METAL DUSTING RESISTANCE OF HIGH TEMPERATURE STEELS AND NICKEL-BASE ALLOYS FOR STRUCTURAL COMPONENTS OF HEAT TREATMENT FURNACES WITH CARBONACEOUS ATMOSPHERES

摘要

Heat treatment processes with carbonaceous atmospheres like case hardening of steels are accomplished in atmospheres with a controlled carbon activity a_c. The functional components of the heat treatment furnaces like gassing lances, gas sensor protection tubes, gratings etc. are unavoidably also exposed to the carburising atmosphere. Generally the materials of these components are high temperature wrought and cast steels or nickel-base alloys with high chromium content. Local carbon activities a_c ? 1 occur if the functional component temperature is locally lower than the over-all furnace temperature or if the composition of the atmosphere locally deviates from the over-all atmosphere in the furnace. Under these conditions the metallic material suffers deterioration by metal dusting corrosion. The metal dusting mechanisms involve carburization and selective oxidation of the material followed by its disintegration into carbides, oxides, metal particles and powdery carbon. The details of the different mechanisms depend on the chemical composition and the metallurgical microstructure of the alloys, the local composition of the atmosphere and the temperature. The attack can be both in the form of pitting and general metal wastage. A laboratory test facility has been developed for the examination of the metal dusting resistance of alloys typically applied in heat treatment furnaces. In this test facility the carbonaceous atmosphere is established by atomizing methanol with nitrogen in a two-fluid nozzle and adding propane simulating a conventional generation process of the atmosphere in case hardening furnaces. A temperature gradient along the alloy specimens is realized and consequently a variation of the carbon activity. Some alloys show corrosion due to metal dusting already after 200 h of exposure. A total exposure time of 1500 h was accomplished. The progress of the corrosion is monitored in intervals of 500 h and the resistance against metal dusting is evaluated for 14 different steels and nickel base-alloys.