Ask the Experts Metals

	Anna Wehr-Aukland Ph.D., Senior Principle Research Engineer

Bright annealing of steels requires conditions that are reducing to steel oxides. Traditionally, the Ellingham diagram has been used to predict the conditions that correspond to oxidation of pure metals or reduction of their oxides. This method can be used to predict the conditions that should be reducing to iron oxides and the oxides of the alloying elements added to steels, such as chromium oxide when stainless steels are considered. This traditional approach is not precise because it only uses thermodynamic data for pure metals and their oxides—it ignores the fact that iron and alloying elements form a solid solution. In addition, you can only determine the approximate equilibrium partial pressure ratio of hydrogen and water vapor for oxidation of a specific metal at a particular temperature.

Alternatively, you can use more accurate and convenient diagrams for steels and other alloys, which are created with the help of modern databases and computer programs, such as FactSage™ (thermochemical software and database package developed jointly between Thermfact/CRCT and GTT-Technologies) or Thermo-Calc software. Using the oxidation-reduction curves, presented as dew point of pure hydrogen or nitrogen-hydrogen atmospheres versus temperature, you can quickly select the atmosphere for annealing steels without formation of oxides. The diagram in Figure 1 was calculated using FactSage. This diagram shows that oxidation-reduction curves for Fe-18%Cr and Fe-18%Cr-8%Ni systems representing stainless steels are higher than the corresponding Cr/Cr2O3 curves. For alloys (e.g. steels), you can achieve more precise calculations using thermodynamic data from both the pure substances (i.e. pure metals and oxides) and solutions databases. Such diagrams can be produced specifically for the steels of interest and variety of atmosphere compositions.

These methods can help you troubleshoot and optimize your annealing operation by balancing hydrogen usage versus product quality. If you would like to learn more about these techniques, send us an e-mail or call 800-654-4567 (press 1 and mention code 749).

Figure 1: Oxidation-reduction curves for pure chromium, Fe-18%Cr and Fe-18%Cr-8%Ni for the total pressure of 1 atm and hydrogen partial pressures of 0.5 atm and 0.05 atm. The hydrogen pressures correspond to the following atmosphere compositions: 100% H2, N2-50% H2 and N2-5% H2. (This diagram has been produced using thermodynamic data for solutions in addition to the data for pure elements and their oxides.)