It is now well established that the phenomenon known as ‘overheating’ in steels is caused by the dissolution of manganese sulphides when a steel is heated at high temperatures, followed by their precipitation during subsequent cooling as fine α-MnS particles on the high-temperature austenite grain boundaries. Crack propagation via these uniformly distributed grain-boundary sulphides is responsible for the faceted fracture surface appearance characteristic of overheating. When viewed under the SEM, these facets are seen to be composed of small, well-defined ductile dimples which collectively give each facet an even, regular appearance.
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