Forging process of duplex stainless steel smelted in vacuum oxygen-argon decarburization converter

Duplex stainless steel combines the excellent characteristics of austenitic and ferritic stainless steels, exhibiting superior corrosion and wear resistance, and is widely used in important fields such as nuclear power, petroleum, chemical industry, and marine engineering.

Compared to austenitic stainless steel, 2205 duplex stainless steel exhibits superior resistance to pitting and crevice corrosion. It has a lower coefficient of thermal expansion, better thermal conductivity, and a compressive strength twice that of austenitic stainless steel. When 2205 duplex stainless steel is used, forging processes eliminate defects such as cast porosity that arise during the smelting process, while optimizing the microstructure and refining the grain size, resulting in excellent comprehensive mechanical properties for the forgings. However, improper forging processes for duplex stainless steel may lead to coarse ferrite grains, which not only reduces the yield strength and toughness of the material but also poses significant challenges for subsequent ultrasonic testing, making quality control of the product more difficult.

During the forging process of 2205 duplex stainless steel, as the heat treatment temperature increases, the corrosion potential first decreases and then increases. With the increase of holding time, the originally balanced ratio of α phase and γ phase changes, and the α phase decomposes into σ phase and secondary austenite phase. In this process, the Cr-rich characteristic of σ phase leads to Cr depletion in the surrounding area, thus forming a potential difference, and the corrosion potential tends to decrease, resulting in a decrease in the hardness and corrosion resistance of 2205 duplex stainless steel. The increase in σ phase content and α/γ phase ratio is the main reason for the decrease in hardness and corrosion resistance of duplex steel.

By utilizing the Vacuum Oxygen-Argon Decarburization Converter (VODC) refining technology for steel ingot smelting, the purity of steel can be enhanced and the loss of alloy elements due to burning can be reduced, ensuring that the alloy composition meets the design requirements. This technology achieves pure steel liquid by reducing carbon and maintaining chromium content, as well as deeply removing impurities such as hydrogen, sulfur, and nitrogen.        

Adopting appropriate forging processes can effectively adjust the ratio of α phase and γ phase in dual-phase steel forgings, thereby enhancing the mechanical properties and corrosion resistance of the forgings:

(1) Heating the cold ingot of duplex stainless steel to 1160~1180℃ for cogging and elongation forging, and crushing the as-cast crystalline structure of duplex stainless steel can refine the grain size of the billet. Water cooling after forging effectively prevents the precipitation of harmful phases such as σ.

(2) After the second heating forging process, the billet undergoes upsetting and drawing, leading to further grain refinement. Various casting defects diffuse at high temperatures, dispersing uniformly or being eliminated.

(3) By maintaining a final forging temperature of 950-1000℃ and properly controlling the forging ratio, the directionality of the phase morphology in the duplex stainless steel's two-phase structure and the distribution of the lamellar duplex structure can be altered. This allows the multi-directional, multi-scale, non-uniform lamellar duplex structure to be retained after cooling following high-temperature deformation. Additionally, it can reduce the transformation of austenite phase to high-temperature ferrite during deformation, thereby enhancing the low-temperature impact toughness of duplex stainless steel.