Zhuojin Pipe Fitting Co., LtdCold Overlay Welding of Wear-Resistant Layer on Concrete Distribution Valve Shell
Literature Overview
Published in "Welding" (1991, Issue 4, pp. 16-18) by Ma Caiwang from Hubei Construction Machinery Factory, this paper describes the cold overlay welding of a high-hardness wear-resistant layer on the inner surface of a concrete distribution valve shell made of HT250 cast iron. The work is classified under TG455 and addresses a specific wear problem in construction machinery.
Application Context and Wear Mechanisms
Concrete distribution valves are critical components in concrete pumps and batching plants. They direct the flow of concrete from the pump cylinder to the delivery pipeline. The inner surface of the valve shell is subjected to severe abrasive wear from the concrete slurry, which contains hard aggregate particles (quartz, granite, etc.) suspended in a cementitious matrix.
The wear mechanisms involved include:
- Abrasive wear: Hard aggregate particles scrape and gouge the valve surface
- Erosive wear: High-velocity concrete flow impacts the surface
- Corrosive wear: The alkaline cement slurry (pH 12-13) can chemically attack the surface
- Fatigue wear: Cyclic loading from concrete pressure fluctuations
HT250 gray cast iron, while adequate for structural purposes, has limited wear resistance due to its relatively low hardness (HB 170-240) and the presence of graphite flakes that act as stress concentrators and reduce surface integrity.
Overlay Material and Process Selection
The paper describes a "cold overlay welding" process, which likely refers to a process that minimizes heat input to the base material, thereby reducing the risk of cracking and distortion in the HT250 cast iron. Common processes for cold overlay welding include:
- Cold metal transfer (CMT): A pulsed arc process with very low heat input
- Cold spray: A solid-state process that deposits material without melting
- Low-heat-input plasma arc welding: Using low current and high travel speed
The overlay material must provide high hardness (typically HRC 50-65) and good adhesion to the HT250 base. Suitable materials include:
| Overlay Material | Hardness (HRC) | Key Features |
|---|---|---|
| High-carbon martensitic steel | 50-60 | Good toughness, moderate wear resistance |
| High-speed steel | 55-65 | Excellent wear resistance, lower toughness |
| Cr-based alloy | 50-60 | Good corrosion resistance, moderate wear resistance |
| Carbide-containing composite | 60-70 | Excellent wear resistance, potential for cracking |
Process Parameters and Quality Control
The cold overlay welding process must be carefully controlled to achieve the required deposit quality while minimizing damage to the base material:
- Preheating: Minimal or no preheating to maintain the "cold" process characteristic
- Heat input: Low heat input (typically < 1.5 kJ/mm) to reduce HAZ softening
- Welding sequence: Designed to minimize distortion and residual stress
- Post-weld treatment: Stress relief may be required if significant residual stresses develop
Quality control requirements include:
- Hardness testing: Verification of overlay hardness (HRC 50+)
- Visual inspection: No cracks, porosity, or slag inclusions
- Bond strength testing: Peel test or shear test to verify overlay adhesion
- Thickness measurement: Minimum overlay thickness of 2-3 mm
Engineering Considerations
The cold overlay welding approach is particularly suitable for HT250 cast iron because:
- Cast iron is highly susceptible to cracking during welding due to its low ductility
- Excessive heat input can cause graphitization and softening of the HAZ
- Distortion can compromise the geometric accuracy of the valve shell
The "cold" process minimizes these risks by reducing the thermal cycle severity. However, the trade-off is that lower heat input may result in reduced bond strength and potentially higher porosity if the process is not carefully controlled.
Study Insights
This work addresses a practical wear problem in construction machinery using a surface engineering approach. The selection of cold overlay welding is appropriate for the HT250 base material, which is challenging to weld using conventional processes. The key challenge is achieving sufficient bond strength while maintaining the "cold" process characteristic that prevents base material damage.
The economic benefit of overlay welding is significant for valve shells, which are relatively large castings that would be expensive to replace entirely. The overlay layer extends service life by providing a hard, wear-resistant surface that protects the base material from abrasive wear.