Combining nitriding with other processes
The benefits of the nitriding surface treatment process have been discussed in previous surface treatment articles in RET-Monitor. While the process gives a significant increase in hardness, it is also used to provide a useful degree of residual compressive stress. Carburising can provide the same benefits, but the temperatures involved mean significant distortion is more likely to occur through the carburising process than in nitriding. Such advantages make nitriding a popular choice for the surface treatment of highly stressed, cyclically loaded components throughout a typical race engine. Piston pins, crankshafts, auxiliary drive gears and camshafts are all regularly nitrided.
There is a wealth of other thermal and surface treatment processes that may be successfully combined with nitriding treatments to real advantage. For example, hard engineering coatings can be successfully applied to all manner of steels, but might suffer premature failure for a number of reasons. Depending on the load applied, the substrate may not be sufficiently strong to resist a small amount of permanent deformation, which would prove too much for a coating to withstand. Coatings again might offer a friction or corrosion resistance advantage, but the substrate might fail owing to subsurface fatigue stresses through repeated contact loads, or possibly cavitation. In both these cases, a nitrided case below the coating has been proven to help.
Nitriding can be carried out simultaneously with some age-hardening processes on certain special steels. I’ve seen cases where such simultaneous treatments have been shown to increase the wear resistance of these special steels without having to resort to multiple processes at different suppliers. This meant that components could be very quickly produced from solution-annealed bar stock and simultaneously aged and nitrided in a short nitriding process. The parts could be machined, heat-treated and delivered for use within a couple of days. Although the steels suitable for such processes are expensive, the fast turnaround of components, combined with a very simple heat treatment, can actually offer very cost-effective components.
Mechanical surface treatments whose aim is to produce residual compressive stresses can be enhanced by being applied to a nitrided surface. The magnitude of the residual stress can be much greater after the shot-peening treatment if the nitriding treatment has increased the surface hardness and strength of the component relative to the non-nitrided material. Racing valve springs are commonly both nitride hardened and subsequently shot-peened.
SAE Paper 2001-01-0834, among others, details one such process and the improvements it can bring for springs of many types. The paper gives an example for a spring, nitrided and then peened twice, whose fatigue strength has been enhanced significantly. The increased allowable stresses made it possible to save more than 50% on the spring mass, although the example given is not a valve spring. However, the allowable stresses for valve springs treated by nitriding and peening are enhanced compared to springs subjected to either nitriding or peening in isolation, and lower-mass valve springs are the result. Low-mass valve springs are valued for allowing more latitude in valve-lift profile design, or for lower friction owing to lower contact stresses between cam and follower.
Written by Wayne Ward