Superalloys

Thursday, January 03, 2013

Tags :  advanced-materials

The class of metallic materials known as superalloys have been developed over several decades primarily to meet the needs of the aero engine industry, in particular their requirement for materials that can operate reliably at high temperatures. Despite a genuine desire in the aerospace industry to limit their use because of cost, they remain stubbornly unbeatable in terms of performance by less expensive alloys.

There are some very obvious applications of superalloys in motorsport, especially where we see high temperatures. The trend in passenger vehicle engine design towards smaller forced-induction engines is being mirrored to some extent in racing: forced induction leads to higher combustion and exhaust gas temperatures.

If we follow the gas path through a typical engine, the exhaust valve is the first component which is routinely made from superalloy materials. As the valve is opened, the ratio of surface area to volume is high, as is the speed of the gas past the valve. These two facts give rise to high heat transfer coefficients from the exhaust gas to the valve, and its rapid heating. While naturally aspirated engines can generally cope using titanium or steel valves, forced-induction engines very often require materials that are better able to withstand the high temperatures. Both Inconel and Nimonic alloys are options for valve materials which are commercially offered by racing valve manufacturers.

Exhaust systems made from superalloy materials are also commonly offered by specialist manufacturers. Here, the strength at temperature is an important consideration, and can mean that superalloy materials offer the lightest possible reliable exhaust system. These are routinely used in many racing classes from Formula One, through NASCAR Sprint Cup to motorcycle racing.

In exhaust system design, considerations of creep are also important. Creep can be summarised as either the increasing strain over time under a fixed stress, or alternatively the reducing level of stress required to maintain a fixed strain in a component. Superalloys are very creep-resistant and are specifically developed with this criterion in mind. Materials which prove themselves over the course of a short dyno test may fall short of expectations when running at the circuit, with exhaust systems ’sagging’ after a while, potentially coming into contact with parts from which they were initially separated by a generous air gap.

For turbocharged engines, where strenuous efforts are made to retain heat in the exhaust gas flow, the turbine wheel has to cope with very high temperatures. This is the application with the greatest similarity to the aero engine applications for which such materials are usually developed, although turbochargers are radial flow devices rather than axial flow. Again these components are routinely made from superalloys. Although ceramic and titanium turbine wheels offer advantages in terms of low inertia and therefore improved transient response, they have proven to be unreliable in the past.

Superalloys are also very corrosion resistant, so much so that there is little difference in the fatigue properties between their use in air and sea water for some alloys. This resistance to corrosion may appear not to be applicable to most race engine components, but there is certainly opportunity for corrosion from the fluids in a race engine. Highly stressed, high-strength fasteners can be susceptible to environmental hydrogen embrittlement over long periods of time. Highly stressed fasteners are often made of superalloys, with con rod bolts being available in a number of different materials of this type.

Written by Wayne Ward

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