From my notebook, of possible interest --- Point of view is often different if from the practical or the academic, precision, etc.
BASICS ABOUT STEEL
This is about simple carbon steel, which is iron with only carbon added. Other steels and most modern steels also have some content of other metals in them such as chromium, molybdenum, nickel etc. to enhance specific characteristics.
Case hardening is usually done to low carbon steels to make it hard on the outside when the carbon content of the steel is not sufficient, or not wanted, to harden throughout. I do it in my home shop using a product called Kasenit. I heat the part to red, push it into the Kasenit powder, take it out and keep it red for half a minute or so, letting the powder 'cook' and bubble, then quench it in water.
The 'cased' exterior is glass hard and the inside condition depends on how much carbon is in its steel. Obviously, if you case harden higher carbon steel, the interior could be glass hard too with probable brittleness.
Carbon content determines what kind of heat treatment can be effective. Any carbon steel can be hardened somewhat by heating red hot and quenching in water. This hardness can give strength and can cause brittleness. Up to a practical point, the more carbon, the more the steel reacts to heating and quenching.
Higher carbon steels are used to make springs, chisels, punches, etc. If you want to make a spring, you can often find suitable steel from old carpenter saws, carbon kitchen knives, files, etc. To make a spring you first anneal (soften) the steel by heating red and allow to cool slowly, not quenched. You then make your spring, fit and shape it, then heat and quench it. It is now hard as glass and brittle. So to "temper", I put my hard spring in a metal dish in a half teaspooon of motor oil, heat it from below till the oil catches fire and burns off. Allowed to cool in air, I have my spring. I have tempered springs in the kitchen oven by turning up to max to get the 560F degrees, watching/knowing that its temper is right when it turn deep blue.
If you want to make a tool needing hardness, like a scraper or chisel, you want to polish it after quenching, then slowly heat it till it just begins to turn to a very light brown, let cool in air. Higher temp gives color changes as it changes to spring temp. Further heating softens the temper. It is worth while to note that higher carbon steels such as SAE 1095 (clock spring steel) should be quenched in oil rather than water. Before working on a steel of unknown carbon content, I do a test heat quench and temper to confirm its useful for my project.
I tried to write something here that the ordinary person can use with a little experimentation and care. I have some experience in that I have done the above scores of times over the past half century. It would take many pages to delve into the details of this subject. A lot more info about steel, including the tempering colors vs. hardness, types of usage can be found on Wikipedia.com.
