Volume 3, Issue 4 (July 1975)
Life Testing of Enclosed Heating Elements
Life tests of 80Ni-20Cr, 70Ni-30Cr, and 35Ni-30Cr-35Fe alloys were conducted in enclosed elements. The life to burnout in accelerated tests at double the rate power, 500 W, gave a life of about 500 h, while tests at rated power, 250 W, were stopped after 20,000 h.
The hot and cold resistance of the magnesium oxide changed during the test but the changes were independent of the type of alloy in the heater. The hot and cold resistance change of the heater wire was larger in the accelerated tests than in tests conducted at rated power. This was the result of a larger loss in chromium from the alloys during the accelerated test. The silicon loss during testing was small but was larger for the rated test than for the accelerated test.
Calculations from electrical data and metallographic examination for both accelerated and rated tests disclose the loss in cross-sectional area of the wire due to oxidation was between 2 and 8%. This very minor change is consistent with previous investigations. The mechanism of change of the heater wire is not similar in the accelerated and rated test. Thus, service life cannot be estimated from accelerated life test data.