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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.

Yes, by PACK CARBURIZING. First, let me clarify. There are several ways to carburize low carbon steels, but in an open atmosphere, box type furnaces like Cress furnaces, there are only two ways. One way is to use a carburizing compound. But buyer beware. Many carburizing compounds contain oxidation chemicals that literally eat other chemical elements in stainless steel. And, using these compounds, even just one time, saturates the porous fire brick to a point that if you ever desire to use stainless foil for decarb protection, the chemicals will literally eat holes in the stainless foil.

Now on to pack hardening: Pack hardening is a two step process that works well for low carbon steel carburization. First, you must have a completely sealable steel box. This is often a bolted together box that fits in your Cress Furnace that has sealing insulation rope between sides and ends of the box. A heavy wall pipe with sealable, removable end caps works well.

The box needs to be filled with any good carbon rich material, such as, cast iron turnings or chips, or something like powdered bone meal. When filling the box, parts can be introduced in layers, with no parts touching one another, then more layers upon layers until the box is completely full. The end of the box must be sealed using insulation sealing rope.

Next, the box can be placed in the furnace and heated as required for the grade of steel being treated, usually in the 1700 to 1900oF range. The whole box must come up to temperature, then soaked for 16 to 48 hours, which is dictated by the depth of case needed; the longer it is soaked, the deeper the case will be. The carbon essentially gets absorbed on all exposed surfaces of the parts during the soak process. Once the soak time is complete, the box can be removed from the furnace and cooled. Once cooled to room temperature, the box is opened and the parts can be removed from the box and are then ready to be put through the second step which is the heat treating process as required for the grade of steel being used.

The packing material can be reused several times before it needs to be refreshed for more available carbon.

COPYRIGHT © May 2007, by Advisor In Metals

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