By -
Date 07-30-2005 12:16
When you weld stainless steel, regardless what welding process you are using, you burn away the surface chromium and leave a chromium-depleted-oxide layer. That is the "color" you see on the weld and heat-affected-zone. This oxide layer must be removed so that the oxidation in the atmosphere can react with the surface of the stainless steel to re-form the chromium layer that is necessary for maximum corossion protection. Depending on the thickness of the part being welded, it is very possible to have a certain amount of discoloration on the back of the piece. This is perfectly normal. If the part being welded is thin enough, and the heat input is high enough, you can actually form an oxide layer on this back side, too, which is commonly called "sugar". If this is the case, this oxide should be removed also. But, just because you have a certain amount of color on the backside normally does not mean you have depleted the surface chromium layer. Due to the electrical conductivity associated with stainless steel, this discoloration of the backside of thinner metal is not unusual nor detrimental to the integrity of the piece. With all due respect to your inspector, it appears that he/she is not as familiar with the welding of stainless steel as he/she should be.
Chuck
are you purging the back of the pipes. if not the cr and c will burn out in a reaction with the oxygen in the atmosphere, usually leaving the black coking. this is not acceptable and can only be overcome by purging, the easiest way is to use a purge monitor to measure the oxygen in the backing gas. the oxygen level should be below 0.1 percent (1000 ppm) to get acceptable results. if you are in the US then con industries will be able to advise on this if not let me know and i can advise a source in your area for advice.
If these are pipe supports, it will probably pretty hard to purge them. Whether one is purging the pipe or not, the heat of the welding process will burn away the chromium on the surface of the area being welded. A purge, if possible, will only affect the inside of the part being welded, reducing the possibility of the oxide layer on the inside. I believe the chromium is much more affected than ony other other element, including carbon.
Chuck
The question was about the back, where if un-protected there will be a reaction between the oxygen in the atmosphere, chromium and carbon. The black crusting is the fromation of Chromium Carbides, which means that the material has been degraded. Little point is using stainless if you are going to let the chromium deplete.
By -
Date 08-05-2005 19:21
Look 3 answers back at my response on 30-Jul-05 at 08:16.
Chuck
Respectfully, I do not think that "sugar" can technically be called Chromium Carbides. This crusting is a chromiume oxide that is formed when the molten weld metal is exposed, unprotected, to the oxidation of the air. I believe the chromium carbides will be found below the surface surrounding the grain boundaries of the chromium molecules in this instance. I don't think the affinity for carbon in this case will allow the carbides to form sugar, the non-metallic oxide layer from the weld heat of the unprotected molten metal.
Chuck
If you are calling the black formations on the back the material 'sugar' then yes chromium carbides are produces and this has a major effect on the weld. It will remove the chromum in a reaction with the carbon and oxygen. This means that the stainless will lose some of it's 'stainlessness'. There are some companies who specialise in post weld cleaning and treatment of stainless steel. These guys reasearch what most people consider to be acceptable, but you need to look at what you are doing with the component ofter welding.
I am quite aware of what sugar is, and what it consists of. Please do not take this wrong, but I'm a Welding Engineer for Avesta Welding Products, and we are the only stainless steel manufacturer in the world that makes a pickling paste. There are "chemical" manufacturers that make a form of pickling, but we are the only stainless steel manufacturer that does this. Also, we have the world's largest Pickling and Cleaning research facility at Malmo, Sweden. I disagree with you about the "sugar" on the back of a weld as being Chromium Carbides. It is a chromium oxide that is definitely detrimental to the corrosion protection and strength of the weld. I would be more than happy to e-mail you papers that have been published and presented at NACE (National Association of Corrosion Engineers) and at the AWS Show in Dallas, Texas this past April explaining about exactly what we're discussing here. Apparently a lot of people have a wrong, or misinformed, preception of exactly what happens when welding stainless steel and the oxide layer that is formed when the unprotected molten weld metal meets the oxidation in the air. If you would like me to share some of this information, you may contact me at chuck.meadows@outokumpu.com and I will gladly send you a couple of papers that you might find interesting.
Chuck
I would be please to see those papers, thanks
I'll email you with details.
