As the tool changes its size by a heat treatment, the investigation of the amount of the change is very important for deciding the dimensions of the tool before the treatment. This subject has varieties of cases, of which some ordinary ones, i.e., concerning a carbon steel and a high speed steel heated in an electric furnace are described in this paper. In these cases, their sizes are altered by : (1) change of structures; (2) oxidation of surfaces; (3) grinding of oxidized and ragged surfaces. On the carbon steel, the first effect is greater than the others, as the ordinary heating temperature is comparatively low. When this steel is quenched in water, its change of dimensions is not equal at different positions, because the differences of structure change and of thermal expansion are not the same at different spots owing to the variation of cooling velocities. On the high speed steel, the change of size due to the structure change is small, but as the best heating temperature for hardening is high, the second and the third effects are considerably great. The writer has investigated the relations between the cooling velocity of specimen and the emissivity of cooling medium used for quenching and developed an empirical formula for the emissivity of air blast. Moreover, he has found that the cooling celocity has almost no effect on the changes of length and of hardness when the high speed steel is quenched by ordinary air blasts or in oil. The oxidation velocity of steels at high temperatures is a function of both temperature and thickness of oxide layer deposited on its surface. This theory has been discussed and empirical formulas for the velocity of oxidation about a high speed steel and some carbon steels are established. The temperature of a tool generally changes continuously when it is heated in a furnace; the amount of oxidation in this case is difficult to be found by any formula, but can be solved by a graphical method, which is described in the paper. As the decarburized portion of a tool must be taken off, the writer has studied about the decarburization of steels at high temperatures and found that the decarburization scarcely happen provided the heating temperature and the time of heating are normal.
View full abstract