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.

Stress relief heat treatment is used to remove stress induced in metals from various manufacturing methods. Some of these methods include: milling, turning, welding, bending, heating, cooling, shearing, forging, sawing, grinding, not to mention the steel making processes that leave the metal full of residual stresses. These stresses can cause harmful distortion, brittle fracture, and stress corrosion cracking near welds and within some grades of metal.

Removing residual stress is a time/temperature related event with a very controlled cooling cycle using your Cress Furnace. If not carried out correctly, new residual stresses can be produced that will result in greater stresses than the part had originally. To remove stresses it is recommended that you consult the mill literature for the grade of metal to determine what the Ac1 temperature is for your application. There are several methods to remove stresses successfully, but the most commonly used method is heat treat stress relief. The main criteria to use for choosing the correct temperature is to heat below the lower austenizing temperature (Ac1). Decarburization will take place above 960oF, so protect the surfaces if the surface is not going to be removed by machining.

Thus, if we have a 4140 steel part, (4140 has a 1380oF Ac1) that we want to stress relieve, we could place it in our Cress furnace, take it up to 1100oF and soak it for 6 hours, followed by controlled cooling at a rate of lowering it at 50 to 75oF per hour, in a closed furnace, to below 400oF, at which point it can be removed from the furnace, and it would be stress relieved. However, if you heated it to 1300oF, you can soak it for just an hour, followed by the controlled cooling and also stress relieve the part.