Zhuojin Pipe Fitting Co., LtdEffect of Plasma Overlay Welding Heat Input on Mechanical Properties of Colmonoy 56 SPL Coated Diesel Engine Valves
Literature Overview
This paper by Zhou Fangming and colleagues from Jiangsu University and Nanjing COSCO Shipping Ship Equipment Parts Co., Ltd. (Journal of Xuzhou University of Technology, 2023, Vol. 38, No. 4, pp. 20–25) investigates the influence of plasma arc surfacing heat input on the microstructure and mechanical properties of Colmonoy 56 SPL overlay deposits applied to 42CrMo alloy steel diesel engine valve blanks. The research is supported by the 2021 Jiangsu Provincial Graduate Practice Innovation Program (SJCX21_1763) and addresses the critical challenge of optimizing overlay welding parameters for valve repair and refurbishment in the marine and power generation industries.
Microstructural Characterization
The Colmonoy 56 SPL overlay deposit exhibits three distinct phases regardless of heat input variation, though their relative proportions change significantly with process parameters:
| Phase | Composition Characteristic | Microhardness Trend | Role in Properties |
|---|---|---|---|
| γ-Ni matrix | Nickel-based austenite | Lower | Ductility, toughness |
| Cr-rich carbide | Chromium carbides | Higher | Wear resistance |
| Eutectic (rosette) | Ni-Cr intermetallic | Intermediate | Hardness contribution |
The rosette-shaped eutectic morphology is characteristic of Ni-Cr alloy solidification and represents a coupled growth structure of Ni solid solution and chromium carbide. The proportion of this eutectic phase is the primary microstructural variable that responds to changes in heat input, and it directly governs the macroscopic hardness of the overlay layer.
Heat Input Effects on Properties
The study demonstrates a clear inverse relationship between eutectic phase proportion and overlay hardness. As heat input increases, the dilution rate rises, and the eutectic phase proportion decreases, leading to a reduction in overall overlay hardness. This behavior can be explained through solidification thermodynamics:
- Low heat input: Rapid solidification promotes constitutional supercooling, favoring eutectic reaction and higher eutectic phase fraction, resulting in higher hardness
- High heat input: Slower cooling allows more complete solute diffusion, suppressing eutectic nucleation and promoting single-phase dendritic growth, resulting in lower hardness
The dilution rate variation with heat input is a critical process parameter. Higher heat input increases the volume of molten base metal that mixes with the deposited powder, diluting the alloy composition away from the nominal Colmonoy 56 SPL chemistry. This dilution effect reduces the chromium and carbon activity in the weld pool, directly affecting the thermodynamic driving force for carbide and eutectic formation.
Valve Application Context
Diesel engine valves operate under extreme conditions involving:
- Temperatures exceeding 700°C in exhaust applications
- Severe thermal cycling at startup and shutdown
- Corrosive combustion products (sulfur, sodium, vanadium)
- Impact loading from valve spring and guide contact
The Colmonoy 56 SPL alloy is specifically designed for these conditions, combining nickel-based corrosion resistance with chromium carbide hardening. The overlay process must therefore achieve a balance between:
- Sufficient hardness for wear resistance at valve seating surfaces
- Adequate toughness to withstand impact loading without cracking
- Low dilution to preserve the alloy's corrosion resistance characteristics
- Thermal stability to maintain microstructure integrity during valve operation
Process Optimization Recommendations
Based on the study findings, the following process optimization strategy is recommended for plasma overlay welding of Colmonoy 56 SPL on diesel engine valves:
- Heat input minimization: Operate at the lowest practical heat input that still ensures complete powder melting and sound bonding. Typical plasma arc parameters might include 100–150 A current, 18–25 V voltage, and travel speeds of 200–400 mm/min, depending on the specific equipment and powder feed rate.
- Dilution control: Target dilution rates below 25% to maintain sufficient alloying element concentration for eutectic phase formation. This can be achieved through preheating control, multi-pass thin layers, or base metal preparation that limits thermal mass.
- Microstructure verification: Include metallographic examination of production welds to confirm the target eutectic phase proportion, correlating observed microstructure with hardness measurements for process validation.
- Post-weld inspection: Perform hardness mapping across the overlay layer to detect compositional banding or dilution variation that may compromise valve performance.
Study Insights and Engineering Value
This research provides valuable quantitative data for process optimization in valve overlay applications. The identification of eutectic phase proportion as the primary hardness-controlling variable offers a clear quality control metric that can be implemented in production environments. However, the study would benefit from additional investigation of the overlay layer's thermal fatigue resistance and corrosion behavior under simulated valve operating conditions, as these properties are equally critical for long-term valve service life as hardness alone.