Hamon and Differential Hardening Theory...

B

barnett25

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Sep 18, 2008
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I have two questions that have been bugging me about differential hardening.

1. What is the actual structure of a normal clay quenched (1095/W1) blade with a hamon? Do you get martensite on along the edge and pearlite along the spine? Is it the border between the two structures that causes the hamon line?

2. It seems that many traditional knife makers believe that a differentially hardened blade will allow flex, but still keep a hard edge. And most metallurgists prefer a fully hardened blade.
Just how much of the steels potential are we giving up by differentially hardening/tempering, and why?
 
YES !
The traditional Japanese blades are made from 1050-1060 .The spine is pearlite and the edge is martensite but at the hamon you might get a mixture of the two and perhaps some additional structures.The Japanese polishing techniques can bring out interesting and beautiful patterns.
Flex ? That's determined by blade thickness not HT. The spine gives a tougher structure but hardly very strong.
Worst case is differential hardening by heating only the edge.This gives a typical soft spheroidized spine.. Best case would be to full harden then temper back the spine - martensite spine and edge.
A good alloy steel doesn't need anything but a full harden/temper.
 
When speaking of flexing I was referring of course to the ABS 90 degree test. My understanding is that they do this to test the individual's ability to properly differentially harden a blade, although I might have incorrectly inferred that in my reading.
One issue I had with that idea is that if the edge is still really hard it would crack just as readily whether or not is was backed with a softer/springier/tougher spine.

Also, is it possible for a trained eye to make an educated guess as to the structure of the steel at different points in a blade based off of the hamon? For instance is it a fairly safe assumption that a well defined hamon has mostly pearlite on one side and martensite on the other, while a whispy hamon has more of a mix of the two at the hamon? If so, is there a major strength advantage to a blade with a whispy hamon that makes a more gradual change from pearlite to martensite?
 
The hamon is the junction between the hardened edge ( ha) and the softer body ( ji). The formation of different crystal structures in the hamon and its boundaries is what makes the line white in some cases, and wispy in others. This is the nie and the nioi.
The old term for the structure in the hamon was troosite ( until around the 1970's when everyone but me abandoned the term). It is a mixture of pearlite, martensite, and some mixed structures. The surface of a nihonto blade can be examined by an expert, and the structure of the steel determined by sight. The large wandering martensite crystals ( bright spots = nioi) in the ji tell of spots where the steel hardened on the surface with breaks or thin spots in the clay. The nie shows where the pearlite wanders into the martensite, caused by the ashi. The darker color of the ha shows the full matensitic structure of the exposed steel.The distinct hamon line shows where the martensite and pearlite meet and how much they intermingle. The soft and more dully reflective ji and shinogi ji show the pearlitic structure of the body of the blade. In places where the shingane ( soft iron core) shows through the ji-gane (harder surface steel) the examiner can tell how well the sword was made, and how much it has been re-polished.

The ABS test blade has nothing to do with a real life blade. No one would create such a blade for general use. It is a test to show the ability to create two vastly different conditions in the same blade, and the ability to control them to a specific function ( bend test). No blade should normally be expected to need this function. The Japanese blade is probably the closest to this type of blade condition, however. It was designed to have a hard, tough, and sharp edge; a soft and ductile core; a strong and flexible spine; and the ability to take a severe bend with only edge cracking. The ancient smiths accomplished all this without any metallurgy. They understood the look and feel of the steel. It has only been in the past 50-75 years that any idea of the actual structures has evolved.

In my opinion, differential hardening by clay coating and full quenching is for developing a hamon. Edge quenching is not an acceptable substitute. The better functioning blade is fully hardened and differentially tempered. You won't get the mystical hamon, but you will get a tough blade.

Stacy
 
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Good post, Stacy. I need to read up on my nomenclature for hamons. After some early playing with edge quenching and some excellent education from other makers, I started doing full quenches with differential tempers. I've just recently started playing with hamons, but it's more for the aesthetics than anything else for me.

--nathan
 
Nathan,
Exactly!
In the old days the hamon told the buyer that the smith understood how to manipulate the structures in the steel ( even though they didn't know what structures were then).
Stacy
 
Thanks for the great information! I am going to have to spend some time looking up the Japanese terms you used, but I've been putting that off too long anyway.
 
Dang Stacy I have been trying to understand all of this and was just getting the hang of it and then you go and throw all these HAs and JIs in there. :) Now I am really confused, but I should be used to it by now. :)
 
To translate all things Japanese ( as far as blades go) try this:
http://home.earthlink.net/~steinrl/glossry.htm

The ha is the hard marternsitic edge.
The ji is the softer pearlitic main body.
nie is the pearlite/martensite junction ,or the hamon.
Nioi is floating spots of martensite on the surface of the pearlitic spine, and on the edge of the hamon.
gane means metal.
Bulchitte is what cows leave in the pasture.
Stacy
 
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