1. Introduction

Accumulator shells, critical components in hydraulic and pneumatic systems, are typically manufactured using hot spinning technology on seamless steel pipes. Among the materials used, 34CrMo4 hot-rolled seamless steel pipe stands out due to its excellent comprehensive mechanical properties—high strength, good plasticity, and toughness—making it ideal for withstanding the pressure and fatigue loads of accumulator operations.

However, a critical quality issue has emerged in industrial production: during the induction heating stage of 34CrMo4 seamless steel pipes (e.g., specification φ356 mm × 14.3 mm), regular ring-shaped hole melting defects often occur at the pipe ends. After cooling, these defects manifest as closed annular holes approximately 5.0 mm from the pipe end, with local melting, outer wall depression, and inner wall protrusion.

To address this problem, this article systematically investigates the causes of ring-shaped hole defects through physical and chemical testing and experimental verification. The findings confirm that induction heating time is the core factor influencing defect formation.

2. Defect Analysis: From Macro to Micro Observation

To identify the origin of ring-shaped holes, a defective 34CrMo4 seamless steel pipe sample was subjected to multi-dimensional testing. The results ruled out metallurgical defects and pointed to external process factors (i.e., induction heating) as the primary cause.

2.1 Macro-Morphology Observation

Longitudinal dissection of the defective pipe end revealed distinct melting and hole characteristics:

• Irregular melting traces: The pipe end showed visible signs of melting, with residual molten material adhering to the inner wall.
• Closed through-holes: Two sawn sections contained well-formed closed through-holes.
• Wall deformation: The outer wall of the pipe end in the hole area exhibited depression, while the inner wall showed protrusion.

2.2 Chemical Composition Analysis

Chemical composition testing was conducted on both the pipe matrix and the molten area to verify if element segregation or impurity contamination caused the defects. The testing followed GB/T 4336-2016.

Key findings confirmed that the pipe’s chemical composition was not the cause of the defects; the element loss in the molten area was merely a secondary effect of high-temperature melting.

2.3 Metallographic Examination

Metallographic analysis was performed on the molten area, hole edges, and distant non-defective areas to evaluate microstructure and non-metallic inclusions. The testing adhered to GB/T 10561-2005.

Key observations included microstructure uniformity, inclusion compliance with standards, and overburn characteristics like intergranular cracking near the defects.

2.4 SEM and EDS Analysis

To further clarify the defect mechanism, a representative sample was analyzed using SEM and EDS. These microscale analyses confirmed that the ring-shaped holes were caused by local overburn and melting rather than metallurgical defects in the 34CrMo4 steel.

3. Experimental Verification: Induction Heating Time as the Core Factor

To confirm that induction heating was responsible for the defects, two controlled experiments were conducted on defect-free 34CrMo4 seamless steel pipes using the same production line, with heating time as the only variable.

3.3 Experiment Conclusion

Both experiments reproduced the ring-shaped hole defects, with defect severity increasing with heating time. This directly confirmed that excessive induction heating time was the root cause of the ring-shaped holes.

4. Mechanism Discussion: Why Does Overburn Occur in the Pipe Wall Middle Layer?

The experimental results raise a critical question: Why does overburn occur in the middle of the pipe wall during induction heating? The answer lies in the combined effects of induction heating physics and heat transfer lag.

4.1 Basic Principles of Induction Heating

Induction heating works by passing an alternating current through an induction coil, generating an alternating magnetic field around the coil. This magnetic field induces eddy currents in the metal pipe, with eddy current density determining heat generation.

4.2 Magnetic Properties of 34CrMo4 Steel and Temperature Distribution

34CrMo4 steel is a ferromagnetic material at room temperature, but its relative permeability decreases as temperature increases. When heated above the Curie temperature (approximately 760°C), it loses ferromagnetism and becomes non-magnetic.

This magnetic transition drastically changes the eddy current distribution, creating a “secondary heat source” below the surface that explains the temperature distribution patterns observed.

5. Conclusions and Practical Recommendations

5.1 Key Conclusions

1. Defect cause: The ring-shaped hole defects are caused by excessive induction heating time—not metallurgical defects or contamination.
2. Temperature distribution mechanism: During induction heating, the peak temperature moves from the surface to the subsurface and then to the middle layer due to magnetic transition and heat transfer lag.
3. Critical parameter: Controlling induction heating time is the most effective way to prevent middle-layer overburn.

5.2 Practical Recommendations for Industrial Production

To eliminate ring-shaped hole defects, the following process optimizations are proposed:

• Standardize heating time: Establish a heating time table based on pipe specifications.
• Real-time temperature monitoring: Install infrared thermometers to monitor both surface and subsurface temperatures.
• Optimize induction frequency: Adjust the induction frequency based on pipe thickness.
• Pre-heating inspection: Conduct 100% ultrasonic testing on pipes before induction heating.

6. FAQ (Frequently Asked Questions)