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

Abstract

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.

Effects of Base Metal Preheating on the Microstructure, Mechanical Properties, and Corrosion Resistance of UNS S32750 SDSS Pulsed Nd:YAG Laser Welding

“…Studies of specialized welding have explored SDDS laser welding, which involves the use of austenite-promoting elements to increase the volume fraction of austenite [3,10,12,15,18,19]. The addition of elements or preheating can increase the austenite volume fraction.…”

“…Although it did not achieve phase balance, the T100, T200, and T300 samples exhibited considerable increases in the proportion of austenite. Since industrial applications require a minimum austenite proportion of 25-30%, all heat-treated samples are suitable for industrial use [3,[21][22][23].…”

“…Duplex and super duplex stainless steels (DSS and SDSS, respectively) are widely used in industries such as marine, nuclear, and petrochemical plants due to their mechanical properties, good weldability, and high corrosion resistance. SDSSs also have an excellent combination of heat transfer capability and thermal expansion coefficient, i.e., high heat transfer and low thermal expansion of the material [1][2][3]. DSSs and SDSSs have a balanced bimetallic microstructure of 50% ferrite and 50% austenite, which is responsible for the material good properties, as well as its excellent applicability, even in aggressive environments [3][4][5].…”

“…Da Cruz Junior et al [10] noted that there was an increase in austenite, mainly in the lamellar, acicular, and Widmanstatten forms after the insertion of a nickel sheet compared to the autogenous welding of UNS S32750. Da Cruz Junior et al [3] found that adding a cobalt layer to the weld region resulted in an increase in the amount of austenite as the thickness of the cobalt layer increased, while the hardness of the weld bead decreased with the increasing thickness of the cobalt layer. In their research, Sung, Shin, and Chung [11] observed that the welded region was mostly ferritized and the amount of energy in the process directly influenced the size of the weld bead.…”

“…SDSSs also have an excellent combination of heat transfer capability and thermal expansion coefficient, i.e., high heat transfer and low thermal expansion of the material [1][2][3]. DSSs and SDSSs have a balanced bimetallic microstructure of 50% ferrite and 50% austenite, which is responsible for the material good properties, as well as its excellent applicability, even in aggressive environments [3][4][5].…”