The critical cooling rate was first studied to inquire into the mass effect in quenching of steels. The specimens were of the open-hearth nickel-chromium and nickel-chromium-molybdenum steels, which were put in the Sato quenching testing machine and cooled in different cooling rates. Change of the transformation point with the difference of cooling rate and the transformation velocity at each temperature were determined. From these results and observation of the microstructure, quenching diagrams were made and the critical cooling rate determined. The present paper deals with the explanation of the quenching diagram as well as the appearance of the transformation. Then the Charpy and tensile specimens were cooled in different cooling rates, and tempered to temperatures between 20 to 700°C, and then subjected to the mechanical test to inquire the retation of the cooling rate and the mechanical properties. Lastly, the mass effect was presumed, seeing from the experimental results described above and the measured value of the cooling curve with regard to the oilquenched round bar of 20-300mm in diameter. It was recognized that although the larger sections have been liable to be influenced by segregarion, ghosts, hydrogen inclusion, manner of forgings and other complex effects, the mass effect due to the heat treatment alone free from such influences may be detected by the present method.

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The present study was made for the purpose of obtaining a high fatigue strength constructional steel by adding different low percentages of manganese. Fatigue tests were carried out on 19 rolled specimens of the open hearth steel containing 0·10-0·36% carbon and less than 1·56% manganese, and also tests on 33 rolled specimens of the crucible steel containing 0·10-0·37% carbon and less than 234% manganese.
The resulte of the tests were represented in space diagrams taking the fatigue limit, the carbon and the manganese content as coordinates in three perpendicular directions. From the results of a number of mechanical tests, space diagrams which show the relation between the mechanical properties, the carbon and the manganese contents were also obtained.
Specifying a certain fatigue limit from the viewpoint of strength and the lowest elongation from the techno ogical standpoint, a range for the carbon and the manganese content was determined from the spece diagram.
When the carbon content was low, a constant fatigue limit was obtained by increasing the manganese to a proper amount.
Comparing two steels of a definite carbon content, the open hearth steel required a higher percentage of the manganese than the crucible steel to develop the same fatigue strength.
Microscopic study showed that specimens with unifom grains were superior in fatigue strength to those with banded structure, s.ag lines or segregation, although the static tensile strength was the same.

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