Effect of Heat Treatment on the Low-temperature Resistance of 42CrMo Steel in Electric Power Fittings

Transcription

1 nd International Conference on Material Engineering and Application (ICMEA 2015) ISBN: Effect of Heat Treatment on the Low-temperature Resistance of 42CrMo Steel in Electric Power Fittings Dongqing Li 1,a, Jiajun Si 1, Haijun Niu 1, Kuanjun Zhu 1 and Shengchun Liu 1 1 Department of Transmission and Transformation Engineering Mechanics, China Electric Power Research Institute, Beijing , China a lidongqing1016@163.com ABSTRACT: The mechanical properties and low-temperature resistance of the 42CrMo steels utilized in electric power fittings after different heat treatments were comparatively investigated. Results indicated that a tempered sorbite with refined structure was obtained in the steels and the mechanical properties and low-temperature resistance were consequently improved after quenching + quenching + tempering and cyclic quenching and tempering, especially after the latter. The product of strength and elongation reached up to MPa% and the impact energy at -50 C increased to 48 J after cyclic quenching and tempering as compared to MPa% and 19 J after conventional quenching and tempering. The 42CrMo steel after cyclic quenching and tempering can be considered as a suitable candidate for connection fittings in electric power transmission line. 1 INTRODUCTION Building power bases in the western region of China and transmitting power to the eastern region is an important channel for meeting power demand and resolving resource conflict since the energy bases are mostly located in the western region but the electric load centres are mainly concentrated in the eastern region. It is well known that the weather in the western region is extremely cold in winter and the temperature can even decrease below -40 C in some areas. Therefore, the electric power wires and fittings serving in the above region should have good low-temperature resistance. As one important constituent part of electric power fittings, the connection fittings are mainly responsible for connecting insulators with strain clamps or suspension clamps. Recently, 42CrMo steel has been considered as a potential candidate for connection fittings mostly due to its high strength, low temper brittleness and good hardening capacity (Chen J.D. et al., 2012). However, the poor low-temperature resistance associated with high ductile-brittle transition temperature is an obstacle to its application under extreme climate condition. Since the final service performance of steels is realized and improved by heat treatment, compiling reasonable heat treatment process is an important method to improve the 552

2 low-temperature resistance of the 42CrMo steel. In the present work, the mechanical properties and low-temperature resistance of the 42CrMo steels after different heat treatments were comparatively investigated, aiming to compile reasonable heat treatment process to make the steel realize suitable match between mechanical strength and low-temperature impact toughness and meet service requirements. 2 EXPERIMENTAL DETAILS 42CrMo steel ingots manufactured by the Hangzhou steel plant were used as starting materials. The ingots were produced by vacuum induction melting and casting. The actual chemical compositions of the as-cast steel are listed in Table 1. Specimens were cut from the plum blossom-shaped ingots into mm 3. Table 1. Actual chemical compositions of the as-cast 42CrMo steel (wt.%). Fe C Si Mn P S Cr Mo Ni Cu A1 Bal Heat treatment process was conducted in an SX chamber-type resistance furnace. It is well accepted that the steel with a tempered sorbite structure has excellent comprehensive mechanical properties and impact toughness which can be obtained by quenching and tempering, denoted as conventional heat treatment. Moreover, recent studies indicated that repeated quenching process further improved the service performance of the steel (Wang L. et al., 2011). Therefore, two modified heat treatment processes were carried out in the present work: (1) quenching + quenching + tempering, denoted as repeated heat treatment; (2) cyclic quenching and tempering, denoted as cyclic heat treatment. Corresponding heat treatment process curves are shown in Figure 1. Note that the heating time (unit: min) during quenching was determined by a formula of specimen diameter (1.5~1.8) filling coefficient (1~3), and that during tempering was determined by a formula of quenching heating time 1.5. Besides, all of the specimens were cooled by oil. All of the specimens after heat treatment were divided into three groups. The first group specimens were cut into mm 3, embedded in epoxy and finely polished. Then the surface morphologies of the specimens were characterized by an optical microscope after etching in 4 % nital. The second group specimens were cut into standard samples with the diameter of 10 mm and the gauge length of 50 mm for tensile testing at room temperature 553

3 according to GB/T The third group specimens were cut into mm 3 with V-notch for impact testing at low temperature according to GB/T Test temperatures were -20 C, -40 C, -50 C and -70 C, respectively. Figure 1. Designed heat treatment process curves: (1) conventional heat treatment, (2) repeated heat treatment and (3) cyclic heat treatment. 3 RESULTS AND DISCUSSION 3.1 Microstructural observation Prior to testing, the microstructure of the 42CrMo steels was characterized. Figure 2 shows the optical micrographs of the as-heat-treated 42CrMo steels at room temperature. All the steels consisted predominantly of a typical tempered sorbite and no other phases were found. In the quenching process, the phase in the steels firstly transformed into austenite during heating and then into martensite during cooling. And finally the phase transformation from martensite to sorbite occurred in the subsequent tempering process. For the steel after conventional heat treatment, most of the tempered sorbite was lath-shaped which retained the morphology of quenched martensite. While in the steel after repeated heat treatment, polygonal tempered sorbite was observed and the structure was finer than that 554

4 observed in the former. This is because that the formation of new phases is a process of nucleation and growth. In the repeated heat treatment process, the lath-shaped martensite nucleated and grew during the first quenching step, and then the same process occurred again during the second quenching step. Consequently, the first-formed martensite was split into smaller grain regions, leading to refined structure. Similar structure was also observed in the steel after cyclic heat treatment and the tempered sorbite was further refined, probably due to repeated nucleation and growth during the additional tempering process. Figure 2. Optical micrographs of the 42CrMo steels under different heat treatment processes: (1) conventional heat treatment, (2) repeated heat treatment and (3) cyclic heat treatment. 3.2 Tensile testing at room temperature Six parallel samples after each heat treatment were tested and the average values were calculated as testing results which are shown in Table 2. The product of strength and elongation is a multiple value of tensile strength and elongation rate which can be utilized to represent the comprehensive mechanical properties of the steel (Xiao H.L. et al., 2010). The tensile strength of the steel after conventional heat treatment was 1183 MPa and the elongation rate was 14 %, and the product of strength and elongation was MPa% by calculation. With the same method, the products of strength and elongation of the steels after repeated and cyclic heat treatment were obtained which were MPa% and MPa%, respectively. It is obvious 555

5 that both of the 42CrMo steels after repeated and cyclic heat treatment showed better comprehensive mechanical performance than the steel after conventional heat treatment, especially the steel after cyclic heat treatment. Table 2. Mechanical properties of 42CrMo steels under different heat treatment processes. Heat treatment process Conventional heat treatment Repeated heat treatment Cyclic heat treatment Yield strength σ 0.2 (MPa) Tensile strength σ b (MPa) Elongation rate δ 5 (%) Reduction rate of area ψ (%) As is mentioned above, the structure and grains of the tempered sorbite in the steels after repeated and cyclic heat treatment were finer and smaller than that after conventional heat treatment. Grain refinement is an important method to enhance the strength and ductility of the steel simultaneously at room temperature. Previous studies suggested that there existed a certain relationship between yield strength and grain size in conventional polycrystalline materials and nanocrystalline materials, namely Hall-Petch formula which can be described as follows (Lu K. et al., 1994): y k y 0 (1) d where σ y is the yield strength, σ 0 is the friction resistance between crystal lattice when a single dislocation is moving, k y is a constant which is related to the nature of materials, and d is the average diameter of grains. It can be concluded from Formula (1) that the yield strength increases with the reduction of grain size for the same material. Therefore, structure and grain refinement is the main reason that the 42CrMo steels after repeated and cyclic heat treatment exhibited better comprehensive mechanical properties. 3.3 Impact testing at low temperature Similar to tensile testing, six parallel samples were tested and the average values were calculated as testing results. Figure 3 shows the low-temperature impact absorbing energy curves of the 42CrMo steels. The impact absorbing energy of all the 42CrMo steels after 556

6 different heat treatment presented a declining trend with the decrease of test temperature. At the test temperature points of -20 C, -40 C, -50 C and -70 C, the average impact absorbing energies of the steels after conventional heat treatment were 43 J, 21 J, 19 J and 14 J, respectively, all of which were lower than that after repeated heat treatment (54 J, 46 J, 41 J and 35 J, respectively) and after cyclic heat treatment (59 J, 52 J, 48 J and 42 J, respectively). Therefore, both of the 42CrMo steels after repeated and cyclic heat treatment showed better low-temperature performance than the steel after conventional heat treatment, especially the steel after cyclic heat treatment. Figure 3. Low-temperature impact absorbing energy curves of the 42CrMo steels under different heat treatment processes at: (a) -20 C, (b) -40 C, (c) -50 C and (d) -70 C. It is mentioned above that the tempered sorbite in the steels after repeated and cyclic heat treatment had finer structure and smaller grains than the steel after conventional heat treatment. Generally, the formation of close tempered sorbite can significantly improve the low-temperature resistance of steels. Besides, it is reported that the impact toughness of steels was highly sensitive to grain size, and the relationship between the ductile-brittle transition temperature and grain size can be described as follows (Li L. et al., 2007): 1 Tc ln B ln (2) d where T c is the ductile-brittle transition temperature, β and B are constants which are related to 557

7 the nature of materials, and d is the average diameter of grains. It can be inferred from formula (2) that the ductile-brittle transition temperature decreases with the reduction of grain size. In other words, the steel with smaller grain size has high impact toughness in broader temperature range. Therefore, structure and grain refinement caused by repeated and cyclic heat treatment led to improved low-temperature resistance of the 42CrMo steel. The low-temperature stability of the 42CrMo steels after different heat treatments were compared and results are shown in Figure 4. The degree of distribution density of impact absorbing energy points at different test temperatures reflects the property stability of the 42CrMo steel with the changes in temperature. It is obvious that the impact absorbing energy points of the steel after conventional heat treatment showed a sparse distribution and those of the steels after repeated and cyclic heat treatment distributed densely. This indicated that the low-temperature stability of the 42CrMo steels after repeated and cyclic heat treatment was better than that after conventional heat treatment. Figure 4. Low-temperature stability curves of the 42CrMo steels under different heat treatment processes. 4 SUMMARY The microstructure, mechanical properties and low-temperature resistance of the 42CrMo steels after different heat treatments were compared. Conclusions can be drawn as follows: 1. A tempered sorbite was obtained in all of the steels after heat treatment, and the structure and grains formed after repeated and cyclic heat treatment (especially after the latter) were finer and smaller than those formed after conventional heat treatment. 2. The product of strength and elongation of the steel after cyclic heat treatment reached up to MPa% and the impact energy at -50 C increased to 48 J, as compared to those after conventional heat treatment (16562 MPa% and 19 J, respectively). Structure and grain refinement caused by repeated nucleation and growth led to the improvement of 558

8 mechanical strength and low-temperature impact toughness of the steel. 3. Due to the above merits, the 42CrMo steel after cyclic heat treatment can be considered as a potential candidate for low-temperature resistant connection fittings in electric power transmission line. REFERENCES Chen J.D., Mo W.L., Wang P. & Lu S.P. (2012) Effects of tempering temperature on the impact toughness of steel 42CrMo. Acta Metallurgica Sinica, 48(10), Wang L., Sun Y.F., Zhao J.Y., Ma Y.H., Xiao Z.Y., Yang H.L. & Wang L.L. (2011) Effect of heat treatment on microstructure and mechanical properties of Nb-containing cryogenic steels. Heat Treatment of Metals, 36(11), Xiao H.L., Shi J. & Yong Q.L. (2010) Ballistic performance and failure mode of thin steel plate with the same product of strength and ductility. Ordnance Material Science and Engineering, 33(5), Lu K., Liu X.D. & Hu Z.Q. (1994) The Hall-Petch relation in nanocrystalline materials. Chinese Journal of Materials Research, 8(5), Li L., Tang Z.Y., Ding H., Du L.X., Song H.M. & Zhang P.J. (2007) Strengthening and toughening mechanism of low carbon manganese steels. Heat Treatment of Metals, 32(11),