Material choices for high-temperature applications
The components used in internal combustion engines, even highly tuned engines that we find in motorsport, are not routinely subjected to very high temperatures. Certainly the combustion temperatures are hot, but not so hot that aluminium can’t be successfully used for pistons, cylinder heads and cylinder bores in most applications. The only internal engine component we need to consider as a high-temperature application is probably the exhaust valve. Even so, it is usually only boosted engines that require a valve made from a high-temperature material such as Nimonic or Inconel. We rarely therefore need to consider high-temperature materials for fasteners in automotive applications.
The increasing use of boosted engines in motorsport, especially turbocharged engines, means that exhaust temperatures are much hotter than usual, often necessitating more exotic exhaust materials. Where there are applications for fasteners in such exhaust systems, we therefore need to consider the use of something more exotic than a conventional tempered steel fastener; while these are generally excellent quality, they are designed only for use at moderate temperatures.
Austenitic stainless steel is less affected by heat but starts off at a lower strength level, and a lot of commercially available fasteners have poor-quality threads. We probably won’t want to consider off-the-shelf austenitic steel fasteners for highly stressed, high-temperature applications.
Martensitic stainless and duplex stainless grades are capable of being hardened to a high-strength condition, and so are used where conventional tempered steels are unsuitable and where austenitic grades are not strong enough. Fasteners in martensitic and duplex stainless materials are custom-made only.
Titanium alloys can be used for higher temperature applications than conventional steels, but the cost of titanium fasteners often makes them difficult to justify when bespoke designs are required. However, they are widely available commercially in a variety of bolt styles, from cap-head screws to bolts with flanged heads. So, if you can find the correct fastener for your application, the cost may not be as bad as you might fear.
If we are considering the use of threaded fasteners for high temperatures (possibly 600 C or higher), we should start to consider superalloys. There are three common types of superalloy, based on iron, nickel and chromium. Such materials are generally quite expensive though, owing to the high proportion of costly alloying elements and the expense of processing.
Materials such as Inconel and Nimonic alloys are the same types of materials that we might consider for other ‘hot’ components such as turbine wheels or exhaust valves; they suffer very little loss of properties at high temperatures relative to ‘normal’ fastener steels. Without special processing though, they may not offer very high strength at room temperature, but it is the level of strength available at working temperatures that will interest us for fasteners used on the exhaust system of a boosted engine. We might normally see superalloys such as Nimonics (sometimes used for exhaust valves) being used for fasteners and Inconel alloys as sheet metal being turned into exhaust systems for teams with large budgets.
Written by Wayne Ward