kerisman

Time between hardening and tempering

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I made a blacksmith tool (a hack) with the business end made from A-2. I hardened it, then ran out of time and it still hasn't been tempered. I just read that tempering should be performed immediately after hardening. I know that tempering absolutely must be done before using or the tool will fail from internal stresses, but I'm not convinced that it's going to be such a problem if it sits around for a few days in the hardened, pre-tempered stage without being used...

By letting it sit between hardening and tempering, have I caused it to form a fragile grain structure? Should I anneal, re-harden, and temper immediately or will I be okay if I go ahead and temper it now, then use it?

By the way, what tempering temperature would be best? I guess I'll use an oven.

Thanks.

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When you harden steel, the grain structure changes from austentite (soft) to martensite (hard). To my knowledge, martensite does not revert back to any other structure at room temperature.

Tempering immediately after hardening pretty much is a precaution so you don't have time to drop the tool or break it by accident. They don't call it "glass hard" for nothing!

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Maillemaker is correct in that you will be able to just temper it now. However the precaution to temper immediately is more intended to prevent spontaneous breakage from cooling stresses than from dropping. As the tool or blade cools it shrinks and in some cases (particularly when the metal is high carbon and hardened to a very high hardness) this change in dimension can be enough to crack or break the metal. It is a particular concern in cases where the tool or blade is unusually large in any dimension or very thin in any dimension. Immediate tempering can also help to minimize warpage which can also be caused by cooling shrinkage. For many medium carbon items of common shapes tempering can be delayed with little risk of problems.

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For a lot of years I never ever worried about that. I would harden how ever many items I had ready and temper them the next time the wife was going to be gone shopping for a few hours so I could use her oven. However this site caused me to change my evil ways. When I read about folks that had cracks form during that delay and not from dropping I decided to not try and see how long before it happened to me. Now when I heat treat I turn the small oven on in the shop before I fire forge or heat treat oven. Then as I take blades from the oil I go right into the oven after a wipe down with a shop rag. The old saying comes to mind. If we ignore history we are doomed to repeat it!

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I have left a blade on the bench after quenching and went to have lunch only to come back to a two piece blade.
Quenched high carbon steel should be tempered right away.
The charts i have say that A2 should be tempered when the steal gets to 120f-150f and a double temper to
help transform any retained austentite.

Bob

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Okay, so it sounds like immediate tempering is ideal (but delayed tempering might work out anyway, depending on the tool's application). I want to minimize failure in my tools, so how do I recover? Anneal, re-harden, and temper? Or is it simply too late to return to the initial grain structure, so I'll have to temper now and hope for the best?

When I do temper it, how does 550 in the oven sound?

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Okay, so it sounds like immediate tempering is ideal (but delayed tempering might work out anyway, depending on the tool's application). I want to minimize failure in my tools, so how do I recover? Anneal, re-harden, and temper? Or is it simply too late to return to the initial grain structure, so I'll have to temper now and hope for the best?

When I do temper it, how does 550 in the oven sound?


What does the final intended usage, have to do with stress fractures occurring due to not tempering soon enough? any delay is pressing our lucky with our project. Temper temperature does depend on intended use, as well as steel used.

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I tell my people that if they fumdiddle any of the steps, they should normalize (or anneal). It depends on the steel. Then harden and temper all over again. Leaving hardened steel laying around means leaving a stressed piece laying around. Besides Grant's tink description, the steel could be on the verge of "tink." I consider leaving hardened steel for a while without tempering right away, a fumdiddle.

If you delay the temper and you still get a successful tool, you lucked out.

http://www.turleyforge.com Granddaddy of Blacksmith Schools

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Steve, I was not suggesting that our intended use of a tool would in some way influence its stress response to remaining in the pre-tempered stage for an extended period (psychic consciousness of tool steel?), but that we may get away with "fumdiddling" (to steal Frank's term) if it's a tool that won't be subjected to a high degree of stress. I'm just speculating here, not claiming to know anything; this is actually my first foray into heat treatment.

Frank, the steel is A2. I read that A2 is not to be normalized. So, I guess the plan is to anneal, harden, and temper.

Thanks, all. Learning from mistakes and putting the "BS" into blacksmithing...fun as always!

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Perhaps you missed the point we are making about hardening: the procees in it self is a stress to the piece. End use of said item has nothing to do with it already being ruined due to fractures from that stress. Not all of these are visible to your eye.

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Keris
Don't pay any attention to these "play it safe" or "Do it right" guys here.
You had fun making the tool the first time..I am sure you will have fun making another one and you may even have smaller pieces for other projects without the need to actually cut the bar yourself...it may just do it on your behalf. Of course those smaller bits may not be the exact size you need and riddled with micro-cracks, but I am sure you can work it out...your creative.


There is a well known knife-maker who discovered a very simple and fast way to harden and temper his axe heads. He was on the phone telling another smith how you can just heat the head and quench the tool and let the latent heat bleed back into the edge...and never have to do anything else. During the conversation the 12 axes on the table went ..tink..tink...tink...tink all in a nice little row....timed about like he had quenched them.
I think he was able to save the last two or so in time.


Ric

you may find this of use..or not:

http://forums.dfoggknives.com/index.php?showtopic=19428&pid=182054&st=40&#entry182054

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Forget for a moment about grain structure and concnetrate on cracks. Grain structure stays the same. Cracks propagete. You can't uncrack something once it's cracked, unless you weld it back together. Ever try to forge-weld A2?

There is NO application I can think of where a crack won't be bad. (I've never made fragmentation grenade casings.) You say you are making a hack. A hack is for use under a power hammer - that's about as "low stress" as an IRS audit during a sloppy divorce.

Cracks can be internal - inside where you won't see them. Cracks can be microscopic - little spiderweb crack networks running along the grain boundries.

So... maybe you don't have any cracks. Maybe you do. You gonna stick that hack under a power hammer, call it a "low stress application" and cross your fingers? I hope for your spouse's sake your life insurance is up to date.

Also, 550F is in the temper embrittlement range for A2. Temper below 400F or above 700F.

-

Now a question in the hope for some good news: How did you measure your 1700 - 1780 F austinizing temperature? And how long did you soak at temp for the carbides to dissolve? If your answer involves a magnet, you might not have hardened the steel at all: ergo no cracks!

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Too bad anyone ever came up with the term "normalize". All of our other cooling processes are defined by the alloy involved and the cross-section to obtain a certain structure. Quench in a given medium for a certain cooling rate, water, oil or air depending. Annealing requires a certain rate of cooing depending on the alloy.

Then along comes "nomalizing" - cool in air. That's all? No cooing rate? No desired structure? Isn't that called "hardening" when you're working with air-hardening steel? Seems to me that normalizing is a term that can only be used with carbon and low-alloy steels. Should probably be called "pearlizing" as that's the objective and then you would come up with different cooling rates for different steel to obtain a pearlitic structure.

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Should probably be called "pearlizing" as that the objective and then you would come up with different cooling rates for different steel to obtain a pearlitic structure.


Never thought of it quite like that Grant.
I'm willing to change my use of the term.

Can we also go after the guy who started saying to quench at "cherry red"? Always looks orange to me...never seen an orange cherry.

Ric

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Changing hearts and changing minds, one blacksmith at a time!

nope..my heart is still the withered black cinder it always was.....

Ric

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A2 is a steel that produce a lot of retained austenite, the higher the austenitazing temp, the higher the retained austenite rate. In his book dedicated to the bladesmithes J. Verhoeven says that steels that have a good response to cryo must be cryo-cooled/tempered (some may be cryo before tempering, some just after the first temper) have to be treated as soon as they reach room temp to avoid retained austenite stabilization.
If this applies to A2 (a steel I don't know much about) your retained austenite may be stabilized and tempering it would not change anything...
I don't know about A2 response to normalizing (the standard way) as an air hardening steel, but if you didn't austenitize too high and soak too long, you might just have to austenitize and quench once more. 52100 is often triple quenched where the two primary quenches are some kind of 'ultra-normalizations'...
What do you think about that guys ?
I hope that my English properly translates what I mean (French Frog inside tongue.gif) and that I don't make things more complicated than necessary rolleyes.gif

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Good points Madmike! One in particular that I overlooked is that normalizing is also used to refine the grain.


Grain size reduction when normalizing is the most desired effect for damascus makers when the blade must be ground to show the desired pattern (Ws explosions...) as the bevels won't be forged so grain size refinement due to dynamic recrystallization while forging in decreasing heats will not occur as it would on a forged to shape 'simple' random pattern...
In this case the ONLY way to refine the pumpkin size grain that came from the successive welding heats is normalizing.
And in any case, a bladesmith who doesn't normalize is to me a too self-confident bladesmith rolleyes.gif

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Cripes. Y'all made me get out an old* booklet,"Bethlehem Tool and Die Steel Manual." Re A2, I quote:
Forging 2000-2050ºF. Stop at 1700F.
Normalizing Do not normalize.
Annealing 1650F. Furnace cool. Brinell 223 max.
Preheating 1200 prior to hardening.
Hardening 1775F Air quench to 150F.
Tempering 350-400F, resulting hardness Rockwell C 60-61.

The booklet further elaborates. "After the pieces have been cooled in the quench down to 150F they should be tempered immediately. For most applications [A2] should be tempered at 350 to 400F. A minimum holding time of two hours per inch of greatest cross section should be used."

On pieces, say, 4" to 6" square for dies, an "oil flash quench" is used. The hot piece is quenched in oil until it reaches 1000F. It is removed to air cool to 150F, and immediately tempered.

*CAVEAT. The information in this booklet (Handbook 2531-F) may be outdated.

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Many are the smiths who left a hardened piece laying with the idea of tempering it "later". And then they hear a little "tink". No boom, no fanfare, just "tink".


Been there, the tempering was done immediately and it still broke. "IT" being several class projects. I was unlucky and had 100% failure in heat treat on the first go around. Several of my projects I did not even touch during heat treat as I handed them off to the lab assistant.


Can we also go after the guy who started saying to quench at "cherry red"? Always looks orange to me...never seen an orange cherry.

Ric


Different varieties of cherries are different colors. The more popular (and generally sweeter) varieties of cherry are dark red. There are cherries that range in color from near white to near black.

There is also the bit about color looking different to the eye under different conditions and such.

Phil

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The old "cherry red" came in before the almost black red Bing cherry cultivar became widespread and so it refers to the *pie* cherries that are more an orange colour when picked (and get dyed red for canned use...)

Unfortunately folks don't think about how things change over time and so methods designed for working wrought iron are still being applied to mild steel and the color of "modern" bing cherries are misleading new folks into working too cold.

If there is a problem with the piece hardened and untempered any repeat of the heat treat will not help it the piece of steel is "toast". If there is not a problem then re-heat treating it will not help it any more than just going to the tempering.

I would certainly not sell or give away a piece that had such a large "maybe" in it; but I have been known to use such goof-ups before and sometimes won and sometimes lost and ALWAYS made sure I was wearing my PPE when trying them out.

I had a college student yesterday forge a blade and harden it and then broke it trying to work on it before tempering, sigh.

I have also picked up a thermometer that goes to 400 deg F after one student heated the quench oil to hotter than the drawing temp and then wanted to know why his blade was too soft after quenching. Now when I tell them to heat it to 140 degF they can actually measure it!

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