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The Damascus Forum The art and study of Damascus steel making.

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  #1  
Old 02-08-2008, 08:51 PM
Doug Lester Doug Lester is offline
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Cool Low fold damascus

I've got a question to ask about the low fold/no fold damascus made from steel shim. I've just finished the part of Verhoeven's "Steel Metallurgy for the Non-metallurgist" that delt with carbon migration in steel. I understand that this is another of those things that is a funtion of both time and temperature. Looking at the graf representing carbon migration I have a bit of a hard time getting my mind around just exactly how fast the carbon molecules are moving at around 2000-2100 degree. It looks like it is moving about 500 microns in about 8 minutes at 2100 degrees. That looks like it is pretty much the thickness of a piece of shim. Now I know that the carbon is going to be moving randomly in all directions; the carbon will be moving out of the low carbon steel at the same time it is moving out of the high carbon steel. However is doesn't seem like it would take very long for the billet to become homogonized. Am I missing something here or is that a real problem?

Doug Lester


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  #2  
Old 02-08-2008, 10:06 PM
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Woodchuck Forge Woodchuck Forge is offline
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It does not take that long to become completely homogonized. If using simple carbon steels it only takes about 3 welds. If using shim probably only 1. This is why most are using all high carbon mixes. This way it does not make a difference. You only loose due to decarb and if your atmosphere is correct it will be held to a minimum. Also Nickel acts as a barrier but pure nickel is not good for edges.

Chuck


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  #3  
Old 02-09-2008, 02:46 AM
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The proper term is carbon diffusion . It is always a one way deal, the carbon diffuses to a lower carbon area.Many variables which makes it difficult to predict. Some elements such as nickel slow down diffusion. For a user knife the average carbon should be .70 or better so don't use .20 C steel !!
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  #4  
Old 02-14-2008, 08:02 AM
Sergio Segre Sergio Segre is offline
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If after etching there is a pattern with clear and dark areas I think that it means that the piece is not homogenized. Am I right or could it be that the pattern is due to alloy elements other that carbon?
Sergio
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Old 02-14-2008, 09:50 AM
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Personally, I don't care if it's homogenized or not just so long as there is a good bold pattern ....


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  #6  
Old 02-14-2008, 10:00 AM
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Sergio, folded or damascus steel is there for appearance not for any practical reason. So what it looks like is the important thing . As long as you get a good weld you want contrast between layers regardless of composition. Small diameter alloying elements like carbon [interstitial alloying element ] will diffuse much faster than substitutional alloying elements like Cr, Mo, Ni, etc. .The primary effect is then due to alloying elements other than carbon.
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  #7  
Old 02-14-2008, 12:18 PM
Doug Lester Doug Lester is offline
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After posting this to Ed Caffrey's site and thinking over some of the things that I have read about and seen presentations on, I'll go back and answere my question from the leading post. It's ovious that I'm missing something here. The rate of diffusion of carbon through an austinite matrix isn't the only thing here or I'm interpreting it wrong. I remember the Nova program on PBS showing a tagahamone (sp) master making steel by the traditional Japanese method. He had the iron and carbon in the furnace for three days and still didn't come up with an homogenous mass of steel. Even the large granuals that were broken off it weren't homogenous because the master sword smith had to repeatedly fold the steel to assure an even mix.

In retrospect, I think that Verhoeven's book is something on the order of a "Metalurgy for Dummys" that gives a good funtional overview but nothing that is really in depth. I think that I was guilty of reading that a carbon atom is capable of traveling 500 microns through an austinite matrix at 2100 degrees and jumped to a wrong conclusion, lacking information on other factors at play.

Doug Lester


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  #8  
Old 02-14-2008, 02:48 PM
AchimW AchimW is offline
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Quote:
Originally Posted by mete
Sergio, folded or damascus steel is there for appearance not for any practical reason.
This is not true. It is perfectly possible (and done by quiet q few people) to make combinations of steels that have properties that are not only different but also beyond the homogenous steels that presented the basic materials for the mix.

For example will a very high nickel steel influence the hardening of a manganese steel like O2 a lot in a damascus steel.

The things that are going on in tamahagane? or any other bloomery iron are completely different from any block of damascus steel. There are several reasons for it. A bloom is not a homogenous mass of steel, but consists of large grains and layers of steel separated by slag, ashes and residual chacoal. Another resaon is that there's a permanent air flow through the bloomery furnace which changes direction a lot due to flowing liquid slag, falling charcoal and other reasons. This air flow at high heat will decarb parts of the bloom or cement it if there's a lot of monoxide in it..

In a billet of damascus steel, you do not have any of these effects. You just have the layers that are welded together. And even in our large billet with 320 layers that were welded in a single weld, the carbon content is very even in all the layers. And they had between 0.75 and 1.1 % carbon before the weld.

Achim
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  #9  
Old 02-14-2008, 08:16 PM
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Achim, I was making a generalization there. I know you are very knowledgeable and skilled so perhaps you could expand on your comments. There are questions I have had about things like - alternating hard and soft steels give a 'toothy'and better edge. I am also of the 'harden all the way through ' group ,rather than edge hardening.Let's have some discussion ,this forum has been slow recently.
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  #10  
Old 02-14-2008, 09:10 PM
Doug Lester Doug Lester is offline
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Another thing that I had read about including a low carbon "tough" steel in the mix is that it will tend to stop cracks that start in the harder but more brittle high carbon bands. I would also love to read a discussion on the pro and cons of selective hardening/tempering as opposed to a homogenous heat treatment but I think that that might need to be started on another board.

Doug Lester


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  #11  
Old 02-15-2008, 10:58 AM
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Quote:
Originally Posted by Doug Lester
Another thing that I had read about including a low carbon "tough" steel in the mix is that it will tend to stop cracks that start in the harder but more brittle high carbon bands. I would also love to read a discussion on the pro and cons of selective hardening/tempering as opposed to a homogenous heat treatment but I think that that might need to be started on another board.

Doug Lester
Yes, but the added toughness would need to come from somewhere besides a difference in carbon content, as that will even itself out with a few welds? Nickel, chrome, and vanadium diffuse much more slowly, if at all, I think. Any difference would come from these elements?

At what point does carbon start moving? Is carbon migration occurring during grain refinement/normalizing cycles, and during the soak before quenching (not to mention during forging)? Maybe even while tempering? I think that the higher the temp the faster it diffuses, but is it happening, to some small degree, at room temperature?

I would think that variables during the welding cycles would have the greatest impact, things like how hot, how long it takes to get hot, is it soaked at these temperatures, how many welding heats, how many and how hot forging heats...
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  #12  
Old 02-15-2008, 11:52 AM
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Nature always wants to be at equilibrium so if two pieces of different carbon content are forged together eventually both sections will have the same carbon level. The diffusion [with rare exception] doesn't occur until you have heated to the austenite stage. The higher the temperature the faster the diffusion.Carbon will always diffuse much faster than other elements....Nickel is a great element to add to increase toughness , as does a bit of retained austenite !...For me if it has cracked, even if only microcracks , it has failed !! Micro cracks have a habit of causing catastrophic failure sometime after the blade has been made , from the stresses in use !!
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  #13  
Old 02-15-2008, 02:16 PM
AchimW AchimW is offline
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mete is 100 % right.

The only way to block the carbon migration completely is to put a layer of pure nickel into the billet. But even then, strange things will happen. If you go up to a real high layer count with this (about 10000 layers is a good number to start with) and use, say, something like O2 with it, the nickel will start to diffuse into the steel. Hence, after a while, you will have very, very few pure nickel left (if any at all) and a transition area with increasing nickel content. The carbon will do strange things in this steel. It will try to migrate out of the O2 steel and will create a carbon "wave" or "peak" somewhere in the transition area and drop near to zero in the higher nickel area. Now if you make a blade from such a damascus steel and harden it, it will present different hardness levels at the edge and produce something like a "toothy" edge when it was used for some time. At the same time, such a damascus steel will be very tough due to the no-more-pure-nickel layers.

Something similar will happen to a smaller degree with more or less any kind of damascus steel even if the carbon content is the same in all areas, because

a) not only carbon, but also other alloy elements have an influence on the abrasion resistance of a steel and

b) not only carbon, but also some elements have an influence on the hardness level, too. And this not only counts for the given steel, but sometimes for the "neighbour" steels, too.

But if you will ever feel this when using a knife (exception: damascus with pure nickel) is questionable, because the overall level and cutting ability will be high and we're talking about very small margins here. In the monster damascus block for example, the hardness level of the different layers is, when correctly HTed, between 60 and 62 HRc and all of them will cut correctly for a VERY long time.

I will give a nice example for b) too. I made a damascus steel using Ultrafort (a carbon free maraging steel with lots of nickel and cobalt), O2 and 1.2519 (a 1.1 % carbon and 1.3 % tungsten steel). According to the technical sheets, the maraging steel hardens to 60 HRc maximum (no tempering needed) and both tool steels harden to 64 HRc maximum before tempering. I made a blade from this material, hardened it, tempered it two times for 1 hour and rockwell tested about 20 spots. The maraging steel was 60 HRc everywhere. The others were 65 to 66 HRc for the O2 and 66 to 67 HRc for the tungsten steel. At the same time, this mix is very tough. This seems to be strange, but happens to a high degree due to the very high nickel content in one of the steels, because the nickel will transport heat a lot faster than iron. There is other, more complex stuff, but this alone works a lot.

Achim
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  #14  
Old 02-17-2008, 05:32 PM
Doug Lester Doug Lester is offline
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Adding alloying elements to a steel can increase toughness by inhibiting grain growth but lowering carbon content also increases the toughness and ductility. It's the oposite of adding carbons to increase hardness and wear resistance. It's why low carbon steel is used in auto body frames. Higher carbon steel would crack and fail. The less carbon in a steel, the less martinsite and bainite can be formed in quenching; also less pearlite and more ferite in normal cooling.

Doug Lester


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  #15  
Old 02-17-2008, 09:12 PM
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Question

Achim, since we use borax for flux ,have you or anyone you know of examined damascus to see if boron has diffused into the steel ?
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