利用TEM和SEM研究了回火时间(10、20、40和120 min)对不同V含量(0、0.08%、0.14%,质量分数) Fe-Cr-Ni-Mo高强钢碳化物转变和力学性能的影响。结果表明,淬火态0V钢在马氏体板条间析出了少量的M₇C₃型碳化物,而含V钢中无碳化物析出,因此淬火态0V钢的强度最高(2060 MPa)。回火处理过程中,短时间(20 min)回火时,0V钢仅在板条间析出了M₃C型碳化物,随着回火时间延长,M₃C型碳化物逐渐转变为M₂₃C₆,这2种碳化物尺寸均较粗大(150~300 nm),对合金钢强度的贡献相对较弱,导致0V钢的强度逐渐下降,由回火20 min时的1197 MPa下降到回火120 min后的1088 MPa。加入V后,合金钢经短时间(20 min)回火后不仅在晶界析出M₃C,还在晶内析出了数量较多的M₂C,且尺寸细小(不大于80 nm),随着回火时间的延长,M₃C逐渐分解并形成了数量较多的M₆C和更稳定的MC,对合金钢的沉淀强化效果较强,且对塑韧性的影响相对较小。因此随着回火时间的延长,含V钢的强度基本保持不变,而塑韧性呈现增加的趋势,获得了良好的强韧性配合。

Fe-Cr-Ni-Mo steel is widely used in various industrial fields, such as water turbine in hydroelectric power station, pressure vessel and shipbuilding section etc. due to its excellent performance in strength and impact toughness. In order to fulfill the needs of high-strength and good toughness, the quenching and following tempering are often used for this kind of Fe-Cr-Ni-Mo steel. In particular, the carbide precipitation in the tempering process is the key to determine the strength and toughness. In this work, TEM and SEM were used to investigate the effect of tempering time (10, 20, 40 and 120 min) on carbide evolution and mechanical properties of Fe-Cr-Ni-Mo steel with different V contents (0, 0.08% and 0.14%, mass fraction) after quenched at 860 ℃ and following tempering at 610 ℃. The results show that some M₇C₃ type carbides precipitated along martensite lath boundaries in quenched 0V steel, but no carbide in the quenched 008V and 014V steels. As a result, the strength of 0V steel (2060 MPa) is higher than 008V and 014V (1906 and 1857 MPa, respectively). After tempering for 20 min, a small amount of M₃C type carbides were found on the lath boundaries in 0V steel. With tempering time increasing, M₃C will transform into M₂₃C₆ carbide gradually. Both M₃C and M₂₃C₆ type carbides exhibited a large size in range from 150 nm to 300 nm which were unfavorable to strength. As a result, the tensile strength of 0V steel decreases from 1197 MPa to 1088 MPa when tempering time increases from 20 min to 120 min. As for the 008V and 014V steels tempered for 20min, there are not only M₃C type carbides precipitated in the grain boundary, but also M₂C type carbides found inside the grains. The size of both carbides is no larger than 80 nm. With increasing tempering time, the M₃C will dissolve gradually and there will precipitate much more M₆C and MC. Compared with coarse M₃C, the finer M₂C, M₆C and MC have better precipitation strengthening effect and less deterioration of ductility and toughness. Therefore, with increasing tempering time the strengthes of 008V and 014V steels keep stable and the elongation and impact toughnesses increase gradually. This indicates that the excellent combination of strength and impact toughness can be obtained in 008V and 014V steels.