High-quality crankshaft has high strength, ductility and high fatigue strength. However, these characteristics are not only determined by the material used to make cranks but also achieved by the manufacturing techniques applied to produce them. And the very manufacturing technique—the secret I am talking about is ‘heat treatment’. Now I would like to explain how ‘heat treatment’ works in improving the strength of the crank.
The first step for the crank, at that time, carbon-steel alloys to get the high strength is to transform the structure of the rough-machined part into the face-centered-cubic austenite crystalline structure. In order to make it, we should heat the part in an oven until the temperature throughtout the part stabilizes in the neighborhood of 1550 degrees F to 1650 degrees F (depending on the specific material). Next, the part is removed from the heating oven and rapidly quenched to extract heat from the part at a rate sufficient to transform a large percentage of the austenitic into fine-grained martensite.
After this, cryogenic treatment, if used, directly follows quenching. It is believed that this process does transform most of the retained austenite to martensite, relax the crystalline distortions and produce helpful eta particles at the grain boundaries.
After quenching and cryogenic treatment, the alloys have reached a very high strength and hardness, but at the hardness level, it lacks sufficient ductility and impact resistance for most applications. So in order to achieve these properties suitable for the applications, the part is placed in a ‘tempering’ oven and soaked for a specific amount of time at a specific temperature to reduce the hardness to the desired level, hence producing the desired properties of the alloys.
Though the alloys now have all the high strength, ductility, and impact resistance, the fatigue strength of the material is still insuffient and this requires the process of nitriding to work for more improvement. There are three common nitriding processes: gas nitriding, molten salt-bath nitriding, and the more precise plasma-ion nitriding and all of them share the process of diffusing elemental nitrogen onto the surface of a steel, producing iron nitrides, which, of course, is the aim of nitriding. This very process results in the hard, high strength alloys along with residual surface compressive stresses.
Well, that is all for the heat treatment of crankshafts. Hope you enjoy it.