Microstructural evolution and mechanical properties of low alloy steel tempered by induction heating

Transcription

1 Journal of Materials Processing Technology 160 (2005) Microstructural evolution and mechanical properties of low alloy steel tempered by induction heating Soon Tae Ahn a, Dae Sung Kim b, Won Jong Nam b, a Samhwa R&D Center, Pusan , Republic of Korea b Department of Materials Science and Engineering, Kookmin University, Jeongneung-dong, Sungbuk-Ku, Seoul , Republic of Korea Received 15 May 2003; received in revised form 10 March 2004; accepted 12 March 2004 Abstract In comparison to conventionally processed, quenched and tempered steels, induction quenched-and-tempered steels offer the potential advantage for significant cost savings. The present study was carried out to investigate the microstructural evolution and the corresponding variation of the mechanical properties in a low alloyed steel tempered by induction heating, compared with those of the steel tempered by salt bath heating. While the cementite particles began to change their shape from a needle type to a fine spheroidal type at the tempering temperature of 600 C in the induction-tempered steels, the spheroidization already started at the tempering temperature of 500 C in the steels tempered in a salt bath. A superior combination of the mechanical properties of tensile strength above 1000 MPa, reduction of area of 65% and Charpy impact value above 120 J/cm 2, was obtained for the steel, induction-tempered at 600 C. Furthermore, the high ratio of YS/TS above 0.9 in induction-tempered steels indicates that work hardening during the deformation was not significant. Although, the cold forgeability test was not performed in this work, the high values of the reduction of area and Charpy impact energy imply that the induction quenched-and-tempered steels would be applicable to cold forging for manufacturing automotive components Elsevier B.V. All rights reserved. Keywords: Low alloyed steels; Induction heating; Quenching and tempering; Spheroidization 1. Introduction Induction heat treatment has been widely used for manufacturing the automotive and other engineering components. In addition to surface hardening [1], which hardens the surface layer and gives the surface a high compressive residual stress to improve fatigue characteristics, an induction quenching-and-tempering (IQT) process, as a through-hardening technique for steels is known to provide a good combination of high strength and toughness, and less deviation of mechanical properties. In addition, the advantages of IQT process include the accurate control of temperature, the short processing time, the less decarburization and the convenience in obtaining refined microstructure. Meanwhile, the elimination of intermediate annealing during cold forging process provides the significant costand time-savings in manufacturing automotive components. However, for the steels with high strength above 1000 MPa, it is difficult to obtain the good ductility and toughness required to prevent fracture during the high speed deformation Corresponding author. of cold forging. Tempered martensite, transformed from refined austenite, with finely distributed spheroidal cementite produced by IQT process is regarded as one of the optimum microstructures which can satisfy these requirements. Then, it can be applied directly to cold forging process for manufacturing automotive components without intermediate annealing. Furthermore, it is not necessary to change the chemical composition of steels by using IQT process. Additionally, hardness changes produced by conventional tempering can be obtained by the shorter times of induction tempering at higher temperature [2]. In addition to IQT process, alloying elements also have a significant effect on microstructures and mechanical properties in tempered steels. Recently, hot coiling in the production of automotive suspension springs has been replaced by cold coiling through the application of IQT process [3,4]. Kawasaki et al. [4] found that refined austenite grain size and the distribution of fine cementite particles at boundaries could be obtained by IQT process due to the rapid heating and short heating time. However, their results were limited to the steels containing high Si content above 1.2 wt.%. Generally, Si is known to be effective in refining tempered carbides by delaying the conversion of ε-carbide to cemen /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.jmatprotec

2 S.T. Ahn et al. / Journal of Materials Processing Technology 160 (2005) tite during tempering [5,6], although its deleterious effect on surface decarburization and toughness. Moreover, the behavior of carbide particles in low alloy steels including Cr and Mo would react more sensitively to tempering conditions. Although the addition of alloying elements such as Cr and Mo does not affect the behavior of tempered carbides at low tempering temperatures, the rates of spheroidization and the coarsening of cementite are reduced at a high temperature above 400 C, due to their slow diffusivity [7,8]. Thus, the combined effect of rapid heating of IQT process and the addition of alloying elements in steels could provide excellent mechanical properties through the formation of refined microstructure with the finer inter-particle spacing of cementite. However, detailed information on the effects of tempering temperatures on microstructures and mechanical properties of IQT processed low alloy steels has not been clarified. In view of the foregoing, the present work was carried out to investigate the microstructural evolution and corresponding variation of mechanical properties of the low alloyed steel tempered by induction heating, compared with those by salt bath heating, to examine the possibility of induction heating for industrial application. 2. Experimental procedure The chemical composition of the steel used in this study is given in Table 1. Hot-rolled steels with 16 mm diameter was induction-heated at 950 C for 50 s, and quenched to room temperature by jet spraying water. Samples were etched in a mixture of 100 ml distilled water, 3 g picric acid and 1 g sodium tridecylbenzene sulfonate. The austenite grain size (AGS) was measured on at least five optical micrographs with a magnification of 400. The measured AGS, according to ASTM E112, was ASTM No To compare the mechanical properties between induction-tempered steels and steels tempered in a salt bath, quenched specimens were tempered at various temperatures of C by induction-heating for 40 s (IT steel) or in a molten salt bath for 30 min (SBT steel). For a detailed understanding of the microstructural evolution during the heat-treatment, a transmission electron microscope (TEM) was used to analyze thin foils obtained from the samples tempered at various temperatures. TEM samples were prepared by utilizing a conventional jet polishing technique in a mixture of 10% perchloric acid and Table 1 Chemical composition of a steel used in this study (wt.%) C 0.21 Si 0.22 Mn 0.73 P S Cr 1.08 Mo 0.19 glacial acetic acid. For tensile tests, rods tempered at various temperatures were machined in the ASTM subsize form of 6.25 mm gauge diameter and 25 mm gauge length. Uniaxial tensile tests were conducted operating at a constant crosshead speed of 1 mm/min. Charpy V-notch specimens were machined from tempered rods, in the ASTM standard form of 10 mm 10 mm 55 mm. Both tensile and impact data reported in this work are the average values of the data obtained from 5 IT steel samples and 10 SBT steel samples. 3. Results and discussion 3.1. Microstructural evolution during tempering TEM micrographs in Fig. 1 show the distribution and morphology of tempered carbides formed at different tempering temperatures. For the IT steel tempered at 400 C(Fig. 1a), many cementite particles which formed at the third stage of tempering were located within martensite laths. Upto the tempering temperature of 500 C(Fig. 1b), most cementite particles maintain a rod shape with the aspect ratio over 7, and the aspect ratio decreased slightly with increasing tempering temperature (Table 2). However, at the high tempering temperature of 600 C(Fig. 1c), cementite particles began to change their shape from a needle type to a fine spheroidal type, through a gradual spheroidization. At 700 C tempering (Fig. 1d), most cementites were spheroidized with the aspect ratio of 1.5, and randomly distributed within martensite laths. The spheroidization behavior of tempered carbides during tempering is controlled by the diffusion of carbon and alloying elements. In earlier work, Babu et al. [9] and Thomson and Miller [10] showed, by using atom probe field ion microscope (FIM), that the cementite growth is controlled by para-equilibrium carbon diffusion during early stages of tempering but it became controlled by a local-equilibrium mode at higher temperatures. The distribution of carbide particles in the SBT steel was quite different from that of the IT steel. The presence of spheroidal cementite particles in Fig. 1f indicates that spheroidization already started at the tempering temperature of 500 C. Furthermore, at 700 C tempering (Fig. 1h), the presence of coarsened cementite particles within martensite laths as well as at lath boundaries indicates that cementite Table 2 The characteristics of carbide particles in the IT or SBT steels Tempering temperature ( C) Inter-particle spacing (nm) Aspect ratio of carbides IT SBT IT SBT ± ± ± ± ± ± ± ± ± ±

3 56 S.T. Ahn et al. / Journal of Materials Processing Technology 160 (2005) Fig. 1. TEM micrographs showing the morphology and distribution of tempered carbides during tempering. (a) IT: 400 C; (b) IT: 500 C; (c) IT: 600 C; (d) IT: 700 C; (e) SBT: 400 C; (f) SBT: 500 C; (g) SBT: 600 C; (h) SBT: 700 C. particles grow rapidly at this temperature. Since temperature and time are inter-dependent variables in tempering, the longer holding time in the SBT steel would induce the similar effect to the increment of tempering temperature. Accordingly, it is obvious that not only the spheroidization but also the growth of the cementite would start at the lower temperature for the SBT steel compared to the IT steel, due to the longer holding time at the same tempering temperatures Mechanical properties Fig. 2 shows the variation of tensile strength (TS) and yield strength (YS) as a function of tempering temperature. Fig. 2. The variation of tensile strength and yield strength of the steels with tempering temperature. TS decreased continuously with increasing tempering temperature. The matrix softening, due to carbon depletion and recovery, as well as the increase of inter-particle spacing during tempering mainly attributed to the TS decrement of tempered steels. As seen in Fig. 2, the decrement rate of TS of the IT steel is less than that of the SBT steel, due to the short tempering time. Additionally, the difference of TS between the IT steel and the SBT steel became more pronounced at high tempering temperatures above 500 C. Although the TS difference was only 70 MPa at 300 C, it reached 250 MPa at 700 C. This is attributed to the fact that, for the IT steel, the tempering time was insufficient for the matrix softening associated with dislocation annihilation and recovery at high tempering temperatures. YS responded to the tempering temperature in a different manner compared to TS. YS of the IT steel first maintained the high strength level of about 1200 MPa upto 500 C, and then continuously decreased with increasing tempering temperature. Generally, YS of tempered steels can be expressed as the sum of several contributions such as substructure hardening, precipitate hardening, etc. Among various microstructural features in tempered martensite, the distribution of precipitates (especially cementite particles) is the most effective one to control YS [11], since YS is directly affected by the distribution of obstacles for dislocation motion. The refinement of tempered carbides provides more effective barrier to dislocation motion, and then increases YS. Accordingly, the values of YS would be strongly related to the characteristics of tempered carbides formed at each tempering temperature listed in Table 2. The inter-particle distance in the IT steel decreased from 62.8 to 57.7 nm as tempering temperature increased from 300 to 400 C. And then, it increased upto

4 S.T. Ahn et al. / Journal of Materials Processing Technology 160 (2005) nm at 500 C. At low temperatures of C, the small inter-particle spacing of rod-shaped cementite particles would effectively hinder the movement of mobile dislocations during straining. Consequently, YS becomes high at low tempering temperatures. The decrease of YS by tempering at higher temperatures above 500 C is primarily caused by the increase of inter-particle spacing ( nm) due to spheroidization and coarsening of cementite particles. As mentioned before, the longer tempering time in salt bath would result in the lower YS for the SBT steel, compared with the IT steel. Besides, the difference of YS between the two steels became more pronounced at high tempering temperatures. YS of the SBT steel tempered at 700 Cwas lower than that of the IT steel by 250 MPa. This implies that the softening due to the spheroidization and coarsening of cementite particles proceeded more significantly at high temperatures in the SBT steel. Meanwhile, assuming that there is no strengthening contribution from grain size or subgrain size, YS can be expressed in a similar form of the Hall Petch relationship by using data of YS in Fig. 2 and inter-particle distance in Table 2 YS = λ 1 where YS is in MPa and λ represents the inter-particle distance in nm. Fig. 3 shows the effect of tempering temperature on reduction of area (RA) and Charpy impact energy. The RA variation in the tempered steels is closely related to the matrix softening and coarsening of carbide particles during tempering. RA of the IT steel increased steadily from 60 to 70% when tempering temperature increased from 300 to 700 C. The SBT steel also showed a similar trend, except the higher RA than the IT steel by about 5% upto tempering temperature of 600 C. It is interesting to note that the difference of RA between the two steels increased abruptly at tempering temperatures above 600 C. This implies that the significant microstructural change occurred in the SBT steel when tempering temperature was increased above 600 C. Although, Fig. 3. Reduction of area (RA) and Charpy impact energy as a function of tempering temperature. Fig. 4. TEM micrograph of the SBT steel tempered at 700 C, showing the presence of recrystallized grains. in the present work, a quantitative analysis regarding the effect of tempering temperature on the dislocation density was not performed, the apparent increase of RA of the SBT steel implies that polygonization and annihilation of dislocations were operative as a recovery process during tempering. Furthermore, the presence of newly recrystallized equi-axed grains with high angle boundaries in the SBT steel (Fig. 4, tempered at temperatures above 600 C) is directly related to the abnormal increase of RA and Charpy impact energy in Fig. 3. The results of Charpy impact test, performed at a room temperature to evaluate toughness of the steels, are presented in Fig. 3. Charpy impact energy responds to tempering temperature in the similar manner to RA. It is interesting to note that the minimum Charpy impact energy was measured at the intermediate tempering temperature for the IT steel. The minima of impact energy curves were 400 C for the IT steel and about 300 C for the SBT steel. This attendant loss of impact toughness is known as tempered martensite embrittlement (TME). This TME behavior is closely related to the microstructural changes at lath or martensite grain boundaries. According to Speich and Leslie [12], as tempering temperature increases, retained austenite decomposes into the film-type cementite at lath or martensite grain boundaries during the second stage of tempering. Subsequently, cementite film would grow into thick cementite film or coarsen into stringer-type cementite particles. Thus, it is anticipated that the coarsening or lateral thickening of cementite at lath boundaries would be responsible for the occurrence of TME [13,14]. As mentioned above, the abnormal increase of impact energy above 600 C is associated with the process of softening, especially the occurrence of recrystallization in the SBT steel. From the above results, it is obvious that good mechanical properties of TS above 1000 MPa, RA of 65% and Charpy impact value above 120 J/cm 2, could be obtained for the IT steel, tempered at 600 C for 40 s. Although the direct test

5 58 S.T. Ahn et al. / Journal of Materials Processing Technology 160 (2005) Fig. 5. The ratio of YS/TS as a function of tensile strength. for cold forgeability was not performed in this work, the high values of measured RA and Charpy impact energy imply that the induction-tempered steels would be possibly applicable to cold forging for manufacturing automotive components. Furthermore, the high ratio of YS/TS above 0.9 (Fig. 5) indicates that the small amount of work hardening occurred during the deformation. The small amount of work hardening results in the uniform distribution of hardness in automotive components even after cold forging. This advantage would accelerate the application of induction-tempered low alloy steels to manufacturing automotive components by cold forging. 4. Conclusions The present work was carried out to investigate the microstructural evolution and corresponding mechanical properties in the low alloyed steel tempered by induction heating, compared with those by salt bath heating. The following specific conclusions can be made: (1) While the cementite particles began to change their shape from a needle type to a fine spheroidal type at the tempering temperature of 600 C with tempering time of 40 s in the induction-tempered steels (IT steel), the spheroidization already started at the tempering temperature of 500 C with tempering time of 40 min in the steels tempered in a salt bath (SBT steel). (2) The difference of tensile strength between the IT and SBT steels became more pronounced at high tempering temperatures above 500 C. For the IT steel, insufficient tempering time for the matrix softening including dislocation annihilation and recovery resulted in the retardation of the softening even at high tempering temperatures. (3) At low tempering temperatures of C, the small inter-particle spacing of rod-shaped cementite particles would effectively hinder the movement of mobile dislocations during straining. Consequently, YS was high at low tempering temperatures for the IT steel. However, the decrease of YS during tempering at higher temperatures above 500 C was primarily caused by the increased inter-particle spacing due to the spheroidization and coarsening of cementite particles. (4) The minima of impact energy curves, which were associated with tempered martensite embrittlement, would be 400 C for the IT steels and about 300 C for the SBT steel. The occurrence of the abnormal increase of RA and Charpy impact energy of the SBT steel tempered above 600 C was directly related to the presence of new equi-axed grains with high angle boundaries. (5) The good mechanical properties of TS above 1000 MPa, RA of 65% and Charpy impact value above 120 J/cm 2, could be obtained for the IT steel, tempered at 600 C for 40 s. In addition, the high ratio of YS/TS above 0.9 implies that the induction quenched-and-tempered steels would be possibly applicable to cold forging for manufacturing automotive components. References [1] K.Z. Shepelyakovskii, F.V. Bezmenov, New induction hardening technology, Adv. Mater. Process. 154 (1998) 225. [2] J.D. Wong, D.K. Matlock, G. Krauss, Effects of induction tempering on microstructure, properties and fracture of hardened carbon steels, in: Proceedings of the 43rd Mechanical Working and Steel Processing Conference, ISS, vol. 39, 2001, pp [3] K. Kawasaki, T. Chiba, N. Takaoka, T. Yamazaki, Microstructure and mechanical properties of induction heating quenched and tempered spring steel, Tetsu-to-Hagane 73 (1987) [4] K. Kawasaki, T. Chiba, T. Yamazaki, Characteristics of microstructure of induction heating tempered spring steel, Tetsu-to-Hagane 74 (1988) 342. [5] M. Assefpour-Dezfuly, A. Brownrigg, Parameters affecting sag resistance in spring steels, Met. Trans. 20A (1989) [6] R.M. Hobbs, G.W. Lorimer, N. Ridley, Effect of silicon on the microstructure of quenched and tempered medium-carbon steels, JISI 210 (1972) 757. [7] N. Saito, K. Abiko, H. Kimura, Effects of small addition of titanium, Mater. Trans. JIM 36 (1995) 601. [8] W.J. Nam, C.S. Lee, D.Y. Ban, Effects of alloy additions and tempering temperature on the sag resistance of Si Cr spring steels, Mater. Sci. Eng. A 289 (2000) 8. [9] S.S. Babu, K. Hono, T. Sakurai, Atom probe field ion microscopy study of the partitioning of substitutional elements during tempering of a low-alloy steel martensite, Met. Mater. Trans. A 25A (1994) 499. [10] R.C. Thomson, M.K. Miller, The partitioning of substitutional solute elements during the tempering of martensite in Cr and Mo containing steels, Appl. Surf. Sci. 87/88 (1995) 185. [11] T. Sakuma, N. Watanabe, T. Nishizawa, The effect of alloying element on the coarsening behavior of cementite particles in ferrite, Trans. JIM 21 (1980) 159. [12] G.R. Speich, W.C. Leslie, Tempering of steel, Met. Trans. 3A (1972) [13] K.A. Peters, J.V. Bee, B. Kolk, G.G. Garret, On the mechanisms of tempered martensite embrittlement, Acta Metall. 37 (1989) 675. [14] W.J. Nam, H.C. Choi, Effects of silicon, nickel and vanadium on impact toughness in spring steels, Mater. Sci. Tech. 13 (1997) 568.