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The effect of the test load on the sensitivity to stress of the diamond pyramid hardness (DPH) was investigated on a martensitic spring steel, a Type 304 stainless steel, and an aluminum bronze with duplex microstructure. For this purpose, a loading frame and load cell of 1200-N capacity were fitted to the microhardness tester, which provided for application of uniaxial tensile loads to a flat test specimen. The test materials were heat treated differently and had different surface preparations, ranging from polished before vacuum annealing to electropolished following the annealing treatment.
Within the load range of 15 to 500 gf, the hardness increased in most cases to a peak from which it leveled off to a more or less constant value as the test load increased. Applied uniaxial tensile stresses did not change this trend except when plasticity initiated. In general, the hardness decreased linearly with increasing applied tensile stress. The rate of decrease was independent of the load for the hardness measurement. Yielding affected the hardness sensitively, particularly at small loads, thus changing the trend of the effect of applied stress on hardness as well as the hardness-load relation. Surface films or residual stresses do substantially affect the hardness-load relationship and the trend of the stress effect.
hardness load dependence, hardness stress effect, yield stress, aluminum bronze, stainless steel, spring steel, microindentation hardness testing
Professor, University of Alberta, Edmonton, Alberta