Abstract
In the previous paper, it was shown that carbon distribution in austenite was not uniform in low hardenability zone in HAZ. This heterogeneous distribution of carbon was estimated to be the main factor for the formation of the microstructure in the low hardenability zone.In this paper, mechanism of the formation of the microstructure in the low hardenability zone was investigated on the basis of these results.Main results obtained were as follows.(1) The amount of high carbon region formed during the transformation to austenite decreased as peak temperature increased. This decrease in the amount of high carbon region was found to correspond with the increase in hardenability.(2) On cooling, polygonal ferrite was nucleated at low carbon region in austenite, when the peak temperature was within the low hardenability region (that is, high carbon region still remained). As temperature decreased, ferrite grew and the remaining austenite (including high carbon region) became M-A constituent.(3) As the results, resultant microstructure in the low hardenability zone was composed of polygonal ferrite and M-A constituent. The distribution of M-A constituent in this zone was similar to that in the zone heated to austenite-ferrite two phase temperature range.
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