Hydrogen effect on mechanical properties of creep-resistant steel P92/X10CrWMoVNb9-2: (a) load-displacement-diagram with different hydrogen concentrations “HD”, fracture topography for (b) high HD of 7.2 ml/100 g Fe, (c) for medium HD of 3.7 ml/100 g Fe, and (d) hydrogen-free reference.
Source: BAM
Martensitic 9% Cr steels like P91 (X10CrMoVNb9-1) or P92 (X10CrWMoVNb9-2) represent materials with excellent creep resistance. Currently, they are widely used in thermal power plants and high-temperature applications in the chemical industry. With focus on the energy transition to CO2-neutral systems, these steel grades are also candidate structural materials for latent heat storage systems, fusion reactor applications and as potential interconnector materials for solid oxide fuel cells (SOFC) or for the steam/hot water supply. The superior mechanical creep-strength is achieved by the martensitic microstructure and complex nanosized precipitates . Hence, the steels require a careful welding processing. After welding, a multi-step heat procedure must be conducted. (1) The steel martensitic 9% Cr steels can show an increased susceptibility to delayed hydrogen-assisted cracking. For that reason, 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 was 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/P92 weld joints must be considered before any heat treatment is conducted. This paper the final part of a series of three papers that focused in detail on the microstructure influence on the hydrogen diffusion and its effect on the mechanical properties. The papers had been the outcome of a long-term collaboration with the Technical University of Chemnitz / Technical University Munich.
Hydrogen effect on mechanical properties and cracking of creep-resistant 9% Cr P92 steel and P91 weld metal
Michael Rhode, Jonathan Nietzke, Tim Richter, Tobias Mente, Peter Mayr, Alexander Nitsche
published in Welding in the World, 67, pages 183–194, 2023.
BAM Department Component Safety
BAM Division Testing Devices and Equipment