Hardening steel - what methods are there and how do they work?


Steel can be hardened in different ways. Which hardening methods are there, and how they work exactly, you can learn in detail in this post. In addition, what is still important after hardening, and why tempering the steel is necessary in many cases.

Purpose of curing

The purpose of curing is always to increase the strength of steel or its mechanical resistance. In some cases this may be necessary for certain applications if the achievable hardness can not be achieved by appropriate selection of the steel alloy.

curing

By hardening is understood to mean a structural change of the steel for greater strength, which extends over the entire cross section of the workpiece. This is not necessarily the case with other hardening methods. In the case of so-called case-hardening, on the other hand, the steel is not through-hardened, but only the surface of the steel is hardened.

Main curing process

Apart from case hardening, which is a special form of hardening, there are some important other hardening methods:

  • Umwandlungshärtung
  • hardening
  • precipitation

Umwandlungshärtung

The transformation hardening process is the most important and frequently used form of steel hardening. It works by transforming the ferritic iron into another structure, namely austenitic iron.

Process during transformation hardening

The transformation process is chemically very complex. At the end of the process, the increased amount of freely available carbon in the iron causes increased formation of so-called martensite (a special microstructure with high strength).

The martensite formation can be controlled by the temperature difference (cooling temperature). The lower the cooling temperature, and the higher the cooling rate, the more martensite is formed, and the harder the steel becomes after hardening has been completed. Different cooling media are used:

  • water
  • oil
  • air
  • pure gases
Requirements for hardenability

The carbon content of the steel grade determines how well steel can harden. Therefore, unalloyed low carbon steels must first be carburized (as described under case hardened steel). The level of chromium content in a steel grade, on the other hand, determines how deep a workpiece can be hardened. The higher the chromium content, the better the through-hardenability.

hardening

Cold work hardening is a process that comparatively less influences the hardness of steel. When deforming steel by certain methods, its dislocation density changes. It then becomes more difficult to deform and increases in strength. Its yield strength also increases. Such pre-deformation can be carried out, for example, by rolling the steel.

precipitation

Precipitation hardening works only under certain conditions, and not with all alloys. The alloying metals must have certain properties, so that this type of curing is feasible.

method

First, the alloy is heated to the extent that the planned precipitation elements go into solution. The temperature must be neither too high nor too low, so that the result of the hardening or the cohesion of the alloy is not endangered. Then it is quenched, and the structure inside changes. The previously dissolved elements are now finely distributed in the metal and hinder the dislocations in their movement. This increases the strength of the metal.

Tips & Tricks

After hardening, a steel must be tempered. The tempering is a heating up to a certain temperature to release the steel from its internal stresses after hardening. Depending on the temperature, tempering also changes the properties of the steel. Too high temperatures lead to the loss of the toughness of the steel.


Video Board: Heat Treatment -The Science of Forging (feat. Alec Steele)