Microstructure and Mechanical Properties of Pilger Cold Rolled 304 Stainless Steel Pipe

Literature Overview

The paper by Li Wei, Chu Zhibing, Shuai Meirong, Wang Huanzhu, Xue Zhanyuan, Li Yugui, Xue Chun, and Su Hui from the Ministry of Education Engineering Research Center of Heavy Machinery at Taiyuan University of Science and Technology, published in Forging Technology (2019, Vol. 44, Issue 6, pp. 53-58), presents a comprehensive study on the microstructure evolution and mechanical property changes during the pilger cold rolling process of 304 stainless steel pipe. The research combines experimental pilger rolling with numerical simulation to analyze metal flow, grain structure, and mechanical property variations.

Core Technical Content

Pilger Cold Rolling Process Parameters

Parameter	Value	Description
Material	304 Stainless Steel	Austenitic stainless steel
Initial pipe diameter	60 mm	Before rolling
Final pipe diameter	50 mm	After rolling
Initial wall thickness	4 mm	Before rolling
Final wall thickness	5 mm	After rolling
Rolling reduction	16.7%	Diameter reduction ratio
Wall thickening	25%	Thickness increase ratio
Rolling temperature	Room temperature	Cold rolling condition

Microstructure Evolution

The research reveals several important microstructural changes during pilger rolling:

Metal flow direction: The metal flow direction at any point on the pipe is continuously changing during the rolling process, resulting in irregular distribution of grain internal flow lines.
Stress distribution: The inner wall experiences higher equivalent stress than the outer wall, leading to more pronounced shear slip bands and more severe deformation on the inner surface.
Grain refinement: Dynamic recrystallization and grain fragmentation occur during rolling, reducing the average grain size. The initial grain size of approximately 25-30 micrometers is refined to 15-20 micrometers after rolling.
Deformation bands: Shear deformation bands are more prominent on the inner wall due to the higher stress state, indicating asymmetric deformation between inner and outer surfaces.

Mechanical Property Changes

Property	Before Rolling	After Rolling	Improvement
Vickers Hardness (HV)	236	403	+70.8%
Yield Strength (MPa)	352	745	+111.6%
Tensile Strength (MPa)	794	1209	+52.3%
Elongation (%)	55	35	-36.4%
Average Grain Size (μm)	25-30	15-20	-33%

Numerical Simulation Insights

The finite element simulation provides additional insights into the deformation process:

The equivalent strain distribution is non-uniform across the wall thickness, with higher values near the inner surface
The stress state transitions from biaxial to triaxial compression during the rolling process
The deformation zone is localized near the roller contact area, with elastic recovery in non-contact regions
Temperature rise due to plastic deformation is relatively small in cold rolling conditions, typically 20-40 degrees Celsius above ambient

Engineering Practice Analysis

The pilger cold rolling process offers several advantages for stainless steel pipe production:

Dimensional accuracy: Cold rolling achieves tighter dimensional tolerances compared to hot rolling, typically ±0.1 mm for diameter and ±0.05 mm for wall thickness
Surface finish: The process produces a smooth surface finish with surface roughness typically below Ra 0.8 micrometers
Mechanical property enhancement: The significant improvement in strength and hardness makes the product suitable for high-pressure applications
Grain structure control: The process allows control over grain size and orientation, which can be optimized for specific applications
Material efficiency: The reduction in diameter while increasing wall thickness provides material efficiency advantages

Quality Control Considerations

Quality Parameter	Acceptance Criteria	Inspection Method	Frequency
Diameter tolerance	±0.1 mm	Optical measurement	100%
Wall thickness tolerance	±0.05 mm	Ultrasonic measurement	100%
Surface finish	Ra < 0.8 μm	Surface profilometer	10% sampling
Hardness	380-420 HV	Vickers hardness test	10% sampling
Grain size	15-20 μm	Metallographic examination	5% sampling
Straightness	< 1 mm/m	Optical comparator	100%

Key Questions and Reflections

A critical consideration is the effect of cold rolling on the corrosion resistance of 304 stainless steel. While the mechanical properties improve significantly, the increased dislocation density and residual stresses may affect the passive film formation and corrosion resistance. Post-rolling stress relief annealing may be necessary for applications in corrosive environments.

Another important aspect is the anisotropy introduced by the rolling process. The non-uniform deformation and metal flow patterns may result in direction-dependent mechanical properties, which must be considered in applications where the pipe is subjected to multi-axial loading.

Study Insights and Implications

This research provides valuable insights into the microstructure-property relationships during pilger cold rolling of 304 stainless steel pipe. The significant improvement in mechanical properties, particularly the 111.6% increase in yield strength, demonstrates the potential of cold working for enhancing pipe performance. The work also highlights the importance of understanding metal flow patterns and stress distributions for optimizing the rolling process. For engineering practice, this research supports the use of pilger cold rolling for high-strength stainless steel pipe applications in oil and gas, chemical processing, and nuclear industries. The combination of experimental and numerical methods provides a comprehensive understanding of the process, enabling better process control and quality assurance in production environments. The findings contribute to the ongoing development of advanced manufacturing techniques for high-performance stainless steel pipe products.