chromemoly welding

[Billet Benny]

First, seems like there is no agreement WRT to actual numbers for different grades of steel…. To me it seems that if it’s X, it should have properties Y, or at least in some range Y, but there doesn’t appear to be agreement there.

I agree this is a big issue. And the numbers for the materials I used aren't really perfect because they are solid rod specs. What needs to be used is the material strength as it's used in your particular tube whether it's hot or colled rolled, cold-worked, stress-relieved, etc. Spec sheets are the very best answer here. I also think herein lies a big answer to the question about the racing rules. I'll hit that at the end of this post.

To make things simple I just stuck with the numbers that you used and didn’t debate them but to give you an example how this affects this argument, if you use the properties given in the second link given in this thread: http://exp-aircraft.com/library/heintz/material.html you would draw conclusions significantly different then if you used the properties that you gave or that I gave from the Mark’s engineering handbook. In that link they give you numbers that result in a 52% difference between tensile and yeald, and a smaller, 50% difference for 4130N (and imply that 4140 is the same), and that mild steel has an elongagion at ultimate stress a full 50% greater then 4130N.

As if that doesn’t confuse the issue badly enough, tensile and yield strengths published there are MUCH lower then anything I’ve seen anywhere else, for example, they list yield for 1026 at 25kg/mm^2, or about 35KSI, almost half of what appears to be the accepted value anywhere else.

Is is such an "it depends" situation. Truth be told that 1026 can be in conditions that pretty much fit it anywhere in that range. There's no great standard here and I think that's the problem. But not the only one.

FWIW, my wife is a librarian at the engineering library at Johns Hopkins and she’s supposed to look into this for me tomorrow.

Very cool. I do need to drop the coin on a few ASTM books for myself. I've been wanting them bad, just haven't ponied up yet.

Second… for the sake of smoothing out the conversation, going back to using the numbers that you posted, what you state here is true for the material as 4130N, but I don’t believe that it is still true at all for a welded structure that may have the areas around the welds hardened to say a tensile strength of >150KSI but the heat of the weld. In that case I believe that although the structure would crack near the welds well before a similar MS structure would.

This can be the case. But I don't think you'll see 150ksi or even close near the weld. You'd have to quench the bejeezus out of it. This issue, however, is about reason #1 why I believe 4130 is a pro's tool. If it's build correctly (this includes, design, fit, and fab) both with ms and 4130 I cannot accept that the 4130 structure would "crack near the welds" BEFORE the ms structure fails. I have no physical tests to back this up only my experience. HOWEVER one of the biggest keys with either structure is to be wary of overly stressed joints... If you have a joint that even may see a load heavy enough to near its yield or especially tensile strength then properly gusseting the area is very important to spread this load. This is important with any material though because you never want to concentrate stress at a welded joint.

That’s a given since the added weight will be on the inside of the tube with a smaller moment, or I guess you could consider it having a smaller lever arm from the center of the shape so it does not impart the same strength to the shape… I’m not sure we need to get into that, but, not taking the time to do the math, I’d be surprised if the difference ends up being that high, since we’re talking about a .037” difference on the ID of a 1.75” tube, about a change of 2%, comparing the tube OD to the change…

I'm working on a simple spreadsheet in excel that will allow you to put in the tube size and strengths along with moments and loads to see bending stresses, max load before failure, etc. Could make it easy to compare different things quickly. However the yield and tensile strengths are still the big variable.

I think that we’re all good, but I was never worried about that. Again, nutshell, I’m wondering how sanctioning bodies could decide that a CM structure is safer under their rules than the same structure made of 40-50% heavier wall MS?

I think I have a decent reason to start the answer to this question. I don't think they think it's safer, I think in the grand scheme of things they must consider it equivalent under worst case scenarios.

I agree 1026 dom is going to be stronger than A519 4130N if the 4130's wall thickness is 40-50% smaller. However, I'm guessing in the grand scheme of their rules making, they make no distinction between types of mild steel. In other words they must compare 1008 dom to 4130. Herein lies probably very equavalent strength structures if the 4130 is 40% thinner wall.

So what I'm saying is they're broadly lumping 1008 and higher grade carbon steels into a ms group. When they do that you kind of have to take the worst case scenario of tube in order to make sure a ms structure theoretically equals a 4130 structure you have already deemed sufficient.

So, the thing is mild steel can be so many things from material composition to condition etc. 4130 can only be one composition and you're left with a small choices of conditions. So 4130 in essenence is much more uniform in its strengths while just saying "mild steel" is all over the place.

I bet if they spec'd 1026 in the rule book and not mild steel you wouldn't see mild steel structures needing to be such heavy wall according to the rules.

I guess I think the rule should be a material and size they feel is adequate and if you can provide spec sheets and data to show equivalency then you should be able to build your chassis out of whatever material and size fits the bill. I think the whole problem seems to come from expecting mild steel to be something like 1008 and comparing that to 4130. There is a huge difference in strength there.

[Billet Benny]

Here's what I whipped up. Feel free to ask me any questions and I'll be glad to help out. If anyone wants to double check the math, please, feel free. This will make comparing tubes quick and easy if you have accurate strength numbers.

Doh! Can't upload excel spreadsheets.. Holly!!! Can we enable uploading of .xls files?? .doc's are allowed.

*Removed to clarify some cells*