4130 chassis, braze or TIG?
David Keyzer
I'm at odds with my faculty advisor on the FSAE team here at Brown U.
He's certain that brazing our 4130 race car chassis is the best option
because it avoids most of the cracking problems associated with TIG.
I've only been welding for a year so I don't have much experience to
draw from, but it seems that TIG would be a better way to go. I've been
told that almost all of the teams in our competition TIG, and that most
don't heat-treat afterwards. This doesn't seem like an especially good
practice to me. My advisor is concerned that heat treating with a torch
is unreliable and that uneven cooling of the joint is a big problem.
Brazing also allows us to accomodate poor fit-up from time to time, but
that's certainly a secondary issue.
The brazing alloy we use is a "super high strength alloy", Xuper Alloy
from a company called Eutectic, I believe. It claims tensile strength
"up to" about 110,000 psi, but I'm sceptical. What's the difference
between "up to" and "in reality" turn out to be?
I'm a first-year student, but according to other team members, we have
never broken a weld or a tube on the chassis in the four or so years
that we've been brazing it together. However, I've been told that the
judges at our competition criticize our practice of brazing the chassis.
Any input (or follow-up questions) would be appreciated.
Thanks,
Dave
Blake Mantel
> Hey guys,
> I'm at odds with my faculty advisor on the FSAE team here at Brown U.
> He's certain that brazing our 4130 race car chassis is the best option
> because it avoids most of the cracking problems associated with TIG.
Many race teams MIG weld their chassis together with NO stress relieve cycle
afterwards!
Personally I would not use MIG on highly critical applications, TIG would be my
choice.
You can also oxy-actelyene the 4130 but that is much more difficult. I was
trained by some nice old coot's to do aircraft grade welding on 4130 tube
fuselages. I am not as pretty as they were, but I get penetration. This
technique is, however, difficult to master.
Brazing will require VERY close cutting and mating of the tubing. I'd say even
closer than 1/16 gap for aircraft welding work.
I'd TIG it together. If this guy is so fuggin paranoid about stress crack
formation, id go over the welds with an O/A rosebud heating tip and reheat to a
very dull red (in dim light!) and let air cool in still room temp air.
Some professors really suffer from recto-cranial-inversions!
Good luck,
Blake
--
CUM CATAPULTAE PROSCRIPTAE ERUNT TUM SOLI PROSCRIPTI CATAPULTAS HABEBUNT.
(When catapults are outlawed, only outlaws will have catapults....)
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My Email address is altered due to the prevalence of bulk Email senders.
To send me mail remove the two *'s before the TIAC.NET.
Randy Zimmerman
things. There indeed are brazing alloys that exceed 100,000 psi but that is
with very small clearances between parts.
I understand that Jaguar "E" types were brazed tube construction and
suffered no ill effects. If brazing has worked for you and you are not
having joint failures why add an unknown into the equation?? I agree with
your advisor.
My guess is that you would produce just as much heat problems with TIG
on a tube cluster as you would brazing. At least with brazing you don't
have to contend with Heat Affected Zones. The gentle heat of brazing plus
a slow coolling is quite different from the large gradient of temperature
next to a welded joint.
You might try asking Handy Harman for technical advice.
http://www.handyharmancanada.com/
David Keyzer wrote in message <38ED4D1F...@brown.edu>...
>Hey guys,
snip
GaryPasson
As usual there's a couple of sides to every story. First brazing has
been used in England and europe for race cars (particuallry light weight
formula cars) forever. The theory has been that the joints were strong
enough (if properly designed) and the lower tempiture of the process
minimized the heat affacted zone and reduced the risk of cracking while
in service. In my experience I seen some well streched joints that
didnot fail and only a very few "cracked" chassis joints (motormounts
and exhaust joints not withstanding). The most important items are joint
design, choice of brazing material and extremly close joint fitups.
On the other hand... Almost all high perfance race cars (excepting alot
of Nascar type cars) are TIG welded. The Racers (and aircraft guys in
general) are looking for the MAX strength, "sorta" production speed and
tidy-ness of the welds. Great TIG welds look the best. The most
important items in TIG are pretty much the same as brazing (although you
can be a bit more sloppy in the fitups - not in my shop though!). In
my humble opnion, no heat treating of the TIG joints is required. Yes
the textbooks and engineering specs would suggest it is necessary, but I
have not seen a chassis or airframe crack I felt would have not occured
even if the joint would of been "relieved". (An exception to the
prior statements would be in highly stressed joints and/or where the
engineer or drawings call out a specifc heat treating or stress
relieving process). The "heavy" guys with thick walled tubing (read
stockcar guys) like Mig because its fast and (arguably) requires the
least skills in fittingup the joints and in the welding process.
After all that I'd say (given your doing a light weight formula car)
that brazing is just fine.
Good luck!!
GaryP (racer and aircraft builder)
---------------------------------------------------------
In article <38ED4D1F...@brown.edu>,
David Keyzer <David_...@brown.edu> wrote:
> Hey guys,
>
> I'm at odds with my faculty advisor on the FSAE team here at Brown U.
> He's certain that brazing our 4130 race car chassis is the best option
> because it avoids most of the cracking problems associated with TIG.
> I've only been welding for a year so I don't have much experience to
> draw from, but it seems that TIG would be a better way to go. I've
been
> told that almost all of the teams in our competition TIG, and that
most
> don't heat-treat afterwards. This doesn't seem like an especially
good
> practice to me. My advisor is concerned that heat treating with a
torch
> is unreliable and that uneven cooling of the joint is a big problem.
> Brazing also allows us to accomodate poor fit-up from time to time,
but
> that's certainly a secondary issue.
> The brazing alloy we use is a "super high strength alloy", Xuper Alloy
> from a company called Eutectic, I believe. It claims tensile strength
> "up to" about 110,000 psi, but I'm sceptical. What's the difference
> between "up to" and "in reality" turn out to be?
>
> I'm a first-year student, but according to other team members, we have
> never broken a weld or a tube on the chassis in the four or so years
> that we've been brazing it together. However, I've been told that the
> judges at our competition criticize our practice of brazing the
chassis.
>
> Any input (or follow-up questions) would be appreciated.
>
> Thanks,
> Dave
>
>
Sent via Deja.com http://www.deja.com/
Before you buy.
Ted Edwards
thread on this a year or so ago in rec.aircraft.homebuilt - you might
try a deja search.
Ted
Gary Coffman
>I'm at odds with my faculty advisor on the FSAE team here at Brown U.
>He's certain that brazing our 4130 race car chassis is the best option
>because it avoids most of the cracking problems associated with TIG.
>I've only been welding for a year so I don't have much experience to
>draw from, but it seems that TIG would be a better way to go. I've been
>told that almost all of the teams in our competition TIG, and that most
>don't heat-treat afterwards. This doesn't seem like an especially good
>practice to me. My advisor is concerned that heat treating with a torch
>is unreliable and that uneven cooling of the joint is a big problem.
IMHO the better practice is to preheat to dull red before welding, then
let cool slowly in still air. By preheating, you avoid the quenching due
to the cold metal surrounding the weld joint. That's what causes stresses
which may lead to cracking. Post-welding heat treatment is less effective
because the major stresses have already "set".
>Brazing also allows us to accomodate poor fit-up from time to time, but
>that's certainly a secondary issue.
>The brazing alloy we use is a "super high strength alloy", Xuper Alloy
>from a company called Eutectic, I believe. It claims tensile strength
>"up to" about 110,000 psi, but I'm sceptical. What's the difference
>between "up to" and "in reality" turn out to be?
Depends a lot on the fit up. If the fits are really good, and you have good
capillary action drawing in the braze, brazing can be strong. But with a
sloppy fit, brazed joints generally aren't very strong. Your comment about
compensating for sloppy fit troubles me in this regard. That may also
explain the criticism by the judges.
With brazing and soldering, the tensile strength of the braze material isn't
the real issue. The real issue is, how strong is the bond between the braze
material and the base material? Brazing and soldering work by "wetting" to
the parent materials. Van der Waals forces are what secure the bond. Ideally,
these forces should span the joint, and that requires a very good fit up.
Welding actually fuses the parent metals, so you can compensate for a bad
fit up with filler. It all becomes one homogeneous piece when it cools since
the filler is the same material as the parent. But you can't do that with brazing.
You'll wind up with 2 weak braze bonds instead of one strong one.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Rich Grise
...
> With brazing and soldering, the tensile strength of the braze material isn't
> the real issue. The real issue is, how strong is the bond between the braze
> material and the base material? Brazing and soldering work by "wetting" to
> the parent materials. Van der Waals forces are what secure the bond. Ideally,
> these forces should span the joint, and that requires a very good fit up.
>
> Welding actually fuses the parent metals, so you can compensate for a bad
> fit up with filler. It all becomes one homogeneous piece when it cools since
> the filler is the same material as the parent. But you can't do that with brazing.
> You'll wind up with 2 weak braze bonds instead of one strong one.
I brazed a piece of my clutch linkage once - I presume it was just mild
steel - about 3/8" diameter rod stock, with bends on either end, some
kind
of shoulder apparently forged into it, and then a portion that fit into
a crank arm, again, 3/8" diameter, but kind of polished, but probably
due to the crank action. Well, it broke at one of the bends. (90
degrees,
of course.) Luckily, I was able to drive home, because it was a '72
Ford,
and I could shift into neutral by feathering the accelerator, and turn
off the motor, and put it into first, and the starter motor would start
the motor while also moving the whole van (of course, a little later, I
had to replace the Bendix) - anyway, I OA brazed the joint that had
opened
up, but with a LOT of coaching, by about three guys who knew what the
heck
was up - I ground a HUGE (relatively) V-groove on the inside, where the
bend had cracked, and one of the fellows said, "You have to puddle the
iron ..." and ever since then, I've assumed that brazing at a
temperature
that would puddle the iron would form some kind of alloy or something -
BTW, this was my second try on this part - the first time, I treated it
like soldering, and it failed first time out.
Did I just get lucky? (BTW, the part continued to work until I sold the
van.)
Thanks!
Rich
brad
will pick up enough carbon from the base metal to gain additional
strength.(annealed 4130 will be in the 85-90ksi tensile range.) Remember
that the HAZ (heat affected zone) will be in the annealed (soft) condition.
Hopefully you will be welding on annealed tubing.Warming the joint a
bit(150-250 degF) and wrapping with fiberglas insulation to slow cool is
helpful if cracks develop. Welding on heat treated tubing can cause
cracks,regardless. If you want max heat treated strength in all the joint
areas,the entire chassis will have to be heat treated after welding-a
difficult and expensive process and is rarely used.(You cannot heat treat "a
joint at a time" with a torch and expect absolute reliability. Any slight
variations will result in some joints having better mechanical properties
than others.-"weak link in a chain".) If heat treating,the entire structure
must be brought up to the same heat at the same time.Annealed tubing must be
used. Heat treating an already heat treated tubing can result in
overhardening of the unwelded tubing,if the welded joints are to be brought
up to standard HT strength. If heat treatment will be done a er90SB-2,3
wire(L grades of this rod are more crack sensitive) or 4130 wire would a
good choice. These will approximate heat treated strength AFTER heat
treatment.Another thing to keep in mind-if the weld is critical,stay away
from the proprietary alloys (eutectic,sodel,hi-alloy,harris,etc and many
others). You cannot reference them to any common welding code as acceptable
for your job.The lawyers will have a field day if a failure occurs. The AWS
(American Welding Society) has much material to help you through your
project. Look in their website.(www.amweld.org)One other thing to keep in
mind that will transcend all the advice you will get.(including mine). If
you are planning to race in some organized,sanctioned races-you better get
their rulebook. Many race organizations dictate how frames will be welded
and with what. These often change from year to year. It would be a shame to
build a good car,only to have it disqualified in the tech inspection.Hope
this helps a little. Good Luck-brad
>He's certain that brazing our 4130 race car chassis is the best option
>because it avoids most of the cracking problems associated with TIG.
>I've only been welding for a year so I don't have much experience to
>draw from, but it seems that TIG would be a better way to go. I've been
>told that almost all of the teams in our competition TIG, and that most
>don't heat-treat afterwards. This doesn't seem like an especially good
>practice to me. My advisor is concerned that heat treating with a torch
>is unreliable and that uneven cooling of the joint is a big problem.
>that's certainly a secondary issue.
>The brazing alloy we use is a "super high strength alloy", Xuper Alloy
>from a company called Eutectic, I believe. It claims tensile strength
>"up to" about 110,000 psi, but I'm sceptical. What's the difference
>between "up to" and "in reality" turn out to be?
>
stri...@telusplanet.net
>
>
Dave...I built a Pitts aerobatic aircraft from scratch using o/a on the
frame and fittings. Each 4130 joint was welded with mild steel rod and
individually heat treated with the torch.
I can't tell you how much tensile strength was gained or lost by this
method but I do know that the pilots crashed the plane twice after it
was finished.
When we checked the chasis we found that no weld had failed in either
crash. In fact, in the first instance a fitting tore (outside the haz)
and the the welds held. The plane is still flying with its third owner.
An old aircraft welder told me long ago that we could probably fly the
darn things tack welded as long as we didn't try anything stupid. Do a
good job of braze or fusion welding and either method will be fine.
Ian
Jon F. Lenander
When I first read the question, I thought there would be a
lot of replies advising not to braze 4130. At least I always read not
to. I have read it in more than one book, however, I only have one of
my references to hand. Richard Finch says in Performance Welding
"Always avoid brazing 4130 steel. The reason not to braze chromemoly
is that this steel has a definite grain structure that actually opens
up at medium red brazing temperatures. When brazing alloy is melted
onto the steel surface, it flows easily into the many small cracks and
crevices in the chromemoly steel. Then, as the braze joint cools, the
brass will not compress and it forces major cracks to form in the 4130
steel. Often, a brazed 4130 steel part will crack completely in two
before your eyes as it cools."
Note that I have never actually tried to do this so I am only adding
secondhand, theoretical knowledge!! Best of luck on your project! Jon
Ted Edwards
> When I first read the question, I thought there would be a
> lot of replies advising not to braze 4130. At least I always read not
> to.
I followed a thread on welding 4130 in rec.aircraft.homebuilt for a
while because of my welding interest. It seems the only acceptable
method for joining 4130 tubing in homebuilt aircraft is TIG or O/A
welding using mild steel filler. No MIG, no brazing. Betcha there's a
reason.
Ted
Gary Coffman
> anyway, I OA brazed the joint that had
>opened
>up, but with a LOT of coaching, by about three guys who knew what the
>heck
>was up - I ground a HUGE (relatively) V-groove on the inside, where the
>bend had cracked, and one of the fellows said, "You have to puddle the
>iron ..." and ever since then, I've assumed that brazing at a
>temperature
>that would puddle the iron would form some kind of alloy or something -
>BTW, this was my second try on this part - the first time, I treated it
>like soldering, and it failed first time out.
>
>Did I just get lucky? (BTW, the part continued to work until I sold the
>van.)
The term brazing spans a lot of territory. It is basically any dissimilar
metals joining technique done at temperatures in excess of 800F.
What you describe is sometimes called braze welding. It is at the
high temperature end of the brazing spectrum. The joint is actually
a fusion joint, though unlike regular welding, dissimilar metals are
being fused. Lower temperature brazing doesn't melt the parent
metal, and works as I described, as does soldering which is joining
done at an even lower temperature (below 800F).
jj...@dow.com
strength of braze joints done with a torch is unpredictable. According
to Bruhn's "Analysis and Design of Flight Vehicle Structures" torch
brazed joints are only allowed 10,000 psi tensile strength for the
purposes of stress analysis.
Blake Mantel
> IMHO the better practice is to preheat to dull red before welding, then
> let cool slowly in still air. By preheating, you avoid the quenching due
> to the cold metal surrounding the weld joint. That's what causes stresses
> which may lead to cracking. Post-welding heat treatment is less effective
> because the major stresses have already "set".
HUH????!!!!
DON'T DO THAT!!!
Gary, I guess that you don't depend on your welding to hold yourself up at 5-10,000
feet and a few hundred miles per hours.
One of the goals of welding is to introduce as little of a contaminant (in this case
iron oxide) into the welded area. That's why you also cut off the end of the rod when
you stop welding with it and it leaves the protective gas cone (either argon/he or
oxy/actyl gas). You also keep the gas cone on the last welded area to prevent
oxidizing the steel as it cools.
A heat to dull red will remove most of the internal stress. There is no time that
metal's crystalline structure is ever "set". It can be modified at any time with the
proper techniques.
Gary Coffman
>Gary Coffman wrote:
>
>> IMHO the better practice is to preheat to dull red before welding, then
>> let cool slowly in still air. By preheating, you avoid the quenching due
>> to the cold metal surrounding the weld joint. That's what causes stresses
>> which may lead to cracking. Post-welding heat treatment is less effective
>> because the major stresses have already "set".
>
>HUH????!!!!
>
>DON'T DO THAT!!!
>
>Gary, I guess that you don't depend on your welding to hold yourself up at 5-10,000
>feet and a few hundred miles per hours.
True, only a few inches above the ground, but my welds have to stand pounding
that would shake an aircraft apart in a few miles.
>One of the goals of welding is to introduce as little of a contaminant (in this case
>iron oxide) into the welded area. That's why you also cut off the end of the rod when
>you stop welding with it and it leaves the protective gas cone (either argon/he or
>oxy/actyl gas). You also keep the gas cone on the last welded area to prevent
>oxidizing the steel as it cools.
>
>A heat to dull red will remove most of the internal stress. There is no time that
>metal's crystalline structure is ever "set". It can be modified at any time with the
>proper techniques.
What I am mostly concerned about is the joint cracking as it cools because
of the stresses set into the material by the welding. If it makes it down to room
temperature before cracking, then reheating can relieve the stresses. But that
won't prevent cracking in the first place. Preheating helps avoid that problem
by eliminating the quench heat sink of cold metal surrounding the joint.
Heating to red is going to produce scale, whether you do it before or after
the weld. The welding heat should float any oxides out of the joint itself.
At least it seems to do so on the relatively thick sections I weld. It might
not with very thin sections, but I have no experience with aircraft practices.
I do acknowledge that different practices may be required for different
jobs. I certainly don't claim to be an expert in aircraft fabrication. I will
note that Babcock & Wilcox preheated nuclear reactor containment
vessel sections before welding them together. That's about as critical
a weld as I can imagine. So I don't think I'm totally out to lunch in suggesting
that preheating can help avoid cracking.
Blake Mantel
> On Sun, 09 Apr 2000 01:20:10 -0400, Blake Mantel <bla...@tiac.net> wrote:
> >Gary Coffman wrote:
> >
> >> IMHO the better practice is to preheat to dull red before welding, then
> >> let cool slowly in still air. By preheating, you avoid the quenching due
> >> to the cold metal surrounding the weld joint. That's what causes stresses
> >> which may lead to cracking. Post-welding heat treatment is less effective
> >> because the major stresses have already "set".
> >
> >HUH????!!!!
> >
> >DON'T DO THAT!!!
> >
> >Gary, I guess that you don't depend on your welding to hold yourself up at 5-10,000
> >feet and a few hundred miles per hours.
>
> True, only a few inches above the ground, but my welds have to stand pounding
> that would shake an aircraft apart in a few miles.
Actually, they don't. And I'm not just being a smart ass!
In aircraft the lack of tires being on a nice firm surface causes greatly increased
vibrations. The contact with the road, suspension components, general over building of
wheeled vehicles (even race breeds) causes it to be not as critical as for aircraft.
A aircraft component that vibrates/resonates has to bleed off energy to other components
in the plane or to the air around it. No great dampening medium is available. Vibration
wreaks havoc.
> >One of the goals of welding is to introduce as little of a contaminant (in this case
> >iron oxide) into the welded area. That's why you also cut off the end of the rod when
> >you stop welding with it and it leaves the protective gas cone (either argon/he or
> >oxy/actyl gas). You also keep the gas cone on the last welded area to prevent
> >oxidizing the steel as it cools.
> >
> >A heat to dull red will remove most of the internal stress. There is no time that
> >metal's crystalline structure is ever "set". It can be modified at any time with the
> >proper techniques.
>
> What I am mostly concerned about is the joint cracking as it cools because
> of the stresses set into the material by the welding. If it makes it down to room
> temperature before cracking, then reheating can relieve the stresses. But that
> won't prevent cracking in the first place. Preheating helps avoid that problem
> by eliminating the quench heat sink of cold metal surrounding the joint.
>
> Heating to red is going to produce scale, whether you do it before or after
> the weld. The welding heat should float any oxides out of the joint itself.
> At least it seems to do so on the relatively thick sections I weld. It might
> not with very thin sections, but I have no experience with aircraft practices.
No problem, they are sorta a bit ridged but the idea is to get every last bit of strength
out of the manufactured part while keeping it light enough to still fly.
Although it looks like the scale is removed it still "contaminates" the weld with
increased amounts of iron oxide. While the part is not going to fall apart by any means,
it is still not the ultimate that it could be.
> I do acknowledge that different practices may be required for different
> jobs. I certainly don't claim to be an expert in aircraft fabrication. I will
> note that Babcock & Wilcox preheated nuclear reactor containment
> vessel sections before welding them together. That's about as critical
> a weld as I can imagine. So I don't think I'm totally out to lunch in suggesting
> that preheating can help avoid cracking.
> Gary
I never have done any large scale welding such as you have. Most of the pieces I have done
mass less than 300# or so.
The reasons for preheat of these large sections might just be due to it's large size.
(coefficients of expansions are measured in thousands of an inch per inch, but this
obviously add up as the workpiece size increases) So just by working on a large section
this might be required to get the process to work.
Keep it up!
Gary Coffman
>Gary Coffman wrote:
>> On Sun, 09 Apr 2000 01:20:10 -0400, Blake Mantel <bla...@tiac.net> wrote:
>> >Gary, I guess that you don't depend on your welding to hold yourself up at 5-10,000
>> >feet and a few hundred miles per hours.
>>
>> True, only a few inches above the ground, but my welds have to stand pounding
>> that would shake an aircraft apart in a few miles.
>
>Actually, they don't. And I'm not just being a smart ass!
>
>In aircraft the lack of tires being on a nice firm surface causes greatly increased
>vibrations. The contact with the road, suspension components, general over building of
>wheeled vehicles (even race breeds) causes it to be not as critical as for aircraft.
>
>A aircraft component that vibrates/resonates has to bleed off energy to other components
>in the plane or to the air around it. No great dampening medium is available. Vibration
>wreaks havoc.
Well, I've never actually driven an aircraft down a washboard gravel road, or
over railroad tracks, or down a potholed street. But what I've always been told
is that the G forces are more severe doing those things than any aircraft is
stressed to withstand on a regular basis. I understand that hard landings do
put a lot of stress on an aircraft, but they aren't doing hard landings every
few seconds. That's why cars and trucks have to be built stronger than planes.
Of course what I've been told could be wrong.
>> I do acknowledge that different practices may be required for different
>> jobs. I certainly don't claim to be an expert in aircraft fabrication. I will
>> note that Babcock & Wilcox preheated nuclear reactor containment
>> vessel sections before welding them together. That's about as critical
>> a weld as I can imagine. So I don't think I'm totally out to lunch in suggesting
>> that preheating can help avoid cracking.
>> Gary
>
>I never have done any large scale welding such as you have. Most of the pieces I have done
>mass less than 300# or so.
I didn't weld reactor vessels, but I did watch it being done at B&W's Mt Vernon plant.
>The reasons for preheat of these large sections might just be due to it's large size.
>(coefficients of expansions are measured in thousands of an inch per inch, but this
>obviously add up as the workpiece size increases) So just by working on a large section
>this might be required to get the process to work.
That may be part of it. The reactor vessel sections being joined were 20 feet
long, 20 feet in diameter, and a foot thick. B&W kept gas jets playing on them to
keep them hot while being welded together to form the 60 foot long vessel. (It
wound up being vertical in use, but was fabricated lying on its side.) But they
said the reason that they did it was to prevent cracking. They didn't want that
mass of metal to act as a heatsink which would quench the weld area and
produce very high stresses which might cause it to crack while cooling, or
later in use. That made sense to me then, and it still does.
(They didn't have a heat treating oven big enough to anneal the whole reactor
vessel after it was welded. But they did have a lathe big enough to turn and true
the vessel after it was welded. That was the biggest machine tool I ever saw.
I'm sure that if they thought that post-welding heat treatment was the better
way to do things, they could have built an oven big enough for the job.)
Now this is certainly a long way from welding together pieces of thin wall
tubing. What works for the heavier stuff might not be the right thing to do
with the thin stuff. The idea of preheating to prevent the surrounding metal
from quenching the joint is a compelling one, however.
Rex the Wrench
tools that was given out is a "Welding Preheat and Interpass Temp
Calculator.
(Lincoln Part# WC-8) This device allows the weldor to determine the
proper
preheat temp for the specific alloy being used.
The CroMo tube we were TIG welding required preheat to 100F. To achieve
proper temp I held the tube ends in bare hands before welding.
Rex the Wrench
Blake Mantel wroted:
>
> Gary Coffman wrote:
>
> > On Sun, 09 Apr 2000 01:20:10 -0400, Blake Mantel <bla...@tiac.net> wrote:
> > >Gary Coffman wrote:
> > >
> > >> IMHO the better practice is to preheat to dull red before welding, then
> > >> let cool slowly in still air. By preheating, you avoid the quenching due
> > >> to the cold metal surrounding the weld joint. That's what causes stresses
> > >> which may lead to cracking. Post-welding heat treatment is less effective
> > >> because the major stresses have already "set".
> > >
> > >HUH????!!!!
> > >
> > >DON'T DO THAT!!!
> > >
> > >Gary, I guess that you don't depend on your welding to hold yourself up at 5-10,000
> > >feet and a few hundred miles per hours.
> >
> > True, only a few inches above the ground, but my welds have to stand pounding
> > that would shake an aircraft apart in a few miles.
>
> Actually, they don't. And I'm not just being a smart ass!
>
> In aircraft the lack of tires being on a nice firm surface causes greatly increased
> vibrations. The contact with the road, suspension components, general over building of
> wheeled vehicles (even race breeds) causes it to be not as critical as for aircraft.
>
> A aircraft component that vibrates/resonates has to bleed off energy to other components
> in the plane or to the air around it. No great dampening medium is available. Vibration
> wreaks havoc.
>
> > >One of the goals of welding is to introduce as little of a contaminant (in this case
> > >iron oxide) into the welded area. That's why you also cut off the end of the rod when
> > >you stop welding with it and it leaves the protective gas cone (either argon/he or
> > >oxy/actyl gas). You also keep the gas cone on the last welded area to prevent
> > >oxidizing the steel as it cools.
> > >
> > >A heat to dull red will remove most of the internal stress. There is no time that
> > >metal's crystalline structure is ever "set". It can be modified at any time with the
> > >proper techniques.
> >
> > What I am mostly concerned about is the joint cracking as it cools because
> > of the stresses set into the material by the welding. If it makes it down to room
> > temperature before cracking, then reheating can relieve the stresses. But that
> > won't prevent cracking in the first place. Preheating helps avoid that problem
> > by eliminating the quench heat sink of cold metal surrounding the joint.
> >
> > Heating to red is going to produce scale, whether you do it before or after
> > the weld. The welding heat should float any oxides out of the joint itself.
> > At least it seems to do so on the relatively thick sections I weld. It might
> > not with very thin sections, but I have no experience with aircraft practices.
>
> No problem, they are sorta a bit ridged but the idea is to get every last bit of strength
> out of the manufactured part while keeping it light enough to still fly.
>
> Although it looks like the scale is removed it still "contaminates" the weld with
> increased amounts of iron oxide. While the part is not going to fall apart by any means,
> it is still not the ultimate that it could be.
>
> > I do acknowledge that different practices may be required for different
> > jobs. I certainly don't claim to be an expert in aircraft fabrication. I will
> > note that Babcock & Wilcox preheated nuclear reactor containment
> > vessel sections before welding them together. That's about as critical
> > a weld as I can imagine. So I don't think I'm totally out to lunch in suggesting
> > that preheating can help avoid cracking.
> > Gary
>
> I never have done any large scale welding such as you have. Most of the pieces I have done
> mass less than 300# or so.
>
> The reasons for preheat of these large sections might just be due to it's large size.
> (coefficients of expansions are measured in thousands of an inch per inch, but this
> obviously add up as the workpiece size increases) So just by working on a large section
> this might be required to get the process to work.
>
Blake Mantel
> Last week I took Lincoln's Motorsports Welding Class and one of the cool
> tools that was given out is a "Welding Preheat and Interpass Temp
> Calculator.
> (Lincoln Part# WC-8) This device allows the weldor to determine the
> proper
> preheat temp for the specific alloy being used.
>
> The CroMo tube we were TIG welding required preheat to 100F. To achieve
> proper temp I held the tube ends in bare hands before welding.
> Rex the Wrench
Hmm, I should get me one of thoes nifty calculators! 100'F seems pretty reasonable.
Though I am a Miller man, I'm sure the welder wont mind a Lincoln product...
Dan Timberlake
<David_...@brown.edu> wrote:
>Hey guys,
>
>I'm at odds with my faculty advisor on the FSAE team here at Brown U.
>He's certain that brazing our 4130 race car chassis is the best option
I tend to agree with him.
Sounds like he read Carrol Smith's Prepare to win, but not any FAA
approved aircraft repair manuals.
Nor has he spent much time looking at bicycle frames in the last 20
years.
>because it avoids most of the cracking problems associated with TIG.
Brittle steel cracks right away if stretched much at all.
Ductile steel cracks right away if stretched too far.
Any steel cracks if subjected to cyclic stress above the fatigue
limit. Mechanical features can act as Stress concentations or stress
raisers and can increase the stress in a tiny local section WAY above
the average stress for the part.
Any welded joint is at serious risk from all 3 factors.
Cracks appear from restraining a weld as it cools, forcing it to
stretch too far. Similarly, if steel is ends up hardened (rapid
cooling) it has low ductility, and will crack if forced to stretch
more than a few percent.
4130 is air hardenable, but must cool off pretty quickly to be
hardened. OxyAcetylene welding pumps extra heat into the weld area,
slowing the rate at which the joint cools, preventing hardening. It
tends to heat the entire joint, making it more likely that that part
of the assembly will cool more like a unit, reducing the high
localized stress.
Aircraft specs have allowed electric or gas welding thin 4130 tubes as
long as proper filler is used. Aircraft design would never put welds
in an area of high stress (especially bending stress) because welds
invariably are full of stress raisers and unknown metallurgy anyway.
Full 3D triangulation and lugs inline with all concentrated loads
accomplish this. I bet your chassis subjects the welded joints to
bending loads.
Ken Sprayson (engineer who worked for Reynolds) wrote an article about
motorcycle frames that appeared in the Feb 1974 Motorcylist magazine.
Reynolds, the steel tubing manufacturer whose "531" products were the
ultimate for bicycle and motorcycle use late in the last millennium,
advocated "bronze welding" which is related to brazing. Theoretical
advantages were
- lower temps that were less damaging to the tubes' metallurgy
- broader softer fillet that reduces stress concentrations
- broader heat affectes zones.
>I've only been welding for a year so I don't have much experience to
>draw from, but it seems that TIG would be a better way to go. I've been
>told that almost all of the teams in our competition TIG, and that most
>don't heat-treat afterwards. This doesn't seem like an especially good
>practice to me.
In theory it risks embrittlement (uncontrolled too rapid cooling) and
higher locked in stresses.
Carrol Smith (Prepare to win, Tune to win, worked for Ford and Shelby)
says it is crazy not to properly heat treat any steel part made of
anything but mild steel. He's probably right. In his earliest book
he felt bronze welding was the "correct" way to join 4130 (and
similar) tubing. Not many race cars are made that way today.
In the same MC article Sprayson described the processes that (then)
modern frame builders were using. Heli-arc (TIG) was the favorite.
Maybe with torch "stress relief." In my mind it is hard to know
whether they were stress relieving (500 - 1200 degrees F) or
annealing/tempering (similar temps).
Heating the joint properly after welding can relieve locked in
stresses (good).
Heating the joint properly after welding can temper or soften a
hardened area to reduce brittleness (good).
Pre-heating the joint properly can serve to do both (good).
>My advisor is concerned that heat treating with a torch
>is unreliable and that uneven cooling of the joint is a big problem.
Yes it is, but Think about what kind of cooling occurs immediately
after welding or brazing.
>Brazing also allows us to accomodate poor fit-up from time to time, but
>that's certainly a secondary issue.
>The brazing alloy we use is a "super high strength alloy", Xuper Alloy
>from a company called Eutectic, I believe. It claims tensile strength
>"up to" about 110,000 psi, but I'm sceptical. What's the difference
>between "up to" and "in reality" turn out to be?
With the variations in fillet thickness and unknown state of stress
after welding, ultimate strength is really not much use. The forces
created by your tires must come through material rated just several
hundred PSI. Weld Ductility, to accomodate any internal situation, is
a much better friend.
>
>I'm a first-year student, but according to other team members, we have
>never broken a weld or a tube on the chassis in the four or so years
>that we've been brazing it together. However, I've been told that the
>judges at our competition criticize our practice of brazing the chassis.
It sounds like they have only read Ron Fournier's welding book, that
condemns brazing Chrome Moly because it "opens up the grain."
I don't question Ron's experience, but I am confused about his and
their conclusion. Please see earlier comments from Reynolds, and the
construction of tube frame race cars from the 60's and 70's, and the
exotic MC frames from Seely and others, and the brazed lugged bicycle
frames.
>
>Any input (or follow-up questions) would be appreciated.
Some times there are several good answers to the same question.
Is it correct to cling to one, and condemn the others?
It is also hard to "try" something when a satisfactory solution
exists. Such experimentation can be a waste of time and money that
could be spent better somewhere else.
BVD
weld performance motorcycles, bicycles, and military components and they
were all welded using the TIG process. The ductility argument is fine as
long as you don't need the strength nor hardness.Of course, we pre-heated,
and post-heated as well. Anyone who tells me that brazing will give you the
same mechanical properties of TIG have been inhaling a little too much
Argon.