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1 Journal of Achievements in Materials and Manufacturing Engineering

1 2008

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Retained austenite in the cracking process of 70MnCrMoV9-2-4-2 tempered steel

Kokosza A., Pacyna J.

Journal of Achievements in Materials and Manufacturing Engineering

2008

Vol. 29, nr 1

39-46

Purpose: The aim of this study was a review of opinions molded within last 35 years on a part of steel cracking process played by retained austenite. Design/methodology/approach: Dependencies between volume fraction of retained austenite, its stability, hardness, fracture toughness, and tempering temperature of hardened 70MnCrMoV9-2-4-2 steel were determined. On the ground of analysis of those dependencies the influence of retained austenite on fracture toughness of tested steel was investigated. Findings: It was found that retained austenite remaining in the structure of tested steel after quenching increased its fracture toughness on directly proportional way to its volume fraction. Advantageous influence of this phase was also found after tempering tested steel at temperatures within the range of 120-400*C. Research limitations/implications: The results of investigation presented in this paper may enrich and complement the knowledge about the part played by this phase in steel cracking process. It was pointed out that most beneficial influence of retained austenite exists when tested steel after hardening is low-tempered. At that moment the highest stabilization of the phase occurs. While at tempering temperatures above 220*C it was indicated that it is possible to combine retained austenite transition and irreversible tempering brittleness. Practical implications: Research results presented in this paper let hope that know-how of such heat treatment, which would make possible to stabilize this phase in the structure of hardened steels, would contribute not only to restrain temper brittleness in these steels but even to complete elimination of this adverse phenomenon. Originality/value: On the basis of own research, the authors have verified the opinions, presenting original point of view on the issue of presence of retained austenite in the structure, its stability and the influence on fracture toughness of tool steel.