CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Repair Technology for Sealing Surfaces
Technical Background and Application Scope
Sealing surface overlay welding repair is a critical maintenance and fabrication technique applied to flange faces, valve seats, pump casings, heat exchanger tube sheets, and other pressure-containing components where the original sealing surface has been damaged by erosion, corrosion, galling, or improper machining. The objective is to restore the sealing surface to its original geometry, material specification, and surface finish while ensuring metallurgical compatibility with the base material. This technique is widely used in petrochemical, power generation, and marine industries where component downtime is costly and replacement parts may have long lead times.
Process Selection and Material Matching
The selection of the overlay welding process and consumable material depends on several factors including the base material, the service environment, the required sealing surface hardness, and the available equipment. GTAW is generally preferred for sealing surface repairs due to its precise heat input control, low dilution, and excellent weld quality. For larger areas or thicker buildup, GMAW or FCAW may be used for intermediate passes with a GTAW cap pass to achieve the final surface finish.
| Base Material | Recommended Overlay Material | Welding Process | Typical Overlay Thickness |
|---|---|---|---|
| Carbon Steel (Q235, 20G) | 304L, 316L | GTAW + GMAW | 2.0-4.0 mm |
| Low-Alloy Steel (15CrMo, 12Cr1MoV) | 309L (transition) + 316L | GTAW | 3.0-5.0 mm |
| Stainless Steel (304, 316) | 308L, 316L | GTAW | 1.0-3.0 mm |
| Nickel Alloy (Inconel 625) | Inconel 625, Inconel 82 | GTAW | 1.5-3.0 mm |
| Copper Alloy (CuNi 90/10) | CuNi 90/10, Bronze | GTAW | 2.0-4.0 mm |
The material matching principle requires that the overlay material have a coefficient of thermal expansion compatible with the base metal to minimize residual stresses and cracking risk. For dissimilar material combinations, a transition layer with high ductility and low carbon sensitivity (such as 309L) is typically inserted between the base metal and the final overlay layer.
Repair Procedure and Quality Control
A systematic repair procedure is essential for successful sealing surface overlay. The process begins with thorough preparation of the damaged surface, which includes removal of all existing overlay material, corrosion products, and contaminants by grinding or machining. The surface is then cleaned to a minimum of Sa 2.5 standard (near-white metal) and inspected for any underlying defects such as cracks or deep corrosion pits. Any detected defects must be repaired by gouging and re-welding before the overlay is applied.
The welding sequence for a flange face repair typically follows a spiral or zigzag pattern starting from the center and working outward, or vice versa, to distribute heat input uniformly and minimize distortion. The interpass temperature should be maintained below 150°C for stainless steel overlays and below 250°C for carbon steel overlays. A back-purging system using argon is essential for GTAW on stainless steel and nickel alloy overlays to prevent oxidation of the backside of the weld.
Post-weld machining is a critical step that must be performed to achieve the required surface geometry and finish. The machining allowance should be calculated based on the maximum expected overlay thickness plus a minimum of 0.5 mm for surface irregularities. The final surface finish should meet the specification of the original component, typically Ra ≤ 1.6 μm for gasketed joints and Ra ≤ 0.8 μm for ring-joint (RTJ) faces.
Defect Prevention and Troubleshooting
Common defects encountered in sealing surface overlay repair include porosity, cracking, lack of fusion, excessive dilution, and surface irregularities. Each defect type has distinct causes and countermeasures:
| Defect | Primary Cause | Countermeasure |
|---|---|---|
| Porosity | Inadequate shielding gas flow or contaminated surface | Verify gas flow rate (8-12 L/min for GTAW), ensure surface cleanliness |
| Cracking | Excessive carbon pickup or thermal stress | Use transition layer, preheat, control interpass temperature |
| Lack of Fusion | Insufficient heat input or poor technique | Increase current, reduce travel speed, ensure proper fit-up |
| Excessive Dilution | High heat input, thin base metal | Use lower current, higher travel speed, add transition layer |
| Surface Irregularities | Inconsistent bead profile, high travel speed | Optimize torch angle, reduce speed, use multi-pass buildup |
Engineering Practice and Reflections
Sealing surface overlay repair is a technique that demands both technical knowledge and practical skill. The engineer must understand the metallurgical interactions at the fusion boundary, the thermal behavior of the component during welding, and the mechanical requirements of the sealing interface. A systematic approach using the PDCA cycle is recommended for each repair: Plan the repair by assessing the damage, selecting materials, and defining the procedure; Do the repair with careful attention to process parameters and technique; Check the result through visual inspection, dimensional verification, and non-destructive testing; and Act by documenting the repair and updating procedures based on lessons learned. This disciplined approach ensures that each repair contributes to the cumulative knowledge base of the organization and improves future repair outcomes.