Distortion is what every heat-treatment practitioner dreads. Yet it is an aspect of the heat-treatment process that cannot be avoided, particularly when austenitizing and quenching. It will happen, and nothing can stop it. Steps can be taken to reduce the distortion, but it cannot be eliminated.
Distortion can be divided into three groups with each one either individually or collectively contributing to the final result of distortion.
- Size change (dimensional stability)
- Phase change (caused as a result of application of heat above the A1 and A3 or the ACM line)
- Residual stress resulting from forging, rolling, fabrication and machining
Size change will occur as a result of the phase change, which is due to the application of heat to the steel.
Phase changes will occur as a direct result of heating the steel to attain austenite temperatures. The steel will change from a Body Centered Cubic lattice structure (ferrite) to Face Centered Cubic (austenite) to a Body Centered Tetragonal lattice structure (martensite) (if rapid cooling is employed).
Each of these phases has its own volumetric size, which varies – with the steel chemistry – from one steel grade to another. Thus size change will occur.
This further assumes that all of the transformation from the formed austenite to the freshly formed martensite is complete. Variations in the rapid cooling rate will create the potential for retained austenite. Retained austenite is not stable and will progressively transform to untempered martensite. This means that there will be both a progressive dimensional changes and hardness value increase.
Induced Residual Stress
As discussed in January’s blog entry, induced residual stress occurs as a result of:
Good pre-heat treatment (normalizing after forging where appropriate) will contribute to a reduction but not elimination of the distortion.