CLADDING TECHNOLOGY SHANXI CO., LTDPlasma Cladding of Stellite Alloy on 1Cr12Ni2W1Mo1V Stainless Steel Microstructure and Properties
Literature Overview
This research by Zhang Youyi, Sun Xuejie, and Ran Chuanhai from Sichuan Engineering Vocational and Technical College (2019) examines the application of plasma transferred arc (PTA) powder cladding to deposit Stellite alloy overlay layers onto 1Cr12Ni2W1Mo1V martensitic stainless steel substrates. The study focuses on the microstructural evolution, hardness distribution, and wear resistance of the resulting composite material system, which is relevant to applications in mining, power generation, and chemical processing equipment.
Core Technical Points
1Cr12Ni2W1Mo1V is a cold-work tool steel with excellent wear resistance and good toughness, commonly used for dies, molds, and cutting tools. However, its surface hardness and wear resistance can be further enhanced through overlay cladding with Stellite alloy. Stellite alloys (particularly Stellite 6, which is the most commonly used grade) are cobalt-chromium-tungsten based alloys with exceptional resistance to hot corrosion, erosion, and wear at elevated temperatures.
The plasma arc cladding process uses a high-velocity plasma jet (typically 8000-12000 K) to melt the powder feedstock and the substrate surface simultaneously. The powder particles are injected into the plasma plume through a lance nozzle and are melted before impacting the substrate, resulting in a metallurgical bond with minimal dilution. Typical dilution rates for PTA cladding are 5-15%, significantly lower than conventional arc welding processes.
Microstructure Characteristics
The microstructure of the PTA Stellite overlay on 1Cr12Ni2W1Mo1V substrate exhibits several important features:
| Layer | Microstructure | Hardness (HV) |
|---|---|---|
| Overlay surface | M7C3 and M23C6 carbides in FCC matrix | 450-550 |
| Overlay interior | Coarse carbides, columnar dendrites | 400-500 |
| Dilution zone | Mixed structure with base metal elements | 350-450 |
| Interface | Transition zone with dissolved carbides | 300-400 |
| Base metal HAZ | Tempered martensite with retained austenite | 280-350 |
The carbide morphology in the Stellite overlay is critical for wear resistance. M7C3 carbides (Cr7C3) are the primary wear-resistant phase in Stellite alloys, while M23C6 carbides form preferentially at grain boundaries and can act as crack initiation sites if overly concentrated. The distribution and size of these carbides are strongly influenced by the cooling rate, which in turn depends on the plasma power, travel speed, and powder feed rate.
Process Parameters and Optimization
PTA cladding requires careful control of multiple parameters to achieve optimal results:
| Parameter | Typical Range | Impact |
|---|---|---|
| Plasma current | 100-300 A | Controls melting rate and penetration |
| Arc voltage | 20-35 V | Affects plasma jet characteristics |
| Powder feed rate | 200-800 g/min | Controls dilution and deposit thickness |
| Travel speed | 100-500 mm/min | Affects cooling rate and microstructure |
| Shielding gas | Ar or Ar+H2 | Prevents oxidation and affects arc stability |
| Powder particle size | 45-150 microns | Affects flowability and melting efficiency |
The powder particle size distribution is particularly important. Uniform particle sizes (typically 45-75 microns for fine powder or 75-150 microns for medium powder) ensure consistent flow through the lance and uniform melting. Irregular or oversized particles can lead to unmelted inclusions in the deposit, which become stress concentrators and reduce fatigue life.
Dilution Control and Its Importance
Dilution is the primary concern in PTA cladding of Stellite on stainless steel substrates. Excessive dilution introduces iron and chromium from the base metal into the overlay, which can:
- Reduce the melting point of the overlay alloy, decreasing hot corrosion resistance.
- Change the carbide composition from M7C3 to M23C6, which is less wear-resistant.
- Introduce martensite-forming elements that can lead to cracking during cooling.
- Reduce the cobalt content, which is essential for the alloy's high-temperature strength.
The study demonstrates that maintaining dilution below 10% is achievable with proper parameter selection. The key to low dilution is using high powder feed rates relative to the plasma power, which ensures that the powder particles are fully melted before reaching the substrate surface.
Wear Resistance Evaluation
The wear resistance of the PTA Stellite overlay was evaluated through standard testing methods. The results typically show:
| Test Method | Overlay Wear Rate | Base Metal Wear Rate | Improvement Factor |
|---|---|---|---|
| Pin-on-disc (dry) | 0.5-1.5 x 10^-3 mm3/Nm | 3.0-5.0 x 10^-3 mm3/Nm | 3-5x |
| Abrasive wear (sand) | 0.8-2.0 x 10^-3 mm3/Nm | 4.0-6.0 x 10^-3 mm3/Nm | 3-4x |
| Erosion wear | 1.0-2.5 x 10^-3 mm3/Nm | 5.0-8.0 x 10^-3 mm3/Nm | 4-6x |
The wear resistance improvement is primarily attributed to the high hardness of the carbide phases in the Stellite overlay. However, the improvement factor decreases at elevated temperatures due to carbide dissolution and softening of the matrix.
Engineering Applications and Practice
The combination of 1Cr12Ni2W1Mo1V substrate with Stellite overlay is particularly valuable for components subjected to severe abrasive and erosive conditions. Typical applications include:
- Mining equipment (crusher liners, conveyor pulleys)
- Power plant components (turbine blades, burner nozzles)
- Chemical processing equipment (valve seats, pump impellers)
- Cement industry equipment (grinding rollers, kiln wear plates)
The PTA process is well-suited for these applications because it produces dense, pore-free deposits with excellent metallurgical bonding. The overlay thickness can be controlled from 0.5 mm to 5.0 mm per pass, with multiple passes used to achieve thicker deposits.
Quality Assurance Considerations
The quality of PTA cladding is verified through several methods:
| Test Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection | Surface defects | No visible cracks, porosity, or unmelted powder |
| Magnetic particle testing | Surface cracks | No indications per ASME V |
| Ultrasonic testing | Internal defects | No indications > 3 mm equivalent |
| Hardness testing | Uniformity | Within specified range, no soft spots |
| Metallographic examination | Microstructure | No unmelted inclusions, acceptable porosity |
The hardness profile across the overlay thickness is an important quality indicator. A uniform hardness distribution indicates consistent melting and solidification conditions. Localized soft spots may indicate incomplete melting of powder particles or excessive dilution in that region.
Study Insights and Reflections
The PTA cladding of Stellite on martensitic stainless steel represents a mature technology with well-established process parameters. The key engineering insight from this study is that the dilution control is the primary factor determining the performance of the overlay. Engineers must understand that achieving low dilution requires a balance between plasma power and powder feed rate, and that the powder particle size distribution significantly affects the process stability.
The study also highlights an important consideration that is often overlooked: the substrate preparation. The surface roughness and cleanliness of the 1Cr12Ni2W1Mo1V substrate directly affect the bond strength and the dilution rate. A well-prepared substrate (machined to Ra 3.2 microns or better, cleaned of oil and contaminants) is essential for achieving reliable results.
From a practical standpoint, the PTA process requires skilled operators and well-maintained equipment. The plasma lance is a consumable component that must be replaced regularly to maintain consistent results. The process is also relatively expensive compared to conventional welding methods, which limits its application to high-value components where the performance benefits justify the cost.