440C Stainless fastener problem 7

[PrecisionT]

Hi All,

I am having a major problem with a 440C fastener heat treated to 53-55HRC. It does have a sharp corner at the bottom of the slot so not sure if this is causing a stress riser or if I am just overlooking something but I have about a 10% fail rate. When put under lite torque (just by hand screw drivers) the heads seem to shatter on the male side only. Sometimes they can set there for 1 hour or more and then fail. But we have put some under decent pull force to try and create a fail and could not. What am I missing?

Thanks in advance,

 

[PrecisionT]

I have added a pic to show just what is happening.

 

[tbuelna]

PrecisionT,

440C is not a good choice as a fastener material. Since you are probably machining the part 100% before heat treat, the fracture failures are likely due to quench cracking. The quench cracks will form on the surface of the part, so they can be easily detected with a quick penetrant inspection.

If you need a high-strength, corrosion-resistant material for a threaded fastener, you might consider something like PH 13-8 Mo or 718 Inconel instead.

Hope that helps.
Terry

 

[PrecisionT]

Thanks Terry,

Yes, we are machining the part 100% before heat treat. They are pretty small so are being quenched in air. I will look at those two materials but at max HRC, IIRC, is high 40's and may not be hard enough for my customers requirements. We use to make them out of 416SS at 40 HRC and the only complaint we had then was they marred up to easy on assembly and they wanted a little better corrosion resistance.

Thanks that did help and had a feeling it was material but couldn't help but wonder if it was geometry at that hardness.

 

[KENAT]

Any scope to add an undercut under the head of the fastener?

At a previous employer we had some larger threaded components we were over hardening which cracked under the 'head' and found that adding a radius ed undercut helped. However, I'm not sure you'd have the space/material.

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[PrecisionT]

Hey Kenat,

No not much. It is threaded 8-32. We could possibly get a very small undercut in there. It has been a very weird case. Most, 95%, have broke in connection with the slot on top but a few have just broke off the threaded portion/stud. We have done a pull test on a select few with no failures but 10% still seem to break in application. Has been frustrating to say the least.

 

[PrecisionT]

Hi tbuelna,

Is it just something about a fastener that makes 440C a poor choice? I only ask because we have made dowel pins from it with good results and also other parts out of plate in the 58-60HRC range with good results also. And it also seems to be used a lot in cutlery. Is it just the geometry of the fastener or the type of stress it is under as a fastener?

 

[israelkk]

If you quench it in air it should had a colored heat tint. I assume you later on cleaned it and passivated it. If the cleaning process was done chemically then you probably caused hydrogen embrittlement problem. Cleaning of such high strength steels must be made by a mechanical cleaning process such as high mesh (180-220) aluminum oxide grit blasting but never by chemical means. No matter the process of cleaning, make sure you do not remove too much material because it will affect the bolt thread dimensions. Even with a chemical cleaning (to remove the heat tint) there is a big risk of material removal resulting in a bolt that is no longer comply with the thread dimensions spec.

 

[CoryPad]

At that hardness level, the part is highly susceptible to environment assisted cracking, including hydrogen embrittlement. While that can be suitable for pin or other applications, the tensile stresses in screws means the cracks can propagate.

 

[EdStainless]

Get rid of the sharp corners.
Do not thread right up to the head, leave a little smooth undercut.
check for machining marks, they may be where the cracks are starting.
add a cryo step, use a higher temper temperature, and maybe double temper.
With an air quench you may not be getting good transformation.

Corrosion resistance? 416 is junk because of the inclusions, but 440C is no better than 410.
None of these grades have much corrosion resistance.

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Plymouth Tube

 

[TVP]

You may want to consider reverting to Type 416 at 40 HRC and treating the parts with a low temperature interstitial hardening process such as SAT12 from Swagelok or Kolsterising from Bodycote. Substantially higher hardness and wear resistance without degrading corrosion resistance. Improved corrosion resistance can be obtained with a different stainless steel grade.

 

[EdStainless]

My hunch is that you are leaving machining marks in the slot, getting some microcracks at them in HT, and then failure.
The marks could be trapping impurities also making it worse.
If you decide to try the surface treatment route use 410, not 416.

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Plymouth Tube

 

[PrecisionT]

Israelkk,

Yes, we get a colored heat tint. We didn't passivate it only ran them through a vibe to even out the color.

EdStainless,

Yes, we are looking at better grades now. (416 is just so easy to machine)

TVP, Thanks I will look into that, as I like the machining rates of 416.

EdStainless, Is 410 really a big difference? 416 is so much easier to get in round bar. And yes we definitely have some machining marks in there, but not a really cost effective way to eliminate it.

Thanks ALL, You guys are a MAJOR help.

And this may deserve a whole new thread but is there a really good way to passivate 416 that is safe on environment and employees/mishaps?

 

[Screwman1]

Take a look at those fractured parts. The one in the foreground looks like it has an existing crack all the way around the circumference of the part. That looks a lot like ESCC. The failures all look like they are dead brittle. The fact that the failed parts are cracking at very light loads seems to indicate that there is an existing flaw that is making the parts more likely to fail. You could look at the fracture surface under a scope to see if it has intergranular cracking which would target some form of embrittlement or SCC.
I would want to put the max possible radius on any corners (including the bottom of the slot) and be sure that there are no machining marks on the parts.

 

[EdStainless]

Huge difference in corrosion resistance of 416 and 410.
The S in 416 that makes it machine easily creates many locations that corrode easily as well.

Passivation: First you have to eliminate the heat tint. Either heat treat in protective atmosphere or pickle are the only options.
You can then passivate, check ASTM A380 and A967 for guidance on solutions. In general either nitric acid or citric/ammonium citrate solutions.
If you pickle any of them are fine. If you don't pickle then nitric may be only workable option to get real passivation.

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Plymouth Tube

 

[rconner]

[Am curious what specific materials are you "fastening"?]

 

[BiPolarMoment]

I may be leaping to conclusions but to me that fastener looks like it's intended to be a bushing that captures (or is captured) something else when assembled. If that's the case, perhaps only the female (bearing) side needs to be 440C? Then a similarly strong but less-brittle SS alloy could be used for the male side. Side-note: Did you try 440A or any other lower carbon grade between 416 and 440C?

I'm also curious what is it about this case that wouldn't permit an undercut/thread relief. With the countersink on the female side it looks like there's probably one full thread that is not threaded anyhow.

 

[PrecisionT]

ScrewMan1,

Excuse my ignorance but what is ESCC and SCC?


EdStainless,

Thanks for the info!

Rconner,

Carbon fiber [ am curious why that matters?]

BiPolarMonent,

Yes it's intended to be a bushing that captures something and no, only 416 & 440C so far. And at this exact moment while we work on a permanent fix is exactly what we are doing, using 416 for the male side. We have the room for an undercut, would it help that much and eliminate the issue in your opinion?

 

[cloa]

SCC- would be Stress Corrosion Cracking.

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[tbuelna]

PrecisionT-

Attached is some material property data for PH13-8Mo, Custom 465 and 440C. Custom 465 cond. H950 will get you the strength you are looking for, and PH13-8Mo cond. H950 will come close. Custom 465 and PH13-8Mo both have far better SCC resistance than 440C at the strength level you are considering. You will also note that Custom 465 and PH13-8Mo have far greater elongation rates than 440C at >Rc45 hardness (10% vs 2%), which can be beneficial for a threaded fastener application.

Hope that helps.
Terry