Abstract
For various kinds of matrix-microstructure, embrittlement by high carbon martensite islands (M*) was evaluated as the difference of the transition temperatures between two specimens; one specimen holding M* and the other containing no M*. The transition temperatures were measured for CTOD test, the standard Charpy test and the press notched Charpy test. The results are summarized as follows.
(1) The embrittlement by M* in CTOD depends on the matrix-microstructure through the M* size. Lower bainite has smaller M* size and less susceptibility to M* embrittlement than other microstructures.
(2) The embrittlement by M* in CTOD was remarkable in comparison with those of the other tests for the mixed microstructure of ferrite and upper bainite, while all test gave the similar embrittlement for specimens of ferrite-pearlite or martensite, i.e. the uniform microstructure.
