Abstract

The growth of crystal grains in metals caused by annealing and the influence of the crystal size upon their mechanical properties have been studied by several investigators. Recently experiments have also been carried out for finding some relation between the grain-size and mechanical properties. According to Stead, the mechanical disease is not due to any change in the crystals themselves, but is due to the intercrystalline weakness. There are, however, another type of weakness, so called the intracrystalline one. According to the first hypothesis, the greater is the number of grain, the stronger should be the resistance to the external force, H.T. Angus has experimentally found an empirical formula concerning with the hardness H and the number of grains n in the case of copper, that-is, a law "H is proportional to √<n>." But according to the second hypothesis, the result should be contrary. In the case of steel, there has, however, been found, up to the present, no definite relation : between the grain-size and the mechanical properties, except a qualitative relation that the decrease in grain-size is accompanied by an increase in the hardness and an improvement in the tensile properties. The properties of steel varies considerably with a slight changes in the carbon content and also with heat treatments. The grain-growth in steel is complicated by virtue of the existence of transformation. In my experiment, after necessary heat treatments of the mild steels, their grain-size was determined by Jeffries' method, and at the same time some mechanical tests, such as hardness test, tensile test and repeated impact test, were made. The results of these tests were then discussed in connection with the grain-size. They also suggest that the material may be exposed to an unfavorable condition, when it is heated beyond a certain limit of temperature.