Differential Hardening?

[Timothy Mankins]

I've heard a lot about the process of differential hardening used in the forging of Japanese blades, where the bladesmith coats the sides and back of the blade with a clay-like substance, and then heats the blade to a very high temperature before quenching it.
The clay insulates the back of the blade, allowing it to cool more slowly than the edge. The rapid cooling of the edge of the blade causes the carbon in the steel to become trapped in the steel, making the edge very hard, but the slow cooling in the back of the blade somehow allows the metal to be more flexible, therefore making the blade more resistant to breakage.
Does anyone know if there was a system or systems in use in medieval or renaissance Europe, which achieved a similar effect? BTW, I've already read the article "How Were Swords Really Made?". Unless I missed something, it did not appear to say anything about differential hardening. I am very interested to know if this was used in Europe during the middle ages and/or renaissance.

[Greg Coffman]

I don't exactly know how all it was done, but the edges of medieval and renaissance sword were indeed hardened more than the middle. This would be a good question for the forum at MyArmoury.com, http://www.myarmoury.com/home.php.

[Webmaster]

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Blade toughness and flexibility was achieved mainly through the tempering process in Europe. The outer layers of steel were generally made harder than the core, though methods varied widely. European swordmakers were very aware of the need for a sword to rebound from a blow without taking a permanent bend or snapping off, and the good ones built their products well enough to keep their customers coming back.

[Tom Kinder]

I know a smith named Dan Davis who recently posted about how every Medieval European sword he has ever seen (several of them) have hamon on them but most people miss it because of the state of polish they are in and because they don't expect it.

another smith, Sam Salvati, posted a link to a web page I cannot read but it appears they gave an antique Euro sword of some type to a Japanese sword polisher (togishi?). there are pictures of both the welding pattern and what seems to clearly be a hamon link: http://www.archaeologie-online.de/magaz ... t/seite-3/

[Timothy Mankins]

Thanks. My follow-up question then is: what sort of method or methods would have been used to achieve a differential temper on European blades during the middle ages and/or renaissance? There are four methods of which I currently know, which can currently be used to achieve this effect in a blade:

#1: After the blade is forged, a powerful heat source such as a blowtorch is applied to the edge or edges of the blade, but not to the back or center of the blade. That way, only the parts that you want hardened are affected by the heat, and therefore, only the parts that you want hardened become hardened when the blade is quenched.

#2: After the blade is forged, the entire blade is rapidly heated before being quenched. Because the blade is thinner at the tip and edges than at the core, the tip and edges absorb heat faster than the core, and are cooled faster than the core during the quenching, again achieving a blade that is harder at the edges than at the core.

#3: After the blade is forged, the core of the blade is insulated with clay. Then the entire blade is heated and quenched. The clay insulates the core of the blade, allowing it to remain soft, while allowing the tip and edges to be hardened.

#4: After the blade is forged the whole blade is heated to a high temperature, but only the tip and edges are quenched, allowing them to be hardened.

Were any of these methods in use in medieval or renaissance Europe? OK, I doubt they used blowtorches back then, but was there another method in use, of applying heat to the tip and edges of a blade without applying heat to the center?

What about the second method? I expect it would take a rather powerful heat source to heat the entire blade, and have the tip and edges reach a high temperature before the core. Was this method in use back then?

What about the third method: the method which is used on Japanese blades? Was that in use?

What about the fourth method? This method sounds like the simplest of the above mentioned methods, as well as potentially a very versatile and controllable method.

[Vincent Le Chevalier]

As far as I know #2 is how it was done (not really differential tempering but rather a natural effect of uniform temper on a particular geometry). #3 is specifically Japanese I believe. #1 and #4 would set too much contrast in temperature perhaps, which could be dangerous and wrap the blade or create faults. At least I'm not aware of them being traditionally used. Even #3 (which is a more uniform process than #1 and #4, because the parts covered by clay are still tempered) curves the blade...

The effect of differential temper (hard edges, resilient cores) was sometimes achieved by pattern-welding. The Japanese smiths also used some of it. See this page for some details:
http://www.templ.net/english/making-blades.php

I've heard it said that these techniques (pattern-welding, differential tempering) are mainly useful when you don't have access to quality ore and must refine it. They can improve a lot when you work from relatively poor materials, but they may not be worth it if you have good steel in sufficient quantities. That might explain why they were not widely used in Europe in later periods (except for ornamental purposes, in the case of pattern-welding).

Regards,

[Timothy Mankins]

Thanks a lot!

[Tom Kinder]

one thing to remember is that tempering and hardening are both parts of Heat Treating but very different things.

as I understand it differential tempering was done in Europe but that does not make a visible line very often that pretty much only happens on differential Hardening.

method #2 is a common method I think (I'm not a smith yet, but I just talk to them and am learning what I can)

tempering is a heat and quench cycle that is done after hardening designed to make the steel a little bit softer and a lot tougher. the amout of heat, how long that heat is held in the steel and how quickly it cools all changes what happens in the steel. in the old days smiths used steel color to know what the temperature of the steel is and when to quench. smiths used to make the steel softer in te center of the sword and on the tang through selective tempering. one method I know of to heat for selective tempering is to heat a rod or iron of some sort and use it to heat a specific area of the steel leaving the hardened edges relatively cool so they will not lose hardness.

#4 IS Japanese and is a god example of differential HARDENING.

another method used to get hard edges and soft spine was done by Norse/vikings. they would make the core of their swords out of steel or iron with lower cabon content than the edges so that the steel in the middle would not get as hard no matter what you did. the Vikings were famous (even in the middle ages) for their wonderful swords and their pattern welding.

there are a lot of tricks and most of them were used in antiquity.

sorry to carry on so.

[Timothy Mankins]

Thanks again! This is all excellent information.

[Mike Adams]

There is also a technique called 'case hardening', where a medium carbon steel is heated to 'cherry red' colour, and placed in a bath containing carbon rich materials.
The carbon migrates into the hot steel, and when quenched gives a very thin layer of higher carbon steel than the inner core.

I believe this is what the movies try to depict when they show horse hair or something being sprinkled onto hot blades, then shoved back into the forge.
This case hardening would give a 'hamon look' to steel when polished,
and certainly would show when etched to show the grain structure for eg microscopic analysis.

Long, long ago, when I was studying to be a metallurgist, one of the experiments we did was to heat a 1" diameter steel bolt to bright red heat,
put it into a holder so its axis was vertical, and impinge a water jet onto the bottom end, until the whole bolt was room temperature.
We then cut the bolt in half along its axis, polished the inner surface, etched it, and examined the grain structure,
where you could clearly see the fine martensitic grain structure on the outer surface
where the water had hit it, and larger grains toward the top end of the bolt, that had cooled more slowly.

Combining the case hardening technique, and knowing the grain structure would be smaller ( and therefore harder) from water quenching,
may I respectfully suggest the above could have been a method of simulating differential hardening, and certainly making a poor piece of steel perform better.

[Mike Sheffield]

The "hamon" on those ancient blades were due more to the fact the steel was very shallow hardening. It was the aspects of the steel not a clay coat that caused the line.
Case hardening does not work for blades. The extra carbon and therefore the hardened portion only goes 1 to 2 mm thick. As soon as you have to sharpen the edge you will wear away the hardened portion.
The temp at which high carbon steel austenizes is at about 1500 deg F so really not that high a temp.
The use of straw...or horse hair(never heard that before...cool learn something new every day) would be used to help in making welds stick. Steel when heated and exposed to oxygen gets a scale on the surface. This scale does not weld. Putting something on the billet that might help burn away the extra 02 helps welds to stick. Yes is does kind of add very little not worth the mention of carbon into the steel.
As for if the Occidental cultures developed a clay hardening system all I can say is maybe. The vikings were way ahead of their time at understanding steel. They may have done some blades but stopped doing it for some reason. Maybe not.

[Adam Humphrys]

I'm not an expert in this, though I do have a fair amount of experience working with blade steels. In my opinion deferential hardening would not have been used. What I believe would be most common would be the use of different alloys, and deferential tempering.
For example, if the core of a sword was made with a medium carbon steel, and the edge was made of a high carbon steel, when the sword is quenched the core would be softer than the edge. My reasons: rapidly heating the sword in order to heat only the edge would be a very very tricky endeavor. A smith would have to work extremely fast in order to get the blade into a quenchant before the atmosphere and the cooler core sucked the heat from the edge. Second, if the smith happened to make it in time, the amount of stress caused by the phase changes would be momentous.

Differential tempering seems to me to be the best answer to the question. One method I have seen, is that after a blade is fully hardened, uniformly, the smith heats up a large bar of iron/steel to a healthy glow. The bar is placed along the spine/core of the blade and the smith watches the temper colours run on the blade. If done properly this can result in a very hard edge, and a spring tempered core.