Dimensional change considerations wtih 4140HT
Dimensional change considerations wtih 4140HT
(OP)
I am trying to determine target machined dimensions for a part being machined from 4140 Annealed. We will specify the heat treat and temper to HRC48-50.
I have read some doc referring to scale losses but it was my understanding that all commercial treaters operate in a vacuum furnace to reduce that concern?
I need to make sure I have all considerations covered. The only critical dimensions are really just a .450" diam hole and .159" hole. They have .001" of tolerance but I would rather hit the target values dead on.
I have read some doc referring to scale losses but it was my understanding that all commercial treaters operate in a vacuum furnace to reduce that concern?
I need to make sure I have all considerations covered. The only critical dimensions are really just a .450" diam hole and .159" hole. They have .001" of tolerance but I would rather hit the target values dead on.





RE: Dimensional change considerations wtih 4140HT
Second sentence is not always true, altho you can specify vacuum or inert furnace but it costs more. Typically you leave 1/8 - 1/4 inch excess for final machining, both to hit the required geometry and remove any decarburized layer. Otherwise consider 'no-carb' or 'stop-off' paint to minimize decarburizing.
RE: Dimensional change considerations wtih 4140HT
It all depends on size, geometry, quality of material, heat treatment conditions (such as how long the part has been allowed to soak at the quenching temperature and the quench oil temperature at the time of part insertion) etc.
If it's a critical part then I feel all you can do in this situation is do a test run...........even then the results will not be conclusive as there will always be too many unaccounted variables to be able to guarantee a result.
Wrenchbender
"Typically you leave 1/8 - 1/4 inch excess for final machining"
I don't understand how you can make this generalisation as the amount of material to be left behind for final machining is determined by the size and geometry of the part. The process of removing the amount of stock you are suggesting from a small component can in itself distort the part.
Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181
RE: Dimensional change considerations wtih 4140HT
You may also want to review your tolerances. The hardness range is rather tight, 49 HRC +/- 1 HRC. Considering that most Rockewll testers only verified to +/- 1.0 HRC in this hardness range, you could find parts that are in range on one hardness tester and out of range on another tester. A range of 45-50 would be much more reasonable. If you could live with a +/- 0.005" on the diameters, you just might be able to get by by machining them to the small side of the tolerance band and machining them afterwards.
rp
RE: Dimensional change considerations wtih 4140HT