Interesting video about the basics of hardening steel

[Surfson]

This video was very interesting to me and I think it is relevant to sword making.  Forgive me if the same video or something like it has already been discussed.  

 

 

[ROKUJURO]

Robert,

interesting indeed, but not relevant in sword making as the TATARA produces high-carbon low alloy steel. TAMAHAGANE is even being decarburized in the forging process to an extent.

[Surfson]

Jean, are you saying that the clay slurry that swordmakers put on the sword doesn't have a carbon source in it?  

[ROKUJURO]

Robert,

yes, indeed. The clay slurry has no protective task in this stage of forging (= fire-welding), but together with the rice-straw ash it works as a flux.

Before the YAKIIRE, the blade is covered with a clay mixture (= YAKIBA TSUCHI) which can contain ground stone (from the grinding/polishing process) and possibly (not always/not every smith) ground charcoal. This TSUCHI-OKI process is done to protect the steel from losing carbon but it cannot increase the carbon content as the carbon migration requires temperatures of above 900°C to work. Quenching is done at about 800°C.

[Surfson]

So there is no element of case hardening also going on during Yakiire then?  It does sound like you are saying that there is some transfer of carbon from the flux to the steel, even if there is no net addition.  Sorry, but I haven't tried to study the inorganic chemistry of the process.

[ROKUJURO]

Robert,

there are two steps:

1)  Clay slurry together with rice-straw ash as flux (no considerable carbon content). A flux protects a welding zone from the surrounding oxygen as it has a low melting temperature and provides a kind of glaze coating at the necessary temperatures. 

2)  The clay applied before YAKIIRE has two purposes: a) it protects the hot steel from surface decarburization  b) Differential hardening: in the quenching, it slows down the temperature decrease in the blade where the clay is applied in a thicker layer, preventing hardening. Where the layer is only thin (on the edge), the cooling is fast enough to harden the steel.

The TAMAHAGANE base steel is a low alloy high carbon steel which is further refined in the forging process. It requires a rapid cooling for full hardness which is only possible with water. Modern steels contain other elements like silica, manganese and chromium. This leads to a better/stronger through-hardening, but makes the steel delicate (and damageable) if the cooling is performed too rapidly. Those industrial steels have to be quenched in oil (or other viscous liquids), otherwise there is a risk of cracks if cooled in water.

The whole quenching process depends to an extent on the cross-section of the workpieces. A cube of 10 x 10 x 10 cm behaves differently than a knife or a sword blade with a cross-section of max. 8 mm.