The effect of cooling rate on the embrittlement of a 12 wt-%Cr steel was investigated. It was found that when the cooling rate is not as fast as that produced by a liquid quench, the carbides have a tendency to segregate to the prior austenite grain boundaries. Carbide coarsening occurred after prolonged exposure at the operating temperature of 538°C. The carbides precipitated at the prior grain boundaries or within the acicular martensitic structure were found to act as nucleation sites for microcracks. It was also postulated that the precipitation and segregation of carbides was promoted by applied stresses at high temperatures.
MST/59
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