CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Technology for Flange Forging Inner Wall
Literature Overview
This study note examines the technical approach to weld overlay on the inner wall of flange forgings, a common requirement in pressure vessel and piping flange manufacturing where corrosion-resistant or wear-resistant surfaces are needed while retaining the structural strength of a carbon or low-alloy steel base. The inner wall geometry of a flange presents unique challenges: the surface is often curved, partially enclosed, and subject to geometric constraints that limit welding access. Understanding the process parameters, weld procedure qualification requirements, and quality control measures is essential for producing reliable cladding layers in this configuration.
Core Technical Challenges
The inner wall of a flange forging typically has a diameter-to-thickness ratio that varies significantly depending on the flange type (WN, BL, SO, IF, RF, etc.). The primary challenges include:
- Access limitation: The welding torch or electrode must be positioned within the bore of the flange, which restricts the range of applicable processes.
- Heat input control: The confined geometry creates uneven heat dissipation, leading to potential distortion, residual stress concentration, and metallurgical issues in the cladding layer.
- Bond strength assurance: Achieving sufficient metallurgical bonding between the overlay and the base forging, especially at the root pass, is critical.
- Uniformity of overlay thickness: The inner surface must be covered uniformly to ensure consistent corrosion or wear resistance around the entire circumference.
Process Selection and Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Base material | Q345R, 16Mn, 20# | Common forging steels |
| Overlay material | 304, 316L, 321, Inconel 625 | Depends on service conditions |
| Process | SAW, GMAW, GTAW | SAW preferred for thick layers; GTAW for root pass |
| Heat input | 0.5–2.5 kJ/mm | Lower for thin walls to avoid distortion |
| Layer thickness | 3–10 mm | Typically 2–4 layers |
| Interpass temperature | <150°C | Prevents grain coarsening in base |
| Preheat temperature | 100–200°C | Reduces HAZ hardness and cracking risk |
Root Pass Considerations
The root pass is the most critical in flange inner wall cladding. A GTAW root pass with a low heat input (typically 0.3–0.8 kJ/mm) ensures full penetration into the base metal without excessive dilution. The root pass wire diameter is usually 1.6 mm or 2.0 mm, with a current range of 80–120 A and arc voltage of 15–20 V. The travel speed should be maintained at 40–70 mm/min to ensure proper wetting and bonding.
Fill and Cap Passes
After the root pass, subsequent passes are typically deposited using SAW or GMAW with solid or flux-cored wire. The flux composition in SAW is critical for controlling dilution and ensuring clean weld metal. A typical dilution ratio for the first fill pass is 20–30%, which should be controlled by adjusting the root pass width and the fill pass overlap.
Weld Procedure Qualification
According to NB/T 47014 and ASME IX, the welding procedure qualification for cladding on forgings requires demonstration of both bond strength and overlay mechanical properties. The bond strength test (per ASTM A263 or A264) typically requires a minimum value of 145 MPa for ferrous-to-ferrous cladding. The overlay layer must also meet chemical composition, hardness, and corrosion resistance requirements per the applicable product standard.
Quality Control Measures
- Visual inspection (VT): Check for undercut, lack of fusion, porosity, and uniform coverage around the entire inner wall.
- Magnetic particle testing (MT): Applied to the cladding surface to detect surface and near-surface cracks.
- Ultrasonic testing (UT): Used to assess bond quality and detect subsurface defects at the interface.
- Hardness survey: Hardness measurements across the overlay layers and into the base to confirm acceptable HAZ properties.
- Corrosion testing: Potentiodynamic polarization or salt spray testing to verify the overlay meets the specified corrosion resistance.
Engineering Practice Insights
In practice, one of the most common failure modes in flange inner wall cladding is incomplete coverage at the junction between the inner wall and the bolt circle or the sealing face. This geometric transition creates a "shadow zone" where the torch cannot be positioned at the optimal angle. The solution is to machine a small chamfer or groove at this junction before cladding, or to use a multi-position welding approach where the flange is rotated to allow access from different angles.
Another practical consideration is the effect of the forging grain structure on cladding quality. Large-grain forgings can create localized areas of high dilution due to preferential melting at grain boundaries. Solution treatment of the forging before cladding, or the use of a higher-nickel overlay alloy with lower dilution sensitivity, can mitigate this issue.
Key Reflections
The flange inner wall cladding problem is fundamentally a geometric challenge rather than a metallurgical one. The metallurgy of the overlay material and the base metal are well-understood; the difficulty lies in achieving consistent weld quality in a confined, curved geometry. Process development should focus on fixture design, torch positioning, and multi-pass strategies that accommodate the geometry. A well-designed welding fixture that rotates and tilts the flange during deposition can reduce the number of weld positions and improve overall consistency.