Failure analysis of stellite dies for brass extrusion: The effect of aging

Improving the performance of the extrusion tools and predicting their life postulates knowledge of their failure mechanisms. In this work, actual failure cases of brass extrusion dies made from stellite 4B are presented and their mechanisms are studied. It is found that exposed parts of the die suffer from cyclic loading which, in combination with manufacturing defects and design imperfections, promote the initiation of fatigue cracks. Aging during service of stellite 4B inserts reduces die life due to the formation of a network of brittle carbides surrounding the ductile cobalt dendrites. During extrusion, the M7C3 carbide dissolves in the cobalt matrix leading to precipitation of new M23C6 and η-carbide types. This occurs at relatively low working temperatures, in the range of 600–650 °C, instead of 850–1100 °C as referred in the resent literature, and is probably assisted by high stresses prevailing during the extrusion process.