EPS Heat Treat Headlines

Years ago mills supplied alloy and tool steels with a spherodized annealed structure. Due to economic constraints most, if not all, mills have short cut that process. They still anneal, but it isn’t a sphereodize annealed any longer. Today’s annealing yields a pearlitic microstructure with an increased hardness. The result, or effect, is that the metals generally don’t machine as well as they used to a few years back and surface finishes also suffer. In applications that require machining in tough, tight tolerances, difficult design areas, it often would make sense to properly anneal the material either before machining begins or, better still, once the part is roughed out to a near net shape. It is better after being roughed out because it also eliminates most of the stresses and reduces some of the deformation.

To properly spherodize anneal any steel, use the annealing temperature as stated by the manufacture of your steel, or refer to the ASM Standards. The best method is to put the steel in your Cress furnace and heat the steel to the annealing temperature. Then lower the temperature slowly (preferably 25 degrees per hour) to 900 F and then shut off the furnace. Do not open the furnace at all, and allow the furnace to return to room temperature. The process will take 22 to 24 hours but will produce a totally uniform grain structure which contain small, neat and orderly globular shaped carbides in a smooth flowing ferritic matrix.

Low carbon steels are not normally spherodized for machining since they become soft and gummy, but can be spherodized when increased ductility is desired for bending or forming parts.

COPYRIGHT © May 2007, by Advisor In Metals

ALL RIGHTS RESERVED. No part of this publication may be reproduced, transmitted or copied without prior written permission of the author and publisher.

Question 6: I have an A2 part that has a main body that is 2” cubed, but it has a part that sticks out another 2” but is only ¼” thick on one side. The print calls for a 60 Rc but I can’t figure out how to make it work. How do I heat treat it and not over cook or under cook it?

Actually it’s not that difficult. Before you wrap it in SST foil and put it in your Cress Furnace, clamp a block onto the ¼” section and fool it into thinking it’s a 2” thick section. When you heat treat the part, it is all uniformly soaked as though it is a 2” thick block that is 4” long.

Question 7: My Cress furnace is set up for the 2400F higher temperature. Can I heat treat high speed steels and powdered steels?

Yes, a Cress furnace set up for 2400F can heat treat high speed steels and powder metal tooling. But first, I generally don’t recommend heat treating these metals if you only do them occasionally. The reason I take this route is because high speed steels have extremely short austenizing soak times. Most all of them need to be soaked for 5 minutes maximum per inch of thickness once they reach austenization temperature. So, over soaking them in the length of time they are in the furnace is extremely critical, and that soak time literally comes down to seconds. In fact, every second in excess time takes away longevity of the tool’s life. For that reason, I generally suggest sending the tools to a good commercial house that works on high speed tools everyday.

But, if you have more than just an occasional piece to do and your operator is able to do some scrap pieces and record how he does them, then by all means, you can heat treat these steels very successfully.

One further word of caution. When the parts are removed from the austenizing temperature, if you are using the oil quench method, they must be quenched in the oil immediately, but the door to the furnace should also be closed as fast as possible. A furnace at 2400oF receiving an extended incoming flow of room temperature air receives a heavy thermal shock, which over time will effect the longevity of the ceramic and fire brick liner.

Question 8: Cress offers a Draw furnace but I can’t afford to spend the money. How important is it to have a draw furnace?

When a piece of metal comes out of the furnace and goes through the quench, whether by air, oil or water quenching, and the temperature drops below 150oF it should be placed in a pre-heated tempering oven or the grain structure will be effected. That will cause premature tool failure. If the furnace is at 1750oF, it takes a long time for the heat to dissipate and to get it down to 400oF for tempering. Opening the furnace and force cooling will cut short the life of the fire brick and ceramic plus, may have an affect on the elements. Thus it makes sense to have a draw furnace that can be pre-heated and ready to complete the process.

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