Why specify soft iron ring gaskets?
At one time Soft Iron was available at a price competitive with Carbon Steel that had gone through an annealing process to make it about the same hardness as Soft Iron. Today the demand for Soft Iron has become so low that manufacturing it now has risen to more than annealed Carbon Steel processed to the same hardness requirements for Soft Iron.
Soft Iron gaskets were previously preferred for API 45K yield strength weld neck flanges because of the usual lower hardness of 45K material. Today properly annealed carbon steel ring gaskets can meet the hardness maximum originally designated for Soft Iron ring gaskets. Soft Iron gaskets can still be purchased when still preferred.