Small-angle neutron scattering study on phase separation in a super duplex stainless steel at 300 °C – Comparing hot-rolled and TIG welded material

Abstract

The evolution of nanoscale phase separation in the ferrite phase of super duplex stainless steel 25Cr 7Ni (wt%) (SDSS 2507) and two SDSS TIG (tungsten inert gas) weldments have been quantitatively investigated by small-angle neutron scattering (SANS). The results show that the phase separation is more pronounced in the SDSS weldments in comparison to the base metal SDSS 2507, especially after aging for 35,000 h at 300 ° C. These results correlate with the higher ferrite micro-hardness in the aged TIG weldments than in the SDSS 2507. The enhanced phase separation is partly due to the higher contents of Cr and Ni in the ferrite of TIG weldments compared to SDSS 2507 base metal, revealed by energy-dispersive X-ray spectroscopy (EDS). Additionally, the residual strain measurements through focused ion beam and digital image correlation (FIB-DIC), indicate larger residual strains in the ferrite of weldments than in the base metal SDSS 2507. This is also believed to contribute to the enhanced phase separation. • TIG weldments show a faster rate of phase separation at 300 ° C than the base metal of SDSS 2507. • Higher contents of Cr and Ni in the ferrite are partly responsible for the accelerated kinetics of phase separation in the weldments. • Higher heat input during TIG welding leads to higher residual stresses which enhance phase separation