Microstructure and fracture toughness of multipass friction stir welded joints of API-5L-X80 steel plates

Abstract

Friction stir welding with the optimum parameters offers high toughness and mechanical properties for pipeline steel applications; however, at subzero temperatures such joints have shown decreased toughness relative to room temperatures values as well as predominantly brittle behavior. This paper relates certain microstructural features to fracture toughness for the different regions of one- and two-pass friction stir welded joints, including base material, heat-affected, stir and hard zones. Toughness was assessed at 0°C, −20°C and −40°C by crack tip opening displacement testing. The microstructures in the stir and hard zones were composed mainly of granular bainite, and bainite packets with irregular and straight ferrite plates. Hardness mapping distinguished the hard zone from the base material and stir zone, and from the heat affected zone that undergoes a softening relative to base material. The effects of low temperatures on the toughness were more pronounced in notches located at the stir zone and hard zone than for other notches. Detailed SEM fractography revealed the complementary roles of martensite-austenite particles, and bainite packets in the initiation and propagation of cleavage fracture in the hard zone at −40°C.