Carburization of high temperature PM-materials

Abstract

Carburization of powder metallurgically processed materials for high temperature use is interesting for several reasons: Production of carbide powders e.g. for hard metals, formation of carbide layers on bulk materials to improve corrosion and/or wear resistance and degradation of material properties like strength and ductility by metal-carbon reactions. Molybdenum, TZM (Mo-0.5Ti-0.07Zr-0.05C) and tungsten are finding wide application as construction material in high temperature furnaces which are operated under high vacuum or inert/protective gas conditions. If grafite is used in the same system reactions of the refractory metal components with carbon containing species have to be considered. Therefore a variety of examinations was performed on the carburization characteristics of molybdenum and tungsten (and alloys), especially at temperatures above 1200°C where carburization rates become technically relevant. High temperature oxidation resistant alloys like steels or Ni- and Co-based superalloys can withstand severe carburization as long as their surfaces are covered with tight, protective chromia scales. In case of porous or cracked scales or conditions where chromia is not stable anymore a front of carburization proceeds through the materials - frequently along the grain boundaries. The PM-materials Ducropur (pure chromium) and PM 2000 (Fe-19Cr-5.5Al -0.5Ti-0.5Y 2 O 3 ) show distinctly lower carburization rates: Ducropur forms tight chromium carbide layers, whereas PM 2000 is nearly unaffected up to 1100°C because of its tight and stable alumina scale.