Stress relief of machined castings 1

After rough machining the casting,how about introducing stress relief as a step. In the past when inventories was not a concern,castings were allowed to lie in the yards for nearly a year. It was natural seasoning or stress relief for the parts.

We don't have that mentioned on the drawings but I'm certainly open to it. We generally have the castings and machining handled by the same vendor and so it doesn't particularly matter when the stress relief operation takes place.

Have you found stress relief done in the machining phase to be more effective than when done on the raw casting?

Cast iron is normally a very stable material, that is why it was used as machine beds 60 years ago and was the industry standard for machine tools. Shifting tolerances after machining occur when large amounts of material are removed from the casting and deformation is bound to occur.

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.

Thank you for the detailed input.

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.

Vibration machines can do a lot of damage, crack the welds etc., even the material, I personally would never use them.

Geesamand:
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.

Geesamand

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

Nope, nobody has time anymore. But I suspect we as a company lost sight of the value of a thoroughly relieved casting because we might not have been very aware of how bad it can be to machine a fresh casting.

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.

How can you tell there is some stress relieve already done, eg. during a delivery ride (as dhengr said), or be it because the part has already been outside for a certain time ?

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?

Stress relieving should be recorded on documentation to do with Quality Assurance, it costs money to stress relieve and there should be a paper trail with reputable companies. Any company that does not have quality assurance procedures in place is best avoided.

Hi,
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?

Hi bobble, it would have been cheaper to have bought in black bar normalised then ground or milled one side. Bright bar is much dearer then black bar.

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.

Hi, ABritTurner,

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?

hahaha maybe he hoped that you would pay him because he was so uhm loyal to your method, maybe he was in deep discussions with his secretary at the time and flushed or maybe he likes bananas. But we have all done it and repented at leisure. Now he writes on this forum and is treated as an expert.

Oh I forget to mention to the originator of this thread that Heat treatment specialists will advise you on method for any of your stress relieving situations. Some will even dig out the relevant technical information and fax it onto you if you ask nicely and pay promptly.

We do the rough machining, stress relief, finish machine on some invar parts (or rather we put it on the drawing and have vendors do it). Finding how much to allow them to machine after stress relief took a little effort.

Under certain conditions:
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