Oxidation.  It is the chemical reaction of oxyegen in the atmosphere, with the stainless base metal, due to intense heat.  It is caused on stainless if you do not back purge with Argon.

Charles

Hello Larry D Morey, to expand just a little bit further on the information you have already received. If you are doing any sort of full-penetration welds on stainless steel alloys or welds that have a speed progression that allows the stainless to remain above it's reactive temperature after the shielding gas has dissipated as a result of weld progression. You can experience what you have referred to as "sugaring"(oxidation is really a better description) unless you provide some sort of protection for the backside of the weld joint. In many cases this may mean using a backing or purge gas, argon would be most common as opposed to using an argon/helium combination or a straight helium backing, the argon is likely the most economical, the main idea is to provide an inert shield to prevent the oxidation from taking place. In other instances there are fluxing agents that can be applied to the backside of these weld joints that will basically provide the same sort of protection, in this case however, these fluxing agents will leave residues so you need to consider whether this would be allowable for the particular application. In yet other instances you can use backing fixtures that "may" be able to contain and provide enough shielding gas that has been introduced from the front side of the weld to also serve to protect the backside, this can be a hit or miss proposition depending on a number of variables( joint type, weld progression, shape of fixturing bars, flow rates, and possibly a few other items). There are really a number of variables that can influence oxidation of stainless steel due to welding. Temperature has everything to do with this and prevention of exposure to atmosphere while the stainless is above it's oxidation temperature also has a direct effect.
     I would also make an additional comment here, this could be argueable amongst others, but here goes, surface oxidation can be dealt with by use of means such as brushing, grinding, polishing, chemical passivation, and probably a few others that I can't think of or don't know about. I would generally say that the "sugaring" that you have made mention of typically would be considered as a condition that affects the stainless to a depth that it can't readily be dealt with by using the methods I have mentioned above. In all instances you should consider carefully the use and function of the material/weldment or whatever it is a part of and use the appropriate procedures to ensure your process meets the requirements of these items. My $.02 Best regards, aevald