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chromo vs. mild

Greg

Well-Known Member
#1
Pat's asking for chromoly got me thinking; lets get everyone's opinion on why, or why NOT, chromo is better than mild steel for fabbing cages and suspension parts. And what about stress relieving and heat treating? Do people buy there tubing normalized or annealed? Does any one worry about their weld/ grind zones becoming austenitic and cracking? I know chormo is stronger in bending than mild steel but then wouldn't it be better to use square tubing in stead of round to build something that would see a bending force; i.e. trailing arm? And in tension (space frame) chromo is only slightly stronger than mild with the risk the weld zones being brittle unless heat treated and stress relived. Is my info correct or have I been missinformed?

Greg
 

Jimmy8

Well-Known Member
#2
Heres the deal. Chromoly is a very brittle metal, while mild steel is not brittle at all. The thing about chromoly is that it takes so much more to break it than steel so it takes a whole lot of punishment before it actually breaks, but when it does it really breaks. Steel tends to bend instead if breaking and is a whole lot softer than chromoly is. As far as strength goes chromoly hands down, because it takes so much for it to finally reach its breaking point, and steel bends much before chromoly breaks. i dont know if this is anything like what you are looking for so let me know.
 

TimHayosh

Well-Known Member
#3
This might be completely redundant to what Jimmy just said but...As an example of the brittle vs. not-brittle argument, my car has aluminum tie rods. My car was recently in an "unplanned contact" with another car. Very hard impact was suffered by the front end. The R/H tie rod was bent, but didn't break. In fact, it yielded and saved the whole rest of the system from complete failure. I believe if I had ChroMo tie rods, something would have snapped and ended my race.

I would think the same priciple applies to cage construction. I would rather my cage deformed around me (absorbing energy) in a nasty accident than for it to snap and leave shattered tubes available for me to be impaled upon.
 

Stan

Well-Known Member
#5
Well then, I'll forgo the part of mentioning heat treating due to nightmares from the Prerunner List discussion on this subject from a few years ago.

"Rehabilitation begins at autopsy."
 

Greg

Well-Known Member
#6
Stan, since I am not privy to the Prerunner List can you please shed some info on what was said. It seems there are plenty of opinions on this subject but what about facts. I've heard plent of people say "it was mild then it broke but we ain't never broke this here chromo peice" even though the chromo piece was fitted better and welded nicer cause the guy just spent 5x the price for the alloy. If most mild steel parts were made with the same care would they be stronger or not? Is it worth the extra money?

Greg
 

Stan

Well-Known Member
#7
<blockquote><font size=1>In reply to:</font><hr>

Stan, since I am not privy to the Prerunner List can you please shed some info on what was said. It seems there are plenty of opinions on this subject but what about facts. I've heard plent of people say "it was mild then it broke but we ain't never broke this here chromo peice" even though the chromo piece was fitted better and welded nicer cause the guy just spent 5x the price for the alloy. If most mild steel parts were made with the same care would they be stronger or not? Is it worth the extra money?

<hr></blockquote>

Someone mentioned that chromemoly has to be heat treated as a whole chassis and it just broke down into the largest flame war on a list I've ever seen. Of course it seemed like the guys whole have the most experience with the stuff was the ones who knew what they were talking about and who I learned the most from (I'm not a fabricator, racer or otherwise, just wanting knowledge to do my own stuff) but they were getting flamed by a few people being the southend of a northbound horse. The guys who know their stuff stated that heat treating a whole chrome moly chassis is inpracticalble and too expensive. If I remember right, it was the same sort of people who were flaming these people were the same ones who made cracks about a certain fabricator's welding after a crash at Laughlin. What I learned was if one had the money and/or talent, chromemoly is the way to go, but mild steel will be a sufficient substitute. If any of the participants in this flame war that read this (I know of a couple who are on here, but I'm not naming names
http://www.race-dezert.com/cgi-bin/wwwthreads/images/icons/smile.gif</A>">) and see if I got any facts screwed up, feel free to correct me.


"Rehabilitation begins at autopsy."
 

Donahoe

Well-Known Member
#8
Good memory Stan... Gregg for your stuff stick with dom...It will last longer and is lots cheeper. on suspension parts go with 4130 but chassis wise use mild... You jump your truck way too much to use 4130.

NEVER LIFT!!!!!
 

Dylan

Well-Known Member
#9
One misconception that a lot of people have is that different alloys of steel have different stiffness. This is not true!!! All steels have almost the same stiffness (modulus of elasticity) within 5%, (stainless is about 10% less). If you want a different modulus of elasticity then you need to use a different material ie: aluminum, titanium ect. The modulus of elasticity determines how far the material will flex under a given load (ie: rubber low modulus, glass high modulus). The modulus for steel is 30,000,000 PSI

Different alloys of steel in different conditions of heat tread and work hardening do however have different strengths. There are to values for strength that we need to look at, Ultimate strength and yield strength. The yield strength tells how much stress the material can handle before it permanently deforms. This is the spring back or memory that the material has. Ultimate strength is the stress level that the material can take before it finally fails, at this point however the part has already bent and is ruined. The other material property that we should look at is Elongation, it is described as a percent and is the amount of permanent deformation that the material will have before failure (ie: silly putty very high elongation, Glass almost zero elongation). More elongation will allow the part to get a pretzeled up and not fracture. On our tubing bender we have had some problems in the past with braking the martial during the bending process, the solution was finding a metal distributor that had the same alloy of material but with a higher elongation value.

The strength of steels is largely dependent on the heat treat. The difference between the alloys is how heat treatable they are. Lower alloys can’t attain as high a level of heat treat.

As far as cromo vs. mild steel here are some specs to look at.

1020 DOM Yield 70 Ksi Ultimate 80 Ksi Elongation 15%
4130 NORM Yield 70 Ksi Ultimate 90 Ksi Elongation 20%
4130 [email protected] Yield 173 Ksi Ultimate 186 Ksi Elongation 13%
ERW tube Yield 25 Ksi Ultimate 45 Ksi Elongation 35%

As you can see 4130 is only slightly better than DOM, also note that it has a higher elongation value. This means that everyone’s conception that 4130 will splinter on impact is false.

As far a weld zone effects there are a lot of conflicting theories regarding this, filler rod type, normalizing stress reliving, ect. One welding practice that is recommended for 4130 and rarely done is preheating. 1020 will be more forgiving to your booger welds.

I would put more energy in to the design rather than the material, avoid single sheer mounting and structures in bending. Don’t use ERW tube, use 4130 for bragging rights or smaller part that you can send to heat treat, other wise use 1020 DOM.
 

FABRICATOR

Well-Known Member
#10
Hello Greg,
This is really a short answer to a bunch of huge questions.

It’s not really a matter of which one is better. It’s which one is right for the particular application.
CrMo has at about 25% higher tensile strength than 1020 cold rolled steel. Generally speaking,
anything that can be successfully made out of mild steel can be made thinner and lighter with
CrMo because of it’s increased strength. Almost all CrMo tubing for fabricating is in the
normalized state. Sheet and plate can be found either normalized or annealed. If you plan on
bending it very sharp or shearing it, it should be in the annealed state for most shears (and to the
relief of the shear owner). Parts that are made from annealed 4130 should be heat treated if they
are going to be under much stress. Otherwise they will flex and may work harden.

CrMo is more brittle than mild steel but certainly not “super brittle.” One reason people think it is
extra brittle is because it work hardens, as do a many alloyed metals. In a poor application, with
flexing at the joint, the metal can crack. Since there is flexing at the joint, the metal ahead of the
crack work hardens and can cause jagged shapes like a piece of broken glass. In a proper joint,
the metal is still fine and the structure often will fatigue somewhere else. Properly designed, fitted,
and welded pieces do not always require stress relieving. If you introduce stresses into the tubing
joints, such as using a pipe clamp or cum-a-long to close a joint or make it fit up better you will
need to stress relive it and all the surrounding joints. (You also should start over again and do it
right) Stresses can also build up if the base of your chassis is tightly clamped to a highly rigid
chassis jig. Quality tube fitting is critical to get the most strength and least warpage during
welding. If you find yourself filling any gaps or holes, the joint will have built up stresses and
should be relieved. I know of some fairly complex parts made from 4130 tubing that go into
production aircraft, that do not receive any stress relieving. Fit quality and welding of these parts
is nothing short of outstanding.

Any part that is subject to high stress such as a front trailing arm or spindle must be stress relieved
and should also be heat treated. Mild steel is great for simple roll cages and offers at least as
much, if not more protection to the occupants as one made of CrMo. The roll cages found on
TT’s and larger Class 1 cars often have anywhere from 12 to 17 upright tubes and are a MAJOR
part of the chassis strength. This type of cage is super strong no matter what is used, and is better
off being CrMo so a lot of weight can be saved.

As far as welding CrMo, some welders preheat the weld zone. This is done to prevent “thermal
shock” to the metal and reduce warpage. This is usually done with an oxy-acetylene torch (about
6 inches away) to about 500 degrees. This can be done easily using a temperature indicating
marker (tempilstick or painted on tempilaq, or others). Stress relieving includes heating the joint
to about 1250 degrees (again using temperature indicators or a dull red color in a semi-darkened
room) and letting it slowly air cool by itself. Never, ever quench with water! The best rod to use
is that which matches the metal being welded. Many use stainless rod because it is easier and
looks good. It will not make a joint as strong as 4130 rod can. Neither will MIG welding. This is
not to say that they can’t be used. These are just some facts. There are companies that make 4130
wire for MIG welding. An improvement, but still not as good as TIG.

There are never situations where mild steel is stronger than CrMo, but there are situations where
it holds up better to ultimate destructive forces by not breaking, but instead just bending and
twisting. It is sort of like a bolt. Ever try breaking a grade 3 or 5 bolt? It usually can’t be done
without bending it back and forth. How many places on your car would you use a grade 3 bolt? Or try bending a piece of normalized CrMo about 1" wide by 6" long and 1/8" thick with your hands (if you can). Then try it with a piece of cold rolled same size. Dramatic difference. CrMo has the characteristic of holding it all together stronger, with more rigidly and doing the same job with less weight than mild steel.

As far as square tubing, it works great in certain places. Most sanctioning bodies don’t have the
time to stare at each car and figure out which tube is OK to be square and which is not, so they
don’t allow any in the critical structure. It is never allowed in roll cages because of it’s rigid
nature. It is either in fine shape or kinked and useless. This rigidity also causes it to be torn off at
or next to welded joints when over stressed. It can only be used in a very rigid structure. Square
or rectangular shapes are used more than we give credit for. Nearly all trailing arms are made up
of square or rectangular sections. Even rear ones made of only round tubes rarely make up a
round shape. The 3 or 4 tubes that make up a racing, VW style rear trailing arm really make up a
triangle or rectangle cross section. The fact that they are made with only tubing or only boxed
sheet metal is of little consequence. The same is true of most front lower control arms. They are
either boxed-in round tubes or a full box. Either way, they make up a rectangular cross section.

Is it worth the extra money and work? As with most items mentioned here, it all depends on the
application.
 

PATCO

Well-Known Member
#11
Greg,

Good question. There are about a million different opinions on this subject. I'd also like to hear what people think.

PATCO
 

PATCO

Well-Known Member
#12
I have a question related to this subject. What exactly makes 1020 D.O.M. stronger than 1020 E.R.W. tubing ? Shapiro supply (www.shapirosupply.com</A>) describes D.O.M. as this " This grade of mechanical tubing is the fifth of six types classified in ASTM A513. Produced from steel strip by cold forming, electric resistance welding (E.R.W.) and cold drawing to finished dimensions, D.O.M. is the most versatile and widely sold mechanical tubing grade..... The E.R.W. process guarantees the weld to be as strong or stronger than the rest of the tube body. The origin from flat strip results in a more concentric product than cold drawn seamless." Sounds like it starts out as E.R.W. and then is drawn over a mandrel to remove the seam. D.O.M. looks like it has scale. Is this from a heat process ? Can someone please clarify this. I've also been told that E.R.W. splits at the seam, but I've never seen this, nor do I know anyone who has.

PATCO
 

FABRICATOR

Well-Known Member
#13
PATCO
Seamless DOM is made from solid bar. There may be some DOM's that do not start out as a solid bar. The seam in welded tube (ERW) may very well be as strong or stronger than the adjacent material. But that in itself should raise concern. The last thing we need, in a highly stressed application, is tubing with one or more sections stronger or harder than other sections. At best, this is a huge stress riser.

Shock mounts on a disco truck, maybe. Anywhere on a serious race car, never.

Again this goes back to application.
 

rdc

- users no longer part of the rdc family -
#14
DOM is not made from solid bar, CDS (cold drawn seamless) is made from solid bar. DOM is formed from strip and electric-resistance welded, then cold drawn to size. The cold drawing process causes the weld line to virtually disappear and gives the tubing it strength by cold working and allows consistent dimensions and tolerance. The carbon content also gives DOM(1020 or 1026) better mechanical properties than the ERW tubing that is usually 1010.

In my opinion, you are wasting you time with 4130 fabrications by not heat treating (you end up with an expensive part with the same strength of 1020 and brittle weld areas). Just by looking at the material properties of 1020 DOM and 4130 CDSN, the two are very comperable. 4130 has only slightly better mechanical properties and by the time you weld it and not stress relieved it, you have lost any strength advantage the 4130 had. The only advantage for 4130 is its ability to be heat treated.

-4130 fabrications are NOT! any stiffer or more rigid that one made from mild steel.

-4130 is in fact more ductile than 1020 DOM, by the definition of elongation: the difference between the original length and length measured after the part has ruptured (expressed as a %). As Dylan mentioned prevoiusly 4130 CDSN is 20%, 1020 DOM is 15% and 1026 DOM is 10%. This means that 4130 CDSN will bend further than 1020 DOM before it breaks.

-4130 is not any lighter than other carbon steels, your parts can be made lighter by using 4130 in thinner sections because 4130 is stronger---but only if it is heat treated.

Just my two cents worth.
 
#16
The guy that used to prep big OLE built a class 4 truck in the winter of 89 out of mild and the damn thing was so heavy it never worked. It was down right laughed at.I remember him preaching that Mild was WAY better then Chromo and so on and so on...Thru the years ive seen alot of unsuccesful mild builds.Maybe guys do build top rides out of mild, i would like to know which ones...
 

Dave_G

Well-Known Member
#17
Ya know, this thread's got me thinking. Next time I build a homebuilt airplane I'll save me a few bucks and ignore the plans and build it with 1020 DOM and mild steel plate instead of that expensive 4130 stuff. Anyone want to go for a ride when it's done? The bright side is if it fails and crashes it'll just "fold" up instead of being "brittle" and impale us with a bunch of broken tubes and bell cranks.

LOL!

This thread cracks me up...
 

ErikShallbetter

Well-Known Member
#18
Dave_G said:
Ya know, this thread's got me thinking. Next time I build a homebuilt airplane I'll save me a few bucks and ignore the plans and build it with 1020 DOM and mild steel plate instead of that expensive 4130 stuff. Anyone want to go for a ride when it's done? The bright side is if it fails and crashes it'll just "fold" up instead of being "brittle" and impale us with a bunch of broken tubes and bell cranks.

LOL!

This thread cracks me up...
Mild steel homebuilt huh. So it's going to have safe glide? I think thats only funny if you are a pilot though:rolleyes: . We've got a sonex in the works.
 

ChuckH

Well-Known Member
#19
Dave_G said:
Ya know, this thread's got me thinking. Next time I build a homebuilt airplane I'll save me a few bucks and ignore the plans and build it with 1020 DOM and mild steel plate instead of that expensive 4130 stuff. Anyone want to go for a ride when it's done? The bright side is if it fails and crashes it'll just "fold" up instead of being "brittle" and impale us with a bunch of broken tubes and bell cranks.

LOL!

This thread cracks me up...
Ha, i will ride in your DOM plane .... with my softie parachute strapped on :D
 

CRAIG_HALL

Well-Known Member
#20
Certain threads shouldn't be brought back up....
 
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