Because the distortion occurs at heat treatment, however, it is not necessarily that the heat-treatment procedure is at fault. Distortion or size change will occur because of the following:
- Phase change from room temperature up to the process temperature
- Phase change (if a quench is necessary) from austenite to martensite
Prior machining and induced residual stress will cause distortion. The induced prior stress, which occurs during the pre-machining procedures, will stress relieve themselves on heat up to the appropriate heat-treatment temperature.
The heat treater cannot stop the volumetric phase change, nor can he stop the stress relieving as the part heats up to its appropriate heat-treatment temperature.
Previously performed thermal treatment such as annealing, normalizing and austenitizing will affect the prior grain size. Remember that grain size is dependent on the time at temperature. Prior grain size will affect the results of the final heat-treatment procedure.
The heat treater can contribute to distortion by:
- Inconsistent load preparation
- Random and inconsistent load orientation
- Inconsistent and uncontrolled quench-medium temperature
- Inconsistent austenitizing temperature for the case hardening of carburized case
- The potential for retained austenite to form
The furnace itself can also contribute to distortion. The furnace will need to be optimized to present consistent and repeatable process conditions such as:
- Temperature uniformity in the process chamber
- Temperature accuracy of the process chamber
- Location of the process thermocouple as close to the workload as it is mechanically feasible without interfering with the movement of the load, in or out of the chamber
- Consistent and repeatable quench-medium temperature (heat exchanger) and temperature rise on quench to be consistent and repeatable
- Control of the chemistry and the cleanliness of the quench medium
- Consistent and repeatable quench-medium flow direction