Delayed Cracking 304L SS Deep Drawn

I would check the carbon content and keep it below .04%...Also would check on optimizing the profile and finish of the draw die, which can eliminate cracking.

try following the weld with a oxy/ acet torch with a rose bud tip. they put out alot of heat over a large surface area. move the torch side to side while the weld is still hot. if you move slowly, the metal should anneal behind the torch and relieve any stresses from the welding process.

Sounds good.

You two duck brains need to wake up and get with the program, or just go back to la la land.

Go easy on them. They know not what they know not. Much the same as ex-President Bush junior.

Delayed cracking after welding? OP says nothing about welding. Anyway, welded delayed cracking use to be related to H₂ in the weld metal.

If one just read the written words... cracking seem to be after deep drawing.

Then google about "strain induced martensitic transformation" and look about the strain induced, temperatures, etc. Anyway a solution anneal will solve the problem recovering the initial austenitic structure.

kind regards

Masher, are you having trouble with "Material is .250" thick. Approx. 4" circular blank. Needs to be drawn/stamped to make 1.9" dia. x 2" long cup shape while maintaining a bottom thickness of .2" .."

You don't provide a lot of feedback to your questions. It will help if you would do so.

Hi Doorman. You hit it right on the head. The die was built and tried out. The results were very good at 1st anaysis based on visual and targeted dimensions etc.. Some parts (raw drawn cups ) were then cylindrically ground removing approx. .016"~ .020" off the OD..

What we thought was a beautiful thing. Was transformed a week or two later. When we realized any raw drawn cups that had not had the OD ground after words. Began to exhibit cracking in several locations around the circumference. The cracks propigate from the open end down along the sides. But none of the ground cups have cracked so far. Into week 4.

Could the grinding and thus the removal of the surface skin with the increased "strain induced martensitic transformation" be the difference. Could this allow us to continue without the fear of cracking in the future. To be safe. We are looking at adding a process to strain anneal. But we are being given mixed messages of the proper temp and duration. I don't want to get back to the original softness/strength. As long as enough stress is removed to prevent cracking/fracture.

Also. I saw the comment about keeping the carbon content down below .04%.. None of the SS mills have mentioned that. They have however proposed increasing the nickel content from approx. 18% to 19~19.3% . But may not have it commercially available in NA.. And of course the cost of nickel is like taxes.

MoMash

Is the 304L your selection, or a customer/performance requirement?

The parts was previously made from 1008 seel. The customer wanted stainless. The steel mill made recomendations of 304L..

Well if the specification is 'stainless' how about type 301, 202, or even 102?

Have you spoken with the metallurgist at the mill about the fabrication, or is 304L the suggestion of a sales representative of some type?

We have 4 or 5 really good material science guys here who will give you the straight poop. If you are lucky, one or two will come along.

Originally the "sales rep guy". (on the head again Doorman) suggected 301. Then he came back when I needed to build prototypes and said that was a mistake and should have been 304L. Now when I ask about 301. It is not available in the coil form i need.

Advise from metalurgists would be very appreciated.

The main reason for cahnge from 1008 steel to stainless was for heat transfer. To slow down the transfer of heat trhough the part to other components of the assembly. The part gets nickel coated. So surf. hardness for wear and corrosion resistance is not the issue.

Why would you want to increase the Ni content when the thing's already cracking?

Milo or Dr. Galala, and few others will know. If not for strength or corrosion resistance, why 304L? Why not use the most malleable grade. I don't think the R factor changes much.

Adding nickel adds ductility AND reduces workhardening in stainless. Milo

I knew there must be a reason.

Thanks.

I'm curious what the % elongation of the maximum moved material is? 40 percent elongation is probably best expectation. You dont describe if the cracks are longitudinal or at an angle. 304/304L is not a nominal substitute for 1008 for cold working applications. I am not convinced that you created martensite or if your grinding is removing microcracks from exceeding yield and materials max % elongation. What is the maximum elongation the workpiece saw? I am travelling but will check back. Milo

I don't have the number for the elongation %.. It was not actually measured. Partially because as I said way back. The die and part seemed great. Not a lot unlike 1008 steel to the naked eye other than it ran hotter. Approx. 450f.. But that was to be expected due to the material & the "making do with draw oils on hand conditions". Rush, Rush, Rush.. I'm sure you guys can relate.

The total dia. draw ratio is 2.09 by blank dia./cup dia.. Or if you like. 48%. Thats over 3 draws. After the 3 draws. The part is tranfered to the next stage of the die (within seconds) and sized. Therefore subjected to high forces to thicken the walls and decrease the outside bot. corner radius thus length to OD & bottom face intersection. And the over all length is set.

Oh. And yes the cracks are longitudinal along the length. Propagating from the open end of course.

There are no micro cracks that we have identified. These cracks go through the entire wall section of approx. .25" thickness.

Are there stainless steels that are more suited for deep drawing heavy gauge. That still have the heat resistant features.

Hi Masher.

Here are a couple more thoughts.

1)I don't think an anneal will do you any good, I don't think that you are really creating martensite. So if you take some unground cups and anneal them i suspect that they will crack regardless.

2) Since they are longitudinal and not at an angle they may be remnants of original hot rolled steel imperfections. WHile seams come to mind, ferrite fingers or other process anomalies could orient themselves in the way you described. These could serve as nucleation sites for the cracking or "stress risers" as many people like to call them. I dont think this is the case unless the cracks all align with a particular direction or orientation of the blank, but it is something to look at.

The fact that removing the outside surface stops the cracking tells me that this is something to do with removing surface conditions that can cause cracking- i see that as pre existing seams, ferrite fingers, die scratches etc.

3) your existing tooling- what is its grade. If it is D-2 that could be a potential cause, as d-2 in particular likes to gall with 304. your statement about run of the mill oil makes this a real possibility, as does just outright oil film failure. however since you didnt say anything about scratching, this is discounted in my opinion.

4) a 302 HQ would be a better choice for severe cold work if my memory serves me right.I'll be back in my office friday (I m currently in LA) and can look at some of my reference materials then.

If there are bright scratches on the unground parts, then those scratches are the proximate cause of the cracking and your removing them by grinding is preventing th e ground parts from cracking. If not, then I go with my original idea about exceeding the material's elongation.

I do not believe that there is any marteniste being created, and by the way a good matallographic examination of the cracks would tell us the real cause/ mechanism.

Hope that you find this helpful.

I''ll be in an airplane all day tomorrow, I'll likely be back on line friday.

Milo

Hi Milo. Quick question. Did your memory serve you well. IS 302HQ a better suited material? If you don't mind. Why? What are the differences from 304L that make it better suited? I would have to sell this to my associates and to our customer. That may be difficult without good reasons.

By the way. Thanks for everyones inputs. Appreciated.

Good morning Masher.

Yes It did.

Here is what Carpenter book says regarding cold working of Carpenter stainless type 302:

" Carpenter Stainless Type 302 is readily fabricated by coldworking. It is extremely tough and ductile and responds well to deep drawing, bending, forming, and upsetting. After cold working it is slightly magnetic. The tensile strength and hardness of Carpenter type 302 can be significantly increased by cold working."

1 caveat: You mentioned earlier something about heat in the application. If the part is heated between 800 F and 1650 F you may very well run into intergranular corrosion.

In selecting materials, its always something.

We used to draw this into wire. It drew nicely. It machines terribly, as it produces a long gummy chip (there's that ductility!) that just won't break.

So back to your original problem- why can't a piece of insulation be placed in the prior 1008 design to retard heat transfer?

Milo

In reply to. "Why can't a piece of insulation be placed in the prior 1008 design to retard heat transfer?" That question had been asked a while back. Apparently an insulator pad was tried. But created too much vibration/noise. Or so it goes.

I am going to attempt to locate some 302HQ and set up a trial run for testing.

Thanks for your advise.

Masher