Shot peening, tight tolerances

[lfw618]

I am relatively new to shot peening, and have some questions regarding some parts I am currently designing that may need shot peening. I understand the general big picture of shot peening, but am getting a bit hung up on some specifics. Hoping this is the right forum for the question.

I am peening to AMS2340, my parts are essentially a round rod, and would require a cast steel shot .010A to .015A intensity according to the table 8 guidelines. As I understand it, this intensity refers to how much an Almen strip will bend in a free state, after being subject to shot peening while constrained.

I am dealing with tight tolerances. Tolerance on final part diameter is +/-.0001" I am getting a bit hung up on the dimensional changes that will come along with peening. It's my understanding that the dimples from shot peening will induce some variability(and even growth?) on the surface of the diameter. I am not sure how to calculate how much variability there could be. I see that in 3.9.3 it is stated that not more than 10% of the specified nominal (.012A) may be removed after peening (so in my case you would not remove more than .0012"?). This makes sense to me as you don't want to remove the residual compressive stress layer. I am trying to determine if the amount of material I am able to remove, will be enough to remove the variability in dimples caused by the shot peening, to get within +/-.0001" tolerance.

I have gotten stuck trying to figure out how to evaluate this. If there will be more than .0012" dimensional variability induced, how can I determine how to adjust intensity/size/media so that I can get the final part in tolerance, while still retaining enough of the residual compressive stress?

Thanks in advance for any help you are able to give.

 

[WKTaylor]

I hate assuming... but do You mean "...regarding some parts"... that they are MF medium to high strength steel or SStl or HRA?

IF so then obtaining a 'finer shot-blasted finish' with Your SP process and controlling distortion and surface finish quality shouldn't be too hard.

Some useful starting points...

ARL-TR-4095 Shot-Peening Sensitivity of Aerospace Materials

AMS2430 Shot Peening [Duuuuh]
AMS2431/* Peening media [Duuuuh]
ARP7488 Peening Design and Process Control Guidelines
SAE HS-84 Manual on Shot Peening

MIL-P-81985 [CX] Peening of Metals

Comparator Gages for Surface Roughness (Machined Finish Quality) Evaluation [Source I am familiar with => GAR Electoforming Division, Electroformers Inc, P.O. Box 340, Danbury, CT 06813]
SH-6 Shot-Blast Microfinish Comparator to evaluate bead-blasted surface finish quality (general purpose).

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[lfw618]

Sorry for my ignorance on this, what does MF stand for? The parts would be medium carbon steel like AISI 1045, at about 20-25 hrc.

Thank you very much, this gives me a lot more to look into.

 

[WKTaylor]

IFW618...

Your steel is heat treated [HT] a bit softer than I expected for such close-tolerance. Let me get my thoughts together on this.

Is this a shaft-surface [round/uniform] or a curved/shaft-surface [such as a 'cam'] or a dead-flat-surface or a hole-surface [etc] that requires exceptionally close-tolerance?
----------
Hmmmmmmm... 'MF' good point...

I forget that what I use simple abbreviations in my quick emails/dispositions that can 'read' like nonsense unless defined in context... especially 'MF'...

In context of materials... I use 'MF' => 'Made[or Make] From'.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[lfw618]

It is essentially a shaft surface with some notches and cutouts. The tolerance is after plating, so before plating the part could have looser tolerance than +/-.0001" but would still need to be held tightly. I also would not want a rough surface finish before plating. That is why the part is softer, to avoid hydrogen embrittlement during the plating process.

Ahh that makes sense, thank you!

 

[WKTaylor]

OK... please advise...

What is Your 'plating'... and how thick?

Cadmium, zinc, zinc-nickel, nickel, 'other nickel alloy', chromium, 'other chromium-alloy', etc...?

Flash? thin? engineering?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[MagBen]

Such soft material will not be susceptible to H2 embrittlement. Normally you need to worry about such embrittlement for material >38 RC. yet there is a way to baking H out, ASTM specifies two baking receipts for material with hardness between 39-44RC and >44RC.
Instead of SP very low strength material, why not use cold-worked material, or other alloy steels to increase fatigue strengths? For such a tight tolerance application, SP will absolutely damage and rough your surface, and you will have to remove too much of surface... by the way, are you sure tolerance is .0001'', not .001''?! what is the diameter of your shaft?

 

[lfw618]

It will be chrome plated, thickness will be .0006-.001. I have been referencing AMS2460A. Yes i am sure the tolerance is +/-.0001", I'm aware that is very very tight. Part is about 3/4" in diameter. I could make slightly harder, but going off the standard I need to keep parts under 34HRC to avoid hydrogen embrittlement risks.

Shot peening came from the AMS2460A as class 2c plating for parts under 40 hrc. As I understand it, shot peening is on the standard to improve fatigue life due to the residual tensile stresses from the chrome plating process.

Recently I've tried to dig more into it and have found some sources say that the tensile stresses come from hydrogen embrittlement, though I have not been able to confirm that. If it is true that the tensile stresses are almost all as a result of hydrogen embrittlement, then I may be able to justify getting rid of the shot peen, since my parts are soft enough that that may not be an issue. I just have not been able to find a reliable source that attributes the tensile stresses from the chrome plating process solely to hydrogen embrittlement.

 

[MagBen]

But won't you plate after shot peen? so you won't remove residual stress anyway by SP anyway.

Wondering how you achieve the tolerance. 1/4 STD for precision grind at .75'' is +/- .0005'', surface roughness can reach to 50Ra, or .00005'', which alone can lead to .0001 variation in diameter. Even if polish (mechanical), roughness can be up to 20 micro-inch. Plating can get a very tight tolerance, but could not it depend on tolerance prior to plating?
Do you use electro-polishing perhaps?

 

[tbuelna]

lfw618,

Surface coatings like your hard chrome plating (or hard anodize) often contain small cracks. Creating residual compressive stress in the underlying metal surface by shot peening inhibits those coating cracks from propagating downward into the substrate.

You can obtain a smoother surface texture by using larger size shot. Shot size is typically limited by the surface geometry of your part (fillet radii, etc). If your shot peening is performed properly, there should be no problem controlling dimensional changes and surface texture to meet your requirements. 0.0012"/side should be adequate stock removal allowance for the grind operation prior to plating.

You also mentioned the finished shaft diameter tolerance is +/-.0001", and the plating "thickness will be .0006-.0010". But is the specified plating thickness before or after finish grinding? Seems like you need to develop a controlled manufacturing process for this component.

 

[lfw618]

The thickness I put will be after finish grinding. For the stock removal after shot peening, on the AMS2430 standard it seems to promote lapping, honing and polishing over grinding. Is this because grinding can create residual tensile stresses where the other processes do not? Due to our shop load we won't be making the entire part here. I believe we will be getting the pre-plated "blank" part from a supplier, so they would be doing the stock removal. Is there a good way to specify a controlled grinding process that will not induce residual tensile stresses, and how would you be able to QC a process like that from a vendor?

 

[Blank man]

Correct me if I'm wrong, but based on your plating tolerances, it seems like size variation due to shot peening might be a non-issue. If you're plating the surface after shot peening, you'll be outside of your final tolerance range and would need to do some finish machining or polishing either way. You mentioned above that you're considering polishing the shot peened surface, but that might not be the recommended approach since you could potentially lose the benefit that shot peening provides in some areas of the part, and it just sounds difficult and expensive. Maybe the shot peening supplier could send you a sample part, or tell you what the surface finish is on a shot peened steel part at that intensity so that you could build that into your stack up?