Surface hardening

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The surface hardening  - even surface hardening  - is a method by which the outer layer of metallic components cured can be.

The term surface hardening but includes DIN 10052 only the methods in which the surface layer is austenitized is: flame hardening, induction hardening, laser beam and electron beam curing. During austenitizing, the structure is transformed, with the steel being heated up to the austenite area.

In contrast to this, with nitriding, hardening usually takes place without structural transformation.

Not all metals are suitable for hardening. For example, steels with a low carbon content are hardly hardenable, but they can be made hardenable by so-called carburization .

General

As with a composite material, the surface hardening results in different material properties in some areas of the starting material of the workpiece . The high toughness of the raw material remains inside a workpiece, while the surface becomes hard and wear-resistant . Typical application examples are, for example, camshafts and gear wheels .

Advantages of surface hardening are:

  • Increase of the fatigue strength, which arises from the tension or compressive residual stresses in the surface of the component.
  • Increasing the resilience, especially of the hardened work surfaces such as B. at the so-called point of engagement in which two gear flanks roll on each other. See Pitting .
  • Significant increase in precision and surface quality through subsequent grinding , which is hardly worthwhile with an unhardened surface due to the rapid wear.
  • Increase in durability and fatigue strength through the reduction of abrasion, wear and deformation as well as through the higher durability of the shape and dimensional accuracy achieved through the advantages mentioned.

After surface hardening, the workpiece can be tempered or annealed in order to reduce the tension and brittleness caused by hardening - with reduced hardness. In this way, the workpiece can be specifically "set" optimized for use in several work steps.

Procedure

Inductive

In the inductive process, the workpiece is briefly exposed to an alternating magnetic field. As a result, the surface of the workpiece heats up to the point of red heat. The layer depth depends on the frequency; the higher the frequency, the smaller the layer depth that is sufficiently heated. It is then quenched and thus hardened.

The process is very often used in mass production because it can be reliably integrated into automated processes with a high throughput and very good control options.

Flame hardening

Similar to inductive hardening, with flame hardening the surface layer is quickly heated to hardening temperature with strong burner flames and quenched with a water shower. To do this, the heating flames and water showers arranged one behind the other are slowly passed over the workpiece. The depth of the hardened surface layer can be adjusted by the advance speed of the burner. The distance between the burner and the shower determines the holding time, which also influences the hardness. The shape of the burner and the shower are adapted to the shape of the workpiece.

Case hardening and nitriding

Nitriding and case hardening is based on the process of solid diffusion .

The workpieces are heated in a sealed furnace to at least half the melting temperature in order to accelerate the diffusion due to the increase in temperature. A nitrogen (nitriding, embroidery) or carbon ( carburizing ) atmosphere is then created inside the furnace . This causes nitrogen or carbon atoms to diffuse into the outermost layers of the workpiece. The hardening depth depends on the square of the time. In order to achieve double the hardening depth, the workpiece must be left in the oven four times as long.

The diffusion atoms embedded in the metal lattice as interstitial atoms produce three-dimensional lattice defects during cooling , which in turn impede the movement of dislocations due to their crystal structure deviating from the matrix and thus increase the strength in the edge area of ​​the workpiece surface.

Laser and electron beam hardening

Is used for surface hardening for small areas and low hardening depths.

High-energy laser or electron beams enable a surface to be hardened to be heated to austenitizing temperature in a very short time. The quenching process takes place directly afterwards due to the very rapid heating by the workpiece itself, which is not heated in the short time due to the inertia of the heat conduction.

Electron beam hardening must be carried out in a vacuum. The easy deflectability of the electron beam means that areas or patterns can be hardened with high precision. Application example: technical knives .

The disadvantage of the process is the required, complex system technology and the correspondingly high costs.

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