Zhuojin Pipe Fitting Co., LtdDomestication Trial of Cobalt-based Hard Alloy Electrode Arc Surfacing
Literature Overview
The paper by Ma Ming, Li Yinan, and Li Chunguang (2010, Electric Welder, Vol. 40, No. 2, pp. 132-135) documents the domestication effort of cobalt-based hard alloy surfacing welding electrodes at Harbin Boiler Works Co., Ltd. The research addresses the economic and supply chain challenges associated with imported cobalt-based surfacing electrodes, which are critical for wear-resistant applications in the modern coal chemical industry. The work represents a practical engineering effort to achieve localization of a high-value welding consumable that had previously been dependent on foreign suppliers.
Technical Background
Cobalt-based hard alloys are among the most effective wear-resistant materials available for industrial applications. Their exceptional performance stems from:
- High hardness: Typically 700-900 HV in the as-deposited condition
- Excellent hot hardness retention: Cobalt maintains its hardness at elevated temperatures due to its high melting point (1,495°C)
- Good thermal shock resistance: The combination of cobalt's thermal conductivity and the thermal expansion match with carbide phases
- Chemical stability: Resistance to oxidation and corrosion at elevated temperatures
- Toughness: Cobalt matrix provides better toughness than nickel or iron-based alternatives
Common Cobalt-based Alloy Systems
| Alloy Type | Typical Composition | Hardness (HV) | Application |
|---|---|---|---|
| Stellite 6 | Co-Cr-W-C | 350-450 | General wear resistance |
| Stellite 21 | Co-Ni-Cr-W-C | 350-450 | Corrosion + wear |
| Hard alloy type | Co-Cr-W-C (high C, W) | 700-900 | Severe abrasion |
| Bimetallic | Co + carbide particles | 800-1000 | Extreme wear conditions |
Domestication Process Development
Weldability Assessment
The primary challenge in domesticating cobalt-based surfacing electrodes is achieving consistent weldability. Cobalt alloys are known for:
- Hot cracking susceptibility: Due to the solidification range and presence of low-melting eutectics
- Cracking sensitivity: Thermal stresses during cooling can cause transverse cracking
- Porosity tendency: High carbon and alloy content promotes gas evolution
- Dilution effects: Interaction with base metal can alter the alloy composition and properties
The authors systematically evaluated the weldability of domestically produced cobalt-based hard alloy electrodes through:
- Chemical composition analysis: Verification of alloy content against specification
- Macroscopic cross-section examination: Assessment of deposition profile, lack of fusion, and cracking
- Surface hardness testing: Confirmation of wear-resistant hardness levels in the deposited layer
Process Parameter Development
Based on the weldability assessment, the following process parameters were established:
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Current type | DCEP (DC electrode positive) | Deep penetration, stable arc, reduced spatter |
| Current range | 80-150 A (for 3.2 mm electrode) | Adequate melting without excessive dilution |
| Arc voltage | 22-28 V | Controlled arc length and deposition profile |
| Travel speed | 150-250 mm/min | Balance between deposition rate and cooling rate |
| Preheat temperature | 150-250°C | Reduce thermal gradient, minimize cracking |
| Interpass temperature | ≤300°C | Prevent excessive grain growth and cracking |
| Post-weld treatment | Slow cooling in insulated box | Reduce residual stress, prevent delayed cracking |
Quality Verification
Chemical Composition Verification
The domestic electrode deposits were verified to contain the required cobalt, chromium, tungsten, and carbon content within acceptable tolerances. The key alloying elements and their functions are:
- Cobalt: Matrix element providing hot hardness and toughness
- Chromium: Solid solution strengthening and carbide formation
- Tungsten: Carbide formation (WC), enhancing hardness and wear resistance
- Carbon: Carbide formation, primary hardening element
- Molybdenum: Additional solid solution strengthening (in some grades)
Hardness Profile Assessment
The surface hardness of the deposited layer was measured to confirm wear-resistant performance. Acceptable hardness levels for cobalt-based hard alloy surfacing typically range from 700 to 900 HV, with the specific target depending on the application requirements.
Macroscopic Examination
Cross-section examination of deposited layers revealed:
- Uniform deposition profile without excessive build-up or undercut
- Absence of macroscopic cracks in the deposited layer and heat-affected zone
- Good fusion with the base material
- Acceptable dilution levels (typically 5-15% for single-pass deposition)
Engineering Practice Implications
Cost-Benefit Analysis
| Factor | Imported Electrode | Domestic Electrode |
|---|---|---|
| Unit cost | High (import premium) | Significantly reduced |
| Lead time | Long (international shipping) | Short (local supply) |
| Technical support | Limited | Direct manufacturer support |
| Quality consistency | High (established manufacturer) | Requires qualification |
| Customization | Difficult | Feasible |
Implementation Considerations
For industrial adoption of domestically produced cobalt-based surfacing electrodes, the following considerations are important:
- Qualification testing: Full qualification according to applicable standards (ASME Section IX, AWS D10.6, or relevant Chinese standards)
- Welder training: Specialized training on cobalt alloy welding techniques, including proper preheat, interpass temperature control, and post-weld treatment
- Equipment requirements: DC welding power sources with current stability; wire feeders if using FCAW process
- Consumable storage: Protection from moisture and contamination; controlled storage conditions
- In-process monitoring: Regular visual inspection of deposited layers for cracking, porosity, or irregular profile
Common Defects and Remediation
| Defect | Root Cause | Prevention |
|---|---|---|
| Transverse cracking | Excessive cooling rate, high thermal stress | Preheating, controlled interpass temperature, post-weld stress relief |
| Longitudinal cracking | Impurity segregation, low-melting eutectics | Clean base metal, proper consumable composition |
| Porosity | Moisture in consumable, gas evolution | Consumable drying, proper shielding, clean work area |
| Excessive dilution | High current, slow travel speed | Process parameter optimization, multi-pass with thinner layers |
| Incomplete fusion | Insufficient heat input | Increase current, reduce travel speed, proper joint preparation |
Study Insights and Reflections
This domestication effort represents a practical and economically significant contribution to the Chinese welding consumable industry. Cobalt-based hard alloy electrodes are high-value consumables where import dependency creates both cost and supply chain vulnerabilities.
The systematic approach taken by the authors—weldability assessment, process parameter development, and quality verification—provides a template for similar domestication efforts in other specialized welding consumable categories. The methodology is essentially an application of the PDCA (Plan-Do-Check-Act) cycle adapted to welding consumable development.
From a technical perspective, the successful domestication of cobalt-based electrodes requires mastery of several challenging aspects:
- Alloy chemistry control: Precise control of cobalt, chromium, tungsten, and carbon content to achieve target properties
- Arc characteristics: Stable arc with appropriate penetration and deposition profile
- Cracking resistance: Management of solidification cracking through composition design and process control
- Batch consistency: Reproducible quality across production lots
The work also highlights the broader strategic importance of developing domestic capabilities in specialized welding materials. In the context of China's growing coal chemical industry and its demand for wear-resistant equipment, the availability of domestically produced cobalt-based surfacing electrodes provides both economic benefits and supply security.
Future development should focus on expanding the range of available cobalt-based alloys to cover different service conditions (high temperature, corrosion, extreme abrasion) and on developing automated surfacing processes (GMAW, plasma transfer arc) that can achieve higher deposition rates and more consistent quality than manual SMAW.