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: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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