Prediction of Reheat Cracking Behavior in a Service Exposed 316H Steam Header

Reheat cracking in an ex-service Type 316H stainless steel steam header component has been investigated in this study. The examined steam header was in service for 87,790[Formula: see text]h and the cracks in this component were found in the vicinity of the weld toe. The root cause of this type of failure was due to the welding residual stresses. The welding-induced residual stresses had been present in the header at the early stage of the operation and were released during service. In this paper, a novel technique has been proposed to simulate the residual stress distribution normal to the crack direction by applying remote fixed displacement boundary conditions in an axisymmetric model. This approach can simulate the presence of residual stresses in actual components without the need to develop full weld simulation to quantify them. The predicted residual stress levels and distributions normal to the crack direction have been found in good agreement with the measured residual stresses available in the literature for a similar header. The creep crack growth (CCG) rates have been characterized using the fracture mechanics [Formula: see text] parameter and estimated using predictive models.

Method and Criteria to Evaluate Reheat Cracking Susceptibility

Due to a lack of facile methods for evaluating the reheat cracking susceptibility of the coarse grain heat-affected zone (CGHAZ), a simple notched C-ring reheat cracking test method and evaluation criteria were proposed. Accordingly, a suitable C-ring sampling method and heat treatment procedure were established. The C-ring specimens were taken from thick plate joints welded under five different heat inputs and testing was carried out. Reheat cracks were observed and identified under different stress loadings. Subsequently, related with the evaluation criteria of the Gleeble® test, the evaluation criteria based on notched C-ring tests were developed. This method can reflect both the microstructure and restraint stress of actual welded joints, and the stress relaxation during heat treatment. Moreover, the experimental operation is simple and repeat-able. It is expected that an evaluation standard for reheat cracking sensitivity in CGHAZ will be established.

Research on Reheat Cracking Criterion of CGHAZ in 2.25Cr1Mo0.25V Steel

During the final post welding heat treatment (PWHT), residual stress relieves gradually with the accumulation of creep strain. However, reheat cracking with intergranular characteristic will occur when grain boundary cannot accommodate this kind of strain for some special steel welding, such as the welding coarse grained heat affected zone (CGHAZ) of 2.25Cr1Mo0.25V steel. Based on the principle of stress relaxation similar to the process of PWHT, two methods are applied to study the strain criterion of reheat cracking. Stress relaxation testing is performed on CGHAZ materials prepared by Gleeble thermomechanical simulator. The critical strain is calculated using the relationship between stress reduction and creep deformation. Self-loaded notched C-ring specimens are tested taken from the welding structure, coupled with finite element modeling and multiaxial creep coefficient to determine the critical strain. The results show that there is a large numerical difference between the critical strains from two methods. The possible reasons for the difference are given. Regarding the PWHT as a service process, whether the critical strain values obtained exceed the strain limits in ASME-NH is discussed.