Material crack in Stressproof hex bar

[mgproducts]

Our company has machined thousands of a hydraulic component we call a banjo bolt, using round Stressproof (tm) without any failure. Our customer decided to award this job to another vendor who would produce the same component using hex Stressproof material, for a lower cost to our customer. The job has returned to us after several of the components have failed that were produced by the new vendor. We asked for samples of the failed components and sure enough the parts show a crack the entire length of the part even to the centerline of the bolt head. These parts are not heat treated. Is it possible that the drawing of the material to the hexagonal shape is creating greater stresses in the material and that subsequent efforts to relieve are not as effective in hex as they are in round? Years ago we had an association with a screw machine shop that experienced failures using Stressproof hex bar for some items as well and determination was made to change over to round bar. We would like to be able to help our customer understand why they have seen part failure as well as understand for our interest what might contribute to these issues. Thanks in advance for any insights.

 

[Sparweb]

Perhaps this offers some clues:
From Niagara-Lasalle:

StressProof® can also be induction hardened but should be watched for quench cracks, and, being a resulfurized steel, a magnetic particle inspection operation should be performed after heat treating.

Have you done a hardness test to confirm that they have not been heat-treated? Perhaps the information you are getting third-hand is... hmmm... inaccurate?

STF

 

[mgproducts]

Good suggestion SparWeb. I had noted the Niagara-LaSalle information about the quench cracks but had not actually performed a hardness test. As you graciously pointed out my acceptance of third hand information should be validated

 

[MikeHalloran]

I had a bad experience with StressProof, many years ago.

We were making shear pins approx 2.5" dia x 10" long, and ordered Stressproof rounds just a little oversize. That was our mistake. Stressproof gets its strength from _severe_ cold working, and that leaves a 'bark' that's typically 1/8" or more thick. The bark showed as microcracks in our finish ground surface. The steel center would gladly have told us about that, had we known to ask. We didn't. So, it you're contemplating using Stressproof, order the Stressproof G/P variant, already ground to size, or order rounds at least 1/4" oversize on the diameter so you can cut the bark off.

As for hex bar, I'm having a hard time imagining how you could make a Stressproof hex to size without bark or something similar.




Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

It is possible that the bar forming process forced slag into the bar as it was processed. I had this happen with a material used in a turning process to create a component for a pressure-containing component. The flaw went the full length of the bar right in the center end to end.

Ted

 

[moon161]

One of my thoughts is that by helping your customer understand why the bolts they sourced from another vendor broke, will also help that vendor take another pass at your business dinner when they find out via your customer. I'm sure there's a polite way to say it's because you've been in the game a little longer than the other guy.