The case hardening process, also known as carburising is very old, and pre-dates controlled nitriding processes by some decades; I have seen drawings of racing crankshafts from the 1920s that were hardened by this method. Other race engine parts that are commonly carburised are camshafts, piston pins and gears. The case hardening of highly stressed gears has been the most popular surface hardening process for decades, not only for racing gears but for highly loaded gears in general.
The process itself involves the diffusion of carbon into the surface of a low or medium carbon steel. Raising the 'carbon' potential of the carburising medium (which may be a gaseous atmosphere, liquid or even solid) allows carbon to diffuse into the surface. Carburising offers a hard, deep case in combination with a tough substrate. Compared to a process such as nitriding, deep cases may be formed in a relatively short time.
So why don't we carburise more components? There are a number of reasons, most of which are less valid now than a decade or two ago.
One problem conventionally associated with carburising has been distortion. This can come from a number of sources. Two of the main contributors are the stresses that are built up from manufacturing processes being relieved in heat treatment, and careless heat treatment. This second factor is far less likely to occur if you use the services of a reputable heat treatment company that is used to treating components with a mixture of thick and thin sections, slender components and so on.
Another contributor to distortion comes from the process itself. Areas where more carbon has diffused into the surface of the component tend to 'grow' more than the average for the whole component. Even where distortion due to manufacturing stresses was very small, the effect of variability in carbon content and case depth has often meant that grinding is needed. The problem is most pronounced on sharp edges and corners - gear teeth, whose function is impaired by inaccuracies in the profile of the flanks, are particularly affected where the top flank meets the top of the tooth. Conversely, on 'inside' corners of gear teeth roots, there is a lower than average case depth.
A relatively new process, known as low-pressure carburising (LPC) or vacuum carburising, aims to minimise distortion by more careful control of the concentration of carbon in the case layer. In conventional carburising processes, the carbon-rich medium surrounds the parts while they are heating, and because the rate of diffusion depends on the temperature, those parts that heat up quickest start to take on carbon earlier and at a more rapid rate than average, while those that heat up slowest have a depleted case depth. In the LPC process, the carburising medium is a gas that is introduced only when the part is up to an even temperature.
LPC has a number of other benefits: compared to conventional processes, the rate of diffusion is higher, leading to faster processing or a greater case depth. Higher compressive stresses are also said to be possible - the value of compressive stress in improving fatigue life is well known. The depth to which carburising is effective in small bores is also improved.
It is likely that we will see this process used more widely for motorsport engines and transmissions in the years to come.
Fig. 1 - LPC might have been a godsend to the engineers working on the Bristol Hercules engine!
Written by Wayne Ward