Implant Test Modeling for Risk of Cold Cracking Assessment During Welding Operations

Abstract

Filler material used for welding operations can lead to the occlusion of hydrogen gas in the arc atmosphere into the solidifying weld metal. This amount of hydrogen as well as the one originally present in the parent metal rapidly diffuses into the various regions of the weldment due to the high temperature depending also on the microstructure evolution and trapping effects. As the welded component cools down, depending on the metal’s microstructure in the heat-affected zone, the concentration of hydrogen in weld and the level of residual stresses, the risk of hydrogen assisted cold cracking in ferritic steel can arise. One of the most effective precautions against weld hydrogen cracking is to use of preheating and post-heating in order to reduce the hydrogen content, by diffusion in the structure and degassing, when residual stresses reach higher values at the end of cooling. The implant test is a stress controlled test applied on small specimen during welding to assess the susceptibility to heat affected zone hydrogen cracking. It may be used to define preheating temperature and postheating duration in order to prevent nuclear component assemblies from cold cracking risk. This paper will first present how to couple hydrogen diffusion, thermo-metallurgical and mechanical modeling in order to simulate the implant test. Finally, a Weibull type probabilistic criterion based on numerical approaches will be proposed to improve the implant test predictive capability in the case of multi-pass welding processes involving dissimilar materials.