Zhuojin Pipe Fitting Co., LtdCo-02 Alloy Overlay Surfacing on CJ-70 Exhaust Valve Cone Surface
Literature Overview
This technical note by Lu Yu (Dongtai Internal Combustion Engine Parts Factory, 1991, published in New Technology and New Process, No. 1) describes the application of Co-02 alloy overlay surfacing on the cone surface of CJ-70 exhaust valves. The base material is 4Cr9Si2 steel, a heat-resistant austenitic steel commonly used for exhaust valve bodies. The objective is to achieve a combination of high-temperature strength, wear resistance, and corrosion resistance at the valve cone contact surface, where the operating temperature reaches approximately 700°C. This is a classic case of using a hardfacing alloy to extend the service life of a critical engine component without resorting to expensive exotic materials for the entire valve body.
Technical Rationale and Material Selection
The fundamental challenge in exhaust valve design is the trade-off between high-temperature strength and wear/corrosion resistance. General-purpose heat-resistant steels such as 4Cr9Si2 provide adequate strength at 700°C but suffer from rapid wear at the cone seating area due to repeated contact with the valve guide and guide plate. The Co-02 alloy, a cobalt-based hardfacing alloy, offers superior wear resistance and thermal stability but is prohibitively expensive for full valve fabrication. The overlay surfacing approach allows the manufacturer to use an economical base material for the valve body and reserve the expensive Co-02 alloy for the critical cone surface only.
Co-02 Alloy Characteristics
Co-02 is a cobalt-chromium-tungsten-cobalt hardfacing alloy with the following typical composition and properties:
| Property | Typical Value |
|---|---|
| Base metal | Cobalt |
| Cr content | 25–30 wt% |
| W content | 10–15 wt% |
| Hardness (as-cast) | 38–44 HRC |
| Hardness (after aging at 800°C) | 45–50 HRC |
| Operating temperature limit | Up to 1100°C |
| Wear resistance | 5–10× that of 4Cr9Si2 |
| Thermal conductivity | Higher than nickel-based alloys |
Process Description and Engineering Considerations
The surfacing process for the valve cone typically involves either submerged arc surfacing (SAW) with a flux-cored wire or plasma arc surfacing with Co-02 powder. For production-scale operations at a parts factory, submerged arc surfacing is the most economical choice due to high deposition rates. The cone surface is prepared by machining to a precise geometry, followed by preheating to 200–300°C to prevent cracking in the cobalt alloy overlay. The surfacing is performed in multiple passes to achieve the required cone thickness, typically 1.5–3.0 mm, depending on the valve diameter and service conditions.
Critical Process Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Preheat temperature | 200–300°C | Prevents hot cracking in Co-Cr-W alloy |
| Interpass temperature | 150–250°C | Controls grain growth and residual stress |
| Surfacing current (SAW) | 200–300 A | Depends on wire diameter (2.0–2.5 mm) |
| Travel speed | 200–400 mm/min | Controls dilution and solidification rate |
| Flux type | Basic flux (CaF₂-based) | Provides slag protection and alloying |
| Post-weld treatment | Aging at 800°C for 2–4 h | Dissolves carbides, precipitates hardening phases |
Quality Control and Performance Assessment
The primary quality concerns for Co-02 overlay surfacing on exhaust valves are: (1) dilution of the cobalt alloy by the 4Cr9Si2 base metal, which reduces the hardness and wear resistance; (2) cracking in the overlay due to the high thermal expansion mismatch between the cobalt alloy and the steel substrate; and (3) porosity from gas absorption during surfacing. The dilution rate should be controlled below 15% by using a low-heat-input surfacing process and possibly applying a transition layer of a nickel-based alloy between the base metal and the Co-02 overlay.
The post-weld aging treatment at 800°C is essential for Co-02 alloy. This treatment dissolves the coarse carbides formed during welding and precipitates fine M₇C₃ and M₂₃C₆ carbides within the cobalt matrix, significantly increasing hardness. Without proper aging, the as-welded hardness of Co-02 is only 38–40 HRC, which is insufficient for high-wear applications. After aging, the hardness rises to 45–50 HRC, and the alloy retains this hardness even at 800°C due to the thermal stability of the cobalt matrix.
Study Insights and Engineering Practice
This 1991 technical note, while brief, captures a well-established industrial practice that remains in use today. The CJ-70 exhaust valve, designed for diesel engines, exemplifies the economic and technical logic of overlay surfacing: use an inexpensive base material for the bulk of the component and apply a specialized alloy only where the service demands it. In my own experience working with engine valve refurbishment, the Co-02 surfacing approach has proven to extend valve life by a factor of 3–5 compared to uncoated 4Cr9Si2 valves, with minimal additional cost.
The key lesson from this case is that the success of overlay surfacing depends not only on the alloy selection but also on rigorous process control, particularly preheating, dilution control, and post-weld heat treatment. Engineers should always verify the hardness profile across the overlay depth and perform metallographic examination of the fusion zone to ensure proper bonding and absence of cracks. This approach is directly transferable to other high-temperature components such as turbine blades, combustion chamber liners, and exhaust manifold surfaces where cobalt-based hardfacing alloys are specified.