CLADDING TECHNOLOGY SHANXI CO., LTDDilution Rate Control of Stellite Alloy Plasma Arc Cladding Through Process Parameter Optimization
Literature Overview
This 2014 study published in Welding Technology by Zhu Kai, Que Meifu, Zhu Zhiyuan, Dai Anlun, Chen Hui, and Niu Wenming from Jiangsu University and KSB Valve Industry Changzhou Co., Ltd. investigates the influence of plasma transferred arc (PTA) cladding process parameters on the dilution rate of Stellite alloy coatings. The research was supported by the Jiangsu University Advantageous Discipline Construction Project and the Jiangsu Provincial Key Laboratory of Advanced Welding Technology, reflecting the collaborative effort between academia and industry in advancing hard-facing technology for critical valve components.
Fundamentals of Dilution Rate in PTA Cladding
The dilution rate in PTA cladding is defined as the volume fraction of base metal melted and incorporated into the cladding layer relative to the total volume of the cladding layer. For Stellite alloy coatings, which typically contain 50–60% Co, 25–35% Cr, and 10–20% W (as Stellite 6 or similar grades), the dilution rate directly affects the final composition and, consequently, the corrosion resistance, hardness, and wear resistance of the overlay.
In PTA cladding, the dilution rate is inherently lower than in conventional arc welding overlay methods due to the focused plasma arc and high energy density. Typical dilution rates for PTA cladding range from 10–30%, compared to 30–50% for submerged arc welding (SAW) and 40–60% for electroslag welding (ESW). However, even within the PTA range, variations of 10–15 percentage points can significantly alter the coating properties.
| Process Parameter | Low Setting | High Setting | Dilution Rate Trend |
|---|---|---|---|
| Arc current (A) | 120 | 250 | Increases with current |
| Powder feed rate (g/min) | 150 | 500 | Decreases with feed rate |
| Travel speed (mm/min) | 80 | 250 | Decreases with speed |
| Arc-to-powder distance (mm) | 3 | 8 | Increases with distance |
| Nozzle-to-workpiece distance (mm) | 5 | 15 | Increases with distance |
| Shielding gas flow (L/min) | 5 | 15 | Minimal effect |
Parameter Interaction and Optimization
The authors conducted a systematic parametric study using orthogonal experimental design to identify the most influential process parameters on dilution rate. The results revealed that arc current and powder feed rate are the two most significant factors, followed by travel speed. The interaction between arc current and powder feed rate is particularly important: at low feed rates, increasing current significantly increases dilution, while at high feed rates, the effect of current is partially masked by the excess powder.
The optimal parameter window for achieving a dilution rate below 20% for Stellite 6 PTA cladding was identified as follows: arc current 180–200 A, powder feed rate 350–450 g/min, travel speed 150–200 mm/min, and arc-to-powder distance 5–6 mm. Under these conditions, the resulting coating composition closely matches the powder composition, with Co content within 2–3% of the nominal value and Cr content within 1–2% of the nominal value.
A critical finding was the effect of arc-to-powder distance on dilution rate. At very short distances (less than 3 mm), the plasma arc directly impinges on the substrate, creating a deep penetration and high dilution. At very long distances (greater than 8 mm), the arc becomes unstable and the powder feeding efficiency decreases, leading to inconsistent layer quality. The optimal distance of 5–6 mm provides a balance between arc stability, powder melting efficiency, and controlled substrate melting.
Metallurgical Consequences of Dilution Rate Variation
The dilution rate has profound effects on the microstructure and properties of the Stellite cladding layer. At low dilution rates (below 15%), the coating retains its full Stellite composition, exhibiting a high hardness (38–45 HRC) and excellent corrosion resistance in oxidizing and reducing acids. The microstructure consists of a Co-based solid solution matrix with Cr-rich carbides (Cr7C3, Cr23C6) and W-rich carbides (WC, W2C).
At moderate dilution rates (15–25%), the incorporation of iron from the base metal promotes the formation of additional iron carbides and modifies the eutectic microstructure. The hardness may decrease slightly (35–40 HRC), but the toughness improves due to the increased ductility of the iron-rich phases. This range is often acceptable for many industrial applications where a balance between hardness and toughness is desired.
At high dilution rates (above 25%), the coating composition deviates significantly from the Stellite alloy, potentially compromising the corrosion resistance and wear resistance. The increased iron content promotes the formation of martensite and pearlite in the matrix, which may be acceptable for wear applications but detrimental for corrosion service. The authors emphasize that for critical valve seat applications, the dilution rate should be maintained below 20% to ensure the required performance.
Engineering Application and Quality Assurance
For valve seat cladding applications, such as those in high-pressure steam shutoff valves or control valves, the dilution rate control is critical for ensuring long-term sealing performance. The authors recommend a multi-pass cladding strategy where the first pass establishes a controlled dilution layer, and subsequent passes are deposited with optimized parameters to achieve the desired final composition.
Quality assurance procedures should include:
- Chemical analysis of the first and last cladding passes to verify dilution rate compliance
- Hardness testing across the full thickness of the cladding layer to detect compositional gradients
- Metallographic examination to assess microstructural homogeneity and the absence of defects such as cracks, pores, and unmelted particles
- Corrosion testing (intergranular corrosion, pitting resistance) for applications requiring high corrosion resistance
The study provides a practical framework for WPS development and qualification testing for Stellite PTA cladding, which can be adapted for other Co-based and Ni-based alloy coatings used in similar applications.
Study Insights and Practical Recommendations
This research underscores the importance of systematic process parameter optimization in achieving consistent cladding quality. The orthogonal experimental design approach used by the authors is an efficient methodology that can be applied to other cladding systems with different alloys and substrates. The identified parameter windows provide a starting point for WPS development, but it is essential to validate these parameters under actual production conditions, considering factors such as substrate geometry, joint configuration, and environmental conditions.
The collaboration between Jiangsu University and KSB Valve Industry demonstrates the value of industry-academia partnerships in advancing cladding technology. The practical insights gained from this research have direct implications for improving the reliability and service life of critical valve components in the oil, gas, and chemical processing industries.