When the heat exchanger was operating at a normal temperature of 1000°F (about 283°C), the bolts made of stainless steel rusted! Why was that? It turns out that at higher temperatures, thermal cycling can turn certain stainless steels into non-stainless steels. Therefore, great care must be taken in choosing the material for fasteners to avoid unexpected incidents.
Stainless Steel Fasteners
By definition, the chromium content in stainless steel should be no less than 10.5%, but the author believes that this content is still low. To prevent stainless steel from rusting at room temperature, the chromium content in the stainless steel matrix should be around 12%.
Contrary to the common view, stainless steel is not stainless forever. Thermal cycling can reduce the chromium content in some stainless steels to a level that causes rusting. However, there are many types of stainless steel, so it is usually possible to select one that meets the special requirements.
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 pipelines. 304 type stainless steel is the most common material for stainless steel fasteners. 18-8 steel is the nominal content of chromium and nickel in 300 series stainless steel.
These materials appear to have good corrosion resistance on the surface, but when 304 stainless steel is heated above 850°F (about 199°C), the chromium content will decrease due to the deposition of carbon. At high temperatures, chromium combines with carbon to form chromium carbide, and chromium carbide is not rust-proof. Using 304L stainless steel can alleviate this problem.
The carbon content of 304L stainless steel is one-third that of 304 type stainless steel. Using 321 stainless steel or 347 stainless steel can eliminate the formation of chromium carbide. 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. In ASTM A193, 300 series stainless steel used for bolts includes 304, 321 and 347 stainless steel.
Since the strength of 300 series stainless steel is only strengthened by 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 reduction is unacceptable, then the stable materials 321 or 347 are also unsatisfactory. Therefore, other materials need to be considered.
400 Series Stainless Steel
The chromium content of 400 series stainless steel is lower than that of 300 series stainless steel, but it does not have the carbon deposition problem like 300 series. It can be heat treated and is suitable for working environments with temperatures up to 1200°F (about 393°C).
However, due to its chromium content of only 12% to 14%, it may corrode in environments with severe chemical atmospheres, while 300 series stainless steel, with 16% to 20% chromium, does not corrode.
The strength of 300 series stainless steel and 400 series stainless steel is the same. 300 series stainless steel is non-magnetic, while 400 series stainless steel is magnetic. According to ASTM F593, 410, 416, and 430 series of 400 series stainless steel are used for fasteners.
Nickel-based materials such as Inconel series (provided by Inconel International) or Haynes series (by Haynes International) are excellent materials for high-temperature applications. Most of these alloy steels contain at least 16% chromium for corrosion resistance.
They can also be heat treated. They have high strength at high temperatures. Of course, 718 alloy steel is probably the most commonly used material for fasteners and is the standard fine-thread fastener for spacecraft. Monel (about 65% nickel and 33% copper) is also used for fasteners, but it has very low strength. Haynes International names their materials Hastelloy or Haynes XX. Both Inconel and Haynes provide 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 an iron-based stainless steel containing 15% chromium, which can be heat-treated and cold-worked hardened. Without cold working, its tensile strength is 140 – 180 ksi.
With heat treatment and cold working, the tensile strength can reach 220 ksi, but the elongation at this time is relatively low for fatigue loads. A-286 can operate at temperatures ranging from -423°C to 1200°C. Almost all aerospace fastener suppliers can provide A-286 fasteners.
The MP35N and MPl59 materials provided by SPS Technology Company are excellent choices for applications in high-temperature and corrosive environments, containing 19% chromium. These materials are considered expensive even in the aerospace industry.
Waspalloy is another material in this group used for extremely high temperatures. These materials should only be used when there is no alternative, as they are both costly and difficult to obtain.
Summary
It is unwise to use 304 stainless steel in a working environment of 1000°F (283°C). If the material strength decline to an almost annealed state is acceptable, stable materials such as 321 or 347 stainless steel can be chosen. If high strength is required and the environmental corrosion is not very severe, 400 series stainless steel should be able to meet the requirements.
If neither 300 series nor 400 series stainless steel can solve the problem, A-286 can be adopted. Inconel, Haynes, MP35N and MPl59 are all excellent materials, but they are expensive and not readily available. The general design rule is: use expensive materials only when necessary to achieve satisfactory design performance.
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