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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 12-20-2021, 08:03 AM
Johnnyjump Johnnyjump is offline
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Heat Treat 1084 Questions

I have a set of 1084 blades that I created via stock removal, and am wondering if they need stress relief (normalization?) prior to heat treat or not. (I am using a kiln for Heat Treatment) I have heard many different opinions on this, and some insist stress relief is needed even from grinding bevels to stabilize grain structure. I plan on foil wrapping them to minimize carbonization, as my blade tolerances don't allow for much more steel to be removed following heat treat. To relieve stress, I follow a three step process of bringing the blade to critical temperature, letting it soak for five minutes, and cooling it in air as the blade color changes from red to black. I know 1084 is one of the easier steels to heat treat, and I have had success with it in the past. I am stuck on the "normalizing" or "stress relieving" steps which seem to be used synonymously by some. Or is that step necessary for 1084? Also, I have always placed my blades edge up during heat treat, and quench horizontally. Your experienced opinions would be much appreciated! Thanks for your help!
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  #2  
Old 12-20-2021, 11:40 AM
KenH KenH is offline
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First of all please allow me to say I am NOT an expert. I rely on folks like Ed to keep me straight. If anything is drastically wrong below I trust Ed or others will chime in and correct.

I agree with you on the confusion that can some in with all the different terms involved with HT'ing steel. It's my understanding there are 3 basic phases of HT'ing simple alloy steels (1070, 1080, 1084 ):

Normalizing: that's heating to well above "red hot" to around 1600F and hold for no longer than 5 minutes, then allow to air cool. This gets everything "normalized" as far as carbides, etc. Since it was heated so hot the grain structure will most likely be large which leads to the next step.

Thermal Cycling (grain reduction): 3 heats, holding at each temp only long enough for complete heating thru the blade. 3X@1500F (I think that is what Kevin is saying below. Or, should 2nd and 3rd heat be around 1400F?), . Allow to cool in still air to 900F (to black) or less between cycles.

Quenching (Hardening):#1500F and quench. As quenched, you should see Rc hardness of 64-65.#NOTE:#1084 is a shallow hardening steel. It requires a very fast quenchant. I think Ed uses Parks 50.

Quenching is of course followed up by tempering.

Since you're doing stock removal all the above steps might not be required, but won't hurt anything either. Steel is delivered from vendors in different forms, some which do require the above steps for proper hardening, other vendors don't require the above step.

Last edited by KenH; 12-25-2021 at 01:24 PM.
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  #3  
Old 12-20-2021, 02:53 PM
Johnnyjump Johnnyjump is offline
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Boy, I love it when things are explained in a simple, bullet point format! Makes sense to me! And I too have conferred with Ed in the past, and agree he is definitely the authority on such matters. Hopefully he agrees with your reply!
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  #4  
Old 12-20-2021, 06:29 PM
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billyO billyO is offline
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I'm not the expert that Ed is, either, but what I do is slightly different that what Ken suggests after reading the book, Knife Engineering by Dr Larrin Thomas.
He says that for simple steels like 1084, successively lower temps during the 3 thermal cycling steps isn't necessary, and the 1st at 1500F isn't really a grain reduction temp (IIRC).
When I'm finished forging, I normalize at about 1700F, then do 3 grain reduction cycles at 1350F.

Do you need to do this when doing stock removal? That depends on where you get your steel from. I'd call the vendor and ask them if this has been done or not.
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Old 12-20-2021, 07:09 PM
Johnnyjump Johnnyjump is offline
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The steel is from Alpha Knife Supply. Interestingly enough, they reference Dr. Larrin Thomas in their description. Sounds like I need to get his book."Condition: This alloy been properly annealed and is in the soft condition. You do not have to do any non-standard wasteful processes before heat treating. The steel must be heat treated to get to a hardened state. Follow the heat treat recipe and the steel will harden.. Choose a heat treat recipe that results in the attributes you want. Thank you to Dr Larrin Thomas for all the testing he performed and information he has provided. Buy his book Knife Engineering to learn more about knifemaking." So according to Alpha Knife Supply, no further "processes" are needed before heat treating, although they do not specify a recipe for HT. So I take that to mean to harden, 1500 and quench, then temper at 425 degrees twice for two hours each, which should result in a Rockwell Hardness of about 57-60.
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Old 12-21-2021, 10:52 AM
vlegski vlegski is offline
 
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Because each oven heats differently i generally dial in my tempering numbers. If the book says 425f, I might temper at 380 or 400f, clean the edge, and check hardness with hardness files. If it checks out too hard then I dial oven up by 20 degrees to say 380/400f and temper again.. The files aren't exact but they get me close to the Rockwell I want.
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Old 12-21-2021, 03:51 PM
KenH KenH is offline
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Quote:
Originally Posted by Johnnyjump View Post
The steel is from Alpha Knife Supply.
Enough said - Just heat 'n quench, you should be good. You might try a small coupon (2X2"?) to test, but AKS's steels are pretty darn good.
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Old 12-22-2021, 06:05 AM
Johnnyjump Johnnyjump is offline
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Agreed! Thanks to all for their insight and advice!
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Old 12-22-2021, 05:43 PM
Kevin R. Cashen Kevin R. Cashen is offline
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Im not sure how we define expert these days, but I will give it a stab. Knifemakers will always use the wrong terms interchangeably, some folks have tried for years to clear things up but as soon as the next wave of internet enthusiasts come along the confusion recycles.

Normalizing has always been confused by knifemakers as almost any heat that does not involve quenching or vermiculite. When, in fact, it is a very specific operation with very targeted results in its effects. Normalizing is one of the highest in temperature of our standard thermal treatments and is the ultimate reset switch for the condition of the steel. The resulting effects on phases and internal conditions is homogeneity, providing uniformity in grain size but, more importantly, uniform distribution of proeutectoid materials, ie ferrite or carbide, depending on the carbon content of the steel. To use it as often as it is recommended by too many knifemakers would be gross overkill if not specifically addressing these aforementioned concerns. Normalizing is best used as a starting point for subsequent heat treatments after operations as intense as something like forging.

Stress relieving would be practically the opposite of normalizing or, at least the opposite end of the high heat spectrum. Normalizing would typically be above Ac3 or AcCm Which could be anywhere from 100 to 500 higher than the standard stress relieving range. Proper stress relieving is done for the purpose of annihilating strain effects in the atomic lattice. It occurs in the heating range known as the recovery phase, well before recrystallization occurs, so it will have no appreciable effects on grain size, and quite limited effects on existing carbide condition. There may be limited formation of additional spheroidal cement in higher carbon contents but no dissolution of carbides.

If you are looking to keep your blades straight after heavy machining stress relieve. If you are looking to keep your blades straight after heavy forging- normalize. Stress relieving is in the range of 1150F to 1250F so it does not scale or decarb as bad as other operations. Be aware that while Ac1 is at around 1335F, anything under 1400F will result in duplex phases, giving you a mix of prior proeutectoid phases (more undissolved pearlite in 1084) and newly formed grains. Unless you are prepared to work with this condition I would not recommend cycles, even for simple grain refinement, in the range below 1400F down to Ac1.

Last edited by Kevin R. Cashen; 12-23-2021 at 03:52 PM.
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Old 12-22-2021, 08:50 PM
vlegski vlegski is offline
 
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Thanks Kevin. Last paragraph helped alot!
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  #11  
Old 12-23-2021, 08:35 AM
Johnnyjump Johnnyjump is offline
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Thanks so much for your explanation, Kevin! I hope you won't mind, but could you define the terms, AC1, AC3, and AcCM? Just beginning to understand the chemistry behind the heat treat process. I'm trying to better understand your last paragraph. You say that stress relief could be applied anywhere fro 1150 F to 1250 F, then seem to say you would not recommend cycles for grain refinement below 1400 F.
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  #12  
Old 12-23-2021, 11:24 AM
Kevin R. Cashen Kevin R. Cashen is offline
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Quote:
Originally Posted by Johnnyjump View Post
Thanks so much for your explanation, Kevin! I hope you won't mind, but could you define the terms, AC1, AC3, and AcCM? Just beginning to understand the chemistry behind the heat treat process. I'm trying to better understand your last paragraph. You say that stress relief could be applied anywhere fro 1150 F to 1250 F, then seem to say you would not recommend cycles for grain refinement below 1400 F.
Since recovery only deals with reordering strain related defects in the atomic lattice (residual stress, if you will*), it is well below Ac1, so no phases are dissolved. Ac1 is the point at which these iron-carbon phases will begin to go into solution, in a pure iron-carbon system it is around 1335F. To give you perspective, non-magnetic (the Currie point for iron) is 1414F. Ac3 is the temperature at which full solution is finally achieved in a steel with less than .8% carbon after the initiation of that transformation at 1335F. AcCm is the temperature at which total solution is achieved with a steel greater than .8% carbon. I use these designations because exact temperatures would depend in the individual steel, e.g.- AISI 1075 will have an entirely different Ac3 temperature than AISI 1050 due to the carbon level differences.

For operations such as stress relieving and, sometimes, spheroidizing, the temperatures are kept below Ac1 so that there are no effects of recrystallization, and no phases going into solution. In 1084, from Ac1 up to 1375F, there will be partial solutions with a rapid increase to complete solution from 1375F to 1400F. Cycling in the range from 1335F to 1375F will give a mix of new austenite grains with the old partially dissolved phases, not a problem if you plan on dissolving them all on the next treatment, but a serious problem if you are hoping for homogeneity. Once again, operations below Ac1 are not included in this since they do not involve dissolution of phases.

Here is one of the problems with confusing normalizing with lower temperature treatments. With a piece of heavily spheroidized stock that you wish to refine to a finer condition that will harden more readily with shorter soak times. If you cycle at temperatures well below Ac3 or AcCm, you could make the problem worse by simply growing the existing carbides, rather than dissolving them.



*to those with knowledge of material stress/strain, yes I know the difference in the engineering terms of stress and strain, but I have found that the average person does not nor really needs to.

Last edited by Kevin R. Cashen; 12-23-2021 at 04:24 PM.
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Old 12-23-2021, 04:42 PM
Doug Lester Doug Lester is offline
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Thanks for the refresher, Kevin. Maybe I need to review your video again.

Doug


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  #14  
Old 12-24-2021, 07:06 AM
Johnnyjump Johnnyjump is offline
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I'm definitely needing to become better educated on the chemistry behind heat treat. Knife Engineering by Dr Larrin Thomas was recommended earlier. Any other written or video sources you could recommend? Thanks.
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Old 12-24-2021, 01:39 PM
Kevin R. Cashen Kevin R. Cashen is offline
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Originally Posted by Doug Lester View Post
Thanks for the refresher, Kevin. Maybe I need to review your video again.

Doug
You are welcome Doug, there is only so much one can fit on a disk, so for the 1080/1084 DVD I have files and files of metallographic images that didn't even get used. I have more time staring at that steel through a microscope that most have working with it. But then, I made blades from it for about 25 years before that... I guess I am getting old.
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