What is weathering Steel and where do we use it?
This edition of the Fastener Training Minute with Carmen Vertullo was originally published May 16, 2020 as “What are Grade 3 Rivets” during episode 152 of Fully Threaded Radio.
As often is the case, my Fastener Training Minute discussion comes from an email question. One of our very smart Certified Fastener Specialists asked me about Grade 3 rivets. And of course once we talk about a grade, we need to know where that grade comes from. In this case it comes from the standard ASTM A502, the standard specification for steel structural rivets. It turns out that Grade 3 is made from a steel called “weathering steel”. In some other standards, it’s called something else. But what is weathering Steel and where do we use it? And why is it important to the Fastener industry?
Weathering steel is a peculiar type of Steel in that it has some additional elements in it, primarily copper, but some others too. these elements give the steel two properties: one thing it gives the steel is a modicum of corrosion resistance (and we’ll see what a modicum is in a minute), and the property is a little bit higher tensile strength than just standard low carbon steel. What happens with weathering steel is it does corrode, it’s not corrosion-resistant in the same sense that stainless steel is. It does rust and it pretty much looks the same as any other steel when it rusts, except it does one important thing: it will stop rusting. And so the rust that is on the surface actually acts as a preventative to further rusting. Some would even call it a patina. The people that call it a patina are those who use this weathering steel to make art objects. You might have seen them out in the world in various plazas in various cities.
Now weathering steel is good in most environments on planet Earth, but it doesn’t work too good where there’s too much rain or in tropical environments very popular throughout the United States. So now in the Fastener world, we see several standards that refer to weathering Steel. In this rivet standard, it’s called Grade 3. In all the other ASTM fastener standards, it’s called Type 3. Some of the standards in which we run into Type 3 steel is ASTM A394 for transmission tower bolts, ASTM A449 which is sort of a general steel bolt spec about the grade 5 strength 120K PSI level. F3125, our standard for structural bolts is probably where we mostly would encounter Type 3 steel, and that includes all four products of either A325 or A490 grade structural bolts and the F1852 and F2280 tension control bolts. We also see it in F959 direct tension indicators, direct tension indicating washers, sometimes called F436 flat washers, hardened flat washers, and A563 nuts which accompany all of those other bolts.
Now when we see a product that has been made from weathering steel or Type 3, it will be marked differently. In most cases that mark is that it’ll have an underline under the grade mark to indicate that it is Type 3. In the case of F436 flat washers, it will have a number 3 on it. In the case of A563 nuts, it’s up to the manufacturer to decide how they want to market or distinguish the product. For the A502 rivet, the marking is just a number 3.
So that’s how we run into weathering steel or Type 3 Steel in the various fastener standards.
Interestingly enough, I’ll tell you a quick story from my youth when I was growing up in Pittsburgh. They were building the U.S. Steel building. It’s right in the middle of downtown Pittsburgh. It’s a triangular shaped skyscraper about 60 stories tall and it’s made of Core 10 steel. So it’s rusty and the idea was to not have to ever paint it. That’s the whole idea of this product that U.S. Steel invented in the thirties called Core 10 Steel, generically called weathering steel that you don’t have to paint. If you ever drive through Colorado, for example, you’ll notice that their guardrails are rusty because they’re made of Core 10 steel. You drive through California and the guardrails are mostly galvanized steel. And I think that’s just a strategy Core 10 steel is more expensive than regular steel. I don’t know if it’s more expensive maybe than galvanized steel, but that’s a strategy that the engineers decide when they choose their steel.
So that is actually about as much as there is to know or you need to know about weathering steel I guess, unless you happen to be running a steel mill. In that case there’s a lot more to know. But just in terms of using and understanding the uses of Core 10 steel or Type 3 steel or weathering steel in fasteners, this is pretty much it.