Turbine Housing Failure Due to Sigma Phase Precipitation and Embrittlement of Niobium-Stabilized Austenitic Steel Casting

摘要

Failure investigation of Nb-stabilized GX40CrNiSi25-12 austenitic steel turbine housing casting subjected to high-temperature degradation under cyclic thermal and strain loading was performed. Crucial, temperature-dependent microstructural degradation due to intermetallic sigma (sigma) phase precipitation was observed. Despite the typical for as cast condition stable spheroidal, also needle-like and square-like sigma phase precipitation was detected. The sigma phase increased the hardness and decreased the toughness, as well as the elongation of the turbine housing casting. Embrittlement caused by sigma phase precipitation determined cracking initiation and propagation processes. As a result, turbine housing failed during industrial testing due to intergranular multicracking mechanism.