A study of the softening mechanisms of laser-welded DP1000 steel butt joints

• Two kinds of softening mechanism act in the heat affected zone (HAZ) of DP1000 steel laser welding.
• One is in the sub-critical HAZ with carbide precipitation from martensite and dislocation density decreasing of ferrite.
• Another occurs in the inter-critical HAZ for martensite being transformed to softer microstructure.

Joint softening occurs in the laser-welded joints of many types of dual-phase steels that are used in automobile structures and weakens the joints' mechanical properties. This paper presents a study of the softening mechanisms in laser-welded joints of DP1000 steel. A Gleeble-3500 thermal simulator was used to simulate the welding thermal cycles of all of the sub-zones of the heat-affected zone (HAZ) of laser-welded specimens of DP1000 steel. The hardness and microstructure of the specimens were then tested and analysed. The results indicate that the softening resulted from two changes in the microstructure. The first change was the transformation of the pre-existing harder martensite into softer tempered martensite and the precipitation of carbides when the DP1000 steel was heated to the tempering temperature during the welding process. The other change was the decrease in the percentage of martensite, while the percentage of softer phases increased because the DP1000 steel was heated to the inter-critical temperature. At this temperature, the ferrite transformed from the martensite and some of the ferrite in the original microstructure were transformed into austenite. During the subsequent cooling process, the austenite was transformed into polygonal ferrite, bainite and martensite-austenite phases, therefore, the percentage of softer phases increased.

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