When designing an optimized carburizing operation, considerations should be given to the following:

1. Edge rounding – Break sharp corners into slightly rounded corners so that the carbon diffusion is uniform even around the rounded corner. This will reduce the risk of massive carbide formation.

2. Alloying element selection

3. Avoidance of slow cooling from carburizing process temperature, which is likely to create carbide networking

4. Care and consideration should be given to pre-annealing and normalizing. (Do not consider very high process temperatures for pre-annealing and normalizing.)

5. The creation of the various carbide forms. Intergranular carbides, as the name implies, means that the carbides will form around grain boundaries. When examined microscopically, they do tend to be seen in a cylindrical form.

Film carbides will form during cooling from elevated temperatures and will tend to congregate at the junctions of austenite grains. Film carbides will form as a result of slow cooling from elevated process temperatures and will be seen as small, thin platelets that form into what can be seen and interpreted as a film layer.

6. The surface hardness will be dependent upon the amount of carbon present in the steel surface from diffusion during carburizing. A further influence to this will be the cooling rate from the austenitizing temperature to transform the carbon to martensite. This means careful selection of the quenched medium is necessary in order to accomplish the appropriate transformation to martensite.

7. Residual stress – the surface carbides can form as a result of high carburizing process temperatures. When cooling from the austenitizing temperature, martensite begins to form at the MS temperature on the isothermal transformation diagram. Martensite cools at a different rate from the austenite grains. This has the tendency to form micro stresses at the grain boundaries.

8. Bending Fatigue – continuous carbide networks can begin to reduce the fatigue strength of, for example, carburized gears.

9. Carbide-forming elements

As previously mentioned, most of the carbide-forming elements are:
  • Chromium
  • Molybdenum
  • Vanadium
  • Tungsten
  • Titanium
  • Zirconium
  • Niobium
It is most important to control the process-atmosphere carbon potential to as close to the eutectoid line of the Iron-Carbon Equilibrium diagram as is possible. This reduces the risk of carbide networks forming as a result of the carbon diffusion from the furnace atmosphere. Operating high carbon potentials will strongly contribute to the formation of carbide networks.