Improving supermartensitic stainless steel weld metal toughness: eliminating ferrite, maximizing austenite, and softening martensite through PWHT markedly improved toughness with respect to the as-welded condition

Abstract

Welding of supermartensitic stainless steel plays a crucial role in fabricated components, influencing their toughness, weldability and resistance to sulphide stress cracking. Post weld heat treatment adjusts the final properties of the weldments, bearing on microstructural evolution. The objective of this work was to maximize all weld metal toughness by microstructural modifications achieved by means of post-weld heat treatments (PWHTs). Two all-weld metal test coupons were prepared according to standard ANSI/AWS A5.22-95, using a 1.2 mm diameter tubular metal-cored wire under Ar-5%He and Ar-18%CO2 gas shielding mixtures in the flat position, with a nominal heat input of 1.6 kJ mm-1. Single tempering, solution annealing, solution annealing plus single tempering and solution annealing plus double tempering treatments were carried out at different times and temperatures. All-weld metal chemical composition analysis, metallurgical characterization, hardness and tensile property measurements and Charpy-V tests were carried out. It was found that eliminating ferrite, maximizing austenite and softening martensite through PWHT, improved toughness up to almost three times with respect of the as-welded condition, for both shielding gases used. When welding under Ar-18%CO2 shielding gas, it was detected a) higher all-weld metal contents of C, O and N and slightly lower contents of Mn, Si, Cr, Ni, Mo, Cu; this fact produced slightly lower both ferrite and austenite contents in as-welded condition and b) lower toughness and ductility, and higher strength and hardness, regarding the samples welded under Ar-5%He mixture.