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I need to get a better understanding as I use an electric kiln for heat treating. I am not sure what it looks like. I cleaned it (1075 steel) thoroughly with acetone and put 2 coats of titanium dioxide containing white paint on. I heated to 885C for normalizing and cooled slowly in the kiln. The paint did not live up to the no-peel promise. I have sanded the flat part of the blade, but I am not sure if I am seeing is decarb or something else.

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Hello Andrew,

 

First of, 1075 doesn't need to be slowly cooled in a kiln. This operation is called a lamellar anneal and is used to soften specific hardened steels for machining. 

 

When doing a normalization, just bring the blade to temp and hold it around 5 minutes and air cool to black and repeat. Your first heat temperature seems ok, but you'll need grain refinement heats after that. Those are done at quenching temperature. You put the blade in the kiln, wait until the kiln has regained it's temperature and wait another 5 minutes to equalize the temp throughout the blade. Air cool to black and repeat. After 3 cycle or more, you can quench the blade instead of air cooling it. 

 

Now, for the decarb part, I heard the paint can work for propane forges, but I strongly advise you to buy ATP-641. An electric kiln has much more oxidizing atmosphere than a propane forge and the steel must be adequately protected. ATP is relatively cheap, considering a single pint can coat dozens and dozens of blades. It can be bought directly at Brownells.com

 

Decarb will etch light gray with ferric chloride and will be easy to file, even on a hardened blade. 

Edited by Joël Mercier
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To add to that, decarb is the result of carbon being more attracted to oxygen than it is to iron.  That's the chemistry behind both rusting and smelting, going in opposite directions.  Oxygen prefers to bond with iron, and carbon prefers to bond with oxygen.  If you put a piece of steel in a sealed bottle of pure oxygen for several days, when you opened it you'd find a rusty bar in an atmosphere of carbon monoxide.  

 

What this translates to in an electric kiln is a loss of carbon from the surface of the steel, since unlike in a forge you can't maintain an oxygen-poor atmosphere.  Since carbon moves freely through steel at high temperatures, up to 1mm per hour at heat-treating temps, the longer the steel sits in there the more carbon is lost from the surface, to be slowly replaced by carbon from deeper within, thus lowering the overall carbon content of the steel.  

 

The ways to get around this involve either a controlled atmosphere or a protective coating.  I don't know the TiO2 content of that particular paint, but I suspect it's not as high as in a high-hiding white primer or simple white-out correction fluid.  Some electric kilns allow you to hook up an inert gas flow, but this can be hard on the elements.  Industrial heat-treaters use either gas-fired ovens or an endothermic gas injection that maintains a neutral or reducing atmosphere.  We can approximate that with stainless steel tool wrap foil and oily blades, but I know in ZA you can't find stainless foil or commercial anti-scale compounds easily or cheaply.  I also know there are professional makers there, and it would be worthwhile to look them up and see what they do to approach the issue.  

 

Finally, Joel is right about the cooling.  Don't ever leave any steel to just slow cool in the kiln or forge.  Proper annealing involved cooling at a rate not to exceed 6 degrees C per hour down to a certain point, often taking six to twelve hours.  Same for sticking it in ashes/lime/vermiculite.  That's a blacksmith myth like quenching aligned to magnetic north, except it's actually less helpful.  For simple steels like 1075 no soak time is ever needed once the blade is up to temperature, just get it there and remove from the heat.  Let cool in still air to normalize, or quench to harden.

 

Finally finally, what you're seeing on that blade looks like a combination of decarb and grain growth.  Follow Joel's advice and do a few thermal cycles across the critical temperature, then air cool without putting it down until it cools to black in the dark.  This will redistribute the carbon somewhat and refine the grain.  

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What they said, and I'll add a few bits. 

Decarb basically looks like surface scale as you would typically see coming out of the forge, What I see in your photos looks more like baked on enamel from the paint.

I see you are in South Africa. Look up a knife maker named Stuart Anthony Smith (he is on FB and IG) and a Bladesmithing School at Heavin' Forge in Belfast. They can both steer you in good directions.

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Just to add a little more to this:  Oxygen goes after a lot of things, not just carbon and iron (obviously).  If you are curious as to the aggressiveness of any given element's affinity for oxygen, you can look up a chart called an Ellingham diagram.  That will show you the propensity for elements to react with oxygen, and it is temperature dependent, so depending on temperature one element may be more likely to oxidize than another, but at a different temperature that could be reversed.  Here is one such diagram.  Note the reaction formulae with each line.  This does not take into account bonds that are not explicitly stated.  So the below diagram doesn't show any carbides being broken down due to oxidation.  Different charts are available when looking online.  I use this one as it covers most of what I need in liquid steel.  

Ellingham_Richardson-diagram.png

 

Also, the annealing process for 1075 is to heat to 815C (1500F) and furnace cool to 650C (1200F) at a rate not to exceed 28C (50F) per hour.  After that cooling rate shouldn't matter, but I would do it in still air, or possibly use a fan, not a liquid quench.  

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I know that you will read to heat the steel up to a little above critical and cool to black before reheating.  Actually, I would recommend that you cool to black and then check it with a magnet to be sure that it has magnetic again thus indicating that it has changed states back to a body centered cube.

 

Doug

Edited by Doug Lester
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Posted (edited)

Thanks all, I am starting to understand better. What was confusing me was the large grain size which was my mistake by letting it cool in the kiln.

Edited by Andrew Gillespie
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On 8/16/2021 at 8:59 PM, Doug Lester said:

I know that you will read to heat the steel up to a little above critical and cool to black before reheating.  Actually, I would recommend that you cool to black and then check it with a magnet to be sure that it has magnetic again thus indicating that it has changed states back to a body centered cube.

 

Doug

Black is about 500F below the curie point.  This is why it is important to do these things in low light.  You need to get well below magnetic for best grain refinement, and black in low light is a much better indicator of where you want to be.  

Edited by Jerrod Miller
Fixed a typo.
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On 8/15/2021 at 1:30 PM, Joël Mercier said:

...need grain refinement heats after that. Those are done at quenching temperature.

The quenching is temp 815°C for 1075, right?

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1 hour ago, Andrew Gillespie said:

The quenching is temp 815°C for 1075, right?

You're gonna have to run your own tests, I've never worked with that steel and I've seen contradictory info. AKS mentions 850°c and I've seen lower elsewhere.

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Do you know the Manganese level?  The lower the Mn the hotter it likes to be to get depth of hardening.  Contrarily, if you are going for hamon on low-Mn steel, use the lowest temperature that works.  Anywhere between 815 and 850 C will work, just don't soak it.   

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