Effect of Indirect Transformation of Retained Austenite During Tempering on the Charpy Impact Toughness of a Low-Alloy Cr–Mo–V Steel

Abstract

A modified tempering treatment has been designed in order to avoid the direct transformation of retained austenite (Ar) during tempering of a low-alloy Cr-Mo-V steel. Instead of the direct transformation of Ar into ferrite and M 23 C 6 carbides during conventional tempering at 700 °C, transformation into aggregate of ferrite and cementite has been forced by a pre-tempering at 455 °C before conventional tempering. Experiments have been performed on specimens quenched with cooling rates 1.5, 3 and 12 °C/s, providing different types of Ar within the as-quenched microstructures. The results show that the tempering modification does not improve the Charpy impact toughness at the highest quenching rate of 12 °C/s, where the specimens cannot incur cleavage cracking induced from fine and discontinuous M 23 C 6 carbides along lath interfaces. For the lowest quenching rate 1.5 °C/s, the Charpy impact toughness can be improved, and the failure is dominated by carbide aggregates, which originate from the decomposed products of blocky Ar. This is because the tempering modification effectively suppresses the formation of coarse M 23 C 6 carbides at interfaces between the carbide aggregate and bainitic matrix, thereby resulting in a relatively homogeneous distribution of M 23 C 6 carbides inside carbide aggregates. Therefore, the tempering modification is recommended for large-scale forgings, in which relatively high quenching rates are difficult to achieve.

Sensitivity of the Impact Toughness and Microstructure of 15CrNi3MoV Steel Under Different Quenching Rates

“…However, the low-temperature impact energy of this steel is not sensitive to the change in the cooling rate, which is mainly attributed to the softening of the martensite matrix and the appearance of spherical carbides during the tempering process. Jiang et al [17] and Li et al [18] studied the effect of different quenching rates on the microstructure and properties of Cr-Mo-V steel after tempering. Jiang et al believed that the decomposition of martensite-austenite (M-A) constituents and the reduction of effective grain size contribute to improving the low-temperature toughness of the material, while Li et al considered that the lath matrix obtained from the sample with higher quenching rate will cause the dispersion distribution of carbides during tempering, thus increasing the toughness.…”

Study on impact fracture behavior of G18CrMo2-6 steel after tempering

“…The main reason for this phenomenon is the evolution of second phase during tempering heat treatment and service. 6,7 Many research 2,3,[8][9][10][11][12] has been conducted on microstructure, mechanical properties and evolution of precipitates of CrMo/CrMoV steel. Jiang et al [13][14][15] indicated that the decomposition of martensiteaustenite (M-A) constituents and the precipitation of carbides in the 2.25Cr1Mo0.25V steel during the tempering process are the primary reasons for the changes in the mechanical properties.…”