Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or cathodic protection. Thus, hydrogen assisted stress corrosion and cold cracking is still a major topic regarding the reliability of welded steel components, as for instance offshore platforms and pipelines. In order to support conclusive testing and life time evaluation of welded steel components, a numerical model for hydrogen assisted cracking has been developed, particularly taking into consideration the geometrical effects of crack propagation on the respective hydrogen distribution alongside and ahead of the crack. Numerical calculations were based on finite element analysis of the hydrogen and stress-strain distribution by using a commercially available program. The model has been verified experimentally by slow strain rate experiments of supermartensitic stainless steels which are intended to be used more extensively as materials for welded flowlines in North Sea oil and gas production. As first results of such simulations the influence of the subsurface concentration provided by different H2S saturation levels in the NACE TM 0177-96 standard solution on crack propagation and the effect of crack shape on the hydrogen distribution profile are presented in this contribution.
Industry faces a growing demand for high-strength structural steels with yield strengths of up to 1,300 MPa in order to cope with increasingly higher strength requirements in engineering. Higher strength levels are achieved by a special coordinated production process and an adapted chemical composition. Nevertheless, disastrous damage cases with high-strength steels have occurred in the past. The sensitivity to mechanical property degradation by hydrogen increases dramatically with strength. This phenomenon leads to hydrogen-assisted cold cracking. T-joints with fillet welds made from one side with an included angle of 60° were examined for their cold cracking behavior. Based on the T-joint, a modified heat input, even interpass temperature, plate thickness, and length ones were examined. The diffusion behavior and the effectiveness of different post-weld heat treatments in joints were simulated. The results of post-weld heat treatments are illustrated in practical hydrogen removal heat treatment diagrams. It is noticed that the T-joint is subject to a very high risk of hydrogen-assisted cold cracking (HACC). Contrary to other joints, its most critical area for cracking is not the weld metal but the heat-affected zone surrounding area of the root pass. The simulation shows that HACC in the T-joint can only be avoided by applying a sufficient post-weld heat treatment.
Numerical simulation of hydrogen removal heat treatment procedures in high strength steel welds
(2007)
Duplex stainless steels have been used for a long time in the offshore industry, since they have higher strength than conventional austenitic stainless steels and they exhibit a better ductility as well as an improved corrosion resistance in harsh environments compared to ferritic stainless steels. However, despite these good properties the literature shows some failure cases of duplex stainless steels in which hydrogen plays a crucial role for the cause of the damage. Numerical simulations can give a significant contribution in clarifying the damage mechanisms. Therefore, a numerical model of a duplex stainless steel microstructure was developed enabling simulation of crack initiation and propagation in both phases. The phase specific stress strain analysis revealed that local plastic deformation occurs in both austenite and δ-ferrite already in the macroscopically elastic range. Altogether, phase specific hydrogen-assisted material damage was simulated for the first time taking into account all main factors influencing hydrogen assisted cracking process. The results agree well with experimental observations and thus allow a better insight in the mechanism of hydrogen-assisted material damage.