| Popis: |
Carbon-containing iron alloys are generally known to possibly contain large quantities of hard phases or compounds able to bring high hardness to the alloys, as cementite, pearlite or martensite. If their chemical compositions also contain chromium such alloys may reach high levels of hardness, thanks to the formation of numerous interdendritic chromium carbides, as it can be met in some iron-based bulk alloys or hardfacing coatings hardened by tungsten carbides for example. Simultaneous high levels of carbon and chromium may lead to bulk alloys, easy to elaborate by casting, and which may display first high volume fractions of carbides for an enhanced hardness and wear resistance and also a good resistance to high temperature oxidation useful in case of substantial increase in temperature during service (e.g. due to friction).The topic of this study is to elaborate by HF induction melting several alloys with very high carbon contents (from 2.5 to 5.0wt.%C), to characterize their microstructures after stabilization at different high temperatures, to assess their fusion temperature range, and finally to compare all these experimental results to predicted results issued from thermodynamic calculations.The obtained microstructures are hypo-eutectic (2.5wt.%C), eutectic (3%C) and hyper-eutectic (3.5 to 5wt.%), and carbides are essentially Cr7C3 (except for 2.5wt.%: Cr23C6). The carbides fraction increases with the carbon content and tends to decrease with temperature. Carbides can become the main phase in the carbon-richest alloys in term of volume fraction. The results of thermodynamic calculations are consistent about the microstructures. A good agreement was also found concerning the liquidus temperatures but mismatches were noticed about the solidus temperatures. The obtained hardness are generally of very high level (more than 800Hv) but it tends to decrease when the heat-treatment temperature increases, as consequence of the decrease in carbide volume fraction when the exposure temperature increases. |
