CorTen vs XTen-50

[metman]

This is a take-off from the following thread in the stainless steel for engineers forum:

cost saving grades
thread1135-116731

At a previous job, we had several heavy equipment product lines. One of these required CorTen and some of the others spec'd XTen 50. Both of these alloys have a minimum YS of 50ksi so I attempted to standardize with CorTen since the prices had very little if any difference at the time. I lost the battle early because one product chief engineer said, "oh please don't take my XTen-50 from me" and I could not convince him that he would be sacraficing nothing by substituting CorTen.

Is there a reason that USS did/does not offer only CorTen or am I missing something?

 

[swall]

Metman--as you are undoubtedly aware, Corten's forte is its enhanced resistance to atmospheric corrosion by virtue of its slight copper addition (I can't remember the exact amount--was thinking .50). So, I agree that your attempt at standardization and getting rid of the X-ten 50 was a good move.But, sometimes, you just can't win these battles. The only technical (metallurgical) issue I could see with going to all Corten would be a possible weldability issue because of the copper content.

 

[HgTX]

Can your product chief get any more specific about his fears?

I assume you're talking about plate rather than sheet.

Cor-Ten was a USS brand name for ASMT A 588 weathering steel; now that USS swapped its last plate mill with ISG, I don't think there's any more "Cor-Ten" in plates (I guess there still is in sheet), but A 588 is still alive and well without the Cor-Ten name.

Ex-Ten 50 was ASTM A 572 Gr. 50, for plate.

I haven't heard about any weldability issues with A 588, but in the bridge world we only use certain electrodes; I can't guarantee that it'll work well with every single consumable that works well with A 572. Check AWS D1.1 to see if they have different consumables listed for the two steels, but I'm pretty sure they list them in the same category.

A 588 has slightly lower carbon than A 572, and additions of nickel, chromium, and copper.

A 588 doesn't like getting wet (I don't mean rain, which is fine, but rather immersion cycles), and the nature of the protective patina it forms means that it particularly doesn't do well with wet crevices, because the corrosion product expands more than that of A 572 (or so I'm told).

In bridges, they don't particularly design any differently for weathering & nonweathering steel (other than they paint the nonweathering). Unless you're looking at immersion service, I can't think of any problems with the substitution.

Is there any difference between the two products that would have required weathering steel only for one and not the other? Is one painted and the other unpainted?

Hg

 

[steelgreg]

Cor-Ten
Actually there are two different Cor-Ten steels. The original one was developed to enhance corrosion resistance without much regard to the ablility to form the material.
Therefore, for heavy plates or beam structurals, the original Cor-Ten was fine. The problem with the forming was because of the high phosphorous content which was there because of its contribution to the corrosion resistance.

If the application was a formed part made from sheets, that is 1/2" or less, then cracking would occur on the outside of a longitudinal bend, that is, with the bend being in the rolling direction. It wasn't too good when bent transverse to the rolling direction either. Much to the USS Steel Research Labs consternation, the mills altered the Cor-Ten chemistry to a lower phosphorous level. I don't remember the level for each of the grades or the other slight chemistry modifications that had to be made. I would have to find my 50 yr old internal grade book to do that. But from memory, the orignial Cor-Ten had a phos content up around .130 to .150% and the formable Cor-Ten had a phos level down around .050 to .080%. Normal plain carbon steels have phos contents of around .008%.
So the answer lies is whether or not the Cor-Ten is to be formed by drawing for bending over a small radius. If yes, the original Cor-Ten will crack.
Both the original and the formable grade has a YS of 50Ksi.
However, in the formable grade there was also a lighter strength modification that was sold as a 45 Ksi min YS.
All of the above is from memory of the time spent in the 1950s at USSteel.
My tag says Mechanical but I am also a Met Eng.

 

[metman]

Thanks for all the interesting comments. This is all academic now because it was 12 years ago that I worked for that company but just for the record, it was all plate over 1/2" so the questions raised about cross over to other product lines answers to the point that there was no reason not to standardize. The resistive chief engineer had no good excuse.

I had forgotten about the relvant specs and appreciate these comments for future reference.

During that same standardization attempt I tried to substutute another ASTM material call out for A36 plate that would have suited just fine and would have been much more ductile for forming but we had an obnoxious purchasing manager that threatened to hang the plate around my neck if he ever had trouble obtaining that particular call out so I hastily backed off. We could have just called out A36 as an alternate but I was too dumb to think of it at the time or maybe he had some other inane reason that I can't remember.

Earlier versions of A36 used to be more formable but they modified the chemistry at some time to allow a broader range of carbon content making it more brittle when the C was at the high end.