Short-Process Manufacturing of 1Cr25Ni20Si2 Heat-Resistant Stainless Steel Tubes

Literature Overview

This paper published in Journal of Iron and Steel Research (2006, Vol. 18, No. 5, pp. 26-28) by Mao Pingli, Li Yingmin, Su Guoyue, and Yang Ke from Shenyang University of Technology and the Institute of Metals, Chinese Academy of Sciences presents a novel short-process manufacturing technology for heat-resistant stainless steel tubes. The research introduces a direct hot extrusion process from hollow cast billets to produce seamless pipe blanks, offering a significant departure from traditional multi-step manufacturing routes while maintaining comparable material quality.

Traditional versus Short-Process Manufacturing

Conventional Manufacturing Route

The traditional process for producing 1Cr25Ni20Si2 heat-resistant stainless steel tubes involves multiple sequential operations:

Step	Process	Key Parameters
1	Solid ingot casting	Ladle furnace melting, controlled cooling
2	Homogenization heat treatment	High temperature soaking for segregation elimination
3	Open-die forging	Multiple passes to reduce ingot to solid bar
4	Drilling	Formation of hollow core in solid bar
5	Extrusion	Final pipe blank production
6	Heat treatment	Solution and aging for final microstructure

This traditional route involves substantial material loss at each step, particularly during the drilling operation which removes the solid core material. The multiple heating and cooling cycles also introduce additional costs and potential quality issues related to grain growth and scale formation.

Short-Process Technology

The proposed short-process technology eliminates the solid forging and drilling steps by directly extruding hollow cast billets:

Step	Process	Key Parameters
1	Hollow billet casting	Metal mold casting with internal cavity
2	Preheating	Controlled heating to extrusion temperature
3	Hot extrusion	Direct extrusion to pipe blank
4	Heat treatment	Solution and aging for final microstructure

The critical innovation lies in the metal mold casting technique that produces hollow billets with controlled cavity dimensions, wall thickness uniformity, and surface quality suitable for direct hot extrusion. This approach significantly reduces the number of processing steps, material waste, and energy consumption.

Microstructure and Property Evaluation

Microstructural Comparison

The study conducted comprehensive metallographic analysis comparing the microstructure of pipe blanks produced by both methods. Key observations include:

Microstructural Feature	Traditional Process	Short Process	Assessment
Grain size	Fine, uniform	Fine, uniform	Comparable
Grain boundary characteristics	Clean, well-defined	Clean, well-defined	Comparable
Inclusion distribution	Uniform	Uniform	Comparable
Segregation	Minimal	Minimal	Comparable
Texture	Moderate	Moderate	Comparable

Mechanical and Physical Properties

The mechanical properties of the short-process pipe blanks demonstrated equivalence with traditional process products:

Property	Traditional Process	Short Process	Acceptance Criteria
Tensile strength	Meets specification	Meets specification	Per applicable standard
Yield strength	Meets specification	Meets specification	Per applicable standard
Elongation	Meets specification	Meets specification	Per applicable standard
Impact toughness	Meets specification	Meets specification	Per applicable standard
High-temperature creep	Meets specification	Meets specification	Per applicable standard

Engineering Significance and Cost Analysis

Economic Benefits

The short-process technology offers substantial cost advantages:

Cost Component	Traditional Process	Short Process	Savings
Raw material utilization	Lower due to drilling waste	Higher due to hollow casting	Significant
Energy consumption	High (multiple heating cycles)	Lower (reduced heating cycles)	Moderate
Processing time	Longer (more steps)	Shorter (fewer steps)	Moderate
Equipment requirements	Standard	Specialized casting molds	Variable
Overall production cost	Baseline	Reduced	Substantial

Quality Assurance Considerations

For engineering applications requiring high integrity, the following quality assurance measures are recommended:

Hollow billet quality control: Strict control of cavity dimensions, wall thickness uniformity, and surface quality of hollow cast billets to ensure consistent extrusion behavior.
Extrusion process monitoring: Real-time monitoring of extrusion temperature, speed, and reduction ratio to maintain microstructural control.
Non-destructive testing: Comprehensive NDT including ultrasonic testing for internal defects and eddy current testing for surface discontinuities.
Metallographic verification: Systematic microstructural examination to confirm grain size, inclusion distribution, and absence of processing defects.

Study Insights and Reflections

This research demonstrates that process innovation can achieve material quality equivalence while substantially reducing manufacturing costs and environmental impact. The short-process technology is particularly attractive for high-alloy stainless steel tubes where material costs are significant and traditional processes involve substantial material waste during drilling operations.

The successful application of metal mold casting for hollow billet production represents a notable advancement in casting technology for high-alloy materials. The controlled solidification conditions in metal molds enable the production of billets with microstructural characteristics suitable for subsequent hot working, overcoming historical challenges associated with casting defects in high-alloy stainless steels.

From a metallurgical perspective, the elimination of the solid forging and drilling steps is particularly significant for maintaining material continuity and minimizing the introduction of processing defects. Each additional processing step in traditional routes introduces potential opportunities for defect formation, including cracking during forging, drilling-induced surface damage, and scale inclusion during reheating. The short-process approach inherently reduces these quality risks.