Tackling the toughness of steel pipes produced by high frequency induction welding and heat-treatment

Steel linepipes produced by high frequency induction welding can result in a low-toughness zone at the weld junction, even after a heat treatment which reaustenitises the affected region. The possible causes for low toughness are explored, including microstructure, retained austenite, inclusions and crystallographic texture. It is found that the toughness is reduced primarily by the tendency for cleavage planes of ferrite crystals to align and hence create a macroscopic plane on which cleavage can propagate easily with little resistance from grain boundaries. This mechanism suggests that an appropriate heat treatment may alter the texture sufficiently to enhance the toughness of the zone concerned.

► We investigated steel linepipes produced by high frequency induction welding.
► The possible causes for low toughness at the heat-treated weld junction are explored.
► Crystallographic texture at the weld junction was investigated in detail.
► Grain orientations favouring fracture are substantially related to low toughness.
► Besides, non-metallic inclusions and grain growth also lower the toughness.