Internal Crack Analysis in GCr15 Bearing Steel Pipe Produced from Square Billets

Literature Overview

This study by Liu Hongyi and Li Xifeng from Bensteel Group's Special Steel Development Department, published in Metal World in 2015, addresses a critical quality issue in bearing steel pipe production. The authors investigate the root cause of internal cracks found in GCr15 bearing steel pipe manufactured from square billets with dimensions of 235 mm × 265 mm. The research is significant because bearing steel pipes are used to produce rolling elements and raceways in rolling bearings, where internal integrity is paramount for fatigue life and service reliability.

Technical Background and Problem Statement

GCr15 is the standard bearing steel grade used in China for high-quality bearing components, corresponding to AISI 52100 in the American system. The steel contains approximately 1.00% carbon and 1.50% chromium, providing excellent hardness, wear resistance, and fatigue strength after proper heat treatment. In this case, the square billets were produced via continuous casting and subsequently processed into bearing steel pipes through hot rolling and cold drawing operations.

The critical issue was that internal cracks were discovered during end-use, presenting as straight, through-length defects. This crack morphology is highly diagnostic—it suggests a mechanical origin rather than a metallurgical one, as casting defects typically manifest as branched, irregular patterns. The fact that the cracks ran longitudinally and continuously along the pipe length pointed toward a process-induced mechanism rather than inherent material weakness.

Investigation Methodology

The investigation followed a systematic approach combining product inspection review with microstructural analysis. The team first verified that all product ex-factory inspection results fully complied with GB/T 18254, the Chinese national standard for bearing steel. This confirmed that conventional inspection methods—such as ultrasonic testing, eddy current testing, and surface magnetic particle inspection—failed to detect the internal crack, highlighting a significant limitation in standard quality assurance protocols.

The authors then performed a longitudinal section cut perpendicular to the crack direction to expose the crack face for detailed examination. Metallographic analysis and scanning electron microscopy were employed to characterize both the crack morphology and the base metal microstructure surrounding the defect.

Key Findings and Root Cause Analysis

The metallurgical examination revealed that the base metal microstructure was essentially sound, with no evidence of internal porosity, segregation, or casting defects that would typically initiate internal cracking. The general looseness observed in the matrix phase was within acceptable limits for continuously cast bearing steel.

The scanning electron microscopy analysis of the crack surface provided the definitive evidence. The crack exhibited features characteristic of mechanical scratching during the cold drawing process. During cold drawing, the pipe is pulled through a die, and if there is improper lubrication, die surface damage, or contamination such as oxide scale fragments, the tube wall can be mechanically scored. These scratches act as stress concentrators that can propagate under subsequent mechanical loading, particularly in bearing applications where cyclic contact stresses are present.

Engineering Implications and Countermeasures

The findings carry important implications for quality control in bearing steel pipe manufacturing. The fact that standard ex-factory inspections passed despite the presence of internal cracks indicates that additional inspection methods may be required for critical applications. The following countermeasures are recommended based on the root cause analysis:

1. Die condition monitoring: Implement regular inspection of cold drawing dies for surface damage, pitting, or buildup of oxide scale that could scratch the pipe surface.
2. Lubrication system optimization: Ensure adequate lubricant coverage and film thickness during the cold drawing operation to prevent direct metal-to-die contact.
3. Pre-drawing surface preparation: Employ thorough pickling and passivation of the pipe surface before cold drawing to remove any loose oxide scale that could act as an abrasive agent.
4. Supplementary NDT: Consider adding longitudinal ultrasonic scanning or phased array UT for critical bearing steel pipe applications where internal integrity is essential.
5. Process parameter control: Monitor and control drawing speed, reduction per pass, and die clearance to minimize the risk of mechanical scoring.

Summary

This case study provides a valuable example of how seemingly sound products can harbor critical defects that escape conventional quality inspection. The root cause—mechanical scratching during cold drawing—is a process-related issue rather than a material defect, which shifts the focus of quality improvement from the steelmaking process to the mechanical working operations. Engineers working with bearing steel pipes should pay particular attention to the cold drawing process parameters and die condition monitoring, as these factors directly influence the internal surface integrity of the finished product. The study reinforces the importance of combining metallurgical analysis with process review when investigating field failures, and it underscores the need for enhanced inspection protocols in applications where internal defects can lead to catastrophic bearing failure.