Improved resistance to hydrogen embrittlement of friction stir welded high carbon steel plates

Abstract

The hydrogen embrittlement of the friction stir welded high carbon steel plates was evaluated by the cathodic hydrogen charging method. The welding was performed below Ac1 transformation point and therefore the stir zone of the welds contained refined ferrite matrix distributed with cementite particles. After hydrogen charging for 4 hours, large quantities of irreversible dome-shaped blisters were formed on the surface of the base metal. However, no blisters were observed in the stir zone even after charging for 16 hours. In addition, hydrogen-induced internal cracks were formed throughout the thickness of the base metal. But the development of the internal cracks in the base metal was restricted when getting close to the stir zone. The hydrogen charged stir zone showed less reduction of ductility than the base metal during tensile testing, which reveals that the friction stir welded steel joints showed higher resistance to hydrogen embrittelment than base metal.

Microstructure and mechanical properties of autobody steel joined by friction stir spot welding

“…According to Sun and Fujii [11], higher resistance to hydrogen embrittlement was observed in the stirred region of the SK 4 high-carbon steel. The FSW process caused no phase transformation in the weld of SK 4 high-carbon steel and the stirred region (of the weld) consisted of a homogeneously refined ferrite matrix with evenly dispersed cementite particles [11].…”

“…According to Sun and Fujii [11], higher resistance to hydrogen embrittlement was observed in the stirred region of the SK 4 high-carbon steel. The FSW process caused no phase transformation in the weld of SK 4 high-carbon steel and the stirred region (of the weld) consisted of a homogeneously refined ferrite matrix with evenly dispersed cementite particles [11]. Fujii et al [12] revealed that the FSW process carried out on the ferriteaustenite two-phase region of carbon steels produced refined microstructure and the highest weld strength.…”

Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

“…FSW does not significantly increase the hydrogen content in API 5 L X80 high strength pipeline steel during dry and underwater welding [1,9]. This suggests that FSW reduces the risk of hydrogen embrittlement in comparison to fusion welding processes during welding.…”

“…For pipeline steels, the influence of the hydrogen in the yield strength and tensile strength cannot be deemed significant [14,15,25,26]. Nevertheless, the ductility (fracture elongation) [14,27,28] and fracture toughness decrease significantly in hydrogen environments [9,12,[14][15][16][17][18][19][20][21][22][23][24]29]. In addition, the loss of ductility increases when the strength level of the steel is high [14,27].…”

“…Few studies have reported the hydrogen effects on fracture toughness on FSW welded joints of pipeline steels [9,29]. The hydrogen decreased the ductility (elongation at fracture) of both the BM and stir zone (SZ) [9] and accelerated fatigue crack growth rate of the BM, SZ and HAZ [29]. Sun and Fujii [9], showed that the SZ showed higher resistance to hydrogen embrittlement than the BM.…”