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Jan Ysselstein

What Am I Looking At?

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I recently purchased some file blanks ( http://www.bladesmithsforum.com/index.php?showtopic=27982&hl=blanks ) which have very little or no carbon , based on a spark test and an etch . The blanks were either intended to be case hardened (after cutting) or were used to tune in the blank making press dies and were never intended to be used.
Having multiple pieces of same sized flat stock I thought I would use the method similar to the one demonstrated long ago at a CBA Conference by Phil Baldwin ..very similar to the one Ric teaches ( http://www.bladesmithsforum.com/index.php?showtopic=27487&hl=blister ) in his blister steel making class , minus the blisters . Thoug very familiar with what decarburization looks like, I am totally unfamiliar with what to expect when one adds carbon to the steel.
Phil placed the steel in a crucible and added some charcoal , covered the crucible and heated to a yellow. Ric shows his method on a video located on his website...basically they are the same.... iron, heat and carbon.
My sample was heated twice (for about 1 hour each time) using a method very close to Phil's . This material sparks as a high carbon steel.
Here is what I am seeing under a simple microscope: What am I looking at? Is that cementite? The 3 similar pics show an area of about 2mm left to right, the other is about 7mm.
blister1.jpg blister2.jpg
blister3.jpg blister4.jpg
Jan

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Certainly looks like pearlite to me. I did a study on a similar scenario in school here are the pictures I got from it. 1020 in a carbon packed pouch at 900 C for 2 hours. Polished and etched with 2% Nitol solution. I apologize for the somewhat blurry images. I was still a student and short on lab time to do better. All samples were taken from the same initial rod that was case hardened. Amazing what a little quenching does to your microstructure.

 

Annealed case

Annealed Case.jpg

 

Annealed mix

Annealed Mix.jpg

 

Annealed center

Annealed Middle.jpg

 

Quenched case

Quenched Case.jpg

 

Quenched mix

Quenched Mixed.jpg

 

Quenched middle

Quenched Middle.jpg

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Jerod,

 

Thank you for the reply and the posts of the carburized samples ( for those reading the post your pictures are about 50X more detailed than mine. I may quench this sample to pearlite and dig up some almost wootz pieces I should still have...I also have a book on this subject (I think). The problem is most case carburizing is done with regard to being able to use the material directly ...while I just went wild at a very high temperature knowing the material would be reprocessed later. I have a better microscope which I will pull out ( not sure I can take pictures with it).

 

AS part of this thread I will attempt to make some blister steel as well.

Jan

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I am not overly proud of the technical writing in the paper (I am much better now), so I won't post much from it; but this may be interesting as regards case hardening and diffusion in general.

 

(cx-c0)/(cs-c0) = 1-erf(x/(2√(2Dt)))

D = D0*exp(-Q/(RT))


x=depth

cx=the carbon content at x
c0=the original carbon content in the steel

cs=the carbon content at the surface

D0 is a material constant

t=time
T=temperature

Q=energy constant for the material

 

Carburization depth for this study was found to be 130 microns in 2 hours, 82 microns in 8 hours. That is .005 inches in 2 hours at 1650 F. This is why I do not believe in soaking for diffusion. It just doesn't happen in appreciable amounts.

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Jerrod,

erf?

I will have to stick with approximations based on what I think I am reading in easy to read books on Metallurgy....in this case it is the little blue book by John D. Verheoven, titled " Steel Metallurgy for the Non-Metallurgist". John's book is suggested reading for anyone thinking of being a bladesmith/knife maker.

 

Based on the diagram of how long it takes a carbon atom to travel 1mm ( I am converting this to case depth being 1mm ) , it takes 18 min. at 1150 C. Two sessions at 1hr each =/- due to a long lag time involved in heating metal surrounded by carbon ( all guess work ) . Just to pick a number I will say it took 40 minutes at my operating conditions ( probably a little longer). My sample is 3mm at the start and a bit less prior to the second heating as a did a coarse grind to clean the surface and to create more surface area. The case depth needs to be 1.5 mm deep ( carbon is coming from both sides and hopefully meeting in the middle.... so I am hoping to find carbon in the middle after a bit of a soak and a air quench.

 

The expected structure (I think) will be pearlite grains ( large ones ) surrounded by cementite grain boundaries and perhaps some pro-eutectoid cementite..I will quench next week. I have a lot of low carbon bloomery iron I plan to process this way. Whether this will add more or less soul to the steel will have to be determined.

 

Jan

 

Edit, I did find the little book on Carburizing and Carbonitriding and the rate of case depth at 1000 C is 1mm/hr for low carbon steels.

Edited by Jan Ysselstein

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Yeah, the good old error function: http://mathworld.wolfram.com/Erf.html

Generally speaking: erf(z) = some number. In this case z=x/(2√(2Dt)). There are tables that list the erf(z) value for given z values.

 

For giggles, lets do your situation:

Assumptions: File blanks are 0.20% C, and you want 0.80% at the center line. The carbon content of your packing is "pretty good" at 70%C, as there will certainly be air gaps and not pure carbon.

Cs=70%

Cx=0.80%

C0=0.20%

x=1.5 mm

D=1.28*10^-11 m^2/s (this is for carbon in austenite at 927 C, 1700 F)

 

(cx-c0)/(cs-c0) = 1-erf(x/(2√(2Dt)))

(0.80-0.20)/(70-0.20)=1-erf(1.5/2√(2*1.28x10^-3*t)) (note that 1.5 is in "mm" and 1.28*10^-11 is in "m^2/s". I am leaving units off until the end, but I am factoring them in during the calculations.

.0086=1-erf(148.23/√t)

.9914=erf(148.23/√t)

Here we look up .9914 in the erf table (and have to do a little interpolation) to find z = 1.862

Now we know 148.23/√t = 1.862

so t = 633.7 seconds = 105 minutes

 

Hotter means you don't have to go as long, but I think the 70% C packing around the blanks may be a bit generous, but I don't know for sure. Keep in mind that you will have a concentration gradient. While you will be at 0.80% at the center, the surface will be much higher.

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Jerrod,

 

That is beautiful, wow...and probably close to reality...the carbon potential of the atmosphere probably peaked early in the process as I was working way above the decomposition temperature of carbonates (added as oyster shell powder). I have located a nearly wootz samples and will cut it with a grinder giving us a chance to compare the carbon potential of cast iron to atmospheric CO. Here is a picture of an old wootz sample where the process was stopped too early for who knows what reason ( probably impatience).

 

A very pure wrought iron rod absorbing carbon from a white cast iron bath, this is the whole spectrum from ferrite to cementite.

clayc copy.jpg

 

Jan

Edited by Jan Ysselstein

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That is just beautiful. Throughout the outer section you can see a lot of dendritic structure (dark ferns in light material). So cool, thank you for sharing that image.

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Good photo Jan...real good.



Carbon moves, yes, but it has a tendency to move slower than one would predict when carburizing.


Remember it is a gas process till it masses on the outside of the bar and then diffuses in. Many things can interfere with the uptake and migration through the bar.


If you wish to run a study...like a Frenchman did in the 1800's... I suggest pure iron and go from there. You can try different carburizing compounds and temps and such.



Took me a while to work through some variations, but I am comfortable with what I do now with the stock of wrought I have...having 1,000 pounds of the same stock helps.



If it is always a sliding scale of variability then it is difficult to progress as every attempt is a hit or miss adventure.



Ric

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Ric,

Thank you , I am glad you like the photo.

 

I am a little startled,............. as a maker of Wootz, I am carburizing all the time( very quickly) in a porcelain crucible via atmosphere and/or cast iron as shown in the photo. I am glad you made reference to that Frenchman, as he and I share many interests including the movement of carbon, grey/white irons and porcelain. All of which I use in the process of carburizing steel. The greatest variability should come when the iron is not uniform in carbon content..I think the process will be self regulating in terms of the final carbon content distribution..raw , clean, unprocessed bloomery iron should pick up carbon faster where there is none and slower where there is lots.

 

Jan

Edited by Jan Ysselstein

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Federal file blanks carburized ,

 

Found that little piece of metal again and got it hot for about 40 seconds and quenched it in the airflow from my forge blower. The amount of carbon is still not to clear..so I will get out a different microscope which will allow me to see in greater detail ( not sure if I can take pics with it..). I will take a blank untreated and subject it to the same heat as well . Here is what I see using a magnifier and here is what I see on the net at 1.3% carbon...I am not stating any correlation as the magnification and the heat treatment are quite different...this is the area away from the center which I think still has a lot of ferrite.

frame10.jpg air cooled sample of carburized file blank ( 7mm from lft edge to rt edge )

1.3%C.jpg google images picture of 1.3% carbon steel ( annealed )

 

Jan

Edited by Jan Ysselstein

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Jan,

The intermediate structure on your carburized bar will be pearlite with widmanstatten and allotriomorphic(GB) ferrite. A polished section will look similar to the upper half of the example below; this is a hearth steel axe-bit welded(poorly) to a bloomery iron body.

 

 

100_3599 600x450.jpg

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Skip,

 

Thank you, much appreciated, that is what I see in the photos in post 1..(that was done quickly, no polish , FerricSulfate etch ) I will attempt a better job at sample prep. I am bring up a better microscope but taking pictures will be a challenge ( for me ) ...I will post the configuration a maybe someone will have an idea of how to do it. Any idea of the carbon content of that hearth steel? I will search the images posted in some of the books I have to see at what carbon concentrations these pearlite/ferrite structures exist.

 

Since we are on the topic of identifying what we have, I will bring up a topic I consider frequently...I save all very magnetic slag and bits and pieces of bloomery iron..sometimes they area a mix of iron and slag...I also save and often discard less magnetic material which may in fact be easier to reduce to iron than what I am saving. Some of the textbooks on slags are very expensive, very expensive. Are you able to readily identify slags by microscopic methods..any suggestions?

 

Jan

Edited by Jan Ysselstein

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