Study on the Martensite-Austenite Constituent in Weld-Heat Affected Zone of High Strength Steel

Abstract

Microstructure in weld-heat affected zone of high strength steel was investigated mainly from the viewpoint of martensite-austenite (M-A) constituent. Main results obtained were as follows.
(1) Microstructure could be easily classified by scanning electron microscopy if suitable etching technique was used.
(2) Main structure changed from martensite through BIII type bainite+BI III type bainite (mixed structure of BI and B III type) to BI type bainite, with increasing of cooling time.
(3) Morphology of M-A constituent in BI type bainite changed from elongated shape to massive shape with increasing of cooling time. When cooling time increased further, the decomposed structure of ferrite and carbide appeared. Therefore, the amount of M-A constituent decreased when cooling time was very long.
(4) M-A constituent was formed from the austenite in which carbon had been condensed up to 0.5-0.8% on cooling to about 400°C.
(5) M-A constituent consisted of lath martensite, twin martensite and retained austenite.