It is necessary to select the correct austenitizing temperature after carburizing in order to ensure good transformation of the formed carburized case from austenite to martensite. The surface carbon content and its diffusion are critical to the success of the austenitize and quench.
The control of the carbon potential is critical to the final success of the treatment. Too high of a carbon potential creates retained austenite, and too low of a carbon potential would result in low as-quenched surface-hardness values.
The ideal carbon potential, which would be dependent on the steel analysis, would be around the eutectoid line on the Iron-Carbon Equilibrium Diagram, say 0.80% up to 1.10%.
Once the carbon is diffused into the surface, one has created a new steel analysis at the surface. From the surface into the core of the steel, a dilution effect has occurred. It is the surface that will exhibit the highest hardness value provided surface oxidation is not present.
The next step would be to locate the surface carbon content along the horizontal line of the Iron-Carbon Equilibrium diagram. Once this has been located, then scribe a vertical line to where the line intersects the upper critical line and add approximately 50°F(10°C).
The austenitizing temperature will also be influenced by the alloying elements in the steel, such as chromium, molybdenum, vanadium and others. Nickel will improve the core strength, but it will contribute to the retention of the austenite phase.
Once the case austenitizing temperature has been established, the soaking time at that temperature can be set. When the austenitizing temperature has been completed, quenching can be considered.
Generally the quenching medium will be at room temperature. If the quench medium is oil, however, the quench oil temperature can be raised. The reason for quenching in hot oil is twofold: reduce distortion and reduce the risk of cracking.
The higher the quench-medium temperature is, the distortion will be reduced. The reason for the reduction in distortion is because of a reduction in the delta T between the case austenitizing temperature and the quench-medium temperature.
Distortion will not and cannot be eliminated. It will occur as a result of any prior induced stress from machining and growth due to the carbon diffusion into the surface of the steel. The selected case austenitizing temperature will also determine what the core hardness and core strength will be. This applies to gas carburizing, salt-bath carburizing, plasma carburizing and low-pressure carburizing.
The degree of distortion will be determined by:
- Induced prior machining stress
- Grain size as a result of the carburizing temperature and time at temperature
- Case austenitizing temperature
- Quench-medium temperature