Internal Surface Defect Analysis and Prevention Measures for 34CrMo4 Gas Cylinder Seamless Steel Tubes

Literature Overview

The paper by Wang En'na and colleagues, published in Sichuan Chemical Industry (2014, Vol. 17, Issue 4, pp. 43-46), presents a detailed analysis of internal surface "bubble" defects found in 34CrMo4 seamless steel tubes intended for gas cylinder applications, and proposes effective preventive measures. The study was conducted at Pangang Group Chengdu Steel Vanadium Co., Ltd., and it addresses a critical quality issue that can compromise the safety and performance of gas cylinders, which are pressure vessels subject to strict regulatory requirements. The defect analysis employs a systematic approach that combines chemical composition analysis, metallographic examination, and production process review.

Defect Characterisation and Chemical Analysis

The "bubble" defects observed on the internal surface of the 34CrMo4 tubes manifest as raised, blister-like protrusions that are typically circular or oval in shape, with diameters ranging from a few millimetres to several tens of millimetres. These defects are distinct from the more common internal surface defects such as scratches, pits, and cracks, and they pose a particular challenge because they can initiate fatigue cracks under cyclic pressure loading, leading to catastrophic failure of the gas cylinder.

The chemical composition analysis of the defective tubes revealed that the composition was within the specified range for 34CrMo4 steel:

The chemical composition was found to be normal, indicating that the defect was not caused by chemical segregation or abnormal composition. This finding directed the investigation towards the production process as the root cause of the defect.

Defect Formation Mechanism

Through metallographic examination and production process review, the authors identified the root cause of the bubble defects: localised heating of the billet end during the reheating process. The mechanism is as follows:

1. During the reheating of the steel billet for the subsequent hot working operation, the end of the billet is subjected to localised heating, either due to uneven furnace temperature distribution or due to the use of localised heating devices to compensate for end-of-billet heat loss.
2. The localised heating causes a temperature gradient across the billet end, with the heated zone reaching a temperature significantly higher than the surrounding material.
3. The high-temperature zone undergoes excessive austenite grain growth, and the thermal stress generated by the temperature gradient can cause microcracking in the surface layer of the billet.
4. During the subsequent hot working (piercing and rolling), the microcracks in the heated zone are opened and flattened, forming the observed bubble defects on the internal surface of the tube.

The metallographic examination of the defective area revealed:

• Coarse austenite grain structure in the defect zone, with grain sizes significantly larger than in the surrounding material.
• Presence of microcracks along the grain boundaries, indicating thermal cracking.
• Oxidation inclusions and non-metallic inclusions concentrated in the defect zone, suggesting that the localised heating also promoted the oxidation of the surface.

Prevention Measures and Their Effectiveness

Based on the identified root cause, the authors proposed and implemented the following preventive measures:

1. Uniform reheating: The reheating furnace temperature profile was optimised to ensure uniform heating of the entire billet, including the ends. This was achieved by adjusting the burner configuration and the fuel-air ratio, and by using thermocouples to monitor the temperature at multiple points along the billet length.
2. Elimination of localised heating: Any localised heating devices or practices that caused uneven heating of the billet end were discontinued. The billet was reheated uniformly throughout its length, and the temperature difference between the end and the centre was limited to within ±20°C.
3. Improved reheating practice: The reheating time and temperature were optimised to achieve the required working temperature without excessive grain growth. The reheating temperature was reduced from the previous practice to 1150-1200°C, which is sufficient for hot working but avoids excessive grain growth.
4. Enhanced quality control: Additional inspection steps were introduced at the reheating stage, including visual inspection of the billet surface for signs of overheating and metallographic examination of sample billets to verify the grain structure.

The implementation of these measures resulted in the complete elimination of the bubble defects from the production of 34CrMo4 gas cylinder tubes. The yield rate of the product was improved and stabilised, and the quality consistency was significantly enhanced.

Engineering Practice Integration

For engineers involved in the production of gas cylinder tubes and other high-integrity seamless tubes, this case study provides valuable lessons on the importance of understanding the root cause of defects and implementing systematic preventive measures. The key takeaways are:

• Chemical composition analysis alone is not sufficient to identify the root cause of defects; a comprehensive investigation that includes metallographic examination and production process review is essential.
• The reheating process is a critical control point in seamless tube production, and any deviation from the optimal reheating practice can lead to quality defects.
• The implementation of preventive measures requires a systematic approach that addresses the root cause rather than treating the symptoms.

The study also highlights the importance of quality management systems in seamless tube production. The implementation of the preventive measures was supported by a robust quality management system that included process documentation, operator training, and regular auditing. This systematic approach ensured that the preventive measures were consistently implemented and that the quality improvements were sustained over time.

Study Insights and Implications

This paper demonstrates the power of a systematic defect analysis approach in identifying and eliminating quality problems in seamless tube production. The authors employed a logical and methodical investigation that started with defect characterisation, proceeded to chemical analysis, and culminated in a production process review that identified the root cause. This approach is consistent with established quality management methodologies such as the 5-Why analysis and the Fishbone diagram, and it is a model for other quality investigations in the metallurgical industry.

The specific finding that localised reheating of the billet end causes bubble defects has broader implications for the seamless tube industry. Many production facilities use localised heating to compensate for end-of-billet heat loss, and this practice may be contributing to quality defects in other tube grades and applications. The study recommends that all producers of high-integrity seamless tubes review their reheating practices and implement uniform reheating procedures.

For gas cylinder manufacturers, the elimination of internal surface defects is critical for ensuring the safety and reliability of the product. Gas cylinders are pressure vessels that are subject to cyclic loading and are regulated by strict safety standards. The presence of internal surface defects can initiate fatigue cracks that propagate under cyclic pressure loading, leading to catastrophic failure. The preventive measures described in this study provide a practical solution for ensuring the quality of gas cylinder tubes and maintaining compliance with safety regulations. Future work should focus on the long-term performance evaluation of the improved tubes under actual service conditions, including fatigue testing, corrosion testing, and pressure cycling testing, to provide a comprehensive basis for material qualification and safety assessment.