How can the service life of high-pressure valves be extended?

The primary causes of failure in ultra-high-pressure valves are cavitation and erosion wear; however, there are numerous factors that influence cavitation and erosion, the main ones being the mechanical properties of the materials, fluid dynamics and environmental influences. There are many methods that can be employed to improve a valve’s resistance to cavitation and erosion wear.

I. Material Selection

To improve the erosion resistance of ultra-high-pressure valves, corrosion-resistant materials are typically selected:

1. Materials with high hardness
2. Materials with a protective film resistant to acid corrosion
3. Materials with a high yield point and good stability
4. Materials with high fatigue strength
To improve the various properties of materials, two approaches are employed: alloying and appropriate heat treatment. Alloying involves altering the chemical composition of steel to develop new materials with specific properties. Heat treatment, on the other hand, does not alter the chemical composition of the steel; instead, it involves subjecting the steel to different heating, holding and cooling processes whilst in the solid state to modify its microstructure and enhance its properties.

II. Heat Treatment and Surface Hardening 1. Vacuum Heat Treatment

This process prevents oxidation, decarburisation and other forms of corrosion during heating, whilst also serving to clean the surface by removing oil and grease. In a vacuum, gases such as hydrogen, nitrogen and oxygen absorbed by the material during smelting can be expelled, thereby improving the material’s quality and performance. For example, subjecting an ultra-high-pressure needle valve made from W18Cr4V to vacuum heat treatment effectively increases the valve’s impact toughness, whilst simultaneously improving its mechanical properties and service life.
2. Surface Hardening Treatments
These include surface hardening (flame hardening, medium- and high-frequency induction hardening, contact electric heating hardening, electrolytic hardening, laser and electron beam hardening, etc.), carburising, nitriding, cyaniding, boronising, metal diffusion (TD method), laser hardening, chemical vapour deposition (CVD), physical vapour deposition (PVD), plasma chemical vapour deposition (PCVD), plasma spraying, etc.

III. Adoption of New Engineering Materials
When selecting materials for the flow-through components of ultra-high-pressure valves, consideration must be given to the fact that flow velocity (maximum operating pressure) varies, as does the weight loss. 
At higher pressures (above 400 MPa), materials with high hardness and good red hardness, such as tool steel or cemented carbide, should be selected.
At lower pressures (100–400 MPa), materials are required to possess both good plasticity and toughness, as well as high hardness. For example, HIP Corporation’s ultra-high-pressure needle valves utilise austenitic 316 stainless steel for operating pressures of 690 MPa, and martensitic precipitation-hardening 17-4PH stainless steel for operating pressures of 1034 MPa. Internationally, materials for components subjected to cavitation—such as valve discs and seats—are predominantly martensitic stainless steel and tool steel, whilst valve seat bodies are typically made of chromium-aluminium steel or stainless steel.
With the successful development of industrial ceramic technology, ceramic valves have also emerged. Ceramic materials exhibit high erosion resistance at low impact angles; however, as the taper of the valve needle decreases, so does its end strength, and the reaction force between the valve needle and seat is reduced, thereby affecting sealing reliability. Therefore, when selecting ceramic materials for valve needles, consideration must be given not only to the size of the taper but also to the material’s strength.