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

doi:10.1007/s40195-020-01055-7

Abstract

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 M₂₃C₆ 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 M₂₃C₆ 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 M₂₃C₆ carbides at interfaces between the carbide aggregate and bainitic matrix, thereby resulting in a relatively homogeneous distribution of M₂₃C₆ carbides inside carbide aggregates. Therefore, the tempering modification is recommended for large-scale forgings, in which relatively high quenching rates are difficult to achieve.

Key words: Bainite steel, Retained austenite, Impact toughness

Yong-Han Li, Zhong-Hua Jiang, Zhen-Dan Yang, Jue-Shun Zhu. Effect of Indirect Transformation of Retained Austenite During Tempering on the Charpy Impact Toughness of a Low-Alloy Cr-Mo-V Steel[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(10): 1346-1358.

Figures/Tables 13

Table 1 Chemical composition of low-alloy Cr-Mo-V steel (wt%)

C	Si	Mn	Cr	Mo	V	Fe
0.15	0.08	0.57	2.24	0.88	0.24	Bal.

Fig. 1 Sketches of the hot working process corresponding to the conventional and modified tempering treatments

Fig. 2 XRD patterns of as-quenched specimens with different quenching rates: a 1.5 °C/s; b 3 °C/s; c 12 °C/s

Fig. 3 As-quenched microstructures of low-alloy Cr-Mo-V steel at different quenching rates: a SEM micrograph of 1.5 °C/s; b TEM bright-field image of 1.5 °C/s; c SAED pattern corresponding to the bright image in b; d SEM micrograph of 3 °C/s; e TEM bright-field image and inserted SAED pattern of 3 °C/s; f TEM dark-field image corresponding to the bright image in e; g SEM micrograph of 12 °C/s; h TEM bright-field image and inserted dark-field image of 12 °C/s; i TEM bright-field image of 12 °C/s

Fig. 4 SEM and TEM micrographs of the steels with different quenching rates after pre-tempering: a, d 1.5 °C/s; b, e 3 °C/s; c, f 12 °C/s

Fig. 5 XRD patterns of the carbides extracted from different tempering specimens with quenching rate of 1.5 °C/s: a modified tempering; b conventional tempering

Fig. 6 Microstructural analysis of the conventional a, b and modified c, d tempered specimens corresponding to the quenching rate of 1.5 °C/s

Fig. 7 Different tempered microstructures of low-alloy Cr-Mo-V steel at the quenching rate of 12 °C/s: a SEM micrograph of the conventional tempering; b SEM micrograph of the modified tempering; c TEM bright-field image of conventional tempering; d SAED pattern corresponding to the bright image in c

Fig. 8 a Dilatation during tempering at 455 °C corresponding to the specimens quenched with V = 1.5, 3 and 12 °C/s; b dilatation during 2 h tempering stage at 455 °C corresponding to the specimens quenched with V = 1.5, 3 and 12 °C/s

Fig. 9 Dilatation during tempering of the V = 1.5 °C/s quenched specimens corresponding to the modified a and conventional b tempering

Fig. 10 Charpy impact toughness and Vickers hardness values in dependent of the quenching rates corresponding to the conventional and modified tempering

Fig. 11 SEM micrographs of the fracture surface of Charpy impact samples of the conventional tempering a-c and modified tempering d-f: a, d, b, e, c, f with quenching rates of V = 1.5, 3 and 12 °C/s, respectively

Fig. 12 SEM micrographs of the cross-section area beneath the fracture surface of Charpy impact samples of the conventional a, b and modified c, d tempering: a, c and b, d with quenching rates of 1.5 °C/s and 12 °C/s, respectively

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