Annealing

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 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.

Oxidation-reduction curves for pure chromium, Fe-18%Cr and Fe-18%Cr-8%Ni for the total pressure of 1 atm with a hydrogen partial pressure of 0.05 atm, corresponding N₂-5% H₂. (This diagram has been produced using thermodynamic data for solutions in addition to the data for pure elements and their oxides.)

Figure 1:

Formation of oxide film is a function of hydrogen/water partial pressure ratio, temperature, and time, all of which can change significantly at the furnace exit. An elevated dew point due to air ingress reacting with hydrogen, combined with reduced temperature, will lead to oxidation if the time within the exit area is sufficiently long. Additionally, the ID surface of the tubing may cool more slowly than the OD, causing inconsistent oxidation. Countermeasures include increasing the tubing’s travel speed, increasing the hydrogen flow rate over all tubing surfaces, and applying a nitrogen curtain at the exit to dilute air ingress and assist in cooling. There are technological, safety, and cost considerations associated with each countermeasure.