Welding of stainless steel and the color of the weld!
Posted: Mon Mar 09, 2015 4:18 pm
Well I have to say that I'm very surprised and little disappointed.
I read this guide from Miller's site:
"Figure 7A shows discoloration on a stainless steel weld caused by overheating, which not only affects a material’s color, but degrades its corrosion resistance and mechanical properties as well. Unfortunately, once this error is made, there is nothing that can be done to fix it except for scrapping the part and starting over. To prevent overheating, reduce amperage, slightly increase travel speed or shorten the arc length. If your welding equipment features pulsing capabilities, now is the time to learn how to use them. Pulsing reduces heat input, and it offers excellent control of the weld puddle. Figure 7B shows proper coloration of stainless."
[http://www.millerwelds.com/resources/ar ... e-graphic/]
7A
7B
Now I understand why people in States relies so much on surface color. I would too, if my studies would base only to this knowledge like this. Don't get me wrong. It's always better to get as little color as possible... but telling to toss part due to surface color. That is a very strong statement and I couldn't agree less.
There is absolutely no reason for tossing your part, if you color is something else than pretty pink or like that. Stainless steel is not active metal like Titanium is. Yes corrosion resistance is lowered once stainless is reacted with oxygen in high temperatures. This reaction will make two layers on top of the stainless surface. One very chromium rich layer and underneath that chromium poor layer. This happens due to diffusion of chromium. In other words chromium diffuses faster than other elements. like iron for example.
[Pickling and passivating of stainless steel. Page 8-9. http://www.euro-inox.org/]
[Practical guidelines for the fabrication of high performance austenitic stainless steels. Euro Inox. Page 58. http://www.euro-inox.org/]
Well this layer must be removed either with polishing or pickling for example! Because chromium rich layer is very porous and won't protect stainless steel from rusting. Notice that pickling actually removes material too. This is often misunderstood and that's why there is maximum pickling times for each alloy groups.
[Practical guidelines for the fabrication of high performance austenitic stainless steels. Euro Inox. Page 59. http://www.euro-inox.org/]
This was just a quick 30 min write up, so this won't include more specific metallurgical facts etc. However I hope it will help welders like me to understand what is good weld and what is not.
I read this guide from Miller's site:
"Figure 7A shows discoloration on a stainless steel weld caused by overheating, which not only affects a material’s color, but degrades its corrosion resistance and mechanical properties as well. Unfortunately, once this error is made, there is nothing that can be done to fix it except for scrapping the part and starting over. To prevent overheating, reduce amperage, slightly increase travel speed or shorten the arc length. If your welding equipment features pulsing capabilities, now is the time to learn how to use them. Pulsing reduces heat input, and it offers excellent control of the weld puddle. Figure 7B shows proper coloration of stainless."
[http://www.millerwelds.com/resources/ar ... e-graphic/]
7A
7B
Now I understand why people in States relies so much on surface color. I would too, if my studies would base only to this knowledge like this. Don't get me wrong. It's always better to get as little color as possible... but telling to toss part due to surface color. That is a very strong statement and I couldn't agree less.
There is absolutely no reason for tossing your part, if you color is something else than pretty pink or like that. Stainless steel is not active metal like Titanium is. Yes corrosion resistance is lowered once stainless is reacted with oxygen in high temperatures. This reaction will make two layers on top of the stainless surface. One very chromium rich layer and underneath that chromium poor layer. This happens due to diffusion of chromium. In other words chromium diffuses faster than other elements. like iron for example.
[Pickling and passivating of stainless steel. Page 8-9. http://www.euro-inox.org/]
- Weld.png (17.9 KiB) Viewed 812 times
Well this layer must be removed either with polishing or pickling for example! Because chromium rich layer is very porous and won't protect stainless steel from rusting. Notice that pickling actually removes material too. This is often misunderstood and that's why there is maximum pickling times for each alloy groups.
- weld 2.png (259.04 KiB) Viewed 812 times
This was just a quick 30 min write up, so this won't include more specific metallurgical facts etc. However I hope it will help welders like me to understand what is good weld and what is not.