Manufacturing Process and Technical Improvement for Large U-Shaped Elbows

Literature Overview

This paper by Niu Yanming and Wang Fangping from Dongfang Electric Group Dongfang Boiler Corporation addresses the manufacturing challenges of large U-shaped elbow shells used in hairpin-type heat exchangers. The U-shaped elbow outer shell is a core component in the overall shell fabrication, requiring multiple complex welding operations and forming processes. The study focuses on the technical difficulties and solutions for the U-bend section manufacturing and optimization.

Manufacturing Process Chain

The complete manufacturing sequence for the U-shaped elbow shell includes:

Operation Step	Technical Description	Key Quality Concern
Single plate forming (press forming)	U-bend section formed from flat plate by press	Dimensional accuracy and surface quality
Longitudinal seam assembly	Welding of plate edges to form half-shell	Weld quality, alignment tolerance
Circumferential seam assembly	Joining longitudinal segments	Fit-up accuracy, residual stress
Shell-to-cylinder junction	Welding U-bend section to straight shell segment	Stress concentration, transition geometry
Final inspection	NDT and dimensional verification	Defect detection, dimensional compliance

Critical Process Challenge: Single Plate Forming

The single plate forming (press forming) step is identified as the most critical process, as its dimensional accuracy directly determines whether subsequent assembly operations can be completed. For large U-shaped elbows, the forming process must achieve:

Consistent bend radius throughout the U-profile
Minimal springback after unloading
Acceptable wall thinning at the inner bend radius
Surface quality without cracking or excessive wrinkling

Technical Solutions and Improvements

The paper discusses process optimization measures, which typically include:

Forming die design optimization: Multi-stage forming with progressive reduction of bend angle to minimize springback and wall thinning
Material selection and preparation: Selection of steel grades with appropriate formability characteristics (high elongation, low yield-to-tensile ratio)
Process parameter control: Optimization of forming force, speed, and temperature (if warm forming is employed)
Compensation techniques: Pre-compensation of anticipated springback by overspecifying the initial bend angle
Post-forming correction: Mechanical or thermal straightening of residual dimensional deviations

Engineering Practice Considerations

From a manufacturing engineering perspective, the production of large U-shaped elbows presents unique challenges compared to conventional pipe elbows. The hairpin-type heat exchanger application requires tight dimensional tolerances because the U-bend must fit precisely within the exchanger header arrangement, and any dimensional deviation affects the overall assembly fit-up.

The quality of the longitudinal and circumferential weld seams is paramount because these welds will be subjected to cyclic thermal stresses during heat exchanger operation. The welding procedure must be carefully qualified, with particular attention to:

Preheat temperature control to prevent cold cracking in thick-section material
Interpass temperature management to control grain growth in the heat-affected zone
Post-weld heat treatment (PWHT) to relieve residual stresses and temper the weld metal
Weld procedure qualification per applicable codes (ASME Section IX, ISO 15614)

The study's focus on process improvement reflects the broader industry trend toward optimizing manufacturing efficiency while maintaining or improving quality. In the context of large power generation equipment, where component weight and fabrication cost are significant, process optimization that reduces material waste, rework, and inspection time provides substantial economic benefit.

A key insight from this work is that dimensional accuracy at the forming stage cascades through the entire manufacturing sequence. Poor forming accuracy leads to difficult fit-up, increased welding distortion, and ultimately compromised dimensional compliance of the finished component. This underscores the importance of front-end process control in complex fabrication operations—investing in forming accuracy is far more cost-effective than attempting to correct deviations in subsequent operations.