There are many applications for high-strength steels in the modern racing powertrain. Their use allows us to make lighter parts and, given that race engine and transmission designers spend much time, effort and money on reducing mass, it should come as no surprise that they are so popular.
There are some drawbacks with such materials though. As strength increases, it becomes more difficult to machine the component from the material in its hardened state. There are manufacturing processes that allow the machining of such hard, high-strength steels, but these can present their own pitfalls. Some of the processes that come under this heading include grinding, hard-turning and electro-discharge machining.
The most common way of producing components from very high-strength steels is to machine the component before hardening. However, if the component is finish-machined, the distortion resulting from the harden and temper process can often render the component scrap. If is very often necessary to anticipate an amount of distortion and leave an amount of machining stock on important surfaces to be removed after hardening and tempering. Again we are faced with a requirement to remove hard material, but on a limited scale. Grinding is often used for such processes.
There is though an alternative to machining hard materials and coping with high levels of distortion. Maraging steels are easy to machine and can be hardened without significant distortion. Conventional steels are heated to a critical temperature (called the austenitising temperature) and then rapidly quenched. This is the stage at which distortion is likely to occur. Not only are significant machining stresses which can cause distortion relieved, but the highly uneven temperature distribution, particularly during quenching, can introduce high and uneven levels of residual stress also.
After hardening and tempering, the steel can often only be machined using abrasive processes such as grinding, or requires much slower material removal rates using conventional machining methods such as milling and turning. By removing much of the material from a billet before hardening, the overall manufacturing time is reduced. The machining operations prior to hardening are partly responsible for distortion during the hardening process.
After quenching, the steel is mainly composed of a very hard and brittle phase called martensite. This requires tempering to give us a more ductile and fatigue-resistant steel. However, in doing so, we lose some of the original as-hardened strength.
Maraging steels, following austenitising and quenching, also have a martensitic structure but the martensite is soft and ductile and can easily be machined. They can even be welded relatively easily, which is certainly not the case with conventional hardened steels. After machining, when we want to produce the final level of strength in the material, the steel is ‘aged’. The ageing process is carried out at a relatively modest temperature (in the region of 480 C/900 F), and distortion is consequently lower as the thermally induced stresses are much lower.
The production of a soft phase after initial heating and quenching, followed by the development of the final hardness in the material, is similar to the way heat-treatable aluminium alloys behave.
I did a project to investigate the possibility of simultaneously ageing and surface hardening components made from maraging steels, and there were a number of processes that proved successful in this respect. Maraging steels may not be cheap to buy, but for some components the production process is simplified sufficiently that such materials prove to be economical. They are available with tensile strengths to 2400 MPa (350 ksi).
Written by Wayne Ward