The Heat Treat is a Salt Bath Hardening, Oil Quench, Salt Bath Tempering. Materials are alloy steels, 4140,8650,4352 types, and the end result is Q&T to 40-44Rc.
For alloy steels, I typically follow the recommendation referenced in most Standards and Codes, which is 1 hour at temperature per inch of material thickness, with 15 minutes as the absolute minimum time at temperature for material thickness at or below 1/4". Over 1/4" and up to 1/2" in thickness, I would go with 30 minutes at temperature.
The advantage with the salt bath is that it provides a uniform temperature gradient for austenitizing and tempering operations
Salt bath heating is much faster than heating in a gaseous furnace atmosphere. Consequently, the soak times will be much less than the rule of thumb, "one hour per inch of section". But, just off the top of my head, I can't tell you how much faster. I will look in my heat treat file in my other office and see if I can find anything.
Good article, metengr. Let's see--the quoted 20-25 seconds per mm of section translates into approximately 500-625 seconds for 25mm of thickness, or 8-10 minutes per inch of section. All I could dig out of my files was a salt bath time of 40 minutes at 1600F we used for a positioning collar on the landing gear for the C5A aircraft. Material was either 4330V or 300M and it was about a 50 pound part, if I recall. Roughly an inch section size.
Some extra thoughts added to the above excellent posts.
Watch the temperature of your bath.
Watch the temperature recovery after adding parts especially if there is any mass. We had two salt baths with the same volume of salt except that one was a much higher wattage and recovered very quickly. While the other took 4-5 times as long for the same size part/parts.
Keep the bath clean and follow the manufacturers recommendation on salt rectification.
Now that you have the soak times down pat you should look at the quenching. The use of a "fast quench oil" with your materials should give you some better properties.
Safety:
Have a good set of safety rules and well maintained equipment. Enforce the safety rules, especially the personal protective gear.
I walked into a shop that was using molten salt baths just as an accident was happening. There was bath eruption, cause unknown, that burned 5 personnel. One later died from secondary infections. I never will never forget trying to get the salt off the personnel.
Within weeks of the incident we started abandoning a very extensive molten salt (Hydride) cleaning process.
