Rolling Process of 35CrMo Steel Ultra-Heavy-Wall Seamless Pipe

Literature Overview

This paper, authored by Li Yuanrui, Hui Zhiqiang, Zhang Ying, and Zheng Shijian from Chongqing University School of Materials Science and Engineering, published in Journal of Chongqing University (Natural Science Edition, Vol. 31, No. 12, 2008, pp. 1360-1364), documents the development and optimization of a hot rolling process for producing ultra-heavy-wall seamless pipes from 35CrMo alloy steel. The project was a collaborative effort between Panzhihua Chengde Steel Company and Chongqing University. The specific product targeted was a Φ215 mm × 72.5 mm seamless pipe with a diameter-to-wall ratio of only 2.97, which represents a significant manufacturing challenge due to the extreme wall thickness.

Technical Background and Challenges

Ultra-heavy-wall seamless pipes find applications in high-pressure piping systems, hydraulic cylinders, and heavy equipment manufacturing where the combination of high strength, toughness, and thick walls is required. The 35CrMo steel grade offers excellent mechanical properties including high yield strength (typically 785 MPa minimum in quenched and tempered condition), good fatigue resistance, and favorable weldability, making it suitable for demanding applications.

The primary manufacturing challenges for this product include:

Process Development and Optimization

Initial Process Route

The manufacturing process begins with a 305 mm × 305 mm precision forged square billet of 35CrMo steel, which is thermally shaped into a cylindrical tube blank, followed by piercing to create the center hole. The tube blank is then hot rolled on a Φ216 mm Pilger cycle rolling mill to achieve the final dimensions of Φ215 mm × 72.5 mm.

Trial Production and Problem Identification

Two batches of trial rolling were conducted using 20 steel billets of the same dimensions (12 pieces per batch) to develop and validate the process parameters before committing to the more expensive 35CrMo alloy steel. The primary quality issue identified was wall thickness eccentricity, which is the deviation of actual wall thickness from the nominal value measured at different angular positions around the pipe circumference.

Root Cause Analysis and Corrective Actions

Using a systematic FMEA approach, the following root causes and corrective measures were identified:

1. Billet heating uniformity: Non-uniform temperature distribution in the billet caused differential deformation during rolling. Corrective action: improved heating furnace temperature control and extended soaking time to ensure uniform core temperature.
2. Piercing quality: Incomplete or uneven piercing created initial wall thickness variation that was amplified during subsequent rolling. Corrective action: optimization of piercing parameters including plug shape, piercing speed, and temperature.
3. Equipment adjustment: Misalignment of the Pilger mill rolls caused systematic eccentricity. Corrective action: precision alignment of the rolling mill and verification of roll geometry.
4. Roll groove design: The initial roll groove shape was not optimized for the extreme diameter-to-wall ratio. Corrective action: modification of roll groove profile to better control the deformation zone and material flow.
5. Feed amount adjustment: Excessive feed per revolution caused unstable deformation and increased eccentricity. Corrective action: reduction of feed amount to allow more uniform material flow.

Quality Verification

The successfully produced 35CrMo ultra-heavy-wall seamless pipes were subjected to comprehensive quality verification:

• Geometric dimensions: Outer diameter, wall thickness, and wall eccentricity measurements confirmed compliance with dimensional tolerances.
• Metallographic examination: Microstructure analysis at different radial positions across the wall thickness confirmed uniform grain structure and absence of segregation or banding.
• Mechanical properties: Tensile strength, yield strength, elongation, and impact toughness were verified at multiple locations on the pipe cross-section to ensure homogeneity.

Engineering Practice Insights

This case study exemplifies the systematic approach required for developing new rolling processes for challenging products. Several lessons are particularly relevant to steel pipe manufacturing engineers:

1. Trial production with substitute material: Using 20 steel for process development before committing to expensive alloy steel is a cost-effective and practical approach. The deformation behavior of plain carbon steel is similar enough to 35CrMo for process parameter optimization, while the lower material cost allows for more extensive experimentation.
2. Wall eccentricity control: For ultra-heavy-wall pipes, wall eccentricity is the dominant quality risk. The multi-factor approach to its control (heating, piercing, equipment alignment, roll design, and feed parameters) demonstrates that no single measure is sufficient.
3. Diameter-to-wall ratio limitations: The achieved ratio of 2.97 is near the practical limit for Pilger rolling technology. For even thicker walls, alternative processes such as extrusion or forging may be required. Engineers should establish process capability limits early in project planning.
4. Alloy steel considerations: 35CrMo requires careful temperature control during hot working to avoid excessive grain growth and to ensure proper austenitization. The rolling temperature window is narrower than for plain carbon steels, and mill scale formation is more pronounced.

The successful development of this process demonstrates the feasibility of producing ultra-heavy-wall seamless pipes through optimized hot rolling, providing an alternative to more expensive forging or extrusion methods for specific size ranges.