Structural Integrity of Bi-Metallic Welds in Piping Fracture Testing and Analysis

The French field experience in stainless steel bi-metallic welds (BMW) has shown different degradations like external surface corrosion cracks close to the low alloy steel/stainless steel interface or fabrication defects in different other locations. In many countries, some degradation has been encountered in different type of bi-metallic welds: stainless steel BMW or Ni-based alloy BMW through different degradation mechanisms (corrosions). The critical crack size in different location of a BMW is a key safety issue. To-day, there is no flaw evaluation procedure for this type of components in existing operation codes, like ASME XI [7], RSE-M [6] or R6 rule [5]. Consequently a fracture mechanic procedures is under preparation in the French RSE-M operation Code [6] in order to evaluate the critical crack sizes of defects in different area of a bi-metallic weld. The procedure validation is based on 2 specific experimental projects that have been performed on 6" and 16" bi-metallic welds at room temperature and 300°C. Detailed residual stress measurements and simulation have been done, in order to check their influence on the critical crack size. The major results of these projects are: • no instable crack growth for cracks closed to the fusion line, • crack ductile tearing growth take place in the lower strength material and not in the lower toughness material, • the residual stresses have a negligible effect on the critical crack size, • the crack initiation take a place close to the maximum limit loads, • the ductile crack growth is mainly at the deepest point and negligible on the outer surface (for the cracks considered in these test programs), • no existing engineering methods are really available for this type of cracked components, • the crack distance to the interface is a key parameter in term of toughness, • comparison of notched and pre-crack specimen has been done and confirm a small increase of toughness in the case of electro-eroded crack. After a brief summary of the validation programs, the paper ends with a proposed procedure to analyse the critical crack size in a bi-metallic weld through two methods: an engineering method (1) with large safety factor and an elasto-plastic FEM (Finite Element Method) (2) with lower safety factors. They will be included soon in the RCC-M [8] for design consideration of BMW and the RSE-M [6] codes for service behaviour of BMW. Similar method can be used for different type of BMW (VVER, Ni-based alloy...).