Differential Hardening

[SStefun]

I've read that in chinese swords, specifically the jian, the edges were differentially hardened without using clay. This creates the soft, water like temper line as opposed to the more clearly defined hamon of japanese blades which are made using clay.

I was hoping somebody here knew how this was done on chinese blades. I thought first of an edge quench but I can't think of how that would be practical on a double edged blade. I would think that by the time the first edge is quenched and you flip it over, the other edge will have cooled too much to be effectively hardened. Anybody have some info?

[Shane Justice]

Good question. I was wondering about this myself.

If I had to venture a guess, I would say...

take advantage of blade geometry(thinner edges) and heat very carefully, raising both edges to critical temp...then do a tip first quench in a deep trough.

This would take supreme heat control and a skilled hand.

I bet one or all of the Sword guys will have a better idea or experience at doing this.

Still a good question.

Shane

[RHGraham]

There are a number of ways to approach that effect, in the context of period Chinese pieces Shane has it right, in heating the blade with care the edges can be brought to a somewhat higher temp than the body of the blade... edges are austenized and in a condition to harden, the rest of the blade is slightly below that temperature, so it doesn't harden.

Other factors include the steel itsself, Chinese steel in this context is a very shallow-hardening steel, meaning it hardens very quickly and only shallowly.

So using a combination of the two, with some practice, it's actually not terribly difficult to achieve selective hardening along the edges.

This effect can be enhanced by using a slower quench medium like Oil as well, but from what I've studied about Chinese weaponsmaking I can't determine if Oil was regularly used as a quench medium. I'd be inclined to assume it was at least sometimes, since Chinese tech in the area of steel and weapons remained pretty advanced and at the fore well into the 1700's. I'd think it was one of the tools in the toolbox in any case.

[Shane Justice]

Maybe this next thought belongs on the Metallurgy thread...but it does have a place here...

I am still studying this subject...when I originally answered I was considering a simple piece of monosteel. What I failed to do was take into account the Sanmei or Quian Gang lamination techniques.Randall check me on this...doesn't higher carbon steel hit crit faster(lower temp) than lower carbon steel?

If that is so then getting the edges to crit is a bunch easier on a blade with low carbon steel center and high carbon edges.

Does anybody know of any commmon trace elements that appear in Chinese swords that might have an effect on the hardening process ?

I have heard that some Japanese blades have moly in them. I forget whart moly does in the way of hardening but if I understand it, moly loans toughness to steel.

Shane

[RHGraham]

I think basic understanding of the swords and thier steels is important in any context/forum.

Yes, the laminated construction can have effects on the blade and how it reacts in a heat-treat, for sure. Depending on a lot of factors, such as carbon levels overall in the seperate sections, the side-plates may not really harden at all, the core *may* harden up to the edges of the side-plates, or it might creep in under some lending some stiffness to the whole, there are a lot of variables.

We think today in terms of alloys and thier affects, but if you jump back in time and you are a bladesmith in China say 400 years ago, there is a different outlook on things. You'd be looking at things like what ores, from what area, does what things as it's refined and forged, and as it's heat-treated. Sadly much has been lost, but these guys were very into recordkeeping and observation, and testing things, so they'd have a collection of information that would indicate what ore sources were preferred for particular uses, what refinements and manipulation techniques resulted in desirable materials, and what ones didn't.

So in a way, they might be utilizing the different attributes of ores and thier locallized alloying additions to create blades with specific attributes.
This is true the world over untill mettalurgy took on a more element-specific bent in the 1800's and then finally into our present time where we know a very great deal about individual elements and thier effects on the steel they are present in. And now, of course, steels are created with specific additions of alloys to create specific effects.

The alloying of ancient steels was engineered by Mother nature and usually constitues very small differences overall from place-to-place and culture-to-culture, but it's obvious that it was recognized and utilized by pretty much everybody, from the Chinese to the Norse who were doing decorative pattern-welding very, very early...which would indicate that they all had a grasp on the fact that different materials did different things.

And an overall factor everybody dealt with was making the best use of the local materials at hand, which shows up in fundamental differences from place-to-place in the refinement of the raw materials and how they were utalized to make weapons and tools.