On the Ductility of Martensite in an Ausformed Fe-Ni-C Alloy

In order to account for the excellent ductility of a ausformed steel, the tensile properties of ausformed martensite were compared with those of deformed austenite using an Fe-25%Ni-0.4%C alloy. The results obtained were as follows.(1) Although the size of martensite plate became smaller by ausforming, there were still lenticular martensite plates in the ausformed steel as large as in the non-ausformed one. In such a large ausformed martensite which showed a characteristic morphology, there were several islands or layers of retained austenite in the plate.(2) The elongation of martensite was hardly effected by the amount of ausforming, while that of austenite decreased with an increase in the amount of prior-deformation. The elongation of deformed austenite, therefore, became smaller than that of ausformed martensite, when the prior-deformation (ausforming) was more than about 30%. The heavily deformed austenite was enhanced in strength and elongation by subzero-cooling to produce martensite.(3) In the fractured surfaces of austenitic specimens, some kinds of large or elongated dimples were observed and their number increased with the degree of prior-deformation, while the size and shape of dimples were hardly changed by the degree of prior-deformation (ausforming) in martensitic specimens.