CLADDING TECHNOLOGY SHANXI CO., LTDCorrosion-Resistant Weld Overlay of 20MnMo Dished Heads
Literature Overview and Engineering Challenge
20MnMo is a low-alloy steel widely used for pressure vessel heads and shells in high-temperature service, particularly in hydrogenation reactors, reformers, and high-temperature heat exchangers. The challenge of providing corrosion resistance to 20MnMo components arises in applications where the vessel interior is exposed to corrosive media such as sulfuric acid, hydrochloric acid, or sour gas environments. This study addresses the technical challenges of applying corrosion-resistant weld overlay to 20MnMo dished heads, which present unique difficulties due to their geometry, residual stress state from forming, and the metallurgical characteristics of the base material.
Base Material Characteristics
20MnMo contains approximately 0.20% C, 0.90-1.20% Mn, and 0.40-0.60% Mo. The molybdenum provides good high-temperature strength and creep resistance, while the manganese contributes to hardenability. The carbon equivalent (CE) of 20MnMo is approximately 0.45-0.55, placing it in the category of materials requiring careful welding control to prevent hydrogen-induced cracking.
The dished head geometry introduces additional complexity. The forming process creates significant plastic deformation, particularly at the knuckle radius where the curvature is greatest. This deformation produces a residual stress state with compressive stresses on the inner surface and tensile stresses on the outer surface. The grain structure at the knuckle is elongated in the circumferential direction, creating anisotropic properties that must be considered when designing the overlay welding sequence.
Overlay Material Selection
| Overlay Material | Corrosion Resistance | Dilution Tolerance | Compatibility with 20MnMo | Typical Application |
|---|---|---|---|---|
| 316L Stainless Steel | Good (general acid) | Moderate | Excellent | Mildly corrosive environments |
| 316L + 2205 Duplex | Excellent | Low | Good with transition layer | Highly corrosive environments |
| Inconel 625 | Excellent (all acids) | Very Low | Requires transition layer | Severe corrosion + high temperature |
| 309L Stainless Steel | Moderate | High | Excellent | General purpose overlay |
For most applications involving 20MnMo dished heads, 316L or 309L is recommended as the primary overlay material. The 309L offers higher dilution tolerance, making it suitable for the first layer where dilution from the 20MnMo substrate is highest. A two-layer approach using 309L as the first layer and 316L as the cover layer provides an optimal balance of metallurgical compatibility and corrosion resistance.
Welding Procedure Development
Preheating and Interpass Temperature
The CE of 20MnMo necessitates preheating to prevent hydrogen-induced cracking in the heat-affected zone. The recommended preheat temperature is 200-250°C, applied uniformly across the welding area and a minimum 50 mm beyond the weld zone. The interpass temperature should be maintained between 200°C and 300°C to control cooling rates and prevent excessive hardness in the HAZ.
| Process Parameter | Value |
|---|---|
| Preheat Temperature | 200-250°C |
| Interpass Temperature | 200-300°C |
| Welding Current (GMAW, 1.2 mm wire) | 160-200 A |
| Arc Voltage | 24-27 V |
| Travel Speed | 50-80 cm/min |
| Heat Input | 1.0-1.5 kJ/mm |
| Shielding Gas | 98% Ar + 2% CO2 or 100% Ar |
Welding Sequence for Dished Heads
The geometry of dished heads requires a carefully planned welding sequence to manage distortion and residual stress. The recommended approach is to start at the pole (centre) and work outward in a spiral or circumferential pattern. For large heads, the overlay is divided into sectors of approximately 60-90 degrees, with each sector welded in a continuous pass to minimise the number of starts and stops.
The critical consideration is the interaction between the overlay weld residual stresses and the existing forming residual stresses. Welding from the pole outward allows the overlay shrinkage stresses to be partially relieved by the flexibility of the unclad outer regions. Welding from the edge inward would concentrate stresses at the pole, potentially causing distortion or cracking.
Quality Control and Inspection
Non-Destructive Testing Requirements
| Inspection Method | Coverage | Acceptance Criteria | Purpose |
|---|---|---|---|
| Visual Testing (VT) | 100% | ASME Sec. IX QW-191.85 | Surface quality, undercut, overlap |
| Dye Penetrant (PT) | 100% | ASME Sec. V Art. 7, Level II | Surface-breaking cracks |
| Magnetic Particle (MT) | 100% (if applicable) | ASME Sec. V Art. 7, Level II | Surface and near-surface defects |
| Ultrasonic (UT) | 10-25% (risk-based) | ASME Sec. V Art. 4 | Bond integrity, internal defects |
| Hardness Test | 100% (HAZ) | ≤ 250 HV (normalized) | HAZ hardenability check |
Metallurgical Examination
Cross-sectional metallographic examination of the overlay-to-substrate interface is essential for verifying bond integrity and assessing dilution. The bond line should be free of cracks, lack of fusion, and excessive intermetallic compound formation. For 316L overlay on 20MnMo, the dilution at the first layer interface typically ranges from 25-40%, resulting in a martensitic or martensitic-ferritic microstructure in the transition zone. This is acceptable provided the hardness remains below 250 HV and no cracking is observed.
Engineering Practice Considerations
Post-Weld Heat Treatment
For 20MnMo components operating above 250°C, post-weld heat treatment (PWHT) is generally required to relieve residual stresses and reduce HAZ hardness. The PWHT temperature for 20MnMo is typically 620-650°C for 2 hours per 25 mm thickness, followed by furnace cooling. However, PWHT can affect the overlay layer properties — austenitic stainless steel overlays may experience sensitisation if held at temperatures between 450-850°C for extended periods. The recommended approach is to perform PWHT before applying the final cover layer of the overlay, or to use a low-carbon overlay material (316L rather than 316) that is resistant to sensitisation.
Hydrogen-Induced Cracking Prevention
Given the CE of 20MnMo, hydrogen-induced cracking (HIC) in the HAZ is a significant risk. The following measures are recommended:
- Use low-hydrogen electrodes or wires with hydrogen diffusion coefficient below 5 ml/100g
- Bake electrodes at 300°C for 2 hours before use
- Maintain preheat and interpass temperatures as specified
- Avoid rapid cooling by allowing natural air cooling or covering with insulation blankets
- Perform delayed inspection (24 hours after welding) for HIC detection
Study Insights and Implications
The corrosion-resistant overlay of 20MnMo dished heads is a well-established practice, but the specific challenges of dished head geometry and the metallurgical characteristics of 20MnMo require careful attention to detail. The key engineering insight is that the overlay welding procedure must be designed as an integrated system considering base material properties, geometry-induced residual stresses, dilution effects, and post-weld treatment requirements. A two-layer approach using 309L as the first layer and 316L as the cover layer provides the best balance of metallurgical compatibility and corrosion resistance. The welding sequence starting from the pole and working outward is critical for managing distortion on curved geometries. Engineers should ensure that procedure qualification testing includes specimens that replicate the actual geometry and residual stress state of the component, as flat coupon testing alone may not capture the true performance of the overlay on a formed dished head.