Abstract
The microstructure of chromium steels markedly changes, according to the composition and the coolling condition, i.e. maximum heating temperature and cooling rate. Studying the microstructure for 46 specimens of chromium steels containing less than 28 percent of chromium and 16 percent of carbon, cooled under the following conditions, the structural diagrams for the respective cases were, constructed:-(1) vefy slowly cooled from 900°, (2) cooled in furnace from 900°, (3) cooled in air from 900°, (4) cooled in furnace from 1200°, and (5) cooled in air from 1200°. In these diagrams the composition range of steels showing the self-hardening property is clearly shown. Moreover, from these diagrams, the following facts are concluded:-(1) the carbon content at the eutectoid points and the solubility of carbon at the eutectic point decreases, as tbe chromium content increases: (2) the pearlitic region in the normal structure increases, as the carbon content increases: (3) When the cooling condition is equal, the micro- stucture changes from pearlite or troostite to martensite and austenite in addition of chromium, and in further increasing chromium, ferrite containing high chromium appesrs: (4) As the cooliog rate increases, or the maximum heating temperature rises, martensite or austenite readily appears, pearlite or troostite tending to be hardly formed: (5) In high carbon alloys, the martensite or austenite hardly appears, unless the cooling rabe is rapid or the maximum heating temperature is high; as the cooling rate and the maximum temperature increases, the austenite is easily retained.
