Laser welding of thin-walled products with a rotation axis of the pipe-nipple type made of corrosion-resistant high-alloy steels
DOI:
https://doi.org/10.33216/1998-7927-2025-287-1-12-20Keywords:
laser welding, thin-walled products, dissimilar steels, parts with a rotation axis, problems, defectsAbstract
Thin-walled products with a rotational axis made of different corrosion-resistant steels, including austenitic and martensitic-ferritic steels, are widely used in industry. Austenitic steels are characterized by high corrosion resistance and mechanical properties at elevated temperatures, while martensitic-ferritic steels demonstrate better resistance to corrosion cracking and are a more economical alternative for use in aqueous environments. Welding of such heterogeneous materials is accompanied by difficulties associated with differences in physical, mechanical and metallurgical properties, which leads to the formation of areas with increased stresses and possible structural defects. Laser welding minimizes the thermal impact and allows controlling the structure of the welded joint, but its application for welding thin-walled products made of dissimilar steels requires further research. Determining the effect of the thermal cycle on the properties of welded joints remains an urgent problem, since a mismatch in the geometric parameters of the welded joint can lead to a decrease in the fatigue strength of parts. As part of the study, butt welds of thin-walled products with a rotation axis made of AISI 321 steel (pipe) and AISI 431 steel (fitting) were performed using a DY044 Nd:YAG laser (power 4.4 kW, wavelength 1.06 µm). The following optimal welding mode was chosen: laser power P = 1 kW, welding speed Vw = 600 mm/min, laser defocusing value ∆F = +2 mm. To protect the welded joint, two shielding gases were used: argon was supplied from below the welded parts at a flow rate of 14 l/min, and helium was supplied from above at a flow rate of 30 l/min. It was determined that all samples demonstrate fracture in the welded joint zone under the influence of vibration loads in the stress range of 14-28 kgf/mm². The analysis of the fracture microstructure confirmed the development of fatigue fracture, which begins in the welded joint zone and, in some cases, moves to the pipe body. Defects in the geometry of the welded joint were detected, in particular, the concavity of the root to a depth of 0.2 mm, which exceeds the permissible values. The width of the welded joint was also found to be excessive in some samples (up to 1.5 mm, while the standard is 1-1.1 mm). Metallographic tests did not reveal any defects in the form of pores, shells or slag inclusions, and the microstructure of the material of the parts was assessed as satisfactory. The material hardness of the fittings is HRC31, which meets the technical requirements. Overheating of the welding zone and high internal stresses during welding were identified as key factors contributing to the cracking. To eliminate the detected defects, it is recommended to optimize the laser welding parameters, in particular, the radiation power and welding speed. In addition, measures are proposed to reduce the likelihood of hot crack formation, such as grain refinement by introducing microalloying elements (Ti, B), using additional laser scanning or ultrasonic vibration during welding.
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