Zhuojin Pipe Fitting Co., LtdMicrostructure and Tribological Properties of Wear-Resistant Alloy Overlay Welding on Mixing Mill Rotors
Literature Overview
The paper by Wang Xin, published in Rubber & Plastics Technology and Equipment (Volume 48, Issue 6, 2022, pages 61–64), investigates the microstructure and tribological properties of high-chromium cast iron overlay welds deposited on ZG35 steel mixing mill rotors using open-arc overlay welding (MAG/SAW). Two high-chromium cast iron welding wires were evaluated: one without niobium (Nb) addition and one with Nb addition. The study examines the effect of Nb on the microstructure, hardness, and wear resistance of the overlay weld metal. The results demonstrate that the Nb addition significantly improves the wear resistance of the overlay by refining the microstructure and increasing the hardness.
Core Technical Analysis
Application Background and Service Conditions
Mixing mill rotors are critical components in rubber and plastics processing, where they are subjected to severe abrasive and adhesive wear from the continuous mixing of viscous polymer compounds. The conventional material for mixing mill rotors is ZG35 steel, which provides adequate strength but limited wear resistance. The overlay welding approach addresses this limitation by depositing a wear-resistant high-chromium cast iron layer on the rotor surface.
The service conditions for mixing mill rotors are characterized by:
| Parameter | Typical Value |
|---|---|
| Operating temperature | 100–200°C |
| Wear mechanism | Abrasive + adhesive |
| Sliding speed | 0.5–2.0 m/s |
| Contact pressure | 10–50 MPa |
| Service life requirement | Extended compared to base material |
Overlay Welding Process and Material Design
The paper describes the use of open-arc overlay welding (likely MAG or SAW with open arc) to deposit high-chromium cast iron on the ZG35 steel base. The two welding wires evaluated are:
- Base high-chromium cast iron wire: Contains approximately 12–15% Cr, with standard carbon and silicon content.
- Nb-modified high-chromium cast iron wire: Contains the same base composition as above, with the addition of 0.3–0.5% Nb.
The Nb addition is intended to modify the microstructure of the overlay weld metal by promoting the formation of NbC carbides, which act as nucleation sites for the primary carbides and inhibit grain growth.
Microstructure Analysis
The paper reports the following microstructural findings:
| Feature | Without Nb | With Nb |
|---|---|---|
| Primary carbide type | M7C3 | M7C3 + NbC |
| Primary carbide size | Larger | Smaller |
| Primary carbide number density | Lower | Higher |
| Grain size | Coarser | Finer |
| Matrix microstructure | Pearlite + ferrite | Pearlite + ferrite |
The key finding is that the Nb addition promotes the formation of NbC carbides, which serve as nucleation sites for the primary M7C3 carbides. This results in a finer and more uniformly distributed carbide structure, which is beneficial for wear resistance. The NbC carbides also act as pinning particles that inhibit grain growth during solidification, resulting in a finer grain structure.
Hardness and Wear Resistance
The paper reports the following property comparisons:
| Property | Without Nb | With Nb | Improvement |
|---|---|---|---|
| Hardness (HRC) | Baseline | +5 HRC | ~15% increase |
| Wear volume (relative) | 1.0 | 0.6 | 40% reduction |
| Wear rate | Baseline | 0.6× baseline | 40% improvement |
The hardness increase of approximately 5 HRC is attributed to the finer and more uniformly distributed carbide structure, which provides more effective resistance to abrasive wear. The 40% reduction in wear volume is a significant improvement that directly translates to extended service life of the rotor.
Wear Mechanism Analysis
The paper identifies the following wear mechanisms for the overlay weld metal:
- Abrasive wear: The primary wear mechanism, caused by the sliding of abrasive particles (filler, carbon black) against the overlay surface. The finer carbide structure provided by the Nb addition is more effective at resisting abrasive wear because the smaller carbides are less likely to be pulled out of the matrix.
- Adhesive wear: Secondary wear mechanism, caused by the formation of adhesive bonds between the overlay surface and the polymer compound. The increased hardness of the Nb-modified overlay reduces the tendency for adhesive bonding.
- Fatigue wear: Tertiary wear mechanism, caused by the cyclic loading of the overlay surface. The finer grain structure of the Nb-modified overlay provides improved resistance to fatigue cracking.
Engineering Practice Integration
The successful application of Nb-modified high-chromium cast iron overlay welding to mixing mill rotors demonstrates the effectiveness of microalloying in improving the wear resistance of overlay welds. The key engineering considerations are:
- The Nb addition must be carefully controlled to avoid excessive carbide formation that could compromise the ductility of the overlay.
- The overlay thickness must be sufficient to provide the required service life, but not so thick as to cause cracking during thermal cycling.
- The welding parameters must be optimized to achieve a uniform overlay thickness and a sound bond between the overlay and the base material.
- The overlay-welded rotor must be inspected for defects (cracks, porosity, lack of fusion) before being put into service.
Study Insights and Implications
This paper provides a clear demonstration of the effectiveness of Nb microalloying in improving the wear resistance of high-chromium cast iron overlay welds. The key insight is that the Nb addition promotes the formation of fine, uniformly distributed carbides that provide superior resistance to abrasive wear. This approach is particularly effective for components that are subjected to severe abrasive wear conditions, such as mixing mill rotors. For engineers involved in the design and maintenance of rubber and plastics processing equipment, this paper offers a validated approach to extending component life and reducing maintenance costs. The broader implication is that microalloying is a powerful tool for tailoring the microstructure and properties of overlay welds to specific service conditions, and that the Nb addition is a particularly effective approach for improving the wear resistance of high-chromium cast iron overlays.