Effects of Heat Treatment Variations on Surface Conditions of Iron-Nickel Alloys and the influence on Mercury Switch Behavior

Various iron-nickel alloys are widely used in telecommunication applications, particularly in switching apparatus. They find application where soft magnetic properties, moderate mechanical strength, and good glass-sealing characteristics are required. Heat treatments are normally employed for decarburizing and/or developing required magnetic properties. Heat treatment in moist hydrogen atmspheres as used for decarburizing can result in undesirable oxidation of dissolved impurities which segregate at the surfaces of the alloys. The dissolved impurities are generally deoxidants from the refining process of the iron-nickel alloys. Certain problems in electrical contact members can be attributed to the presence of these reaction products on the surfaces of the iron-nickel alloys. In the case of mercury switches, poor plating adherence, leaking, and inadequate wetting are problems associated with these reaction products. Commercial iron-nickel alloy parts employed in mercury switches were heat treated in manufacturing facilities at various time, temperature, and moisture conditions. The resulting surfaces were analyzed with a scanning electron microscope-microprobe. Annealing the components near 900°C for 20-30 min in dried line hydrogen is adequate for producing proper magnetic characteristics as well as clean surfaces suitable for overplating, mercury wetting, and glass sealing. Tabulated thermodynamic data were used to calculate appropriate equilibrium relationships for selected impurity elements. The calculated results were plotted in a manner to serve as a practical guideline for heat treating iron-nickel alloys for any application.