Protecting Against Decarburization with Cress Furnaces

Decarburization is an oxidizing surface condition caused whenever ferrous (carbon based) metal is heated to temperatures above the visible heat (960oF) zone, and is exposed to atmosphere. The depth of decarburization penetrates deeper as temperature and time of exposure increases. The surface in this condition has lost carbon composition (de-carbon-ized) and scale (loose flaking surface which resembles the scales on a fish) will also become evident. Hardness in this layer is poor to none, and tool life is definitely and dramatically sacrificed. In fact, if a tool is put into service with any appreciable decarburization, it is guaranteed to fail and can show surface breakdown quickly.

Cress Furnaces are atmospheric box furnaces which means they have air, or heated air in them and there is no protection from decarburization in the furnace. There are several optional methods to protect against decarburization when using a Cress Furnace. They are:

Cress Stainless Steel Inert Atmosphere Tube: You can order your Cress Furnace  with a stainless steel inert atmosphere tube used to pump Nitrogen or inert gas into the chamber. It helps, but does not solve the decarb problem because box furnaces are not sealed vessels and are built using fire bricks and contain ceramic element walls that are very porous and full of air and water molecules that cause the surface corrosion.  Purging the chamber does not remove all the air and water molecules and thus decarb will take place. Installing a retort (metal box) inside the chamber and pumping inert gas into the retort works better because the porous fire brick is taken out of the equation; but a thin gray, smutty surface will still prevail because the retort is not air tight.

Stainless Steel Foil, formed by triple folding the edges into an envelope, gives relatively safe atmospheric protection. The disadvantage of SST foil is that the SST is razor sharp, and not particularly inexpensive to use. It provides very good protection, but there is often a grayish surface color that needs to be removed before the tool is used.

Protective Powder is applied to the metal when it is 450oF which, when it melts, creates a protective barrier around the part. After the heat treat and tempering is completed, the powder may be washed off with hot water for clean up.  Results reported are bright shiny parts, but it’s imperative in all protection methods to remove all traces of cutting fluids, oils or even finger prints.

Protective Paints can be applied to the parts.  After heat treatment is complete, the paint is removed by sand blasting.

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 in¬tended 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

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