1. Superhardness obtained during high-frequency quenching is not due to any specific feature of the martensite structure, but to the high cooling rate of the thin, induction-heated layers. A uniformly high cooling rate during hardening produces the same hardness both after furnace and induction heating. 2. The size of the blocks in the mosaic structure of the martensite does not vary with an increase in the induction-heating rate prior to quenching and is the same as for furnace hardening. 3. The inhomogeneity of martensite in induction hardening at heating rates below 200°/sec for hypoeutectoid steel can only be detected by X-ray methods at underheating temperatures. At quenching temperatures producing maximum hardness, the martensite is close in homogeneity to furnace hardened martensite. An increase in the heating rate to 850–1200°/sec leads to inhomogeneity of the martensite at quenching temperatures corresponding to maximum hardness. However, this does not lead to an increase in hardness, compared with that of homogeneous martensite.