In earlier times steel was heat-treated by craftsmen who depended largely upon their accumulated knowledge to ensure that the properties of the metal matched the application. Present day requirements demand more exact control, not only of temperatures but also of furnace atmospheres, for if the latter is chemically unsuitable the composition and hence the physical properties of the steel will be adversely affected.

To provide a reliable and accurate indication of available carbon in furnace gases Corning Glass Works, of Corning, New York, U.S.A., have now developed a solid state oxygen measuring probe. Somewhat similar probes find application in chemical process technology, and can assist in vehicle emission control

The Corning Glass probe includes a yttria stabilised zirconia solid electrolyte and a platinum electrode which is exposed to the furnace atmosphere. Oxygen molecules in the furnace are turned into ions by the catalytic effect of the platinum and are then conducted through the zirconia where they recombine into molecules, the electrical potential generated depending upon the difference in the amounts of oxygen inside and outside the furnace. The oxygen produced in the furnace is directly related to the concentration of carbon monoxide and carbon dioxide, and thus to the amount of carbon available for hardening steel by carburising and carbonitriding treatments.

Operating in the range 760 to 1100°C, the probe can be used with furnace atmospheres generated from natural gas, nitrogen+methanol mixtures and propane-derived gases.

In the U.K. the Corning Glass probe is marketed by Land Pyrometers, Dronfield, Sheffield S18 6DJ, from whom additional information is available.