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The Newbies Arena Are you new to knife making? Here is all the help you will need.

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  #1  
Old 03-31-2016, 06:22 AM
WNC Goater WNC Goater is offline
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School me on decarb and scale

Could someone explain decarb and scale. I thought I had an understanding of this being the residual effects of heat treating, that black ugliness we grind and sand off to get the finish. I assumed it was on the finish but does it go deeper than the surface?

Is scale and decarb the same thing?

My assumption was it is something that builds UP on the blade, a type of oxydation. The last two blades I did in 1095, (using a brine quench for the first time) after final grind and finishing to remove this stuff, now I realize if I turn it and get a reflection just right, there seems to be a type of orange peel texture though the blade has been finished and is smooth.

Now I'm wondering if it burns into the blade rather than building up on the blade?


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Old 03-31-2016, 06:49 AM
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Ed Caffrey Ed Caffrey is offline
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As simply as it can be put..... Scale is basically "steel ash", its the waste product of combustion. As steel is heated and/or forged, a bit of the steel is consumed/burned away....how much depends on the level of heat, and the duration of exposure.

"Decarb" is a bit more complicated process overall, but can be simplified in it's terms.... What element is present in steel that can be consumed via combustion? Most prevalent is carbon. With excessive heat and longer exposure to that heat, carbon can be consumed in combustion (burnt out). This can happen a various levels. Generally it's not a major issue....all it takes is paying attention to how hot the steel gets, and how long you allow to remain at higher temps. Mild decarb doesn't give any distinct visible signs to the naked eye.... usually it manifests itself in a lower hardening of the steel, and reduced cutting ability/edge retention. In severe cases, if combustion is allowed to consume the majority of the carbon in a given piece of steel.... the steel will turn into what I call "black cottage cheese".... that's essentially what it will appear as.

There are many different methods to overcome/minimize scale.... probably the easiest for most to grasp is to remember...... leave your blades thicker/heavier before heat treat/final finishing....remember, you can always grind it off.....but if you run yourself out of steel, you can never put it back. Unless you heat steel in an inert atmosphere (no oxygen) scale IS going to happen.

The "orange peel" COULD have something to do with the heating/quenching process you use, but more often then not "orange peel" is a result of something done during finishing.... for example over buffing, or in some cases if a scotchbrite finishing product is used incorrectly/over used.


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Old 03-31-2016, 10:27 AM
samuraistuart samuraistuart is offline
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What is decarb? My definition.......decarb is the answer to 95% of the threads with titles "why didn't my steel harden after I followed the HT carefully?".

Scale and decarb are too often confused as being the same thing, and they are not.

Steel is heated towards and past Ac1, carbon leaches out of the steel and is deposited onto the surface. That carbon layer on the surface is known as scale. Can be removed by vinegar soak, or grinding it off with a wire brush even.

Because the carbon has leeched out of the steel and formed a scale layer on the surface, below the surface is going to be a thin layer of steel that has NO carbon in it. This is the decarb layer, and can NOT be removed with vinegar. You must sand/grind through it to get to hard steel. How thick this decarb layer is will depend on a few factors, how hot, how long, maybe carbon % not sure.

Sometimes the orange peel look is a decarb layer that has not been thoroughly sanded thru, only lightly sanded into, and thus can resemble the orange peel look. Also, it can resemble an auto hamon. If you had a blade that had a fairly thick decarb layer on it, and you only sanded thru the decarb layer (evenly) near the edge of the blade, and etched it with vinegar or FC, it can resemble a hamon or lamination line.
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Old 04-01-2016, 01:15 PM
WNC Goater WNC Goater is offline
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Okay thanks for the info. Well it COULD be the finish with a scotch brite wheel though I've never experienced that before, or it COULD be that I did not sand through the decarb layer. I did final grinding after HT with 240 then 400, then scotchbrite. But it isn't so much grinding as it is finishing. But still I would have thought it would be deep enough to get rid of the decarb.

Unless the decarb was deeper than normal.
When quenching this blade (1095) I realized I wasn't getting hardness as before and ended up using a brine quench, after two previous attempts with what I now realize, must have been "used up" canola oil.
I don't know if 3 heating/quenching cycles would have caused deeper decarb or not.


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Old 04-01-2016, 02:42 PM
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Ray Rogers Ray Rogers is offline
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QUOTE: I don't know if 3 heating/quenching cycles would have caused deeper decarb or not.

I would certainly think it would increase your opportunity for decarb. I don't do any grinding until the HT is complete. That way, I know I got rid of any possible decarb and it reduces the chances of warping too ...


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Old 04-17-2016, 01:06 AM
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Why do I see no mention of Turco here?


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