I contacted the manufacturer of the case hardening compound. Their chemist replied:
Contact with acids liberates very toxic gas. Hydrogen cyanide (HCN).
Thermal decomposition or combustion products may include the following substances:
Hydrogen cyanide (HCN).
I think chlorine trifluoride deserves a mention
One of the issues with relying on published NPFA triangles to judge the answer to this question is that some of them don't seem to be very reliable given the known properties of compounds.
According to one of its suppliers ClF3 has a rating of 4-0-3. So it would not count very highly here. This seems about as ...
I am guessing you mean a fused salt bath at 1400 to 1600 F to diffuse both carbon and nitrogen into the surface of the steel. The other salts in the bath likely have an affect on fumes. I never worked with molten salts but it is definitely not for amateurs. Find a commercial heat-treat shop that performs this process.
Potassium ferrocyanide cannot burn, because it is the product of a combustion. When meat or blood is overheated, carbonized and burnt, the remaining ashes contain potassium ferrocyanide. And this product was initially recovered by washing these ashes and evaporating the obtained solution. In German, this compound is called Blutlaugensalz, which means "...