Abstract: In this paper, the failure of a large Pelton turbine, arising immediately after receiving a quenching and tempering heat treatment, is analysed. The turbine was manufactured in ASTM CA-6NM soft martensitic stainless steel. A thermo-mechanical finite elements model was developed to evaluate the homogeneity of the heat treatment; for this purpose, the thermal histories of internal and external points of the component were compared, and the possible existence of temper embrittlement was assessed. Moreover, the thermal stresses during heat treatment were obtained in order to perform a failure analysis. After analysing the fracture surface through visual inspection and scanning electron microscope fractography, the material properties were characterised all around the fracture surface including optical and transmission electron microscopy, chemical composition and Vickers and Charpy impact tests. All this experimental information allowed a satisfactory understanding of the phenomenon to be obtained and a failure analysis to be performed in order to justify the fracture of the component.

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