Tempering Blades

[Gadge]

I have a basic question which I hope an expert can answer.

 

A knife maker friend asked me whether katana were tempered after quenching. I said as far as I know there is no further heat treatment after quenching. I've always thought that quite low carbon content of hagane and reasonably low quenching temperature produced an edge which was not glass hard and of course the spine is protected by clay and is softer.

He argued that a heated copper block was used to temper the blade but my understanding is that the copper was only used to improve the consistency of the curve and did not heat through to the cutting edge.

 

Any clarification would be appreciated.

 

Regards,

Garry

 

[Surfson]

I think that you have it right Garry, the quenching is the tempering. 

[John A Stuart]

There can be tempering after the quench yakiire called yakimodoshi. I had a convo with our Montreal swordsmith and he confirmed it. Oh, it is 焼戻 John

[paulb]

I agree with John

I have seen on several videos and referenced in several books that blades were heated after quenching (to a relatively low temperature, I think 200 deg. C was quoted) and then tempered. The stresses created by quenching are huge hence the failure rate of about 10%. the secondary tempering is aimed at relieving some of that stress.

The heated copper block is as you say used to adjust the curve.

regards

Paul

[ROKUJURO]

Garry,

while the carbon content of TAMAHAGANE may come up to 1.4%, the carbon content of the HAGANE in a blade is generally about 0.7%, so not really low. A steel alloy of this quality can reach a max. hardness of 67 HRc after quenching. To my knowledge there is no standardized tempering process, however in some videos on Japanese blade forging you can see the smith moving his already quenched blade in the fire again, so there may be some variation in the techniques. 

A ''reasonably low quenching temperature'' has always to be above Curie temperature (en.wikipedia.org/wiki/Curie_temperature) (which is roughly 770°C for steel) to produce an hardening effect. This is the necessary temperature when the blades goes into the water, so you have to have about 800°C in the blade when it comes out of the fire to get there, and there is no 'moderate' hardening below that temperature! The literature says that the edges of traditionally forged Japanese sword blades are measured at about 60 HRc in many cases.

I think that the ratio of SHINGANE and KAWAGANE versus HAGANE in a blade has a huge effect of how the blade reacts (deforms) in the quenching and what the requirements of tempering are afterwards. This ratio also decides the performance (resilience) of the finished blade in a slicing slash against a hard or soft medium.   

[vajo]

I have some very old books from wartimes about heating.

 

My english is not perfekt to explain but i will copy some sides.

 

 

Maybe it helps 

 

read from the last to the first site.. upload "durcheinander"...

[ROKUJURO]

Chris,

these pages are not very old (egyptian papyrus are very old ), but they contain the actual basics of industrial metallurgy, especially quenching and tempering.

However, Japanese swordsmiths have their own requirements and procedures which differ a bit from the above.

[vajo]

Jean,

 

this book is older than i. It must be old

 

It was spoken about quenching and tempering. I wanted to clear the difference between.

[O-Yumi]

  I believe modern Bladesmiths refer to this process as "annealing".      John

[Nihonto Chicken]

It may be somewhat confusing that what has been translated from the Japanese as "tempering" is really in western parlance "quench hardening".  In the west, tempering is a second heat treatment in which the hardened metal is reheated to a few hundred degrees F with the result that some hardness is lost, but with a great increase in ductility (resistance to brittle fracture). 

 

Metallurgically, the blade is first heated to above the phase transformation temperature, and the previous room temperature structure of ferrite (body-centered cubic iron) and pearlite (a lamellar structure of ferrite and cementite, Fe3C) is all converted to a solution of carbon in austenite (face-centered cubic iron).  When the blade is quenched, there is not enough time for the original forms to return through a diffusion process, and the metal steps off the equilibrium phase diagram, and instead of pearlite, martensite is formed (body centered tetragonal iron with carbon straining the lattice).  Martensite is very hard, but also very brittle.  In the west, martensite is usually tempered and is transformed to a number of different end products, such as spheroidite (spherical particles of Fe3C in ferrite), the nature and amount of which depend on the starting mix, the result of the quench, and the time and temperature of the tempering process, but usually resulting in a much less brittle metal with much greater strength than at the start (ferrite and pearlite).

 

[John A Stuart]

This was my trade for some years. Heat treatment of big industrial parts, crankshafts, webs of which are man sized, shafts and spindles. Hardening and tempering were never confused, and I speak in a western parlance. To mistake one for the other is only for those not educated in the craft. As I said before, yakimodoshi is quite common after yakiire. John

[Gadge]

Thank you all for contributing. John, my take away from this is your comment that yakimodoshi is quite common after yakiire.

 

Garry

[ROKUJURO]

Rick,

may I underline what you wrote: Martensite appears in two different forms, and the spheric form is visible on the surface of a traditionally quenched steel blade as NIOI or NIE.

 

...... martensite is formed (body centered tetragonal iron with carbon straining the lattice).....