chromemoly welding

[Silverback]

Ok guys… good info (just printed the second article to peruse during some free time tomorrow at work maybe ).

I think that I can cover most of what I’m thinking now by replying to benny’s post:

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.

Grrr… that’s frustrating. I guess you need to understand that part of my issue here is about NHRA and IHRA rules and their impact on all this. In theory, I could build 2 of the same cage/chassis, one out of .083” wall CM (4130) and one out of .120 wall MS (probably 1018, 1026 would be nice but real world 1018 is a **** of a lot more common), and the CM one would be legal for cars running faster then 8.50 where the MS one wouldn’t. Now most people will tell you that the CM one is more rigid (it isn’t) and stronger (no matter how you shake it, the CM isn’t 40-44% stronger then the MS to make up for the difference in typical material thickness)

Here is a problem. 4130 normalized is in fact a good bit stronger than comparable 1026. I pulled some numbers from matweb.com here using 1 inch solid rounds for material strengths just for comparison purposes.

1026 cold rolled specs out at about 60,000 psi yield strength and 71,000 psi ultimate strength.
1026 hot rolled specs out at about 35,000 psi yield strength and 64,000 psi ultimate strength.
4130 normalized specs out at 63,000 yield strength and 97,000 psi ultimate strength.

Now, I'll be the first to say it's obvious that 1026 cold rolled and 4130 normalized look pretty equivalent so far because they yield around the same amount of stress. However, taking into account the ultimate strengths 4130 becomes a much tougher material.

I’m not sure that comparing tensile is valid here… Once you get to yield, once you’ve deformed the chassis it needs to be repaired in most cases to continue using it. Here you’re taking about a 5% difference in the strength of the structure _if_ you used the same wall thickness, but you don’t, again, you’re typically using 44% thinner material. Even if you’re comparing tensile/ultimate strength, the heavier MS tube will be much stronger before it fails, EVEN if the CM tube didn’t have the additional issue of hard/brittle points in the HAZ.

Also if you look at the 2 elongation percentages for each, 4130 specs out at 25% and 1026 cold rolled just 15%. This is proof that 4130 is much tougher than the comparable 1026 because toughness is the total area under the stress/strain curve.

Again, for identical pieces that’s true, but applying the same force to similar pieces of that .120” MS and .083 CM, which one will yield first? Which one will fail first?

Cold rolled 1026 is actually a much less ductile material than 4130 in the normalized condition. This is why I disagree with your assumption that 4130 is a bad decision because it "breaks" rather than bends and absorbs energy. This is actually not the case. In fact 4130 N will able to plastically deform and absorb more energy than 1026 cold rolled. It is not correct to look at 4130 as a material that will inherently fail drastically and break apart.

I’m not looking at the material failing, I’m looking at the welded structure cracking around the welded joints and spearing the driver. If we were talking about a structure that was made of the same weight material and the CM structure re-normalized after it is welded then I would agree entirely, but that is not what we’re discussing here.

Sure it's a little more difficult to cut and weld, but nailing down those procedures will net you a strength or (if you choose) a weight advantage.

Is there really a weight advantage? It’s rare to see class rules for race cars where it’s not possible to build a significantly lighter car then what is allowed as a minimum race weight anyway, even taking the weight penalty for using MS tube in your structure. In that case it would seem obvious to add the additional weight in the form of additional structure, but real world you see a lot of gym weights welded into structures, concrete filled box stock in between frame rails/bumper supports…. Maybe they don’t leave the chassis shop like that but that’s what is there when they show up at the track.

I do semi-agree with this. 4130 is not the answer in all cases. Nascar vehicles are mass produced and pretty overkill to begin with. In very many cases 1026 or other mild steels are more than adequate.

NASCAR has stated that their use of MS is one of the reasons for their exceptional safety record, and numerous fabricators (Formier comes to mind) and the likes of Carrol Smith have gone on record as saying that 4130 has no business being used in a safety cage structure or anywhere near a driver (mostly because of the condition you end up with when you weld normalized 4130). I still haven’t seen much justification otherwise and much less justification for the way that NHRA and IHRA rules are written WRT to 4130 vs MS cage structures.

I hope I've shed a little thought on the subject. It's pretty easy to work with and very strong in the normalized condition.

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?

[Billet Benny]

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 not the question I'm attempting to answer.

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.

If you think I'm holding 4130 up to be the end all be all I'm not. I feel strongly the other way. I think mild steel is better in most all applications especially for those projects done by green or hobby builders. However, its attributes and properties do give it much more possible advantages even in the normalized condition and used as fabbed.

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. Compare the 1026 cold rolled material to the 4130. Yes, yield strengths are similar, but look at what each can take before failure. What I'm saying is in order to make 1026 come close to 4130 N you have to use it in what can be considered 1026's hardest condition and therefore its least ductile. Now when you compare 4130 N to a material w/ similar ductility numbers like 1018 or 1026 hot rolled there is a large difference in yield strengths. This is truly how much stronger 4130 is than mild steel.

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.

I'm out of time, but that's what I got to say now.

[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,

[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]

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]

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.

[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.

[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

[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?

[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.

[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.

[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]

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]

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)

[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).