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EricJergensen

Burnt Steel - what's happening metallurgically?

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What is happening to the metal? Some kind of lattice change that only melting resolves? Can it become "burnt" in an inert atmosphere (is it oxidizing)? Does pure iron do it (or does it somehow involve the carbon)?

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Burning is oxidizing but fast, rusting is the same process including producing heat. Burning is just faster and a lot hotter. Yes, both iron and carbon burn. When you see a spark shower that's generally mostly iron but the decarburizing the bladesmith guys are always talking about is the carbon burning out of the surface of the steel.

 

No, things can't burn in an inert atmosphere.

 

I probably should've let one of the blade guys answer but I'm feeling impulsive. <wink>

 

Frosty The Lucky.

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Another problem with overheating steel is runaway grain growth. IIRC there was also something bad happening at the grain boundaries.

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What happens at crystal boundaries has to do with how force is conducted, (transmitted?) Any disruption in the smooth or uniform structure, be it molecular or structural focuses the force. A good live example is to have someone stand on an aluminum can on end. The person has to apply the pressure of his/er weight evenly on the can and step up slowly. I figure under 150lbs is plenty. Anyway with the person standing on it the can supports him/er just fine. Now take a sharp pencil and touch the side of the can, about as hard as you'd press to write and the can will collapse flat.

 

The tiny flex/dent is a stress riser and all the weight is conducted to that point and far exceeds the structural strength of the can and it collapses. This is also exactly how they implode large buildings and why in spite of Movies a sky scraper can NOT fall over, once it starts to go it will go straight down. Maybe more messy than a designed demolition but it can't tip over.

 

Back to steel and grain growth, (crystal growth) Steel is a mechanical structure on a molecular level and any defects in a uniformly smooth material provides stress risers, just like a forged square inside corner or nick. The larger the crystals the larger the risers and the more likely one will become a failure initiation point and that's it.

 

The smaller the crystal structure the more flexible it can be so stresses can be carried around or the steel can just yield. The less carbon in steel the less it can crystallize, (harden) the less it'll work harden and the easier it can yield.

 

When flexion exceeds an alloy's rebound strength it starts to yield even though it may return to it's initial shape. What happens internally is the molecules are moved and not just flexed but physically slid around, making them find new arrangements with each other The easiest (lowest energy) way for iron/carbon molecules to arrange themselves is on their valence bond sites and they arrange themselves in orderly sets. Crystals. Crystals have stronger bonds to other crystals and these are the boundary sites and are far less flexible; brittle.

 

Geeze I got windy on that one. <sigh>

 

Frosty The Lucky.

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When I learned oxy/acet cutting there was a lot of talk about the kindling point or auto-ignition temperature. this is the temperature of metal where oxygen will directly bond and oxidize. the kindling point is not fixed in stone it varies depending on the surface area exposed to oxygen (dust and powder will kindle at a very low temperature).

 

There is a couple of things happening in a forge.

firstly burning is exothermic, it releases energy, this extra heat speeds up the burning process.

Secondly the metal surface increases in roughness exposing more molecules to oxygen and lowering the auto-ignition temperature further.

thirdly the large amounts of iron oxide starts to act as a flux and assists other contaminants in the fuel to initiate further side reactions forming things like metal salts.

 

As the burning metal is pulled from the forge there is a few moments where the cooling of the now ruined masterpiece is perfectly balanced by the increased oxygen levels and just to spite us, the burn continues on for a few seconds longer.

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Yeah, I've learned from a practical perspective: when I can see I've overheated the metal, stop the blast and leave it in the fire to cool down. Minimizes the O2 contact and how much grinding I'm gonna be in for ;-).

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the thing that makes burning steel so hard to deal with is the stickyness of the molten oxide, scale formed a lower temperatures will flake off in very thin layers before oxidizing the next layer underneath letting gasses like co2 escape. this stuff is like baking bread or hot syrup and bicarb, the gasses are trapped by the molten layer and form part of the expanding hardening mass.

 

The other problem is chemists like nice neat little balanced equations, while they are focusing on the molecule interaction between iron and oxygen there are hundreds of chemical reactions happening in the burning fuel that are ignored for the sake of simplicity because in theory they are not part of the reaction, I am not entirely convinced that is correct 100% of the time.

 

I have some leftover argon in the shop at the moment, I was going to try blowing some gas over a piece of burning steel while holding it in a bucket to see if it stopped the reaction dead but I keep forgetting.

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You're right on one account Yahoo, NOTHING is always right. Argon will stop oxidization by displacing oxy in the atmosphere but it won't undo it unless you provide carbon to reduce it.

 

Frosty The Lucky.

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Years ago, the Scientfic American magazine had an article about welding ferrous metals in outer space. No heat; it's cold up there. The specimens had flat, highly machine-polished faces for contact. I don't remember the mechanics of the operation. I suspect the pieces were kept in a vacuum until they were placed together. The result was perfect cohesion, not adhesion. The pieces welded because of the lack of gaseous or dirty contaminants.     

 

On earth, we like to breathe oxygen, but it is a bugaboo regarding heated forgings and welding. We must remember that when the steel is heated to burning range, the oxygen "attacks" it in an uncontrolled manner. The metal is not of the right composition for melting nor is it fluxed. Cast iron has a different composition than steel and is melted in a controlled, protected manner.

 

In good ole boy terminology, an old horseshoer friend commented on my burnt shoe, "It looks like the rats have been chewin' on it."

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There are a few mis-conceptions or misundertandings in this post that I'd like to clear up.

1. "Burning" Steel in general means that you have begun to get localized or grain boundary melting without melting the entire part. Both oxidation and carbon depletion of the surface (decarburization) happen at much lower temperature than melting and so they are NOT considered burning in the same way that over heating to a sparkling white heat is. Certainly both oxidation and decarburization are happening during burning and as temperatures increase the rate of these reactions also increases, but burning is separate and distinct phenomenon.

2. Buildings, like cans, can be made to topple over. What is required is careful positioning of the explosives and a removal of a portion of the structure on the side in which the building is to fall. It's just like cutting down a tree. Notch one side, then cut from the back. I'm not sure what this has to do with crystal structures though.

3. Iron and steel are always crystalline unless except when you have a liquid or a metallic glass. Carbon does not make steel "crystalize" . What it does do is allow you to transform from austenite to martensite given sufficiently rapid cooling. Both austentite and martensite are crystals, just different kinds of crystals. Carbon in the microstructure will form carbides which do pin dislocations (see below) so you do find that the rate of work hardening will be greater as the carbon content increases.

4. Grain size does affect yield strength in metals of the same crystal structure and smaller grains do result in higher yield strength. The reason for this is not because small grains provide fewer stress concentrators. It's because grain boundaries provided a barrier to dislocation movement. Since iron and steel are crystalline, the atomic arrangement is by definition organized and repeating within the crystal. When a load or stress is applied to the crystal, that atomic arrangement is disrupted, much like what happens if you have a wrinkle in a rug or bed sheet. That wrinkle moves through the crystal unimpeded until it hits a grain boundary (or something trapped within the grain like a carbide). A dislocation of the crystal is a minute amount of plastic deformation on the atomic level. These dislocations build up at grain boundaries. The more grain boundaries there are (or carbides), the more dislocations can be stopped within, or at the edge, of an individual grain before measurable yield strength of the bulk product has occurred. Once you have plastic deformation, the yield strength has been exceeded, but dislocations continue to be formed and pinned, resulting in a continuing increase in hardness. This is what we commonly call work hardening. Hardness is increasing WITHOUT a change in phase.

5. If you put a load on a piece of metal and it deforms and stays deformed you have exceeded its yield strength. If you put a load on a part and it deforms but springs back to its original size and shape you have not exceeded the yield strength. Instead you are using the metal in its elastic region. That's what springs do. Through a combination of microstructure (crystal type, size etc) and form they can be loaded and unloaded over and over again. This type of loading is called fatigue loading, that is there is cyclic deformation that does NOT exceed the yield strength. This particular application is significantly benifitted by having a small grain size.

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patrick, so the problem with "burning" steel is really going to this semi-molten state which upon cooling results in a messy (and heterogenous?) structure?

If yes, this answers a thought I've had: "How is this different from going from totally molten to solid, at some point it spends time at the same temperature anyways"

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In burning, not only are you partially molten, you are doing that in an oxidizing atmosphere so you have rapid oxidation and loss of carbon much deeper into the piece rather than just on the surface  as I described above. When steel is melted intentionally  at a mill or foundry, they using a slag layer to protect the steel from oxidation. Also note that steel is not uniform in composition, even though we look at a heat chemistry and assume that. In reality, the composition of the steel varies from point to point within its volume. This means that the melting temperature is not uniform, but rather occurs over a range of temperature with the regions of highest carbon having the lowest melting temperature. The problem with burning is actually not just on cooling. Since you have rapid oxidation of localized molten areas, you can end up with oxide penetrating into the forging which means the surfaces tend to crack or even fall apart during deformation. In most cases, just hammering the burned steel back together will not really fix the problem, though for some applications that might be good enough.

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