In the process of valve stem machining

Function of valve stem

1. Power transmission: connect the handwheel and the electric/pneumatic actuator, transmit the operating torque through rotation or reciprocating motion, and control the opening and closing of the valve.
2. Sealing control: through the cooperation with the valve disc and stuffing box, ensure that the medium does not leak during the opening and closing process of the valve, especially bear the friction force and medium pressure of the packing seal.
3. Working condition tolerance: in direct contact with pipeline medium, it is necessary to resist the corrosion, high temperature and wear of the medium to ensure the long-term reliable operation of the valve.

Corrosion-resistant and high-temperature resistant alloy materials and their characteristics

1. Stainless steel series

2. Nickel-based alloys

3. Titanium and its alloys

4. Cobalt-based alloy (Stellite)

• Composition: Co≈50%, Cr≈25%, W≈5-15%, containing C, Ni, etc.
• Characteristics: high temperature wear resistance, corrosion resistance (such as high temperature sulfide), temperature resistance up to 1100℃, but high cost.
• Application: high temperature and high pressure medium containing solid particles (such as steam valve of power plant and valve of garbage incinerator).

Analysis of application differences of different materials

1. Choose according to the corrosiveness of the medium.

• Weakly corrosive environment (such as water, atmosphere and dilute acid): 304/316 stainless steel is preferred, with low cost and satisfactory performance.
• Strong corrosive environment:
  • Containing chloride ion (such as seawater and hydrochloric acid): choose 316L, duplex stainless steel or Hastelloy C-276.
  • Hydrofluoric acid: Only Monel alloy (Ni-Cu alloy) can be used, and other materials are easy to corrode.
  • Concentrated nitric acid and concentrated sulfuric acid: Hastelloy C-276 or titanium alloy (titanium is passivated in concentrated nitric acid).

2. Select according to temperature requirements

• Medium and low temperature (≤ 650℃): 304/316 stainless steel, duplex stainless steel.
• High temperature (650-1000℃): Inconel 625, Hastelloy C-276, which need to give consideration to oxidation resistance.
• Ultra-high temperature (> 1000℃): Stellite cobalt-based alloy, which is resistant to high temperature oxidation and wear.

3. Select according to the special requirements of working conditions

• High pressure+corrosion: duplex stainless steel (high strength) or titanium alloy (stress corrosion resistance).
• Serious wear: Stellite alloy (high surface hardness, erosion resistance).
• Food/pharmaceutical industry: 316L stainless steel (nontoxic and easy to clean).

summary

The selection of valve stem material needs to integrate media characteristics (composition and concentration), temperature, pressure and cost factors. Stainless steel is the most commonly used basic material. Nickel-based alloys and titanium alloys are suitable for harsh working conditions, while cobalt-based alloys are suitable for high-temperature wear scenarios. In practical application, it is necessary to verify the compatibility of materials through corrosion tests (such as intergranular corrosion test and salt spray test) to ensure the long-term reliable operation of the valve.