Development of Φ508mm×50mm WB36 Steel Pipe Using Cyclical Rolling Process

Literature Overview

This paper by Wang Yong, Ran Xu, and Zhang Min, published in Special Steel (2011, Vol. 32, No. 5, pp. 60–62), documents the successful development of large-diameter WB36 steel pipes with dimensions of Φ508mm×50mm using a cyclical rolling (cyclo-roller) process. The research was conducted at Hunan Hengyang Steel Pipe (Group) Co., Ltd., representing a significant industrial achievement in the production of large-diameter alloy steel pipes. WB36 is a high-strength low-alloy (HSLA) steel grade widely used in demanding applications such as pressure vessels, heat exchangers, and nuclear power plant components.

Material Specification and Metallurgical Flow

The WB36 steel grade specified in this study has the following chemical composition:

The metallurgical flow was as follows:

1. 45 t EAF melting: Electric arc furnace melting with precise alloy addition to achieve target composition.
2. LF refining: Ladle furnace refining for chemistry adjustment, desulfurization, and temperature control.
3. VD treatment: Vacuum degassing for hydrogen removal and final chemistry refinement.
4. Φ600 mm casting: Continuous casting to produce square billets of 600 mm × 600 mm cross-section.
5. Piercing: Hot piercing of billets to produce seamless pipe blanks (Φ600 mm × 70 mm).
6. Cyclical rolling: Multi-pass rolling on a cyclical rolling mill to produce the final Φ508 mm × 50 mm pipe dimensions.

Cyclical Rolling Process Parameters

The cyclical rolling process is a specialized manufacturing technique that uses a rotating rolling mill with multiple passes to gradually reduce the pipe diameter while increasing the wall thickness. Key process parameters were carefully controlled:

The triaxial compressive stress state during cyclical rolling is a critical process feature that distinguishes this method from conventional piercing and rolling operations. The compressive stress state:

• Suppresses the initiation and propagation of internal defects such as cracks and inclusions.
• Promotes uniform deformation throughout the pipe cross-section.
• Enhances the transverse mechanical properties by breaking up any columnar grain structures.
• Improves the overall material homogeneity and isotropy.

Heat Treatment and Microstructure

After cyclical rolling, the WB36 pipes underwent quenching and tempering heat treatment to achieve the target mechanical properties. The resulting microstructure consisted of:

• Ferrite matrix: Providing ductility and toughness.
• Bainite phases: Contributing strength through fine-scale transformation products.

The ferrite-bainite microstructure is characteristic of HSLA steels in the medium carbon range with Mo, Ni, and Nb additions. The microalloying elements play specific roles:

Quality Control Considerations

Producing large-diameter alloy steel pipes like Φ508 mm × 50 mm WB36 presents several quality control challenges:

• Chemistry control: Achieving tight composition tolerances, particularly for microalloying elements like Nb (0.02%) and Al (0.05%), requires precise addition control during melting and refining.
• Homogeneity: Large-diameter pipes are susceptible to macrosegregation from the casting process. The homogenization heat treatment at 1250°C is critical for dissolving segregation bands and achieving uniform composition.
• Surface quality: The controlled heating atmosphere in the ring furnace prevents excessive scale formation, which is particularly important for large-diameter pipes where scale removal can be challenging.
• Dimensional accuracy: The cyclical rolling process must be precisely controlled to achieve uniform wall thickness and roundness across the entire pipe length.
• Mechanical properties: The final quenched-and-tempered condition must deliver consistent mechanical properties throughout the pipe cross-section, including the critical transverse and longitudinal directions.

Engineering Practice Implications

The successful development of Φ508 mm × 50 mm WB36 pipes using cyclical rolling has significant implications for industrial applications:

• Pressure vessel manufacturing: Large-diameter WB36 pipes are essential for manufacturing large pressure vessels used in petrochemical, power generation, and nuclear industries.
• Heat exchanger tubes: The combination of high strength, good toughness, and corrosion resistance makes WB36 suitable for demanding heat exchanger applications.
• Nuclear applications: The controlled chemistry (low P, S) and excellent mechanical properties make WB36 suitable for nuclear power plant components, subject to appropriate qualification testing.
• Manufacturing efficiency: The cyclical rolling process offers advantages over conventional piercing and rotary piercing methods for large-diameter pipes, including better control of internal quality and reduced material waste.

Key Reflections and Insights

This paper documents a significant industrial achievement in the production of large-diameter HSLA steel pipes. The cyclical rolling process, while conceptually simple, requires precise control of multiple process parameters to achieve the desired product quality. The triaxial compressive stress state during rolling is a key metallurgical advantage that contributes to superior internal quality compared to conventional rolling methods.

From a metallurgical perspective, the WB36 composition represents a well-balanced HSLA design. The combination of Mo, Ni, Cu, and Nb provides a synergistic strengthening mechanism that delivers good strength-toughness combinations at reasonable carbon levels. The low carbon content (0.16%) ensures good weldability, which is essential for pipe applications where field welding is common.

The controlled heating atmosphere during the ring furnace stage is particularly noteworthy. For large-diameter pipes, surface oxidation during heating can lead to scale defects that may not be fully removed during descaling, potentially initiating fatigue cracks or corrosion damage in service. The controlled atmosphere approach demonstrates a sophisticated understanding of the interaction between processing and final product quality.

The homogenization temperature of 1250°C is at the upper end of typical practice for HSLA steels. While this temperature effectively dissolves segregation bands, it also risks excessive grain coarsening, which can adversely affect toughness. The balance between homogenization effectiveness and grain size control represents a critical process optimization challenge.

This work exemplifies the integration of metallurgical science with industrial manufacturing practice. The successful production of Φ508 mm × 50 mm WB36 pipes demonstrates that the cyclical rolling process can be scaled to produce large-diameter alloy steel pipes with controlled quality, opening up new possibilities for applications that previously required imported or custom-fabricated pipe products. Future work should focus on further optimizing the heat treatment parameters to maximize the strength-toughness combination and on developing qualification databases for nuclear and other high-integrity applications.

The development of domestic production capability for large-diameter WB36 pipes represents a significant step forward in reducing dependence on imports and ensuring supply chain security for critical industrial applications, particularly in the nuclear power and petrochemical sectors where WB36 is a preferred material grade.