R. K. Rao et al.

Effect of laser surface melting on erosion of low alloy steel

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could be due to a reduction in carbon content in spite of the finer cell size at the melt region. A possible reason for the above behaviour could be that, during the 650 tempering before laser treatment, a substantial amount of carbon was lost from the near-surface layers due to

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According to Ashby and Easterling [21], the hardness of quenched steel essentially depends on the carbon content and volume fraction of martensite. The difference in hardness cannot be attributed to a difference in volume fraction of martensite since both microstructural examination and subzero treatment in liquid nitrogen indicate that the structure in both steels is fully martensite. In fact, the hardness value of HV 660 obtained on L200T samples matches very closely the predicted value of HV 668 obtained on the basis of the analysis given by Ashby and Easterling [21]. It should also be noted that the hardness achieved by laser surface melting in the case of the L200T sample is consistent with the values reported in the literature for similar steels. For example, microhardness values of HV 800 ±100 and HV 600 ±50 have been reported for a laser-surfacemelted AISI 4340 steel with 25% and 75% overlap between laser passes respectively, by Fastow et a!. [17]. The hardness obtained in the present study for L200 steel, namely HK 650 ±50 (Fig. 5), is consistent with the value reported by Fastow et al. [17] considering 50% overlap provided between passes. According to Ashby and Easterling [21], the hardness of the melt region should be HV 668 in the case of L650T also. However, the hardness value of L650T is only HV 500 as shown in Table 1 which is not consistent with the predicted value. The carbon content obtained based on the analysis of Ashby and Easterling [21] is 0.27 wt.%. Hence the low hardness in the case of L650T

decarburization. The microhardness—depth profiles illustrated in Fig. 5 were taken in such a way that only the melt zone and the HAZ below it were sampled by the microindentations. Thus the overlap zone has not been characterized in the present study. The low microhardness values in the HAZ in the L200T sample is to be expected since it is a lightly tempered sample (before laser treatment) and thus susceptible to tempering-induced softening. In contrast, in the L650T sample, the HAZ does not exhibit a dramatic reduction in hardness since it is a heavily tempered sample having a stable microstructure. The most interesting result of the present study is that the erosion resistance of steel is not improved by laser surface melting in spite of an increase in hardness. The most obvious explanation for such behaviour, also suggested by Yerramareddy and Bahadur [14] for the lack of influence of laser treatment of Ti~6Al4Valloy on its erosion resistance, is that the laser-modified layer is removed completely during the erosion test itself. To check this postulate, the maximum depth of the crater formed on the sample surface was measured and was found to never exceed about 50 ~m as opposed to a laser-melted zone 800 tim thick. In addition, the plastic deformation induced by the impinging particles extends only to a depth of the order of the particle radius, i.e. about 120 tim in the present study [22]. Thus it can be concluded that the erosion rates obtained on the L200T and L650T samples are characteristic of a steel with a melt zone microstructure and that the base or HAZ microstructure will not have any influence on the erosion rate of laser-treated steels. Given the above conclusion, it is obvious that an increase in hardness alone is not sufficient to improve the erosion resistance. It has been suggested that either a high ductility or a high strain hardening capability, which will postpone the onset of localization leading to lip formation and hence erosion [12], is required for obtaining high erosion resistance. Thus the negligible effect of hardness on erosion resistance can be rationalized on the basis that the increase in hardness has been achieved at the expense of ductility and/or strain hardening capability and these two effects offset each other, leading to a nearly hardness-invariant erosion resistance. The recent study by Yerramareddy and Bahadur [14] in the case of Ti—6Al—4V also suggests that laser surface melting does not improve the erosion resistance. In the case of cast iron, the presence of graphite flakes gives rise to the notch effect, accounting for poor ductility. On laser surface melting, the graphite flakes are replaced

92

D. R. K. Rao et a).

Effect of laser surface melting on erosion of low alloy steel

by a fine leduberite structure [13] which will most probably increase the ductility. Thus the laser melting of cast iron results in an increase in both hardness and ductility. The hardness of steels due to conventional quenching and low temperature tempering does not improve the erosion resistance in spite of a dramatic increase in hardness compared with that of Armco iron as demonstrated in the present study (Fig. 10) as well as by the earlier studies [6—10]. Such behaviour has been attributed to the reduction in ductility. Given the fact that the laser melting has not resulted in very fine structure, it is likely that the hardness increase due to laser surface melting has been achieved at the expense of ductility as in the case of conventional quenching and tempering, To conclude, laser surface melting can be expected to enhance erosion resistance only if it can increase the hardness of a material without impairing its ductility.

5. Conclusions Low alloy steel samples with hardnesses of MV 550 and HV 350 were laser surface melted with resultant surface hardnesses of HV 660 and HV 500 respectively. Erosion studies of these materials indicate that laser surface melting does not improve or impair the erosion resistance of a 0.4% C steel.

Acknowledgments The authors wish to express their gratitude to Mr. Babu Vishwanathan for his help in carrying out the

laser treatment and to the Director, Defence Metallurgical Research Laboratory, for granting permission to publish this work.

References I E. Raask, Wear, 13 (1968) 301. W. Sage and G. P. Tilly, Aeronaut. J., 73 (1969) 429. J. G. A. Bitter, Wear, 6 (1963) 169. G. P Tilly, in D. Scott (ed). Treatise on Materials Science and

23 4

Technology, Vol. 13, Academic Press, New York. 1979, p. 287. 5 G. Sundararajan, Trans. Indian Inst. Met., 36 (6) (1983) 474. 6 I. Finnie, J. Wolak and Y. Kabil, J. Mater., 2(1967) 682. 78 I.M.R.F.Kleis, Wear, Gulden, in J.13E.(l969) Field199. (ed), Proc. 5th mt. Conf. on Erosion by Liquid and Solid Impact, Cambridge University Press. Cambridge, 1979, p. 19. 9 T. Foley and A. Levy, Wear, 91(1983) 45. 10 L. P. McCabe, G. A. Sargent and H. Conrad. Wear, 105 (1985) 257. II L. Ambrosini and S. Bahadur, Wear. 117 (1987) 37. 12 G. Sundararajan, Wear, 149 (1991) Ill. 13 P. A. Molian and M. Baldwin, J. Tribol., 110 (1988) 462. 14 5. Yerramareddy and S. Bahadur, in K. C. Ludema (ed.). Proc. Conf on Wear of Materials 1991, American Society of Mechanical Engineers, New York, 1991, p. 531. 15 A. Bharti, unpublished work, Defence Metallurgical Research Laboratory, Hyderabad, 1991. 16 A. w. Ruff and L. K. Ives, Wear, 35 (1975) 195. 17 M. Fastow, M. Bamberger, N. Nir and M. Landkof, Mater. Sci. 900. 18 Technol., M. Cohen,6 B.(1990) H. Kear and R. Mehrabian, in R. Mehrabian, B. H. Kear and M. Cohen (eds.), Rapid Solidification Processing; Principles and Technologies, Vol. 2, Claitors, Los Angeles, CA, 1980, p. I. 19 M. C. Flemings, Solidification Processing, McGraw-Hill, New York, 1974. 20 W. M. Steen, Laser Processing of Materials, Springer, London, 1991. 21 M. F. Ashby and K. E. Easterling, Acta Metall., 32 (1984) 1935. 22 G. Sundararajan, Wear, 149 (1991) 129.