Effect Of Technological Conditions On Microstructure And Properties Of Chromium Cast Irons

Superheating treatment and compound modification were applied to treat a series of chromium white irons and control the thermodymics and kinetic process of solidification, aiming to control effectively the phase structure of matrix, dimension, morphology and distrubution of carbides, thus obtaining a new type of wear resistant chromium alloy.The results have been shown that: (1) After superheat treatment and compound modification, the effect of grain refinement of matrix and carbides in chromium white irons is very evident compared with the ordinary grain size. The carbides are fined to be isolated and counteract the adverse effect of sharp angle on toughness, therefore toughness of chromium white irons increased greatly. (2) For the hypoeutectic chromium white irons, when chromium content keeps at 12% and with the increase of carbon content, the amount of proeutectic austenite reduced, chromium content in proeutectic austenite droped correspondingly, the stability of austenite descended: when carbon content is 2.2%, 2.4% respectively, cast structure is composed of mainly austenite + a small amount of eutectic (austenite+carbide);when carbon content is 2.6%, proeutectic austenite began to transform to martensite, when carbon content is 2.8%, 3.0%, cast structure is composed of martensite + eutectic (martensite+carbide);When carbon content keeps the same and with the chromium content raised from 12% to 15%, the chromium solubility in proeutectic austenite increased correspondingly, the stability of proeutectic austenite enhanced, the amount of retained austenite increased. Proeutectic austenite began to transfer to martensite obviously only when carbon content is more than 2.6%. (3) For the hypereutectic chromium white irons, the grade of overeutectic weaked and the grade of pseudoeutectic intensified after compound modification treatment under the condition of superheating, which imply that solidification went on by nonequilibrium freezing. Thermodynamics supercooling stunted the growth of carbides and made them to be rodlike and isolated;In the meantime, the intensification of pseudoeutectic degree and the formationof pseudoeutectic were beneficial for the refinement of eutectic carbides, and therefore beneficial for toughness. (4) Carbides in hypereutectic chromium white irons are inclined to be rodlike and isolated after compound inoculation under the condition of superheating, which is due to the rise of Cr/C in micro-area of molten cast irons. Under the condition of carbon content keeping at 3.8%, when chromium content is 12%, the improvement of carbide morphology is significant;but when chromium content is 15%, the morphology improved little.