Improving the toughness of welded joints by post-weld heat treatment: HAZ toughness in 780 and 980 MPa class HSLA steels welded with high heat inputs (4th report)

摘要

To determine optimum PWHT (post-weld heat treatment) parameters for improvement of the HAZ toughness of 780 and 980 MPa class high-strength steels welded with high heat inputs, this paper describes a systematic investigation of the effect of PWHT on the toughness of the simulated weld HAZ of the steels in relation to decomposition of the M-A (martensite-aus-tenite) constituent. The optimum PWHT time and temperature proposed with regard to practical workability are 623 K x 8 hr for the weld CGHAZ (coarse-grained HAZ for high heat input welding) and ICCGHAZ (intercritically reheated CGHAZ for multipass welding) of these steels. The optimum PWHT parameters are tested on specimens actually welded with a heat input twice as high as that conventionally applied. The toughness of the welds specimens as estimated from the CTOD value is significantly improved by the proposed PWHT conditions and meets practical workability requirements. The tensile strength of the specimens welded with a heat input twice as high as that conventionally applied, as estimated from high-width tensile specimens, is assumed to satisfy the specification for 780 MPa class high-strength steel welded joints. Although the joints include a HAZ softening zone, this is so provided that the specimen width is more than five-fold the thickness.