Microstructure and Mechanical Properties of 15CrMo Steel Pipe During Reducing Mouth Forming

Literature Overview

The paper by Hua Longlong, Liu Tao, Hong Lei, and Hong Liang, published in 2025 in the journal Hot Working Technology (Vol. 54, No. 4, pp. 137-141), investigates the microstructural evolution and mechanical property changes in 15CrMo seamless steel pipe during cold extrusion reducing mouth forming. Funded by the National Natural Science Foundation of China (Youth Fund, Grant No. 51601078) and the Jiangsu Provincial Government Overseas Scholarship Program (JS-2019-322), this study bridges academic research with industrial practice through collaboration between Jiangsu Institute of Technology and Jiangsu Bete Pipe Fittings Co., Ltd. The work addresses a critical manufacturing process in pipe fitting production where dimensional reduction and shaping are essential.

Process Description and Technical Context

Reducing mouth forming (also referred to as necking or cold extrusion) is a fundamental process in pipe fitting manufacturing, particularly for producing reducers, transitions, and shaped connections. The process involves cold working the end section of a steel pipe to reduce its diameter while maintaining structural integrity. For alloy steels such as 15CrMo—a low-alloy martensitic steel widely used in power generation, petrochemical, and high-temperature service applications—the cold working process introduces significant metallurgical changes that must be carefully managed.

15CrMo steel contains approximately 1.25% Cr and 0.50% Mo, providing excellent high-temperature strength and creep resistance. The cold forming process must be designed to avoid:

• Excessive strain hardening that compromises subsequent welding or service performance
• Microstructural damage (microcracks, inclusion-induced defects)
• Residual stress accumulation that could trigger stress corrosion cracking in service

Microstructural Analysis

The study's metallographic examination revealed characteristic cold work features in the reducing mouth region:

The formation of inter-lamellar slip within the deformed grains indicates that the deformation occurred through dislocation glide and pile-up at grain boundaries and phase interfaces. This is consistent with the strain hardening mechanism observed in the mechanical property data.

Mechanical Property Evolution

The mechanical testing results demonstrate a clear strain hardening response:

The increase in dislocation density during cold forming creates strong resistance to further deformation, manifesting as increased tensile strength and hardness. The corresponding decrease in ductility reflects the reduced capacity for further plastic deformation. The fracture surface analysis showed a transition from purely ductile (cup-and-cone) to mixed ductile-brittle morphology, indicating the onset of limited ductility.

Annealing Treatment and Property Recovery

The critical finding of this study is that annealing the cold-formed region restores the microstructure and mechanical properties to near-original levels. The annealing process accomplishes:

1. Recovery: Dislocation annihilation and rearrangement, reducing internal stresses
2. Recrystallization: Formation of new, strain-free equiaxed grains replacing deformed grains
3. Grain growth: Limited grain coarsening if annealing temperature and time are properly controlled

The post-annealing microstructure closely resembled the original forged state, with equiaxed grains and low dislocation density. Mechanical properties returned to values comparable to the original material, indicating that the cold forming damage was fully reversible through proper heat treatment.

Engineering Practice Implications

This research has direct practical significance for pipe fitting manufacturers:

1. Process design: Cold reducing mouth forming can be used to achieve desired geometries without requiring expensive hot forming equipment, provided that subsequent annealing is included in the process route.
2. Quality control: The study provides a framework for quality assessment:

• Microstructural examination to verify proper annealing
• Mechanical testing to confirm property recovery
• Hardness mapping to detect incomplete annealing

1. Process optimization: The annealing parameters (temperature, time, cooling rate) must be carefully controlled to:

• Ensure complete recrystallization
• Avoid excessive grain growth
• Maintain the desired chemical homogeneity of the 15CrMo matrix

1. Service performance: For applications requiring high-temperature service (power plants, refineries), the annealed reducing mouth region will perform equivalently to the parent pipe material, ensuring consistent service life across the entire fitting.

Study Insights and Reflections

This work exemplifies the integration of metallurgical understanding with manufacturing practice. The demonstration that cold forming followed by annealing produces a microstructure and property profile equivalent to the original forged material is a powerful result for production engineers. It validates the use of cold forming as a cost-effective shaping method for alloy steel pipes, provided that the complete process cycle (forming + annealing) is implemented.

From a welding perspective, the annealed reducing mouth region presents no special challenges for subsequent welding operations. The restored ductility and low residual stress state make the region weldable under standard procedures for 15CrMo steel. Engineers should, however, verify that the annealing temperature does not approach the critical temperature range that could affect subsequent welding heat-affected zone behavior.

This study provides a clear metallurgical basis for the cold forming and annealing process route in 15CrMo steel pipe fitting production, demonstrating that proper process design can achieve the desired geometry without compromising the material's service performance.