Quenching is perhaps the area in a heat-treat operation where the problem of distortion will occur. However, it is not always the quenching that is the cause of the distortion; it is usually “the effect” because it is immediately after quenching that distortion is seen.
First, we need to understand the term "distortion" as it applies to heat-treatment operations. You have all heard the following comments from a 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, the distortion at quenching will always occur.
Causes of Distortion
There are many causes of distortion, but the primary cause is machining and residual induced stresses. 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 the removal of induced stress is by the application of heat.
As soon as the steel (or metal) begins to see heat applied, 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, then a phase change will occur. This means that the lattice structure size and construction will change from a 9-atom 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 (TTT) diagram. If the steel is cooled in such a manner, then full transformation can be achieved. 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.