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Help with 304SS

Help with 304SS

Help with 304SS

(OP)
I have run into a problem with manufacturing a part and i am looking for some help to determine what went wrong.

First off we manufactured 2 perfect working parts about 6 months ago. We have now tried to duplicate the process and are having problems. The problem is that the 304SS material is not taking a texture from sandblasting the same as the first time. After the original time sandblasting the parts new parts we found they did not take the required texture we decided to anneal them and try again. We have just received a report saying the parts are still not taking the required texture. I have verified that the sandblasting medium is new and it is the same physical person doing the sandblasting and they are using the same pressure and gun as last time.

We only changed one thing we know of between the first set and this set we are having problems with. The newest set was subcontracted out to a different machine shop. I don't know if this could actually be causing the problem or not. And if it is is there anyway we can fix it.

Any help or suggestions would be much appreciated. Thanks

RE: Help with 304SS

Is the problem appearing on the original wrought surface or on machined ones?
Can you test superficial hardness of both successful and failed parts?
The description of "not taking texture" is not much informative.
Would you mean that the surface is too hard to be indented by the sandblast?
How was annealing performed?
Any more hints?

Regards,

http://www.welding-advisers.com/

RE: Help with 304SS

(OP)
The problem is not apprearing until we try and sandblast the parts. Everything looks perfect until then. The sandblasting is done after the machining.

I can not easily test for hardness on these parts as the original part is at our customers currently and they don't want to give it up. I can test the current part i have but it will not give me any immediate results since i have nothing else to compare it to.

We are sandblasting the parts with Aluminum Oxide #14 which should give it a feeling similar to #200 sand paper. On the latest parts it is feeling more similar to a #800 sand paper.

I do believe the 304SS is harder now and that is why it is not taking the texture as we thought it would.

Is it possible for material to harden like this. It was machined on a lathe and polished only. As far as i know that was the only processes done to it.

The annealing was done for 40min at 2050 deg F in a vaccume furnace. The part was nitrogen quenched.

I don't know if any other information is required but if you ask questions i will do my best to answer.

I hope that gives you a better picture.

RE: Help with 304SS

The anneal should be fine.
It sure is possible to work harden surfaces when machining.  Different tooling and methods can cause big differences.

Were the pieces made from the same piece of raw material?  There can also be large variations in properties.  Different Nitrogen levels are the first thing that comes to mind.  You might have actually nitrided the parts some when you annealed them.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Help with 304SS

Is the SS from different lots or suppliers?

I read somewhere that SS from 2005 often has lower levels of expensive Ni & Mo levels than previously; producers are at the lowest allowable composition limits.  Maybe, adding more Cr, Mn & Si to compensate. Mn increases work hardening. EdStainless, you must see lots of mill specs; can you comment on the composition?

RE: Help with 304SS

(OP)
The pieces were made from different pieces of material but from the same supplier. I would have to assume the raw material was from different lots as we bought them about 6 months apart.

We have actually somewhat solved this problem now. We bought a new bag of aluminum oxide to blast the parts with and the parts are taking on the proper texture.

I do not know if this was a combination of problems or if the bag of blasting media was the original problem.

I would really like some help from people here to determine if it is possible the surface did harden when being turned on a lathe. It would be great to know how hard the surface could have got too. (Rockwell scale would be great.)

If it is a combined problem then the media might have been broken down by the hard surface and then after annealing they used the same media rather then using new media.

We also seem to have some problems with the annealing that was done. Our parts seemed to have warped more then i was expecting from the heat. Is there anyway to help limit the warp in the future? Or maybe i should be normalizing the parts rather then annealing them.

Thanks for the help everybody.

RE: Help with 304SS

In no specific order....

The mills have been running minimum Ni for may years now.  It is not uncommon to see 8.05% Ni.
The cheapest replacement for Ni in the alloy is Nitrogen.  It is an austenite stabalizer, it is low cost, and it is not resticted as an addition.  The N will raise the strength of the material.

MAchined surface issues are hard to deal with.  Surface hardening has to be checked using microhardness or low load superficial scale.  I have only used microhardness for this.  It is not difficult to get surface layers in machining 304L that are the equivalent of Rc40.

If you could, you would be better off not annealing the parts.  This introduces a lot additional variables.  You can look at lower anneal temp.  It depends on what you can live with for properties.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Help with 304SS

(OP)
Ok that gives me some help.

My sandblaster mentioned to me that we might want to look at cryogenically treating the parts. Does anybody have experience with this. I am told that it should help normalize the part and reduce the surface hardness somewhat without inducing heat that will chance the physical apperience of the part. If anybody has experience with this i would appreciate some help.

We really do need to get our part back into a more normal range of hardness somewhere between Rc18-22 is what i am looking for.

Thanks

RE: Help with 304SS

Hi,
I would recommend lowering your annealing temperature to 1800°F from the 2050°F and also try slowing down your quench a bit with an argon gas cool, or even a slow vacuum cool to a lower temperature something like 1000°F before force cooling with nitrogen.
Not knowing the cross-sectional shape of the part you are making it is difficult to say for sure, but if you are introducing stresses into the material by your machining methods and work hardening it as well, the part may move a bit in the furnace as a result.

RE: Help with 304SS

(OP)
Jsokal thanks for the help.

The part is a ring with about a 24" Inner diameter. If you take a cross section it looks like a U but with the opening facing the outer diameter. The outer diameter is 27.5" and the thickness is 0.75" Also the inner diameter thickness is very thin only about 0.047" thick.

Any suggestions for fixturing including what material to make a fixture out of is much appreciated.

Thanks

RE: Help with 304SS

Cryo-treatment will do nothing except remove money from your pocket. No matter what anybody says. No matter how they do it. No matter how hard they try to tell you it will solve all your problems.

RE: Help with 304SS

A variable that has not been mentionned is the degradation of the grit. Fresh abrasive will give a rougher surface. This happens when using abrasive belts to produce a #4 polished finish on coils of stainless. The Ra drops from over 30 micro-inch at the beginning of the coil with fresh belts to 10 to 15 after several thousand feet.
 

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Help with 304SS

1. Don't bother with cryo, there are no phase changes to work with in this alloy.

2. Are there any corrosion test requirements?  A lower anneal temp, maybe even 1750F, will work IF this is L (low carbon) material.

2.a. In low carbon grades the cooling rate is not significant.  You don't need to force it.  Let it cool gently until it is dark, then it is strong enough to not distort much.

2.b. Do you really need to anneal?  If the only reason was related to the surface finish then maybe you don't need it.

3. Your abrasive should have a fixed replacement rate.  Such as 'remove 3# or used abrasive and add 4# of fresh after each part' (or some such numbers).  That way the mixture will stay constant.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Help with 304SS

(OP)
EdStainless,

Thanks for the info on the cryo. I also looked into it and talked with a specialist and they suggested that the cryo would not give the result i was looking for.

The corrosion is fairly minimal. I am specifically working with 304L SS here so it is low carbon.

The annealing is only done to remove the surface hardness. Are there other ways to remove the hardness without risking any distortion?

We are going to change our replacment rate of the media. This is a fairly low volume part so we will change it for every new part we do, which will add very little cost overall but should ensure we don't run into this problem again.

I am interested to find out if there are other ways to remove the surface hardness on this material. If you can elaborate i would appreciate it.

RE: Help with 304SS

Does the superficial hardness from machining impact anything other than the way that the part takes the blast?  If not, then look at modifying the blasting and forget the anneal.

Oh, one more thing.  Make sure that the parts get passivated after blasting.  It will prevent suface rusting.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Help with 304SS

(OP)
The superficial hardness does not affect anything other then the blasting. I agree it would be much better if we didn't have to anneal the parts but i don't know what media or process would be able to remove the superficial hardness so the softer material will take the required texture. We have tried blasting the surface of the part many many times with the very course media we want to use without effect.

Do you have a suggesting of something i am missing?

I don't believe we need to passivate the surface since these parts are used as a friction device any iron on the surface will be removed as the parts run in daily operation.

Thanks for the help.

RE: Help with 304SS

Friction device?  Now I'm beginning to understand the finish requirement.

Some alternatives to blasting come to mind:

EDM- normally run very slow to improve surface finish, but in this case it might be competitive at a high speed.

Spray metallizing- add metal melted from powder on the fly.  Again, normally optimized for good finish, but capable of making a very abrasive surfaace.

Mike Halloran
Pembroke Pines, FL, USA

RE: Help with 304SS

What would pickling do for you?
It sure will remove the surface layers and also start texturing the surface.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Help with 304SS

(OP)
Mike,

Thanks for the ideas. However it will be difficult to talk people here into changing the process at this point. It will only be an option if we can not find a suitable solution to the current problem.

Ed,

Is there anyway to test for the thickness of the superficial hardness in a non-destructive test so i know the minimum amount of material to take off? This does sound like a good posiblity. How fast of a turn around is this process?

Thanks again for the help.

RE: Help with 304SS

I hate to say it, but trial is the easiest way to find out.
Pickling in hot nitric/HF will take off any surface layers very quickly.
Are you interesetd in sending a few small pieces to play with?

You can find people in your area that pickle.  Often platting houses have this capacity.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Help with 304SS

(OP)
It would be great to have a starting place. I really have no idea how thick the superficial hardness is. Are we talking 0.0001" or 0.001". The less i have to take off the better.

We deal with a few plating houses so i will talk to them about their process.

If you are willing to give me a hand with this that would be great.

RE: Help with 304SS

Some further thoughts.

Nobody can really judge the thickness of the superficial hardness layer; it depends upon all the variables of the final machining passes, plus material removal and heat generated in the polishing.  Heat causes a thicker oxide film on the surface.

Question:  Which surfaces matter?  Is it perhaps the sides only, as in a disk brake rotor?

Don't like the idea of etching SS (except to reveal grain boundaries for metallography) to remove metal.  You might not like the appearance afterwards, anyhow.  As alternatives, I suggest wet polishing or electropolishing or electrochemical machining ( A simpler process than EDM; you can even do by hand by brush plating process, with saltwater electrolyte and the workpiece as anode.).

First though, try a mild stress relief at or below 480oC (900oF), with slow cooling.  Then, dip in the nitric acid/HF (or nitric/ammonium bifluoride) long enough to descale (remove when it starts to etch).

Besides platers, Al anodizers and anyone who passivates SS should have the nitric/HF solution available.  Don't think anyone uses except at ambient temperature (nasty fumes when heated).
Ken

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