heat treatment, annealing, and tempering

Sword typology and Edge Weapons forms of the Chinese Empire and related cultures with an emphasis on their relationship to Swordsmanship.

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josh stout
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heat treatment, annealing, and tempering

Post by josh stout » Fri Jun 29, 2007 12:51 pm

Well I have done enough research now to know a bit of what is going on and to ask some questions. I have tried to address only issues of heating and cooling, and have left the interaction of carbon and iron alone.

Heat treatment involves heating a blade until it is quite hot (glowing) and then thrusting it quickly into a much cooler liquid. This causes a rapid crystallization that makes a blade very hard and brittle. Tempering is when the hard and brittle blade is then put into a somewhat hot (say 350 degrees or so) liquid or oven for enough time to cause new types of (larger) crystals to form. This makes a blade softer but less brittle. Annealing is like tempering but involves making the steel as soft as possible through heating it up to very hot again and then letting it cool as slowly as possible. Thus the largest (softest, least brittle) crystals are encouraged to grow. So tempering is like partial and more finely controlled annealing. Because tempering is done at a relatively low temperature, blades can have their tempers ruined at similarly low temperatures.

So as far as I can tell a “temper line” has nothing to do with tempering. It is caused by what people call “differential heat treatment” which is, as far as I can tell, differential cooling. The blade is evenly heated and through clay or quenching technique, the edge cools much faster than the body producing many more small crystals and a hard brittle edge than the slowly cooling body of the blade with large crystals and soft tough steel.

Would one ever “temper” a blade that has a “temper line”? What happens if you don’t do anything? Ie. What happens if you just take a blade out of a forge and let it cool? Would it be “annealed”? If a blade has a temper line would you call it tempered if it has not been? Does it make any sense to say a blade has “lost its temper”? A blade can loose its hardness due to heat, but since tempering is a process of softening due to heat it seems strange to say a blade can loose its temper due to heat. It is more like it is over tempered. If someone said a blade had a “high temper” I would think it was very hard, but wouldn’t it be very soft? Why do these words all seem to mean their opposites?

Josh
hidup itu silat, silat itu hidup

-Suhu

Kenneth, H.
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Post by Kenneth, H. » Sat Jun 30, 2007 12:28 am

The so called "temper line" is a result of differentially hardening. (covering the body with clay, exposing the edge only; allowing it to cool quicker than the body of the sword).

After a hard quench (water); the blade would often if not always be somewhat bent due to the stress the entire blade is exposed to during the quench when the metal expands and contracts.

After the quench the blade needs to be heated a bit (low temperature, water vaporizes), and gently straightened.

After being straightened the blade rests on a bed of even heated coal; normalizing... Until the blade turns to a slight blue colour.. This is the time to re-quench it and check if the temper is acceptable. If not, re-heat it slightly and hammer it into the correct shape again.

What can also be done is to heat the blade to its correct temperature and then let it rest by itself lying on a flat area; allowing it to cool in room temperature.

Its a little while since I've done this myself; so I'm not 100% sure on all this...

I'll see if I can remember to ask Mr. Zhou this afternoon. ;) haha!

Best regards, Kenneth A.H.
Zheng Wu Knife & Sword Company.
High quality custom blades by Master sword-smith Zhou Zheng Wu of China.

PaulC
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Post by PaulC » Fri Jul 20, 2007 6:55 am

Much of the confusion is a matter of definition.

Temper: Noun -the degree of hardness and elasticity in steel or other metal.
Verb - improve the temper of (a metal) by reheating and then cooling it.

This definition as a noun doesn’t imply the process of tempering which is what you are talking about. Instead it just gives a reference to the qualities of the steel itself i.e., hard temper, steely temper, spring temper etc. The verb ‘temper’ is the heat treatment process to achieve a particular quality in steel.

Looking at temper as a noun, the temper line is a zone of steel that has been treated to affect its hardness and elasticity. So it can be called a temper(n) line. But current usage is geared more toward the process of tempering. To make things more fun, you can temper(v) this hard edge, and should in many cases, and then what do we call it… a tempered line? See, there are a lot of variables being covered by few terms. This is why there are specific and exact scientific terms for the structures and processes.
Temper as a noun is becoming less and less common in modern usage, but since many papers, books written about the sword are from an earlier time we should be familiar with both meanings.

You essentially ‘have it.’ Tempering(v) is a time at temperature type of process. Meaning that a low temp for a long time can be similar to a higher temp for a shorter time. Various elements and structures can affect this relationship. Simply put, keeping the brittle/hard martensite at an elevated temp for a given time will give you a mixture of structures ( tempered martinsite) that will be tougher and softer than the as- quenched structure. The smith can decide how soft to go; imparting what characteristics he feels is best for that particular blade

Cooling without quenching:
If you take a blade out of the forge and let it cool in still air it is called normalizing, as long as an initial specific temperature was reached. Otherwise it’s called….. cooling ;) it’s not all difficult.

josh stout
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Post by josh stout » Fri Jul 20, 2007 9:46 am

Thanks Paul and Keneth, that was helpful and consice.

Amusingly, I ran into an article on tempering chocalat, and it turns out to be the same process of reheating and controled cooling to create a specific crystal formation and hardness.

Back to swords, I think I understand now the basics of heat treatment, in terms of controled cooling, but I also saw a reference to actualy heating different parts of a blade to different temperatures, not just cooling. Is this common? It is not part of the usual descriptions of the process.

Thanks,
Josh
hidup itu silat, silat itu hidup

-Suhu

Philip Tom
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handy reference on iron and steel

Post by Philip Tom » Sun Sep 09, 2007 1:25 am

Hey guys,
I just got through doing a new thread on bronze and brass, and cited a reference that would also be useful to interested readers of this thread here, dealing with heat treating of steel. I'll repeat the citation:

Art Gogan, FIGHTING IRON: A METALS HANDBOOK FOR ARMS COLLECTORS, Lincoln (RI): Andrew Mowbray Publishers, 1999.

Chapter 7 is a survey of heat treating, this like the rest of the book is clearly written for the layman and avoids the overly technical presentation that you find in many professional journals.

Other parts of the book which will be germane to members of this forum are Chap. 2, "Iron in Antiquity", and Chaps. 3 and 4 which cover all the various kinds of iron and steel which have been used over the centuries. I like the way that the multitude of terms that we hear all the time are clearly defined, with links to their application in the armorer's trade. Chap. 8, on corrosion, is useful for the arms collector as opposed to the martial artist -- this material is a good adjunct to H. L. Peterson's classic HOW DO YOU KNOW IT'S OLD?
Phil

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