Cladding of Co-02 Alloy Layer on CJ-70 Exhaust Valve

Literature Overview

This 1991 technical publication by Lu Yu from Dongtai Internal Combustion Engine Parts Factory documents the application of plasma arc cladding with Co-02 alloy powder on CJ-70 exhaust valve surfaces. Exhaust valves in internal combustion engines operate under extreme conditions: temperatures exceeding 700 °C, cyclic thermal loading, oxidation, and mechanical contact with valve seats. The CJ-70 exhaust valve, designed for a specific engine application, required enhanced surface properties to extend service life and improve engine reliability.

Core Technical Content

Exhaust valve cladding is a classic application of surface engineering, where the goal is to improve the surface properties of a component without altering its bulk characteristics. The CJ-70 exhaust valve is typically manufactured from a heat-resistant austenitic stainless steel or a martensitic stainless steel, depending on the engine type and operating temperature. The valve head and stem sealing surfaces require enhanced hardness, wear resistance, and oxidation resistance.

The Co-02 alloy (a cobalt-chromium-tungsten alloy, similar to Stellite 6 or CoCrW) is a well-established overlay material for high-temperature wear applications. Its properties include:

Property	Typical Value
Hardness (as-cast)	30–40 HRC
Hardness (after heat treatment)	45–50 HRC
Density	~8.4 g/cm³
Thermal expansion coefficient	~13 × 10⁻⁶ /K
Oxidation resistance temperature	Up to 1000 °C
Wear resistance	Excellent at elevated temperatures

The Co-02 alloy achieves its wear resistance through a combination of:

Hard carbide phases (WC, Cr₇C₃) dispersed in a cobalt-rich matrix
Solid solution strengthening from chromium and tungsten
Excellent hot hardness retention due to the high melting point of cobalt

Process Development and Welding Parameters

Plasma arc cladding was selected for this application due to its ability to produce narrow, controlled overlay tracks with minimal dilution. The welding parameters for Co-02 cladding on exhaust valve surfaces typically include:

Parameter	Value	Rationale
Plasma current	150–250 A	Sufficient to melt powder and substrate
Arc voltage	25–35 V	Controls heat input and penetration
Travel speed	200–400 mm/min	Balances deposition rate and quality
Powder feed rate	150–300 g/min	Ensures adequate overlay thickness
Shielding gas	Argon (15–20 L/min)	Prevents oxidation of cobalt-rich melt
Preheat temperature	150–200 °C	Reduces cracking risk
Overlay thickness	0.5–1.5 mm	Adequate for valve application

The CJ-70 valve geometry presents unique challenges for cladding. The valve head is a curved surface with limited access for the torch, and the stem-to-head transition requires careful parameter adjustment to maintain consistent overlay quality. The cladding must be applied uniformly to avoid stress concentrations that could initiate cracking under thermal cycling.

Microstructural Considerations

The interface between the Co-02 overlay and the valve base steel is critical for long-term reliability. Key microstructural features include:

Dilution zone: The first layer of Co-02 alloy will have some dilution from the base steel, resulting in a composition gradient. This zone may have slightly lower hardness but generally remains above the required minimum.
Grain structure: The overlay typically exhibits columnar grains growing from the interface, transitioning to equiaxed grains in the upper layers. Columnar grains at the interface can be beneficial for crack resistance if they are fine and well-aligned.
Carbide distribution: The WC and Cr₇C₃ carbides should be uniformly distributed throughout the overlay. Coarse or segregated carbides can act as crack initiation sites.
Residual stress: The thermal contraction mismatch between the cobalt alloy and the steel substrate generates residual tensile stresses at the interface. Post-weld heat treatment (typically 800–900 °C for 1–2 hours) can relieve these stresses and promote carbide precipitation hardening.

Performance Evaluation and Engineering Results

The effectiveness of the Co-02 cladding on CJ-70 exhaust valves was evaluated through:

Hardness testing: Vickers and Rockwell hardness measurements across the overlay thickness to verify uniformity
Wear testing: Pin-on-disk or block-on-ring tests simulating valve seat contact conditions
Thermal cycling: Repeated heating and cooling to simulate engine operating cycles
Service life comparison: Comparison of cladded vs. uncladded valve life in field trials

Typical results for Co-02 cladding on exhaust valves show:

Hardness increase from ~30 HRC (base steel) to ~45 HRC (overlay)
Wear life improvement of 2–5 times compared to uncladded valves
No cracking or delamination after extended thermal cycling

Study Insights and Modern Context

This 1991 work represents an important application of plasma arc cladding technology in the Chinese internal combustion engine industry during a period of rapid industrialization. The methodology employed—selecting a proven overlay material (Co-02), optimizing process parameters through trial and error, and validating through field trials—remains relevant for modern surface engineering applications.

In contemporary practice, the Co-02 cladding approach for exhaust valves would be supplemented by:

Advanced powder metallurgy to produce near-net-shape valves with integrated overlay layers
Laser cladding for more precise thermal control and reduced dilution
Computational modeling of thermal stresses during engine operation
High-throughput characterization using automated metallography and data analysis-assisted phase identification

The enduring value of this work lies in its demonstration that plasma arc cladding with cobalt-based alloys can effectively solve the wear and oxidation challenges of exhaust valve surfaces, a technology that continues to be used in marine diesel engines, industrial gas turbines, and heavy-duty truck engines worldwide.