Structure and hardness of laser-processed Fe-0.2%C-5%Cr and Fe-0.2%C-10%Cr alloys

Abstract

The effects of high quench rates achieved through a laser surface-alloying process on the martensitic transformations of Fe-0.2%C-Cr steels (up to 10wt% chromium) were investigated. The microstructural variables: martensitic morphology and its substructure; amount of retained austenite; and carbide precipitation were characterized by optical metallography and thin foil transmission electron microscopy (TEM). The microstructures exhibited were fully lath martensitic type, the substructure of which consisted of dislocations. The morphology and substructure of martensite were influenced neither by the chromium content of the alloy nor by the laser parameters (or melt depth) employed. Thin films of retained austenite were observed at packet and lath boundaries of martensite and at prior austenite grain boundaries. The amount of retained austenite was found to decrease with decrease in melt depth. TEM studies also revealed the presence of more or less continuous cementite films both at the lath boundaries and within the laths. Microhardness measurements had shown that the hardness increased with increase in chromium content of the alloy but appeared to be independent of melt depth.