Mechanism of ƒ¿'•¨ƒÁ reverse transformation has been examined by means of dilatometric measurements, X-ray analysis and structural observations in an 18% Ni maraging steel.

Synopsis [n order to study the morphology q[ strain-induced martensite and the plasticity induced by the martensitic transformation, tellsile tests at various temperatures were carried out using Fc-31% N i, Fe-29%N i-O.26% C and Fe-15% Cr-13%N i alloys whose Ms temperatures u:ere below room temjlerature and the microstructures were observed. The results obtained are summarized as fo llows. In Fe-Ni and Fe-Ni-C alloys, a characteristic martensite whose morphology is entirely different fiom that of thermalfv transformed martensite is formed ~y tensile deformation mainly at the temperatures between Md and M s. This characteristic martensile is forme d continuously little by Little with increasing strain and contributes to the transformation-induced jJlasticity. The difference of morphology between thermally transformed martensite and stra in-induced martellsite must be caused by the change of transformation mechanism. In the case of Fe-Cr-N i alloy, however, the morjJ!wlogy of strain-induced martensite is not dijJerent from that of thermally transformed martensite. Transform ation-induced plasticity occurs in Fe-N i alLqys and the Fe-Cr-N i alloy. The elongation has a maximum value at a temperature between M d and M s. I n the test in which the maximum elongation is obtained, the strain-hardening exjJonent increases continuously with increase in strain. The main prerequisite for maximum elongation is that the martensite is constantly .f~m7Cd Little by little du ring deformation up to fracture. The difference between the F c-Ni alloys and the Fe-Cr-N i alloy is that the temperature range showing large elollgation is wider in the Fe-Cr-Ni alloy than in Fe-N i alloys. This is caused by the dijJerence in transformation behavior of martensite.

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Synopsis:Embrittlement due to the high temperature solution annealing (H.T.S.A.) in 18%Ni maraging steels has been reported, but the mechanism is not known. A study has been made of the effects of the solution annealing condition, aging condition and cold working on this embrittlement, and discussions are given on the cause of the embrittlement.The embrittlement is not observed in high temperature solution annealed condition. However, it becomes pronounced with increase in the aging temperature or aging time. The degree of the embrittlement becomes larger with higher solution annealing temperature or with increase in the strength level. The embrittled specimens show the characteristic bamboo blind-like fracture surfaces. When the cold working is applied after the H.T.S.A., the toughness recovers. On the other hand, H.T.S.A. specimens as-quenched show a very large uniform elongation in the liquid nitrogen, probably due to the twin deformation. From the above results and the microstructure observations,it is infered that abnormal precipitation on the lath boundary is accelerated during aging in the H.T.S.A. specimens because of the characteristic martensite substructure (lower dislocation density or grid-like dislocation arrangement), and consequently that the cracks can form easily in the H.T.S.A. specimens.It is also made clear that stringer-like martensite is introduced when quenched from the temperatures below austenite recrystallization temperature, while blocky martensite is formed from recrystallized austenite.

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