It is known that steel is generally considered to be an alloy of iron plus carbon. It is also safe to say that (by definition of steel) cast iron can be considered to be steel, albeit iron with considerably more carbon present than what we generally consider steel to have.

We further know that what makes steel (simple definition of steel) hard is the combination of the following facets: carbon content, austenitizing temperature and rate of cooling (quench speed).

Depending on the percentage of carbon present, the austenitizing temperature and the cooling rate, the following properties will be influenced:
  • Hardness
  • Hardenability
  • Tensile strength
  • Impact strength
  • Ductility
  • Yield strength
  • Elasticity
  • Microstructure
The properties will be influenced up to a particular level, depending on the carbon content. So it is safe to say that carbon is the “magic ingredient” that makes the steel hard. The more carbon present, the harder the steel (up to a particular point). Based on plain-carbon steel, the maximum hardness achievable would be approximately 62-64 HRC. If the hardness value needs to be higher, it would be necessary to add alloying elements to raise the hardness value by making it more hardenable.

An analogy can be considered by taking of a glass of plain water. We know that the water will freeze at 0°C (32°F) and boil at 100°C (212°F). Those are the parameters of the physical properties of the water. If we add salt to the water, the physical properties will change. The freezing point will be lowered, and the boiling point will be raised. The change in the freezing and boiling temperature will be completely dependent on how much salt is added. The more salt, the lower the freezing point. Hence the principle of salt on ice during the freezing conditions of winter. We will note that three physical changes occur with the addition of the salt to the water: freezing point, boiling point and taste.

Another simple analogy that will embrace the principle of alloying to change the physical properties is coffee. Coffee can be black, no sugar, no cream; black with sugar; with cream only; and with sugar and cream. Depending on the amount of sugar and/or cream, the taste of the coffee will be modified accordingly from slightly sweet to very sweet.

The next phenomena that will occur when considering the salt and water/coffee and sugar analogy is that the more salt/sugar you add to the basic material you will very quickly reach a level of saturation where no more salt or sugar will dissolve. Then you will have a saturated solution.

Once again, this analogy can be applied to iron. Add carbon and you have made steel. Reach a saturated solution of carbon in the iron and you have made cast iron.

Cast iron is (by definition) steel, but it has carbon added to a level greater than approximately 2.30%. Some tool steels will exceed that carbon content up to approximately 2.70%, such as D7. With the cast irons, it will be noted that the maximum achievable hardness would be (excluding any surface treatment) up to an approximate value of 64 HRC (depending on the intensity of the cooling medium).

The next presentation will discuss the modification of the physical properties a little further.