Superalloys

Wednesday, January 08, 2014

Tags :  advanced-materials

Despite some significant advances in polymer and composite materials in recent decades, race engines remain stubbornly metallic. While Formula One transmissions have embraced composite materials in their main cases, a combination of regulation and experience has deterred engine design engineers from using anything other than metals for most applications.

We use a number of different metals as a matter of routine. Pistons are almost always made from aluminium, crankshafts are normally made from steel, and camshafts are fashioned from steel or cast iron.

Superalloys are an odd fit in a race engine. They were developed for high-temperature use in gas turbine engines, since the operating envelope of conventional materials places limits on the performance of turbine engines. Superalloy development therefore continues, as new designs of turbine engines for transporting people and goods around the globe would simply not have been possible using ‘conventional’ or even ‘old’ high-temperature materials.

There are a few genuine applications though for high-temperature materials in any race engine. In supercharged and turbocharged engines, the exhaust valves are probably the only components that need the high-temperature properties that superalloys offer. In such engines, exhaust gas temperatures are very high and the mass flow rate of the burnt gas makes the rates of heat transfer to the valve also very high.

In many naturally aspirated engines, we can often find components made from superalloys in very specific applications – the most common example is undoubtedly that of con rod bolts. These fasteners are extremely highly stressed, and the consequences of their failure are usually catastrophic and very expensive. Almost all con rods for bespoke race engines, and many aftermarket rods for production engines adapted for racing, will be supplied with superalloy bolts or studs. Various superalloy materials are used for con rod bolts – Inconel 718 is quite commonly found, along with others such as MP35N and MP159 and Custom Age 625+, which is similar to Inconel 625.

Similarly, we can find superalloys such as MP35N or MP159 used for other critical fasteners, such as those used to secure main bearing caps to attach the cylinder head to the engine block. These can’t be described as high-temperature fasteners; it’s more a case that the strength of these materials, and their fatigue and corrosion resistance, allow engines to function reliably. In many cases it isn’t possible to use larger fasteners fashioned from less capable materials to cope with the forces and stresses involved. In the case of production based engines in particular, there simply isn't sufficient material surrounding the fastener to allow a larger stud.

Superalloy materials themselves are expensive, and making bolts and studs from them can be costly too, as the material is very hard and therefore difficult to thread roll. It may be necessary to heat the material and to use special thread rolls in order to form the threads properly.

The other main application for superalloys is for valves in naturally aspirated engines. While the engines don’t appear to warrant such expensive materials in terms of temperature resistance, superalloys do offer much higher strength than the austenitic steel or titanium alternatives.

Written by Wayne Ward

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Comments

I would have thought that the turbines in turbochargers would be a natural location for superalloy use.