CLADDING TECHNOLOGY SHANXI CO., LTDRepair Welding of Cemented Carbide Overlay Layers on Mixing Chambers
Literature Overview
This 1995 publication by Li Xue'an of Yiyang Rubber Machinery Factory, published in the journal Welding, addresses a practical and challenging repair welding problem encountered in rubber mixing equipment. Mixing chambers (intensive mixers) used in rubber processing are subjected to extreme abrasive and erosive conditions, and cemented carbide (WC-Co) overlay layers are applied to their critical surfaces to extend service life. When these overlay layers suffer localized damage—spalling, cracking, or erosion through—a repair welding operation is required to restore the protective layer without compromising the integrity of the surrounding intact overlay.
Technical Challenges of Cemented Carbide Overlay Repair
Repair welding of cemented carbide overlays presents several unique challenges that distinguish it from conventional steel repair welding:
- Extreme hardness mismatch: WC-Co overlay layers typically have a hardness of 1200–1800 HV, while the substrate steel is 200–300 HV. This extreme difference creates severe challenges for achieving metallurgical bonding.
- Thermal sensitivity of the overlay: Excessive heat input during repair welding can cause cracking in the existing overlay layer due to its inherent brittleness.
- Dilution control: The repair weld must maintain adequate carbide content and cobalt binder phase ratio to preserve wear resistance, while avoiding excessive dilution from the base steel.
- Residual stress management: The coefficient of thermal expansion mismatch between WC-Co and steel is significant, and repair welding introduces new residual stresses that can trigger cracking in the existing overlay.
Welding Process Selection and Parameters
| Process | Applicability | Key Parameters |
|---|---|---|
| Submerged arc welding (SAW) | Large area repair | Wire: WC-Co agglomerate; Flux: special low-dilution; Current: 300–500 A |
| Gas metal arc welding (GMAW) | Medium area repair | Wire: WC-Co cored wire; Shielding: Ar + 5% CO2; Current: 150–250 A |
| Gas tungsten arc welding (GTAW) | Small area repair | Electrode: pure tungsten; Filler: WC-Co powder paste; Current: 80–150 A |
| Oxy-fuel welding | Emergency repair | Flame: neutral; Filler: pre-mixed carbide rod |
The selection of process depends primarily on the size of the damaged area, the accessibility of the repair location, and the required quality level. For mixing chambers in rubber processing, the repair typically involves SAW or GMAW with specialized carbide-containing consumables.
Repair Procedure and Quality Control
A systematic repair procedure is essential for successful cemented carbide overlay restoration:
- Damage assessment: Determine the extent of damage using visual inspection and magnetic particle testing (MT). Identify the boundary between intact and damaged overlay.
- Surface preparation: Remove all damaged material by grinding or machining. The preparation should extend into sound overlay by at least 1–2 mm to ensure a clean repair interface. The surface should be ground to bare metal with a slight undercut to facilitate mechanical interlock.
- Preheating: Apply localized preheating to 200–300°C to reduce thermal gradients and minimize cracking risk in the existing overlay.
- Welding execution: Use multiple thin passes to build up the repair layer. Each pass should be no thicker than 2–3 mm to limit heat input. Interpass temperature should be maintained below 350°C.
- Post-weld treatment: Allow slow cooling, preferably under insulation blankets. Avoid water quenching or forced air cooling.
- Inspection: Perform MT for surface cracks and hardness testing to verify the overlay properties. Typical target hardness for the repair area is 1000–1400 HV.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in existing overlay | Excessive heat input during repair | Reduce current, increase travel speed, apply more aggressive preheating |
| Poor bond strength | Incomplete cleaning of repair area | Increase grinding depth, use solvent cleaning between passes |
| Low hardness in repair zone | Excessive dilution | Use lower current, shorter arc length, specialized flux/wire |
| Porosity | Contamination or gas entrapment | Ensure dry consumables, proper flux coverage, clean base metal |
| Spalling of overlay edge | Thermal shock at repair boundary | Apply gradual heat input, use pulsed welding parameters |
Study Insights
This paper represents the practical engineering approach to overlay repair that characterizes the mid-1990s Chinese manufacturing industry. The emphasis is on consumable selection, parameter optimization, and procedural discipline rather than on fundamental metallurgical understanding. From a modern perspective, the work lacks detailed metallurgical analysis of the repair zone, including microstructural characterization and bond strength testing. However, the systematic approach to damage assessment, surface preparation, and multi-pass repair is sound and remains applicable today.
The key insight for contemporary engineers is that repair welding of hardfacing overlays is fundamentally different from new overlay application. The existing overlay acts as a thermally sensitive constraint that limits the available welding parameters. Successful repair requires a conservative approach to heat input, combined with meticulous surface preparation and post-weld cooling control. In modern practice, techniques such as laser cladding or cold spray are increasingly used for overlay repair because they offer significantly lower heat input and better control over dilution, but these methods were not available or economically viable in 1995.