Tempering (metalworking)
Tempering describes the combined heat treatment of metals , consisting of hardening and subsequent tempering . In general, the material steel is meant here, but this type of thermal structure formation and change is also common with non-ferrous metals such as titanium alloys . The tempering process is described here using the example of steel, as it is the most common in practice.
Hardening
The prerequisite for tempering is the hardenability of a material, i.e. the ability to form a stable martensite or bainite structure under certain conditions . A carbon content of 0.2–0.6% of the steel is necessary for classic tempering. Due to their excellent suitability, certain mechanical engineering steels are also referred to as heat-treatable steel (usually 0.35–0.6% carbon). In contrast to this, there are also steels that are more suitable for so-called surface hardening due to their poor hardenability . The thickness of the edge layer can be adjusted by a suitable choice of alloying elements. The grain size of the structure also has an influence on the temperature-dependent transformation processes and thus on the heat treatment.
For hardening, the workpiece must first be heated up quickly (> 4 K / min) above the austenitizing temperature . Heating up too quickly creates the risk of warping and cracking and should be avoided. For hypoeutectoid steels , temperatures of 30–50 ° C above the temperature AC 3 defined in the iron-carbon diagram , and for hypereutectoid steels, temperatures just above AC 1 are recommended before quenching. The holding time t H depends on the workpiece thickness s and can be estimated using the following rule of thumb:
Here the carbon is dissolved in the austenite. An increased austenitizing temperature is required for complete dissolution of carbides . However, this is not recommended due to the later embrittlement of the martensite that forms. If the austenitizing temperature is not reached, soft ferrite nuclei in the hard martensite structure, also known as soft spots, can occur. This has a massive influence on the machinability of the material and thus the service life of the tools used.
Scare off
The quenching, i.e. the rapid cooling of the heated workpiece, takes place through the use of quenching agents, preferably water, oil (polymer bath) or air. The choice here influences the quenching speed v K to be achieved and thus the resulting structure (and the possibility of cracks in the material). With air cooling, 5 to 35 (compressed air) K / s can be achieved. With mineral oils the maximum achievable v K is in the range of 150–200 K / s, with water it is approx. 3 times as high. Normally, a hard structure made of martensite, bainite or a mixture of these two is the goal of hardening.
Starting
After quenching, an immediate tempering stage at approx. 150 ° C is advantageous. Here, the brittle tetragonal martensite (needle martensite) produced during hardening is converted into the cubic martensite structure with the precipitation of fine carbides. This has a smaller volume and therefore relaxes the grain lattice and thus eliminates the “glass hardness” of the material. This process is continued with further tempering stages at a higher temperature (200–350 ° C). In addition, any remaining austenite is further decomposed by diffusion processes and converted into martensite. This leads to a further increase in hardness.
In the case of high-alloy steels, iron carbide is converted into more stable, harder special carbides of the carbide-forming alloy elements (e.g. V, W, Mo, Cr) in a further tempering stage above 500 ° C. This precipitation of even finer carbides makes it more difficult, among other things, for dislocations to slide off due to high loads, inhibits the formation and continuation of cracks and thus increases toughness and hardness ( maximum secondary hardness). A precise overview of the changes in the properties of the tempering can be derived from a material-specific tempering diagram.