The bolt at the temperature of 350 ℃ should choose to use a heat resistant steel. Regulating valve in the heat-resistant steel bolt steel are: low alloy heat-resistant steel, high chromium martensitic stainless steel, austenitic stainless steel, iron, nickel and nickel alloy heat-resistant steel.
Stainless steel is heat-resistant steel, is also corrosion resistant steel.
(1) low alloy heat-resistant steel
Pearlitic heat resistant steel low alloy can be used at higher temperatures. Bolts general working temperature is below 480 ℃ can choose Cr-Mo steel, while the operating temperature is below 540 ℃ are advised to use chrome molybdenum vanadium steel. These two types of valve materials in short-term quenched and tempered with better performance in normalizing and tempering conditions, long-term use of relaxation properties of high resistance. Chrome molybdenum vanadium steel after normalizing and tempering, impact toughness at room temperature is low, there is rupture notch sensitivity at 540 ℃, lasting strength reduction.
(2) martensitic stainless steel
The chromium content of anti-relaxation properties in 12%~17% of the martensitic stainless steel bolt, high durable plastic and low notch sensitivity. Control valve for toughness and heat of a good strong material, but also has good damping performance and a smaller linear expansion coefficient, resistance to atmospheric and marine corrosion. Martensitic stainless steel bolts for shock loads and corrosive medium environment. The maximum operating temperature is not higher than 600 ℃, its strength was lower than the austenitic stainless steel, but there is still considerable strength. But poor low temperature properties of martensitic stainless steel. Below the environment below 0 ℃, the toughness of the material will be lost, the presence of cold brittleness phenomenon.
(3) austenitic stainless steel
Austenitic stainless steel 18-8 bolt has an overall corrosion resistance and good high-temperature mechanical properties, can be used in the high temperature of 600 DEG C. And has good low temperature performance, butterfly valve can still maintain considerable strength at low temperatures. But its drawback is that the yield strength of the material is low, and not by heat treatment to improve its mechanical properties. And austenitic precipitation hardening stainless steel is in the process of heat treatment, precipitation hardening produced by aging treatment, the precipitation of compounds dispersed carbides and metal, so as to improve the strength, while still maintaining the original stainless steel had good corrosion and oxidation resistance, the bolt in the 650 ℃ using relaxation performance is stable, but its higher alloy content, heat treatment process is complex, the price of materials more expensive.
(4) iron nickel and nickel alloy
The bolt temperature over 570 ℃ can choose iron nickel or nickel alloy. Control valves such material containing cobalt, has a particularly good high temperature performance. Such as Ni base alloy waspalog the United States, in 760 ℃; Ni base alloy M252 can be used to 840 ℃, Cr14Ni25Mo iron nickel base alloy can also be used to 650 ℃; high temperature alloy production in our country can also be used as high temperature bolt steel.
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