Heat treatment

Heating seized car parts in order to free them is conventionally accepted as a useful procedure and I have done it many times. However, heat treatment will change the hardness, toughness and brittleness of steel components and I wonder if this might cause future failure? Changes in chemical structure will occur well below red hot temperatures and a visual inspection will not show any differences. However, the durability of the affected component might be severely compromised. Are there any official guidelines about using this technique on safety-critical parts? Robert Talbot This is a very good point and it should be noted that some brake calipers have a rubber insert within the slider pin recess to dampen movement – even low heat applied to the area would damage the rubber insert.

When talking about steel, there are four ways the structure of the metal can be altered: annealing, normalising, tempering and hardening. Annealing relieves the stresses in the metal by heating beyond the critical temperature and then very slowly bringing the temperature of the metal down using a controlled environment. Normalising is very similar, except the metal is allowed to cool at its own rate in the open air. Hardening the metal requires it to be heated up to the critical temperature and then cooled rapidly by quenching in oil or water. Tempering is a more precise process and involves reheating previously hardened metal by heating it below the critical temperature, then cooling it.

Metals in a motor vehicle are normally heated below the critical temperature, but sufficiently to allow expansion of the metal; it is this process which helps free seized components. Aluminium components require only a gentle heat to allow sufficient expansion to free them, while steel and cast components require a higher temperature. Providing the metal is allowed to cool naturally and is not quenched, the structure of the metal should not be adversely affected.

Of course, it is advisable to consider the surrounding components when heating, to ensure no damage is done to rubber or nylon bushes, sleeves, etc.