The main failure reasons of ultra-high pressure valves are: cavitation and erosion wear, and there are many factors affecting cavitation and erosion, mainly including mechanical properties of materials, hydrodynamic factors and environmental influences. To improve the valve’s resistance to cavitation and erosion wear, many methods can be used.
1. Material selection
In order to improve the ability of ultra-high pressure valves to resist erosion and wear, corrosion-resistant materials are usually selected:
1. Materials with high hardness
2. Materials with acid-resistant protective film
3. Materials with high yield point and good stability
4. Materials with high fatigue strength.
To improve various properties of materials, one is to use alloying, and the other is to use appropriate heat treatment. The alloying method is to develop new materials with various special properties by changing the chemical composition of steel. The heat treatment method does not change the chemical composition of the steel, but applies different heating, heat preservation and cooling to 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
1. Vacuum heat treatment
It does not produce oxidation, decarburization and other corrosion during heating, and it has the effect of purifying the surface of oil and grease. In the vacuum, the hydrogen, nitrogen, oxygen and other gases absorbed by the material during the smelting process can be extracted, and the quality and performance of the material can be improved. For example, after vacuum heat treatment of the ultra-high pressure needle valve made of W18Cr4V, the impact resistance of the needle valve is effectively increased, and the mechanical properties and service life are improved at the same time.
2. Surface strengthening treatment
Such as surface quenching (flame heating, high and medium frequency heating surface quenching, contact electric heating surface quenching, electrolyte heating surface quenching, laser electron beam heating surface quenching, etc.), carburizing, nitriding, cyanidation, boronizing, metalizing (TD method), laser strengthening, chemical vapor deposition (CVD method), physical vapor deposition (PVD method), plasma chemical vapor deposition (PCVD method), plasma spraying, etc.
Third, the use of new engineering materials
When choosing the material of the over-current parts of the ultra-high pressure valve, the difference in flow rate (higher operating pressure) should be considered, and the weight loss is also different. Under higher pressure (above 400MPa), select materials with high hardness and good red hardness, such as tool steel or cemented carbide. Under lower pressure (100 ~ 400MPa), the material is required to have both good plasticity and toughness, and high hardness. For example, the ultra-high pressure needle valve of HIP Company in the United States uses austenitic 316 stainless steel with a working pressure of 690MPa, and uses martensitic precipitation hardening stainless steel 17-4PH with a working pressure of 1034MPa. In foreign countries, martensitic stainless steel and tool steel are used for the component materials, valve disc and valve seat to withstand cavitation, and chromium aluminum steel and stainless steel are used for the valve seat base. With the successful development of industrial ceramic technology, ceramic material valves have also appeared. Ceramic materials have high erosion resistance at low attack angles, but due to the reduced taper of the valve needle, the strength of its end is also reduced, and the support reaction force between the valve needle and the valve seat is also reduced, which affects the reliability of the seal. Therefore, when choosing a ceramic material to make a valve needle, not only the size of its taper, but also its strength should be considered.