Color Metallography for Microstructural Characterization of Welded Regions in a Trip 800 Steel

Abstract

This paper presents the results of the effect of Laser CO₂ welding process on the weldability, phase transformations and exhibited tensile properties of a TRIP800 Steel. The resultant microstructures were identified by metallography. In this case, color tint etching was used to reveal the various phases found in the base metal (BM), heat affected zone (HAZ) and weld metal (WM) regions.  It was found that the BM consisted of small discrete bright white retained austenite pockets, brown ferrite and dark brown bainite phases in the HAZ the dominant phase was mostly martensite whereas bainite was predominant in the WM regions. 

In addition, it was found that the tensile strength of welded specimens did not reach 800 MPa and that the elongation fell down to 15 % or below when compared with the BM. In turn, this clearly suggested that the implemented laser welding process lead to a reduction in the TRIP800 steel toughness. Confirmation for the loss of toughness was found by examining the fracture regions.  It was found that fracture occurred in the BM region adjacent to the HAZ. In particular, the fracture appearance was brittle cleavage with what seemed to be chevron markings. Apparently, in this region, coarsening of ferrite and bainite phases occurred during welding.