I'm making a sculptural table base for a client and trying to decide on
the type of stainless to use. I'm currently mig welding 14 gauge 304 with
Air Liquide Blueshield 8 Ar/CO2 mix and .035 308 wire. I'm spot welding to
avoid distortion which is time consuming but ya gotta do what ya gotta do.
The problems are mainly distortion, work hardening and flap disc glazing.
410 is more easily machined so I'm wondering if that's the way to go or
are the drawbacks worse than my current problems. I understand that it
cracks more easily.
If I stick with 304 how do I avoid the glazing and work hardening that
happens with the flap discs? Different discs? less pressure? more cooling?
Should I have a dedicated liner for the welding gun to avoid
contamination. I mostly weld mild steel.
Any tips would be helpful
On Wed, 30 Mar 2011 08:14:50 +0000, orange4boy wrote:
Distortion can be reduced with less heat input (switch to GTAW could
help), more tacks or fixturing to clamp the parts rigidly in place. Back-
stepping or skip-stepping may also help.
Work hardening is an unavoidable characteristic of all austentic
stainless steels, the solution is to take a deeper cut, possibly clean up
with a grinding wheel first then polish with flap wheel then buffer.
410 is an air hardening martensitic or martensite plus ferrite (depending
on heat treatment) alloy which will develop a hard HAZ (Heat Affected
Zone) with poor toughness no matter how you weld it. I doubt if you will
like working with it or the patina it develops outdoors, but why not give
a bit of it a try?
Attempt only very little material removal with the flap disks, try a
really aggressive abrasive like Borazon.
The level of contamination you might get from a liner used for mild steel
will be negligible.
There are some serious and all too often overlooked issues with welding
304 or any of the "normal" grades of austentic SST; not including the
stabilized varieties like 321 or 347 and affecting the low carbon
varieties denoted with an L suffix such as 304L to a much lesser extent.
The issue is "sensitization" (becomes sensitive to corrosion) due to
carbon precipitating at the grain boundaries between about 750 and 1550
F. Needless to say it is impossible to avoid these temperatures in the
weld HAZ. The carbon at the grain boundaries forms carbides preferably
with chromium, and effectively ties up the chromium next to the grain
boundaries so that the grain boundaries lose corrosion resistance.
Corrosion resistance can be restored only by a solution anneal; heating
to 1950 to 2050 F and quenching with water bath or spray, generally not
practical with finished welded assemblies due to cost and distortion
In extreme cases the sensitized SST will crumble apart into individual
grains and fail within days, often only in the weld HAZ. This requires
exposure to corrosive chemicals, and has been seen in dilute acid piping
service and electrochemical machining systems. In less corrosive
environments failure can take years. I recall a case where 304 bolts
were used to assemble roof trusses for an indoor swimming pool, which
collapsed in less than 2 years due to bolt failure from inter-granular
corrosion; the bolts still looked good with no visible sign of corrosion
other than snapping in half. Another classic case was the 304 and 316
piping and safe-ends in the Westinghouse Mark 1 boiling water reactor
which has been making the news lately for other reasons. The water was
deemed essentially non-corrosive by the designers (very pure and
chemically treated to be non-corrosive). All of this piping developed
major cracks within a few years, with some of the smaller piping failing
completely, and all of it including the safe-ends had to be replaced with
304L and 316L. (The safe-ends are the piping stubs attached to the
reactor vessel, which have a wall thickness about 4 times the thickness
of the attached piping, which is turned down to the same thickness as the
pipe at the pipe butt weld end about a foot out from the reactor wall.)
This fiasco resulted in significant changes to the ASME pressure vessel
code and also the applicable weld inspection standards.
Problems with sensitization can be reduced by reducing heat input and
speeding up cooling, to minimize time in the sensitization temperature
range. The best way to do this is to clamp the parts in a heavy fixture
(suitable for conducting heat away from the sides of the weld, copper
works best) and then GTAW very fast with no filler metal. Done correctly
this can result in an extremely good looking weld with very little
distortion which will polish up easily, and which will have excellent
mechanical properties (fast cooling means fine grain structure with good
strength and toughness) and minimal sensitization. Your thin sheet is
ideally suited to this sort of fixtured autogenous weld.
I have only scratched the surface of the issues with welding 304 SST, and
suggest some reading for a better understanding. The AWS welding
handbook has some good info, as do the ASME Pressure Vessel Code and the
ASM Metals Handbooks although the latter two are not really suited to
beginners and cost an arm and a leg (but worth a look if you know someone
who has them). There are no doubt lots of other good sources but I am a
bit out of date here since I haven't been in the business of designing
welded structures for over a decade now (been welding since '67 and
designed weldments from '80 to '00).
On Fri, 01 Apr 2011 13:57:10 -0700, email@example.com wrote:
Helps with what exactly? Higher chrome content has absolutely no effect
on sensitization in the HAZ, which is almost always the biggest problem
when welding the regular carbon grades of SST. The reasons are that the
chrome cannot diffuse more than a few thousandths of an inch from the
puddle (can't get to the HAZ problem area), and higher chrome has almost
no effect on intergranular carbide precipitation. Low carbon in the weld
is good, all available fillers are low carbon AFIK, but again no effect
on the HAZ.
I recently went searching for an approximately 20 year old issue of the
AWS Welding Journal issue dedicated to SST welding which I remember
saving, initially thinking it was mis-piled but then remembered giving it
to a high school student I gave welding lessons to a couple of years ago,
so I have no idea what issue it was. (I did find the notes from a
lecture in intergranular stress corrosion cracking in SST from a seminar
on nuclear power plant pipe welding I attended in 1983 however. :-))
This lost but not forgotten Welding Journal issue had an excellent
article on selecting filler metal for SST, which addressed the old
misconception that you should use a filler with a higher alloy content
designation than the base metal because some of the alloy content is lost
in the welding process. The author attributed the persistence of this
misconception to the grain of truth behind it; higher alloy content can
help in those rare circumstances where weld edge pitting is a problem,
and some alloy content is lost in welding. But the usual result of
blindly applying this rule of thumb is a weld that costs more than it
should and has less toughness and ductility than it should. In most
cases, where an exact matching filler is available, that filler will
produce the best weld, because it was designed to produce the best weld
with the base metal having the same designation.
In most cases you cannot obtain the matching designation since most
welded SST is 304L and matching filler is not readily available, so the
higher alloy 308L filler is used (being the lowest alloy SST filler in
regular production). But where you can get the match, such as for 316
(L), that will be the best filler, and all published filler
recommendations I have seen agree here.
Note that the matching filler *will not* have the same composition as the
base metal it was designed for. The manufacturer adjusts filler alloy
content to produce the desired composition in the weld deposit, taking
into account alloy loss in the welding process. (This is true of all
welding filler metals, not just those for SST.) In the case of austentic
SST, the desired composition of the weld is not the same as the
composition of the base metal either. Any austentic filler which can be
used to produce a weld compliant with the requirements of the ASME
Pressure Vessel Code needs to produce a weld deposit with at least 5%
ferrite (typically around 6%) for improved toughness. The ferrite
content is magnetic, so the structurally ideal weld deposit will be
slightly ferromagnetic; about 6% as magnetic as regular steel, even
though the base metal is not.
It is true that you should not use a filler with a lower alloy content
than the base metal. Where the lower alloy filler will work well, then
so would a cheaper lower alloy base metal.
Regular grades of SST should be welded with the closest matching low
carbon filler (I don't think anyone makes regular carbon content filler),
not a stabilized filler (347) which produces a lower toughness weld, but
regular grades of SST once welded are highly susceptible to corrosion in
the HAZ unless solution annealed, and are only suitable for use indoors
in benign environments where condensation is unlikely to occur.
Low carbon grades should be welded with a match or close match and not a
stabilized filler for the same reason as above. Low carbon grades retain
corrosion resistance after welding without post-weld heat treatment and
are suitable for use outdoors or where condensation may occur, as
prolonged elevated temperatures in the sensitization range are not
expected (not suited for high temperature operation).
High temperature use, in the sensitizing temp range, require a stabilized
grade of SST (321, 347 or 348) and a stabilized filler, normally 347
because 321 and 348 are stabilized with titanium which is prone to
excessive loss in the welding process. The stabilizing agents (titanium,
columbium, or tantalum (subsituted partly or completely for columbium)
completely tie up the carbon for the equivalent of a zero carbon alloy
for corrosion resistance (completely immune to sensitization). The price
paid is higher cost and less toughness and ductility and greater risk of
cracking during welding.
Surface rust can also occur in the HAZ also if not completely protected
from oxidation during welding, but this is not a severe structural
problem like intergranular corrosion (which usually shows no visible
signs until the grains start falling out or the part breaks). This is
easily prevented by mechanical cleaning with abrasives or clean SST wire
brush or pickling with 10 to 20% nitric acid.
Proper welding technique is also required; lowest current which assures
complete fusion and the fastest straight line motion which leaves a well
fused flat bead (stringer bead for stick welding with absolutely no
weave, just like welding high strength steels with low hydrogen rod.)
Additional complexities arise for heavy sections.
I poked around a bit for web general info on welding SST with little
luck. Lincoln used to publish a "weldirectory of stainless steel" with
good basic info, but apparently their marketing department took over the
literature department so that their current literature does not admit to
the existence of anything they do not make, unlike the old (Nov '86 on my
copy) weldirectory which listed the best filler even if they did not make
it - around half of the recommended fillers have a "not manufactured by
Lincoln" note where the current literature provides no info. (It was all
swiped from the AWS Welding Handbook anyhow.)
On Sun, 03 Apr 2011 04:07:11 -0700, firstname.lastname@example.org wrote:
Do you have an example of one of these tables I might be able to find and
Note that I was only talking about the austentic stainless steels (300
series, but also applies to 200 series where the closest 300 series
filler is normally used). The 400 series stainless steels are
different, while most often also welded with 300 series fillers to reduce
risk of cracking, a higher chrome in the filler is generally recommended
for the difficult to weld 400 series, as well as for welding SST to plain
steel, to counter dilution effects.
Also I should have mentioned that loss of alloy content is not a
significant issue for inert gas shielded process or submerged arc, only
for SMAW (stick), so titanium stabilized filler is used for GTAW, GMAW
and submerged arc welding. Most of my SST welding has been SMAW, so I
tend to think of the rules for that first.
Is there any way to hide your joints?
If you could join it from the back side of the metal I would switch to
MIG brazing using silicon bronze wire, and argon/helium shielding gas.
The reduced heat of the brazing eliminates most of the distortion and
sugaring on the backside of the metal.
I have used MIG brazing to assemble hundreds of feet of stainless
Other than that I can't see any way to make MIG welding of stainless
sheet less painful.
It really is a bad combination.
On Wed, 30 Mar 2011 08:14:50 +0000, orange4boy wrote:
I missed this before; don't use a gas mix with CO2 on stainless steel, it
will increase the carbon content of the weld deposit. Straight argon
works, adding 1 or 2% oxygen increases arc stability and improves weld
bead smoothness. I strongly recommend switching to the argon oxygen mix,
whichever your supplier stocks (1 or 2%).
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