chromemoly welding

[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*

[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.

[Silverback]

The basic answer is no for all practical purposes and for your intent in school. You need the same fitup skills and same welding skills.

Heh, yea, the original question… forgot about that, sorry…

Nutshell, as Sberry says, unless you’re constrained to rules (for example NHRA/IHRA and I think that some SCAA rules require TIG welds on 4130) for the most part there is no difference welding CM from MS. The biggest issues are if you put too much heat into it or cool it too fast it will be more prone to cracking then MS, and if your CM gets over about 3/16” thick then you really need to preheat it to about 3-400*F to prevent it from self quenching too fast and cracking.

Billet Benny wrote:
"That's right. It'll never be more rigid. In fact, it will likely be less rigid if you use lighter wall tube to save weight which is one main reason for using 4130."

Don't they typically combine thinner wall with a larger diameter on the tubing? The weight is about the same but the stiffness (2nd moment of inertia) goes up...

Not only does required OD for the thinner section CM not go up, but in some cases it actually goes down (as per the rules). Second issue ends up being that at some point larger diameter/thinner wall, although it should be stiffer, the wall gets thin enough that it becomes prone to oil canning/deforming rather then flexing slightly (think aluminum soda can vs thin wall aluminum tube used in lawn furnature).

[Silverback]

I think one reason to restrict the use of mig on moly is to try to bring the skill level of the fabricators up. A guy goes to the trouble of buying a TIG, the cost and the time to learn to use it is different than letting Jethro loose on roll cages with the Wal-Mart feeder and a 100 ft of 16 ga extension cord in the back yard. When we were kids we welded dirt bikes all the time with mild steel electrodes, they held up even with limited skill level sometimes.

Maybe… but that kind of reasoning just bugs me. To be really blunt, at this point (hopefully in the near future with some more practice this won’t be the case), I’m 100% certain of my ability with a MIG in this sort of application, I’m not so certain with a TIG, although I’m also 100% certain that I could use either to build a structure that would pass tech, even though with an honest assessment of my abilities right now I’d trust the MIGed version more.

Silverback,

You raise some excellent points and have explained them well.

I'll let you and Benny figure out what points to debate between you but

Wow, I hope that this didn’t come across as me just debating with Benny, because of some disagreement… it’s more of me hoping that someone could explain to me or give me a different perspective on something that has bothered me in the motorsports fabrication world, and Benny is doing a very good job of enumerating the qualities of 4130 in this kind of structure giving me a perspective to reexamine what the issue is.

Regarding the weight issue from a performance standpoint, a few pounds of ballast between the frame rails is preferable to the same weight up in the cage structure. I do prefer the mild steel for all the reasons you mention, but lots of builders will intentionally build a car that will require ballast.

Very true, and something I purposefully avoided to avoid bringing another variable that I don’t believe will actually change the final answer. But yes, if you can build a lighter structure and then being able to decide where you put the ballast to get it up to legal weight is a definite advantage (whether we’re talking low and back on most drag chassis or low and towards the center to lower moment in a road race chasses, again a broad generalization)

[M1.30-'06]

I have a picture somewhere of a guy carrying the front half of his 4130 cage out of the wreckage of his drag car after the ride was over...

Ill post it up if i can find it since were on the subject

[Silverback]

That's not the question I'm attempting to answer.

But that is the question that I asked

Well, you took everything I said completely out of context. You are on this comparing .083 wall 4130 to .120 wall 1026 kick. Technically that's probably not a great comparison and maybe I don't even agree with the NHRA rules. I will not be the one to say that this is a good idea. This is not what I was comparing. I was not trying to use all this information in the context of the racing rules you do not like. What I've said in my post is correct, how it is applied by racing bodies may not be. This is why I keep saying 4130 is an experienced builders tool and I do think it can be dangerous in the hands of lots of fabricators. I agree with you in lots of respects.

Benny, I think that mostly we’re on the same page, the thing is that like I clearly said at the beginning of my response, I’m interested within the context of the NHRA/IHRA rules. I don’t believe that you think that CM is the be all and end all or that you’re defending something that you’re not, I’m trying to get correct info in this thread and then apply it within the perspective of motorsports rules/regs (after all, this is the motorsports forum, right?)

The thing is that the NHRA/IHRA rules have always thrown me for a loop as being at best misguided and at worst, stupid/dangerous and I’d love to make some logical sense of them.

You also missed another point. One reason you said you like ms over 4130 is the "giving" thing in an accident. This why I brought ultimate tensile strength into the equation. The gap between yield and tensile strength in a round-about way shows how tough and how much energy a particular steel can absorb.

Now that is an interesting/useful point. But it also brings up 2 problems that I have with this whole debate that I was hoping to clear up:
-
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.

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.

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

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.

Finally, strength % gains is not just a function of material amount when it comes to tube. If same od tubes of mild steel and 4130 are compared their percentage differences in strength are not linearly related to the percent differences in weight. IE if the 4130 is 30% percent lighter you can't just take 30% from its tensile strength and then compare that to the mild steel. To compare strengths it's a little more complex than that, but we can go over it.

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…

Silverback,
You do raise some fine and valid points in my mind. However, I answered this question.
….
That's a difficult question to answer and I've never said the chromoly structure in this case would be safer. The particular cause would require an in depth look. But, what I am saying is it's a factor of taking the facts, rules, and skills to figure what's best in each application
…
All good?

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?

[M1.30-'06]

Don't they typically combine thinner wall with a larger diameter on the tubing? The weight is about the same but the stiffness (2nd moment of inertia) goes up...

Regards,

For the straight line drag race cages we build , the NHRA spec. sheet lists the approved layouts and tubing material , OD , and thickness... from what ive seen the O.D. is always the same between mild steel and 4130.. wall thickness is close to 50% thicker on the mild though. Some gussets can either be folded plate or tubing.

[Sberry]

im want to know if there is a special technique to weld moly?im in school to learn how to weld,my instructors did not know if there was a special way.i would like to know for the reason the reason im going to school is to build race cars.thanks

The basic answer is no for all practical purposes and for your intent in school. You need the same fitup skills and same welding skills. I should have taken a couple pics of the joints. Here is the drop and it was the second one I cut, the first was almost perfect as I was very careful and I got sloppy with this one but it is an example.

[Billet Benny]

Silverback,
You do raise some fine and valid points in my mind. However, I answered this question.

Am I missing some quality of 4130 that makes it better for some applications?

And I think I answered it well and in a general sense where you can use the math and the information I outlined to decide when and how you wish to extract the advantages of 4130.

Now you came back with this.

Yes, but I’m not sure that you’ve answered what my issue is here. In the example at the beginning, the same structure built out of heavier wall MS vs much lighter wall CM, what makes the CM structure safer?

That's a difficult question to answer and I've never said the chromoly structure in this case would be safer. The particular cause would require an in depth look. But, what I am saying is it's a factor of taking the facts, rules, and skills to figure what's best in each application.

I've only attempted to explain the general possible advantages of 4130 over mild steel when chosen and implemented correctly. No disagreements with your specific racing applications, just your broad assumptions.

All good?

[JD in Socal]

Silverback,

You raise some excellent points and have explained them well.

I'll let you and Benny figure out what points to debate between you but, from a motorsports perspective, you make some very good points. The comparison between .120 mild steel and thinner chromoly is one that should receive more attention, because it affects the choices race car builders make almost every time a car or cage is built.

Regarding the weight issue from a performance standpoint, a few pounds of ballast between the frame rails is preferable to the same weight up in the cage structure. I do prefer the mild steel for all the reasons you mention, but lots of builders will intentionally build a car that will require ballast.

JD

[Sberry]

I think one reason to restrict the use of mig on moly is to try to bring the skill level of the fabricators up. A guy goes to the trouble of buying a TIG, the cost and the time to learn to use it is different than letting Jethro loose on roll cages with the Wal-Mart feeder and a 100 ft of 16 ga extension cord in the back yard. When we were kids we welded dirt bikes all the time with mild steel electrodes, they held up even with limited skill level sometimes.

[Billet Benny]

2nd moment of inertia, 2nd moment of area, area moment of inertia, etc... all denoted as "I" in the equations...

I've never seen section moment of inertia, but it is apparently also common, at least in a google search...

Regards,

Gotcha. I checked it out and we're on the same page. Technically it looks like 2nd moment of inertia is the most correct terminology for talking of the moment of inertia of an object in respect to resistance to bending.

[BillC]

What are you referring to as "2nd moment of inertia"?
Do you mean section moment of inertia?

2nd moment of inertia, 2nd moment of area, area moment of inertia, etc... all denoted as "I" in the equations...

I've never seen section moment of inertia, but it is apparently also common, at least in a google search...

Regards,

[Billet Benny]

Billet Benny wrote:
"That's right. It'll never be more rigid. In fact, it will likely be less rigid if you use lighter wall tube to save weight which is one main reason for using 4130."

Don't they typically combine thinner wall with a larger diameter on the tubing? The weight is about the same but the stiffness (2nd moment of inertia) goes up...

Regards,

Sure, this is true in cases. However, for comparison purposes I'm speaking of two exact structures w/ only material and wall thickness as variables to help explain a concept.

What are you referring to as "2nd moment of inertia"?
Do you mean section moment of inertia?

That is one of the important parameters to know when figuring a tube's stiffness.

Section modulus comes into play when we want to start comparing yield strengths like I mentioned in the end of my post above.

[BillC]

Billet Benny wrote:
"That's right. It'll never be more rigid. In fact, it will likely be less rigid if you use lighter wall tube to save weight which is one main reason for using 4130."

Don't they typically combine thinner wall with a larger diameter on the tubing? The weight is about the same but the stiffness (2nd moment of inertia) goes up...

Regards,