Metals (Steel & SSs) change color with enough heat and when they cool rapidly the color stays near the highest temp that area reached. There is also the fact that many SS are poor conductors of heat which makes the color band travel slower.
It would seem what you are seeing are welds that had little or no preheat and the color bands show the gradent of the heat traveling out ward from the puddle.
There are charts available (Pipe fitters book is one) that show the color of different metals at different temps.
The closer they are to the original color of the base metal, the less oxidation occured while cooling. I am basing this on experience with various thicknesses, gas cup dias and welding parameters. A pretty blue weld has oxidized more than a straw colered one.
If you have a machine with remote current/contactor control, set your post flow to max or leave the valve on the torch open, run a bead and when you get to the end slope the current down and out but do not move the torch. Keep it there for a minute or so.
Do the same thing on another bead on similar thickness and when you get to the end move the torch away as soon as the arc is broken.
There should be a difference .
Have a good day
Gerald Austin
Dale,
Regarding your excellent question about coloration of stainless steel after GTA Welding. The short answer is that the colors you see are oxides, which form on the surface of the stainless steel when atmosphere or surface contaminants come into contact with it at elevated temperatures.
Practically what does this mean? We must consider two different points at witch contamination/oxides can color stainless and the performance risks of each.
First and most critical is contamination of the shielding gas over the molten pool. Contrary to popular opinion and shop legend, the amount of contaminant it takes to cause marked loss in performance is NOT necessarily enough to cause discoloration of the weld face. This means that if your torch connections are somehow sucking oxygen, your cover gas is insufficient or moisture has entered your system from dew or a leak in your water cooled torch system, contaminants may be present when the weld metal is most vulnerable <soluble> and your project could be at risk. The point of this lengthy bit of prose is that a weld can look perfect and even Xray perfect yet have less than ideal mechanical properties. The amount of contamination it takes to foul a weld may not cause a color change. (This fact is deadly true with Titanium) By the way, this same risk is present when surface contaminants such as Lead; sulfur, oils and carbon are not removed during pre-weld surface prep operations. Also remember the old stick welding rule of thumb applies-Moisture = hydrogen=embrittlement.
Second. Oxide color caused by hot metal exposed to atmosphere after the trailing gas envelope has expired. This is the more common item and the performance requirements of the part will dictate what color if any is acceptable. Here the weld metal has solidified and as the welding torch progresses forward the argon coverage moves as well, now if the metal is still too hot, oxides will form on the weld face taking the form of discoloration. Most procedures require that oxides are to be brushed off for corrosion prevention and appearance. Also mechanical removal of oxides are generally required if multi pass welds are being produced. Welding multiple passes over the oxides can lead to embrittlement or lack of fusion at the toe of the weld causing "wagon track" or other linear indications.
Stainless steels and super alloys come in literally hundreds of flavors; each different alloy will react in its own fashion, some are virtually impervious and remain tough under the worst of conditions (347), other alloy formulas are far more sensitive (Inco718).
So what can we do? We can pay attention to our specifications and go the extra mile to actually understand why a given specification is requiring specific post weld appearance. We can also attend to process control, especially with manual GTA welding- Operator inputs (Craftsmanship) can make all the difference. Torch angle, cup walking technique with pipe, custom gas trailers as well as measures taken to baffle external breezes from the hot zone will all add to consistent integrity in output. Parent material cannot be too clean, your electrode cannot be too clean, the extra effort to employ the optimum cups or gas lenses can make a difference.
To further editorialize; We all have experienced ourselves or heard tales of welders hanging off scaffolds, hooked up to an R2D2 and scratch starting with a tungsten that has not been sharpened in 6 months and a chipped cup with a mirror in a rainstorm. Or *special* field welding techniques for flammable containers. If only we had an equal passion to know the best way to do things and build that knowledge into habit. Being versatile and making the best of a poor situation is the mark of a true pro. None the less, that same pro must enter the situation knowing the absolute best way to do the job.
More specific source material on stainless steels can be gotten from the "ASM Handbook" Vol. 6 or
AWS "Welding Handbook" Volume 3 Materials and applications part 1 Eighth Edition.
My fellow eggheads who may be interested in the colors in Titanium might enjoy A full report entitled "Development of Titanium Weld Color
Inspection Tool" it can be found in the March 2000 issue of The Welding Journal, Vol. 79, No. 3 (Pg.35-38). This article is a real eye opener for many welders. EWI also has good stuff on their website.