Zhuojin Pipe Fitting Co., LtdTungsten Carbide Overlay Welding on Feed Mill Hammer Plates
Literature Overview
The paper by Liu Xuejun and Ma Songbai from the School of Mechanical Engineering, Beijing Technology and Business University, published in Grain and Feed Industry (2010, Issue 4, pp. 39-41), addresses a practical engineering problem that many equipment maintenance engineers face: the rapid wear of hammer plates in feed grinding mills. The authors compare four hardening processes and focus on tungsten carbide (WC) overlay welding as the most effective solution. The study covers material selection, welding method, preheat requirements, and robotic welding application, making it a compact but technically dense reference for surface engineering in agricultural processing equipment.
Core Technical Points
Material Selection Rationale
The selection of tungsten carbide-based overlay materials is driven by the extreme abrasion mechanism encountered in feed mills, where hammer plates experience high-frequency impact and sliding contact with grain particles. WC particles, with hardness values exceeding HV3000, provide exceptional resistance to adhesive and abrasive wear. However, the brittleness of WC demands careful control of the welding thermal cycle to avoid microcracking in the dilution zone. The authors emphasize that the base material—typically low-carbon steel such as Q235 or Q345—must be compatible with the high carbon equivalent of the overlay filler.
Welding Process Parameters
The study highlights four hardening approaches: high-frequency induction hardening, flame hardening, electroslag hardening, and overlay welding. Among these, WC overlay welding via robotic GTAW or FCAW provides the most controllable and repeatable results. Key process parameters extracted from the study include:
| Parameter | Typical Range | Rationale |
|---|---|---|
| Preheat temperature | 200–300 °C | Reduces cooling rate to minimize HAZ hardening and cracking |
| Interpass temperature | 150–250 °C | Controls dilution and prevents thermal fatigue |
| Welding current (FCAW) | 180–260 A | Balances penetration and dilution ratio |
| Travel speed | 150–300 mm/min | Controls heat input and bead geometry |
| Shielding gas (GTAW) | 100% Ar or Ar/CO₂ mix | Prevents oxide inclusion in the overlay |
| Number of passes | 2–3 | Ensures sufficient WC retention without excessive cracking |
Robotic Welding Application
The integration of robotic welding is a notable contribution. Automated welding eliminates operator variability in travel speed and torch angle, which are critical for maintaining uniform dilution levels in WC overlay. The authors note that a controlled dilution ratio of 15–25% is optimal: below 15%, the overlay becomes excessively brittle; above 25%, the WC particles dissolve into the matrix, reducing the hardness benefit.
Defect Analysis and Countermeasures
A common failure mode in WC overlay welding is transverse cracking in the overlay layer, caused by the high thermal stress from the mismatch between the coefficient of thermal expansion of WC (6.5×10⁻⁶/°C) and the steel matrix (12×10⁻⁶/°C). The authors propose three countermeasures:
- Introducing a transition layer of medium-carbon steel (e.g., D147 or equivalent) between the base and the WC overlay to buffer thermal stress.
- Applying a multi-pass welding strategy with the first pass using a low-carbon steel wire to create a ductile foundation.
- Post-weld stress relief at 550–600 °C for 1–2 hours to relieve residual stresses below 100 MPa.
Another frequently observed defect is porosity, particularly gas porosity from hydrogen absorbed in the flux-coated wire. The study recommends strict flux drying at 300 °C for 2 hours and keeping the welding environment below 80% relative humidity.
Integration with Engineering Practice
From a practical standpoint, the robotic WC overlay process can extend hammer plate service life by 3–5 times compared to the original quenched-and-tempered condition. In a typical feed mill operation, hammer plates are replaced every 2000–3000 operating hours; with WC overlay, this interval can be extended to 8000–10000 hours. The economic analysis should consider not only the overlay cost (typically 15–25% of new plate cost) but also the reduction in downtime for replacement.
When applying this technology to pipe fitting repair—analogous to overlay repair of worn reducer or elbow surfaces in slurry pipelines—the same principles of dilution control and thermal management apply. The key difference is that pipe fittings often have thicker walls and higher carbon equivalent, requiring more aggressive preheating and slower cooling rates.
Key Questions and Reflections
One question that arises is whether laser cladding could replace arc overlay welding for WC coatings on hammer plates. Laser cladding offers lower dilution (5–10%), higher WC retention, and finer microstructure, but the equipment cost and processing speed limitations make it less economical for high-volume agricultural equipment repair. The robotic arc welding approach remains the pragmatic choice for most industrial settings.
Another reflection concerns the long-term stability of WC overlay under cyclic loading. The study does not address fatigue performance, which is critical for hammer plates subjected to repeated impact. Engineers applying this technology should supplement the overlay with fatigue testing under simulated operating conditions.
Study Insights and Implications
This paper serves as a well-structured reference for surface hardening of wear parts, particularly in the context of automated welding. The systematic comparison of four hardening methods and the detailed discussion of robotic welding parameters provide actionable guidance for maintenance engineers. The emphasis on dilution control and transition layer design is particularly valuable, as these are the most common sources of failure in overlay welding practice. For engineers working in piping and fittings, the underlying metallurgical principles—thermal stress management, dilution control, and microstructure optimization—are directly transferable to overlay repair of worn pipeline components.