In hard-sealed ball valves, the hardening treatment of the ball and seat is the core technology to improve the wear resistance, corrosion resistance and service life of the valve. The following are the key technical methods and precautions for hardening the ball and seat:

I. Hardening technology for the ball

1. Surface coating technology

Thermal spraying (HVOF, plasma spraying)

Tungsten carbide (WC) coating: A dense tungsten carbide coating is formed on the surface of the ball through supersonic flame spraying (HVOF), with a hardness of more than HRC 70 and a wear resistance of 5 to 10 times.

Chromium carbide (Cr3C2) coating: Suitable for high temperature (below 800°C) and corrosive environment, with better oxidation resistance than tungsten carbide.

Physical vapor deposition (PVD/CVD)

The PVD process is used to coat hard films such as titanium nitride (TiN) and chromium nitride (CrN), with a surface hardness of up to HRC 80, reducing the friction coefficient, and suitable for precision balls.

2. Surface modification technology

Nitriding/carburizing treatment

Ion nitriding is performed on stainless steel spheres (such as 316L) to form a high-hardness nitride layer (HV 1000~1200) on the surface while retaining the toughness of the matrix.

Laser cladding

Stellite alloy or nickel-based alloy (such as Inconel 625) is clad on the surface of the sphere to achieve a combination of high hardness (HRC 55~60) and corrosion resistance.

3. Material selection

Overall hardening material

High-hardness materials are used to directly process the sphere, such as:

Ceramics (zirconia, silicon carbide): high hardness (HV 1500+), high temperature resistance, but brittle, and impact must be avoided.

Tool steel (such as D2, M2): hardness reaches HRC 60~62 after quenching, low cost, suitable for medium and low pressure conditions.

II. Hardening technology of valve seat

1. Surface hardening process

Overlaying hard alloy

Overlaying Stellite 6 or cobalt-based alloy on the valve seat sealing surface, the hardness can reach HRC 40~55, resistant to high temperature wear and corrosion.

Flame spraying/plasma cladding

Spraying tungsten carbide or nickel-based alloy powder to form a uniform wear-resistant layer with high bonding strength, suitable for high pressure difference working conditions.

2. Structural design optimization

Self-centering valve seat

Design elastic support structure so that the valve seat automatically fits the ball when under pressure to reduce local wear.

Multi-level sealing surface

Adopt stepped or double-bevel sealing structure to disperse the stress of the sealing surface and extend the service life.

3. Material matching

Hardness gradient of valve seat and ball

The hardness of the valve seat should be slightly lower than that of the ball (such as ball HRC 65, valve seat HRC 55~60) to avoid brittle peeling caused by hard-to-hard contact.

High temperature resistant materials

Under high temperature conditions (>500°C), nickel-based alloys (such as Inconel 718) or metal-ceramic composite materials are selected.

III. Key process control

Precision machining and polishing

The roundness error of the sphere must be controlled within ≤0.005mm, and the surface roughness Ra≤0.2μm to ensure uniform contact of the sealing surface.

The valve seat sealing surface is mirror polished (Ra≤0.1μm) to reduce friction and micro-wear.

Heat treatment process

Stress relief annealing is required after coating or surfacing to prevent cracking due to residual stress.

The quenching temperature and time of the integral hardened material must be precisely controlled to avoid deformation or grain coarsening.

Coating bonding strength test

The bonding strength between the coating and the substrate is tested by scratch test or micro-indentation method to ensure ≥70MPa.

IV. Quality verification and testing

Hardness test

The hardness of the coating or hardened layer is tested using a Vickers hardness tester (HV) or a Rockwell hardness tester (HRC).

Wear resistance test

Simulate actual working conditions to conduct cyclic opening and closing tests (such as more than 100,000 times) and observe the wear of the sealing surface.

Sealing performance test

Perform high-pressure gas sealing test according to API 598 standard, and the leakage rate must be ≤ISO 5208 A grade.

V. Precautions

Avoid over-hardening: Excessive hardness may lead to brittle failure, and hardness and toughness need to be balanced.

Environmental matching: The coating material must be compatible with the medium (such as avoiding the use of stainless steel substrate in chloride ion environment).

Cost control: High-end coatings (such as PVD) are expensive and need to be used selectively according to working conditions.

Through scientific material selection, advanced surface hardening process and strict process control, the reliability and life of hard-sealed ball valves can be significantly improved, which is suitable for long-term stable operation under harsh working conditions.