When you are making heavily loaded parts like crankshafts, gears, pistons or pivots you are likely to turn to one of the high carbon steels. These are generally more expensive than mild steels and more difficult to machine to a high finish, so you will only choose them when their special properties are needed.. Let's look at the commonest examples: EN16, EN19 and EN24. Their high carbon content allows them to be hardened through heat treatment, and the can be supplied in as-produced condition or in the T (heat treated) condition which has less stress and better machineability.
EN16 is also known by the European designation 605M36. It is a strong steel that is a step up from EN8 (see mild steels) in terms of its ability to withstand wear and shear loads. In smaller sizes it is usually supplied in the 'T' condition (heat treated) as EN16T which maximises its shock resistance and helps machineability.
Although it is stronger than EN8, it still has a certain degree of give which makes it a good choice for load bearing and stressed parts such as connecting rods. It is not too demanding to machine but you do need to use cutting fluid.
EN19 or 709M40 is a really strong steel ideal for gears and other highly stressed components. It is quite tough to machine and the use of carbide tooling is recommended but not essential. Use plenty of cutting fluid, especially with HSS. Unlike EN16 it is easy to polish to a good finish.
EN 19 is also available in heat treated (EN19T) condition. One specialist product are accurately drawn hard-chromium plated bars which can be useful for things like guide bars.
EN24T, which is also listed as 817M40, is a very strong steel that is usually supplied in the 'T' condition. It is very wear resistant but this can be increased even further by induction hardening or nitriding. It can be hardened in the workshop by heating followed by oil quenching, but it should be tempered at a relatively high temperature to prevent brittleness. For this reasons sharp edges and corners should be avoided - it has less resistance to shock loads than EN16 and EN19.
It is a demanding metal to machine, but with carbide tools and high surface speeds you can achieve a very good finish straight from the tool.
See Mild Steels for more general purpose steels.