Study on Surface Modification of Stainless Steels by Laser Surface Melting (Report 4) Effect of Rapid Solidification on the Microstructures and Delta-ferrite Morphologies*

Abstract

Microstructures of stainless steels rapidly solidified by laser surface melting (LSM) were investigated. In addition to the massively solidified structure and the rod-like eutectic structure which have been reported, characteristic structures, named as film-like ferrite, and rod-like ferrite were observed in LSM treated specimen. TEM and STEM analysis have revealed that there was not specific orientation relationship like K-S relationship between rod-like ferrite and austenite matrix, while there was K-S orientation relationship between film-like ferrite and austenite matrix. Between ferrite and austenite matrix in vermicular ferrite structuer 4 or 5 degree deviation from K-S orientation direction had been observed. Segregation of elements due to solid transformation from ferrite to austenite was observed in vermicular ferrite and film-like ferrite structures, but no segregation due to solid transformation was observed in rod-like ferrite structure. From these results it was deduced that: (1) as for vermicular ferrite structure, delta ferrite is the primary phase of solidification, and austenite solidifies at the terminal stage on solidification and a diffusional solid transformation from ferrite to austenite takes place. (2) as for film-like ferrite, delta ferrite is dominant phase of solidification and a diffusional solid transformation follows with drop temperature than that of vermicular ferrite structure. (3) as for rod-like ferrite, LSM treated specimen solidifies completely to ferrite and a massive (patitionless) solid transformation from ferrite to austenite takes place during cooling. © 1991, JAPAN WELDING SOCIETY. All rights reserved.