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 .
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.