Well no, chromoly is going to be LESS dense.

Like I said I have no idea about grades of metal and what not so lets see how I do if you'll let me take a wild throw at this question.

Man, its gotta be a trick question. Both bars are an inch round, both weigh 10 pounds. Both have the same percentages of elements for alloying. So they are the same length? The only defference is the density?

I gonna feel really dumb arent I?.....

 

Thanks. That was my point exactly. 3 significant might not be enough.

I guess this is not intuitive to everyone.

FWIW:

Nominally, 1018 is about 99.205% Fe (iron) while 4140 is about 98.975% Fe.

With the exception of Mo (molybdenum) which is about 1/3 more dense than Fe, but is only 0.2% of the 4140 (Cr-Mo), all the other elements in both steels are less dense than Fe, so the steel with the highest % Fe is the most dense, and that's the 1018.

Shoot, that's the homework answer!


Another FWIW, but sorta on-topic:

Nextel Cup cars use DOM carbon steel for roll cages. Because the material can vary from 1015 to 1026 there is a strength variation with 1026 material being the strongest.

There was a team that went so far as to get a mill run (1000s of feet) of DOM tubing made to their specs: max carbon (1026), a controlled inside diameter and an oversize outside diameter. They then had all the bars of tubing centerless ground to hold the OD to about .002 tolerance both for size and roundness, and the wall thickness to about the minimum allowed. In the end they had NASCAR-legal DOM with the lightest lbs/ft and maximum strength. It wasn't cheap, but it was extremely nice to work with and every cage made from it was very consistant and very strong/stiff. Chassis stiffness, especially torsionally, is very important. This was quite a few years ago. I wouldn't be surprised if others do this now. Penske was the first, I believe.

Oh yeah, has anyone noticed that the Cup teams that are doing the best have become more "engineering-driven" than just "crew chief driven"? Not to take anything away from the crew chiefs, but better engineered cars win more races in almost every venue. One of the latest teams to adopt this approach is Petty, which went from having trouble qualifying in 2003, I believe, to getting competitive more recently. They are still playing catch-up, of course.

More than you wanted to know...again.

 

It's the different alloying elements and different percentages that make the density (and therefore the lengths) vary in this steel comparison.

Less volume for the same weight means more dense, right? Imagine comparing lead and aluminum. Small volume of lead for 10 lbs, but large volume of aluminum for 10 lbs.

The bottom line is that virtually all steel has about the same lbs/cubic inch, but depending on the alloy, the heat treatment, and the internal structure of the metal, strength can vary a LOT. Materials science is a complex subject. We're barely scratching the surface of it here. Even metallurgists have trouble understanding all of it.

'Bout time to move on?

 

VERY good posts OldSStroker and Injuneer, thanks for the info.

It is, just more than what most people can understand. I'm very interested in your knowledge of these things, as materials are a major part of my undergrad study, and i frequently do much research stemming from the tidbits you throw out here and there. I guess I should mail you a check for all the "tutoring" you've given me through the past couple of years , it's helped me a lot in my materials classes. Thanks again.

 

PAYPAL works for me! You'll have to ask Fred.

Materials science is a badly neglected subject, IMO. I can 't begin to tell you how many designs I see which are either overdesigned, underdesigned or made from "Unobtanium". It's not just a matter of not being able to produce the parts as designed, but to do it economically.

Knowledge is power. Power to the people!

 

Great thread! Thanks for the answer, that's exactly what I needed to know. It's been so long since I've taken any engineering classes, and actually exercised my "engineering brain" I forgot most everything.

 

A lot of body parts atrophy or become less functional if we don't continuously exercise/use them. Ask me how I know. I'm 62...

Memory is the second thing to go...or is it the first?

Forums like this one help keep us young...or at least functional.

 

"OldSStroker", last login 10:30 PM EST, www.masterbationtips.org

(paybacks are fun! Exhaust thread )

How would sonic testing verify roll cage integrity for IHRA/NHRA?

I recall something about acoustics measuring cage thickness?

 

This should clarify the "how" part I'm looking at one of these little beasties now for porting purposes..

As to the actual rules that are followed, someone else will have to chime in. I do know that they look for specific things beyond just the thickness of the tubing such as welding process used, types of welds used in various places, bracing and tubing placement (downbar height as an example).. Once your have a cert to a certain ET though, your good to go for at least a couple years.

One thing I've never understood though... I see cars at the track all the time with a cert sticker placed on the bars AFTER powder coating. Ok, since the tech places the sticker on at the time of testing and the sticker will basically self destruct if you try to peel it off, then how do you get a clean PC job? Masking off the sticker would seem kinda "hokey" and would be messy.. Somebody enlighten me please

Dave C.

 

Cool link. I printed and laminated the picture from that article showing spark pattern. That'll go somewhere on the workbench.


Any idea on what the damage is for one of those? It's never good when it says call for pricing.

 

No I don't know how much, though no price listed is NEVER good. I'm expecting between 1000 and 2500. I can't realistically plop down more than that on a hobby item.

Dave C.

 

If you have some scraps of it, the easiest way to tell if it’s mild steel (1026 or less) or an alloy like 4130 is to heat he scrap till it’s red hot and quench it. The mild steel will not harden significantly and you should still be able to cut it with a file, 4130 will surface harden so you won’t be able to. In thicknesses greater then 1/8-3/16” it’s doubtful if you’ll manage to get 4130 to harden all the way through, but higher grades like 4140 or 300m will.

This will be easier for a lot of people then something somewhat subjective like a spark test.

Which bothers me in a lot of cases, since I don’t think that anybody could give a good reason to allow this unless the parts are heat treated after they are completed and no one seems to heat treat most of the parts available for our cars or similar applications.

1025 or 1026 (I forgot which one right now it’s late and I desperately need sleep…) only exists because of an effort to find a superior material for tubular structures/roll cages, but even in lower grades most pros are pretty adamant that mild steel is better for structures like roll cages then 4130, even though, outside of stock car racing, 4130 is considered better or even mandated.

The problem is that normalized 4130 (most of the tube stock you could buy) is roughly similar in strength to mild steel (slightly stronger then 1018, similar or slightly weaker then 102_). The way to get it stronger is to heat treat it. Well, when you weld it (not that 99% of people use chrome molly filler or anything), the 4130 will self quench and harden around the weld, resulting in a structure that is mostly the same strength as mild steel but with hard, brittle joints that are prone to cracking leaving sharp angles that make perfect spears aimed at the driver.

I would love to know why in drag racing, sanctioning bodies allow thinner wall or smaller tubing if you use 4130, until you finally go fast enough that only 4130 is allowed. I’ve also seen road racing rules written giving preferential treatment to 4130, which again, as far as I can tell makes no sense (I’m pretty sure that I’ve asked about this before on this board with no real answers). I would understand if people were somehow expected to heat treat the entire cassis…

 

Mark,

You raise some interesting points. I'd like to quote Mark Ortiz, who gives credit to Caroll Smith (RIP), two guys I think a lot of:

"The Mark Ortiz Automotive
CHASSIS NEWSLETTER
PRESENTED FREE OF CHARGE
AS A SERVICE TO THE
MOTORSPORTS COMMUNITY

May 2001


WELCOME

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering answers to chassis questions. Readers may submit questions by mail to: 155 Wankel Dr., Kannapolis, NC 28083; by phone at 704-933-8876; by e-mail to: markortiz@vnet.net. Topics are also drawn from my posts on the tech forums at www.racecartech.com and www.rpmnet.com. Readers are invited to check out these sites, and to subscribe to this newsletter by e-mail.
Mark Ortiz


CHROME MOLY IN STOCK CARS

Recent discussions of frontal impact deaths in stock cars have produced allegations that some of the more advanced teams in Winston Cup, which build their own chassis, are now using chrome-moly (4130) steel in their frames, supposedly including even the large rectangular main rails. I have no way of verifying these reports, but I have noticed that various other classes in racing are allowing thinner-walled tubing in roll cages when 4130 is used. I am told the material is making major inroads in dirt Late Model construction as a result of this, as it allows considerably lighter cars.

Most stock car classes, including Winston Cup, neither require 4130 nor give it a wall thickness break. Therefore, there is no weight saving to be had from it. The reported reason for using it in Cup cars is to gain stiffness.

These developments prompt me to offer some information about what 4130 will and won’t do for a car, and some of the less-recognized properties of the material. All this information has been published before. I would particularly like to tip my hat to Carroll Smith, whose book Engineer to Win offers considerable insight into this and other materials-related matters.

STIFFNESS

First thing we need to emphasize, and this will come as a shock to many racers, is that 4130 offers no significant stiffness advantage over mild steel! The gain is 1% or less. That’s assuming equal wall thickness and weight, and identical design. If the frame is built with lighter gauge tubing to save weight, it will actually flex MORE.

Note that stiffness means resistance to deflection under load, short of the point of permanent deformation. Stiffnesses of all steels are remarkably similar. The best spring steels are less than 3% stiffer than mild steel, and 4130 as normally used in frames is actually closer to mild steel than to spring steel. Resistance to permanent deformation is called strength, and that’s where chrome-moly offers potential gains. In normal use, a car frame is a stiffness-limited structure, and therefore does

not gain performance significantly from the use of 4130, unless the rules allow us a frame or cage weight reduction. This applies until we crash the car. At that point, we suddenly have a strength-limited structure.

STRENGTH

Steels may be very similar in stiffness, but they differ dramatically in strength. Strength can be expressed in terms of ultimate tensile strength, yield strength, and impact strength. Ultimate tensile strength refers to tension load required to pull the material completely apart. Yield strength refers to tension load required to permanently deform the material. Both ultimate tensile strength and yield strength are measured by applying a gradually increasing load to the material. Impact strength, on the other hand, is measured by striking a sudden blow, of known magnitude. If the material deforms, but doesn’t fracture, it passes. If it actually breaks, it fails.

The distinctions between these different kinds of strength are of great importance in race car construction. For portions of the structure that are close to the driver, we mainly want to prevent the wall, track surface, or whatever the car hits, from intruding into the cockpit. This means we want yield strength. A little deformation may cushion an impact, but we don’t have room for large deformations. For portions further from the driver, we want a structure that deforms in a controlled manner, preferably outer portions first, absorbing energy as it crumples. To achieve this, we need graduated yield strength – more as the deformation approaches the driver. In both cases, we need impact strength; the structure can only do its job if it doesn’t come apart on impact.

Mild steel does not respond to heat treatment; its carbon content is too low. Consequently, its properties remain about the same no matter how we weld it, heat it, or cool it. It never becomes brittle, as long as we don’t introduce impurities while welding. Its yield strength is moderate, but its impact strength is good. In a crash, it deforms and absorbs energy, while resisting being torn apart or fracturing. This makes mild steel a good choice for most portions of a stock car frame. There is no significant stiffness penalty compared to 4130, and greater deformability when the car takes a hit.

Therefore, there may be a case for using 4130 for the cage in the driver’s compartment of a stock car, but not for the front or rear clips.

4130, on the other hand, will harden when heated above its critical temperature and cooled rapidly. Cooled slowly, it remains soft. In the hardened condition, 4130 has great tensile and yield strength, but POOR IMPACT STRENGTH. It’s hard and strong, but brittle. 4130 tubing, as supplied, is not in the hardened condition. It’s cold drawn and normalized – fairly soft. In this condition, it will usually bend on impact and not fracture, just like mild steel, and it is somewhat stronger. But when we TIG weld 4130, we get a hard zone, not at the weld but half an inch or a little more from the weld. This happens because this region is heated enough to produce hardening, and is close enough to cool metal to be cooled abruptly after welding. The result of this is the failure pattern commonly seen in crashed 4130 frames: the tubes bend, the frame diamonds and twists, but there are some fractures near the welds. Not at the welds, but an inch or less from them.
Most people who build 4130 frames simply live with this. But there is a solution. After welding, heat the joint and the nearby metal to a dull cherry red with an oxy-acetylene torch, and allow to air-cool slowly. In warm ambient temperatures, just letting the area cool naturally is often sufficient. In colder ambient temperatures, or for insurance, a sheet metal or foil shield loosely fitted around the joint will slow the cooling sufficiently to avoid hardening.

Another approach, seldom considered nowadays, is to gas weld the joint instead. 4130 tubing was originally invented about 75 years ago for the aircraft industry. TIG welding was unknown. Aircraft structures were gas welded. Gas welding heats the metal surrounding the weld more than TIG welding does. This compounds distortion problems, but it does alleviate the problem of having very hot metal near cool metal that can quench it. In many cases, this will automatically eliminate the brittle zone."

 

My thoughts are that heating and quenching 1026 and 4130 will give about the same hardness. It's the carbon content that determines the hardness, and .26% and .30% Carbon steel quench out at just about the same. Edit: 1026 is at the very high end of carbon or "mild" steel. Much mild steel sheet is 1008 or 1010 or even 1015-1018. Remember the last two digits are the decimal % carbon. Until lyou approach 30 (points) of carbon, no effective hardness occurs from heating quenching.

OTOH, the Cr and Mo in 4130 help achieve deeper hardness in both 4130 and 4140. 4140 will get harder than 4130 during quenching due to it's extra carbon. You won't get "file hard" Rc60+ until you get to about .60% Carbon, but 4140 can get to Rc50+ before tempering.