FTM 131: Surface Discontinuities

Fasteners are allowed to have a fair amount of these things called surface discontinuities and still be acceptable according to the standard.

This edition of the Fastener Training Minute with Carmen Vertullo was originally published August 23, 2018 as “Surface Discontinuities” during episode 131 of Fully Threaded Radio.

Hi everybody, this is Carmen Vertullo with the Fastener Training Minute coming to you from the Fastener Training Institute and the Carver Facts Center here in beautiful, San Diego.

Today’s topic has to do with a very interesting problem that we find in fasteners. It doesn’t occur very often, but when it does occur it throws lots of folks for a loop, and that’s called surface discontinuities. Otherwise known as crack, marks, things on the surface of the fastener that look like they don’t belong there. When we get these usually the customer complains and they’re not happy, and we have to do something about it. So how do we deal with surface discontinuities? Today we’re going to talk about surface discontinuities on nuts. In a previous episode, we talked about surface discontinuities on bolts. So when we return I’ll tell you how to analyze and deal with these and make your customer satisfied whenever they see these things called surface discontinuities on Nuts.

I had two cases recently where a client sent me a sample of a large hex nut. And one of them had a crack on it the other one had a weird-looking line and in both cases the customers complained about these. One was a silicon bronze nut that had a crack, and the other one was just a common Grade 2 large hex nut with a weird-looking line on the bearing surface. So, interestingly enough, it turns out that fasteners are allowed to have a fair amount of these things called surface discontinuities and still be acceptable according to the standard. If you’re in the inch World, there are two standard for surface discontinuities that we would use. For bolts and screws its ASTM F788, but when it comes to nuts, ASTM F812.

ASTM F812 is a very illustrative standard. It has lots of pictures of nuts with weird-looking lines and cracks and these things called surface discontinuities on them. So we would take a look at our particular fastener and we would say what kind of a line is this? Is it a crack? Is it a fold? Is it a seam? Is it a lap? What’s going on? How did it get there? In some cases we’re allowed to have a surface discontinuity. One particular surface discontinuity that we’re not allowed to have is a thing called a quench crack and that typically is only going to apply to heat treated products. So if it’s not heat-treated, it’s not going to have quench cracking that looks like crack. So any kind of quench crack is not allowed, but other types of surface surface discontinuities such as forging marks, folds, laps or seams are allowed.

You would be surprised at the level of surface to discontinuities that are allowed. They can be pretty ugly and gnarly and actually look like they might make the part not work, but they can still be acceptable. Now regardless of whether or not the specification finds the product acceptable. The customer is still going to have to be willing to keep it. In many cases, the customer simply needs to be motivated to keep the part in play. But as a supplier, you need to show the standard and why it is okay, and that there’s nothing wrong with that part. And hopefully everyone is happy ever after. If you happen to have your IFI 9th Edition in the Fastener standard book. You can look at ASTM F812 and see this illustrated.

If you happen to live more in the metric world, you can use ISO 6157-2 for surface discontinuities for nuts and it looks pretty much the same exact way that ASME F812. So the standards are remarkably similar.

So two things to remember: surface discontinuities on nuts are not always unacceptable; in most cases the standard allows them. And secondly, it doesn’t matter what the standard says if it looks ugly and the customer doesn’t like it, you’re going to take it back. Unless they’re motivated to keep it in play because they have no choice, you can show them how they can do that by referencing the standard.

Well, that was today’s faster training minute. This is Carmen Vertullo from the Fastener Training Institute. Thank you for listening.

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