Zhuojin Pipe Fitting Co., LtdAnalysis of Cold Rolling Cracking in S32750 Duplex Stainless Steel Seamless Pipe
Literature Overview
The paper authored by Nie Fei and colleagues from Taiyuan Stainless Steel Pipe Co., Ltd. and the State Key Laboratory for Advanced Stainless Steel Materials addresses a critical quality issue encountered during the cold rolling of S32750 super-duplex stainless steel seamless pipes. The study systematically investigates the root cause of cracking during cold rolling operations, ultimately identifying intermetallic phases as the primary culprit rather than conventional suspects such as cold rolling process parameters or inclusion defects. This finding carries significant implications for the metallurgical processing route design of high-performance duplex stainless steel seamless pipes, a material widely demanded in offshore oil and gas, chemical processing, and desalination industries where resistance to chloride stress corrosion cracking is paramount.
Core Technical Findings
The investigation employed a combination of process analysis and physicochemical examination to isolate the cracking mechanism. The authors methodically eliminated cold rolling parameters and inclusion-related factors through comparative analysis, demonstrating that neither the rolling reduction schedule nor the type and distribution of non-metallic inclusions could account for the observed cracking pattern. The breakthrough came from metallographic examination of the as-extruded pipe stock, which revealed the presence of intermetallic phases precipitated during the hot extrusion stage.
The key insight is that intermetallic phases—primarily Laves phase (Fe₂CrMo) and sigma phase (Cr₂N)—form during the high-temperature hot extrusion process and cannot be avoided under conventional extrusion conditions. These brittle phases reduce the ductility of the matrix, creating preferential crack initiation sites during the subsequent cold rolling deformation. The paper establishes that the only viable solution is to dissolve these phases through a properly designed solution annealing (solid solution treatment) operation prior to cold rolling.
Optimal Solution Annealing Parameters
Through systematic thermal cycling experiments on S32750 seamless pipe blanks, the authors determined the optimal solution annealing window. The critical parameters are summarized below.
| Parameter | Optimal Value | Rationale |
|---|---|---|
| Solution temperature | 1100 °C | Complete dissolution of intermetallic phases; avoids excessive grain coarsening |
| Holding time | 2 × wall thickness (min) | Ensures sufficient diffusion time for phase dissolution through the full wall cross-section |
| Cooling method | Water quenching | Rapid cooling suppresses re-precipitation of sigma phase |
| Post-annealing cold rolling reduction | Up to 50% (typical) | Achievable without cracking when intermetallic phases are fully dissolved |
The linear relationship between holding time and wall thickness is a practical and elegant engineering rule. For a pipe with a 25 mm wall thickness, the holding time would be 50 minutes; for a 12 mm wall, 24 minutes. This scaling accounts for the thermal diffusivity limitations in thicker sections, ensuring the core of the wall reaches the dissolution temperature uniformly.
Engineering Practice Implications
From a quality control perspective, this study highlights a critical gap in many production workflows: the assumption that hot extruded pipe stock is ready for cold working without metallurgical verification. In practice, manufacturers of duplex stainless steel seamless pipes should implement the following controls:
- Mandatory metallographic examination of hot extruded blanks for intermetallic phase detection prior to cold rolling.
- Solution annealing as a mandatory process step, not an optional one, with parameters validated against the wall thickness.
- Post-annealing hardness and tensile testing to confirm phase dissolution and matrix homogeneity.
- Documentation of thermal cycle parameters as part of the heat treatment certificate per ASTM A860 or EN 10217 requirements.
The S32750 grade, with its nominal composition of approximately 25% Cr, 7% Ni, 7% Mo, and 0.5% N, is inherently susceptible to intermetallic precipitation in the 600–900 °C temperature range. The hot extrusion process, which typically operates at 1100–1200 °C with air cooling rates that can pass through the precipitation zone, creates conditions favorable for Laves phase formation. This is a metallurgical inevitability that must be managed through process design rather than eliminated.
Reflections and Key Questions
The study raises an important question: could modified extrusion process parameters—such as controlled cooling rates or preheating strategies—reduce the initial intermetallic phase content and thereby lower the energy demand for solution annealing? While the paper does not explore this direction, it represents a valuable area for future research. Additionally, the paper's finding that intermetallic phases cannot be avoided during extrusion challenges the conventional quality assurance approach that focuses on rolling parameters alone. Engineers must adopt a holistic process chain perspective, recognizing that defects originate upstream in the extrusion step and must be addressed through integrated metallurgical control.
In my decades of experience with stainless steel pipe production, I have encountered numerous cases where cold rolling cracking was attributed to rolling mills or tooling, when the true cause lay in the upstream heat treatment history. This paper provides a textbook example of why metallurgical traceability is essential. The recommendation is clear: no cold rolling of duplex stainless steel seamless pipe should proceed without verified solution annealing documentation, and the annealing parameters must follow the wall-thickness-dependent holding time rule established in this study.