Significance of fracture toughness for linear friction welded joint of weathering steel

Abstract

This paper examined the characteristics and significance of fracture toughness for LFW (linear friction welding) joints of weathering steel SPA-H processed under different joining conditions to control maximum temperature during LFW process. Two types of LFW joint were prepared; one was jointed under higher compressive pressure so as a maximum temperature during LFW process to be lower than the A₁-transformation temperature (A1-LFW), and the other was jointed under lower compressive pressure to be higher than the A₃-transformation temperature (A3-LFW). The fracture toughness of both joints where a crack was located at the joint interface exhibited a higher value than that of the heat-affected zone of MAG (metal active gas) welds for the same steel. These results indicated that the LFW was more effective for the joining of weathering steel compared with conventional arc welding in terms of fracture toughness. However, A1-LFW exhibited lower fracture toughness (critical CTOD) than that of base metal or A3-LFW. Thus, the significance of the test results was discussed from mechanical and metallurgical viewpoints. The fracture toughness for A1-LFW found to be deteriorated due to work hardening associated with compressive plastic straining during LFW under higher compressive pressure, where the metal heated under A₁-temperature was not completely ejected by friction but remained around the joint interface. On the other hand, the deterioration of fracture toughness for A3-LFW was found to be caused by hardening due to bainitic transformation near the joint interface, whereas the narrowness of the hardened region provided a little bit higher toughness than the intrinsic toughness of the transformed phase due to plastic constraint loss.