The problem of retained austenite is twofold. A lower hardness value will be experienced (depending on the degree of retention of the austenite phase) in addition to dimensional instability. If the problem of retained austenite is not addressed, a natural and progressive dimensional change will begin to occur due to the transformation of the retained austenite into untempered martensite ... hence distortion.
It can be seen that the first distortion will occur as a result of stress relieving as the steel is heated up. The second potential for distortion is the phase changes due to heating and cooling of the steel. The heat treater is fighting a losing battle from the beginning because he has no control over the pre-condition of the steel before it is received for treatment.
Surface treatments such as nitriding, post-oxy-nitride, FNC, carbonitriding, carburizing and boronizaing modify the surface by diffusion. Any process that modifies the surface by the diffusion of an element or combinations of elements into the steel will cause a surface volume change. In other words, the surface will grow – hence distortion – without even quenching!
What can be done to eliminate distortion? The writer believes that distortion cannot be eliminated, but it can be controlled by:
- Controlling the depth of case formed in any of the diffusion processes.
- Controlling the rate of heating when the steel is heated to the austenitizing temperature
- Controlling the quench-medium temperature carefully in relation to the Time-Temperature-Transformation diagram
- Ensuring a clean quench medium free of sediment and sludge
- Ensuring that careful preheating takes place
Lastly, the writer believes that it is most important to get the client to talk BEFORE the work comes to you about how you both can develop a synergy to reduce the risk of distortion in terms of: machining practice, grinding practice, post-heat-treatment machining and the importance of pre-final heat-treatment stress relieving.