Some chromoly questions

(Please note: I've crossposted this to both rec.crafts.metalworking and
sci.engr.joining.welding since it's valid both places, and possibly of
equal interest to readers of both, so set your follow-ups accordingly if
you're concerned about it!)
So as my welding classes are coming to a close, I happened across some
factoids in my welding text that have me wondering about the wonders of
chromoly steel. The main one, though, is this whole thing about post-weld
stress relieving using a torch. Just about every book I've read on
fabrication suggests the same post-weld process, except for one; that book
is "Performance Welding" by Richard Finch. Now previous discussions on
the subject of Mr. Finch have led me to believe that he's not always
playing with a full deck of cards, but my welding textbook for class said
something that actually matches his opinion on the subject.
Finch writes that post-weld stress relieving of a weld in chromoly steel
using a torch is completely worthless because proper stress relieving
requires a six hour long process that simply can't be achieved with a
torch. My textbook makes a vaguely similar assertion in that it says that
proper stress relief of a welded joint can take anywhere from one hour to
six hours for the heating segment of the process, with the point of
diminishing returns on the increase in strength starting at around the six
hour mark. However, its implication is that SOME amount of stress
relieving will still have a benefit, but that the percentage of
effectiveness is based on the total amount of time the part to be stress
relieved is soaked in the heat.
Here, then, is my first question. Who is right on this subject? Is it
worthless to even attempt post-weld stress relieving of a chromoly part,
or can an appreciable amount of strength be regained through using a torch
and allowing the part to cool in still air, or better yet, buried in
sand? Does anyone know of a chart that might exist someplace that shows
the relationship between gained strength and duration of applied heat?
That first question then leads me to my second question. According to Ron
Fournier in his book "Metal Fabricator's Handbook" the best rule to follow
with chromoly is to simply not use it unless you know EXACTLY why it is
needed. And from my reading, I'm beginning to think that he's absolutely
right on the money with that. So, when then would you actually need to
use chromoly? I can only think of two times, that being when weight is a
critical issue and when its strength makes it the only metal appropriate
for the part while its deficiencies do not make for an undesirable failure
mode (see my example in the next paragraph). Does that sound accurate?
As I tinker with cars a lot, I especially think of this in terms of car
parts, and one part in particular where chromoly shows up a lot in the
aftermarket is with suspension and chassis components. Mr. Fournier says
to stay away from chromoly roll cages because they tend to break instead
of bend, and that a broken up cage is infintely more likely to kill a
driver than a bent up cage since bends absorb impact and breaks create
sharp spears that turn a driver into hamburger. This sounds absolutely
reasonable to me, after reading about chromoly's deficiencies. But now I
also wonder, in anything but a track driven race vehicle, couldn't the
decreased weight of a chromoly part have its value offset by the fact that
it would break instead of bend? After all, if you were to, say, break a
chromoly control arm on the track, there'd be a vehicle to tow you back to
the pits. However, if you were offroading in the desert or being an idiot
on the street, a broken control arm could leave you completely stranded
whereas a bent up one might still allow you to limp home. It seems like
chromoly's only place for street and offroad vehicles exists for parts
like sway bars and other things where breakage is either statistically
impossible or not particularly hazardous/lethal.
So those are the questions and my moment of pondering... I look forward to
hear comments from the smart folks out there with more knowledge and
experience on the subject than I. Thanks!
Reply to
The Hurdy Gurdy Man
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I've also spent a fair amount of time trying to sort out the various claims concerning 4130, and I've gotten a lot of contradictory answers.
I went to a lot of sources, ranging from EAA to the AISI and the US Air Force. The remarkable thing is that the kind of destructive testing I want to hear about is in very short supply. There is anecdotal information but I could find no one who knew about systematic, scientific destructive testing of welded joints in 4130. One source I never got around to was the American Society for Metals. I still want to call them some day and see if they have anything.
I smashed some with a hammer when I completed my welding class. My instructor, who was an Air Force reservist certified by the military to do all kinds of aircraft repair welding, TIG welded some samples (0.75 in. dia., 0.065 in. wall) for me, and I welded some similar ones with O/A. No "stress relieving" involved. He used 4130 rod, which Finch says not to do but which is REQUIRED by the Air Force for repair joints; I used high-quality mild steel. After smashing the hell out of them with a big hammer on an anvil, I was convinced that there is nothing at all brittle about those joints, even the ones TIG welded with 4130 rod. I could pound them flat, fold them over, and so on, without a crack at the weld. The pieces did eventually crack in various places, but that was after they were tortured beyond belief.
4130 has twice the strength of mild steel and, according to most sources, much greater impact strength and overall toughness. It is NOT a high-strength alloy that is given to brittleness. It is a medium-strength alloy formulated for reliable welding and high toughness. Although this disagrees with the common understanding of "toughness," it is much tougher than mild steel, in terms of the impact it can tolerate. It has relatively high elongation so ductility is not a limiting factor.
You'd think that someone would have conducted really systematic tests of welded 4130, especially since it was created (in the 1920's) specifically for aircraft use. If you find evidence of any, I'd like to hear about it.
Oh, about brazing it, which Finch says not to do: I found no support for that claim, anywhere. In fact, bicycle and motorcycle frame-makers do it all the time, with no reports I've been able to uncover, of weak joints. The guy who wrote the most widely used book on brazing in the world, who is now close to 80, told me two years ago that he tested brazed 4130 during WWII, for the military, and it was as strong as welded joints.
Ed Huntress
Reply to
Ed Huntress
I am top-posting a reply because I am just too tired to edit and reply inline as usual. Sorry.
Post weld heat treat is an old solution to a known problem, but it is no longer needed. It would be more appropriate to call it "tempering". "Annealing " requires a very slow cool down to completely stress relieve or "normalize" or "spheroidize" a steel. Tempering is simply reducing the possibility of a failure by reducing the overall hardness below a critical level.
If you TIG weld 4130 using a ER80S-B2 filler rod, no post weld heat treat is needed.
Read this page This guy works as a subcontractor for Northrup's Skunkworks. He knows his shit, and teaches airframe repair classes across the country.
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Reply to
Ernie Leimkuhler
--Donald Hutson, of Battlebot fame, has a sticker on one of his bots that says: "Chromoly: the other white meat". Heh.
Reply to
steamer
Lincoln and probably Miller as well, have quite a bit of practical information available on welding cromoly as it is commonly seen in automotive applications.
4130 is, I believe, a SAE (?) designation, this material, and slight variations of it, are commonly used in high pressure piping, especially in high temp steam applications. This is under a different designation than the SAE (?) 4130. In those applications, the material and the welding of it has been tested to a great extent. That information may or may not satisfy Ed's desire for scientific testing of the material, it is normally used in much greater wall thickness in piping applications (as much as 8" wall on main steam leads, but also much thinner than that) than the structural tubing usually found in aircraft and roll cages. The practical understanding of thin wall tubing material is well known, many an NHRA dragster has crashed at high speed, and the damage is studied by all concerned.
JTMcC.
Reply to
JTMcC
4130 is no lighter than mild steel, their density's differ by only .02 g/cc. Young's Modulus is also the same for each, meaning that with the same force applied to an equivalent sample of each, they will both deform the same amount. Where the two metals differ is their yield and ultimate strength. 4130 will take a larger force to both permanently deform it and to break it than 1018 for example.
I don't know much on the finer details of the welding processes, I just wanted to give the facts on the materials.
Josh
The Hurdy Gurdy Man wrote:
Reply to
Josh Fowler
Thanks for top posting.
Dan
Reply to
Dan Caster
I've welded 4130 for years, always TIG, you can stress relieve it with a torch, but not doing it is ok, we weld race car suspension a lot and never have had a problem with welded only parts.
-- David Algie Algie Composite Aircraft
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Reply to
David Algie
Actually what I was wondering about post-weld stress relieving using a torch (which the author of the above link says is required) and not heat treating/annealing, but thank you for the link and the information. It seems like Finch's text is the only one that flat out says that post-weld stress relieving isn't doable... it also says that the heat treating isn't necessary either, but then the only book I've seen that says you do need to do it is in some by Carroll Smith, and he talks of it more in terms of actually treating the part to a certain level of hardness to attain the required mechanical properties for a part's design, which seems valid. I'm sure for his designs and applications he knows exactly what he's doing, and probably has the real world examples to back it up.
I guess I'm still no closer to a real documented answer to the question, unless I can find some actual stress analysis studies of stress relieved joints versus non-stress relieved joints. In the meantime I suppose it's best to simply follow what the majority of experienced folks do, since they very likely have good reasons for doing it the way they do.
Reply to
The Hurdy Gurdy Man
Hmm... well, maybe Finch is actually on to something for a change. Thanks a bunch for sharing your experience with it! I spoke with my welding instructor today, and he apparently spent a lot of time training on chromoly back in the Navy... they didn't do any stress relieving on the coupons either before testing them. In tensile tests, the correctly welded parts never failed at the joint or in the heat affected zone either, which makes me think that quite possibly Finch is completely correct in that the torch heating stress relief rigamaroll isn't effective enough nor required enough to matter. Probably still a safe bit of insurance to do it, but it does make me think that it's not as critical as many of the sources I have read have made it out to be. Thanks again!
Reply to
The Hurdy Gurdy Man
Oh I'm fully aware of that... my understanding is that due to the increased strength of the material, you can get away with using a thinner wall for the part and thereby decrease weight, hence chromoly's use in weight critical situations. But I do realize that for a dimensionally similar component there is no weight savings by using chromoly. Otherwise, things like bicycle frames wouldn't be made out of tubing with such thin walls... they'd have regular old thick walls, and have some sort of magical weight loss associated with them. That, however, is what aluminum is for!
Reply to
The Hurdy Gurdy Man
Without digging out my research, I can say that most reputable sources I've read say that no pre-heat or "stress relieving" is necessary with 4130 tubing in wall thickness of 0.80 in. or less -- one said 0.065 in. Somewhere, the EAA says NOT to do it. Somewhere else, their various independent authors say to do it.
But those are mostly older references. The newer ones tend to say not to try to stress-relieve 4130 with a torch.
Ed Huntress
Reply to
Ed Huntress
Otherwise,
Actually, a lot of custom bicycles are made with thin-wall 4130 today. If you really want to see thin-wall tubing on a bicycle, saw through the frame of an old, high-quality bike made with double-butted Reynolds 531 or Columbus tubing. They're very darned thin.
Aluminum is a mixed bag. I had an aluminum Allegro track bike when I raced bicycles in high school. It was 'way too flexible. Some types of bikes benefit from the flex, but not ones that are raced on roads or flat tracks by well-tuned athletes.
That was 40 years ago, BTW. I'm far from being well-tuned today.
Ed Huntress
Reply to
Ed Huntress
Schwinn Paramount track bike - those things were scary to ride, they were so light.
Jim
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Reply to
jim rozen
You could, but from the equal Young's Moduluses, you would lose stiffness.
Josh
But I do realize that for a dimensionally > similar component there is no weight savings by using chromoly. Otherwise, > things like bicycle frames wouldn't be made out of tubing with such thin > walls... they'd have regular old thick walls, and have some sort of magical > weight loss associated with them. That, however, is what aluminum is for!
Reply to
Josh Fowler
In the days you are talking about, the thinner center sections of a lightweight bike tube were typically 0.032" to 0.028" thick. Time trial frames would be scarry light, 0.024" or even (Gasp) 0.020".
Now 0.024" and 0.020" can be used for touring frames. 0.016" and 0.015" are used for racing frames. Larger diameters provide adequate rigidity.
These steels are still 4130, or closely related to it.
True Temper's air hardening OX Platinum has a tensile strength that exceeds 195,000 psi, before welding. Silver brazing does nothing to alter this strength, but brass brazing or TIG welding INCREASES this strength considerably.
My Paramount track bike has 1 speed, no brakes, derailleurs, levers or cables. It weighs almost 22 lbs. Frame alone is 4.5 lbs.
A friend of mine recently built a lugged, steel, frame for a cyclocross bike. (Cyclocross; Think off-road racing on 10 speeds) With cantlever brakes, rear derailler and knobby tires it's a bit under 19 lbs.
Reply to
Mark Stonich
Higher strength steels allow you to run higher diameter/thickness ratios. A 2.0" x 0.035" tube is only 74% as heavy as a 1.5" x 0.065" tube. However it's 38% stiffer. A 2.0" x 0.035" mild steel tube would dent easily, leading to a cripling failure. A 2.0" x 0.035" 4130 tube is pretty tough. I bet my life on this, every time I hit a bump at high speed on this baby.
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Reply to
Mark Stonich
Wow, nice looking bike, Mark. Did you design it yourself?
Ed Huntress
Reply to
Ed Huntress
"Stress relieving" 4130
True Temper has done a great deal of fatigue testing to determine the proper methods for joining 4130 steels. One of their engineers told me that they found the best fatigue strength came with a low temp pre-heat, and then after welding, just bringing the area up to 400-600F. I heard the same from the guy who taught me TIG welding, and his background was in the Aerospace industry. These temps aren't going to alter the grain structure, just put the area through an expansion/contraction cycle. Theoretically, it shouldn't make any difference, but I'll take test results over theory. BTW There is a whole industry devoted to fatigue testing of bike frames and components. The best way to do this is with a propane torch. When you start to heat steel with propane you get a shiny wet film on the steel. This goes away at about 400F.
4130 filler rod. ER80-S2 is usually used with plain "Aircraft grade" 4130. See
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However, an increasing number of bike builders are using a stainless filler, Weld Mold's Polytensile 880.
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welds very nicely and has good mechanical properties, 110 kpsi and 35% elongation. It's expensive, about $15/lb. But a whole bicycle may only need 3 or 4 sticks of 0.035" diameter rod. (When you are trying to save grams, you don't go broke buying filler ;-)
Reply to
Mark Stonich
Yep, that's teh one. I used to work in Westwood Cycle (formerly owned by Jacky Simes) and they had a track bike like that, used for running errands. Inch pitch chain, solid gearing, no brakes. One day the boss said take this down to the deli and get lunch. Felt like I was driving a maserati, the fork had basically zero rake.
Wow. Times have changed.
Jim
================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================
Reply to
jim rozen

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