Following a few basic rules can often minimize troublesome distortion that takes place during the steel heat treat process. There are three major areas where distortion can take place, assuming the part is straight prior to heat treatment.
1. The pre-heat step is the first critical distortion prevention step. The pre-heat step, as specified by the steel producer, prepares the metal to transform through the heat treat phase changes, but also reduces the stress introduced by machining, forming, or other processes. Just by programming the controller on your Cress furnace for a 10 minute stop, gives the temperature inside the metal’s mass a chance to catch up to its surface temperature, reducing the tugging and pulling distortion from taking place.
2. At the austenizing temperature, the molecules in steel are ‘in-solution’. That is. they are in a molten state within the shape of the part. That also means they will deform, or sag, to the surface of what they are resting upon. Heat treating is not meant to be done on the Cress hearth plate. That’s an insulation and is not going to allow uniform heat penetration of a part if it’s laying on the hearth plate. A rack should be built that allows even temperature transfer. If the rack becomes distorted, then it needs to be straightened to support the parts in a flat manner.
3. Taking the part from the Cress furnace and transporting it to an air quench rack, or to water or oil quench is next. Because the part is coming from the furnace in the ‘in-solution’ condition, picking the part up has to be done with great care; and if it’s an air hardening metal, needs a flat rack for quenching to prevent sag. If it’s being water or oil quenched, it needs to enter the water or oil straight up and down to avoid cooling one side earlier than the other. Oil or water hard metals may still deform, but it will be less.
If you are heat treating very thin flat parts, often the best way to treat them is to press quench them. How that is accomplished is by removing a single part from austenization and quickly placing it on a flat, heavy plate of steel. Immediately place another heavy plate of steel on top of it. As soon as it loses heat color, proceed with your normal quench method. Remove each piece from the furnace and press quench that same way.
If the part is round or of a peculiar shape, consider making a fixture to hold the part through the heat treat and quench process. For instance, if you have a ¼” diameter rod, 16” long, take a steel pipe with a good schedule wall, drill and tap 3 holes around the pipe every 6 to 8”. Using three bolts in these holes will hold the rod straight in the center of the tube, allowing quenching and a straight part.
If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at [email protected] Information about the book or seminars is available on line at: Advisor In Metals
COPYRIGHT © August 2008, 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.