Distortion plagues both engineer and heat treater alike, for no one can heat treat metal – steel in particular – without incurring distortion. When it comes to distortion, the sentiment is typically:
Granted, there can be flaws in the heat-treatment process as there can be with any manufacturing process. But when the heat treater receives the steel component from the client, he has had no control over the machining and manufacturing techniques.
Any time that one manipulates a piece of steel from the mill in any way, shape or form, stresses are introduced into the steel, which will become residual stress. Any time that any one of the following machining and manufacturing practices are conducted, stress is induced:
Even if the heat treater applies all the care in the world to the steel, he is faced with the problem of phase changes that will occur as the steel comes up to an austenitizing temperature, for example. The phase changes that will occur are: ferrite to austenite and austenite to martensite (when the steel is cooled rapidly).
When ferrite changes to austenite, there will be a volumetric size change. This means that the steel component is no longer the pre-heat-treatment size. Once at the austenite (hardening) temperature and after the appropriate temperature soak followed by the quench (rapid cooling), a further volumetric change will occur to martensite.
- It is always the heat treaters fault.
- The heat-treatment process must be flawed because the steel always distorts at heat treatment.
- Don’t the heat treaters know what they are doing?
Granted, there can be flaws in the heat-treatment process as there can be with any manufacturing process. But when the heat treater receives the steel component from the client, he has had no control over the machining and manufacturing techniques.
Any time that one manipulates a piece of steel from the mill in any way, shape or form, stresses are introduced into the steel, which will become residual stress. Any time that any one of the following machining and manufacturing practices are conducted, stress is induced:
- Forging
- Extrusion
- Drawing
- Bending
- Fabrication
- Turning
- Milling
- Grinding
Even if the heat treater applies all the care in the world to the steel, he is faced with the problem of phase changes that will occur as the steel comes up to an austenitizing temperature, for example. The phase changes that will occur are: ferrite to austenite and austenite to martensite (when the steel is cooled rapidly).
When ferrite changes to austenite, there will be a volumetric size change. This means that the steel component is no longer the pre-heat-treatment size. Once at the austenite (hardening) temperature and after the appropriate temperature soak followed by the quench (rapid cooling), a further volumetric change will occur to martensite.
- Ferrite is a nine-atom structure, and it is known as a body-centered cubic (BCC) lattice structure.
- The austenite structure forms as a result of the decomposition of ferrite followed by the regrouping of the atoms into a 14-atom structure, which we know as austenite. So, once again, there is a volumetric change and consequently a size change. The structure of austenite is face-centered cubic.
- On the rapid cooling from the 14-atom structure of austenite, steel is cooled down rapidly to form the phase of martensite. Martensite is a nine-atom structure but now with a completely different shape – tetragonal. This lattice structure is known as body-centered tetragonal.