Tempering talk

kbaknife said:
I'll hold 'em, you flog 'em.
Thanks, Kevin.
Will this sort of discussion be part of your Ashokan dissertation?
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Karl, the Ashokan metallurgy lecture will involve a microscope hooked to a video feed and a large assortment of prepared samples. With thes tools at my disposal we will then have a good Q&A session where I can talk ans show the actual subjects of conversation on the screen that we sould deal with in knives. The format has worked quite well when I have done it before and is much more interesting thatn a pile of dry words and a chalkbaord.
 
Kevin, Mete, RJ, thanks for the good respnses.
Some folks like to pick the fly crap outta the pepper and make things way more complicated than they are.
 
mete said:
"explicitly state" Please understand that published info on HT is a guide not an absolute. Complex alloys will have retained austenite and you can't remove it all just minimize it...As for going right to cryo - if you're the nervous type do a snap temper of 300 F first ! It depends on whether or not you want your bridge to fall down !
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It is wise to listen to what mete is saying, we all (myself included) like to describe things in absolutes, wishful thinking perhaps and just wanting to shoot for 100%. But 100% would be perfection, and there is but one maker that can hope to obtain that level. Everything we do will mostly be to maximize what we are shooting for, hoping for 100% will not be realisitic, I doubt any of us will ever nail 100% martensite in a blade. Even simple alloys will have some retained austenite regardless of how well you did, but we can expect it to be at low enough levels not to ever notice it. Alloying plays a significant role and will seriously effect retained austenite and some steels will just have Mf bellow room temperature and will need some cold treatments. One more reason to totally abandon the idea of a "one size fits all" heat treatment for all steel.
 
Lot of info here, and that's good.
maybe I can get some help on zeroing in on my steel of choice, for now,5160.
Kevin states "In the 375F to 400F range the initial drops in overall hardness will be seen. I rarely need to investigate a persons methods much more when I hear them say that they temper at 375F or less on a steel with .6% or more carbon, it is obvious that they didn’t fully harden it to begin with. "
Well, that puts 5160 right at the bottom of that scenario.
Can I assume that lower than 400 degree tempering is acceptable when using 5160 because the degree of hardness "as quenched" is lower due to the lower amount of carbon?
I've tempered 5160 in the 400-425 range and ended up with a glorified spring that wold not sharpen and tested on a hardness tester up on the spine and only found 48 HRC!
At the same time, I often see 350-375 for tempering temps suggested for 5160.
Is this because of the lower C.?
Or is 400 recommended for ALL non-stainless steels and I have other issues?
 
bladsmth said:
Tommegow
The other way around. Temper at 375F first if you are not sure, and then again at higher temps to attain the target Rc.

Kevin
Yesterday (about 90 minutes before your post) I saw the same confusion and misinformation about tempering that you did. My post was here:
http://www.bladeforums.com/forums/showthread.php?t=490613
What is interesting is that we both addressed it in the same progression and with the same basic information and conclusions.
As always your info was more detailed and technical. Your use of graphics was excellent. A picture is worth a thousand words, especially when the reader isn't sure what the words mean. I ,for one, really appreciate your posts and edification efforts.
Thanks - Stacy
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Yes, indeed.

Tommegow, Stacy hit it on the head, start low and walk in your exact HRC by bumping up your subsequent tempers. In normally start at 400F but I would advise others to start at around 375F-380F then check your hardness and bump it up around 20 degrees and go again, continuing until you get the exact hardness you want. I have found that around the 400F range 20 degrees equals approx 1 point HRC (this is not written in stone buts works with my equipment and proceedures). Dropping temperatures as you go would accomplish little more than using up electricity, hardness will not change, since your first heat would be your highest if you over shoot you are boned!

My times and temps will vary since I temper in the salts, but I normally get things stabalized and down to 60-63HRC with the first temper cycle and then take my hardness readings. I then bump it up 20 degrees and run another cycle before more hardness testing. By the third cycle things normally are right where I want them. I keep records on all my tempering jobs and have found one thing that is a constant in all the cycles I have ever done. The HRC range gets narrower with every tempering done after the first, indicating a very nice homogenizing effect that has caused me to insist on all of my stuff being tempered at least three times, after this here are dimishing returns.
 
kbaknife said:
Lot of info here, and that's good.
maybe I can get some help on zeroing in on my steel of choice, for now,5160.
Kevin states "In the 375F to 400F range the initial drops in overall hardness will be seen. I rarely need to investigate a persons methods much more when I hear them say that they temper at 375F or less on a steel with .6% or more carbon, it is obvious that they didn’t fully harden it to begin with. "
Well, that puts 5160 right at the bottom of that scenario.
Can I assume that lower than 400 degree tempering is acceptable when using 5160 because the degree of hardness "as quenched" is lower due to the lower amount of carbon?
I've tempered 5160 in the 400-425 range and ended up with a glorified spring that wold not sharpen and tested on a hardness tester up on the spine and only found 48 HRC!
At the same time, I often see 350-375 for tempering temps suggested for 5160.
Is this because of the lower C.?
Or is 400 recommended for ALL non-stainless steels and I have other issues?
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Karl I would never recommend a blanket temperature for any group of steels, not even 400F. 5160 should reach 60-61HRC as quenched if the soak was sufficient, and here is where most folks run into trouble. I have done numerous soak time studies and one thing I have noticed is how resistant to softening steel can be if enough carbon was pulled into solution. One could get 60HRC from 5160 by getting just enough carbon into solution do to so and no more, but in tempering this hardness will drop much more rapidly than a piece that had a proper soak, that initial Rockwell hardness seems all "full of sound an fury but signifying nothing".

"At the same time, I often see 350-375 for tempering temps suggested for 5160."

You probably often see these suggestions from nitwit bladesmiths, you can put more trust in the "Heat Treater's Guide", which recommends temps from 400-450F. Too many would tell you that the specs given in that book are for industry and don't work for knifemakers, but I find it really interesting that when I finally started using tools that let me do it the way the book recommends all of those industry numbers suddenly started coinciding with my own:jerkit: hmmm, if you actually soak steel at the recommended temperature for the recommended times, it will result in the properties that people who made steel said it would, WOW! Imagine that!

Now you know me well enough to know I am not directing this at you as you know of the jokers to whom I refer. In all of my experience if I ever see a drop in HRC below 375F tempering, I know I blew it in the hardening. The carbon level will determine how hard it gets initially (at .60% things will begin to level off and maximum levels will be seen at around .80%). Tempering will take some of this carbon out of play, how much you got into play to begin with will determine how much can go before hardness drops.

I often hear well regarded smiths speak of tempering 5160 to only 375F or less, but I have yet to hear any as quenched HRC numbers before they did this, I would not be surprised to find those numbers to be conveniently close to the final ones. Take a hypoeutectoid steel and heat it for a minute until a magnet quits sticking (1414F whether they acknowledge it or not, while 5160 needs 1525F) then plunge it into a bucket of sludge, and nobody should be surprised that very little tempering is needed to toughen things up, heck you may not need to temper at all. Barely enough austenite was formed to transform into anything brittle and then cooling was just enough to avoid making pearlite coarse enough to see with the naked eye! To be honest with many of the recipes I have heard and read I am not sure if any tempering is just a formality and a waste of time.

In fact I have actually encountered the attitude that one can replace the effects of tempering by simply adjusting quench speeds, since it will make a tougher blade:rolleyes:!!. And was another really good reason to do this thread. Pearlite and tempered martensite are two entirely different things and properly tempered martensite will soundly beat pearlite in both hardness and toughness, this is one of the reason there are no edge quenched blades with an old English “KC” stamped on them.
 
magneto said:
Can you elaborate some on this. I have been thinking about tempering in salts since the pot is hot. Does it need less time tempering in the salt pot?
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As with austenitizing and quenching, everything is much more even in the salts. Oven are great but consist of open space that acts as an insulator, and things can be hotter nearer the elements. Salts are a highly conductive medium that is in total and direct contact with the blade. One can see significant tempering effects in just 30 minutes in 400F salts while the same blade would just be reaching the oven temperature in 30 minutes at 400F. The thermal mass of an entire tube of salts completly avoids the huge temperature variations of something like a toaster oven (I always wondered about our fascination with the most poorly calibrated and difficult to regulate heat source we could find).

I do love my salts for tempering, heck you just used them to quench and there they are a 400F heat source waiting to be used that you can go right back into as soon as the blade reaches Mf... it just makes a lot of efficient sense. I can often do my initial temperings in less than an hour in the salts and get onto business. I don't have to worry about purple edges on a straw colored blade, since the edge came up to temp at virtually the same time as the spine and there was no atmosphere to mess with it. The coating that is left, if very well rinsed, is similar to a bluing and will allow the blades to resist a hair more corrosion of I can't get to them right away.
 
Thanks, Kevin! Makes sense to me.
My soak time on my 5160 has been about 15-20 minutes after my oven rebounds to temp, and then another 4-5 minutes for the steel to catch up. Then, that got me thinkong about the interrupted quench on 5160.
I realize you, and others, use a lot of the simpler steels - 1084-1095-L6, etc. and faster oil of #50 speed.
Even though I use the same "recipe" of quench time to get down to Ms of 400 or,so, will my "slower" steel and "slower" oil benefit from this quench time, as far as % of martensite transformation, as will the faster oil/steel combinations?
In other words, does the same thing happen in 4-5 secs with slower oil/steel combinations that happens in 1-2 seconds with the faster oil/steel combinations?
All things being correct, up to the point of tempering, i.e. normalizing, spherodizing, soak time, quench time, oil speed, am I getting my 5160 to be "all it can be" ?
If I am, then 400 should work for tempering.
Right?
 
I guess what I'm saying is, I realize we want to get down to the 400 degree range on quench to get Ms. From things I've seen written on this subject, 7-8 seconds is a good rule of thumb. But, I'm thinkng that temp is attained with the popular Parks #50. This gets the simpler steels below that "nose" in just a second or so, and leaves another 6-7 seconds to get down below 400 or so.
Now, with the mid-range oils and steels, we don't get below that pearlite nose for about 4-5 seconds! That only leaves another 2-3-4 seconds to get donw to Ms at that rate.
Is my problem - possibly! - that I sould leave my 5160 blades in the Type A oil maybe 10-11 seconds to get down to Ms because of the slower oil? I'm pulling out to soon? (You know what I mean!)
 
Karl, have you tried that fancy ray-gun thermometer on one of your blades coming out of quench?
 
fitzo said:
Karl, have you tried that fancy ray-gun thermometer on one of your blades coming out of quench?
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Yes, I have!! But, for the most part, it varies quite a bit and I think it's because of all of the oil/smoke that accompanies the blade right out of quench!
Damn "ray gun" doesn't know what to read!!
I think I'm going to add a few seconds on my next two knives.
 
kbaknife,
The nose for 5160 is about 4 seconds. The time to 400F is 100 seconds. It is the nose that the oil is matched for. For 5160, Parks AAA will get it past the nose in time. The remaining drop will be more than fast enough (assuming a sufficient volume of oil) to meet the Ms deadline. As to your question, YES, your use of 5160 and Parks AAA (for example) will yield the same results as using 1095 and Parks #50. Both situations are properly matched for the steel and the quench speed required. Will quenching 5160 in #50 be better or worse? Probably neither, but that is because 5160 is so forgiving. Quench the wrong steel too fast and it may warp or crack. Quench it too slow and it won't harden properly, ending in a Pearlite mix. Rushing the steel to Ms too fast is not advantageous. That is part of the reason the oil is not kept at room temp. One other reason is that with 120-130F oil, the steel stops at a point before most severe cracking occurs, and holds at just below the Mf.
Stacy
 
bladsmth said:
kbaknife,
The nose for 5160 is about 4 seconds. The time to 400F is 100 seconds. It is the nose that the oil is matched for. For 5160, Parks AAA will get it past the nose in time. The remaining drop will be more than fast enough (assuming a sufficient volume of oil) to meet the Ms deadline. As to your question, YES, your use of 5160 and Parks AAA (for example) will yield the same results as using 1095 and Parks #50. Both situations are properly matched for the steel and the quench speed required. Will quenching 5160 in #50 be better or worse? Probably neither, but that is because 5160 is so forgiving. Quench the wrong steel too fast and it may warp or crack. Quench it too slow and it won't harden properly, ending in a Pearlite mix. Rushing the steel to Ms too fast is not advantageous. That is part of the reason the oil is not kept at room temp. One other reason is that with 120-130F oil, the steel stops at a point before most severe cracking occurs, and holds at just below the Mf.
Stacy
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Correct - I know the nose for 5160 is four seconds. And I use Tex. Type "A". I wouldn't use #50 for 5160. I'm not trying to get there faster.
My question was, that many times I have seen the 7-8 second reference to get to Ms of around 400, and I'm thinking that was using faster/steel/oil combos as the "control". Such as 1084 and parks #50, for example.
My question was, with the slower oils for 5160, is that Ms temp of around 400 attained in 7-8 seconds!?! If it takes about 4 seconds for the Type A oil to get below, say 900-1000 degrees, where is it at 7-8 seconds??!! Might NOT be down to 400! Maybe I should add a few seconds to my quench time!
 
bladsmth said:
kbaknife,
The nose for 5160 is about 4 seconds. The time to 400F is 100 seconds. It is the nose that the oil is matched for. For 5160, Parks AAA will get it past the nose in time. The remaining drop will be more than fast enough (assuming a sufficient volume of oil) to meet the Ms deadline. As to your question, YES, your use of 5160 and Parks AAA (for example) will yield the same results as using 1095 and Parks #50. Both situations are properly matched for the steel and the quench speed required. Will quenching 5160 in #50 be better or worse? Probably neither, but that is because 5160 is so forgiving. Quench the wrong steel too fast and it may warp or crack. Quench it too slow and it won't harden properly, ending in a Pearlite mix. Rushing the steel to Ms too fast is not advantageous. That is part of the reason the oil is not kept at room temp. One other reason is that with 120-130F oil, the steel stops at a point before most severe cracking occurs, and holds at just below the Mf.
Stacy
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Stacy, I re-read that post I made to which you replied, and I can see why you said what you said!
What I wrote was " But, I'm thinking that temp is attained with the popular Parks #50."
And I should have said " But, I'm thinking that temp is attained with the popular Parks #50.... and 10XX and W series steels."
 
I'm not sure if I am missing something here. When you say, "Add a couple seconds to the quench" - Are you taking the blade out of the quench in 7-8 seconds? Just leave it in the tank for several minutes, or until you are ready to temper it. It won't matter. (Unless you are doing an interrupted quench for a specific reason.)

I don't know why anyone would say to get it to 400F in 7-8 seconds, because you have much more time than that once you pass the nose.

Each steel has unique isothermal transformation properties, and the use of ITD or TTT charts will tell you what to look for. For 1095 the pearlite nose is .9 seconds, but the Ms is 500 seconds. With 5160 The nose is 4 seconds and thew Ms is just over 100 seconds.
Stacy
 
No, you are not missing something -= you got it.
Yes, I've been interrupting the quench at what I had hoped to be around 400 degrees.
I was under the impression that the Ms to Mf range sort of went from 400 down to about 200-250 degrees or so. If the blade dropped all the way to the temp of the oil, the full potential of martensite transformation would be halted because the oil temp is below Mf.
I've been wanting to get down below that 400 degree mark and remove the blade to allow martensitic transformation to occur un-inhibited, but at the same time not too hot.
I am NOT trying to get to 400 in any specified amount of time on purpose!
I have read that the Parks #50 will get there in about that time - 7-8 seconds..
I'm trying to figure out how much EXTRA time a slower oil will take to get to that mark. I don't care if it takes two minutes, I'm just trying to figure out when it gets there - 400 - and before it gets totally cool!
 
Unless you want a mixed bag of structures, don't do that. That is basically a quasi-austemper. For getting sori (curved blades = controlled warp?), or making bainite swords, that is what is done. You would gain no advantage in martensitic conversion by stopping the quench at 400F and might create problems you don't want. The reason I pull a blade out above 400F (I shoot for 800-900F) is to give it a quick check for warpage. You can hammer, twist, whatever above 400F and it is like butter. Once it gets to 200 it will break. Now, about what is happening inside your blade. At 400 ,the austenite STARTS converting to martensite. It is done by 200. If you don't let it get to 200, it won't all get converted in a smooth and orderly process. So the martensite and austenite start fighting for who's king of the hill. Bad situation.
Once the blade has cooled to 200 the transformation is complete, or as much as it will for now. Cooling to 100-120F and then immediate tempering at Ms again will relieve the Martensitic stress and allow any retained austenite to convert. Again this won't finish until it drops below Mf again. A final temper will fix the new martensite and make a last conversion for any minute amounts of austenite. Three tempers is the max before there is no net gain. Two will suffice for any simple steel.

If you want to experiment with austempering quench into hot oil and immediately place in a preheated 450F oven (before it gets to 400F). Hold for many hours and you will have a bainite blade. For most knifmakers, this is of no advantage.
Stacy
 
The cooling should be continuous to Mf if you are shooting for martensite as you will want to keep the shear type reaction going. Only If one was austempering would the cooling be halted altogether, and then above Ms. It is the rate of cooling that we are dealing with. If sufficient carbon is trapped within the matrix there is no real benefits in greater martensite by slowing the rate of cooling. Cooling all the way to room temperature in the oil or switching to air cooling at 400F will come down to six with one and half a dozen with the other as far as the carbon rich martensite is concerned. Now, if properly marquenched you will see 1 to 2 points less HRC as quenched but this is not due to lack of hardening but more a result of a good percentage of the fresh martensite being autotempered on the way down.

I do not interrupt a quench or use low temp salts to affect the ammount of martensite formed from Ms to Mf, as you wouldn't since the process is not time dependant, but instead to control the rate at which the transformation occurs. I could slam the shear reaction in at full quench speed or I can let it happen at a rate where the thermal mass can deal with some of the stress factor as it arises.

I know the athermal nature of martensite formation can be confusing (heck the term "athermal" doesn't make much sense when applied here since it is all thermal). The time the steel is held in any range has no effect whatsoever on the martensite formation itself, that is soley dependant upon temperature. If you stop the cooling at 50% martensite that is all you will get until you start coolling again be it a minute or a day. If you cool at 500F degrees per second, or 50F degrees per second Mf will still be Mf. We say Mf instead of M100% because it is not realistic to belive we would ever reach that.

Think of Mf as somwhere in excess of M95%. If you quench all the way to ambient in oil you will reach M95%+ in a few seconds after Ms, but you will hopefully reach it. If you interrupt the faster cooling rate for a slower one in air at 400F, you will reach M95%+ in a few minutes but reach it you will. Either way, fast or slow, you want the cooling from Ms to be continuous if you desire martensite, you don't want to stop and you definitely don't want to go warmer, until you are done.
 
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