Factors for Formation fo Low-Hardenability Zone in Weld-Heat Affected Zone

Study on Distribution of Microstructure and Toughness in Multi-Pass Weld HAZ (Report 4)

Abstract

In the previous paper, it was confirmed that low hardenability zone was formed in the weld-heat affected zone. This zone was found to correspond to that heated to the peak temperature range of about 900-1100°C in weld thermal cycles.
In this paper, factors for the formation of this low hardenability zone were investigated using some high strength steels.
Main results obtained were as follows.
(1) Microstructure in low hardenability zone of all steels used was fundamentally different from that in coarse grain high hardenability zone. That is, microstructure in the low hardenability zone was characterized by both polygonal ferrite and M-A constituent.
(2) Austenite grain size had little effect on the formation of microstructure in the low hardenability zone.
(3) The formation of compounds (for example, BN) or the segregation of some elements along grain boundary during weld thermal cycles was considered to have little effect on the formation of microstructure in the low hardenability zone.
(4) It was found that carbon distribution was not uniform in the peak temperature range corresponding to low hardenability zone. It was estimated that this heterogeneous distribution of carbon in austenite was the main factor for the formation of microstructure in the low hardenability zone.