Grinding and Heat treating thin blades (O1 tool steel)

hathegkla

hathegkla

Joined
Feb 3, 2010
Messages
201
I just started getting interested in kitchen knives.

I'm starting to get the hang of it but are there any "how to" resources specific to extremely thin knives? Either grinding tips or heat treating info. I'm having some warping issues but I think it's mostly a balance of not grinding the bevel too much and straightening immediately after hardening. Grinding has been challenging because my grinder is a bit fast. My 1/16th blades aren't as flexible as I'd like so I'm thinking of going thinner.
 
I sent some thin O1 to JT for HT ... and he had trouble with it warping. I would say profile only before heat treat, and grind bevels after heat treat (leave as much metal for stability during HT as possible). JT will likely be the best person to chime in on this one....
 
Cushing H. said:
I sent some thin O1 to JT for HT ... and he had trouble with it warping. I would say profile only before heat treat, and grind bevels after heat treat (leave as much metal for stability during HT as possible). JT will likely be the best person to chime in on this one....
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I think you're right. I've done quite a few in 1/8" and get ok results, the large chef I did had some warpage, but the two 1/16" blades (paring knives) I did both warped, one with no bevel and one with some bevel. I was able to straighten both of them but feel like I could do better. I think a fillet knife would probably have to be hardened with no bevel.
 
Profile only before HT. Immediately after the quench ( 5-8 seconds) put the blade between thick aluminum quench plates. Grind both sides evenly. Keep the blade cool. A Kool-mist system will help enormously.
 
Stacy E. Apelt - Bladesmith said:
Profile only before HT. Immediately after the quench ( 5-8 seconds) put the blade between thick aluminum quench plates. Grind both sides evenly. Keep the blade cool. A Kool-mist system will help enormously.
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Looks like I need a kool-mist system, any idea what's the smallest size of air compressor to keep it going?
 
Stacy E. Apelt - Bladesmith said:
Profile only before HT. Immediately after the quench ( 5-8 seconds) put the blade between thick aluminum quench plates. Grind both sides evenly. Keep the blade cool. A Kool-mist system will help enormously.
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Awesome, I'll give it a shot. how thick do quench plates have to be? I have some 1/4 scrap I could make them out out. may be double them up?
 
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I have been considering a kool mist system. I already have a big air compressor. I am curious how much of a mess it makes and if it will cause my grinder to rust? I am aware of the positives, just not the negatives.
 
I use the system that Stacy referenced.
It does cause a bit of a mess but honestly I thought it would be much worse than it is.
They sell a solution to use with it ( Kool Mist #77 ) which helps keep your machinery from rusting. I always use that and have not noticed any rust on my grinder.
It keeps the work piece fairly cool, and it keeps my fingers freezing.
I use it when grinding hardened blades and it definitely helps out there.
There are some cheaper Chinese clones available on eBay. Search for "mist coolant lubrication spray system".
They look like just about the same thing.
https://www.ebay.com/itm/Mist-Coola...503566&hash=item42018af608:g:3ecAAOSwlR1c5jhI
 
Most people use quench plates that are 1" to 2" thick. I would say a normal set of plates is two bars 3"x1.5"X18" long. 1" is pretty much the minimum, and more that 2" isn't needed (but is OK). They don't have to be the same size, and often folks use a wider plate for the bottom one. Buy some cut-offs from ebay or a metal supplier. If you have a good scrap yard near you that gets the scrap from industrial machine shops they may have some great pieces. I just looked on ebay and a 1.5X5X20 bar and a 1.5X3.5X19 bar would cost a total of $80.

The idea is that the two plates can be clamped together and hold the blade straight at the same time it cools the metal below the Ms transformation point. Some guys build them as a sort of horizontal vise to clamp down, others just set a big brick or other heavy weight on the top plate. Mine are in an old pipe clamp for cutting large pipe.
 
I agree with Stromberg - thinner blades will harden just fine in the plates alone. If there is any real amount of Mn or CR, the steel will harden.

I will be doing some experiments when the new shop is up and running on "technological" quenching and pushing the structures where you want them to go, not where they will go if you leave them to their own path.

An example is that many guys say they chill their quench plates before the blade is put in them. You have to ask what that does. It speeds the quench, OK, but do you really want to speed up the drop?
Once the steel gets around 400F, it is actually beneficial to have the cooling rate be much slower. Would it perhaps be better to heat the plates to 300F and insert the blade , thus providing a very slow cooling to room temp. Somewhat like marquenching - this would both delay the cooling in order to equalize the stress, prevent warp and cracking, as well as auto-temper the blade.
 
Stacy E. Apelt - Bladesmith said:
Most people use quench plates that are 1" to 2" thick. I would say a normal set of plates is two bars 3"x1.5"X18" long. 1" is pretty much the minimum, and more that 2" isn't needed (but is OK). They don't have to be the same size, and often folks use a wider plate for the bottom one. Buy some cut-offs from ebay or a metal supplier. If you have a good scrap yard near you that gets the scrap from industrial machine shops they may have some great pieces. I just looked on ebay and a 1.5X5X20 bar and a 1.5X3.5X19 bar would cost a total of $80.

The idea is that the two plates can be clamped together and hold the blade straight at the same time it cools the metal below the Ms transformation point. Some guys build them as a sort of horizontal vise to clamp down, others just set a big brick or other heavy weight on the top plate. Mine are in an old pipe clamp for cutting large pipe.
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nice, I just ordered a couple plates, I plan on doing some more 1/16" so I will wait for those to heat treat them. I've been having some luck clamping to a steel plate right after hardening and then fully cooling in a water bucket but maybe slowing it down a little will help a bit.
 
Stacy E. Apelt - Bladesmith said:
OK, but do you really want to speed up the drop?
Once the steel gets around 400F, it is actually beneficial to have the cooling rate be much slower. Would it perhaps be better to heat the plates to 300F and insert the blade , thus providing a very slow cooling to room temp. Somewhat like marquenching - this would both delay the cooling in order to equalize the stress, prevent warp and cracking, as well as auto-temper the blade.
Click to expand...
I make lot of test like what you say with 1.5mm 1.2519 steel . Steel never bend IF I put it in already heated tempering oven BEFORE steel cool to room temperature , never. Same steel let to cool to room temperature and after that in tempering oven have always slightly bend.....
 
Natlek said:
I make lot of test like what you say with 1.5mm 1.2519 steel . Steel never bend IF I put it in already heated tempering oven BEFORE steel cool to room temperature , never. Same steel let to cool to room temperature and after that in tempering oven have always slightly bend.....
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Why do you suppose that is?
 
Natlek said:
I make lot of test like what you say with 1.5mm 1.2519 steel . Steel never bend IF I put it in already heated tempering oven BEFORE steel cool to room temperature , never. Same steel let to cool to room temperature and after that in tempering oven have always slightly bend.....
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I would be carful with this practice as you could be affecting the final outcome of the finished structure. Some steels will say to cool to room temp completely before tempering. By tempering before the steel has a chance to fully convert you could be creating or trapping something you don’t want in a blade steel. But I have never experimented with that so I am open to new ideas. I do know AEBL takes is sweet ass time to fully convert over and some times if not most times it takes the first temper to push it the rest of the way. I can and have hand straightened bowed aebl blades fresh out of the liquid nitrogen. Thy will still take a set by hand. But I have found that if thy have made it that far and bowed a hand straightening will not solve it as it will show back up during the tempering even if it’s clamped straight.

Stacy I do like your idea of slow cooling. I have noticed a trend with my quench plates in that once thy warm up I seam to get more consistent flat blades. But aebl is quite picky and there are other factors that affect it muck more I think then just cold plates. I would be interested in running hot plate quench tests. I have 6 sets of removable quench plates that can be pre heated and pulled for the quench.

but you realized if this does help or affect the blades positively it will require a pid controlled plate quench set up with electricity pre heated heated plates and air cooling to control the rate at which you drop from the 400° start point. You could in fact with a little cleaver set up hold 400 for as long as you wanted. Hell, quench plates could be clamped and moved straight into a ready waiting large tempering oven (wink wink). Way to go now you got me thinking of ways I can shift around liquid hot steel fast enough to control a temp drop accurately lol
 
[COLOR=#ff4d4d said:
"Natlek, post: 19463034, member: 422623"]I make lot of test like what you say with 1.5mm 1.2519 steel . Steel never bend IF I put it in already heated tempering oven BEFORE steel cool to room temperature , never. Same steel let to cool to room temperature and after that in tempering oven have always slightly bend....[/COLOR].
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tkroenlein said:
Why do you suppose that is?
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A bare-bones explanation of steels conversions in HT.
The steel goes into the oven and heats to 1475-1500F (using O-1 as an example). As it crosses around 1414F the structure starts to convert to austenite and the steel becomes non-magnetic. For those who look at charts, this is called the Ac1 point ( austenitic climbing first transformation point)
The reason you heat about 50 degrees past this point is to allow the alloying and carbon to get fully transformed.

The steel is quenched in some medium or method that will drop it past the pearlite nose, at 1000F, so it continues to cool as austenite. If the quench isn't fast enough, the steel will become soft pearlite. With a suitably fast cooling, the steel stays austenite and is a rubbery soft structure called "super-cooled austenite". From this point until it reached around 400F, the cooling rate is not important... but you don't want to rush things too much.

A steel crosses 400F, a point called the Martensitic Start point (Ms for short), it begins converting to the hard martensite we want for a knife blade. This continues until the steel reaches the Martensitic Finish point (Mf for short). The Mf is between 200F and room temperature for most carbon steels. High alloy and stainless steels have a Mf around -100F.

The steel is then tempered at a point around the Ms temperature to release stresses and toughen the steel. The amount above the Ms will determine the final hardness. Normal knife blade tempering for carbon steel is between 400F and 500F

If you don't allow the steel to reach the Mf point before tempering, you trap some of the structures as austenite. This makes a less hard blade. You should ALWAYS cool the metal to the full Mf point if you want a properly hardened blade. As an absolute minimum, it needs to be below 200F before tempering on the simple carbon steels. If the steel has over 1% alloying (52100 for example), it should cool to below 100F before tempering. The old "Stick it in the freezer overnight doesn't perform any magic, but it does assure that a carbon steel blade reached the Mf.
 
tkroenlein said:
Why do you suppose that is?
Click to expand...
Martensite formation leads to an increase in volume and stresses in the material .This is reason why quenching should be interrupted before room temperature has been reached ....
Holding steel just above Ms for short time help temperature to equalize between core and the surface and to transform in martensite in same time so diminishes thermal stresses ......
And when we are already there .....putting cold steel in a hot oven is not best thing to do .Thermal stress should be avoided in any step of HT process ....IF possible
Now i making some test with quenching in horizontal and vertical tank ...so far seems that make difference , but I need more test to do and to borrow for one weekend one Hardness tester from my friend .......
 
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