Abstract
Observations were made on the fracture surface of tempered martensite of a low-carbon nickel steel. Impact test data were correlated with details of fracture surface and microstructure. It is shown that cleavage microcracks initiate in large ferrite grains, i.e., first-formed large martensite plates; the dominant factor leading to the ultimate cleavage fracture from these microcracks is the mean length of a cleavage crack facet, which is much larger than the width of martensite needles or plates and is varied considerably with the prior austenite grain size.
