Stress relief of machined castings
Stress relief of machined castings
(OP)
Hello everyone,
I work regularly with machined cast parts made from A48 Class 30 iron. The parts are cast and machined in various places in the US and worldwide. Yet our incoming inspection regularly finds features out of their locational tolerance. Both their outgoing and our incoming inspection are using sufficiently accurate CMMs. We require stress relieving on the casting drawing but we do not specify method.
Is there a recommended method for stress relief? (Back when we did machining in house, our castings sat outside for weeks/months, and we also had a big vibratory stress relief machine). In the case of these parts there is very little time between casting and machining.
Is there another cast material that is more dimensionally stable?
Thanks for your thoughts.
I work regularly with machined cast parts made from A48 Class 30 iron. The parts are cast and machined in various places in the US and worldwide. Yet our incoming inspection regularly finds features out of their locational tolerance. Both their outgoing and our incoming inspection are using sufficiently accurate CMMs. We require stress relieving on the casting drawing but we do not specify method.
Is there a recommended method for stress relief? (Back when we did machining in house, our castings sat outside for weeks/months, and we also had a big vibratory stress relief machine). In the case of these parts there is very little time between casting and machining.
Is there another cast material that is more dimensionally stable?
Thanks for your thoughts.





RE: Stress relief of machined castings
RE: Stress relief of machined castings
Have you found stress relief done in the machining phase to be more effective than when done on the raw casting?
RE: Stress relief of machined castings
It is the industry standard to always rough machine castings leaving as much as 1/8 for final machining. Stress relieving is best done after rough machining. It can take place before rough machining if it is preferable for queues.
Heat casting for 4 hours and bring to around 500 degrees F leave to soak 4 hours and bring down the temperature slowly over a four hour period. That should relieve all internal stresses.
Personally I would not use vibration stress relieving on casting as they can crack.
RE: Stress relief of machined castings
I believe in the past our vibration machine was primarily used on weldments and the castings were relieved by thermal seasoning outdoors. From what I've heard these vibration machines are rather uncommon these days.
RE: Stress relief of machined castings
RE: Stress relief of machined castings
Black magic, witch craft and vibratory stress relieving are all in the same group of things to be believed or not believed depending upon how gullible you are. There is a figment of truth to the theory of the process. I think I've seen vibratory stress relieving work if you can tune the frequency and vibration wave fronts to exactly address the residual stress you are trying to relieve. Most often it does a little, can crack a few things, and is akin to voodoo, sometimes strange things happen. On more complex castings and welded fabrications you just can't determine how to vibrate them to make it work. I've even heard of castings showing some stress relief, in transit, during a long rough delivery ride.
Long enough aging, with many significant temp. changes will many times do the stress relieving for you. But, the surest method is thermal stress relieving. I'm actually surprise that they can hold the tolerances through the machining process. Or maybe they are real lucky and can do that but then the vibs. during transit, or some aging after machining are unlocking the residual stresses or some additional residual stresses.
You have to do some experimenting with your castings; size, shape, amount of machining, etc. to determine the best sequence of events. As stated by AbritTurner above, you may have to, or want to stress relieve after heavy rough machining. The shame is that this req'rs. several more handling and set-up operations, which you might want to try to avoid. You parts should have just enough time btwn. casting and machining, for thermal stress relieving by that supplier. And then, a guaranteed tolerance at your plant.
RE: Stress relief of machined castings
I agree with the previous post
thermal stress after roughing if cost permits.
I have read good stuff about letting casting sit in the yard
for aging & stress but is there normally time?
Cheers
MfgEnggear
RE: Stress relief of machined castings
To be clear, our casting drawing does specify for the vendor to provide stress relief. We might consider moving it to the machining drawing, however my suspicion is the vendor ignored stress relief completely. We are looking into that possibility now.
However we do sometimes work with our vendors to provide a larger quantity PO and stage the shipments so that we don't have the parts in our inventory. I will discuss with our purchasing department to see if there are mutually agreeable terms that let them pour more castings than they need to ship and let the castings season with the vendor. For the 2nd+ shipments this may help for minimal cost.
Thanks again for all of the input.
RE: Stress relief of machined castings
I'm sorry to hijack this thread, but I've never had to deal with this before, and it certainly sounds interesting...
So, How can you tell a part is already (partly) stress relieved?
RE: Stress relief of machined castings
RE: Stress relief of machined castings
Not referring to castings here, but I drew up a part for a contractor to make (stripper bars for a Harris folder. Involved removing most of one side of the raw stock bright strip. Told him to stress relieve first, before any machining. (he was based in Sheffield, of all places) He didn't arrange stress relief, and was surprised at the banana shapes he ended up with. He wasn't embarrassed, however, and brought the parts in to show me! Prat!
What's happening in British engineering?
RE: Stress relief of machined castings
These days profile specialists will cut you strips from plate and have grinders available to machine 6 metre lengths.
I should imagine engineering is the Cinderella industry at the moment, too many engineers and declining industries, I don't really know as I am retired.
RE: Stress relief of machined castings
I didn't describe the strippers terribly well, hence your reply. Stripper assemblies on print machine folders have a value high enough to justify specifying bright, cold rolled stock and stress relief, and in fact, were designed to utilise standard sections, minimising machine time.
The contractor did not grasp the necessity for stress relief or the consequences of not having it performed, so pitched up with a box of scrap to show me. We all make mistakes, and have experienced that "oh shit! that's why!" moment, but do we advertise the fact that we have? No, we do the job again, quickly, quietly, and correctly, and chalk it up to experience. Why did he make more than one banana shaped piece?
RE: Stress relief of machined castings
RE: Stress relief of machined castings
RE: Stress relief of machined castings
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Stress relief of machined castings
1. size of casting,
2 type of material,
3. amount of material to be removed
4. complexity of shape of component
Some components are best heat treated twice, from memory the old British classed material EN25 (moly steel) required such double heat treatment. Once to the bar in what is called a stabilising operation and secondly after rough machining to 1/8" of diameter a second stabilising operation ensured no movement took place.
Shafts are especially prone to movement and need to be viewed with a critical eye before starting and final machining.machining