questions about tempering...

[bassomatic123]

I am new to this site and blacksmithing as well.
I did a one week blacksmithing camp, with 3 hour sessions. But with this meager experience, I still feel confident enough to buy materials to make a very small forge.

just recently, I got a book from the library about smithing, called "The Modern Blacksmith" by Alexander G. Weygers. It involves mainly tool making, but knifemaking as well. One subject it talks about a lot is tempering the steel, something that I'm confused about. What I do know is that it's quenching the steel at certain color, or temperature. But can anyone give a good description of how to do it, or what it involves? Thanks a lot.

[Rich Hale]

Tempering steel is a process to actually soften the steel to a workable state. It follow the hardening process where a steel that has a usable carbon content is heat treated to make it hard. When it is hardened it is usually to hard to hold an edge without chipping and if bent it may break. If it is tool land not a edged blade the tool may crack or shatter. so the steel it tempered also known as drawn or drawn back. This usually involves heating for a period of time until the metal reaches a certain temperature and may be judged by a color change. Try Blueprint BP0078 for a further explanation.

[ThomasPowers]

Do you mean tempering as in making a hardened steel softer? Or do you really mean the complete heat treat cycle to harden and temper a piece of steel? If the latter then your qustion is best answered in about a 2000 page large scale book published by ASM. For a take slanted towards blademaking may I commend "The Complete Bladesmith" by James Hrisoulas to you---if you live in America the local public library should be able to ILL a copy for you to read. (Inter Library Loan)

Some of the high points would be:
Alloy! what alloy the metal is controls whether it will harden at all, what temp it needs to be raised to before quenching; what the quenchant should be and what temp you draw temper too.

Quenchents: generally you want to use the slowest quenchent that will provide the hardness you need; from slow to fast: Air, Oil, Water, Brine, Superquench

Temperatures: for simple steels the temp to quench at is usually fairly close to the Curie temp---where the steel becomes non-magnetic. For complex alloy steels use the manufacturer's recomendations. After it is hardened you should IMMEDIATELY temper! Tempering is a much lower temp process with common temps being between 375 degF to 600 degF; though some high alloy steels actually temper at over 1000 degF.

If you do not know what steel you have: Heat to non-magnetic, let cool, check with file, if hard it's an air harden steel---fairly rare in the scrap stream; but I have had star drills air harden in knife thicknesses. If soft, Heat to non-magnetic, quench in oil---check hardness with a file if hard it's an oil quenching alloy. If soft Heat to non-magnetic, quench in brine, test. If it doesn't harden enough in brine it is not an alloy suitable for blades but may still be ok for tooling if it hardens a bit in Superquench.

If you try to go the other way and heat an unknown steel and quench in Superquench it may shatter if it's a high carbon steel.

Note too that all this heating may create a decarb layer that will not harden so if it seems soft to the file; file a bit deeper and see if it's any harder.

[oakwoodforge]

Every thing said so far has been good advice, especially coming from 2 of the most experienced folks on this site. Possibly you can tell us what steel you are working with and what you are making and the good folks here can give you specific advice rather that generalizations.

Skal !

Jens

[julian]

If you want to know more about how steel works and what you can do to manipulate it look at this thread; scroll down and there's a link where you can download a large pdf file on metallurgy.

In short: quenching lowers a steel's temperature very rapidly and makes it hard, tempering raises a steel's temperature slowly and softens the steel enough to make it tough but durable.

http://www.iforgeiron.com/forum/f14/metallurgy-steel-bladesmiths-1896/

[bassomatic123]

oh, i thought tempering hardened the steel at a certain point. so you temper it, and then quench it for a desired hardness?

[ThomasPowers]

Backwards------ heat treat for knives usually starts with a normalization or 3; then you quench to harden and then you draw temper perhaps several times to trade some of that hardness for toughness.

Tempering softens the steel.

Thomas

[kbaknife]

Sort a few things out here:
Hardening is hardening. You are taking up a piece of hardenable steel to its Austenite condition, which is the condition where all of the carbides go into "solution" and become atomically liquid = not part of the Iron/carbon matrix. This condition is also the point a little above that temp. where the steel becomes non-magnetic known as "critical" temp. For most simple steels, this temp. can be anywhere from 1413 - 1525 degrees, providing the steel is non-stainless.
By quenching in a medium to reduce this temp and condition to below 1000 in 1-4 seconds, depending on the steel type/amount of carbon/amount to alloys, you change the steels condition to Martensite, which is a hard piece of steel.
The quench medium must remove the temp at a particular speed and rate to match the steels alloy content. Different steels with different alloys and amounts, require different "speeds" of oil/medium to reduce the austenitic temp to below 1000 or so in the specified amount of time.
Along with this hardness, come brittleness and "retained austenite" - austentite that did not "make" the transition to martensite.
A "soak" at a much lower temp of 350 - 400 degrees, completes the "retained austenite" transformation to martensite, and relaxes the brittleness. This lower temp/soak time is kown as "TEMPERING"!!!
Now, with the new martensite being formed from the "retained austenite", the steel needs to be tempered AGAIN to relieve hardening stresses of the new martensite.
Thus, the term you may have heard of double or even triple tempering.
Know your steel, know what industry standards are for THAT steel, what quench medium to use, what tempering temps. to use for what hardness is desired, etc.
This entire process is called "HEAT TREATING", which includes the entire processes of forging, normalizing, annealing, hardening and tempering.
Each of which are distinct and seperate steps of a long and drawn out process with specific times and temperatures for each step.
Individually Handcrafted Knives for the Outdoor Enthusiast

[ThomasPowers]

One added point: back in the "old days" hardening and tempering was often done as a unified process where you heated the piece and then only quenched the working edge and then let heat from the rest of the piece flow back to draw temper on the edge and *then* quenched the entire piece when it was at the right temper colour to stop it from further softening the working edge.

Of course back then the body of the piece was often wrought iron and only the edge was steel--I used to find hammers and chisels done this way at a Columbus OH fleamarket so bac k then was not so far back sometimes!

Note too that modern high alloy steels have greater issues with retained austenite than old shallow hardening low alloy steels.

[kbaknife]

Modern hatchet, pipe 'hawk makers, etc. STILL! do use that method!
Unfortunately for many of the 'smiths in the "good old days", they had a LOT of failures! A lot of good stuff was made, but there were MANY failures. Something that didn't get understood until the modern age of photomicrographs, and spectrometers and the like, is that tempering "COLORS" are only indicators of temp change, and NOT distinctly accurate for specific temperatures! Simply the quality, temp, moisture content, oxygen levels, contaminents, etc. in the ambient air can give you an entirely different color for a given temperature!
Oh, well.
Now we have digitally controlled ovens and high temp salt pots, etc.
I still often do, however, use simple edge quenches and drawing tempering techniques for spine softening, etc.
Depends on the knife's end purpose, and I sure am glad we can now have spec data sheets for each mill run of steel so we know alloy content %, etc.
Sounds too scientific, doesn't it??

[ThomasPowers]

We often romanticize the past thinking that the old time blacksmith coming from a long line of smiths was taught or *knew* a lot of stuff we don't.

Part of this is a selection effect: good old stuff tends to be kept; bad old stuff was recycled or thrown away so today we tend to see the best of the past not the mediocre stuff.

Going through a lot of old scrap hunting for wrought iron I have seen a lot that I would have never let out of the shop; but was good enough to sell to a customer 150 years ago!

However if we know more now it's because of the smiths that came before us and amassed the body of knowledge that lead to modern metallurgy.

[brothermoore]

Regarding Thomas Powers' "old ways" post: You know, I am really going to remember that. I am new to bladesmithing, but I am not green in working with my hands. One thing I have learned is to always listen to the old ways. They made pretty good blades when blades were still the main form of combat (even a bayonet was impportant to a muzzle loader), and artifacts and relics are still good to this day. I might prefer to heat treat the steel that way. If nothing else, it's good to have options, new ways and old ways, whatever way it takes to get the job done, right?

[ThomasPowers]

My blacksmithing mantra: "There is only one right way to do something blacksmithing---and that is ANY WAY THAT WORKS!"

[Finnr]

BINGO!!!! Thomas!
Finnr