Q. Show General Properties of Castings?
1. Fe-Cr (10-30%Cr) alloys offer good resistance to oxidation, low strength at elevated temps, restricted to use in oxidizing or reducing conditions that involves low static loads and uniform heating. Chromium content depends on anticipated service temperature.
2. Fe-Cr-Ni (> 13% Cr, > 7% Ni, {%Cr > %Ni}) alloys are similar to above except they have greater strength and ductility than the straight Cr alloys. These alloys are also used to withstand greater loads and moderate changes in temperature, and in oxidizing and reducing gases that are high in sulphur content.
3. Fe-Ni-Cr (> 25% Ni, > 10% Cr, {%Ni > %Cr}) alloys are used where there is no appreciable content of sulphur, they do not carburize or become brittle and do not take up nitrogen in nitriding atmospheres. The service life of castings, increases with increasing Ni content in carburizing or nitriding atmospheres used extensively under conditions of severe temperature fluctuations.
4. Nickel-base (³ 50% Ni, appreciable amounts of Cr, cobalt (Co), and refractory metals, little or no Fe, possibly some Al and Ti) heat resistant alloys have superior corrosion resistance and mechanical properties superior to other alloys. 50Cr-50Ni and 60Cr-40Ni are used for supports and other firebox fittings due to their resistance to hot-slag corrosion in boilers that fire oil high in vanadium content. Hot slag high in vanadium pentoxide (V2O5) is extremely destructive to most other heat-resistant alloys.
5. Cobalt-base alloys (³ 50% Co, plus appreciable Cr and refractory metals) have good high-temperature mechanical properties. They are not as strong as Ni-base alloys in short-time tests but are competitive in combined strength and corrosion resistance with Ni-base alloys at high temperatures and for long periods of operation.