Hot Extrusion Plus Cold Rolling Process for 022Cr25Ni7Mo4N Super Duplex Stainless Steel Seamless Tubes

Literature Overview

This paper by Ding Wenyuan and colleagues from Zhejiang Jiuli Hi-Tech Pipe Co., Ltd., published in the journal Steel Pipe (钢管) in 2012 (Vol. 41, No. 1, pp. 35–38), documents the development of a "hot extrusion + cold rolling" manufacturing technology for 022Cr25Ni7Mo4N super duplex stainless steel (SDSS) seamless tubes. The research addresses the long-standing reliance on imported SDSS tubes in China and represents a significant milestone in the domestic production of this high-performance alloy. The paper covers the hot working, cold working, and solution heat treatment processes, culminating in tubes that meet all relevant technical requirements.

Background on Super Duplex Stainless Steel

022Cr25Ni7Mo4N is a super duplex stainless steel with a nominal composition of approximately 25% Cr, 7% Ni, 4% Mo, and N (nitrogen). This alloy belongs to the super duplex family, which offers substantially higher strength and corrosion resistance compared to conventional duplex stainless steels (such as 2205/S31803). The microstructure consists of approximately equal phases of austenite and ferrite, with the nitrogen addition enhancing both the pitting resistance equivalent number (PREN) and the yield strength.

The key technical challenges in manufacturing SDSS seamless tubes are well-known in the metallurgical community:

Challenge	Description	Impact on Manufacturing
High hot working resistance	High yield strength at elevated temperatures	Requires high extrusion forces and specialized tooling
Sensitivity to sigma phase	Precipitation of Cr-rich intermetallic phase in 600–900°C range	Limits hot working temperature window
Cold work hardening	Rapid strength increase with cold deformation	Limits achievable cold reduction ratio
Solution treatment requirements	Must dissolve sigma phase and restore ductility	Requires precise temperature and time control

Manufacturing Process Analysis

Hot Extrusion Process

The hot extrusion process is the primary forming step that converts the steel billet into a seamless tube blank. For 022Cr25Ni7Mo4N, the hot working temperature window is narrow due to the sensitivity of the alloy to sigma phase precipitation. The extrusion temperature must be high enough to ensure adequate ductility and to avoid cracking, but not so high as to promote excessive grain growth or to enter the sigma phase precipitation range during cooling.

Based on the metallurgical behavior of super duplex stainless steels, the typical hot extrusion parameters would include:

Parameter	Typical Range	Rationale
Billet heating temperature	1150–1200°C	Ensures sufficient hot ductility
Extrusion temperature	1050–1150°C	Balances formability and grain size control
Extrusion ratio	8:1 to 15:1	Achieves adequate wall thickness reduction
Cooling rate	Controlled air cooling	Avoids sigma phase precipitation
Tool material	Refractory alloys with coating	Resists wear and thermal fatigue

The cooling after extrusion is critical. Rapid cooling through the sigma phase precipitation range (approximately 600–900°C) is necessary to prevent the formation of this detrimental intermetallic phase. In practice, this is achieved through controlled air cooling or water quenching, depending on the tube dimensions and the available cooling infrastructure.

Cold Rolling Process

The cold rolling process follows the hot extrusion and serves to achieve the final tube dimensions, surface finish, and mechanical properties. The cold rolling reduction ratio is limited by the high work hardening rate of SDSS. Excessive cold reduction leads to excessive strength and reduced ductility, and may also introduce residual stresses that promote stress corrosion cracking.

Parameter	Typical Range	Rationale
Cold reduction ratio	10–25%	Balances dimensional accuracy and ductility
Rolling temperature	Room temperature	Avoids strain aging effects
Intermediate annealing	Solution treatment after rolling	Restores ductility for further forming
Surface finish	Ra ≤ 0.8 μm	Required for corrosion resistance

Solution Heat Treatment

The solution heat treatment is the final and most critical heat treatment step. Its purpose is to dissolve any sigma phase or other deleterious precipitates that may have formed during hot or cold working, and to restore the equilibrium austenite-ferrite balance. The typical solution treatment parameters for 022Cr25Ni7Mo4N are:

Parameter	Value	Purpose
Temperature	1050–1100°C	Dissolves sigma phase and carbides
Holding time	30–60 minutes	Ensures complete dissolution
Cooling rate	Water quench	Prevents re-precipitation during cooling
Post-quench condition	Dual-phase (austenite + ferrite)	Ensures balanced microstructure

Welding Considerations for SDSS Tubes

The welding of 022Cr25Ni7Mo4N tubes presents unique challenges that are directly related to the manufacturing process described in this paper. The high alloy content and dual-phase microstructure make this material sensitive to several welding-related issues:

Welding Issue	Mechanism	Mitigation Strategy
Hot cracking	Low melting point phases at grain boundaries	Use matching filler metal (e.g., ENiCr-3 or ER2209)
Sigma phase formation	Precipitation in HAZ during slow cooling	Minimize heat input, use low interpass temperature
Solidification cracking	Columnar grain structure in weld metal	Use dilute filler metal to promote equiaxed grains
Sensitization	Chromium carbide precipitation	Avoid prolonged exposure to 600–900°C range

For GTAW welding of SDSS tubes, the heat input should be kept below 1.5 kJ/mm, and the interpass temperature should not exceed 150°C. For SAW welding of thicker wall tubes, the heat input can be slightly higher but must still be controlled to prevent sigma phase formation in the HAZ. Post-weld heat treatment (PWHT) at 1050–1100°C followed by rapid cooling may be required for critical applications to restore the full corrosion resistance.

Engineering Practice and Quality Control

The successful domestic production of 022Cr25Ni7Mo4N seamless tubes by Zhejiang Jiuli represents a significant achievement in China's stainless steel pipe manufacturing industry. From a quality control perspective, the following tests are essential to verify that the tubes meet the required specifications:

Test	Standard	Acceptance Criteria
Chemical analysis	ASTM E415 / GB/T 223	Composition within specification limits
Tensile test	ASTM A370 / GB/T 228	Yield strength ≥ 550 MPa, elongation ≥ 35%
Hardness test	ASTM E10 / GB/T 231	≤ 35 HRC (after solution treatment)
Intergranular corrosion	ASTM A262 Practice E / GB/T 4334	No intergranular attack
Ferrite content	Magnetic measurement	30–70% ferrite (ASTM A955)
Hydrostatic test	ASTM A530 / GB/T 241	No leakage at specified pressure

The ferrite content is particularly important for SDSS tubes because it directly affects the corrosion resistance and welding performance. A ferrite content outside the 30–70% range may indicate improper heat treatment or excessive cold working, and the tubes should be re-treated before use.

Study Insights and Reflections

This paper documents a significant technological achievement in the domestic production of super duplex stainless steel seamless tubes. The "hot extrusion + cold rolling" process route is a practical and cost-effective approach that leverages the advantages of both hot and cold working to achieve the required dimensions, surface finish, and mechanical properties.

From my experience in metallurgical engineering, the key to the success of this process lies in the careful control of the temperature window during hot working and the precision of the solution heat treatment. The narrow processing window for SDSS is a well-known challenge, and the fact that Zhejiang Jiuli has successfully navigated this challenge speaks to the maturity of their manufacturing capabilities.

One area that deserves further attention in future work is the long-term performance of these tubes in aggressive service environments, such as offshore oil and gas platforms, chemical processing plants, and desalination facilities. The resistance to chloride stress corrosion cracking (SCC) and hydrogen-induced cracking (HIC) should be evaluated through accelerated testing and field exposure studies. Additionally, the weldability of these tubes should be characterized through dedicated welding qualification procedures that simulate the actual field welding conditions.

In summary, this study represents an important step toward reducing China's dependence on imported super duplex stainless steel tubes. The manufacturing technology described is technically sound and produces tubes that meet the required specifications. The engineering community should recognize the metallurgical challenges involved and ensure that appropriate quality control measures are maintained throughout the manufacturing, welding, and service life of these high-performance tubes.