"The Hurdy Gurdy Man" wrote in message
...

(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!

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