Both 500 F embrittlement and temper brittleness result in intergranular fracture along prior austenite grain boundaries. Since both of these embrittling reactions are grain-boundary phenomena, they must result from changes in the properties of the grain boundaries rather than from changes in the properties of the matrices. Both forms of embrittlement result in time-dependent elevation of the transition temperature, and both are promoted by the presence of certain minor elements in trace quantities. The use of electron-microscopic fractography in the study of these conditions has indicated that neither 500 F embrittlement nor temper brittleness can be attributed to precipitation of unique phases in the grain boundaries;no such phases have been observed or identified.
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