Question 4: How do I know if my Cress furnace temperature controller is calibrated correctly?
Simple! Unless you check it, you don’t know. For that reason I always suggest customers purchase the Cress pyrometer with their furnace. It does several things.
First, with the thermocouple supplied, setting directly on the parts being heat treated, the operator will know for certain when the parts are at austenizing temperature instead of relying on ambient air temperature inside the chamber. That means less opportunity of over cooking, or under cooking parts.
Second, it also acts as a check against the temperature recorder readings in the event there is a wide difference, which would indicate either one or the other is out of calibration.
Question 5: I don’t always have time to temper my parts so I put them into my Cress furnace and temper them the next day when I get to work. Is this OK?
NO!!!! After quenching a part, it is loaded with freshly created Martensite. Martensite is a fine grain structure, but in the un-tempered condition it is extremely unstable. It is so unstable it can literally explode and send razor sharp shrapnel like pieces 25’ in all directions. It can be triggered by performing a hardness test, heating it, chilling it, vibrations and especially impact.
In addition, it is a fact that an un-tempered part that sits at room temperature for over 2 hours loses so much in good attributes, that economically it would be best to anneal the part and re-start the heat treat process from the beginning. The un-tempered part will still retain its hardness and will temper and on the surface look just fine; but the fine grain structure will have gotten course, and will refuse to transform additional Martensite as usually happens during the tempering process.
Solution: Never start heat treating unless you have time to complete the entire process. If you absolutely cannot finish, then quench the part to below 150oF, then put it back in your Cress furnace (make sure management, the fire inspector and your insurance company approve of an unattended furnace in operation) and raise the temperature back above 200oF and you may leave it there till the next morning or till the following month, and it will temper and form the proper martensite structure. Just avoid letting it sit at room temperature. In fact, if permitted, you can temper the parts at the correct tempering temperature instead of the over 200oF temperature. Remember, you can under-temper a part, but you can never, ever over-temper a part by holding too long. Tempering temperatures will not over cook a part.
DISCLAIMER OF LIABILITY
The material presented in this article is intended for general educational information only. It should not be used for a specific application without careful analysis and study of the intended use. Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.
COPYRIGHT © April 2007, by Advisor In Metals
The author, Bill Bryson, Advisor In Metals has had numerous years and extensive experience in the heat treating of tool steels. He has conducted over 250 seminars to leading companies in the U.S. to train their tool makers and engineers on proper steel selection and heat treatment practices. He is also the author of the book called “HEAT TREATMENT, SELECTION AND APPLICATION OF TOOL STEELS” published by Hanser-Gardner Publications.
If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at firstname.lastname@example.org. Information about the book or seminars is available on line at: Advisor In Metals