304 or 410 for welded 14ga outdoor sculpture

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 Thanks.
--





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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 problems.
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).
Regards, Glen
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My understanding is that using a filler with higher chrome content and lower carbon content than the alloy being welded helps. Any comment?
Dan
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On Fri, 01 Apr 2011 13:57:10 -0700, snipped-for-privacy@krl.org 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.)
http://www.aws.org/w/a/weldinghandbook/indexnav.html
Regards, Glen
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 The author attributed the persistence of this

but

Thanks for the detailed information. I have seen numerous tables that recommend the use of higher chrome alloys for filler.
Dan
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On Sun, 03 Apr 2011 04:07:11 -0700, snipped-for-privacy@krl.org wrote:

Do you have an example of one of these tables I might be able to find and examine?
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.
Regards, Glen
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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 railings.
Other than that I can't see any way to make MIG welding of stainless sheet less painful. It really is a bad combination.
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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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