Multiaxial thermomechanical creep-fatigue analysis of heat-resistant steels with varying chromium contents

• TMF and multiaxial lives of steels with varying chromium contents were investigated.
• Biaxial loading causes no life reduction under the same equivalent strain range.
• Steels with higher chromium content have shorter TMF lives than isothermal lives.
• Separations observed in TMF lead to a shorter TMF live at 10%Cr steel.
• FEA shows that strain ratcheting is responsible for the formation of separations.

Steels with varying chromium contents are widely used in steam turbine components and have been introduced steadily in the last decades. The initial aim in the development of such steels is to achieve high performance in creep resistance. Due to the fluctuations of electrical power demand nowadays, power plants are increasingly forced to run at varying utilization levels, which can shift the critical load to the fatigue domain by superimposed creep on the heated surface of components. In the current paper, the creep fatigue behavior of 1%-, 2%- and 10%Cr steels under multiaxial loading is described. The experimental investigation was conducted on steels of the types 1Cr–1Mo–Ni–V, 2Cr–1Mo–W–V and 10Cr–1Mo–1 W–V–Nb–N as representative samples for each of the three steel grades. The experimental database consists of uniaxial as well as biaxial creep fatigue experiments which were conducted on a biaxial cruciform testing machine. Of special interest was a lifetime comparison of experiments under thermomechanical and isothermal loading at the maximum application temperature. A unified viscoplastic constitutive material model with an incorporated damage variable was applied for lifetime assessment. Finally, metallographic investigations contribute to a better knowledge of the evolution of damage and its modeling. The investigation shows slightly different effects on lifetime, dependent on the three steel grades.