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Utsuri


Darcy

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I will try to answer some of the interesting questions below:

 

Fro example is the prevalence of nie and Yamashiro and Yamato work just a result of them working at higher temperatures or did the raw material they were working with lend itself more readily to the creation of nie than that used by Bizen smiths?

 

It is a direct result of temperature and carbon content. Bizen smiths are known (in general) to use steel with a different carbon content and to quench at lower temps. So the answer is yes and yes.

 

 

Does utsuri strengthen or weaken the blade?...is it deep? ...does it survive many polishes or is it soon polished off?

 

Utsuri is simply the harder microstructure that results from quenching. It isn't as hard as martensite nor as soft as ferrite. It is somewhere in between. It is in effect, case hardening in that it does not penetrate through the blade. It will change appearance with polishing and can become indistinct after many polishes.

 

It plays a small part in giving a blade a bit more lateral strength thought the actual benefit is thought to be marginal at best. There are those that believe the early Bizen blades are not multipiece construction, i.e., they are monosteel and lack a soft core. Since Bizen blades are usually hardened to a lesser degree, they have more toughness than a higher carbon blade quenched to a higher hardness. The difference in a Soshu blade made with a composite structure is that they were trying to use the brittle high carbon/high hardness edge only in the hagane and gave the blade a lower carbon/lower hardness (softer) core. In other words, the Bizen smiths tried a compromise using one piece of steel, the Soshu smiths tried to engineer a composite solution utilizing the best traits of each constituent.

 

Again, utsuri may not have been seen in and of itself as improving the blade in any way, mechanically or aesthetically by the original smiths. It may have simply resulted from their processing regimen and the edge hardness they were seeking just as nie is a result of higher quench temperatures and nioi a result of lower quench temps.

 

Utsuri occurs under very specific conditions within temperature range and metal compostition...

 

I believe that is what I have been saying....It all depends on the rate of cooling at a specific level of carbon and that in turn is dependent on many factors such as the pre quench temperature, the time at temperature (grain growth), geometry of the blade, quenchant type (specific heat, boiling point, etc.), quenchant temperature, quenchant agitation, clay insulation value (again depends on the constituents of the clay, the geometry of application), etc. It is indeed quite complex but if your process is constant, repeatable, and followed to the letter each time, using the same carbon content materials, it is quite routine metallurgy.

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