Zhuojin Pipe Fitting Co., LtdLarge-Area Surfacing Process for Cobalt-Chromium-Tungsten Cemented Carbide
Literature Overview
Published in 2008 by Li Falin and Long Benren from Jiangxi Jianglian Energy and Environmental Protection Co., Ltd., this paper addresses the practical challenge of applying cobalt-chromium-tungsten cemented carbide surfacing over large areas on gasification equipment components. Cemented carbide surfacing offers exceptional wear resistance but is notoriously difficult to apply over large areas due to cracking, porosity, and dilution issues. This paper presents a systematic approach to welding process parameter optimization for large-area coverage.
Core Technical Findings
Challenges of Large-Area Cemented Carbide Surfacing
The application of cemented carbide surfacing over large areas presents several unique challenges compared to small-area or localized surfacing:
| Challenge | Root Cause | Consequence |
|---|---|---|
| Cracking | High carbon equivalent, rapid solidification, thermal stress | Surface cracks, spalling |
| Porosity | Gas evolution from carbide decomposition, inadequate shielding | Reduced load-bearing capacity |
| Dilution | Base metal melting into the surfacing pool | Reduced hardness and wear resistance |
| Uneven deposition | Large area coverage requires multiple passes | Inconsistent properties |
| Heat input accumulation | Multiple passes on large area | Overheating, microstructural degradation |
Process Parameter Optimization
The study presents a systematic approach to welding process parameter selection for large-area surfacing:
- Preheating: The base material is preheated to 300-400°C to reduce the temperature gradient and minimize thermal stress. This is critical for preventing cracking in the surfacing layer.
- Interpass temperature control: Maintaining the interpass temperature below 200°C prevents excessive heat input accumulation while ensuring adequate wetting of the previous layer.
- Welding current and voltage: Lower current and voltage settings are used to minimize dilution of the base metal into the surfacing pool. Typical parameters include:
- Current: 80-120 A (depending on electrode size)
- Voltage: 18-24 V
- Travel speed: 50-80 mm/min
- Layer thickness control: Each surfacing layer is deposited to a controlled thickness of 1.5-2.5 mm to minimize the volume of material subject to cracking and to ensure adequate dilution control.
- Welding sequence: A systematic welding sequence is employed to ensure uniform heat distribution and minimize distortion. For large flat surfaces, a block-by-block approach with overlap is used.
Microstructural and Performance Characterization
The surfacing layer microstructure consists of:
- Cobalt-chromium matrix: Provides the tough, corrosion-resistant base
- Tungsten carbide (WC) particles: Provide the primary wear resistance; typically 10-20 μm in size
- Chromium carbides (Cr₇C₃, Cr₂₃C₆): Contribute to hardness and oxidation resistance
The resulting hardness is typically 80-90 HRC, with excellent resistance to abrasive and erosive wear. The surface finish is typically Ra 3.2-6.3 μm, which is acceptable for most gasification equipment applications.
Engineering Practice Implications
Quality Control Procedures
| Inspection Stage | Method | Acceptance Criteria |
|---|---|---|
| Pre-weld | Visual inspection of base surface | No rust, oil, or scale; preheated to specified temperature |
| During welding | Parameter monitoring | Current, voltage, travel speed within specified range |
| Post-weld | Visual inspection | No surface cracks, porosity, or undercut |
| Post-weld | Magnetic particle testing (MT) | No cracks longer than 1 mm in the surfacing layer |
| Post-weld | Hardness testing | 80-90 HRC across the entire surfacing area |
| Post-weld | Thickness measurement | Uniform thickness within ±0.5 mm of nominal |
Common Defects and Remediation
- Surface cracking: If surface cracks are detected, they can be ground out and rewelded. However, if the cracking is extensive, the entire surfacing layer may need to be removed and reapplied with modified parameters (lower current, higher preheat).
- Porosity: Porosity is typically caused by inadequate shielding or contamination of the surfacing material. Ensuring proper gas shielding (argon or argon-helium mixture) and using dry, uncontaminated surfacing material are essential.
- Low hardness: Low hardness in localized areas indicates excessive dilution. This can be corrected by using a lower current, faster travel speed, or a thicker surfacing layer.
Study Insights and Reflections
This paper addresses a practical problem that is frequently encountered in industrial surfacing operations: how to apply a high-performance surfacing layer over a large area without compromising quality. The systematic approach to process parameter optimization presented here is directly applicable to similar challenges in other industries, such as the surfacing of large pump housings, turbine casings, and mining equipment components.
The emphasis on preheating and interpass temperature control is particularly noteworthy. In my experience, many surfacing failures on large components are traced back to inadequate preheating or uncontrolled interpass temperatures. This paper reinforces the importance of these often-overlooked parameters.
The application to gasification equipment is also significant. Gasification equipment operates under severe conditions of high temperature, abrasive gas-solid flow, and corrosive atmosphere. The cobalt-chromium-tungsten cemented carbide surfacing provides a robust solution, but only if applied correctly. This paper provides the technical foundation for successful implementation.