As promised, we will cover retained austenite and finish our hints.
The problem of retained austenite is twofold: a lower hardness value will be experienced (depending on how much austenite is retained) in addition to dimensional instability. If the problem of retained austenite is not addressed, a natural and progressive dimensional change will begin to occur – distortion – due to the transformation of the retained austenite into untempered martensite.
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 occurrence of 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-treatment processes such as nitriding, post oxy-nitride, FNC, carbonitriding, carburizing and boronizing that require the surface to be modified 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? I do not believe that distortion can be eliminated. I believe that it can be controlled by doing the following:
- Controlling the depth of case formed in any of the diffusion processes
- Controlling the rate of heating when the steel is being heated up to 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 careful preheating takes place
Lastly, and I believe very important, get the client to talk to you 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 pre-final heat-treatment stress relieving.