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What level of warpage should I anticipate with plasma nitriding?

What level of warpage should I anticipate with plasma nitriding?

Thanks to low treatment temperatures, plasma nitriding is especially low-warpage. In most cases, no rework is necessary after plasma nitriding and you can install the components immediately – saving time and money!

Expert knowledge on the subject

During heat treatment, a dimensional change and/or a change in shape can arise in the treated workpiece.

A dimensional change refers to a change in dimensions without a change in shape. This is unavoidable during nitriding because nitrogen is introduced to the surface of the workpiece. This growth can be compensated for by a prior dimensional correction.

Warpage refers to a change in the dimensions and shape of a workpiece through heat treatment. (DIN 17014). Reasons for the warpage, on the one hand, could be residual stresses in the component from the manufacturing process, which reduce through heat treatment. On the other hand, the warpage may be down to residual stresses arising due to the heat treatment. Form changes can be prevented during plasma nitriding while complying with certain boundary conditions [1].

Dimensional change | Plasma nitriding

Example dimensional change

Change in shape | Plasma nitriding

Example change in shape

Prevention of warpage

To prevent warpage, the components to be nitrided must fulfil the following criteria:
• They should not possess any residual stresses
• They should be thermally stable
• No residues should be bonding which obstruct the nitriding process

If residual stresses become free during treatment, or if joint transformations occur during nitriding (tempering processes, precipitate processes), then these alone could lead to an undesired change in dimension and form. Production residues that are not removed could lead to the formation of an uneven nitride layer, thus leading to form changes.

During the nitriding process itself, form changes can be countered by lowering the treatment temperature. Choosing very low nitriding temperatures is possible at the time of plasma nitriding. However, there is a limit to this efficiency because choosing a lower nitriding temperature increases the treatment time to achieve a specific nitriding hardness depth.

For complex geometries (especially for contours that are not rotationally symmetric), the partial nitriding of a component can counter an unwanted form change.

Example: After nitriding all the sides of a gear (42CrMo4, Nhd> 0.4 mm), the internal contour of this gear became non-circular. The reason for this was the asymmetric cross-sectional change of the contour in the groove area.

Since the internal diameter did not have to be hard, the form change problem was able to be completely solved by partial plasma nitriding.

Mit welchem Verzug muss ich beim Plasmanitrieren rechnen?

Handling dimensional changes

In the case of geometrically simple components – such as a smooth shaft – the dimensional change can easily be estimated. The growth in diameter increases along with:
• An increasing nitriding hardness depth
• A thicker compound layer
• An increase in the content of nitride formers in the steel

The following rule of thumb can be applied for the growth in diameter of a simple shaft:

material Increase of diameter as a function of the nitriding depth sample calculation
un- and low alloyed steels as for instance C45, 16MnCr5 3,00 % a smooth shaft growth, in case of a nitriding depth of 0,5 mm, about 15 µm in diameter
hot forming tool steel as for instance
5,00 % a smooth shaft growth, in case of a nitriding depth of 0,3 mm, about 15 µm in diameter
high alloyed steel with chromium content > 12%
as for instance 1.4301
10,00 % a smooth shaft growth, in case of a nitriding depth (diffusion layer thickness) of 0,05 mm, about 5 µm in diameter

While dimensional changes for geometrically simple parts are relatively easy to estimate, they are considerably more complex for parts with cross-sectional changes or for hollow parts. For example, a pipe can grow, stay the same or shrink depending on treatment specification, wall thickness and diameter. The decisive factor in this case is the state of compressive residual stress resulting from the nitriding, as well as the relationship between nitriding hardness depth and wall thickness of the component. Here too, an optimisation of the dimensional change is possible through partial treatment during plasma nitriding; e.g. through masking the internal diameter of the pipe. For us, it goes without saying that we determine the dimensional change behaviour of your component together with you at the start of the project and to support you in this respect with the prior dimensional correction. We would be happy to advise you regarding a nitrided component with optimal dimensional stability and form retention!

[1]: Technologie der Wärmebehandlung von Stahl, Deutscher Verlag für Grundstoffindustrie, Leipzig, 2nd edition, 1987, p. 71ff