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Overview of common hardening methods

The plasma heat treatment is characterized by low processing temperatures and high dimensional stability. This treatment can be performed selectively and allows hardening of corrosion and acid resistant steels. Below you will find an overview of the most common hardening methods for comparison. We would like to advise you how you can use the plasma nitriding for processing your workpieces and answer all your questions about materials, processes and process combinations. Your contact persons can be found here.

Process

Explanation

Temperature

Hardening

During conventional hardening, the components are heated up to the hardening temperature which is then maintained. The carbon is dissolved in austenite. The aim of the subsequent, rapid cooling is the increase of hardness due to formation of martensite.

780-1200 °C

(depending on material)

Case hardening

Low-carbon steels can be processed better than those with a high carbon content.

For hardening of such materials the case hardening method can be used. After mechanical fabrication, the surface layer of the workpiece is carburized (annealing in carbon-rich atmosphere) and then hardened.

880-980 °C

Carbonitriding

The carbonitriding method is similar to the case hardening. in addition to carbon, nitrogen is enriched in the surface layer of the workpiece.

Thus the hardenability of the steel is increased.

Due to lower process temperature and shorter process time as well as milder quenching, the risk of distortion for carbonitrided workpieces is generally lower than for case-hardened parts.

820-900 °C

Tempering

Tempering performed after a hardening treatment may improve the performance characteristics of hardened parts.
Dislocations and tensions are reduced. As the hardness decreases, the part becomes less brittle. This in turn increases the toughness of the workpiece. Crack and fracture risk decreases

Unalloyed and low-alloyed steels:

150-200 °C

 

Hot-working and high-speed steels:

500-560 °C

Quenching and tempering

This method comprises quenching of the material followed by subsequent tempering.

Due to increased core hardness a good supporting effect for highly stressed nitride layers is provided.
Some steels are supplied in pre-tempered condition.

Case hardening + high tempering

Stress relief annealing

Due to machining and forming processes or welding, mechanical internal stresses are built up in workpieces. These stress conditions manifest themselves as dimensional changes during a heat treatment.

In order to minimize these changes, parts made to final size and subject to a subsequent heat treatment should be annealed between roughing and finishing in the temperature range of 550-650 ° C. This helps to considerably reduce dislocations and stress values of the material. Dimensional changes during hardening processes are minimized

550-650 °C

Salt bath nitriding / nitrocarburizing /

Tenifer®, Tenifer QPQ®

The material to be treated is enriched with nitrogen or carbon in a molten salt that is used as a nitrogen donor. The surface hardness is increased due to formation of nitrides and carbides.

The optional method of post-oxidation, polishing and subsequent re-oxidation is called Tenifer QPQ® (Quench Polish Quench).

480-580 (630) °C

 

Gas nitriding

In this thermochemical diffusion process the surface layer of workpieces is enriched with nitrogen. Iron and alloying elements of the material used form high-hardness nitrides. If additional carbon is diffused into the surface layer, this method is referred to as gas nitrocarburizing.

Gas nitriding: 520 °C

Gas nitrocarburizing: 570 °C

Plasma nitriding /
Plasma nitrocarburizing
The plasma nitriding is a thermochemical diffusion method which is carried out under vacuum.

The surface layer of the parts is enriched with nitrogen.
In combination with diffused nitrogen, iron and nitride forming elements present in the material (Cr, Mo, Al, V, Ti, W) form nitrides and thus increase the hardness. Depending on the alloy, a hardness of up to 1,200 HV can be reached. The resulting increase in hardness of the surface layer improves the running properties, wear resistance as well as antifriction properties.

This method can also be used for hardening of austenitic stainless steels.
This process is also known as pulse plasma nitriding or ion nitriding.
If, in addition, carbon is diffused into the surface layer, this method is referred to as plasma nitrocarburizing
An optional subsequent oxidation can significantly increase corrosion resistance of low-alloy and non-alloyed nitrided steels.
A partial nitriding can be better implemented than other methods since covering of the material is relatively simple.
Although the process temperature of between 350 and 500 ° C is lower compared to other methods, it minimizes the distortion and mostly allows short treatment times.

Plasma nitriding: 350-500 °C

Plasma nitrocarburizing: 570 °C

IONIT OX®

This method is characterized by the following process steps:

nitrocarburizing, plasma activation, oxidation.

570 °C

Superficial layer hardening

Beside case hardening and nitriding, other processes can be referred to as surface layer hardening methods: inductive, laser hardening, flame hardening and electron beam hardening.
These 4 methods are based on transformation hardening.

 

Boronizing

In this process boron diffuses into the workpiece and forms an iron boride layer. For this purpose, a treatment temperature of 850-950° C is required. Depending on the material, hardness values of > 2000 HV can be achieved.
The resistance to adhesive and abrasive wear is significantly increased.
The growth rate of the treated surface layer is about 25%. The possible distortion is higher due to the treatment temperature.

850-950 °C

Kolsterising®

This method allows the increase in hardness of austenitic stainless steels without reducing the corrosion resistance. Carbon diffuses at low temperatures (up to 300° C) and is dissolved in interstitial sites. No carbides are formed.
This method allows only formation of small layer thicknesses and with long treatment times.

up to approx. 300 °C

 

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