Hi Quaterstaff,
Seems to me that "420 Sub Zero Quenched" refers to cryogenically treated 420 grade steel. i may be wrong to say that sub zero quenched refers to cryogenic tempering .......but if i am right, it means that the steel is further quenched in medium below room temperature(down to -300 degrees Fahrenheit) after initial hardening process. The reason for this process is to almost completely transform retained austenite to martensite. This transformation will not be complete, in many instances by just cooling the steel to ambient temperature. The reason being that for most high alloy steel, the temperature for complete austenite to martensite transformation is below ambient temperature.
By using additional quenching(at a controlled rate) to -300 degrees Fahrenheit, many alloy's retained austenite content can be near completely converted into martensite. Typically, the cryogenic tempering/quenching is then proceeded by subsequence re-tempering or draws at +300 to +500 degrees Fahrenheit. This re-tempering serves to further reduce retained stresses, brittleness as well as enhance dimensional stability.
Hardening process for steels, in most cases, strive to achieve a martensitic crystal structure. This particular crystal structure provides the characteristic of high strength and hardness in steels.
The benefits of cryogenic tempering you may ask ------ increased abrasive wear resistance, increased corrosion resistance, reduced brittleness, etc.
Hope this helps.
