Oxidizing Region

[Bully]

I have read that when you place a peice of iron in the fire that you should not angle it down to deep into the fire because that is the "oxidizing region"??? Can anyone explain a little more in relation to this and the adverse effects oxidizing could have on the iron ie Does it effect welding etc. Thanks Bully

[Glenn]

[Dave Budd]

think of it like a ball with shells around it, a bit like the earth's mantle and crust.

The air comes in and you have a 'cool' area immediately infront of the tuyere (or grate if bottom blast), this is the cold air cooling everything down before it burns the fuel.

the next layer or shell is extremely hot and highly oxidizing. This is where the air is imparting the most effect on the fuel being burnt. you get more scale forming here (scale being iron oxide of course) and it tends to be thicker due to the excess of oxygen. welds may not stick due to scaling, but flux helps to prevent that someway. Carbon will migrate OUT of steel in this environment (at normal forging temps).

The next layer is the theoretical best place to forge, a neutral zone. There is neither a deficit nor excess of oxygen. metal gets hot pretty quick as the fuel is still burning efficiently and scaling is fairly average really (not especially thick nor copious). This is the best place to weld and forge as the metal gets hot without much else happening chemically. Carbon will only migrate within the metal (for examppl in a laminated billet of different steels), but not in or out of the bar.

The next shell is the reducing area. this is where there is barely enough oxygen to burn the furl at a forging temperature. Here you will still get scaling but it will be thin and not much of it (assuming you are still getting your metal hot). In this environment carbon migrates INTO the steel (assuming you are burning charcoal and not coal or coke). Welding is unlikely as there is not normally enough heat to get welding temps. In ye olde days, this is where the smith would leave his iron to soak in teh fire to produce steel for inserting into cutting tools.

Outside from that is just fuel that is not getting enough extra oxygen (above what the outside world provides) to get hot enough for forging. The environment is generally oxidising, but not of concern to forging. The main function of the fuel here is as insulation to keep the stuff below it hot.

Obviously that is all the theory. In practice the fire gets disturbed and we end up sticking the metal in all three zones! But it is obvious when you stick the end of your bar too near the air (into the oxidising zone), coz you get thick scale and burn the thing!

I could be wrong, but I have a Distinction on an MA thesis that says otherwise ;)

[Sam Thompson]

I've always been led to believe that the size of the fuel makes a difference to the oxidising/reducing qualities of the fire. The surface area:volume ratio and the size of the gaps between the grains allowing more or less O2 (from blast or atmosphere) into contact with the work As you say Dave the diagrams etc. are true only in an ideal world. Don't Japanese swordsmiths set great store by the size of their grains?

[TASMITH]

Dave is correct in his explanation of the fire zones. One little bit I will add is the zone that he calls reducing. At this point ,most of the oxygen has been consumed, but there is some still remaining. If you are burning coal or coke the remaining oxygen will combine with the carbon in the fuel and produce Carbon Monoxide gas (CO). This is what is called reducing and it is at this point the steel will absorb carbon from the CO. It is the same as what happens in case hardening. Carbon itself cannot be absorbed directly from the fuel. It is absorbed from the CO formed at the elevated temp. and the oxygen and carbon combining first to form the gas.

Terry

[JohnW]

Yip 3 zones, thanks for the great explanation, Dave. How deep does the fire have to be to get 3 zones, about 8"? If the fire's too shallow, there's only one zone. It's all oxidizing.

[ThomasPowers]

Depends on how much air you are forcing through it! Too much air and it will all be oxidizing. Too little and very little of it will be oxidizing (or hot).

Terry I have a book from the early 1900's "The Cementation of Iron and Steel" that in the "history" section goes into detail about the experiments including ones that show that iron *CAN* absorb carbon without the presence of carbon monoxide. IIRC it was done using an inert gas flush so no O2 in the system and they still got carbon migration into the iron where the carbon source was in contact. May not be as efficient but it does work.

[TASMITH]

Thomas;

You are correct that it can absorb carbon directly, but it is only a bare minimum on the surface. I believe(and I may be wrong here) though that the carbon cannot diffuse into the steel and that it is only the surface that absorbs any. I will have to search a bit to find this information again though. When I find it I will post a link.

Terry

[Dave Budd]

nope, diffusion of carbon like anything else moves from an area of high concentration towards an area of low concentration until the two are equal. During my research I did some controlled experiments with making steel though cementation, using a selection of potential forge fuels known archaeologically (Tom Powers actually gave me some pointers initially on this about 6 years ago now).

I packed air tight ceramic containers full of pure iron slivers and one of teh fuels, then soaked them in a kiln at 900C for 4 hours. The fuels looked at were: charcoal (beech wood), horse dung (was only herbivorous poo I could find without feed supliments or other chemical additives), peat and soft coal. The slices of iron/steel were then prepared as metalographic samples and optical and electron (SEM micrographs taken of the cross sections.

The coal sample had a very small amount of carbon just in from the surface and then a clear skin of iron with only traces of sulphur left. The carbon looked like it had migrated (from the CO I guess) into the iron initially, but then the supply was exhausted and continued to move a little. the presence of sulphur also inhibits the absorption of carbon btw.

The charcoal sample had some carbon penetrating to a depth of about 3mm (from memory) and there was a diffusion gradient of approx 1%C at the surface down to 0% beyond the reach of teh carbon migration.

The peat had similarities to the charcoal but had a slightly higher C content at the surface and the depth of penetration was slightly less.

The hosre dung was very strange. Greater depth of penetration, but also the presence of cast iron on the surface! So I managed to increase the rate of absorption from the standard rate of pure carbon migration (approx 0.4mm per hour) and lower the melting temperature of cast iron!

Sadly, the university I was at didn't have the kind of resources I needed to explore the results further Maybe one day when I have an excess of money or I give up on real work I shall go and do a PhD and finish it :rolleyes:

[TASMITH]

Yup! Have to have a little piece of humble pie on this one! After some further research it seems I was wrong,but, I've been wrong before and will be again I'm sure. Most of my understanding had to do with modern methods of carbonizing. It is a field in itself and I have only drawn some conclusions on a limited experience with this. Oh well!.Live and learn!!

Terry

[Bully]

Thanks to all you have clariyfied my question and sum cheers Bully

[David Einhorn]

opps, posted wrong place. Sorry.