Mechanisms of tempered martensite embrittlement in low alloy steels
- Univ. of California, Berkeley
An investigation into the mechanisms of tempered martensite embrittlement (TME), also known as ''500/sup 0/F'' or ''350/sup 0/C'' or one-step temper embrittlement, was made in commercial, ultra-high 4340 and Si-modified 4340 (300-M) alloy steels, with focus given to the role of interlath films of retained austenite. Studies were performed on the variation of strength and toughness, and the morphology, volume fraction and thermal and mechanical stability of retained austenite, as a function of tempering temperature, following oil-quenching, isothermal holding, and continuous air cooling from the austenitizing temperature. TME was observed as a decrease in both K/sub Ic/ and Charpy V-notch impact energy after tempering around 300/sup 0/C in 4340 and 425/sup 0/C in 300-M, where the mechanisms of fracture were either interlath cleavage or largely transgranular cleavage. The embrittlement was concurrent with the interlath precipitation of cementite during tempering and the consequent mechanical instability of interlath films of retained austenite during subsequent loading. The role of silicon in 300-M was seen to retard these processes and hence retard TME to higher tempering temperatures than for 4340. The magnitude of the embrittlement was found to be greater in microstructures containing increasing volume fractions of retained austenite. Specifically, in 300-M the decrease in K/sub Ic/, due to TME, was a 5 MPa..sqrt..m in oil quenched structures with less than 4% austenite, compared to a massive decrease of 70 MPa..sqrt..m in slowly (air) cooled structures containing 25% austenite.A complete mechanism of tempered martensite embrittlement is proposed involving precipitation of interlath cementite due to partial thermal decomposition of interlath films of retained austenite, and subsequent deformation-induced transformation on loading of remaining interlath austenite, destabilized by carbon depletion from carbide precipitation. The relative importance of these effects is discussed.
- OSTI ID:
- 6517224
- Journal Information:
- Metall. Trans., A; (United States), Vol. 9:8
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
NICKEL ALLOYS
TENSILE PROPERTIES
SILICON ALLOYS
STEELS
AUSTENITE
COOLING
CRACKS
DUCTILITY
EMBRITTLEMENT
FAILURES
HEAT TREATMENTS
MARTENSITE
QUENCHING
TEMPERATURE DEPENDENCE
TEMPERING
ALLOYS
CARBON ADDITIONS
IRON ALLOYS
IRON BASE ALLOYS
MECHANICAL PROPERTIES
360103* - Metals & Alloys- Mechanical Properties