What is happening when scale is formed?

tinman207

tinman207

Joined
Jul 27, 2012
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Could one of the more experienced guys give a detailed scientific rundown of scale formation? I was messing around actually trying to create more scale for a non-knife artist piece of metal I am working with. There is a lot of info on preventing it, but not much on intentionally causing it. I figured an understanding of the chemistry behind it would lend to understanding how I can control it a bit.
 
Scale is iron oxide. Oxygen reacts with iron to form iron oxide. At room temp it is fairly slow in comparison to at highly elevated temps. To promote scale formation heat the metal to a red ,or hotter, heat and let cool in still air.
 
Scale is "Just" Oxidation of the Iron in the steel.

Oxidation takes place much faster at elevated temperatures (at school, one of the first things they taught us in Chemistry was that, as a rule-of thumb, the rate of a chemical reaction doubles with every 10 degC (18 degF) rise in temperature).

Oxidation is a reaction between the Iron and the Oxygen in the atmosphere of the forge.

It therefore needs a combination of high temperature and an Oxygen-rich atmosphere.

"We" usually do not want Oxidation, so arrange to have a forge atmosphere with very little Oxygen in it when heating steel. We usually run a gas-rich forge atmosphere inside the forge (if we are using a gas forge) with the excess gas being burnt as the "dragons breath" when it reaches the atmosphere outside the forge.

This gives us the high temperature needed to make scale, but without the Oxygen present little scale forms in the forge. As we take the hot steel out and it meets air (21% Oxygen), it usually starts scaling goodstyle.

If you really want to promote scaling, run your forge leaner, so that there is Oxgen present inside it and no dragons breath.
 
Makes sense.....so could the application of some oxide, like an iron oxide paste prior to heat treat accelerate the scaling during heat treating, and provide some control over where it develops more?
 
You'd really need a source of free Oxygen. Adding Iron Oxide is basically just adding pre-formed scale.

The best way is almost certainly to run the forge leaner (Oxidizing). If you want to persuade it to scale more in some places than others, using anti-scale-paint (the stuff sold for Heat-Treating Carbon Steels) on the areas you want to limit the scale would seem to make most sense; it's well-developed, proven, off-the-shelf technology. Admittedly you'd be using it in a way that was probably not envisaged by the inventor/developer, but it seems a good place to start.
 
also moisture is a factor which promotes scaling; if the air supply which mix with the fuel in the forge is high moisture level you get more scaling, given the other conditions.
If you don't have a forge for your project you will need a torce
 
What is the difference between oxidation and decarburization? And something I've had trouble comprehending; if decarburization is the loss of carbon does it ever come back? Can it actually bring down the carbon content in the steel you buy, say making 1095 lower carbon content to 1080 something?
 
Oxidation is the adding/combining with oxygen. In the process on steel, it may make iron oxide as scale or rust.

If the steel is hot enough, carbon dioxide forms when the hot carbon in the steel combines with the hot oxygen. If CO2 is made, the carbon is lost forever. This is usually only a very thin layer, called de-carb. The steel under this few thousandths of an inch is fine.
 
Both Carbon and Iron have a strong affinity for Oxygen at high temperatures. Carbon's affinity for Oxygen is slightly greater than Iron's, which is why we can reduce Iron Oxide ores to Iron with Carbon; the basis of the iron making process throughout history.

Where there is only a little little Oxygen to be had, the Carbon tends to react with it, producing Carbon Dioxide gas which leaves the surface of the steel with a reduced Carbon content. At the high temperatures involved, Carbon diffusion occurs and Carbon tends to move from areas of high Carbon concentration to areas of low Carbon concentration. The diffusion is very temperature dependent and at typical HT temperatures, the Carbon gradient is quite steep; decarb tends to be near the surface.

At welding temperatures diffusion is much faster and the Carbon gradient much shallower. Decarb can be significant throughout the bar. Taking 1095 down to 1080-something is entirely possible and I am sure it happens to many beginners; working hot, working slowly and using a poorly-adjusted forge.

In general, it is probably fair to say that decarburization is permanent; there are methods of packing iron in a material with very high Carbon concentration, heating it and getting the Carbon to diffuse into the iron to make steel, but these are really part of the steelmaking process and tend to occur long before your average knifemaker starts getting involved. Look up the "cementation" process if you are interested.
 
Stacy, Tim thanks for the great answers, makes much more sense to me now.

When forge welding what can be done to reduce or eliminate decarburization? My novice opinion wants to say that flux helps with this by creating a layer that prevents oxygen from reaching the steel similar to anti scaling powders/liquids. Any truth to that statement? And is there anything outside flux and controlling the oxygen levels in the forge?
 
Your idea is correct. The flux is borax....sodium tetraborate. The water and oxygen are driven off in the high heat leaving behind molten boron-soda glass. This protects the steel from oxygen which would prevent the weld as well as remove carbon.
 
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