Abstract
Attention is drawn to changes which take place in freshly hardened tool steel, enabling tools to be straightened by bending immediately after hardening. A detailed study was made of hardness changes occurring during several hours after the “secondary heat treatment” of high-speed steel, the treatment being applied repeatedly to the same specimen. The ensuing fluctuations appeared to be periodic and characterized by increasing time periodicity, and they were found to be susceptible to magnetism. Similar, but much more rapid, fluctuations were found to occur in the hot steel during heat treatment.
Periodic fluctuations having been observed during the ageing of metals after severe disturbance by thermal, mechanical, or magnetic treatment, an investigation of these phenomena was made in very pure metals, nickel, iron, and gold, and certain general conclusions were reached, the fluctuations being attributed to periodic changes in the state of cohesion, probably electromagnetic in character.
Experiments were resumed on high-speed steel, their object being to find the combination of thermal and magnetic treatment which would set up fluctuations of greatest amplitude capable of being stabilized magnetically at a maximum or minimum phase so as to produce the greatest permanent change in the physical characteristics of the steel.
Drilling tests were carried out with twist drills subjected to various treatments suggested by the foregoing experiments, and it was found possible to increase the life of a drill many times by a suitable combination of magnetic and thermal treatments. Confirmatory evidence is quoted of hardness variations caused by magnetic treatment in the experiments of other workers. The influence of the mechanism of the various hardness tests is discussed.
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