Hygrometers Improve Control of Treatment Furnaces


Measurement of moisture in annealing furnaces is essential to achieve consistent properties of the treated metals. If dew point is uncontrolled or measured with poor quality instruments negative impact on dimensions of the part or its hardness are likely due to carburization or decarburisation occurring.

Improving annealing processes through accurate moisture measurement

Annealing increases ductility and reduce internal material stresses. The metal is heated to just above its critical temperature, then held at a lower temperature for several hours before being cooled. The process is carried out in a controlled atmosphere to ensure a good quality finish.

Measuring the dew point of the annealing furnace gives a vital indication of how reducing or oxidizing the atmosphere is during the heating stages. This allows the furnace operator to accurately control the gas mixture to manipulate the atmosphere to give the best results. Accurate determination of the furnace atmosphere conditions leads to better finishes, reduced gas usage and can enable the annealing process to be carried out at lower temperatures, resulting in lower operating costs.

The dew point of the furnace atmosphere should be measured continuously. The sample would be extracted from the furnace and passed through a cooling coil in order to bring it back down to ambient temperature for safe measurement. Lower dew points mean the reducing potential of the atmosphere is increased, with typical desirable dew points of a reducing atmosphere below -50°C or -70°C depending on the type of metal being treated. This is well within the measurement range of Michell Instruments precision chilled mirror hygrometers.

Control of moisture during sintering processes

Sintering processes use relatively low temperatures (around 600°C) to bond powdered metals. Sintering results in products that are both light and strong, which makes them suitable for a variety of applications from machine parts to shell casings for ammunition. As an example, sintered metal filters are available with many of the Michell moisture sensors. The sintering process creates a strong product, which is still porous. It provides a finer filter than would be possible with gauze or by machining.

Sintering may be carried out in a vacuum, however it can also be carried out at atmospheric pressure in an inert gas, such as hydrogen. Due to the powdered composition of the raw material, exposure to moisture inside the furnace can cause clumping, resulting in large holes being created in the finished product. Accurate measurement and control of moisture content is necessary to avoid this. For sintering carried out 600°C, maintaining a constant dew point of -60°C is necessary. The gas is sampled at several points in the furnace and it is necessary to have a fast response for moisture measurements so action may be taken quickly if moisture levels rise above the maximum.