Zhuojin Pipe Fitting Co., LtdOverlay Welding Flanges as a Substitute for Forged Flanges
Literature Overview
This paper by Chen Yuchuan and Li Jingshi from Harbin Boiler Works, published in "Welding" in 1991, presents a pioneering engineering application of overlay welding technology to manufacture flanges as a replacement for conventional forged flanges. The study covers the forming overlay welding technology, process trials, product welding, and the technical and economic benefits of this approach. This is a notable early example of the "welding replaces forging" philosophy in pressure vessel and boiler manufacturing.
Core Technical Findings
The study demonstrates that overlay welding can produce flanges that meet the same functional requirements as forged flanges, with significant cost and material savings. The forming overlay welding process involves building up the flange geometry (hub, face, and bolt hole areas) on a plate or rolled base by successive welding passes.
Technical and Economic Benefits
| Aspect | Forged Flange | Overlay Welded Flange |
|---|---|---|
| Material utilization | Low (forging yield 40-60%) | High (plate utilization 85-95%) |
| Production flexibility | Limited by forging equipment capacity | Flexible, limited only by welding capability |
| Large diameter capability | Restricted by forging press capacity | Virtually unlimited |
| Corrosion resistance | Uniform forging material | Can use corrosion-resistant overlay on carbon steel base |
| Cost | Higher material and forging costs | Lower material cost, moderate welding cost |
| Certification | Requires forging certification | Requires welding procedure qualification |
Forming Overlay Welding Process
The forming overlay welding process for flange production involves several key steps:
- Base plate preparation with proper surface treatment and fit-up
- Multi-pass welding to build up the hub section to the required diameter and height
- Welding of the flange face with controlled geometry and surface finish
- Drilling and reaming of bolt holes after the overlay is complete
- Non-destructive testing to verify weld quality
- Pressure testing to confirm structural integrity
The welding sequence is critical to minimize distortion and ensure dimensional accuracy. A symmetric welding pattern starting from the center of the hub and progressing outward helps balance thermal input and reduce angular distortion.
Process Analysis
Welding Procedure Qualification
The transition from forged to overlay-welded flanges requires comprehensive welding procedure qualification (WPQ) in accordance with applicable codes such as ASME Section IX or NB/T 47014. The qualification must demonstrate that the overlay weld metal meets the required mechanical properties, including tensile strength, impact toughness, and hardness, and that the weld is free from unacceptable defects.
Quality Control Considerations
Key quality control points in overlay-welded flange production include:
- Pre-weld inspection of base material for surface defects and proper chemistry
- In-process monitoring of welding parameters (current, voltage, travel speed, interpass temperature)
- Post-weld NDT including radiographic testing (RT) of the hub-to-base joint and ultrasonic testing (UT) of overlay layers
- Hardness survey to verify uniform weld metal properties
- Dimensional inspection of bolt hole positions and flange face flatness
- Hydrostatic pressure testing at the required test pressure
Distortion Control
Flange overlay welding introduces significant thermal distortion due to the large thermal gradient between the overlay weld metal and the base plate. Control measures include:
- Back-deformation of the base plate before welding
- Rigid clamping to restrict deformation
- Symmetric welding sequence to balance thermal input
- Post-weld stress relief if required by the applicable code
- Machining of the flange face after welding to achieve final dimensional accuracy
Engineering Practice Integration
The overlay welding approach for flange production has several practical advantages in the boiler and pressure vessel industry. For large-diameter flanges where forging is impractical or prohibitively expensive, overlay welding provides a viable alternative. The ability to use a low-cost carbon steel base plate with a high-performance overlay material also enables the production of flanges with corrosion-resistant faces at a fraction of the cost of solid stainless steel or alloy flanges.
However, the approach also introduces challenges that must be managed. The weld metal in the overlay region may have different mechanical properties from the base material, requiring careful consideration of the joint design and loading conditions. The weld-to-base interface is a potential site for stress concentration and crack initiation under cyclic loading, and the weld must be designed and qualified to withstand the design pressure and temperature.
Study Insights and Implications
This 1991 paper represents an early and influential demonstration of the "welding replaces forging" concept in pressure equipment manufacturing. The approach has since been widely adopted in the industry for large flanges, pipe spools, and other components where forging is impractical. The key insight is that welding technology, when properly qualified and executed, can produce components that meet or exceed the performance of forged parts at significantly lower cost. Engineers evaluating the feasibility of overlay-welded flanges should carefully assess the service conditions, including pressure, temperature, cyclic loading, and corrosion environment, and ensure that the welding procedure qualification covers all relevant variables. The economic benefits of reduced material usage and production flexibility make this approach particularly attractive for large-diameter and low-volume flange requirements.