Zhuojin Pipe Fitting Co., LtdHot Extrusion Plus Cold Rolling Manufacturing of 022Cr25Ni7Mo4N Super Duplex Stainless Steel Seamless Pipe
Literature Overview
This paper by Ding Wenyuan et al. (2012), published in the journal Steel Pipe, documents the development of a "hot extrusion + cold rolling" process technology for manufacturing 022Cr25Ni7Mo4N super duplex stainless steel seamless pipes at Zhejiang Jiuli Special Materials Technology Co., Ltd. The product had long been dependent on imports, and this work represents a successful domestic production achievement. The study covers hot working, cold working, and solution heat treatment process parameters.
Core Technical Content
022Cr25Ni7Mo4N is a super duplex stainless steel with a nominal composition of approximately 25% Cr, 7% Ni, and 4% Mo, with nitrogen added for additional strength and pitting corrosion resistance. The equivalent chromium number (Cr_eq) is approximately 37-38, and the pitting resistance equivalent number (PREN) is approximately 42-44, providing excellent resistance to pitting and crevice corrosion in aggressive chloride environments.
Process Technology Overview
| Process Stage | Key Parameters | Technical Challenges |
|---|---|---|
| Billet preparation | Composition control, ingot quality | Inclusion control, segregation minimization |
| Hot extrusion | Extrusion temperature, reduction ratio, speed | High flow stress, oxide scale control |
| Cold rolling | Rolling temperature, reduction ratio, pass schedule | Work hardening, dimensional accuracy |
| Solution heat treatment | Temperature, time, cooling rate | Phase balance, sigma phase prevention |
Hot Extrusion Process Analysis
The hot extrusion of super duplex stainless steel presents unique challenges due to the high flow stress and limited hot working window. The optimal extrusion temperature range is typically 1100-1250°C, with the upper limit constrained by excessive grain growth and the lower limit by the onset of dynamic strain aging. The reduction ratio during extrusion must be sufficient to achieve adequate grain refinement and to break up any coarse grain structure from the ingot.
Key considerations for hot extrusion include:
- Extrusion temperature control: The temperature must be carefully controlled to avoid both excessive grain growth at high temperatures and work hardening at low temperatures. Preheating the billet to 1150-1200°C before extrusion is recommended.
- Container temperature: The extrusion container should be preheated to 1050-1150°C to reduce the extrusion force and prevent surface cracking.
- Extrusion speed: Moderate extrusion speeds of 20-50 mm/s are recommended to allow adequate dynamic recrystallization while avoiding excessive temperature rise.
- Oxide scale control: Super duplex stainless steels form thick and adherent oxide scales at high temperatures. The scale must be removed after extrusion to prevent surface defects in subsequent cold rolling.
Cold Rolling Process Analysis
Cold rolling is used after hot extrusion to achieve the final dimensional accuracy and to refine the microstructure. The cold rolling process must be carefully controlled to avoid cracking due to the high work hardening rate of super duplex stainless steels.
| Parameter | Typical Range | Notes |
|---|---|---|
| Cold rolling temperature | 20-80°C | Lower temperatures reduce dynamic recovery |
| Total cold reduction | 30-60% | Depends on initial hot extruded dimensions |
| Pass schedule | Multiple passes with intermediate annealing | Prevents cracking |
| Surface roughness | Ra ≤ 1.6 μm | For corrosion resistance requirements |
The high work hardening rate of super duplex stainless steels means that intermediate annealing may be required between cold rolling passes to restore ductility and prevent cracking. The annealing temperature should be 1050-1100°C for a short time to avoid sigma phase formation.
Solution Heat Treatment
The solution heat treatment is critical for achieving the desired austenite-ferrite phase balance and for dissolving any sigma phase that may have formed during hot or cold working. The recommended solution treatment is 1050-1100°C for 1-2 hours followed by rapid water quenching. The cooling rate must be rapid enough to prevent the formation of intermetallic phases such as sigma (Cr-rich) and chi (Ni-rich) phases.
Phase Balance Requirements
| Phase | Target Percentage | Measurement Method |
|---|---|---|
| Ferrite | 40-60% | Magnetic permeability, metallographic analysis |
| Austenite | 40-60% | Metallographic analysis |
| Sigma phase | Not permitted | Metallographic analysis, XRD |
Quality Control and Inspection
For super duplex stainless steel seamless pipes, the following quality control measures are essential:
- Chemical analysis: Verification of Cr, Ni, Mo, N, C, S, P content per ASTM A928 or equivalent.
- Mechanical properties: Tensile strength (minimum 620 MPa), yield strength (minimum 450 MPa), elongation (minimum 15%), and hardness (maximum 32 HRC).
- Corrosion resistance: Pitting corrosion resistance per ASTM G48, crevice corrosion resistance per ASTM G110, and intergranular corrosion resistance per ASTM A262.
- Dimensional inspection: Outer diameter, wall thickness, and straightness per ASTM A928 or EN 10216-5.
- Non-destructive testing: Eddy current testing (ET) for surface and near-surface defects, ultrasonic testing (UT) for internal defects.
Study Insights and Reflections
This paper documents a significant achievement in the domestic production of super duplex stainless steel seamless pipes, which had previously been imported exclusively. The "hot extrusion + cold rolling" route is a well-established approach for producing high-quality seamless pipes of difficult-to-work alloys, but the specific process parameters for 022Cr25Ni7Mo4N require careful optimization due to the alloy's high flow stress and limited processing window. The emphasis on solution heat treatment is particularly important, as the phase balance directly affects both mechanical properties and corrosion resistance. From a manufacturing quality perspective, the key challenge is maintaining consistency across production batches, which requires tight control of billet quality, extrusion parameters, rolling schedules, and heat treatment cycles. The successful domestic production of this alloy represents a meaningful step toward reducing import dependency for critical materials in the oil and gas, chemical processing, and marine industries.