Abstract
In order to measure the hardened depth in induction surface hardned steels easily and non-destructively, we investigated the same electro-magnetic method as already reported as a method for case depth measurement in carburized steels. [See T. Ueda, M. Tanaka, A. Nishimura, J. Jap. Inst. Metals, 16 (1952), 626]. The method consists in that surface hardened steel specimens are excited by alternating current and the induced voltages in the secondary coil are read by valvevolmeter. The hardened depth is obtainable from the relation curve between hardened depth and induced voltage. To make the most sensitive measurement, we changed the frequency of exciting current according to the hardened depth. The following two kinds of induction surface hardened carbon steels with various hardened depth were tested: of 0.7% carbon content, 21 mm dia., 120 mm length and of 0.5% carbon content, 50 mm dia., 100 mm length. The experimental results obtained by this method were as follows: (1) When the exciting current was large, sensitive results were obtainable. (2) The frequency of exciting current must be adjusted to the hardened depth. For example, by using 200∼300 c/s in about 1 mm depth, 100 c/s in about 2 mm depth, below 60 c/s in about 3 mm depth and below 30 c/s in about 4 mm depth, better results could be obtained. (3) The two kinds of test-specimens with different carbon contents and dimensions showed similar results.
