Valve Rod Cladding via Electric Discharge Machining
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Solution Overview
Problem
High-temperature steam in thermal power plants causes oxide film formation on valve apparatus contact surfaces, leading to peeling, sticking, and adhesive wear, which reduces wear resistance and service life, especially when cobalt-based hard alloys are used for cladding due to excessive heat input and similar material properties between valve rod and bushing.
Innovation Solution
The method involves using electric discharge machining to form a cobalt-based hard alloy cladding on the valve rod with a functionally gradient composition to minimize heat input and prevent bending, combined with a chromizing or titanizing process on the bushing to create a chromium carbide or titanium carbide layer, respectively, to enhance wear and oxidation resistance, and using different materials for the sliding surfaces to reduce adhesive wear.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If cobalt-based hard alloy is formed by cladding by welding on the sliding-contact surface of the valve rod, then wear resistance and oxidation resistance are improved, but non-permissive bending easily occurs during manufacture due to large heat input amount
Solution Approach 1:
The patent replaces the conventional welding process with electric discharge machining (EDM) to form the cobalt-based hard alloy cladding layer. EDM uses electrical discharge to erode and deposit material without direct contact or high heat input, thereby achieving wear-resistant coating formation while avoiding the thermal deformation and bending that occur with welding methods.
Solution Approach 2:
The patent changes the processing parameters from high-heat welding to low-heat electric discharge machining. By controlling the electrical discharge energy, the process achieves material deposition with minimal heat input, preventing thermal expansion and subsequent bending deformation of the valve rod while still forming a dense, wear-resistant cladding layer.
2Ease of manufacture
If the same metallic material is formed on both the valve rod and bushing sliding-contact surfaces, then manufacturing is simplified, but adhesive wear easily occurs during sliding movement due to similar material properties
Solution Approach 1:
The patent applies different metallic materials to the sliding-contact surfaces of the valve rod and bushing. Specifically, the valve rod receives a cobalt-based hard alloy cladding while the bushing receives a chromium-based or titanium-based alloy coating. This local differentiation of material properties prevents adhesive wear by ensuring the contacting surfaces have dissimilar metallurgical characteristics, while the overall manufacturing process remains systematic and controlled.
Solution Approach 2:
The patent employs composite material strategies by combining different metallic alloys on mating sliding surfaces. The valve rod uses cobalt-based hard alloy embedded with carbide particles, while the bushing uses chromium or titanium-based alloys, creating a composite material system that leverages the complementary properties of different metals to resist adhesive wear through galvanic and metallurgical incompatibility.
3Reliability
If nitriding treatment is applied to contact surfaces, then wear resistance is enhanced, but oxide film formation occurs at high temperatures causing peeling and sticking
Solution Approach 1:
The patent replaces the nitriding treatment with electric discharge machining-based cladding that forms a more stable, oxidation-resistant metallic coating. Instead of relying on a nitride layer that degrades at high temperatures, the process creates a durable cobalt-based hard alloy coating that maintains its protective function in high-temperature steam environments without forming peeling oxide films.
Solution Approach 2:
The patent changes the surface treatment parameter from chemical nitriding to physical/electrical electric discharge machining. This parameter change transitions the process from forming a chemically-bonded nitride layer (which oxidizes at high temperature) to forming a metallurgically-bonded metallic cladding layer that exhibits superior oxidation resistance and thermal stability in steam turbine operating conditions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces heat input during cladding, prevents adhesive wear, and significantly improves wear resistance and durability by forming high-quality, stress-free coatings with equal linear expansion coefficients, maintaining hardness and oxidation resistance at high temperatures.
Implementation Method 1
a cladding portion is integrally formed on a sliding-contact surface of the valve rod by electric discharge machining
Implementation Method 2
a chromizing or titanizing process on the bushing to create a chromium carbide or titanium carbide layer
Implementation Method 3
create a chromium carbide or titanium carbide layer
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
According to one embodiment, there is provided a valve apparatus including a cladding portion that is integrally formed on a sliding-contact surface of either a valve rod (205) or a valve body (204), which serves as a movable member, wherein the cladding portion is formed by inducing a pulsed discharge between an electrode, which is formed of a molded body consisting mainly of a metal, and a treatment target portion of either the valve rod (205) or the valve body (204), so as to weld and deposit a material of the electrode on a surface of the treatment target portion; and a surface layer that is integrally formed on a sliding-contact surface of either a bushing (201) or a sleeve, which serves as a stationary member, wherein the surface layer is formed by forming a first coating film by surface-hardening heat treatment using a metallic cementation.