Enhancing strength and plasticity in the nugget zone of friction stir welded X100 pipeline steel via back-heating

Here, X100 pipeline steel was subjected to FSW by controlling the peak temperature in the ferrite phase region, and preheating was used to tailor microstructure characteristics and enhance the strength-plasticity combination of the NZ. The conventional NZ contained ultrafine ferrite recrystallized grains (0.6 mu m), and nanometer-sized (Ti, Nb)C and NbC precipitates were dispersed in the ferrite matrix. After preheating, fine ferrite grains (1.3 mu m) were obtained in the preheating NZ, and the amount of precipitates decreased. The high yield strength (YS) of 1049 MPa was achieved in the conventional NZ, but the uniform elongation (UE) of 3.6% decreased markedly, only reaching similar to 61% of UE of the base metal (BM). After preheating, a perfect YS of 852 MPa and UE of 9.1% were obtained in the preheating NZ, reaching 105% and 154% of that of the BM. The abovementioned excellent mechanical properties were attributed to the formation of fine ferrite recrystallized grains and precipitates, which provided a consistently high strain hardening rate.