Heat & surface treatment

Case hardening is one of the thermochemical processes. In this process, the surface layer of components and tools is carburized with a carbon-emitting medium and then quenched, improving the mechanical properties (e.g. wear) of the component's surface layer.
Carburization is usually at temperatures of 880 to 950° C. After the hardening of the carburized parts, annealing is generally required to reduce the stresses resulting from hardening and to achieve the required strength.

A range of different systems such as chamber furnaces and continuous furnaces are available for case hardening. Oils are generally the media used for quenching.

The conventional way to increase hardness or strength is by tempering. A second, somewhat more specific method is austempering, or bainitic hardening.

In this form of heat treatment, the component is austenitized in the same way as for other forms of hardening, i.e. it is heat-treated at temperatures of 800 to 900° C, depending on the material.

Quenching then takes place in a hot salt bath. The temperature of the salt bath depends on the material, and can range from 220 to 450° C. The part remains in the salt bath at a constant temperature (isothermal) until the structural transformation of austenite to bainite is complete (intermediate stage). No martensite is formed. Depending on the material, the transformation can be completed in a few minutes; but can sometimes also take several hours.

Bainite structures have very specific properties, characterized by high strength (hardnesses), maximum toughness and, generally speaking, relatively little distortion. Very low residual austenite contents that can otherwise only be achieved by deep-freezing are also possible.

Nitriding

• Gas nitriding
• Plasma nitriding
• Vacuum nitriding

Nitriding with diffusion of nitrogen and carbon

• Gas nitrocarburizing
• Plasma nitrocarburizing
• Salt bath nitrocarburizing

Advantages of this form of heat treatment

• High wear resistance against adhesion
• Adaptation of layers to wear type
• Reduction of coefficients of friction
• Savings on lubricants
• Creation of corrosion resistant layers
• Heat resistance of the nitrided layer to 400° C
• Partial nitriding possible(with the exception of salt bath nitrocarburizing)

Plasma nitriding is one of the thermochemical heat treatment processes and is carried out at temperatures between 350 and 600° C.

Positively charged ions strike the workpieces connected as a cathode in front of the furnace wall (anode) with a high impact speed.

Initially, this ion bombardment results in a very intense cleaning of the workpiece surface (sputtering), followed by the heating and nitriding of the surface.

Advantages of this form of heat treatment

• High wear resistance against abrasion and adhesion
• Little distortion
• Adaptation of layers to wear type
• Creation of corrosion resistant layers
• Reduction of coefficients of friction
• Partial hardening
• High temperature strength and tempering resistance of the surface
  layers to about 500° C
• Environmentally sound

The term annealing refers to the treatment of a workpiece at a given temperature for a given time period, with the subsequent cooling adapted to achieve the required material properties.

The major annealing processes are as follows

• Normalizing
• Stress-relief annealing
• Soft annealing
• Spheroidized annealing
• Coarse-grain annealing
• Diffusion annealing
• Recrystallization annealing
• Solution annealing

In the process of inductive heating, medium or high frequency alternating electrical current is used to generate an induction current in the workpiece through an inductor adapted to the contour to be hardened, producing heat.

The increase in hardness is caused by a transformation of the heated layer into martensite (during quenching), and the hardness that can be achieved depends on the carbon content and composition of the alloy.

Quenching is controlled, with the time window depending on the material, and is usually done as a supplementary quench with a synthetic polymer solution.