carburizing, neutral or oxidizing flame?

Fred.Rowe

Fred.Rowe

Knifemaker / Craftsman / Service Provider
Joined
May 2, 2004
Messages
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In the past, I have used venturi forges. With the venturi forges, controlling the burn, was accomplished by turning the pressure either up or down giving me a hotter or cooler working temperature.
The blown forge, I have just completed, has control over both gas pressure and volume. Each of the five burner ports has its own needle valve. I have rheostat control, hooked to the air supply.
My question is this; how do I go about getting the optimum burn from my new forge and how do I recognize the three different burn choices: carburizing, neutral or oxidizing flame, as I try adjusting the burn?

What effect do the three different flames have on the steel itself when forging?

Thanks for your input, Fred
 
Oxidizing flame fuel is completely consumed inside the forge. O2 is present and high temps = massive scale and decarb. NOT GOOD, SPARKLER TIME (burnt steel)

Neutral, fuel is just being consumed inside forge no O2 present little scale and decarb. Better but very hard to maintain a true Neutral atmosphere.

Reducing, Fuel is not completely consumed and wisps of flame escape from the forge, This yellow flame is the unburned fuel being consumed in the open atmosphere. No O2 present and easier to maintain than neutral. Very little scale and decarb and this is the atmosphere I try for.
 
A C Richards said:
.............

Reducing, Fuel is not completely consumed and wisps of flame escape from the forge, This yellow flame is the unburned fuel being consumed in the open atmosphere. No O2 present and easier to maintain than neutral. Very little scale and decarb and this is the atmosphere I try for.
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Do you try to accomplish this with more fuel, or less air?
Or is it irrelevant?
 
Both. It is a dance with the lowest air and fuel possible to achieve the temps desired without flashback. I have it down to a sound and the temp probe does not hurt either... Once I started using the thermocouple my results have become much more consistent. I can also see what the changes are doing to the temperatures. it is amazing how little fuel/air you need to achieve 2350f. Also as the forge heats up it becomes even more efficient so adjustments need to be made or you can get too hot. Melted the end of the TC a couple of times.
 
Thanks A.C.! This will help a lot, tuning the new forge.

Fred
 
The full answer to the question of the original post in not that simple, it is quite complex and full of chemistry. I have found legitimate uses for all three flames, with no "all good" or "all bad" involved. That is why I insist on a forge that is fully adjustable to all three with one quick step. As to the details, I would encourage folks to play with it and research the possiblities. To get into the details myself puts me once again into the position of being the messenger killed for the message he bears. Don't ask the questions if you don't want to hear the answers. If avoiding O2 at all costs is working for folks then stick with that, after all if it aint broke don't fix it:).
 
Kevin, I am asking the question, why would you need an oxidizing atmosphere in knife making? I have not found a use for it and am interested in were it might be useful. Well I guess if you are trying to remove carbon from a very high carbon material it would be good for that. I have used it to cause excess scale for collecting to try smelting before too. I do not believe in the not broke adage. We all need to challenge the comfort zone occasionally.
 
A C Richards said:
Kevin, I am asking the question, why would you need an oxidizing atmosphere in knife making? I have not found a use for it and am interested in were it might be useful. Well I guess if you are trying to remove carbon from a very high carbon material it would be good for that. I have used it to cause excess scale for collecting to try smelting before too. I do not believe in the not broke adage. We all need to challenge the comfort zone occasionally.
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I think you answered your own question. You can use it to suck carbon out of the steel and to create scale. Scale can also be powdered and used as a polish.

You also need oxygen for a heat patina, or temper colors, but it's not dependant on oxygen inside the gas forge chamber and is usually done outside the forge.

Outside of that, you need an oxidizing "torch flame" for reticulating sliver, which can be nice for fittings and embellishments.

Generally speaking though,... for the vast majority of bladesmithng and metalsmithing procedures, reducing is best. :)
 
This is one of those concepts that the oversimplified version has become such standard wisdom among smiths that the more detailed explanations are a bit too much of a technicality to challenge to the status quo so it easier to just go with the standard concept. However, oxygen does not necessarily prefer carbon over iron, thus it is possible to actually counteract decarb with an oxidizing flame to some extent, it is even possible to concentrate carbon on the surface by stripping away iron with an oxidizing atmosphere. To put it simply- scale is not decarb it is iron oxide, when iron leaves steel it makes scale, when carbon leaves steel it is via a gas. It is complicated and involves levels of CO and CO2, but is heavily influenced by the moisture content in the atmosphere. For the majority of folks smithing the safest way indeed is just to go neutral or carburizing and leave a skin to grind off later. For those who wish to explore the topic more in-depth I would recommend pages 61 and 62 of “Metallurgy of Steel For Bladesmiths & Others Who Heat Treat and Forge Steel” by John D. Verhoeven, and Chapter 20 of “Tool Steel Simplified” by Palmer and Luerssen, as a starting point; particularly the later.
 
Thanks Kevin. Had not heard that before. Very interesting about the moisture levels. I had noticed a huge difference in the performance of the forge during periods of high humidity. Cool.. Thanks again
 
Kevin R. Cashen said:
This is one of those concepts that the oversimplified version has become such standard wisdom among smiths that the more detailed explanations are a bit too much of a technicality to challenge to the status quo so it easier to just go with the standard concept. However, oxygen does not necessarily prefer carbon over iron, thus it is possible to actually counteract decarb with an oxidizing flame to some extent, it is even possible to concentrate carbon on the surface by stripping away iron with an oxidizing atmosphere. To put it simply- scale is not decarb it is iron oxide, when iron leaves steel it makes scale, when carbon leaves steel it is via a gas. It is complicated and involves levels of CO and CO2, but is heavily influenced by the moisture content in the atmosphere. For the majority of folks smithing the safest way indeed is just to go neutral or carburizing and leave a skin to grind off later. For those who wish to explore the topic more in-depth I would recommend pages 61 and 62 of “Metallurgy of Steel For Bladesmiths & Others Who Heat Treat and Forge Steel” by John D. Verhoeven, and Chapter 20 of “Tool Steel Simplified” by Palmer and Luerssen, as a starting point; particularly the later.
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I have a copy of Tool Steel Simplified and will read chapter 20. In the mean time; as a rule of thumb, as the relative humidity rises, do you lose more iron oxide and less carbon, or is the reverse true?

Fred
 
Chapter 20 lays out how carburizing atmospheres can actually cause decarb when moisture is introduced into the equation. Once again, for the most part the standard ways of dealing with this issue will work but if one has very precise tools and observation methods it is something to play with. I thought it was all beyond my applications until I noticed the two distinct layers on my work under the microscope; the slight decarburized layer, and then a crust of oxidized iron. It was then I realized that if I could slightly oxidize the iron I could use scale to counteract decarb from earlier operations and actually get a practical application out of the industrial technicalities.
 
I am looking forward to the read and to experimenting with this new knowledge.
I didn't realize that carbon was lost in gas form and not as scale. That changes how I will interpret what I am seeing in the forge.

Thanks for your input, Fred:thumbup:
 
Another observation:

For those of us who take our sweet darned time shaping anything worthwhile, is it possible to lose too much iron to scale?

Basically, I am not at all satisfied with how many times I still need to put a knife back into the heat before I'm done hammering. At what point does the scale loss get to be so much that I'm just moving scrap and don't have any worthwhile steel left in the knife I thought I was shaping.

It hasn't happened yet, far from it, but it's something I'm constantly afraid of. For some reason, every time I clean the scale off a new piece and start filing, I am still incredibly relieved when I start seeing nice, shiny steel underneath. Some part of the back of my mind is still afraid that one of these days, I'll file out a toothpick before I get all the scale off.
 
Kevin R. Cashen said:
Chapter 20 lays out how carburizing atmospheres can actually cause decarb when moisture is introduced into the equation. Once again, for the most part the standard ways of dealing with this issue will work but if one has very precise tools and observation methods it is something to play with. I thought it was all beyond my applications until I noticed the two distinct layers on my work under the microscope; the slight decarburized layer, and then a crust of oxidized iron. It was then I realized that if I could slightly oxidize the iron I could use scale to counteract decarb from earlier operations and actually get a practical application out of the industrial technicalities.
Click to expand...

How do you control the moisture content or humidity inside a standard gas forge chamber? It seems like the heat would dry off any moisture anyway.

Also, once the blade is pulled from the forge, it automatically enters an oxidizing atmosphere. Do you really need more oxygen and time than that?

Are we talking inside the forge or outside?
 
I totally agree with Kevin that nothing is all good or all bad,... philosophically speaking. It really depends on what you are trying to do and how you want to do it,... and there is more than one way to skin a cat.

I think it is important that the technology and science be "applicable", but also that it be "practical", under any given set of circumstances....

It always seems to come down to a balancing act.
 
Dan Zawacki said:
Another observation:

For those of us who take our sweet darned time shaping anything worthwhile, is it possible to lose too much iron to scale?

Basically, I am not at all satisfied with how many times I still need to put a knife back into the heat before I'm done hammering. At what point does the scale loss get to be so much that I'm just moving scrap and don't have any worthwhile steel left in the knife I thought I was shaping.

It hasn't happened yet, far from it, but it's something I'm constantly afraid of. For some reason, every time I clean the scale off a new piece and start filing, I am still incredibly relieved when I start seeing nice, shiny steel underneath. Some part of the back of my mind is still afraid that one of these days, I'll file out a toothpick before I get all the scale off.
Click to expand...

Yes, "shrinkage" can be a factor.
 
There is no such thing as having too much, good, information. Sorting it out and applying it to one's own situation can be a bit heady at times.

I look back on the first couple of years I spent learning this craft and recall how much time was wasted in learning and then unlearning many of the myths that are forever alive in the world of knives.

There is so much that is intuitive in making knives and so much knowledge of hard science, that is needed to make a superior product, that at times it can be overwhelming; especially for someone just starting out.
When I learn something new about the science required to make a good knife, I take it to the shop and try to find a practical application, for it, so I can relate it to the intuitive part of my craft.

No one said it was going to be easy; Most things that are worthwhile are not.
If you want to do it well, be prepared to do a good deal of reading and keep a bottle of aspirin handy. I say this to myself, regularly, Fred

A big thanks to all those who share their knowledge on the forums.:thumbup:
 
Fred.Rowe said:
There is no such thing as having too much, good, information. Sorting it out and applying it to one's own situation can be a bit heady at times.

I look back on the first couple of years I spent learning this craft and recall how much time was wasted in learning and then unlearning many of the myths that are forever alive in the world of knives.

There is so much that is intuitive in making knives and so much knowledge of hard science, that is needed to make a superior product, that at times it can be overwhelming; especially for someone just starting out.
When I learn something new about the science required to make a good knife, I take it to the shop and try to find a practical application, for it, so I can relate it to the intuitive part of my craft.

No one said it was going to be easy; Most things that are worthwhile are not.
If you want to do it well, be prepared to do a good deal of reading and keep a bottle of aspirin handy. I say this to myself, regularly, Fred

A big thanks to all those who share their knowledge on the forums.:thumbup:
Click to expand...

Great post Fred!

Factoring it all in isn't an easy job.

It becomes personal and intuitive in the long run...

Science is good, intuition is good, learn as much as you can about both,... but in the end strive for a happy balance.

However, look at the “lead in” to this forum,… it says the “Art of knifemaking”, not the “Science of knifemaking“, and art is primarily subjective and intuitive. Knifemaking/bladesmithing involves science,... but is NOT a science. I’m pretty sure about that! :)
 
I've wondered that perhaps this forum needs to be broken down into areas,... maybe a separate "special" area for "pure metallurgy", without the philosophical B.S... :D

... just the "facts". :)
 
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