QUESTIONS and ANSWERS from CRESS FURNACE OWNERS – Part 1

As a seller of Cress Heat Treat Furnaces we often get questions from end users regarding heat treating. We have taken the time to answer of few of those questions here, and will be posting a few more in the coming weeks so be sure to check back.

Question 1: I heat treated a piece of D2 in my Cress furnace and the part physically shrunk. Now it’s too small to use. Why?

Answer: The only reason a part will shrink in size after being heat treated is because it was over cooked. D2 is meant to be soaked at 1850F for 1 hour per inch of cross section. In many cases people will get busy and often don’t get back to the furnace when the time has expired. As little as 5 or 10 minutes too long and you will experience over cooked parts.

Solution: Use the auxiliary contacts on the temperature controller of your Cress furnace to light a light or ring a bell to make sure you don’t over soak your parts.

If you lose size in a part from over cooking during the heat treat process, the part can be easily fixed by packing it down in dry ice or liquid nitrogen for 8 hours. This will cause the out-of-phase retained austenite to transform and the size will snap back to its original size.

Question 2: I heat treated a part made from A2 and when I put it on the surface grinder, it didn’t have much attracting magnetism. What causes this?

Answer: Essentially, it’s the same over cooking as we described in question 1, only the part may not have shrunk. When steel reaches its austenizing temperature, it’s referred to as being in-solution. When that happens, the crystal structure is between stages and is essentially free floating with a loss of magnetism. If over cooked by too long, or too high a temperature, it never snaps back into phase and thus, magnetism is lost.

Solution:Again packing on dry ice will solve the loss of magnetism.

Question 3: Why do heat treat recipes call for a pre-heat before going to the austenizing temperature? Is it really necessary?

Answer:Yes, it is necessary. It does two things. It gives the temperature on the interior of the part being heat treated time to equalize with the surface temperature. This helps relieve stresses and reduce distortion. But, it’s also essential to get the chemistry ready to transform into a proper austenite structure with better opportunity to form fine grained martensite after the quench.

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 thegateway@metrocast.net. Information about the book or seminars is available on line at: Advisor In Metals