You’ll notice that this blog is not reserved for confirmed metallurgists, although some may have difficulty explaining this physical process in layman’s terms. Since the dawn of time, man has hardened steel without really understanding the subtleties of this atomic-level transformation. Only in the 19th Century did scientists come to understand this phenomenon which was long associated with such things as magic and spiritism. It all depends on the atomic transformation of steel.
Now when we say steel, we mean iron with a pinch of carbon. Because pure iron cannot be quench hardened, 0.2 to 2% carbon must be added to its mass; if we add more, it becomes cast-iron, which is another type of metal. Therefore, construction steel, which contains 0.18% carbon (also known as mild steel), shows very little reaction to tempering and is actually the only steel that can be quenched in water without it becoming brittle like pane glass. As for the other nuances, it depends on the amount of carbon involved.
Carbon atoms, as we know, are infinitely minute; so much so (compared to large iron atoms and the other elements) that they can not only insert themselves between the latter without altering the atomic structure of the part but can also move around inside the structure, regardless of the heat fluctuations at play. Another important fact is that iron can alter the layout of its atomic structure during its solid state, which is remarkable for a metal.
For steel, this atomic-level transformation occurs at around 730°C (1350°F). It is easy to detect because above this temperature, both iron and steel lose their magnetism. During the cooling process, tiny carbon atoms become trapped and are unable to return to their original configuration, as steel was in its initial state. During rapid cooling, such as in ice water for example, carbon atoms stay where they are, and because they are unable to move as fast, this creates distortions in the atomic structure of the matrix, which results in microscopic cracks. When oil is used to cool the part, however, the change in temperature is less dramatic, and because it is less intense, the carbon atoms partially reposition themselves in the right place, with a few atoms blocking and deforming the atomic structure in ambient temperature, which is enough to significantly increase the mechanical properties of the steel without weakening it against the slightest shock. Finally, if the cooling time is long enough, the carbon atoms revert to their initial position and the steel to its original state. Now some specialists might find this explanation too simple, but ultimately, the tempering phenomenon is just that.