Metal Embrittlement in galvanized bolts

[Ductmate]

We use 1/4"-20 Grade 5 galvanized bolts in one of our products. I don't know if they're electroplated or hot-dipped. Recently we've had problems with the heads breaking off during installation. When I do a pull test on them, (at 2"/min) they are breaking around 2500-2800lbs - sometimes even less. Normally my grade 5 bolts break around 4500lbs or more. The problem bolts do not seem to elongate before breaking, but snap off clean.

Doing some net research of my own, it seems that the problem could be coming from either liquid metal embrittlement or hydrogen embrittlement, but i don't know how to tell for sure.

My bolt supplier is ISO9002 and tests all incoming raw mat'ls to make sure they are the proper composition. I don't know if they do any testing after the galvanizing is applied.

My questions are:
What is the most likely cause of this?
How can I tell for sure if it is from the galvanizing?
Would a zinc chromate finish be less likely to cause problems?
Any other advice?

Thanks,
Vince Bloom

 

[TVP]

Vince,

Your supplier is grossly negligent. Hydrogen embrittlement of high strength, heat treated fasteners is a well known problem when acid pickling and/or electroplating is used. If you do not order the plating of these fasteners according to an industry standard, then you should probably reference something like ASTM B 633 Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel in order to insure that they are given a suitable treatment for hydrogen embrittlement relief (Section 6.6).

However, this really doesn't excuse a supplier from not already doing this if they knew that the bolts were heat-treated. The plating supplier and bolt manufacturer/heat treater should have worked together to catch this before it became a problem. By the way, if you intend to order these fasteners as a "Grade 5", then you should reference SAE J429 Mechanical and Material Requirements for Externally Threaded Fasteners as it defines the product requirements. It does not discuss coatings or embrittlement relief, though. Good luck.

 

[EngineerDave]

Gentlemen,

Those pictures are very high quality. Not to detract from the discussion but what camera system did you use to do this?

I would also highly recommend getting ASM's Handbook Volume 11 on Failure Analysis. It covers alot of case histories similar to this and is useful for a company to have.

 

[Ductmate]

The camera is a Fuji Finepix 2650 digital camera. The close ups were done on the "Macro" setting. It can also do video conferencing/webcam and short movie clips without sound. Got the camera, 64mb memory card, case, etc, for about $250 online. So far, I'm very pleased with it.

Only thing is, it eats batteries, but that's probably true of most digital cameras.

Vince

 

[formcast]

Vince,

I'm working for a fastener manufacturer and we make lots of Grade 5 bolts. We reference them to SAE J429 as TVP said and we use, for Grade 5, Boron material (SAE 10B21) as it helps in heat treatment of the parts. For plating we use (usualy) ASTM B633 and we ALWAYS specify "bake" on our drawings. We also send a copy of the material, heat treatment and plating cert's with every shipment. We do test the parts after heat treatment and plating and we keep our records in case our customer wants to see them. So, I sugest that you ask your supplier to provide you cert's and their test records with every shipment.

Good luck!