The Role of Retained Austenite in Tempered Martensite Embrittlement of 4340 and 300-M Steels Investigated through Rapid Tempering

Virginia K Euser,John G Speer,Kip O Findley,Amy J Clarke,Don L Williamson

doi:10.3390/met11091349

Abstract

Tempered martensite embrittlement (TME) is investigated in two medium carbon, high strength steels, 4340 (low silicon) and 300-M (high silicon), via rapid (1, 10, or 100 s) and conventional (3600 s) tempering. Rapid tempering of 4340 diminishes the depth of the TME toughness trough, where improvements in impact toughness correspond to the suppression of retained austenite decomposition. In 300-M, retained austenite decomposition is suppressed to an even greater extent by rapid tempering. While toughness improves overall after rapid tempering, TME severity remains consistent in 300-M across the tempering conditions examined. Through interrupted tensile tests, it was found that the 300-M conditions that exhibit TME are associated with mechanically unstable retained austenite. Unstable retained austenite is shown to mechanically transform early in the deformation process, presumably resulting in fresh martensite adjacent to interlath cementite that ultimately contributes to TME. The present results emphasize the role of both the thermal decomposition and mechanical transformation of retained austenite in the manifestation of TME.

Highlights

Tempered martensite embrittlement (TME) is a phenomenon characterized by a reduction in the room temperature toughness of steels tempered within the regime associated with retained austenite decomposition [1]
The toughness trough associated with TME is reduced through rapid tempering of
Toughness trough is not diminished by rapid tempering in this alloy

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Tempered martensite embrittlement (TME) is a phenomenon characterized by a reduction in the room temperature toughness of steels tempered within the regime associated with retained austenite decomposition [1]. The embrittlement associated with TME limits the strength–toughness properties achievable in medium carbon, high strength steels such as AISI 4340. Rapid tempering on the scale of seconds has recently been shown to reduce TME in 4340 steel, leading to significantly improved strength-toughness combinations [2,3,4,5]. In addition to suppressing TME, rapid tempering is associated with a greater preservation of retained austenite upon tempering compared to more conventional treatments [2,3,6]

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Results

Discussion

Conclusion