Laser welding of 200-series stainless steels: Solidification behavior and microstructure characteristics

The welding of metals and alloys with a CO{dollar}sb2{dollar} laser differs from the arc welding process in that the solidification and cooling of the melt occurs very rapidly. As a result, the final product microstructure of the laser weld metal will vary from that produced by arc welding processes, and from the slow cooled melt produced by other processes such as casting. Also, the rapidly cooled microstructure produced by the laser welding process cannot be predicted by the constituent diagrams as is true in the case of the equilibrium structure produced during slow cooling. Since the welding of some metals and alloys by the CO{dollar}sb2{dollar} laser is relatively new, more studies are needed to understand the effect of rapid cooling rates on the microstructural features, morphologies and solidification behavior.; The role of the cooling and solidification rates on the final product microstructure has been investigated through this research. Slow cooling experiments using melting and casting techniques, and rapid cooling experiments using the laser welding process have been conducted on type 201 and type 202 stainless steels and the resultant structures were investigated and related to the cooling rate. An empirical equation relating cooling rate to the secondary dendrite arm spacing was determined and the 201 and 202 stainless steel laser weld metal cooling rate has been estimated. In the slow cooling experiments, the melt solidified as primary ferrite and the delta-ferrite morphology varied, depending on the cooling rate, from a lacy structure to a skeletal structure. The laser weld metal structure solidified as primary austenite and the structure consists of austenite dendrite columns surrounded by delta-ferrite rims. The change in the solidification mode from primary ferrite to primary austenite was related to the excessive undercooling caused by rapid cooling during laser welding. Moreover, the amounts of austenite and ferrite in the laser weld metal, the primary and secondary dendrite arm spacings, the chemical composition, the size of the heat affected zone, and the depth and width of weld metal, were all related to the solidification rate during laser welding.