Zhuojin Pipe Fitting Co., LtdProcess Development and Application of Small Diameter GCr15 Bearing Steel Pipe Using Hot Continuous Rolling
Literature Overview
The paper by Wang Zenghai, Wen Bo, Gao Heping, and Lin Zhen, published in 2021 in the journal "Steel Pipe," presents the development and application of a hot continuous rolling process for producing small diameter GCr15 bearing steel pipes. The study addresses the geometric dimensions, melting composition, and production process flow, and conducts small-batch trial production. The primary challenge identified was internal folding (internal fold defects), which was attributed to potential defects in the continuous casting billet, unreasonable ring furnace heating practices, and improper piercing mill parameter settings. This research is significant for the bearing steel pipe manufacturing industry, as it explores the feasibility of hot continuous rolling for small diameter applications.
Core Technical Content
GCr15 is a high-carbon chromium bearing steel (equivalent to AISI 52100) that is widely used in rolling element bearings due to its excellent wear resistance, high contact fatigue strength, and good hardenability. The production of small diameter GCr15 tubes requires precise control of composition, microstructure, and internal quality to meet the demanding requirements of bearing applications.
Material Specification and Composition
| Parameter | Specification |
|---|---|
| Grade | GCr15 (GB/T 18254) |
| Carbon content | 0.95–1.05% |
| Chromium content | 1.40–1.65% |
| Silicon content | ≤0.30% |
| Manganese content | ≤0.35% |
| Sulfur content | ≤0.030% |
| Phosphorus content | ≤0.030% |
| Residual elements | Strictly controlled |
The composition must be carefully controlled to ensure:
- Adequate hardenability for bearing applications
- Sufficient carbon for high contact fatigue strength
- Controlled chromium for wear resistance and corrosion resistance
- Low sulfur and phosphorus to prevent inclusion formation and improve transverse properties
- Strict control of residual elements (Ni, Cu, Sn, As, Pb) to prevent detrimental effects on bearing performance
Production Process Flow
The developed production process for small diameter GCr15 bearing steel pipes includes the following steps:
- Continuous casting – production of GCr15 continuous casting billets with controlled composition and internal quality.
- Ring furnace heating – heating of billets to the piercing temperature (typically 1150–1250 °C for GCr15).
- Piercing – formation of the initial tube hole using a mandrel piercing mill.
- Hot continuous rolling – reduction of the pierced tube through a series of rolling stands to achieve the final dimensions.
- Cooling and straightening – controlled cooling and straightening of the rolled tubes.
- Inspection and quality control – dimensional inspection, internal quality verification, and surface quality assessment.
Internal Folding Defect Analysis
The study identified internal folding as the primary quality issue encountered during trial production. Internal folding refers to surface folds or laps that form on the internal surface of the tube, which can serve as crack initiation sites during subsequent processing or service. The root cause analysis identified three potential contributing factors:
| Potential Cause | Mechanism | Countermeasure |
|---|---|---|
| Continuous casting billet defects | Center porosity, center cracks, or segregation in the billet propagate during piercing and rolling | Improve casting quality through optimized cooling rates, controlled solidification, and billet inspection |
| Ring furnace heating practices | Uneven heating, overheating, or underheating causes non-uniform plasticity and flow behavior | Optimize heating temperature, holding time, and temperature uniformity; implement billet preheating |
| Piercing mill parameters | Improper mandrel diameter, roll gap, or piercing angle causes non-uniform deformation | Optimize piercing parameters based on material properties and billet dimensions; conduct parameter trials |
The internal folding defect is particularly concerning for bearing steel pipes because:
- It creates stress concentration points that reduce contact fatigue life
- It can propagate during subsequent cold drawing or cold rolling operations
- It is difficult to detect using conventional non-destructive testing methods
- It may not be visible on the external surface, making detection challenging
Engineering Practice Implications
Process Development Approach
The study demonstrates a systematic approach to process development for a new product:
- Material characterization – understanding the composition, microstructure, and properties of the starting material is essential for process design.
- Pilot production – small-batch trial production allows for process validation and defect identification before full-scale production.
- Defect analysis – systematic analysis of defects encountered during trial production provides valuable insights for process improvement.
- Iterative improvement – the process development should be treated as an iterative process, with each trial providing information for the next iteration.
Quality Control Strategy
For GCr15 bearing steel pipe production, the following quality control measures should be implemented:
- Billet quality control – continuous casting billets must be inspected for internal defects using ultrasonic testing (UT) or other non-destructive methods before piercing.
- Temperature monitoring – the heating temperature must be precisely controlled and monitored throughout the production process.
- Piercing parameter optimization – the piercing parameters must be optimized for each billet size and material condition.
- Internal surface inspection – internal surface quality must be verified using borescope inspection or ultrasonic testing.
- Mechanical property testing – tensile strength, hardness, and impact properties must be verified to ensure compliance with bearing steel requirements.
Study Insights and Reflections
This study highlights the challenges of extending hot continuous rolling technology to small diameter bearing steel pipes. The transition from larger diameter tubes to smaller diameters introduces new challenges related to deformation homogeneity, temperature control, and defect sensitivity. The internal folding defect identified in this study is a common challenge in hot rolled tube production, particularly for high-carbon steels that are more susceptible to surface defects due to their lower ductility at elevated temperatures.
The systematic approach to defect analysis demonstrated in this study is exemplary. Rather than simply reporting the defect, the authors identified potential root causes and proposed specific areas for further investigation. This approach is consistent with best practices in process improvement and quality management, where understanding the root cause is essential for effective problem solving.
For the bearing steel pipe manufacturing industry, this study provides valuable information about the feasibility of hot continuous rolling for small diameter applications. The identified challenges and proposed countermeasures offer a roadmap for further process development. As the demand for smaller diameter bearing steel pipes grows, the continued development and optimization of hot continuous rolling technology will be increasingly important.