Two types of low-transformation-temperature weld metals were devised, one associated with primary austenite solidification, the other primary ferrite solidification. The martensite start temperature of both low-transformation-temperature weld metals was about 125°C. Experimental results showed that low-transformation-temperature weld microstructure associated with primary austenite solidification was martensite with 8.0% retained austenite, whereas that one related to primary ferrite solidification primarily consisted of martensite and δ-ferrite. Accordingly, both welded joints had little distinction between distortion and residual stress, indicating that weld metal associated with primary ferrite solidification played the same function as primary austenite solidification on residual stress reduction. Moreover, the low-transformationtemperature weld metal associated with primary ferrite solidification had higher tensile strength and hardness than that based on primary austenite solidification.
The as-welded low-transformation-temperature (LTT) weld metal with martensite/retained austenite (RA) dual phase exhibits high toughness and ductility, but the yield strength (YS) is very low. After low-temperature postweld heat treatment at 300°C, the YS, toughness and ductility of dual-phase LTT weld metal increase dramatically, while there is a slight effect on mechanical properties of full martensite LTT weld metal. During the low-temperature postweld heat treatment, carbon atoms diffuse from martensite into RA, which increases the stability of RA. The improvements of mechanical properties for dual-phase LTT weld metal after low-temperature postweld heat treatment are attributed to the increased stability of RA and enhanced transformation-induced plasticity effect.
The solidification behaviour and microstructure of welding transition zone between lowtransformation-temperature deposited metals and high-strength low-alloy steels were investigated. It was found that the steep composition gradient provided driving forces for the diffusion of carbon from base metal to weld metal, leading to the hardened and softened regions near fusion boundary. In weld metal near fusion boundary, there were retained δ-ferrites when the base metal dilution rate below 35% and Cr eq /Ni eq value larger than 2.62. Compared with martensite, the mixed microstructures of martensite + δ-ferrite obtained less strain localisation, dislocation density and more percentages of large misorientation, which were more liable to resist microcrack initiation and propagation during deformation.
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