HEAT exchanger normal operating temperature up to 1000°f (about 283°c), made of stainless steel bolts but rust! What is this for? It turns out that at higher temperatures, the heat cycle makes certain stainless steel become stainless. So choose the fastener material must be cautious, lest accident.
Stainless Steel Background:
According to the definition, the chromium content of stainless steel should be no less than 10.5%, but the author believes that this level is still low. To prevent stainless steel rust at room temperature, the chromium content of stainless steel substrate should be about 12%.
Contrary to popular belief, stainless steel will not always be stainless steel. The heat cycle will reduce the chromium content of some stainless steel to the level that makes it rust. But there are many types of stainless steel, so you can usually choose one of the stainless steel for special use.
300 Series Stainless Steel
The 300 series of stainless steel, which is known as 18-8 Steel, is the most commonly used material for fasteners, connectors, pipes and piping. 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 chromium and nickel. These materials have good corrosion resistance on the surface, but heat 304 stainless steel to 850°f (about 199°c) due to the deposition of carbon. Will reduce the content of chromium. High temperature chromium and carbon combined to form chromium carbide, and chromium carbide is not rust. The use of 304L stainless steel can alleviate the problem, 304L stainless steel carbon content is 304 stainless steel 1/3. The formation of chromium carbide can be eliminated with 321 stainless steel or 347 stainless steel. 321 stainless steel and 347 stainless steel respectively containing titanium and niobium, the property is very stable. Because the affinity between titanium and niobium is greater than that of chromium, they are formed by carbon and niobium carbide at elevated temperatures, so chromium is preserved. The 300 series stainless steel used in ASTMAl93 has 304, 321 and 347 stainless steel bolts.
Because the strength of the 300 series stainless steel only in cold forming only to be strengthened, when the temperature reached 1000°f (about 283°c), the thermal cycle will make the strength of the fastener down to the annealing state. If the strength drop is unacceptable, then stable materials 321 or 347 are not satisfactory. So consider other materials.
400 Series Stainless Steel
400 series stainless steel has a lower chromium content than 300 series stainless steel, but not like the 300 series has carbon deposition problem, and can be heat treatment, can be used for temperatures up to 1200°f (about 393 ℃) working environment. However, because chromium only 12% 14%, in a serious chemical atmosphere in the use of corrosion, and 300 series containing 16% 20% of chromium, will not corrode. 300 series stainless steel and 400 series stainless steel in the same strength. 300 series stainless steel non-magnetic, and 400 series stainless steel has magnetism. According to ASTMF593 regulations, 400 series Stainless steel 410, 416 and 430 series for fasteners.
Nickel-based materials such as the Lnconel series (provided by Lnconel International) or the Haynes series (by Haynes International) are excellent materials for high-temperature applications. Most of the alloy steels in these alloys contain chromium at least 16%, used for anti-corrosion. They can also be heat-treated. High strength at high temperatures, of course, 718 alloy steel may be the most used material fasteners, is a spacecraft in the standard fine tooth fasteners. There are also Monel (about 65% nickel and 33% copper) also used for fasteners, but the strength is very low. Haynes International companies name their materials as Haslelloy or haynesxx. Inconel and Haynes companies both offer some of the same fastener materials as 718 and X-750.
A-286 Stainless Steel
This is one of the most widely used stainless steel in the aerospace industry. It is iron base stainless steel, chromium 15%, can heat treatment, can be cold hardening. No cold hardening tensile strength in 140-180ksi, heat treatment plus cold hardening can be the tensile strength of 220ksi, but at this time the elongation relative to the fatigue load is too low. A-286 operating temperature in -423°c-1200°c, almost all aerospace industry fasteners suppliers can provide a 1286 fasteners.
mp35n and MPl59
The mp35n and MPl59 materials provided by the SPS technology company are very good materials for application in high temperature and corrosive environments, including chromium 19%. These materials are expensive even in the aerospace industry. Waspalloy is another material used for extremely high temperatures in this group of materials. These materials are used only in the absence of alternatives, which are expensive and difficult to obtain.
In the 1000°f (283 ℃) working environment using 304 stainless steel is not sensible, such as material strength drop close to the annealing state acceptable, can choose stable materials such as 321 or 347 stainless steel: such as high strength, and environmental corrosion is not very serious, can be used 400 series stainless steel should be able to meet the requirements. such as 300 series and 400 series stainless steel can not solve the problem, the use of A-286. inconel,haynes,mp35n and MPl59 are good materials, but they are expensive. And not easy to get. The general design rule is that expensive materials are used to achieve satisfactory design performance when necessary.
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