Improved toughness of double-pass welding heat affected zone by fine Ti–Ca oxide inclusions for high-strength low-alloy steel

Abstract In order to improve the mechanical performance of double-pass welding heat affected zone, acicular ferrite (AF) induced by Ti–Ca oxide inclusions was used to enhance the toughness properties of the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ). The microstructural evolution and toughness mechanisms in the coarse-grained heat-affected zone (CGHAZ) and ICCGHAZ containing different oxide inclusions were studied by thermo-mechanical simulator, optical and electron microscopy, electron back-scattered diffraction and mechanical testing. The results indicated that CGHAZ toughness was decreased and Vickers hardness was increased compared with the base metal in Ti–Ca deoxidized and Al–Ca killed steels. However, the toughness loss in Al–Ca killed steel was more significant. In Ti–Ca steel, dominant Ti–Ca–O–Mn–S oxide inclusions of size between ∼0.2 and 1.6 μm effectively promoted AF transformation. After the double-pass thermal cycle, AF plates were retained and the prior martensite/bainite structure in CGHAZ transformed into fine ferrite/bainite structure. As a result, the toughness of ICCGHAZ in Ti–Ca steel was significantly enhanced from ∼94 J to ∼209 J. In the Al–Ca steel, dominant Al–Ca oxide inclusions were ineffective for microstructure refinement and acted as a potential crack initiation site. The formation of coarse granular bainite and lath-like martensite/austenite (M/A) constituents led to deterioration of toughness in CGHAZ (∼20 J) and further loss in ICCGHAZ (∼13 J).