5.5%Ni鋼の焼もどし脆性と松かさ状破面

抄録

Pine cone like fracture surfaces ("pine cone") were observed in Charpy and compact tension specimens of cryogenic Ni steels containing retained austenite when temper embrittled and broken in a brittle manner. 5.5Ni steels, especially those bearing no molybdenum, were severely embrittled on slow cooling from the final tempering, and their fracture surfaces were mainly composed of "pine cone". Microscopic observations and various mechanical tests were performed to elucidate the characteristics of this fracture surface.
It was found that "pine cone" is a kind of intergranular fracture surfaces and produced by fracture along P segregated α'/γ interfaces which formed at prior austenitic grain boundaries and its roughness is not caused by precipitates but the reverted austenites at grain boundaries. Intercritical heat treatment promoted austenite formation at grain boundaries and enhanced the roughness of "pine cone" and suppressed "pine cone" fracture. It was also found that the reverted austenite was not stably present on "pine cone" fracture surface through Mössbauer spectroscopy nevertheless reduced susceptibility to temper embrittlement.