How can one ensure that the steel for carburizing will respond in the same manner each time that it is processed? If the steel has been forged, normalizing will be appropriate.

The question then arises at what temperature should the steel be normalized to ensure a consistent and repeatable grain size and metallurgical structure.

There is no “magical” number for the normalize procedure other than to be sure that the normalize temperature selection is above the final heat-treat or carburize temperature. If not, then any residual stresses that are within the steel will be manifested at the carburize temperature.

For example, if the final carburize temperature is 1700°F, you should normalize at 1725°F. As long as the normalize temperature is above the final carburize temperature, any induced residual stress will be reduced. This will not eliminate distortion from the final heat-treatment procedure, but it will certainly reduce the distortion factor.

The major problem with the process of normalizing is ensuring a consistent and repeatable metallurgy after the procedure.

The simple way is to place the parts into the process furnace and then heat up to the selected normalizing temperature. This method is not likely to produce repeatable results due to the load’s mass effect. If the parts are separated and spaced for the procedure, it is not an issue. But, in general, parts are heaped onto the furnace hearth, and there are temperature differentials though the load mass. The parts are heated and cooled at different rates. Hence, different core metallurgies result.

The method to ensure more repeatable metallurgy is to:
  • Normalize (slightly above the final carburize temperature by 25°F)
  • Quench (oil)
  • Temper at 1250°F
This will produce a consistent metallurgy, which would be in the semi-pearlitic structure with a constant and repeatable grain size.