D.P. Wang

Effect of martensite–austenite constituent on toughness of simulated inter-critically reheated coarse-grained heat-affected zone in X70 pipeline steel

The inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of X70 pipeline steel with different second peak temperature and heat input was simulated in this study by means of Gleeble3500 simulator. The volume fraction, size, shape and distribution of martensite–austenite (M–A) constituent were analysed. The toughness of ICCGHAZ and corresponding fractographs were examined. The results showed that the distribution of M–A was strongly influenced by second peak temperature, and M–A constituent with necklace structure at lower second peak temperature led to worse toughness. The volume fraction and size of M–A were strongly affected by heat input, the volume fraction of M–A constituent increased with the increase of heat input; the volume fraction and size of M–A were key factors of toughness deterioration; the interfacial energies and the initiation of crack were related to the shape of M–A constituent.

Effect of different pre- and post-weld heat treatments on microstructures and mechanical properties of variable polarity TIG welded AA2219 joints

The microstructures and mechanical properties of 2219 aluminium alloy joints welded by variable polarity TIG welding in four kinds of different heat treatment conditions, namely, with/without pre-/post-weld heat treatment, were analysed and compared. The post-weld artificial aging treatment rendered fine dispersed precipitates (metastable θ′ phase) separate out of weld metal, which made the hardness of weld metal increase by ∼20%. Meanwhile, the fracture location of joints during tensile test changed from weld zone to heat affected zone (HAZ). The pre-weld solution treatment reduced the overaging degree of HAZ and a large number of θ′ phase reprecipitated in HAZ after post-weld artificial aging treatment. Intermediate state welding, with pre-weld solution treatment and post-weld artificial aging treatment together, could increase 20% of the hardness of weld zone, increase 11.5% of the hardness of HAZ and increase 20.3% of the tensile strength of joint.

Toughening mechanisms of low transformation temperature deposited metals with martensite-austenite dual phases

Four groups of low transformation temperature (LTT) deposited metals with different Ni contents were prepared, and their microstructures were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and electron backscattered diffraction techniques. The relationship between the microstructures of the mixed martensite–retained austenite (RA) phases and their impact toughness were investigated; it was found that the impact toughness of the LTT deposited metals increased with increasing volume fraction of RA. In particular, its magnitude was higher for the specimens containing the lath martensite, interlath RA, and intercellular RA phases than for those composed of the lath martensite and interlath RA. The toughness of the lath martensite–RA mixed microstructure was primarily determined by the presence of the soft RA phase (containing film interlath RA and stringer intercellular RA), while lath martensite phase characterized by a high density of tangled dislocations and relatively small amount of twinned substructures resulted in the embrittlement of the LTT deposited metals. The dislocation absorption by the retained austenite and transformation-induced plasticity (TRIP) effects of RA were found to be main reasons for the improvement in materials toughness during crack initiation stage. The subsequent crack propagation proceeds via the TRIP and the transformation-induced crack termination mechanisms; it is also significantly affected by the increased fraction of martensite/RA boundaries. The optimization of the RA fraction in the martensite–RA dual structure is a potentially effective method for the toughness enhancement of the LTT deposited metals containing martensite–RA dual phases.

Solidification behaviour and microstructure of welding transition zone using low-transformation-temperature welding consumables

The solidification behaviour and microstructure of welding transition zone between low-transformation-temperature deposited metals and high-strength low-alloy steels were investigated. It was found that the steep composition gradient provided driving forces for the diffusion of carbon from base metal to weld metal, leading to the hardened and softened regions near fusion boundary. In weld metal near fusion boundary, there were retained δ-ferrites when the base metal dilution rate below 35% and Creq/Nieq value larger than 2.62. Compared with martensite, the mixed microstructures of martensiteu2009+u2009δ-ferrite obtained less strain localisation, dislocation density and more percentages of large misorientation, which were more liable to resist microcrack initiation and propagation during deformation.

New method of assessing susceptibility of welds to transverse cracking

A method for assessing the susceptibility of a weld to transverse cracking is proposed based on a newly designed test specimen featuring a transverse gap in the centre to control restraint and produce a large residual stress in the weld metal. Through FEM simulation of the effects of the weld bead length on the longitudinal residual tensile stress (σx) after welding, it is confirmed that the transverse gap can significantly increase σx after welding. Furthermore, with a fixed transverse gap length, σx increases monotonically with the length of the weld bead. Two different commercial weld wires were used to check the effectiveness of this method; the experimental results obtained indicate that the critical cracking length of the weld bead differs depending on the weld wire used. The critical length of the weld bead can therefore be used as an effective index of its susceptibility to transverse cracking.