Microstructural development in gas tungsten arc (GTA) welded silicon-and aluminum-based transformation-induced plasticity (TRIP) steels was studied by optical and electron microscopy. The fusion zone (FZ) of both welds contained complex inclusions. Energy-dispersive spectroscopic (EDS) analysis on these inclusions showed that the center of the inclusions contained oxides of silicon and aluminum in silicon-and aluminum-based steels, respectively. Epitaxial enrichment of manganese, sulfur, and phosphorous was found on the oxides in the inclusions, resulting in depletion of solutes in solid solution and thereby reducing the stability of austenite in the fusion zone. The fusion line of aluminum-based steel weldments contained higher amounts of allotriomorphic ferrite than silicon-based steel weldments due the partitioning of aluminum at the fusion line during solidification. The retained austenite (RA) contents in the heat-affected zones (HAZs) and FZs of both TRIP steels were found to be about 8 and 4 pct by volume, respectively.

Ultrasonic impact treatment (UIT) is a relatively novel technique applied to the toe of welded joints to improve the fatigue life by changing the weld geometry and the residual stress state. In this study, the stress relaxation due to ultrasonic impact treatment is investigated on a six pass welded high strength quenched and tempered steel section. Stress measurements in two orthogonal directions were conducted by energy dispersive synchrotron X-ray diffraction. Results show that the application of only ultrasound to a welded component re-distributes the residual stresses more uniformly, while mechanical impacts in combination with ultrasound is an effective way to release the residual stresses. After welding, diffraction peak broadening due to the lattice distortion, characterised by the full width at half maximum (FWHM), is observed in the region of the weld toes. Ultrasonic impact treatment reduces the FWHM at these locations.

A quantitative analysis of retained austenite and nonmetallic inclusions in gas tungsten arc (GTA)-welded aluminum-containing transformation-induced-plasticity (TRIP) steels is presented. The amount of retained austenite in the heat-affected and fusion zones of welded aluminum-containing TRIP steel with different base metal austenite fractions has been measured by magnetic saturation measurements, to study the effect of weld thermal cycles on the stabilization of austenite. It is found that for base metals containing 3 to 14 pct of austenite, 4 to 13 pct of austenite is found in the heat-affected zones and 6 to 10 pct in the fusion zones. The decomposition kinetics of retained austenite in the base metal and welded samples was also studied by thermomagnetic measurements. The decomposition kinetics of the austenite in the fusion zone is found to be slower compared to that in the base metal. Thermomagnetic measurements indicated the formation of ferromagnetic e carbides above 290°C and paramagnetic g(e¢) transient iron carbides at approximately 400°C due to the decomposition of austenite during heating.

Sufficient liquid feeding under constrained solidification conditions like, those experienced in welding and casting, is vital to avoid solidification cracking. We present the results of unique in-situ experimental observations of liquid feeding in a solidifying steel melt pool. Liquid feeding was observed in the inter-cellular regions during the terminal stage of solidification. An average liquid flow speed of 450-500 µm s −1 was found. A pressure difference of the order of 10 4 Pa is calculated to cause the liquid flow. The rate of solidification shrinkage and the rate of deformation were found to be less than the rate of liquid feeding.

The partitioning behaviour of carbon, phosphorous and boron during the solidification of a resistance spot weld pool was studied using experimental simulations and a phase field model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to a range of resistant spot welding thermal cycles. Mechanical properties were evaluated by hardness and cross tension tests and correlated with the weld microstructure. Phase field modelling results and experimental predictions show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt% for a steel with a bulk concentration of 0.08 wt%. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength and plug diameter, the failure mode switches to a complete plug mode, resulting from the phosphorous depletion at the grain/inter-phase boundaries.