Dimensional changes will occur during any of the case-hardening procedures, such as nitriding through to carburizing. Why does this occur?

In addressing the issue with carburizing, there are three phase changes that the steel will go through:

• Ramp-up to the carburizing process temperature, the phase change is from (body-centered cubic lattice) ferrite (9 atoms)
• Austenite (elevated radiant temperature), which is now face-centered cubic lattice (14 atoms; larger structure)
• Quenched to form martensite (body-centered tetragonal lattice with 9 atoms)

Each of the above phases has a different volume. Therefore, what goes into the furnace does not come out of the furnace with its original size. That is assuming that the cooling rate at quench was correct to form the phase of martensite. If there is retained austenite, there will be a progressive dimensional change with time as well as an increase in hardness.

The phase changes will happen as will the physical size change, and there is nothing that the heat-treatment associate can do about it.

The second occurrence of size change will be caused with the surface diffusion of either nitrogen (nitriding) or carbon (carburizing). The amount of growth that will occur will be dependent on the case depth and the time at the process temperature.

It can be likened to a glass of water. If you put stones into the water, the water will be displaced by the volume of the stones. Therefore, a size change will occur. The amount of size change will be dependent primarily on the case depth diffused into the surface. The deeper the case, the greater the size change. Even if the heat-treatment associate does everything correctly, there will be a size change.

Another size-change opportunity will occur on the ramp-up to the process temperature. This will be caused by induced residual machining stresses. Heat will release the residual stress on the ramp-up to the process temperature, thus stress relieving the steel component and causing shape distortion.

Some of the causes for size changes can be summarized in the following manner:

• Variations in case depth of carburizing or nitriding will cause dimensional changes.
• Phase changes that occur as a result of the transformation from ferrite to austenite and the time at temperature will also initiate dimensional changes.
• Quenching from the austenite temperature to form surface (case) martensite will cause volumetric size changes.
• Mixed phases, such as retained austenite, will cause dimensional changes.
• Too high an austenitize temperature selection will initiate dimensional changes.

It is also said that “there is no distortion with the process of nitriding.” That is not a true statement. Any diffusion treatment will cause a size change simply because of the surface gaseous diffusion (nitrogen reacting with nitride-forming elements such as aluminium, chromium, molybdenum, titanium, tungsten, silicon and iron).

Granted, there are no phase changes that will occur with the process of nitriding, just volumetric surface changes and the thickness of the formed compound layer. Then one will have to ramp-up to the process temperature and make the consideration of induced pre-machining stresses and the steel stress reliving itself on its rise to the selected process temperature.

One of the machining techniques is to machine slightly under size and grow the part to size during the nitride procedure. But careful experimentation will need to be conducted to establish the amount of growth that will occur with a reasonable degree of certainty.