Zhuojin Pipe Fitting Co., LtdManual Surfacing of Cobalt-Based Alloy on High-Pressure Valves
Literature Overview
This paper by Wen Xiangjun, published in Petrochemical Equipment (Volume 18, Issue 1, 1989, pp. 47), addresses the challenges of manual surfacing welding of cobalt-based alloys on high-temperature, high-pressure valves used in large-scale chemical plants. The author, representing Shanxi Chemical Fertilizer Plant, identifies a critical problem: conventional surfacing procedures involving preheating, interpass temperature control, and post-weld high-temperature annealing with slow cooling can lead to cold cracking or crystallization cracking when applied to components with high rigidity.
Technical Background
High-temperature, high-pressure valves are essential components in petrochemical processing, where they must withstand extreme temperatures (up to 600°C), pressures (up to 30 MPa), and corrosive media. The sealing surfaces of these valves are commonly surfaced with cobalt-based alloys to provide resistance to wear, impact, and oxidation. Common cobalt-based surfacing alloys include:
| Alloy Type | Composition (typical) | Properties | Application |
|---|---|---|---|
| Stellite 6 (CoCr15W) | Co-6Cr-5W-5Fe-3Mo | High hardness, excellent wear resistance | Valve seats, plug surfaces |
| CoCrMo alloys | Co-15Cr-5Mo-2W | High temperature strength, oxidation resistance | High-temperature valve components |
| CoCrSi alloys | Co-15Cr-5Si-3Mo | Good castability, moderate hardness | Valve guide surfaces |
Cracking Mechanisms in Cobalt Alloy Surfacing
The paper identifies two primary cracking mechanisms:
- Cold cracking (hydrogen-induced cracking): Occurs during cooling when the weld metal is in a brittle phase transformation range. High rigidity of the valve body prevents plastic deformation, concentrating stress at the weld root and leading to cracking.
- Crystallization cracking (hot cracking): Occurs during solidification when the weld metal is in a semi-solid state. Low melting point phases (e.g., Co-S, Co-P) segregate at grain boundaries and form liquid films that crack under tensile stress during solidification.
Modified Surfacing Process
Analysis of Cracking Causes
The author analyzes the root causes of cracking in conventional surfacing procedures:
| Factor | Conventional Practice | Problem |
|---|---|---|
| Preheat temperature | 200–300°C | Insufficient to prevent cold cracking in rigid components |
| Interpass temperature | 200–300°C | Allows excessive cooling between passes |
| Post-weld heat treatment | High-temperature annealing with slow cooling | Creates prolonged exposure to cracking-sensitive temperature range |
| Weld sequence | Sequential pass deposition | Creates asymmetric stress distribution |
Process Modifications
Based on the cracking analysis, the following process modifications are proposed:
- Increased preheat temperature: Raise preheat to 300–400°C to reduce thermal gradient and allow plastic deformation of the valve body, relieving residual stress.
- Controlled interpass temperature: Maintain interpass temperature between 250–350°C to prevent excessive cooling and maintain ductility of the weld metal during deposition.
- Modified post-weld heat treatment: Replace the conventional high-temperature annealing with a lower-temperature stress relief treatment (400–500°C) for a shorter duration, reducing the time spent in the cracking-sensitive range.
- Symmetric welding sequence: Deposit weld passes in a symmetric pattern around the valve seat to minimize asymmetric stress and distortion.
- Reduced heat input per pass: Use smaller diameter electrodes and lower welding current to reduce the thermal cycle severity and minimize the risk of hot cracking.
Welding Consumable Selection
The selection of welding consumables for cobalt-based alloy surfacing is critical:
| Consumable Type | Electrode/Flux | Shielding | Application |
|---|---|---|---|
| SMAW | ECoCr-15 or equivalent | Flux-coated | Small areas, repair work |
| SAW | CoCrMo strip + flux | Flux (covered) | Large areas, automated surfacing |
| GTAW | CoCrMo rod | Argon | Precision surfacing, thin sections |
Quality Control
The quality of the cobalt alloy surfacing is verified through:
- Visual inspection: Check for surface cracks, porosity, and undercut.
- Dye penetrant testing (PT): Detect surface cracks that are not visible to the naked eye.
- Hardness testing: Verify the hardness of the overlay (typically 30–45 HRC for Stellite-type alloys).
- Microstructural examination: Examine the weld metal and HAZ for grain structure, phase distribution, and cracking.
Study Insights and Implications
This paper, though brief, addresses a critical practical problem in the surfacing of high-pressure valves. The key insight is that the rigidity of the valve body fundamentally changes the cracking behavior compared to more flexible components, and conventional surfacing procedures must be modified accordingly. For engineers working on valve repair and refurbishment in petrochemical plants, this work provides a practical approach to preventing cracking by modifying the thermal cycle rather than relying solely on material selection. The emphasis on symmetric welding sequences and controlled heat input is particularly relevant for modern valve manufacturing, where automation and precision are increasingly important. Future developments in cobalt alloy surfacing should focus on the development of low-stress welding consumables, advanced thermal management techniques (e.g., induction heating for localized preheat), and process monitoring systems that can detect cracking in real time.