likely occur. However, it is not always the quenching that is the cause of the distortion. It is usually “the effect” because it distortion is seen immediately after quenching.
First, we need to understand the term of distortion as it applies to heat-treatment operations. You have likely heard these comments from the client when distortion occurs:
- Don't those heat treaters know what they are doing?
- It was straight (or round) when it went into heat treatment, how come it is not straight now?
- We took great care in machining to finish size and now it has shrunk, and we cannot clean it up. That did not happen last time!
Every commercial heat treater has their share of horror stories. Yet, distortion at quenching will always occur.
Causes of Distortion
There are many causes of distortion, but the primary one is residual induced stresses during machining. When steel or any metal is mechanically manipulated by machining or mechanical deformation (such as forging and rolling), stress is induced into the steel. The only really effective method of removing induced stress is by the application of heat. As soon as the steel (or metal) experiences applied heat, it will begin to stress relieve itself by movement. (Guess who applies heat?)
If the temperature that the steel is being taken to is an austenitizing temperature, a phase change will occur. This means that the lattice structure size and construction will change from a 9- to a 14-atom structure. In addition, this means that (amongst other things) the size has changed. It has grown! Will it come back to its original size? Yes, if it is cooled down very slowly. But because the steel is (for example) being hardened and cooled rapidly, then another phase transformation will occur. That is the transformation from austenite to martensite, which means another lattice structure change from 14 atoms back to 9 atoms. But now the shape is tetragonal.
The complete transformation from austenite to martensite will only occur if the cooling medium is such that it will cool the steel according to the Time-Temperature-Transformation diagram. Full transformation can be achieved if the steel is cooled in such a manner. The heat treater can only cool down at the rate that the particular quench medium will allow. If the quench medium cools down a little too slowly, the incomplete phase transformation will occur. This means mixed phases, which is a mixture of martensite and austenite (more commonly known as retained austenite).
Next time we’ll talk more about retained austenite.