Armco Iron: New Concept and Broad-Data Base Justify Its Use as a Thermal Conductivity Reference Material

A new treatment for the data generated over a period of almost 40 years, including results of a recently completed round robin, indicate no reason for not continuing the use of Armco iron as a thermal conductivity reference material in the temperature range 80 to 925 K. The correlation of thermal conductivity and temperature divided by electrical resistivity is linear in this temperature range with changes in slope at reasonable magnetic transition temperatures for α-Fe2O3 and Fe3C. The small amounts of the two compounds present coupled with electrical resistivity trends for the impurity elements hydrogen, nitrogen, and oxygen suggest the thermal and electrical transport properties are measurably influenced by a semicontinuous or continuous network of hydrogen and iron compounds in the grain boundaries. Improved results are effected in the use of Armco iron as a reference material in the range 80 to 925 K by the use of three equations correlating thermal conductivity and temperature divided by electrical resistivity. The equations provide a tool for monitoring changes in thermal conductivity during use of the iron as a reference material and a means of adjusting thermal conductivity values for the normal compositional variations of Armco iron.