FRANK 660 / FRANK REGENERATOR is used in the above working range for hardening various tool steels also used for preheating, high-speed steels and hot work steels before treating in FRANK 970. The bath gives freedom from decarburization similar to that obtained in FRANK 720, but FRANK 660 is less soluble in water. The bath is made up and maintained in a similar way to FRANK 720 / FRANK REGENERATOR. FRANK 660 has a greater density than FRANK 720. The salt is first melted and FRANK REGENERATOR at the rate of 225 gms per 100 kgs of FRANK 660 is added.

FRANK ECONOMISER must not be used on baths used for treating rust-resisting steels or high-speed steels. It causes reduced corrosion resistance on the former and pitting during the hardening of the latter.