Solid state diffusion bonding of superplastic duplex stainless steel with carbon steel

Solid state bonding between superplastic duplex stainless steel with carbon steel under low pressures is studied for the objectives to understand bonding behaviors and to determine mechanisms controlling bonding through the activation energy analysis. Experiments were carried out by combining several bonding parameters such as bonding temperatures, bonding pressures and surface roughness. Bond quality was evaluated by its tensile strength. Bonding strength increased with temperature, time and pressure but decreased with surface roughness. Sound bonds comparable to that of the parent metal were obtained at considerably short bonding time and low pressure. The best bonding condition obtained in this study was 1373 K temperature, 3 MPa pressure, 0.32 mu m R-max and 220 s bonding time, producing the parent metal strength with around 1 % of deformation ratio. Changes in microstructures and hardness were observed across the bonding interface, due to diffusion of atoms, mainly C, from the carbon steel side to the duplex stainless steel side. A hard carburized layer formed at the DSS side and a soft decarburized layer at the carbon steel side. From the value of activation energy and experimental data, bonding was controlled mainly by two mechanisms, which were the superplastic deformation at the early stage and the diffusion of carbon at the second stage.