Microstructural Effects on the Mechanical Integrity of a TRIP-800 Steel Welded by Laser-CO₂ Process

In this study, a TRIP-800 steel was welded using a Laser CO₂ process, and the resultant microstructures were characterized by optical, scanning, and transmission electron microscopy (TEM) means. It was found that the microstructure of the steel in the as-received condition consisted of ferrite, bainite, and retained austenite (RA), including some martensite. In particular, TEM observations indicated that the developed martensites were high carbon twinned martensites. It was found that laser beam welding (LBW) promoted the development of up to 23% martensite in the fusion zone (FZ) and up to 30% in the heat-affected zone (HAZ). In addition, determinations of RA using x-ray diffraction indicated that the amount of RA developed in the FZ was relatively small (<6%). Confirmation for the relatively large amounts of martensite in both the FZ and HAZ was indirectly made by the shape of microhardness profiles, which resembled a “top hat.” Tensile testing in welded strips indicated a loss of strength and ductility. An examination of the fracture surfaces indicated that the steel fractured in a brittle fashion at the HAZ-BM interface. Apparently, the development of relatively large amounts of martensite in the HAZ reduced the steel toughness. In turn, this indicated that LBW leads to martensite embrittlement in the HAZ regions, but not at the parting line of the FZ.