why does scale form on heat treated blades?

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William2112

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Aug 19, 2014
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I just heat treated my first knife today in my new paragon oven. The blade material is 1095 and I didn't have any tool wrap to wrap it in so I cooked it naked. It's no surprise that after it came out of the oven and was quenched it had a ton of scale on it. It then dawned on me that I don't have a truly scientific understanding on why this phenomenon takes place. Could someone please explain to me in the simplest of terms why this happens when a blade is not wrapped up but dose not happen when a blade is wrapped up? Thanks.
 
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I'm not an expert, but it has to do with the oxygen in the kiln. The tool wrap will prevent some oxygen from getting in. Some people put a small piece of paper in the foil to burn the oxygen that is trapped in there when it is sealed. Some kilns have the ability to have Argon pumped in to replace the oxygen. You can also get a solution that you can dunk them in and it will help prevent scale (but I have heard reviews from some that it causes pitting). Usually tool wrap is reserved for stainless steels (air quenched). I think the tool wrap may interfere with an oil quench that 1095 needs. Scale is just something you need to deal with I guess. Some people swear by soaking the HT'd blades in vinegar overnight (after tempering) and that will help loosen the scale. I'm sure someone with much more knowledge will chime in shortly.
 
Now I have another question. It seems to me that scale is mostly made up of carbon and if so dose that rob the steel of any hardness or toughness?
 
The oxygen in the air combines with the iron as well as the carbon in the blades surface. The iron reduces to iron oxide in a hard form similar to magnetite - Fe3O4. The carbon combines to form CO2 and CO, which are gasses. The scale layer isn't very deep, but is really hard. Soaking in a bucket of white vinegar overnight will dissolve it. Scrub the blade off in the morning and grind away. There is no damage done by the scale, and it is an expected part of HT for carbon blades. There are several HT coatings available from Brownell's and other suppliers to coat the blade and prevent/reduce the scale. Most folks don't bother.

Stainless blades are a different thing, and have to be tightly sealed in a stainless steel pouch during HT or the decarb would go deep enough to damage the blade.
 
A great answer Stacy - and right on the money. Scale is HARD!!! and your chemistry is good.

Ken H>
 
May I take this thread off track a bit?

Besides searching stickies on this forum", and doing a general search of the internet . . . Is there a good 10 - 40 page document, book, or website that provides a strong introduction to the the heat treatment of steel, especially tool steels? Something that covers topics like hardening, quenching, annealing, tempering, desired hardness, principle of nose curves, etc?.

I've recently acquired the ASM Handbook, Volume 4, Heat Treating but it is over 2100 pages long. I'd like something a bit more condensed and I misplaced my engineering textbook which covered this adequately.

Thanks,

Mike L.
 
logem said:
May I take this thread off track a bit?

Besides searching stickies on this forum", and doing a general search of the internet . . . Is there a good 10 - 40 page document, book, or website that provides a strong introduction to the the heat treatment of steel, especially tool steels? Something that covers topics like hardening, quenching, annealing, tempering, desired hardness, principle of nose curves, etc?.

I've recently acquired the ASM Handbook, Volume 4, Heat Treating but it is over 2100 pages long. I'd like something a bit more condensed and I misplaced my engineering textbook which covered this adequately.

Thanks,

Mike L.
Click to expand...

Probably the best bet for this is 'Metallurgy of Steel for Bladesmiths & Other who Heat-treat and Forge Steel' which is a paper written by Professor John D. Verhoeven covering heat-treatment of carbon steels quite extensively. It's a great read and it's relatively easy reading given it's subject matter. I re-read it every once in a while and always pick up on new things I missed last time.

The paper says that it's ok to share it, so I have a copy of it hosted on my Google drive, it's accessible here:

https://drive.google.com/file/d/0B5UkG5XX-Mt1M2hJYkxheGhXeUE/edit?usp=sharing
 
Another great read is called 'The New Science of Strong Materials' by J. E. Gordon. It has a great section on the history of steel, and the different ways it's been made as well as the different structures in steel.

That book will also give you a solid understanding of why some materials and strong and some aren't, which is a really great thing to know!

It's only $20 on Amazon which is about the best money you'll spend I reckon! http://www.amazon.com/Science-Materials-through-Princeton-Library/dp/0691125481
 
Aaron,

Thanks for the information. The 210 page is much better than 2100 pages.

I'll look into the "Strong Materials" publication too.

Thanks a lot.

Mike L.

Mike L.
 
logem said:
Aaron,

Thanks for the information. The 210 page is much better than 2100 pages.

I'll look into the "Strong Materials" publication too.

Thanks a lot.

Mike L.

Mike L.
Click to expand...

Haha, wow! I had forgotten it was that long!

It's a complicated subject! I would like to do a video at some point that shows motion graphics of the crystal structures inside the steel, as well as video of the actual heat-treat process...
 
I took the heat treated blade blank that I mentioned in my original post to work so I could Rockwell test it and it only checked about 45 Rockwell. I was shooting for just above 60 and I have no clue what I did wrong. I got all my info on how to harden 1095 off of this forum so I trust the source but not my method. I heated the blade up in my brand new paragon oven to 1475F and let it soak for 17 minutes. Then quenched in 130 degree peanut oil. I then sanded most of the scale off and put it in my kitchen oven at 375 degrees for a little over 2 hours to temper. I took it out of that oven and let it air cool from there. Can anyone please point out anything I may have done wrong that would cause such unimpressive results? Any help what so ever would be tremendously appreciated. Thanks in advance.
 
Check your hardness tester. It might be on the wrong scale or something. Also, be sure the surfaces to be tested are parallel and polished to 220 grit or finer. It's possible you have a decarburized layer on the surface as well. Polishing will help get rid of that.
 
aarongough said:
Another great read is called 'The New Science of Strong Materials' by J. E. Gordon. It has a great section on the history of steel, and the different ways it's been made as well as the different structures in steel.

That book will also give you a solid understanding of why some materials and strong and some aren't, which is a really great thing to know!

It's only $20 on Amazon which is about the best money you'll spend I reckon! http://www.amazon.com/Science-Materials-through-Princeton-Library/dp/0691125481
Click to expand...

Oooo pages... Is there a movie? :) Great references
 
Before I tested the blade I surface ground both sides. I also Rockwell tested a 1 2 3 block just to see if the tester was way off and it checked between 64 to 65 Rockwell. I tested the blade blank in 6 different spots and everyone of them checked almost the same . That's why I'm so baffled! I've been a machinist for just over 20 years, working with metal of all different types is almost all I know.I guess I got more to learn than what I thought.
 
William2112 said:
Can 1095 be reheat treated and still get a good result?
Click to expand...

Yep, keep doing it until you get it right. It's awesome that you have the access to a Rockwell. You didn't use any kind of coating or anything on the blade did you? I use Parks 50, and I'm not familiar with peanut oil as a quenchant (I know it makes an awesome deep fried turkey).
 
Just had the exact same problem...

http://www.bladeforums.com/forums/showthread.php/1221314-1095-heat-treat-failure

It was a quench issue for me; I heated the quenchant (Park50) when I shouldn't have. At any rate, if you follow the same advice Stacy and Chuck gave me it may help you refine your process and zero in on the problem at the same time. If you have any scrap 1095, heat a chunk of it just like you heated your knife, then quench in water. Test the hardness. I was getting 64-66HRc. If you get similar results, you will know the steel is good and your tester is probably accurate, and it is a quench issue. If not, perhaps an oven temp (inaccurate) issue or a steel mix up...
 
When I surface ground the flats I just barely dusted off the surfaces. Not much heat built up because the magnetic chuck drew most of the heat out. I tried heating another blank and quenched it in water and I think I'm getting closer to the hardness I want. I won't be able to Rockwell test it until tomorrow when I go to work. Thank you goose for the advice and thanks to everyone else who gave input without people like you people like me would look like fools.
 
My guess would be decarb, like Me2 suggested, but you said you surface ground both sides AFTER heat treat??? If you barely sanded the surface....you may not have gotten through the small layer of decarb that is present. (The carbon that makes up the scale you sanded off came from a very thin layer of steel on the surface and you'll need to remove that layer by grinding it off before you reach the hard stuff). If you are sure the decarb layer has been removed, the surface was polished up, and was a dead flat surface for the diamond penetrator, calibrated and working, etc yada yada....should be very hard! While 130F peanut oil isn't the ideal 1095 quenchant, it should have hardened the steel decently. I have heard that 1095 isn't always 1095, that the quality control of that steel isn't all that great, depending on the source. Ggoose has a good suggestion. Take a scrap piece, hold it at 1475 for 10 minutes and quench in water. Remove the scale, sand through the decarb layer, polish to 220 and check it on your RC tester. Should get about 66.
 
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