Hardening (metallurgy)

The hardening of steel is to increase its mechanical resistance by selective modification and conversion of its microstructure. It can be done by heat treatment followed by rapid cooling. If metal is plastically deformed, so wide in the workpiece displacements from. In order to increase the strength, measures must be taken to inhibit the movement of dislocations.

Important curing

Umwandlungshärtung

The most important is the hardening Umwandlungshärtung. In this case, the workpiece is heated to the extent that the present at room temperature α - iron (ferrite ) into γ - iron ( austenite ). In the austenite can be solved much more carbon than the ferrite ( see the iron-carbon diagram). By resolution of the existing cementite ( Fe3C ) whose carbon in austenite is in solution. Startled to these carbon-rich austenite now on, can by the kinetic inhibition ( diffusion takes time ), no separation into ferrite and cementite take place. The iron bars can not pass into the body-centered cubic α - iron due to the " trapped " carbon atoms. It works instead in a distorted tetragonal body-centered cubic lattice ( martensite) to which is clamped by the carbon. An important role in this type of curing makes the cooling rate. The greater the supercooling ( temperature difference ), the more martensite forms. The conversion speed is controlled by different cooling media (water, oil, air or pure gas). Also important is the chemical composition of the steel. Carbon carrying because of its high diffusion rate of mainly Aufhärtbarkeit at the substitutional alloying elements such as chromium on the other hand determining the hardenability of the material. So for small components / large quench hardenability can be achieved over the entire cross section of the workpiece. To be cured, can a steel must contain at least 0.2 % carbon. In industrial applications, often working under Schutzgasatmospäre such as nitrogen, noble gases or in a vacuum to avoid surface oxidation and decarburization during warm- up above the austenitizing temperature of 723 ° C.

Precipitation

A further possibility for alloying is the fact that the elements involved while forming a common crystal, which is similar to but not the crystal system of the base elements. The result is a separate crystal system that is constructed contrary to those of the pure metal is very complicated. Such compounds are also very hard and brittle. In addition, these crystals require a fixed atom number ratio. An alloy with intermediate formation of crystals, the alloy elements are only metals, it is called an intermetallic compound or intermetallic phase. Examples of intermediate crystallization are Al2Cu, Mg2Si, Cu4Sn and Ni3Al (see nickel -base superalloys ). Crystallization formula similar to the formula for chemical compounds, but which, unlike alloys have a completely different type of connection. The most common method for introducing foreign atoms alloying. Steel impurity can also be introduced by carbonizing or nitriding.

Work hardening

By increasing the dislocation density in the structure of the sliding processes are impeded. This increases the strength, and is therefore referred to as cold working. Strain hardening is used especially in non-ferrous metal alloys (eg, bronze ) and solid solution alloys.

Hardening by quenching

Two of the above-mentioned methods consist of a heating up to a material-dependent temperature, maintaining the temperature of the workpieces and followed by rapid cooling ( quenching ) in compliance with the critical cooling rate.

As the quenching which possibly corresponding additives are added that change the surface tension of water to suppress the occurrence of the Leidenfrost effect ( the insulating vapor layer under the water drops on a very hot plate ) is used, among other things, water. Further serve as quench oil, salt bath, aqueous polymer solutions (e.g., polyvinylpyrrolidone), air or gases such as nitrogen (N2) or argon (Ar) (the latter during hardening in vacuum). Only steels with more than 0.35 % carbon (C) are suitable for such a curing.

Entirely without quenching does the hardening of the laser beam or the electron beam. In this case, only a small area of a thin surface layer is heated respectively, and the necessary very rapid cooling takes place by the removal of heat in the workpiece. These processes take place at very high speed. With the beam method, workpiece areas can be targeted for treatment locally ( within defined limits ) without heating the entire workpiece.

Physical background

The basis of the heat treatment is the phase diagram for steel. It shows graphically what temperatures are required to warming in the so-called austenite region. These are above a characteristic line in the phase diagram, the temperature values ​​are marked as A1 and A3 transformation points. They are located at 723 ° C or higher.

Depending on the alloy of the steel and the proportion of alloying elements in the steel the critical cooling rate has to be considered at the lower critical cooling rate exceeding occurs first martensite upon reaching or exceeding the upper critical cooling rate is the resulting hardening structure to approximately 99% of martensite.

Tempering of hardened steel

When quenching is formed in the outer regions ( the cool down fast enough ) martensite. At a carbon content of 0.6% is expected to RA retained austenite, as the martensite Mf lower than the room temperature and thus not all of the austenite is converted to martensite. The conversion of this so-called austenite is delayed and is accompanied by an increase in volume. This leads to considerable stresses in the workpiece. Warping and cracking can be the result.

In this as-quenched condition of the steel is very hard and brittle and not useful for technical uses. The condition is called very aptly " glass hard " with.

In a second step, the so-called tempering, also known as annealing, the hardness may be reduced and the desired performance properties (hardness, tensile strength and toughness ) of the steel can be adjusted. This is the steel, depending on the alloy composition, and properties desired, heated again. The result is the desired use hardness. The higher the annealing temperature, the lower the hardness. For increasing the toughness.

The annealing is performed depending on the content of alloying elements and carbon in the temperature range of 100-350 ° C, high alloy steels up to 600 ° C. Some higher -alloy steels (eg, material 1.2379 with 12 % chromium ) have a fairly complicated annealing behavior, namely they reach the third starting with about 500 ° C, a higher hardness than the first time (secondary hardness maximum). In powder metallurgy ( PM ) steels, the required hardness is however set above the starting temperature during quenching, tempering is carried out at uniform temperatures.

The combined operation of the hardening and tempering is referred to as tempering.

Grain refinement

Grain refining is a way to increase the strength of metallic materials. It involves the generation of a finer, smaller grain in the structure by means of appropriate heat treatment.

The grain refinement in addition to increasing the strength and toughness of metallic materials. This reduces inter alia, the tendency of the steel to the formation of hot cracks.

Solid solution strengthening

In metallurgy, an alloy is a mixture with metallic character of two or more chemical elements, one of which is at least one metal. The alloying element to the base element can form a solid solution ( single-phase alloy) or make it several phases. While in single-phase alloys, the properties are mainly determined by the chemical composition, these in multiphase alloys also greatly influenced by the distribution of the phases (microstructure ) can be influenced. Base metal and alloying elements are also called components of an alloy. To use metals as a material better them certain elements ( alloying elements ) are added in the molten state, the (hardness, corrosion resistance examples) should change the " base metals " in the desired manner, the material properties. Example of metal alloys are bronze, brass, amalgam, stainless steel.

Curing

A frequently occurring in practice is the use case hardening steel of which dar. is when the operation of the martensitic hardening to take place over the entire cross section of material. A curing is at larger dimensions of the workpiece can only be guaranteed, if the interior of the workpiece for the cooling rate is always a critical value is exceeded. In the martensitic hardening the attainable hardness and strength values ​​of the austenitizing temperature and duration, of the steel composition, the cooling rate and of the workpiece dimensions depend. The critical cooling rate can be influenced by the selection of the steel alloy elements added in a wide range.

Historical

Already in ancient and medieval times attempts were made to make iron or steel harder. For this purpose, for example, was used to partly magical means such as the use of verbena.