stainless steel fastener breakage 1

[hinged]

I'm looking for some thoughts concerning breakage on a 1/4-20 410 stainless steel shoulder screw. The application involves mounting an aluminum hinge to steel doors and frames. These are exterior applications so weather is involved. On the steel,the doors/frames are always coated/painted and the hinges are anodized. the only base metal would be the drilled hole for the fastener.
The fasteners are HT to RC36-40, zinc plated and baked, and a final coating of xylan for additional corrosion resistance.
There is not a designated torque for the installation application and the fastener drives(#3 phillips) are never returned distorted or abused.
The doors would be distorted if the fasteners are over torqued, most would be installed with portable hand drills.
We had several instances where the heads have popped off with considerable velocity. The fractures are always a clean break at the location where the thread meets the shoulder on the screw.
Is this just a hydrogen problem due the baking process after plating or do we have a hardness concern?

Thanks

 

[mcguire]

If the failures occurred immediately after the fastener was torqued, then it's a hardness/toughness problem. If some time transpired before the failure, that identifies it as a hydrogen-caused failure and you must not have gotten all the hydrogen baked out.

 

[hinged]

Mcguire.
Thank you for the response. The failures usually occur within a time frame of 24 to 72 hours.
It appears that the platers had been baking for 4 hours and now have increased to 12 hours, we'll see if this solves the problem but I also have the issue with the hardness being so high.
An article that I had read involved 410 stainless steel self drilling screws ( ELco Textron)and they referred to the fact that with core hardness above Rc 35 they are more susceptible to failure.
The bulletin lists three factors that could contribute to the failures:1. stress on the fastener 2. presence of hydrogen 3. fastener hardness greater than Rc 35.

A second article " Hydrogen embrittlement testing can prevent big losses" by joe Greenslade states that " commercial categories of fasteners susceptible to hydrogen embrittlement are electroplated parts which are Rc 32 or greater in hardness." He goes on to say that these types of fasteners should be baked within one hour after plating, 4 hours min, 12 hrs max at 400 deg f.
Will we be sacrificing anything by decreasing hardness to 28-32, it appears that we don't sacrifice much strength.
Thanks
HInged

 

[mcguire]

Although susceptibility to hydrogen embrittlement is ususally given as a function of hardness, HRC 22 being the threshold, it is equally a function of yield strength. As you decrease hardness ( and yield strength ) you will exponentially increase the time it takes to fail. That may be a worse situation, however, if it happens in service.
Better to get the baking perfected or go all the way down to a hardness that gives immunity.
Or you could get a higher corrosion resistance material and skip the plating and the baking. Keep the xylan to protect the other materials from galvanic corrosion. See stainlesscenter.com for a list of martensitic stainlesses.