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

Literature Overview

The paper by Wang A'na, Yin Renjie, Chen Jin, Zhang Tao, and Tong Zongsheng, published in Sichuan Chemical Industry (2014, Vol. 17, No. 4, pp. 43–46), investigates the "bulging" (鼓泡) defects observed on the internal surface of 34CrMo4 seamless steel pipes used for gas cylinder manufacturing. The study was conducted by the Steel Research Institute of Pangang Group Chengdu Steel & Wire Co., Ltd., in collaboration with the Jintang Branch of the same group. The paper analyzes the chemical composition, defect formation mechanism, and production process to identify the root cause and propose effective preventive measures.

Defect Description and Impact

The "bulging" defect refers to localized raised areas on the internal surface of the seamless steel pipe. In gas cylinder applications, the internal surface quality is critical because:

1. Gas cylinders are subjected to high internal pressures (typically 15–30 MPa), and any surface irregularity acts as a stress concentrator.
2. Internal bulges can reduce the effective wall thickness at the defect location, compromising the pressure-bearing capacity.
3. Surface defects on the internal surface can initiate fatigue cracks under cyclic pressure loading, leading to catastrophic failure.
4. Regulatory standards for gas cylinders (such as GB/T 34540, ISO 11119, and TSG 23-2021) impose strict requirements on internal surface quality.

Root Cause Analysis

The authors conducted a systematic analysis of the defect using chemical composition examination, metallographic analysis, and production process review. The key findings are as follows:

The root cause was identified as localized heating of the steel billet tail end during the production process. During the hot rolling or piercing process, the tail end of the billet cools faster than the main body due to its smaller cross-section and exposure to ambient air. To compensate for this temperature differential, operators applied local heating to the tail end. However, this local heating caused overheating of the steel in the affected region, leading to grain coarsening and localized softening. During subsequent forming operations, this overheated region deformed more readily than the surrounding material, resulting in the formation of internal bulges.

Defect Formation Mechanism

The defect formation mechanism can be described as follows:

1. Localized overheating: The tail end of the steel billet is locally heated to compensate for temperature loss, causing the steel temperature to exceed the recommended hot working range.
2. Grain coarsening: At elevated temperatures, the austenite grains grow excessively, reducing the material's strength and ductility.
3. Differential deformation: During the piercing or rolling process, the overheated region deforms more than the surrounding material due to its reduced flow stress.
4. Internal bulge formation: The differential deformation creates localized raised areas on the internal surface of the pipe.

Preventive Measures and Effectiveness

The authors propose and implement the following preventive measures:

1. Elimination of local heating: Replace local tail-end heating with a more uniform heating strategy, such as using a preheating furnace or adjusting the heating profile to maintain uniform temperature across the entire billet length.
2. Temperature monitoring: Install thermocouples at multiple points along the billet length to monitor temperature uniformity during heating and rolling.
3. Process parameter optimization: Adjust the rolling mill speed and stand gap settings to accommodate the temperature gradient without requiring local heating.
4. Quality inspection enhancement: Implement ultrasonic or eddy current inspection of the internal surface to detect bulge defects before the pipe proceeds to gas cylinder manufacturing.

The implementation of these measures effectively eliminated the bulge defect, leading to improved and stabilized yield rates for 34CrMo4 gas cylinder seamless steel pipes.

Technical Assessment and Engineering Insights

The root cause analysis in this paper is exemplary in its systematic approach. The authors did not jump to conclusions but methodically examined the chemical composition, microstructure, and production process to identify the true cause of the defect. This is a lesson for all engineers: surface defects in steel pipes are often process-related rather than material-related, and a thorough investigation of the production process is essential for effective defect prevention.

The use of local heating to compensate for temperature loss at the billet tail end is a common practice in steel mills, but it is also a common source of quality problems. The fundamental issue is that local heating creates a localized zone of overheated steel, which is difficult to control and can lead to various defects depending on the subsequent forming process. The better approach is to design the heating process to maintain uniform temperature across the entire billet, which may require additional equipment investment but yields significantly better quality.

For gas cylinder applications, the internal surface quality is paramount. The 34CrMo4 steel is a quenched and tempered low-alloy steel with excellent strength and toughness properties, but these properties are compromised if the microstructure is non-uniform due to overheating. The defect prevention measures proposed in this paper are practical and effective, and they should be adopted by all manufacturers of gas cylinder seamless steel pipes.

In my experience with gas cylinder steel pipe production, the internal surface quality is often the most challenging aspect to control. The defects are not visible from the outside, and they can only be detected through specialized inspection methods. The authors' recommendation to implement internal surface inspection is critical, and I would additionally recommend using borescope inspection or ultrasonic internal surface scanning as part of the quality assurance process.

Summary

This paper provides a clear and systematic analysis of the internal surface bulge defect in 34CrMo4 gas cylinder seamless steel pipes. The root cause—localized overheating of the steel billet tail end—is well identified, and the preventive measures are practical and effective. The study demonstrates the importance of process control in ensuring the quality of critical safety components such as gas cylinders, and it offers valuable lessons for engineers working in the steel pipe manufacturing industry.