Forming Process Research for Short Radius Thick-Walled Elbows

Literature Overview

This paper by Hou Zhiyong (2014), published in China Chemical Equipment, addresses the manufacturing challenge of forming short-radius elbows from thick-walled pipe using the new steel grade A335 P92. The author proposes a two-stage hot pressing forming method to accommodate the large deformation ratio inherent in short-radius elbow geometry, while also introducing process improvements that reduce mold costs, simplify operations, lower labor intensity, and enhance forming quality. The paper represents a practical engineering solution to a significant manufacturing challenge in power plant and petrochemical piping applications.

Technical Challenge of Short Radius Thick-Walled Elbows

Short-radius elbows (SR, R = 1.0D) require significantly more material deformation than long-radius elbows (LR, R = 1.5D). When combined with thick-walled pipe, the forming challenge becomes particularly severe:

The large deformation ratio means that the outer fiber of the elbow must elongate substantially while the inner fiber compresses. For thick-walled material, this creates a high risk of:

• Wall thinning exceeding acceptable limits at the outer radius
• Wrinkling or folding at the inner radius
• Excessive residual stresses that compromise fatigue life
• Dimensional inaccuracy due to springback

Two-Stage Hot Pressing Process

The author's proposed two-stage hot pressing process addresses these challenges by dividing the total deformation into two sequential stages:

Stage 1 - Pre-forming:

• The pipe is heated to the forming temperature (typically 950-1100°C for P92, considering its high hardenability and need for controlled cooling)
• A preliminary bend is formed using a die with a radius slightly larger than the final target radius
• This stage establishes the basic elbow geometry with controlled material flow
• The deformation ratio per stage is reduced, minimizing the risk of excessive thinning or wrinkling

Stage 2 - Final forming:

• The pre-formed elbow is reheated to the forming temperature
• A final die with the exact target radius is used to achieve precise dimensions
• The second pressing corrects any dimensional deviations from the first stage
• Controlled cooling (furnace cooling or controlled air cooling) is applied to minimize residual stresses and avoid undesirable microstructural changes

A335 P92 Material Considerations

The A335 P92 (ASTM A335 Grade P92) steel is a 9Cr-1Mo-V-Nb-B type martensitic steel with excellent high-temperature creep strength, making it ideal for supercritical and ultra-supercritical power plant applications. However, its manufacturing characteristics present specific challenges:

The forming process for P92 elbows must carefully control the heating temperature, heating uniformity, forming speed, and cooling rate to avoid:

• Quenched microstructures in the HAZ or formed regions
• Cracking due to excessive cooling rates
• Incomplete transformation leading to retained austenite or martensite in undesirable locations

Process Improvements and Benefits

The author's process improvements yield several quantifiable benefits:

1. Reduced mold cost: The two-stage approach allows the use of simpler, more economical dies for the pre-forming stage, with the precision die used only in the final stage. This reduces the overall tooling investment.
2. Simplified forming operations: By dividing the deformation into two stages, each stage requires less force and less complex die geometry, reducing the risk of equipment overload and operator difficulty.
3. Lower labor intensity: The process improvements reduce the physical demands on operators, improving workplace safety and reducing fatigue-related errors.
4. Improved forming quality: The controlled two-stage deformation produces elbows with more uniform wall thickness, better dimensional accuracy, and lower residual stress levels compared to single-stage forming.

Engineering Practice Integration

The two-stage hot pressing approach is particularly relevant for modern power plant construction, where P91/P92 piping is increasingly used for its superior high-temperature performance. Key implementation considerations include:

1. Thermal process control: The heating temperature must be precisely controlled using thermocouples embedded in the pipe or die. For P92, temperatures above 1100°C risk grain growth and excessive oxidation, while temperatures below 950°C risk incomplete austenitization and excessive forming forces.
2. Cooling management: After forming, the elbow must be cooled at a controlled rate to achieve the desired microstructure. For P92, furnace cooling to approximately 700°C followed by air cooling is typical, with a subsequent tempering treatment to relieve residual stresses and achieve the target hardness (typically 22-30 HRC).
3. Quality verification: Post-forming inspection should include dimensional measurement (bend radius, wall thickness at multiple locations), hardness testing (to verify microstructural integrity), and non-destructive testing (UT or MT) to detect any internal or surface defects.
4. Springback compensation: The die geometry must account for springback, which is particularly significant for P92 due to its high yield strength. Empirical springback data or finite element simulation should be used to determine the appropriate die overbend angle.

Reflections and Study Insights

This paper addresses a manufacturing challenge that is becoming increasingly important as the power generation industry transitions to supercritical and ultra-supercritical coal-fired power plants. The use of P92 piping for high-temperature steam lines requires manufacturing processes that can handle the material's demanding forming characteristics while producing components that meet stringent quality requirements.

The two-stage hot pressing approach is conceptually straightforward but practically challenging. The key to success lies in the precise control of thermal and mechanical parameters during each stage. In my experience, the transition from single-stage to multi-stage forming is often the difference between achieving acceptable quality and experiencing chronic production problems for thick-walled short-radius elbows.

The paper's emphasis on process improvement benefits (cost reduction, quality enhancement, labor reduction) reflects a mature manufacturing philosophy that balances technical performance with economic viability. For engineers involved in specifying elbow manufacturing processes, the lessons here are clear: the complexity of the forming process should match the complexity of the geometry and material, and a systematic approach to process development is essential for achieving consistent quality.