Adam is more than likely welding some high purity or ultra-high purity lines that require very stringent controls in the amount of sulphur and/or manganese content in order to prevent such problems as well as avoiding certain discontinuities that may result from having higher than recommended levels of these elements in both the base/parent metal as well as the filler material and more than likely with 308, there's too much of a percentage of either or both of those elements I mentioned previously... Remember this usually occurs when one is GTA welding via the use of orbital equipment and usually in high purity or ultra-high purity applications. ;)

He may also be experiencing a problem with the ppm's (Parts Per Millions of Oxygen atoms in the shielding or purging gas) of Oxygen in either the shielding or purging gas being too high which can also cause some undesirable effects in both the arc consistencey as well as the desired shape of the weld pool and the surface conditions of the face of the weld also when in combination of having higher than recommended levels of both sulphur and manganese as well. However, the only way to find out is by process of elimination and as he noted before in his initial post, I believe that Adam has found the cause of the problem he was previously experiencing and no longer faces this issue.

The bottom line is that one needs to read the article I posted in the previous post before this one in order to get a better sense of what I am alluding to in this one. :) :) :)

Here is just an example from arc Machines how the manufacturing process of the materials used alone can make a significant difference in the quality of the weld deposit...

Metallurgical Aspects

Sulfur. Research has shown that elements such as sulfur and oxygen, which cause the temperature coefficient of surface tension to be positive, result in a weld puddle in which heat is transferred from the perimeter inward and downward with good penetration of the weld bead.

Removing sulfur and oxygen, either by refining or by the presence of elements that combine with sulfur or oxygen, such as aluminum, has an opposite effect on penetration. In the latter case, the temperature coefficient of surface tension becomes negative, producing a wide, shallow weld with poor penetration and a tendency toward concavity (see Figure 2). Other elements, including manganese and silicon, have slight effects on penetration, but sulfur has by far the greatest effect.

However, sulfur in stainless steel combines with manganese to form nonmetallic inclusions called manganese sulfide (MnS) "stringers." When tubing is passivated or electropolished, the stringers leave pits in the 0.25- to 1.0-micron size range. The tiny pits show up on SEMs (scanning electron micrographs) used to screen tubing samples for surface finish qualification. Since pitted surfaces are undesirable for high-purity applications and are typically the first places to show evidence of corrosion, tubing manufacturers and distributors have rallied to drive down the sulfur content of 316L tubing.

The American Society for Testing & Materials (ASTM) has recently added a supplement for Pharmaceutical Quality Tubing to the ASTM A270 specification for Seamless and Welded Austenitic Stainless Steel Sanitary Tubing. The supplement (S2) limits the sulfur content of this grade to 0.005 to 0.017 percent. These values allow for ease of welding with lower MnS inclusions than would be found at the higher sulfur values of 316L.

With moderate to high sulfur content, type 316L is easier to machine than the low-sulfur materials, so it is favored by some fitting manufacturers. Thus, engineers, contractors, and welding personnel must take care in ordering tubing and fittings and record and track material heat numbers during fabrication to avoid costly problems.

VIM/VAR and EBR Materials: 316L bar stock made by the vacuum induction melted plus vacuum arc remelted (VIM plus VAR) processes that contains a very low level of sulfide and other inclusions is now available. This "clean" material, called 316L-SCQ™, is intended for use for ultraclean gas supply components such as valves, regulators, fittings, glands, gaskets, pipe, and tubing. The base metal has a finer, more uniform grain structure than conventional type 316L, and orbital welds on this material have a much smoother appearance.

Stainless steel produced by the electron beam refining (EBR) process has been used experimentally for special ultra-high-purity (UHP) semiconductor applications. This EBR process uses entirely virgin materials in the melt, producing an unusually clean material. Manganese and other trace elements are reduced to very low levels, resulting in reduced stringers and improved corrosion resistance.

This material also has less of a tendency to discolor from oxidation during welding. The blue "halo" that typically appears on either side of a weld on some heats of material does not appear on properly purged EBR material.The surface of the EBR weld bead is significantly smoother than welds on even VIM/VAR material. The dendritic crystallization structure of welds on typical argon oxygen decarburization (AOD) and VIM/VAR materials form a rough surface with minute projections seen at high SEM magnifications.

One recurring problem with conventional type 316 is the tendency for some heats to form slag islands or weld dross on the OD or ID weld bead. A change in shielding gas or weld parameters may reduce this problem, but it is difficult to eliminate entirely. Slag typically contains silicon and compounds of calcium and aluminum, which are added to standard melts to remove impurities. The EBR melt is very low in impurities, so additions to remove them are unnecessary, and the slagging problem is nonexistent.

Stainless steel's lack of consistent weldability from heat to heat has kept orbital GTAW from becoming a fully automatic process. Weld programs for each tubing diameter and wall thickness are entered into the power supply memory. A weld program or schedule will produce consistent uniform welds on a particular batch of tubing, but if tubing of a different heat number is introduced, some adjustment of amperage may be required, and parameter verification through test coupons is advisable.

Controlling the chemical composition of 316L materials with advanced refining technologies might eventually minimize heat-to-heat variations in penetration and allow orbital GTAW of tube to become a nearly automatic process. This will save the production time currently spent on optimizing weld programs for individual material heats.

Welding and fabricating tubing may result in a loss of corrosion resistance relative to the unwelded base metal. The HAZ of welds has been implicated in the formation of rouge, a rust-like film containing the products of corrosion, in pharmaceutical water systems. Contamination of stainless steel tubing, particularly with carbon, carbon steel, or chlorides, can severely affect corrosion resistance. Heat tint oxidation produced during welding also severely reduces the corrosion resistance of stainless steel, with the loss of corrosion resistance proportional to the oxygen concentration of the purge gas.

After fabrication, pharmaceutical piping systems are typically passivated with nitric acid or a mixed chelant solution before being placed into service. Passivation, however, is a relatively mild treatment and only affects the outer surface layer to a depth of 30 to 50 Å. If the heat tint extends below this level, and it has been shown to extend to a depth of about 1,000 Å in severe cases, then passivation will not be able to remove the heat tint. For passivation to be effective, the surface must be clean.

Both welding and fabrication must be done carefully to limit damage to levels that can be corrected by passivation. Recent studies have shown that welded 316L tube samples purged at oxygen levels of 108 parts per million (ppm), 8 ppm, and less than 0.1 ppm show visible effects of corrosion in proportion to the levels of oxygen in the argon purger.
The amount of heat tint and corrosion was slightly greater on mechanically polished tubing than on electropolished tubing of the same heat number. This is most likely due to the larger surface area on mechanically polished tubing, which has a greater affinity for oxides.

Orbitally welded 316L tube samples were compared to previously studied unwelded tube samples and found to have comparable pitting potentials(2). Samples passivated with either mixed chelant solutions or with nitric acid were found to have significantly higher pitting potentials than unpassivated samples, whether or not they had been welded (see Figure 1 and Figure 2). The exception to this was the group of mechanically polished tubes welded with the lowest-purity purge gas (108 ppm oxygen). These samples had active polarization curves, indicating a severe loss of corrosion resistance. This severe effect was not observed on electropolished tubes purged with the low-purity gas.

The most heavily corroded parts of the welds subjected to corrosion testing were the areas of overlap and downslope. Since these areas were welded twice, it suggests that the additional heat input caused a localized reduction in corrosion resistance and indicates that careful control of the heat input during welding is important for corrosion control.

Manual Tacking. For some applications, tubing is manually tacked in place with a hand-held GTAW torch to align the tubes or fittings for orbital welding. If this is done, the inside diameter (ID) of the weld joint must be purged during the tacking process. An orbital arc may deflect around an unpurged tack and cause a lack-of-penetration defect, and oxidized tacks may become corrosion initiation sites. Look! read this article:

http://www.arcmachines.com/appPages/fabri02.html

Then read this... Material weldability. Different heats of stainless steel tubing can make it difficult to weld by automatic fusion welding techniques. ASTM specifications for each type of stainless steel such as 304, 316, 304L and 316L may vary in concentrations of alloying elements such as chromium, nickel, molybdenum, copper, sulfur, etc., resulting in no two heats of 304 or 316 being exactly alike. Variations in alloying elements could dramatically affect the weld bead appearance and depth of weld penetration of two samples of 316L stainless steel tubing, both with a 1-in. outer diameter (OD) and wall thickness of 0.083 in., but neither having the same heats as a result of varied alloy concentration.

An unusually wide weld bead relative to the depth of penetration characterizes heats that are low in sulfur content. The large weld pool can be difficult to control and sensitive to gravity. As a result, the weld could become concave on the outside of the tube and lack repeatability. Several irregularities have been observed in the weld bead when sulfur concentrations are above 0.024%. If orbital tube welding is to be employed, it is recommended that sulfur content not vary by more than 0.010% between tubes being welded together.

Then there is the issue of hwat type of shielding/purging gases to use...

Shielding/Purge gas. Shielding gas can be a critical ingredient in the success of a weld. Shielding gas minimizes porosity in a weld and can, in some cases such as mixed gases, almost eliminate porosity. Also, shielding gas is used to purge possible contaminants from a weld area, and manipulation of purge pressure can be used to support a weld while it is molten. For the sake of economy and simplicity, most welds are developed using only argon for shielding and purging. However, some of the more difficult applications necessitate the use of mixed welding gases to achieve the weld quality and results demanded by the X-34's high-pressure applications.

Mixed gases were used for both shielding and purging. Although mixed welding gases have their problems, the benefits allowed the X-34 program to weld difficult - but necessary- applications that would have otherwise forced program costs up and vehicle delivery schedules to slip. Specific benefits of the mixed welding gases include setup of a reducing atmosphere that effectively eliminates moisture from the welds, resulting in little or no porosity; consumption (burning) of the hydrogen component of the gas mixture during welding, which effectively adds heat energy to the weld; heavier wall tubing that can be welded with less amperage; and a more focused weld, with better direct penetration.

Shielding gas was critical in solving problems when welding difficult applications of very heavy wall tubing/fittings with dissimilar chemistries (i.e., different sulfur contents) and in addressing severe pool shift, but its use created other problems related to mixed purge gases in K-bottles and the obtaining of a consistent mixture ratio during purging. For instance, if the bottles yielded a slightly higher hydrogen ratio during welding, the weld would be hotter than expected and the subsequent weld probably would not meet inspection criteria. Therefore, consistent mixture-ratio delivery became a critical factor that had to be controlled.

The gas supplier suggested that, when using a gas mixture of argon and hydrogen, a K-bottle with a siphon tube that stirs the gas mixture as it siphons through the tube should be used. This would help prevent gas mixture stratification. Another vendor recommended using a lamp on one end of the K-bottle to mildly heat it, creating a convective heat flow inside the bottle to stir the gas mixture.

Orbital Science specified gases to have less than 10 ppm of oxygen and 3 ppm of moisture to minimize porosity in the final weld. The X-34 program required the following welding purge gases: 100% argon, 95/5 (95% argon/5% hydrogen), and 92/8 (92% argon/8% hydrogen). The reason 95/5 was chosen was because it is a standard mix and a reasonable starting point. Another particularly difficult application required further weld development, which deter- mined the need for a 92/8 mixture. Once the desired results were obtained, the application was discontinued, but it is believed further potential exists in higher hydrogen mixtures. Practical limits of this approach are in the 12 to 15% hydrogen range. Here's the complete article:

http://www.aws.org/w/a/wj/2001/03/0039/index.html

Finally there's the potential issue of electrodes...

Electrodes. Electrode geometry has always been an important orbital welding parameter because it has such a pronounced effect on weld shape and penetration. The use of properly prepared tungsten helps ensure repeatable welds. The typical geometry is a 22-deg taper with a 0.010 to 0.020 flat tip. A tip without a flat point may create an unstable arc and produce welds that wander from side to side. A flat point allows the arc to come off an edge, thus producing a stable arc. The flat point also has the other advantage of extended tungsten life and, most important, a tungsten-to-work distance not compromised by having a sharp-pointed tungsten tip break or wear back.

The X-34 weld schedules were relatively insensitive to changes in electrode geometry between a wide breadth of weld schedules. The majority of the electrodes have a 20-deg taper with a 0.010- to 0.015-in. flat tip. Electrode material did matter in some cases, depending on joint size (i.e., tube OD, wall thickness, weld head, purge gas, etc.). The program migrated from thoriated to both ceriated and lanthiated tungsten electrodes, which is consistent with what is used in Europe and Japan. Health hazards associated with grinding thoriated tungsten are behind the move to ceriated and lanthiated tungsten. More than half the applications favored ceriated tungsten, and lanthiated worked well when using mixed purge gases.

Now if all of this is accounted for, then the only possibility for problems may originate from this...

Tube preparation. As in all welding, fitup is critical to successfully producing repeatable welds. It is especially critical with orbital welding because specific parameters such as travel speed, welding amperes and arc volts are preset. Any high-low condition or tube ovality problems will have an adverse effect on weld quality. The tubing used for square butt joints must be cut square and the end face machined perpendicular to the tube centerline using a facing tool. A tight butt joint is a critical factor in fitup.

Beveling, or chamfering, the tube ends was not desirable. Material removed by chamfering could result in additional weld joint concavity and cause the wall of the weld joint to thin. All X-34 tubing butt joints used only the base material being welded.

So, in summary, orbital welding involves much more of a focus on many different factors in which all are critical to the successful and consistent production of desirable weld deposits required for high purity and ultra high purity applications, and none of these factors can be taken lightly, or dismissed even more so than in less critical applications... So please re-read the links I posted previously. Like I mentioned before... if you are all still asking questions then you didn't take the time necessary to learn all of the possible causes because there are certainly more than one listed in the article I posted in my previous post. get off your butts and start reading!!! :) :) ;)

Respectfully,
Henry