Behavior of Cracks in Cladded Components under Consideration of Residual Stresses

The present study is concerned with an experimental and numerical investigation of the behavior of sub-clad and surface cracks in ferritic steels with austenitic (welded) cladding. For this purpose, two component tests have been performed. The residual stress field was determined by means of a high precision numerical simulation of the welding and heat treatment processes. Based on the results, a numerical simulation of the component tests was performed for a fracture mechanics assessment of the situation at crack initiation and arrest. In the present tests, it was observed that failure was initiated in the ferritic range whereas the austenitic cladding remained intact even in the case of a limited crack extension in the base metal. Introduction Cladded ferritic pressure vessels with austenitic welded cladding are common in power generation and many other technological fields. In the industrial design process, the fracture assessment of cladded components is usually performed by means of a two-step procedure consisting of a stress analysis of the uncracked component under consideration of the cladding and a subsequent fracture assessment using analytical formulae. In this procedure, the cladding is either assumed to be cracked or neglected (ASME Code XX[2] XX, German KTA rules XX[3] XX, etc.). The assumption of a cracked cladding is conservative. Nevertheless, the safety margin cannot easily be quantified. On the other hand, a more detailed numerical analysis under consideration of the cladding might be difficult due to the different failure modes and loads in both materials as well as due to the complex residual stress field caused by the welding and heat treatment processes as well as by the mismatch in the coefficients of thermal expansion. Aim of the present study is the experimental and numerical investigation of the behavior of cracks in ferritic components supplied with an austenitic welded cladding considering both, subclad and through-clad surface cracks. For this purpose, two large-scale specimens consisting of a ferritic pressure vessel steel were cladded with an austenitic material and supplied with a surface and a sub-clad crack respectively. The specimens were tested under combined thermal and mechanical loading conditions (three-point bending). Both specimens failed by a brittle initiation in the ferritic material. For the specimen with through-clad crack, a first initiation with a subsequent crack extension was followed by a crack arrest before final failure occurred at a significantly higher load level. For the specimen with sub-clad crack, no interim crack extension and arrest prior to final failure was observed. In both cases, the cladding remained intact and stable until the ultimate fracture of the specimens. The local conditions for crack initiation and arrest were analyzed in a numerical simulation of the experiments. In a first analysis, the residual stress field was determined by means of a detailed analysis of the welding process and the subsequent heat treatment process. The residual stress field was found to have a complex periodic character with distinct local variations. In the subsequent analysis of the fracture experiments, it was demonstrated that the brittle failure of the base metal