Correction of Phase Balance on Nd:YAG Pulsed Laser Welded UNS S32750 Using Cobalt Electroplating Technique

Super-duplex stainless steel (SDSS) shows high mechanical and corrosion resistance because of the balanced structure of austenite and ferrite. However, maintaining this phase ratio after welding is a challenge. The use of austenite stabilizing components is recommended to balance the microstructure. The addition of alloying elements presents a challenge because of the characteristics of Nd:YAG pulsed laser welding. An approach, which has proven to be effective, is to use metal electroplating to prepare the surfaces of the mechanical SDSS components that will be welded, therefore promoting the phase balance in the fusion zone. While the effects of metals such as nickel as an austenite stabilizer are well recognized, cobalt’s effects require more research. The present work investigated the influence of the use of cobalt addition in the joining process by preliminary electroplating on UNS S32750 SDSS Nd: YAG pulsed laser welding, specifically regarding microstructure and microhardness. Three conditions were investigated, changing the thickness of the deposited cobalt layer. The addition of cobalt modified the morphology and increased the volume fraction of austenite. An austenite volume fraction of around 48% was obtained using a 35 μm thick cobalt coating. The microhardness was affected by austenite/ferrite proportions. The microhardness dropped from about 375 HV to 345 HV as the cobalt layer’s thickness rose, being similar to that of the base metal. The effect of cobalt as an austenite stabilizer was observed, and the cobalt electroplating technique was effective to correct the phase balance on UNS S32750 laser welding.