In power plant boilers, T91 and TP347H are generally welded using argon arc welding. Traditional argon arc welding requires beveling, a substantial wire filling amount, low welding efficiency, and high overall heat input. Excessive and uneven heat input can lead to the formation of blocky ferrite and ‘island structures’ at the weld joint, promoting component segregation and carbon migration and softening the joint structure. A large bevel angle can increase the unevenness in the thickness direction along the wall of the weld joint, which can affect the performance of dissimilar steel weld joints under high-temperature and high-pressure service conditions.

This study uses a handheld single-mode laser swing welding device with a rated power of 1500 W, swing amplitude of up to 5 mm, and swing frequency of 0?300 Hz. The filler wire used is ERNiCr-3, with a diameter of 1.2 mm. The sample is a tubular sample with dimensions of Φ57 mm×4.5 mm, without beveling, which is welded using a butt joint without preheating and subsequently air-cooled to room temperature. The area within a 10 mm range of the inner and outer walls of the welding position is cleaned before welding, removing water rust, oil stains, and other impurities to expose the metallic luster. Nitrogen is used as the shielding gas and is blown into the molten pool. The test tube is aligned with calipers and fixed with a handheld laser welder for spot welding. The tube sample is fixed and rotated using a three-jaw chuck, and single-pass full-penetration welding is performed.

The weld seam is well formed with no evident macroscopic defects such as concavity, undercut, or surface pores. No defects are observed inside the joint, such as pores, inclusions, cracks, lack of fusion, or lack of penetration (Fig. 4). No “island structure,” “beach structure,” or delta ferrite can be found in the heat-affected zone (HAZ) of the welded joint (Fig. 6). The quantity of fine precipitates in the fine crystal region is significantly reduced, the shape of the carbides is round, and the size increases; however, some small precipitates remain [Fig. 8(b)]. The coarse crystal region has the least amount of precipitates, with only tiny precipitates observed [Fig. 8(c)]. Under the action of high energy factors, the carbides of M₂₃C₆ and MX in the coarse-grained region dissolve, the pinning effect of these precipitates disappears, and the grain boundaries begin to move. The austenite grain sizes in this region increase significantly, leading to coarsening. During rapid cooling, these coarse austenite grains transform into coarse, untempered martensite grains, forming a coarse-grained HAZ. When the overall energy input decreases gradually as the fusion line transitions to the T91 base metal, the M₂₃C₆ carbides trigger the Ostwald ripening phenomenon, manifested by a change in the shape and size of the precipitated phase becoming rounder. MX carbides contain the strong carbide-forming elements V and Nb, which can stabilize the carbides and prevent Ostwald ripening. Additionally, owing to the lower peak temperature experienced and higher cooling rate compared to the coarse-grained HAZ (CGHAZ) region, the recrystallized austenite is refined, resulting in grain sizes smaller than those in the coarse-grain region and T91 base metal. The highest hardness is observed in the HAZ, and the average tensile strength of the joints exceeds 600 MPa.

In this study, T91/TP347H dissimilar steel is welded with oscillating laser filling wire using a handheld single-mode laser, and the microstructure and mechanical properties of the welded joints are studied. The results show that the handheld single-mode oscillating laser can be used to weld dissimilar steel boilers with well-formed welds that are free from defects such as cracks and porosity. No δ-ferrite and island structure or beach structure caused by composition segregation is observed. Numerous precipitates appear in the TP347H side HAZ, while the T91 side HAZ exhibits a CGHAZ and fine-grained HAZ (FGHAZ) owing to the influence of the welding heat cycle. In the CGHAZ, the precipitated phase is almost completely dissolved, and M₂₃C₆ matures and coarsens. Primary austenite grains and untempered martensite are observed in the HAZ, and the highest hardness is obtained. Under the influence of laser oscillation, the weld zone presents a mixture of cellular, columnar, and equiaxed crystals, and the preferred growth direction of the columnar crystals is diversified. The average tensile strength of the joint exceeds 600 MPa. No evident cracks are observed on the surface of the specimen after bending, and the joint performance is satisfactory.