Volume 3, Issue 2 (March 1975)
Evaluation of an 80Ni-20Cr Alloy in an Enclosed Heating Element
Accelerated life tests on enclosed heating elements were conducted using a constant power cyclic type of test that caused failure in about 500 h. The results were compared to those on a cyclic test run near rated power of the element where failure occurred in approximately 5000 h. The change in both the hot and cold resistance of the heater wire in the accelerated test was about five times greater than in the rated power test. This correlated with a much larger loss in chromium and silicon from the heating coil tested under accelerated conditions.
The cold resistance of the MgO was always over 100 Mω during the test. The hot resistance of the MgO decreased during the test but was always over 3000 times the resistance of the heater coil. This correlated with X-ray fluorescence analyses of the MgO which disclosed no chromium in the midpoint of the MgO to form a continuous conductive path.
The emissivity and convection coefficient of the sheath were essentially constant during the test at rated power.
The accelerated test method selected for range-type enclosed elements is not suitable for predicting the service life of an enclosed element. It is usable as a quality control test.