Zhuojin Pipe Fitting Co., LtdApplication and Analysis of Cobalt-Based Alloy Surfacing
Literature Overview
The paper by Yu Jianping, Zhang Yong, and Zhang Yufeng from the Lanzhou Refining and Chemical Machinery Factory, published in Welding Technology (Vol. 29, No. 3, 2000, pp. 7-8), addresses the practical challenges encountered during cobalt-based alloy surfacing of refinery equipment. The authors document the cracking phenomena observed when different surfacing methods are employed, analyze the cracking mechanisms from multiple perspectives, and propose preventive measures. The classification code TG455 confirms the work's relevance to surfacing welding, with specific application to the demanding service conditions of petroleum refining equipment.
Core Technical Points
Cobalt-based alloys, such as Stellite-type alloys (Co-Cr-W), are widely used in surfacing applications where extreme resistance to wear, corrosion, and high-temperature oxidation is required. Refinery equipment components such as pump shafts, valve seats, mixers, and heat exchanger tubes operate under conditions of high temperature, corrosive media, and mechanical abrasion, making cobalt-based surfacing a critical technology. However, the cracking susceptibility of cobalt-based alloys during surfacing is a well-known challenge that can compromise the integrity of the deposit.
The authors identified the following key findings:
- Cracking occurs when different surfacing methods are applied to cobalt-based alloys, with the cracking mode and severity depending on the process parameters and the surfacing sequence.
- Cracking mechanisms include hot cracking, cold cracking, and reheat cracking, each associated with different stages of the cooling cycle and different metallurgical causes.
- Strict adherence to surfacing procedures, including proper surfacing sequence, control of harmful elements in both the consumable and base material, is essential for crack prevention.
Cracking Mechanisms and Prevention Measures
| Cracking Type | Occurrence Stage | Primary Cause | Prevention Measure |
|---|---|---|---|
| Hot cracking | Solidification | Low melting point eutectics at grain boundaries | Preheat, control sulfur and phosphorus |
| Cold cracking | Post-weld cooling | Hydrogen embrittlement, high residual stress | Post-weld heat treatment, low hydrogen consumables |
| Reheat cracking | PWHT | Stress relaxation cracking in susceptible microstructures | Controlled PWHT temperature and rate |
Metallurgical Analysis of Cobalt-Based Surfacing Cracking
Cobalt-based alloys are characterized by an FCC austenitic matrix with a high volume fraction of hard carbide precipitates, primarily Co3W and Co3Cr. The high alloy content (typically 25-30% Cr, 5-7% W, 5-6% Mo) provides excellent corrosion and wear resistance but also contributes to cracking susceptibility through several mechanisms:
- High melting point and wide solidification range: The solidification of cobalt-based alloys occurs over a wide temperature range, promoting the formation of columnar grains with interdendritic segregation of low-melting-point eutectics. These eutectics are prone to hot cracking during solidification.
- High thermal residual stress: The combination of high heat input during surfacing and the high thermal conductivity of cobalt-based alloys generates significant thermal gradients and residual stresses. These stresses, if not relieved, can initiate cold cracking in the presence of hydrogen.
- Hydrogen sensitivity: Cobalt-based alloys are susceptible to hydrogen embrittlement, particularly when hydrogen is introduced from the atmosphere, the base material, or the consumable. The high alloy content reduces the solubility of hydrogen in the solid, promoting hydrogen accumulation at grain boundaries and microcrack initiation.
- Dilution effects: The dilution of the cobalt-based consumable by the carbon steel base material introduces carbon, manganese, and sulfur, which can form low-melting-point eutectics and promote hot cracking.
The authors emphasized that the surfacing sequence is a critical process variable. Multi-pass surfacing without proper interpass temperature control can lead to cumulative stress buildup and cracking. The recommended approach is to use a compatible transition layer between the carbon steel base and the cobalt-based surfacing layer, followed by multiple thin passes with controlled interpass temperatures.
Process Parameters and Quality Control
The surfacing of cobalt-based alloys requires careful control of the following process parameters:
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Preheat temperature | 200-400 degrees C | Reduces thermal gradient, slows cooling rate |
| Interpass temperature | 150-300 degrees C | Prevents cold cracking, maintains plasticity |
| Heat input | Low to moderate | Minimizes dilution, reduces residual stress |
| Travel speed | Moderate | Balances penetration and dilution |
| PWHT temperature | 800-900 degrees C | Relieves residual stress without over-aging |
Quality control for cobalt-based surfacing should include:
- Visual inspection: Check for surface cracks, undercut, and incomplete fusion.
- Magnetic particle testing (MT): Detect surface and near-surface cracks, particularly important for cobalt-based alloys that may be slightly magnetic.
- Ultrasonic testing (UT): Detect subsurface cracks and lack of fusion.
- Hardness testing: Verify that the hardness of the surfacing layer meets specification, typically 35-50 HRC for cobalt-based alloys.
- Chemical analysis: Verify the composition of the surfacing layer, paying particular attention to Cr, W, Mo, C, S, and P content.
Engineering Practice Implications
In refinery equipment repair, cobalt-based surfacing is often applied to components that have experienced severe wear or corrosion damage. The cracking susceptibility of cobalt-based alloys makes the surfacing process challenging, and the consequences of cracking can be severe, leading to component failure and unplanned shutdowns. The authors' emphasis on strict adherence to surfacing procedures and control of harmful elements is therefore of practical importance.
The use of a transition layer is a critical practice that should be standardized in refinery repair procedures. The transition layer, typically an austenitic stainless steel or a nickel-based alloy, acts as a buffer between the carbon steel base and the cobalt-based surfacing layer, reducing the dilution rate and preventing the formation of brittle martensitic phases at the interface.
The control of harmful elements, particularly sulfur and phosphorus, in both the consumable and the base material is another critical practice. These elements promote hot cracking by forming low-melting-point eutectics at grain boundaries. The base material should be inspected for sulfur and phosphorus content, and consumables with low sulfur and phosphorus levels should be selected.
Study Insights and Reflections
This paper, while concise, provides a valuable practical perspective on the challenges of cobalt-based alloy surfacing in refinery applications. The authors' experience-based analysis of cracking mechanisms and preventive measures reflects deep practical knowledge of the process. The emphasis on surfacing sequence and harmful element control, rather than solely on process parameters, is a mature and insightful approach that recognizes the multifactorial nature of cracking.
The work also underscores the importance of process discipline in surfacing operations. Even with the correct consumable and equipment, cracking can occur if the surfacing sequence is not followed, if interpass temperatures are not controlled, or if harmful elements in the base material are not addressed. This is a reminder that surfacing quality is not solely determined by the consumable or the welding parameters but by the entire process system, including preparation, execution, and post-weld treatment.
The overall contribution of this study lies in providing a practical framework for the successful application of cobalt-based alloy surfacing in refinery equipment repair, emphasizing the importance of process discipline, material control, and systematic crack prevention.