Microstructure investigation on the fusion zone of steel/nickel-alloy dissimilar weld joint for nozzle buttering in nuclear power industry

Microstructure of the fusion zone of steel/nickel-alloy dissimilar metal welds (DMWs) for nozzle buttering was investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron back-scattered microscopy (EBSD). The results showed that the dissimilar joint was complete, without welding defects. The structures of the fusion zone included the beach structure along the fusion boundary, the peninsula structures connected with the fusion boundary, and the island structures in the weld. The distribution of these three types of structures near the fusion boundary was uneven. The beach structure was formed because of the insufficient mixing and solidification of the molten liquid base material and the filler metal, with the width ranging between 0 and 150 μm. The peninsula and island structures were formed by the undercooling of the insufficiently mixed liquid base material and filler metal that entered the weld because of the convection and scouring of the weld pool. The composition of the three structures depended on the degree of mixing of the liquid base material and the filler metal, with a dilution ratio between 40 and 60%. The degree of dilution for the beach, peninsula, and island structures decreased in turn. With an increase in the dilution ratio, the initial solidification temperature of the corresponding composition increased significantly. When the dilution ratio exceeded 94.5%, the initial solidification phase transformed from the face-centered cubic γ-austenite into a body-centered cubic ferrite, with island structures solidified in the form of ferrites in the weld near the fusion boundary. The austenite grain orientations at weld side are dispersed with 75% large (> 15°) misorientation in frequency and the overall texture orientation distributes dispersedly with deviating from the < 100 > direction.

Influence of M-EMS on Fluid Flow and Initial Solidification in Slab Continuous Casting

A mathematical model coupled with electromagnetic field has been developed to simulate the transient turbulence flow and initial solidification in a slab continuous casting mold under different electromagnetic stirring (EMS) currents and casting speeds. Through comparing the magnetic flux density, flow field with measured results, the reliability of the mathematical model is proved. The uniform index of solidified shell thickness has been introduced to judge the uniformity of the solidified shell. The results show that a horizonal recirculation flow has been generated when EMS is applied, and either accelerated or decelerated regions of flow field are formed in the liquid pool. Large EMS current and low casting speed may cause the plug flow near the mold narrow face and a suitable EMS current can benefit to the uniform growth of solidified shell. Meanwhile, an industrial test exhibits that EMS can weaken the level fluctuation and number density of inclusion. Overall, a rational EMS current range is gained, when the casting speed is 1.2 m/min, the rational EMS current is 500–600 A.

Analysis of Peritectic Transformation Contraction of 304 Stainless Steel Using Surface Roughness

Peritectic transformation contraction of ferrite to austenite plays an important role in the formation of cracks for steels. In order to evaluate the peritectic transformation contraction of steels at the initial solidification, the solidification of 304 stainless steel under different cooling rates were carried out by using high temperature laser confocal microscopy, and then the surface roughness and peritectic transformation contraction were analysed in combination with the microstructure of solidified steel. The result shows that the solidification model of 304 stainless steel was ferrite-austenite model in the experiments, and peritectic transformation occurred during solidification. The residual ferrite in the as-cast structure were vermicular, skeletal and reticular in turn with the increase of cooling rate. The volume contraction caused by peritectic transformation resulted in wrinkles (surface roughness) appearing on the grain surface. The peritectic transformation contraction that was affected by surface roughness increased first and then decreased with cooling rate increasing, indicating the peritectic transformation contraction can be evaluated by the surface roughness.