The application of tool steels in race engines
The use of steels in race engines continues to be significant, owing to the combination of properties such as fatigue strength and stiffness these alloys offer. Within the larger category of steel is a still wide range of materials which are known as tool steels. These often have properties that go beyond the usual standards set by low-alloy steels, and were developed for use as tools, often for cutting or forming metals. Although metal-cutting tools are now often made from pure carbides, there is still a thriving market for steel tools for operations such as forming, blanking, extrusion and moulding.
The applications for which these tool steels were developed means there is a range of materials with the ability to resist cracking or loss of strength at moderately elevated temperatures, or steels that can be hardened in air rather than oil or water, and by which method they can minimise distortion. Most tool steels are designed to be resistant to wear from a number of mechanisms – cutting tools need to resist abrasive and adhesive wear under intense sliding conditions with poor lubrication, while mould tools need to resist wear under the action of viscous, hot fluids flowing over their surfaces.
The combination of wear resistance and high fatigue strength, along with a degree of temperature resistance, makes some tools steels attractive candidates for manufacturing race engine components. This is often not a trivial undertaking, as the highly alloyed nature of these steels means the heat treatment processes involved are very specialised. Many tool steels require double and even triple tempering steps in order to gain the full benefit of their enhanced properties, and some steels also incorporate cryogenic treatments to fully transform their structures. The benefits of using the materials therefore needs to be substantial.
Tool steel camshafts are specified where a combination of high stress and endurance is required. Some also make good candidate materials for camshafts coated with DLC, as they do not to soften at the temperatures used during the coating process. As we might expect, tool steels are also sometimes used for cam followers.
Continuing with valvetrains, several suppliers specify tool steels for pushrods used in overhead valve race engines, while others who use tool steel ends fitted to pushrods produced from more conventional steels.
Gears for pump and camshaft drives are sometimes also made from tool steels. Traditionally these gears are made from carburised steels, but the carburising process (also known as case hardening) often has the disadvantage of introducing significant distortion. There is often very little difference in cost between tool steel and carburised gears made in small batches.
Tool steels are sometimes used for piston pins, and again we see that the materials they replace are often those where carburising or nitriding has been used previously.
A number of tool steels are used where high-strength fasteners are used, and con rod bolts are sometimes specified in tool steels such as H13.
Some tool steels, especially powder metallurgy types, can be so highly alloyed that their elastic moduli are significantly increased relative to traditional steels, combined with a reduced density. Several of them show a 10% increase in modulus, and I have seen at least one which has a specific modulus (elastic modulus divided by density) that is 20% higher than a typical steel. Increased stiffness is very attractive to design and development engineers, especially where valvetrain and cranktrain components are concerned.
Written by Wayne Ward