Study on extrusion process of UNS S32750 super duplex stainless steel seamless pipe

DataTime:[2020-2-17]

Duplex stainless steel is a kind of stainless steel which has both austenite and ferrite structure and two phases are independent and contain a large amount. The third generation duplex stainless steel is also called super duplex stainless steel pre (CR% + 3.3 × (Mo + 0.5W)% + 16N%) ≥ 40.

UNS S32750 (022cr25ni7mo4n) is a typical super duplex stainless steel with high content of Cr, Mo and N, which has good resistance to pitting and crevice corrosion. It is suitable for the fields of petrochemical industry, oil refining, vacuum salt making, flue gas desulfurization and other more severe corrosion environment.

UNS S32750 (022cr25ni7mo4n) super duplex stainless steel has low plasticity and high resistance to hot deformation. According to the characteristics of steel grades, the hot extrusion process of sms-meer 6000 extruder was studied. The main research contents are preheating heat treatment system, expanding deformation, induction heating system, hot extrusion deformation, cooling system and hot extrusion tube blank performance.

The test material is smelted in electric furnace + AOD + LF furnace. After radial forging, it is turned to Φ 219mm. See Table 1 for the main components of UNS S32750.

Table.1 Chemical composition of UNS S32750

Element	C	Si	Mn	P	S	Cr	Ni	Mo	N
Standard requirements	≤0.030	≤0.80	≤1.20	≤0.035	≤0.020	24.00～26.00	6.00～8.00	3.00～5.00	0.24～0.32
Actual	0.022	0.054	1.03	0.029	0.001	24.94	7.15	3.93	0.28

Process flow: tube preparation → ring furnace preheating → primary induction heating → reaming → secondary induction heating → extrusion → cooling.

The extrusion equipment is 6000 ton horizontal extruder. According to the size requirements of the finished pipe, the extrusion barrel is Φ 230mm, the extrusion needle is Φ 75mm, the extrusion die is Φ 89mm, and the specification of the finished steel pipe is Φ 89 × 7mm. The maximum safe load of this extruder is 38mn. At the same time, special glass powder lubrication pad and glass powder for extrusion duplex stainless steel are prepared.

In this hot extrusion process, UNS S32750 is extruded and the surface quality is good. Take the head and tail of the UNS S32750 super duplex stainless steel seamless pipe for tensile test according to ASTM A370 standard, and the test results meet the requirements of the standard.

According to different composition design, the strength increases with the increase of ferrite, the toughness increases with the increase of austenite, and the strength decreases. However, for the same steel, the influence of the example on the mechanical properties has no obvious change.

The metallographic analysis of the extruded tube shows that the ferrite and austenite are banded, the proportion of ferrite is 49.52%, and the grain boundary is pure without precipitation, which meets the requirements of ASME code. The hardness and corrosion test results of the UNS S32750 super duplex stainless steel seamless pipe meet the requirements of the standard. The results show that:

1) UNS S32750 (022cr25ni7mo4n) super duplex stainless steel σ phase harmful phase precipitation interval is 800-1020 ℃, preheating is 750 ℃, 2.75h, no harmful phase precipitation, temperature uniformity;
2) The hot working temperature is 1100-1200 ℃, the elongation coefficient is 1.056, the extrusion ratio is 19.9, and the surface quality of hot extrusion is good;
3) After extrusion, it cools rapidly and has good structure;
4) The hot extrusion state performance meets the standard requirements and can be delivered directly.

Source: China Super Duplex Seamless Pipe Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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