"Fired at 1000°C in a hydrogen atmosphere..." "Boiled in a solution of sodium cyanide..."


Joined Sep 30, 2009
Sodium cyanide in the melted condition is or used to be a pretty common method of case hardening steel, /We used to get thin parts done that way back in the 1970s -80s where I worked. It takes less time to get a good depth of hardness so keeps distortion to a low amount.

Cyaniding is a case-hardening process that is fast and efficient; it is mainly used on low-carbon steels. The part is heated to 871-954 °C (1600-1750 °F) in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide.

2NaCN + O2 → 2NaCNO
2NaCNO + O2 → Na2CO3 + CO + N2
2CO → CO2 + C
This process produces a thin, hard shell (between 0.25 - 0.75 mm, 0.01 and 0.03 inches) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours so the parts have less opportunity to become distorted. It is typically used on small parts such as bolts, nuts, screws and small gears. The major drawback of cyaniding is that cyanide salts are poisonous. From - https://en.wikipedia.org/wiki/Case-hardening#Cyaniding