Defect Assessment of Brazed Steel Components

High temperature brazing is widely applied in industry as a quick and cost-effective method for joining large components used in the aerospace and the chemical industry as well as for power generation, e.g. compressor impellers or turbine parts. These components are subjected to complex loading conditions in service, comprising mechanical, thermal or thermo-mechanical loads. Brazed joints form a heterogeneous anisotropic system consisting of base material, diffusion zone and filler metal. During brazing, defects such as pores or incomplete gap filling may arise and act as stress concentration sites, leading to crack initiation, propagation and spontaneous failure. Thus, defect assessment of brazed components is very important, but up to now only little information on the applicability of defect assessment procedures developed for bulk materials or weld joints on brazed components is available.

In this work, the defect tolerance of brazed martensitic stainless steel joints subject to static or cycling loading was investigated in detail. Therefore, the quasi-static and cyclic mechanical behaviour of standard round specimens as well as of component like specimens with T-joint geometry under tensile loading conditions was characterized. In order to consider the effect of brazing defects on the structural integrity, typical defects with different sizes and geometries were introduced in the brazing zone. In addition, a numerical deformation analysis by means of finite elements was performed and compared with the experimental results. For static loading, a defect assessment procedure based on failure assessment diagrams (FAD) is proposed. For cycling loading, the life time of the defect containing specimens in the low cycle fatigue range can be estimated using K_I,N-curves with K_I being a notch stress intensity factor of the corresponding defect.