By Eric Feeney
Date 01-05-2010 00:47
Edited 01-05-2010 02:59
Why does steel have such a violent reaction when reaching its molten state without a shielding gas when welding verses it being in a molten state in a steel mill with no shielding gas. For example, you strike your arc using GTAW and forget to turn your gas on, well the oxidization to the base metal is extreme in a fraction of a second. And in a steel mill the metal is heated and becomes molten, yet although the surface area oxidizes, there is not the same reaction. Is this maybe because the heating of the metal in the welding process is different. Please forgive my ignorance in this area, and would appreciate any insight to this.
Hot steel scales like mad in a steel mill. Visitors do not get close enough to see how much it scales, but some industrial films show it in detail.
I guess I am just trying to understand the difference in the two scenarios. Why would one rapidly scale, and the other rapidly oxidize? They are the same material reaching the same molten state, just in different ways and in VERY different times.
Scale is iron oxide, they are the same thing.
By Eric Feeney
Date 01-05-2010 03:45
Edited 01-05-2010 12:04
Thanks
Two things at play here. Iron (and most metals) will oxidize EXTREMELY fast at high temperatures and even more so when liquid.
In a modern furnace a layer of slag is kept on top of the molten load during the entire process and another layer (mill scale) forms during hot rolling. This is a sacrificial coating that protects the molten metal. Sometimes differing atmospheres, or vacuums are used to further change the metal composition such as in vacuum remelted steel used to prevent bubbles from being trapped inside the metal.
Now when we are welding the conditions change:
First all the protective coatings and large volume in of a furnace are gone. most metals are welded in a bare condition. In the simplest forms stick electrode fluxes mimic the slag and mill scale found in a furnace and do the same thing. Protect the still molten and hot metal from oxidation. since we are welding on a bare metal even a small amount of oxygen will cause it to negatively oxidize and create problems.
The second issue is that in almost all forms of arc welding an electric arc must gap through a working gas. Different shielding gas properties GREATLY affect the properties of the electrical arc. As it so happens to be, among other things oxygen has an increased "digging" affect compared to other gasses. To top it off when it oxidizes the metal, the metal oxides have a higher electron work potential, meaning they increase the resistance which in turn increases energy input. Which does nasty things.
Long story short the conditions of relatively clean metal being welded on an exposed surface are much different than the middle of a Crucible carrying a 30 ton charge beneath a blanket of flux 12" thick.
Eric, something to consider is the speed of the arc. Like a sonic boom it draws air (oxy) in behind it and in to the puddle.
very true, the electric arc has a force that actually penetrates the steel. Also the arc creates a plasma pressure at the surface that can be known to literally inject gas into the molten metal. (co2 entrainment)
Thank you Metarinka, I beleive your post answered my question precisely on the level that I was trying to ask the question. And the "sonic boom" effect of the arc is another interesting point. This is why I love the AWS forum. Many thanks
cheers, glad I could be of some help.
Not sure about your level precisely, Maybe I should have went in to more depth and explained how the arc moves through the air so fast it leaves a void or for another level precisely a presure differential or for even higher level precisely a Delta V. That said void is very quickly filled by what ever source is available (in your case air containing Oxygen).
