An odd tempering question

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me2

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Has anyone tried tempering at higher temperatures than normal for shorter times to get the same hardness? Say for instance, normally O1 gives 58 HRc at 450 F after one hour (example, I don't really know). Can you temper at 600 F for 15 minutes (or whatever combo works) to get the same hardness? I was just wondering if the higher temperature would have any beneficial stress relieving effects.
 
Higher temps will result in lower Rc numbers.Tempering is a result of time and temp.not just temp.
Stan
 
That's essentially what I'm asking. Does anyone use or has anyone tried higher temps for shorter times?
 
i have, under uncontrolled conditions [wood-fire].

it gives unpredictable temper, and uneven heating.
 
If I got it right, one of the things that happens during tempering is that retained austentisite converts to martensite and martensite refines its structure. Both of these take time. Higher temps will result in lower RC without either of these happening.
 
While tempering is a function of time and temperature, the ratio is not fixed and is because they determine different things. The temperature determines how much and what type of transformation is occurring. The time is a factor of allowing the transformation to happen ( it is not instantaneous) ,not to what degree it happens.
Tempering for longer time periods will slightly over-temper the steel, because it allows more transformation, but it is almost a logarithmic scale, not linear. If you were trying to temper at 450F for one hour to get Rc59, but forgot to take it out of the oven until the next day ( happens more than you would think), after 10-12 hours the Rc may be about 58. The steel may actually be tougher,too. The opposite is not true with shorter tempers. You are not giving the structures time to convert, so little or no change occurs except in the thinest sections....the edge, which is exactly where you don't want over-tempering. The thin edge may well get tempered to Rc52.
 
Tempering is a diffusion process and we want a thorough tempering so we usually say 2 hours , certainly 1 hour minimum.
A complex steel needs much more care in tempering and 2+2 or 2+2+2 is suggested depending on the steel .A complex steel also can temper very differently at high temperatures , such as secondary hardening where different carbides are formed.
 
With that said, how effective is select tempering (as in, tortch, hot tongs, or plate.)?

There is almost no soak time with these methods. Would just the outer skin be getting tempered?
 
Standard tempering times are assumed to be about an hour. Is there any data out there for say, 1/2 hour? I know once temperature is reached, the reaction is not instantaneous, but how fast is it? My question centers around an idea I had to temper the surface at a higher temperature than the interior, giving sort of a laminated structure without having to use different steels. Probably the best way to do something like this (I'm sure industry has tried it somewhere) is with induction heating. I just haven't seen any data on it and don't know of anyone doing induction tempering. Induction hardening (austenizing) is fairly common.
 
Rick Marchand said:
With that said, how effective is select tempering (as in, tortch, hot tongs, or plate.)?

There is almost no soak time with these methods. Would just the outer skin be getting tempered?
Click to expand...

That's just what I was going to ask.:D Good question. I would think that using a torch along the spine of a blade would be a form of tempering with high temps in a very short time.
 
With carbon steels like 1095, soak time is only a function of getting the steel at an even temperature all the way through. Once saturated to temperature (either in hardening or tempering) there is not much benefit to soaking.
 
With that said, how effective is select tempering (as in, tortch, hot tongs, or plate.)?

There is almost no soak time with these methods. Would just the outer skin be getting tempered?
Click to expand...

It's not very effective at all. Many of the failures I've witnessed while administering ABS JS tests were caused by attempting these methods of tempering, or in other words "soft back drawing". What occurs in nearly every case is that the blade breaks during the bending portion of the test, and when examined closely, you can visually see a "skin" of tempered material .010-.020 thick, but the remainder of the blade never transformed, leaving the core of the blade "as hardened".

I've experimented with the soft back draw for a lot of years...it can be successful, but most do not have the patience to do it the way it needs to be done. Most use the visual colors on the exterior of the steel to judge when using a plate, tongs, or a torch....that's when you'll only achieve a tempered "skin" on a blade. The most effective method I have found is to place the edge in water, and work the spine for AT LEAST 20 mins with the heat source. Considering that tongs or a plate will not hold heat long enough to this, the torch is about your only option. BUT, even then the spine never gets tempered all the way through. In most cases you can get the blade to bend to 90 degrees...once. If you go back the other way the blade will usually break, and upon examining the broken ends, you will see that the tempering effect still didn't make it all the way through the blade's thickness.
So, in reality, it's all academic, between the time, effort, and room for error that this method imparts, I personally don't think it's worth it.

If you want a soft backed blade, a much more controllable and effective method is edge hardening. Once you understand how to accomplish it, it gives tremendous control over many aspects of a blade, and offers a level of control you could never achieve with the soft back drawing method.
 
Sorry to disagree with you Bill, but the time at temper is very important. It doesn't really start until it passes about 200F and needs at least an hour ( even for 1095) to allow the diffusion to take place. At austenitization temperatures ( 1350-1550F), the change is quite fast, and once fully heated evenly, time at soak is not as important for simple steels. However, at tempering temps ( 350-550F) the transformations occur at a very slow speed......for all steels.

Any tempering method that does not allow enough time for the transformation may do something to the steel's structure, but it will be incomplete and may be very uneven.

Before anyone gets all excited and says " But that is how it was done for thousands of years", I would like to point out that for thousands of years people with health problems were bleed to "let out the bad blood". Now days we know what it take to make sick people better.....and we also know what it takes to temper steel better.
 
I like Ed's take on this. And like he said "if you want a SOFT back".

One additional option if you don't care for the edge quench/soft back, either because of cosmetics or performance, is to do a full temper--say 3 two-hour cycles at your desired temperature. If your blade is at say 58 RC, and that is where you want the edge, then use a torch on the back from there. I've seen Burt Foster and some others do this to very good effect.
 
Disagree if you wish. I can make a 1095 spring, harden it with a propane torch, quench in room temperature canola oil, temper with a propane torch with no soak time. These springs cycle hundreds, even thousands of times- bending from an arc of over 30 degrees to almost flat. I have made many hundreds of them. None has ever broken. Tells me I'm on to SOMETHING.
 
True, your results will work.
I reread my post, and it sounded a bit like an argument. I was just stating why the temper time mattered.
Also, I was addressing the question about O-1 and making knife blades.
 
me2 said:
Has anyone tried tempering at higher temperatures than normal for shorter times to get the same hardness? Say for instance, normally O1 gives 58 HRc at 450 F after one hour (example, I don't really know). Can you temper at 600 F for 15 minutes (or whatever combo works) to get the same hardness? I was just wondering if the higher temperature would have any beneficial stress relieving effects.
Click to expand...

Mr. 2, or can I call you me? No wait then you would be Kevin:confused:, oh the heck with it… me2, due to the way the tempering process works, the direction involved in your question would be the opposite of what you would want to go in order to achieve gains in the desired properties with tempering.

Tempering is a very complex process, although it is often an afterthought in knifemaking it is actually the most complex of all the heat treating operations in terms of what all happens inside the steel. From 275F to 350F there is very little that happens in the way of noticeable changes from the outside but on the inside a very dramatic shift in the atomic stacking begins to happen as carbon atoms are VERY slowly allowed to move via diffusion. From 350F on up enough of this shift has actually occurred to allow us to notice drops in hardness, it will be most dramatic at first but will continue as the desired effects become more thorough.

Above 400F there is an effect on any retained austenite that can affect both hardness and any subsequent cryo efforts if one had hopes of converting it to more complete martensite. At 450F to 550F degrees there can be a very obnoxious effect from secondary carbides forming within martensite packets which results in a loss of toughness while still losing hardness at an alarming rate. This is a nasty no win zone where you are paradoxically losing both desirable properties at the same time. While this phenomenon, known as TEM (tempered martensite embrittlement), does happen in simple steels such as the ones we use, fortunately it is not a real problem in all steels except ones with certain combinations of basic alloying. This issue should not be confused with TE (tempering embrittlement) which occurs in richer alloys at temps above 600F, but will result in heavy secondary carbide precipitation with an unexpected spike in hardness and brittleness.

At any rate all of these effects are diffusion dependant which means they are a factor of time and temperature, but as always temperature is by far the greater power. But at temperatures as low as used for blade tempering the effects of diffusion take vast amounts of time to occur, making this an operation that cannot be rushed. As with soak times at high temperatures the industrial specs should not be ignored simply because we are working with fractions of an inch in thickness, the chemistry involved requires time to happen even after the blade fully assumes temperature. In fact some gains in toughness at a higher hardness can be achieved by going in the opposite direction from what the thread topic suggests. Longer tempering at lower temps can achieve much more thorough diffusion while avoiding many of the complications mentioned above, and could give some extra toughness at a relatively higher Rockwell.

I have independently confirmed the concepts outlined in this post myself, both in actual blade use and also in lab type analysis, but for an excellent source of information on all of the concepts I have outlined here I suggest “Principles of Heat Treatment of Steel” by George Krauss. It is just one of many writings on the topic but one of my favorites due to the in-depth coverage of the mechanisms; it is however a bit more advanced metallurgical reading.
 
Aww, Kevin... you beat me too it... I was just about to say the same thing... honestly, I was.:p


Rick
 
It was just an idea I was floating out there. Basically, I wondered if one could temper at higher temps for shorter times in order to get a progression of hardness going from the surface to the center. The surface would, ideally, be spring tempered, while leaving the center tempered in the 350 - 400 F range. It seems the results would be unpredictable. I have found no reference to the effects of such heat treatments in any of my reading, so I thought I'd get a bigger sample than the few authors whose books I have.
 
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