Zhuojin Pipe Fitting Co., LtdRepair Welding of Cemented Carbide Surfacing Layer on Mixing Chamber
Literature Overview
This paper, published in Welding (No. 2, 1995, pp. 22-23) by Li Xuean from Yiyang Rubber Machinery Factory, addresses the repair welding of a cemented carbide surfacing layer on the mixing chamber of a rubber mixing machine. The mixing chamber is a critical component in rubber compounding, subjected to severe abrasive and adhesive wear from the continuous mixing of rubber compounds with fillers and additives. The original design included a 5 mm thick cemented carbide surfacing layer (HRC 56-60) on the inner surface of the chamber liner to extend service life, but the high hardness and low plasticity of the carbide layer presented significant challenges for repair welding.
Core Technical Content
The mixing chamber described in the paper has the following configuration:
| Component | Specification |
|---|---|
| Overall dimensions | 889 mm × 590 mm × 1000 mm |
| Base material | 20Mn steel |
| Construction | Inner liner + wear lining plate + rib plates + wall plates (welded assembly) |
| Surfacing layer | 5 mm thick cemented carbide |
| Surfacing hardness | HRC 56-60 |
| Surfacing application | Inner surface of chamber liner |
Cemented Carbide Surfacing Characteristics
Cemented carbide surfacing layers, typically composed of tungsten carbide (WC) or chromium carbide (Cr₃C₂) particles bonded in a cobalt or nickel matrix, offer exceptional wear resistance but present unique welding challenges:
- High hardness (HRC 56-60) — The extreme hardness makes the surfacing layer difficult to machine and repair.
- Low plasticity — The brittle nature of the carbide layer makes it susceptible to cracking under thermal stress.
- Thermal expansion mismatch — The coefficient of thermal expansion of cemented carbide is significantly different from that of the 20Mn base steel, leading to high residual stresses at the interface.
- Low fracture toughness — The carbide layer has limited ability to absorb energy, making it prone to spalling under impact or thermal shock.
Repair Welding Challenges and Solutions
The repair welding of cemented carbide surfacing layers is one of the most challenging tasks in industrial surfacing repair. The fundamental difficulty lies in the fact that the repair weld must be deposited onto a brittle, high-hardness substrate that is prone to cracking under thermal stress. The paper discusses the following key aspects of the repair process:
Pre-Weld Preparation
- Defect assessment — Thorough inspection of the damaged area to determine the extent of damage and the condition of the remaining surfacing layer.
- Surface preparation — Grinding or machining of the damaged area to remove loose or cracked carbide material and expose sound base material.
- Preheating — Careful consideration of preheating requirements to manage thermal stress, although the optimal preheat temperature for cemented carbide repair is not explicitly stated in the paper.
Welding Process Selection
The selection of welding process for cemented carbide repair is critical. The following factors must be considered:
| Factor | Consideration |
|---|---|
| Heat input | Must be minimized to reduce thermal stress in carbide layer |
| Process type | Low-heat-input processes preferred (GTAW, cold spray) |
| Consumable selection | Must be compatible with both carbide layer and base steel |
| Interpass temperature | Must be controlled to prevent cracking |
| Post-weld cooling | Controlled cooling rate to minimize residual stress |
Common Defects in Carbide Surfacing Repair
| Defect | Cause | Prevention |
|---|---|---|
| Cracking at interface | Thermal stress from expansion mismatch | Preheating, low heat input, controlled cooling |
| Spalling of carbide layer | Thermal shock, excessive heat input | Gradual heating, minimal heat input |
| Incomplete fusion | Low wettability of carbide surface | Surface preparation, appropriate consumable |
| Porosity | Gas entrapment, poor wetting | Clean surface, proper shielding |
Engineering Practice Integration
The repair of cemented carbide surfacing layers on mixing chambers is a common maintenance task in rubber manufacturing. The frequency of repair depends on several factors:
- Rubber compound composition — Compounds containing abrasive fillers (silica, carbon black) cause faster wear.
- Mixing temperature — Higher temperatures accelerate wear and can cause thermal degradation of the carbide layer.
- Mixing cycle — The number of mixing cycles per day directly affects the wear rate.
- Surface finish of carbide layer — A smoother surface reduces adhesive wear from rubber compound.
Repair Strategy Recommendations
Based on the technical content of this paper and general engineering practice, the following repair strategy is recommended for cemented carbide surfacing layers on mixing chambers:
- Minor damage (surface spalling, <1 mm depth) — Local repair using GTAW with a nickel-based filler metal, followed by controlled cooling.
- Moderate damage (1-3 mm depth) — Removal of damaged carbide layer, surface preparation, and re-surfacing with a compatible carbide-based consumable.
- Severe damage (>3 mm depth or through-thickness) — Complete removal of the carbide layer, inspection of base material for cracking, and re-surfacing with a new carbide layer.
Key Questions and Reflections
The paper does not provide detailed information on the specific welding consumable used for the repair, which is a critical piece of information for engineers attempting to replicate the process. The selection of filler metal for cemented carbide repair is non-trivial — common choices include nickel-based alloys (ENi-CrMo, ENi-Fe), cobalt-based alloys, and carbide-containing consumables, each with different advantages and limitations.
Another important consideration is the long-term reliability of the repair. Cemented carbide surfacing layers are inherently brittle, and the repair weld introduces a new interface that may be a preferential site for crack initiation. Engineers should consider the use of non-destructive testing (NDT) methods such as magnetic particle inspection (MT) or dye penetrant testing (PT) to verify the integrity of the repair before returning the component to service.
Study Insights and Implications
This paper highlights the challenges and practical considerations of repairing cemented carbide surfacing layers in industrial equipment. The key insight is that repair welding of brittle, high-hardness surfacing layers requires a careful balance between heat input, cooling rate, and consumable selection to minimize the risk of cracking and spalling. For engineers working on similar repair tasks, the paper provides a valuable case study that emphasizes the importance of thorough defect assessment, careful process planning, and rigorous quality verification.