Bolts with an operating temperature above 350°C should be made of heat-resistant steel. The heat-resistant steels in the steel for regulating valve bolts mainly include: low-alloy pearlitic heat-resistant steel, high-chromium martensitic stainless steel, austenitic stainless steel, iron-nickel-based and nickel-based alloy heat-resistant steel.
Stainless steel is both a heat-resistant steel and a corrosion-resistant steel.
(1) Low-alloy pearlitic heat-resistant steel
Low-alloy pearlitic heat-resistant steel can be used at higher temperatures. Generally, chrome-molybdenum steel can be selected for bolts whose working temperature is lower than 480°C, and chrome-molybdenum-vanadium steel is suitable for working temperatures below 540°C. These two types of valve materials have better performance in short-term use in the quenched and tempered state, and have better anti-relaxation performance in long-term use in the normalized and tempered state. After normalizing and tempering, chromium molybdenum vanadium steel has low impact toughness at room temperature, and has persistent notch sensitivity at 540°C, resulting in reduced durable strength.
(2) Martensitic stainless steel
Martensitic stainless steel bolts with a chromium content of 12% to 17% have better relaxation resistance, higher permanent plasticity and lower notch sensitivity. In addition to the good toughness and thermal strength of the material, the control valve also has good damping performance and a small linear expansion coefficient, and can resist corrosion in atmospheric and seawater environments. Martensitic stainless steel bolts are suitable for environmental conditions of shock loads and corrosive media. When the highest working temperature does not exceed 600°C, although its strength is lower than that of austenitic stainless steel, it still has considerable strength. However, the low temperature performance of martensitic stainless steel is poor. In an environment below 0°C, the toughness of the material will be lost, and there will be cold and brittle phenomena.
(3) Austenitic stainless steel
Austenitic 18-8 stainless steel bolts have good general corrosion resistance and high temperature mechanical properties, and can be used at high temperatures of 600 °C. And it has good low-temperature performance, and the butterfly valve can still maintain considerable strength at lower temperatures. But its disadvantage is that the yield strength of the material is low, and its mechanical properties cannot be improved by heat treatment. The austenitic precipitation hardening stainless steel is in the heat treatment process, through the aging treatment to produce precipitation hardening, precipitate dispersed carbides and intermetallic compounds, thereby improving the strength, while still maintaining the original good corrosion resistance and oxidation resistance of the original stainless steel The anti-relaxation performance of this kind of bolt is relatively stable when used below 650°C, but its alloy content is high, the heat treatment process is complicated, and the price of the material is relatively expensive.
(4) Iron-nickel-based and nickel-based alloys
Bolts with a working temperature above 570°C can choose iron-nickel-based or nickel-based alloys. Materials such as control valves generally contain cobalt and have particularly good high temperature performance. For example, the waspalog nickel-based alloy in the United States can work below 760°C; the M252 nickel-based alloy can be used up to 840°C, and the Cr14Ni25Mo iron-nickel-based alloy can also be used up to 650°C; the superalloy produced in my country can also be used as steel for high-temperature bolts.