When the normal working temperature of the heat exchanger reaches 1000°F(about 283°C), the bolts made of stainless steel are rusty! Why is that? It turns out that at higher temperatures, thermal cycling makes some stainless steels non-stainless. Therefore, the selection of fastener materials must be careful to avoid accidents.

Stainless steel background

By definition, stainless steel should contain no less than 10.5% chromium, but the author believes that the content is still low. In order to prevent stainless steel from rusting at room temperature, the chromium content of the stainless steel matrix should be about 12%.

Contrary to popular belief, stainless steel will not always be stainless steel. Thermal cycling can reduce the chromium content of some stainless steels to a level that causes them to rust. But there are many kinds of stainless steel, so you can usually choose one of the stainless steel to meet special purposes.

300 series Stainless steel

300 series stainless steel, also known as 18-8 steel, 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 300 series stainless steel with nominal chromium and nickel content. These materials appear to have good corrosion resistance on the surface, but 304 stainless steel is heated to more than 850°F(about 199°C) due to the deposition of carbon. It reduces the chromium content. At high temperatures, chromium combines with carbon to form chromium carbide, which is not rust-proof. The use of 304L stainless steel can alleviate the problem, 304L stainless steel carbon content is 304 type stainless steel 1/3. Chromium carbide formation can be eliminated with 321 stainless steel or 347 stainless steel. 321 stainless steel and 347 stainless steel contain titanium and niobium, respectively, and their properties are very stable. Because titanium and niobium have a greater affinity for carbon than chromium, they form titanium carbide and niobium carbide respectively with carbon at high temperatures, so chromium is retained. The 300 series stainless steels in ASTMAl93 for bolts are 304, 321 and 347 stainless steels.

Since the strength of the 300 Series stainless steel is only strengthened when cold forming, when the temperature reaches 1000°F(about 283°C), the thermal cycle will reduce the strength of the fastener to an annealed state. If the decline in strength is not acceptable, then stable material 321 or 347 is also not satisfactory. So consider other materials.

400 series stainless steel

The chromium content of the 400 series stainless steel is lower than that of the 300 series stainless steel, but there is no carbon deposition problem 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, because the chromium content is only 12%-14%, it will corrode when used in an environment with a severe chemical atmosphere, while the 300 series contains 16%-20% chromium, which will not corrode. The 300 series stainless steel has the same strength as the 400 series stainless steel. 300 series stainless steel is non-magnetic, while 400 series stainless steel is magnetic. According to ASTMF593, the 410, 416 and 430 series of 400 series stainless steel are used for fasteners.

Nickel-based materials such as the lnconel series (supplied by lnconel International) or the Haynes series (supplied 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 is probably the most used material for fasteners, and is the standard fine-tooth fastener in spacecraft. Monel(about 65% nickel and 33% copper) is also used for fasteners, but the strength is very low. Haynes International names their material Haslelloy or HaynesXX. Both Inconel and Haynes offer some of the same fastener materials such as 718 and X-750.

A-286 stainless steel

This is the most widely used stainless steel in the aerospace industry. It is iron-based stainless steel with 15% chromium and can be heat-treated and cold-hardened. The tensile strength is 140-180ksi without cold hardening, and the tensile strength can reach 220ksi with heat treatment and cold hardening, but the elongation at this time is too low relative to the fatigue load. A-286 operating temperature of -423°C - 1200°C, almost all aerospace industry fastener suppliers can provide A-286 fasteners.

MP35N and MPl59

The MP35N and MPl59 materials from SPS Technologies are excellent materials for applications in high temperature and corrosive environments and contain 19% chromium. These materials are expensive even for the aerospace industry. Waspalloy is another material in this group for extremely high temperatures. These materials should only be used if there is no alternative; they are expensive and not readily available.

summarize

It is not wise to use 304 stainless steel in the working environment of 1000°F(283 ° C), such as the material strength decline close to the annealing state is acceptable, can choose stable materials such as 321 or 347 stainless steel: if the strength requirements are high, and the environmental corrosion is not very serious, can use 400 series stainless steel should be able to meet the requirements. If 300 series and 400 series stainless steel can not solve the problem, A-286 is used. Inconel, Haynes, MP35N and MPl59 are all good materials, but they cost a lot. And not easy to get. The general design rule is to use expensive materials only when necessary to achieve satisfactory design performance.