Blade Article on Forging

[Beau Erwin]

Was reading through some of the Blade for Dec 2008 and have been reading on the article about forging and it mentions atmospheres for gas forges.

 

Author states that "for most of the simple tool steels you will want the forge atmosphere to be neutral to slightly oxidising. You will keep more carbon. I know this sounds backward but read Tool Steel Simplified (pages 437-445) for more information."

 

I don't have that book, and I was kind of curious why having it slightly oxidising would keep more carbon. It does seem backwards, and I thought maybe someone might know why.

[Alan Longmire]

Well, since I have a copy of that book, I looked it up. He's not talking about carbon, that chapter is about microstructures revealed in etching quenched pieces . I think he's confused, because his assertion, if you are presenting it accurately, is not supported. I admit, some of the etched samples show decarb, but there's no mention of furnace atmosphere being anything other than oxidizing on those samples.

 

I suspect he just reversed the terms "oxidizing" and "reducing."

[Mike Blue]

Well this is a good lesson. First, don't believe everything you read. Even books with good reputations can have typos. Second, don't believe everything that other author's cite, because they might just be propagating the original error and didn't do their own research. You can't trust everyone.

 

But, you've asked a good question. I pulled my copy of TSS and found a decent discussion of carb/decarb on page 479. Palmer and Luerssen state: "In reducing atmospheres the presence of even small quantities of water vapor tends to cause decarburization. In oxidizing atmospheres, however, this tendency is eliminated. It becomes obvious now why in manually controlled furnances, in which the presence of water vapor cannot be controlled, oxidizing atmospheres are recommended." (Author's italics.)

 

Further along on page 495 the author's of TSS list the recommended oxidizing atmosphere content for their proprietary steels from the Carpenter Steel Company. This book was originally written in 1937, revised in 1948 and again in 1960. We should all be slightly cautious due to the changes in metallurgical knowledge now available to us.

 

The atmospheric oxygen content list ranges from 1.5% to 5% depending on the steel being treated. Normal atmospheric oxygen is near 18%. Compared to normal atmosphere the environment listed is still a reducing atmosphere. Further discussion of Pages 494-495 indicates that the amount of carburization, in the steels listed, in what the author's define as an oxidizing fire, are limited to a skin of 0.002-0.004 inch depth. For the steels they list, in a decarburizing atmosphere, the depth of the effect is 0.005 to 0.016 inch.

 

Sadly that is significantly less than other claims as to decarburization published in the popular knife magazines, and for either case more than likely any one of us who is doing finish grinding on blades will remove at least that much material during the clean up. Even the authors of TSS admit: "The decarburizing effect in reducing atmospheres in manually controlled furnaces is thus much more pronounced than the carburizing effect in oxidizing atmospheres - and it is quite unreliable."

 

The caveat here as well is, we are working on knife shaped objects, where the tool steel books are generally talking about blocks of material.

 

More stuff to read and digest for us all I suspect.

[Doug Lester]

Ya, I wondered about that too. My experience is that a slightly gas rich atmospher gives me less scaling, which is something that I can obsurve. Can't see decarboration but I'd agree that it should be restricted to the sacrificial steel that will be ground away.

 

Doug Lester

[Beau Erwin]

*nods* It just seemed backwards like he'd said, but didn't really make sense.

I'm not too worried about it, was just curious as I'd prefer less scale and pitting so reducing for me.

 

Makes sense how things in that book could be inaccurate now if it was written that long ago.

[Mike Blue]

I'd encourage having a copy of that book if you can find one. There is a chapter in there about spark testing that is quite good even if it is about Carpenter's steels. You could generalize the information to your own collected set of steel samples and that's something every maker should be accumulating. For its day, it really did simplify the study of steel.

[Todd Gdula]

Tim Zowada once gave a talk on how decarb can occur in a reducing atmosphere in a propane forge. I wasn't there, but I did confirm with Tim that he gave the talk. Unfortunately, I didn't have a chance to ask Tim any questions about it because we were at Ashokan and he was pretty busy.

 

Fortunately a friend of mine was there to hear it first hand - and my friend is a chemist, so he actually understood what Tim was saying. If I remember correctly, it has something to do with a chemical byproduct of burning propane which has an affinity for carbon - it attaches to the carbon in the steel and removes it.

 

After having it explained to me I do believe it's possible. Like I said, I'm not a chemist, but I did take a few years of college biology and I can tell you for sure that things that seem to make no sense can be true when dealing with chemistry (and physics even more so).

 

-Todd

[Bennett]

I've always wondered how to make a SAFE endothermic atmosphere in the home shop. That's how we control the chemistry of the steel in our ovens at work.

 

You don't want your endothermic atmosphere to turn exothermic though.

[Mike Blue]

.... If I remember correctly, it has something to do with a chemical byproduct of burning propane which has an affinity for carbon - it attaches to the carbon in the steel and removes it. ...

 

Propane burns to form carbon dioxide and water vapor. And the authors above also include the water vapor in the air taken in by the burner.

 

In an oxygen poor environment (reducing), the main products are carbon monoxide some carbon dioxide and water vapor. The carbon monoxide will either give up the last oxygen to another carbon monoxide to form CO2 which is then exhausted and the free carbon will either find an iron or oxygen on the surface of the billet to form an iron carbide (carburizing) or attach to an oxygen on the billet to form another carbon monoxide etc. (decarburizing). The water vapor will break down at heat to form a hydrogen that burns to form more water vapor and release oxygen to form more carbon dioxide or find an iron to form iron oxide as scale.

 

The real question is how much of a gradient exists to push the equation either toward carbon uptake by the billet or carbon loss by the billet combined with carbide breakdown leaving iron to bind to oxygen and be lost as decarb'd scale. A carbon excess atmosphere will allow for more carbon flow into the billet and an oxygen rich atmosphere, whether excess oxygen or from water vapor, will attract more carbon away from the billet. Even so, the amount gained or lost is dependent both on temperature and time. Generally the authors of TSS hold that 1700 degrees F is the line of effect. Below 1700 the effects are more or less negligible. Above that line, the effects can be more aggressive for either process, if, given enough time.

 

Even then there are some conditions where the general rules do not apply. I'd be encouraging that most of us will not discover those using normal shop practices.

[kb0fhp]

Water vapor is very decarburizing because it releases hydrogen and oxygen. The oxygen reacts with the carbon on the surface making CO. That is the reason why you want a low dew point in your atmosphere. That is one reason why people tend to add a small amount of natural gas to the furnace atmosphere - to scavenge the dissociated water....