Good day everyone,
I just wanted to bring up a couple of points regarding steel composition and hardening.
I would prefer to describe tamahagane as a "simple" steel rather than "pure" in that the two significant elements are iron and carbon with some other elements as impurities existing in insignificant amounts (hopefully).Modern steels have a higher degree of purity in a sense.
As we add alloying elements like manganese,molybdenum and chromium (just to name a few) the nose of the TTT moves further away in time so we can cool at a slower and safer rate while still obtaining sufficient martensite.
With respect to the "hard spots" observed in gunto; I feel this is an artifact of the steel composition rather than the method of quenching.
I also think that nie is not an indicator of the quenching medium but of how the steel was smelted,forged and heat treated.
The reason that smiths chose fresh water as a quenchant was because it perfectly matched the time constraints with simple steels. If they used saltwater or seawater, as I'm sure many tried, the blade did not survive the thermal shock and if they used animal fat or vegetable oils the blades did not get hard.
Now on top of all that you can through some clay on the blade to make the ridge and spine cool even slower so there is little chance of martensite forming elsewhere (differential hardening).I have heard it postulated that the thin layer of clay on the ha reduces the effect of the vapor barrier that is formed and increases the local cooling rate even further.