Effect of M-A constituent on toughness of weld-heat affected zone of high tensile steel was investigated by Charpy impact test. Main results obtained were as follows.
(1) Both the coarsenig of structure and the M-A constituent were factors of loss of toughness.
(2) M-A constituent in case of long cooling time exerted less influence on loss of toughness than that in case of short cooling time.
(3) Ferrite-carbide structure formed by the decomposition of austenite in case of long cooling time exerted less influence on loss of toughness than M-A constituent in case of short cooling time, but that exerted almost the same influence as M-A constituent in case of long cooling time.
(4) Harmful effect of M-A constituent on toughness was mitigated by tempering at low temperature.