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New crankshaft steels

Although there are a great many race engines that carry over the stock crankshaft from the production engine on which the competition unit is based, many will choose to replace the production crankshaft with something more accurate and better engineered. Where there are changes to the production engine stroke, or where a bespoke race engine is concerned, there are rarely production car options, so a specifically designed part is required.

In the vast majority of cases, racing crankshafts are made from steels that are subsequently nitride hardened. Although processes like carburising (also known as case hardening) have been used in the past, nitriding is now used to almost the complete exclusion of other processes for highly stressed crankshafts.

There are several advantages to nitride hardening. The obvious one that we might draw from the name of the process is that nitrides are formed at the surface to create a hard, wear-resistant layer. However, just as important is the level and depth of residual compressive stress created. This has a significant and positive effect on the endurance of the part, and for this reason there are lots of applications for nitride hardening where there is actually no requirement for a hard surface.

The same can be said of carburising. Compared to carburising, however, nitriding takes place at a relatively low temperature, and there are no quenches in the treatment. Distortion is therefore minimised.

There are some drawbacks to nitriding though. The process generally takes a very long time and so can be quite costly. The longest cycles I have heard of for racing parts take more than seven days in the furnace; although a week or more in a furnace is not typical for a nitriding process, it can form a significant part of the cost of a component.

Perhaps of greater importance is the fact that the nitriding process, when one takes into account shipping times and so on, can constitute a large proportion of the overall manufacturing time, and this needs to be planned for. Unless your nitriding supplier has multiple furnaces running long cycles at staggered intervals, your wait for parts to be turned around can be much longer than the nitriding process itself. For example, a 90-hour cycle is not unusual, and missing the start of this cycle by a day means you will need to wait an extra three days for the next process to start - assuming of course that another 90-hour cycle is scheduled to run.

There is significant interest therefore in steels that can be nitrided more quickly. The rate of diffusion of nitrogen into the surface of the component and through the material immediately beneath the surface is controlled in part by the process temperature. This is often limited by the tempering temperature of the steel. If the tempering temperature is exceeded, the steel will begin to soften. During a long cycle, the effect is significant.

Nitriding steels have been developed that have higher tempering temperatures. This means they can be safely nitrided at higher process temperatures without risking softening of the substrate material. Such steels offer the possibility of much reduced nitriding process times, and therefore faster overall manufacturing times for key components. It is quite possible that the component costs can be reduced.

Written by Wayne Ward

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