Austenitic Cr–Ni Alloyed Heat Resistant Steels with Nb Additions Are Used for Intensively Thermo-Mechanically Loaded Cast Components Working in Extreme High Temperature Oxidizing Environment. their Performance during Static Oxidation and Transient Thermo-Mechanical Loading Was Investigated to Recommend an Optimal Cost-Effective Cr/Ni Composition of Nb-Alloyed Austenitic Class Steels. the Static Oxidation and Transient Thermo-Mechanical Behavior of Three Austenitic Steels with Different Cr/Ni Alloying Levels Were Investigated and Compared for Variety of Working Conditions. Static Oxidation Was Performed between 900 °C and 1000 °C in Air for 400 H. the Critical Temperature Which Increases Spallation during Static Oxidation Was Determined for Each of the Steel Alloying Levels. in Addition, Thermal Cycling of a Constrained Specimen Was Done with Varying Upper Cycling Temperatures between 850 °C and 1000 °C. SEM and TEM Analyses Were Supported by Thermodynamic Simulation of the Phases Precipitated in the Metal Matrix and the Structure of Formed Oxide Layers. These Studies Were Used to Determine the Mechanisms of Degradation of Thermo-Mechanically Loaded Cr/Ni Austenitic Steels at Extreme High Temperatures. a Recommendation for a Cost-Effective Cr/Ni Alloying Level for Different Working Conditions Was Determined.