S32750 (022Cr25Ni7Mo4N) super duplex stainless steel seamless knowledge sharing

Duplex stainless steel is a type of stainless steel that has both austenite and ferrite structure and two phases exist independently and has a relatively large content. The third-generation duplex stainless steel is also referred to as super duplex stainless steel PRE (Cr%+3.3*(Mo+0.5W)%+16N%)≥40.
S32750 (022Cr25Ni7Mo4N) is a typical super duplex stainless steel with high content of Cr, Mo, and N in the steel and good resistance to pitting and crevice corrosion. Applicable to petrochemical, oil refining, vacuum salt, flue gas desulfurization and other more corrosive environments.
S32750 (022Cr25Ni7Mo4N) super duplex stainless steel has low thermal plasticity and high thermal deformation resistance. In view of the properties of steel materials, the 6000-extruder hot extrusion process of TISCO SMS-MEER was studied. The main research contents are preheating heat treatment system, expanding hole deformation amount, induction heating system, hot extrusion deformation amount, cooling system and hot extrusion tube blank performance.
The test materials were smelted using an electric furnace + AOD + LF furnace, and the vehicle light after forging was Φ219mm. The main components of S32750 are shown in Table 1.
Table 1 Chemical composition elements of S32750 CSiMnPSCrNiMoN
Standard requirements ≤0.030≤0.80≤1.20≤0.035≤0.02024.00~26.006.00~8.003.00~5.000.24~0.32
Actual 0.0220.0541.030.0290.00124.947.153.930.28
Process flow: Tube preparation → Ring furnace preheating → Induction heating → Expanding → Secondary induction heating → Extrusion → Cooling.
The extrusion equipment is a 6000 tons horizontal extruder. According to the size requirements of the finished pipe, the extrusion cylinder is selected as Φ230mm, the extrusion needle is Φ75mm, the extrusion die is Φ89mm, and the finished steel pipe specification is Φ89×7mm. The maximum safety load of this specification extruder is 38MN. At the same time prepared for extrusion of stainless steel special glass powder lubrication pads and glass powder.
Under the hot extrusion process, S32750 completes the extrusion and the surface quality is good. The tensile test was performed at the head and tail of the steel pipe and the ASTM A370 standard was implemented. The test results met the standard requirements.
For different component designs, as the ferrite increases, the strength increases, and as the austenite increases, the toughness increases and the strength decreases. However, for the same steel grade, the effect of the comparative example on the mechanical properties did not change significantly.
The metallographic analysis of the extruded tube shows that the ferrite and austenite phases are in strips, the proportion of ferrite phase is 49.52%, and the grain boundary is pure and free from precipitation, meeting the requirements of the ASME standard. The hardness and corrosion test results of the steel pipe meet the standard requirements. Research indicates:
1) S32750 (022Cr25Ni7Mo4N) super duplex stainless steel σ phase harmful phase precipitation interval of 800 ~ 1020 °C, preheating using 750 °C, 2.75h, no harmful phase precipitation, temperature uniformity;
2) The thermal processing temperature is 1100-1200°C, the extension coefficient is 1.056, the extrusion ratio is 19.9, and the hot extrusion surface quality is good;
3) Rapid cooling after extrusion, good organization;
4) Hot extrusion performance meets standard requirements and can be delivered directly.
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