Heat Treatment Shrinkage

What sort of tolerance do you need on the 42mm bore?

If its going to be around 0.005mm (0.0002") then you need to jig grind or some such (ID Grind) anyway.

If its around 0.050mm (0.0020") then you can likely get close with a bit of trial and/or a bit of cutting with a carbide tool.

If its around 0.5mm (0.0200") then machine and heat treat and dont worry.

Some steels are better at this, and soem machinists can pull off a drawing note of MTS. (Make To Size)



Nick
I love materials science!

td1racer,

There is no formula for this, because there are competing effects: growth due to martensite formation, shrinkage due to temperature reduction & tempering, and then the unknown of residual stress distribution in the bar/billet/machined component and what type of distortion will occur due to uneven temperature distribution during quenching. NickE gave some recommendations for how to account for this.

You should completely exclude the idea of formulas - unless you are individually heath treating using an automatized system induction and/or tube furnace type, in this case you maybe able to develop some empirical data and/or formulations - in real life there are multiple factors affecting growth of the heat treted parts - some very well explained by one of the responders - another important factor is load size & distribution - this will affect in terms of growth - " pick up " and also in terms of distorsion - associated with quenching a long product ( how do you have these parts in the fixture and not just the density ). Last but not least when you look at the " pick up " aspect the furnace uniformity will be the main factor - higher temperature ( uneven temp in the furnace ) first and then exposure ( atmosphere agitation ) in the furnace as a secondary factor.