Hydrogen effect on mechanical properties and cracking of creep-resistant 9% Cr P92 steel and P91 weld metal

Martensitic 9% Cr steels like P91 and P92 can show an increased susceptibility to delayed hydrogen-assisted cracking. The focus of this study was the microstructure and heat treatment effect on the mechanical properties of P92 base material and P91 multi-layer weld metal in both as-welded and post weld heat treated (PWHT) condition. Tensile tests with hydrogen-free reference samples and electrochemically hydrogen charged samples were carried out; the mechanical properties were assessed and supported by detailed fractographic analysis. Finally, a hydrogen and microstructure-dependent fracture criterion is established. All investigated microstructures showed a hydrogen-influenced degradation of the mechanical properties compared to the hydrogen-free reference samples. The as-welded martensitic P91 weld metal had the highest degree of degradation in the presence of hydrogen. The P91 PWHT weld metal and the P92 base material had comparable properties. From that point of view, a significantly increased risk for hydrogen-assisted cold cracking during welding fabrication of P91 weld joints must be considered before any heat treatment is conducted.