SOLID PARTICLE EROSION OF WELD HARDFACING CAST IRONS

Transcription

1 SOLID PARTICLE EROSION OF WELD HARDFACING CAST IRONS S. G. SAPATE Department of Metallurgical Engineering, VR College of Engineering, South Ambazari Road, Nagpur 4411, INDIA; A. V. RAMARAO, N. K. GARG Diffusion Engineers Pvt. Limited, T/65, MIDC, Nagpur 4422, INDIA; SUMMARY Five commercial hardfacing high chromium cast iron alloys were deposited by flux cored arc-welding methods. The solid particle erosion studies were carried out using air blast type erosion test rig with µm cement clinker, µm blast furnace sinter, 1-15 µm silica sand and µm alumina particles at a velocity of 5 ms -1 and at impingement angles of 15-9 o. The observed erosion rates were rationalised in terms of relative hardness of erodent particles and ability of erodent particle to cause gross fracture of the carbides. The dependence of erosion rate on impingement angle was found to be quite weak for hardfacing high chromium cast iron alloys. However significant differences were observed in the ranking of the alloys when eroded with different erodent particles. The presence of large volume fraction of carbides proved to be beneficial to the erosion resistance when the erodent particle were softer than the carbides. With silica sand particles at normal impact and with alumina particles at both 3 o and 9 o, large volume fraction of carbides were detrimental to the erosion resistance. The operating erosion mechanisms involved small-scale chipping, edge effect, indentation and fracture and fatigue. Keywords: Hardfacing, Erodent, Edge effect, Carbide volume fraction, Indentation 1 INTRODUCTION The strategy of dispersing a ceramic phase into a more ductile matrix to improve the wear resistance has been successfully employed in abrasion, e.g. in WC-Co alloys and high chromium white irons. High chromium cast irons exhibit twenty to twenty five times more abrasion resistance than mild steels under two body and three body abrasive wear situations, when the abrasive particles are softer than the carbide particles [1-2]. The excellent abrasion resistance of high chromium irons is attributed to the presence of large volume fraction of M7C3 carbides (13-18 HV) in the microstructure. The abrasion resistance of high chromium white irons in cast as well as in weld deposited form is well documented in the literature [3-7], whereas very little data is available on erosion behavior of white irons in either cast or weld deposited form. Kosel and Ahmed [8] performed extensive erosion studies on model alloys; Cu-Al 2 O 3 and Cu-WC (W 2 C), under different erosion conditions with quartz particles and established edge effect as one of the erosion mechanisms of material removal from second phase particles. Hansen [9] compared erosion rates of a large number of metallic alloys when eroded with 27 µm alumina particles and observed that erosion rate of 25 % chromium iron was 1.25 times as compared to that of Stellite 6 B alloy at normal impact. Erosion investigation by Aptekar and Kosel [1], Stevenson and Hutchings [11] and Katavic [12] revealed weaker dependence of erosion rate on angle of incidence for high chromium cast irons and a peak in erosion rate was observed at impingement angles between 5 o -9 o. Stevenson and Hutchings [11] indicated significant differences in ranking of hardafcing white irons depending upon the erodent particle hardness and the impingement angle. High chromium white irons are widely used in weld deposited form, for protection of engineering components against abrasion and erosion in cement plants, iron and steel making industries and mining industries. The present work was undertaken to evaluate the erosion resistance of weld hardfacing high chromium iron alloys with different erodent particles. 2 EXPERIMENTAL 2.1 Materials Erosion tests were performed on five different grades of high chromium iron weld hardfacing alloys, which were deposited by flux cored arc welding technique on a mild steel base plate, typically 1 mm thick. In each case two layers of alloys, designated as,,, and were deposited to produce a deposit thickness of 5 mm. A low alloy steel, S1, in the weld deposited condition was used as the reference material. The chemical composition and hardness of weld hardfacing alloys and that of the reference material are given in Tables 1 and 2 respectively Metallography The specimems measuring 15 x 15 mm were prepared using standard metallographic techniques. The polished specimens of high chromium iron hardafacing alloys were subsequently etched with Kallings etching reagent whereas weld deposited low alloy steel was etched with 2 % Nital. The SEM observations revealed difference in scale of the microstructure of hardfacing alloys. Alloy had hypoeutectic structure consisting of fine eutectic carbides and primary austenite, whereas large primary carbides of M 7 C 3 type were observed in the microstructure of alloy. The primary carbides were surrounded by matrix of eutectic carbides and martensite. The M 7 C 3 carbides were also present in the microstructure of alloys, and. The back-scattered SEM image of alloy and revealed white regions which were recognized as niobium carbides, which was indicated by EDX analysis of these regions.

2 2.1.2 Hardness Measurement The bulk hardness was measured at a load of 3 kg, using Indentec make Vickers Indentation Hardness Tester. The microhardness of hardfacing high chromium iron alloys was measured using Shimadzu make Vickers Microhardness Tester. The polished and etched specimens were used for microhardness measurements. The relatively small size of the eutectic carbides present in the microstructure of the high chromium iron alloys made it difficult to measure the microhardness of the eutectic carbides. An average of the ten readings is reported in the results. The hardness and microhardness values are given in Table Characterization of Erodent Particles In the present investigation, erodent particles used were cement clinker, blast furnace sinter, silica sand and alumina. The erodent particles were dry sieved using sieve shaker to obtain the required fractions. The shape and size of the erodent particles was examined under scanning electron microscope. For indentation hardness testing, the erodent particles were mounted in a cold curing resin and were subsequently ground using successive abrasive papers followed by polishing with alumina. The indentation hardness was measured using Shimadzu make Vickers Micro-indentation Tester using diamond pyramid indenter at a load of 1 g. Table 3 gives the physical properties of the erodent particles. 2.2 Erosion Testing The erosion experiments were performed using air blast type erosion test rig. The specimens for erosion testing were rectangular blocks measuring 2 mm long, 15 mm wide and 5 mm thick. The specimens were mounted directly below the nozzle with a stand off distance of 1 mm between the end of the nozzle and the test surface. The particle velocity was measured using double disk apparatus [13]. The particle feed rate was kept constant throughout the erosion studies and was nominally 5 g min -1. All the erosion experiments were conducted at room temperature. The erosion experiments were performed using cement clinker, blast furnace sinter, silica sand and alumina particles, at impingement angles of 15,3,6 and 9 o and at a velocity of 5 ms -1. The steady state erosion rate of alloys was determined by exposing the specimens to erodent particles in successive increments of 25-1 g, depending upon the erodent and the target material. The specimens were weighed before and after the erosion testing. The mass loss measurements were used to represent the erosion rate (g/g of erodent particle) through out this work. Eroded surfaces were observed under SEM to study the operating erosion mechanisms. 3 RESULTS Fig.1 (a-d) shows the variation in erosion rate of hardfacing high chromium irons with impingement angle at a velocity of 5 ms -1 with µm cement clinker, µm blast furnace sinter, 1-15 µm silica sand and µm alumina particles, respectively. A peak in erosion rate at 3 o was observed for weld deposited steel whereas for hardfacing high chromium iron alloys, maximum erosion rate occurred at impingement angles of 6-9 o. In general, erosion rate showed a weak dependence on impingement angle, the largest and lowest difference was observed with silica and particles at normal impact and cement clinker particles respectively. With µm cement clinker particles, alloy exhibited highest erosion rate, than rest of the alloys, irrespective of the impingement angle. At normal impact, alloy had lowest erosion rate. With blast furnace sinter particles, alloy exhibited highest erosion rate than rest of the hardfacing alloys irrespective of the impingement angle. With 1-15 µm silica sand particles the peak in erosion rate for hardfacing alloys shifted to 9 o ; alloys and, which were having higher volume fraction of carbides, showed continuous increases in erosion rate with impact angle. Alloy, which had lowest erosion rate with cement clinker and blast furnace sinter particles, exhibited highest erosion rate of all the hardfacing alloys at normal impact. Alloys and exhibited lowest erosion rate of all the hardfacing alloys, at impingement angles of 3 o and 9 o respectively, with silica sand and alumina particles. With µm alumina particles the erosion rates of hardfacing alloys were observed to be times as compared to that with 1-15 µm silica sand particles under similar erosion conditions. 4 DISCUSSION The results of the present investigation suggest that E-α characteristics of weld hardfacing white irons depend upon the relative hardness of the erodent particles which in turn decides the relative contribution of matrix and the carbides to the erosion process. The dependence of erosion rate on impingement angle for hardfacing high chromium iron alloys was found to be weak, which is consistent with previous observations on erosion of high chromium cast irons [1-12]. With softer cement clinker particles at a velocity of 5 ms -1, the peak in erosion curve for hardfacing high chromium iron alloys was observed to be at 6 o impingement angle. Zhu and Mao [14] reported similar observations for erosion of harder materials with softer erodents like Fe 2 O 3 and glass particles. With blast furnace sinter particles, similar dependence of erosion rate on impact angle was noted with lower wear rates being observed at 9 degrees as compared to that at 3 degrees. The differential hardness of erodents with respect to carbides and matrix could very well explain the observed erosion rates of hardfacing alloys. For an erodent particle to be able to indent a target plastically its hardness should be about 1.2 times greater than that of the target [15]. With cement clinker particles and blast furnace sinter particles, the ratio of erodent hardness to carbide hardness i.e. H e /H c is not more than.3 and.6 respectively. Cement clinker and blast furnace sinter particles both, can neither indent nor cause fracture of the primary carbides in high chromium irons, thereby offering protection to the matrix resulting in decreased wear rates at 9 degrees. At 3 degrees, cement clinker can not cause micromachining of the matrix, instead it itself fractures upon impact due to its lower hardness. The ratio of H e /H mat for blast furnace sinter particles is close to 1.2, hence it is expected that matrix will be worn to a greater extent as compared to that with softer cement clinker particles. This is reflected in higher erosion rates with blast furnace sinter particles, which were

3 nearly four times as compared to that with cement clinker particles. Thus it can be suggested that with cement clinker particles, the erosion rate of hardfacing alloys depend upon bulk hardness of the alloys whereas with blast furnace sinter particles, the matrix hardness is expected to be a controlling factor in deciding the erosion rate of the hardfacing alloys. Alloy, containing large primary carbides as compared to other alloys, showed lowest erosion rate with cement clinker and blast furnace sinter particles. It is beneficial for erosion resistance if the harder second phase particles are larger than the extent of rounding caused by the edge chipping [11]. The relative erosion resistance of hardfacing alloys at 3 o was observed to be 1-14 times better as compared to that of alloy S1 (153 µg/g), under similar erosion conditions with cement clinker particles. With blast furnace sinter particles, the relative erosion resistance of hardfacing alloys dropped by a factor of three as compared to that with cement clinker particles, at normal impact. In comparison with cement clinker and blast furnace particles, which have hardness less than that of the carbides, silica sand particles have hardness intermediate between that of primary carbides and the matrix in the high chromium iron hardfacing alloys. The silica sand particles can indent and cause fracture of the primary carbides at 9 degrees as shown in fig. 2. The edge rounding and edge fracture of carbides can also be seen. The high volume fraction of carbides close to the surface will tend to enhance the plastic strain in the matrix, leading to rapid removal of carbides [11]. This resulted in increase in erosion rate of hardfacing alloys containing higher volume fraction of carbide, e.g. alloy, at higher angles. In fact, the erosion rate of alloy with silica sand particles at 9 degrees exceeds that of the weld deposited low alloy steel (159.3 µg/g) suggesting that hardfacing alloys do not offer any advantage over weld deposited low alloy steel under these erosion conditions. At an impact angle of 3 degrees, carbides are not readily fractured as carbide particles protrude from the surface, thus offering some protection to the matrix. At lower angles the material removal mechanism from the carbides involved small scale chipping as opposed to gross fracture at normal impact, resulting in less rapid erosion of carbide particles. The comparison of erosion rates of hardfacing alloys at 3 o angle with that of weld deposited low alloy steel (285 µg/g) showed that hardfacing alloys exhibit times better erosion resistance with silica sand particles at 3 o impact angle. The hardness of alumina particles is greater than that of the matrix as well as that of the carbides. With alumina particles, primary carbides were no longer effective in protecting the matrix against erodent particle attack. It is also evident from SEM observations that the carbides were removed by the process of lateral cracking and fracture as it can be seen from fig.3, which shows eroded surface of alloy when eroded with µm alumina particles at an impingement angle of 3 o. It can also observed that most of the carbides were seen depressed or at the same level as that of the matrix. The matrix erosion rate is also expected to be higher with alumina particles, due to its greater angularity. This is reflected in erosion rates of hardfacing alloys, which were observed to be 1.5 times greater as compared to those with silica sand particles, at 3 o impingement angle. The material removal mechanism from the matrix involved ductile cutting at lower impingement angles with alumina particles, which can also be seen from fig.3, as opposed to ploughing with softer erodent particles. At normal impact, the material was removed by the process of fatigue as described by Hutchings [16]. The erosion rates of hardfacing alloys with alumina particles, at 9 o angle were more than two times as compared to that of weld deposited low alloy steel S1 (195.4 µg/g) under similar erosion conditions, suggesting the detrimental effect of large carbides to the erosion resistance at normal impact. 5 CONCLUSION 1. The dependence of erosion rate on impingement angle was in general weak for weld hardfacing high chromium cast irons. 2. The peak in erosion rate occurred at impingement angle of 6-9 o depending upon the alloy and the erodent particle hardness. 3. Significant differences were seen in ranking of weld hardfacing alloys depending upon erodent particle hardness and the impingement angle. 4. Large volume fraction of carbides proved beneficial to the erosion resistance with softer erodent particles. With silica sand particles at normal impact and with alumina particles, large volume fraction of carbides was detrimental to the erosion resistance. 5. Relative hardness of erodent particles has great influence on erosion rate of weld hardfacing cast irons. 6. The material removal mechanisms from the carbides were strongly influenced by the erodent particle hardness and the impingement angle. Alloy C Si Mn Cr Mo Other Ni B B Nb B Nb S Ni Table 1: Chemical composition of hardfacing alloys Bulk Hardness Microhardness Hv.1 Alloy HRc Matrix Carbide 691 ± ± ± ± ± ± ± ± ± ± ± ± ± ± 131 S ± 2 - Table 2: Hardness and Microhardness of hardfacing alloys

4 Erodent Cement clinker µm Blast furnace sinter µm Silica sand 1-15 µm Alumina µm Density Hardness kg m -3 Hv.1 Morphology ± 123 Amorphous, ± 179 Crystalline, ± 139 Crystalline, Sub-angular ± 195 Crystalline, Table 3: Physical properties of erodent particles Figure 2: SEM photograph showing eroded surface of alloy when eroded with 1-15 µm silica sand particles at velocity of 5 m s -1 and at normal impact. 6 REFERENCES [1] J.Xing et al: An investigation on the wear resistance of high chromium cast irons. Proc. Int. Conf. on Wear of Materials, April 11-14, 1983, 45-61, Reston Virginia [2] J. Shu Sun: The abrasion characteristics of some carbide containing alloys. Proc. Int. Conf. on Wear of Materials, April , 79-86, Reston Virginia [3] K.H. Zum Gahr: Microstructure and Wear of Materials. Elsevier, 1987 [4] S. Turenne et al. : Matrix microstructure effect on the abrasion wear resistance of high chromium white cast irons. Materials Science, 24, (1989), [5] D.N. Noble: Abrasive wear resistance of hardfacing weld deposits. Metal Construction, 17, (1985) 9, [6] S. Atamert and J.Stekly: Microstructure, wear resistance, and stability of cobalt based and alternative iron based hardfacing alloys. Surface Engineering, 9, (1993) 3, [7] H.Berns and A.Fischer: Abrasive wear resistance and microstructure of Fe-Cr-C-B hard-surfacing weld deposits. Wear, 112, (1986), [8] T.H. Kosel, T. Ahmed: Erosion of Ceramic Materials. Trans.Tech. Publication, 1992, [9] J.S. Hansen: Relative erosion resistance of several materials. Erosion: Prevention and useful Applications, ASTM STP 664, ASTM, 1979, [1] S.S.Aptekar and T.H. Kosel: Erosion of white cast irons. Proc. of Int. Conf. on Wear of Materials, 1985, [11] A.N.J. Stevenson and I.M. Hutchings: Wear of hardfacing white cast irons by solid particles erosion. Wear, , (1995), [12] I.Katavic: Investigation of erosion wear of white irons, Proc. 7 th Int. Conf. On Erosion by Liquid and Solid Impact, Cambridge, UK, 1987, [13] A.W.Ruff and L.K.Ives: Measurement of solid particle velocity in erosive wear. Wear 35, (1975), [14] W.Zhu and Z.Y.Mao: Study of erosion by relatively soft particles. Proc. Int. Conf. on Wear of Materials, 1987, , Newyork [15] I.M.Hutchings: Tribolgy, Friction and Wear of Engineering Materials. Edward Arnold, 1992 [16] I.M.Hutchings: A model for the erosion of metals by spherical particles at normal incidence. Wear, 7, (1981), Figure 3: SEM photograph of alloy when eroded with µm alumina particles at an impact angle of 3 o and at a velocity of 5 ms -1. Alumina particle can plastically indent and cause fracture of carbide particles. 7 ACKNOWLEDGEMENTS The authors are grateful to Principal VRCE for providing necessary facilities in carrying out this investigation. The authors are also thankful to M/s Diffusion Engineers Pvt. Limited, Nagpur for supplying the samples for present investigation. The authors are grateful to Mr. Rajshekar Rao, JNARDDC Nagpur for assistance during SEM work.

5 a b Impingement angle ( degrees) c d Figure 1: Erosion rate of weld hardfacing alloys as a function of impingement angle at a velocity of 5 m s -1 with: a) µm cement clinker particles b) µm blast furnace sinter particles. c) 1-15 µm silica sand d) µm alumina particles.