Static Tensile Properties of Mixed Structure of Martensite and Residual Ferrite in Ni-Cr-Mo Steels Containing Medium Carbon

Abstract

A study has been made to clarify the effect of residual ferrite on static tensile properties of Ni-Cr-Mo steels containing medium carbon. Results obtained are summarized as follows:
(1) The 0.2% proof stress approximately follows the calculated law of mixture up to 14% of residual ferrite, but becomes to deviate significantly from it beyond 14%.
(2) Both elongation and reduction in area remarkably decrease with an increase in residual ferrite.
(3) From the analyses of true stress-strain diagrams and microfractographs, a detrimental effect on ductility results from the fact that, in low volume fraction of residual ferrite, microcracks are initiated by fracturing of the residual ferrite in a brittle manner due to higher plastic restraining of it by martensite in the process of plastic deformation, and that with increasing in the residual ferrite microcracks which are initiated by fracturing of it, this mainly resulting from stress concentration in the neiborhood of interfaces between martensite and residual ferrite due to ununiform deformation, become to be simultaneously propagated through embrittled martensite.