Dimensional change considerations wtih 4140HT

[fastline12]

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.

 

[Wrenchbender]

With a 1-mil tolerance, holes have to be 'reamed' to size after HT due to possible bulk part warpage and scale formation as you mentioned.

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.

 

[gearcutter]

Your question is like 'how long is this piece of string I'm holding if front of me'..........that we can't see.
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

 

[redpicker]

I doubt you will be able to hold +/- 0.001" tolerance on holes machined in annealed material after a quench and temper heat treatment. As has been menitoned, there are just too many variables that you won't be able to control. The best you are going to be able to do is reduce the amount of stock you have to leave that you will have to ream or hone after heat treatment. Because of the high hardenss, 48-50 HRC, you really don't want to have to remove any more than needed, but you will need to leave enough to insure you will hold tolerance after.

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

 

[fastline12]

Obviously a lot of variables and I think what I am mostly needing is data. We don't spec much heat treating and don't have many standards to lean on. Post machining is not an option. We recently had some parts done from 8620 and cased and a .118 hole close up by about .0005. All other dims were the same.