When the normal operating temperature of the heat exchanger reaches 1000°F (about 283°C), the bolt made of stainless steel is rusted! Why is this? Originally, at higher temperatures, the thermal cycle will change some stainless steel. Non-stainless steel. Therefore, the selection of fastener materials must be careful to avoid accidents.
The chromium content of stainless steel should not be less than 10.5%, but the author believes that the content is still low. In order to prevent rusting of stainless steel at room temperature, the chromium content of the stainless steel substrate should be around 12%.
Contrary to popular belief, stainless steel will not always be stainless steel. Thermal cycling reduces the chromium content of some stainless steels to levels that cause them to rust. However, there are many types of stainless steel, so it is usually possible to choose one of them for special purpose stainless steel.
300 series stainless steel
300 series stainless steel is also known as 18-8 steel, which is the most commonly used material for fasteners, joints, pipes and pipes. Type 304 stainless steel is the most common material for stainless steel fasteners. 18-8 steel is the nominal content of 300 series stainless steel chrome and nickel. These materials have good corrosion resistance on the surface, but the 304 stainless steel is heated above 850 °F (about 199 ° C) due to carbon deposition. Will reduce the chromium content. At high temperatures, chromium combines with carbon to form chromium carbide, while chromium carbide is not rust-proof. This problem can be alleviated by using 304L stainless steel. The carbon content of 304L stainless steel is 1/3 of that of 304 stainless steel. The formation of chromium carbide can be eliminated by using 321 stainless steel or 347 stainless steel. 321 stainless steel and 347 stainless steel contain titanium and tantalum, respectively, and their properties are very stable. Since titanium and niobium have a higher affinity for carbon than chromium, they form titanium carbide and tantalum carbide with carbon at high temperatures, respectively, and thus the chromium remains. ASTMAl9 300 series stainless steel is available in 304, 321 and 347 stainless steel for bolts.
Since the strength of the 300 series stainless steel is only strengthened during cold forming, when the temperature reaches 1000 °F (about 283 ° C), the thermal cycle will reduce the strength of the fastener to the annealed state. If the strength drop is unacceptable, the stable material 321 or 347 is also unsatisfactory. Therefore, other materials should be considered.
400 series stainless steel
The 400 series stainless steel has a lower chromium content than the 300 series stainless steel, but it does not have carbon deposition problems like the 300 series, and it can be heat treated and can be used in working environments with temperatures up to 1200 °F (about 393 °C). However, since chromium is only 12% to 14%, it will corrode in an environment with a severe chemical atmosphere, while the 300 series contains 16% to 20% chromium, which will not corrode. The 300 series stainless steel is the same strength as the 400 series stainless steel. The 300 series stainless steel is non-magnetic, while the 400 series stainless steel is magnetic. The 410, 416 and 430 series of 400 series stainless steel are used for fasteners according to ASTM F593.
Nickel-based materials such as the lnconel series (provided by lnconel International) or the Haynes Hastelloy series (by Haynes International) are excellent materials for high temperature applications. Most of these alloy steels contain at least 16% chromium and are used for corrosion protection. They can also be heat treated. High strength at high temperatures, of course, 718 alloy steel may be the most used material for fasteners, and is the standard fine tooth fastener in spacecraft. Also Monel Montessori (about 65% nickel and 33% copper) is also used for fasteners, but the strength is very low. Haynes International named their materials Haslelloy or HaynesXX. Both Inconel and Haynes offer the same fastener materials such as the 718 and X-750.
A-286 stainless steel
This is the most widely used stainless steel in the aerospace industry. It is an iron-based stainless steel containing 15% chromium, heat treatable and cold hardenable. The tensile strength is 140-180 ksi (965-1241 MPa) without cold work hardening, and the tensile strength reaches 220 ksi (1517 MPa) after heat treatment and cold work hardening, but the elongation at this time is too low relative to the fatigue load. A-286 operating temperature is -423 ° C - 1200 ° C, almost all aerospace industry fastener suppliers can provide A-286 fasteners.
MP35N and MPl59
The MP35N and MPl59 materials supplied by SPS Technologies are very good materials for high temperature and corrosive environments, containing 19% chromium. These materials are expensive even in the aerospace industry. Waspalloy is another material in this group for extremely high temperatures. These materials are used only when there is no choice, and these materials are expensive and not easily available.
to sum up
It is not wise to use 304 stainless steel in a working environment of 1000 °F (283 °C). If the material strength is close to the annealing state, it is acceptable to choose a stable material such as 321 or 347 stainless steel: if the strength is high, and the environmental corrosion is not serious. 400 series stainless steel can be used to meet the requirements. For example, 300 series and 400 series stainless steel can not solve the problem, using A-286. Inconel, Haynes, MP35N and MPl59 are all good materials, but at a high cost. And not easy to get. The general design rule is to use expensive materials when necessary to achieve satisfactory design performance.
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