The main failure causes of ultra-high pressure valves are cavitation and erosive wear. Many factors affect cavitation and erosion, mainly the mechanical properties of materials, fluid mechanics factors and environmental influences. Many methods can be used for improving the resistance to cavitation and erosion for valves.
1. The selection of materials
In order to improve the erosion and wear resistance for ultra-high pressure valves, the following materials which can resist corrosion are usually selected:
- Materials with good hardness
- Materials with acid corrosion-resistant protective films
- Materials with high yield point and good stability
- Materials with high fatigue strength
To improve the various properties of materials, one is to use alloying, and the other is to adopt appropriate heat treatment. The alloying method is to develop new materials with various special properties by changing the chemical composition of the steel. The heat treatment method does not change the chemical composition of the steel, but different heating, heat preservation and cooling are adopted for the steel in the solid-state to change the structure of the steel and improve the performance of the material.
2. Heat treatment and surface hardening treatment
(1) Vacuum heat treatment does not produce oxidation, decarburization and other corrosion during heating, and it has the function of degreasing and purifying the surface. In the vacuum, the hydrogen, nitrogen, oxygen and other gases absorbed in the smelting process can be removed to improve the quality and performance of the material. For example, after the ultra-high pressure needle valve made from W18Cr4V is subjected to vacuum heat treatment, the impact toughness of the needle valve is effectively improved, and the mechanical properties and service life are improved at the same time.
(2) Surface strengthening treatment
There are surface strengthening treatments such as surface quenching (flame heating, high-medium frequency heating surface quenching, contact electric heating surface quenching, electrolyte heating surface quenching and laser electron beam heating surface quenching), carburizing, nitriding, cyanidation, boronizing, metalizing (TD Methods), laser strengthening, chemical vapor deposition (CVD methods), physical vapor deposition (PVD methods), plasma chemical vapor deposition (PCVD methods), plasma spraying, etc.
3. Adopting new engineering materials
When the material of the flow-through parts of the ultra-high pressure valve is selected, the weight loss will be different due to different flow rates (maximum operating pressure). Under high pressure above 400MPa, materials with good hardness and red hardness should be selected, such as tool steel or cemented carbide. Under lower pressure between 100 to 400MPa, the material is required to have good plasticity, good toughness and good hardness. For example, the ultra-high pressure needle valve with working pressure of 690MPa in the United States adopts austenitic 316 stainless steel, and that of 1034MPa martensitic precipitation hardening stainless steel 17-4PH. In foreign countries, martensitic stainless steel and tool steel are mostly used for materials for components, valve discs and valve seats for withstanding cavitation, and chromium aluminum steel and stainless steel are used for the valve seat base. With the successful development of industrial ceramic technology, valves made from ceramic materials have also appeared. Ceramic materials have good erosion resistance at low angles of attack. The end strength is also reduced due to the reduction of the valve needle taper; the supporting force between the valve needle and valve seat is also reduced, which affects the reliability of the sealing. Therefore, when choosing ceramic materials to make the valve needle, not only the taper but also the strength should be considered.
