Surface properties and activation energy analysis for superplastic carburizing of duplex stainless steel

Abstract

A new surface carburizing technique which combines superplastic phenomenon and carburizing process called superplastic carburizing (SPC) was introduced and compared with conventional carburizing (CC) process. Both processes were conducted on duplex stainless steel (DSS), which has been thermo-mechanically treated to obtain fine grain microstructure and exhibit superplasticity. Both processes were carried out at temperatures ranging from 1123 to 1223 K for various durations. Metallographic studies revealed a uniform, dense and smooth morphology of carbon layer was formed on all carburized specimens. For CC process, the thickness of carbon layer were in the range of 15–65 μm, while a much higher carbon layer thickness in the range of 17–75 μm was formed on the surface of carburized DSS through SPC process. Surface hardness values of carburized specimens through CC process were between 682 and 1300 HV, while through SPC process the surface hardness was increased significantly between 737 and 1648 HV. Activation energy for SPC was determined as 151.87 kJ/mol, which is lower compare to 198.58 kJ/mol for CC process. The results indicate that SPC process accelerates the diffusion of carbon atoms into the surface of DSS, thus increasing the thickness of carburized layer as well as the surface hardness and lower activation energy.