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Atmosphere Carburizing Control Using Shim Analysis

Shim Analysis

Control of the carburizing potential of the atmosphere in any heat treat operation is critical to the success of the procedure, particularly during the carburize procedure. The current ‘state of the art’ method of atmosphere control is of course, the oxygen probe.

However the oxygen probe requires a substantial investment and careful monitoring of the output signal from the probe to the readout instrument.

A simple and yet inexpensive method of carbon potential control is the method called: Shim analysis

Many seem to believe that it is a complex method of atmosphere control, yet it is perhaps one of the simplest methods.

Its problem is, that the result is not a real time result, and is perhaps some 45 minutes old when the actual result is obtained. This is the major disadvantage with the procedure.

The procedure of shim testing is accomplished by using low carbon steel shim with a known carbon content and known weight followed by inserting the shim into the furnace atmosphere. The shim is left in the atmosphere for a period of time and if the atmosphere is carburizing, there will be again in weight of the shim. Conversely, if the atmosphere is decarburizing, there will be a loss in weight. The method is as simple as that!!
The shim size that is required is that the shim sock should be approximately 0.010” thick X approximately 0.50” wide X approximately 3” long. A hole will need to be drilled through the furnace wall and a 6” maximum long pipe X 2” bore diameter should be welded to the furnace casing. On the end of the tube a gate valve will be fitted (gas tight). And a further tube should be fitted to the gate valve with a nipple cap with a hole through the center of the cap. This is to take a stainless steel probe on which the shim stock sample is to be fitted.

The following is the procedure for the shim stock test preparation, test and evaluation:

Cut the shim stock sample to the size described above

Using  a two hole paper punch, punch two holes into the shim sample

Using a pair of tweezers, wash the sample shim in alcohol to remove any surface contaminants. Do not touch the shim with your fingers, as your fingers will leave a body oil finger print on the shim surface

After the  wash (alcohol or acetone)  weigh the sample using micro gram balance accurate to 4 decimal places of a gram

Using the tweezers, thread the shim on to the sample probe rod which has the nipple cap on it

Screw the nipple cap onto the gate valve. The shim will now be sealed on the air side of the gate valve.

Open the gate valve and allow the atmosphere to purge out the full tube

Then push the rod with the shim into the furnace so that it goes into the furnace atmosphere by about 4” to 6”. The shim is now exposed to the furnace atmosphere

Leave in that location for approximately 35 to 40 minutes in the furnace atmosphere

After the cycle time (shim in the furnace atmosphere) remove the probe back to the air side of the gate valve and close the gate valve. Allow the shim to cool (approximately 5 minutes maximum)

Close the gate valve

Unscrew the nipple cap and remove the sample holding rod

Take a pair of tweezers once again and remove the shim gently from the sample rod

Using the tweezers to hold the sample, gently wash the sample in either alcohol or acetone and allow to dry

Reweigh the shim very accurately using the microgram balance

Use the following formula to calculate the carbon potential of the furnace atmosphere: (Carbon Potential= final weight-original weight  X 100+%of original  carbon)/Final weight

This will be the carbon potential of the carburizing furnace atmosphere. The procedure does work albeit not in real time such as the oxygen probe does. But it is accurate.