Carburization of high temperature PM-materials

摘要

Carburization of powder metallurgically processed materials for high temperature use is interestingfor several reasons: Production of carbide powders e.g. for hard metals, formation of carbide layerson bulk materials to improve corrosion and/or wear resistance and degradation of material propertieslike strength and ductility by metal-carbon reactions. Molybdenum, TZM (Mo-0.5Ti-0.07Zr-0.05C) and tungsten are finding wide application asconstruction material in high temperature furnaces which are operated under high vacuum orinert/protective gas conditions. If grafite is used in the same system reactions of the refractory metalcomponents with carbon containing species have to be considered. Therefore a variety ofexaminations was performed on the carburization characteristics of molybdenum and tungsten (andalloys), especially at temperatures above 1200℃ where carburization rates become technicallyrelevant. High temperature oxidation resistant alloys like steels or Ni- and Co-based superalloys canwithstand severe carburization as long as their surfaces are covered with tight, protective chromiascales. In case of porous or cracked scales or conditions where chromia is not stable anymore a frontof carburization proceeds through the materials - frequently along the grain boundaries. The PM-materials Ducropur (pure chromium) and PM 2000 (Fe-l9Cr-5.5Al-0.5Ti-0.5Y{sub}2O{sub}3)show distinctly lower carburization rates: Ducropur forms tight chromium carbide layers, whereasPM 2000 is nearly unaffected up to 1100℃ because of its tight and stable alumina scale.