High Temperature Deformation Behavior of a Fe-25Ni-20Cr (Wt Pct) Austenitic Stainless Steel

Structural components of Gen-IV nuclear reactors are expected to endure higher operating temperatures for longer service life. Therefore, understanding micromechanisms governing the high temperature deformation and establishing associated constitutive equations are essential for safety and design considerations. Due to its distinctive combination of high temperature strength, thermal stability, weldability, and corrosion resistance, a highly alloyed advanced austenitic stainless steel Fe-25wtpctNi-20Cr (Alloy 709) is being considered for application in next generation nuclear reactors. In this study, the deformation behavior and constitutive equation of the Alloy 709 were analyzed by conducting isothermal-uniaxial tensile tests at elevated temperatures ranging from 700 °C to 800 °C and at strain rates ranging from 10 −5 to 10 -3 s −1 followed by microstructure assessments. Considering the modified peak stress values for each test condition, the following hyperbolic sine constitutive relation was found to be applicable for the alloy: 1 ε̇ = ( 5.5 ± 1.3) × 10^9 [ sinh( ασ)]^ 6.11 ± 0.3exp( - Q/RT), The extrapolation of the developed constitutive equation to lower strain rates agreed with the experimental data obtained from creep tests for the same alloy, implying the validity of the proposed equation. In addition, micro mechanistic parameters were utilized to estimate the dislocation jump distance, which was found to be around 20b (where b is Burger’s vector) suggesting that the climb of edge dislocations is the rate controlling deformation mechanism. This rate controlling process was further supported by the presence of well-defined subgrains under TEM microstructure evaluations following deformation. Subgrain size tends to decrease with increasing strain rate as the peak stress increases. The constitutive relationship developed here will help to understand the deformation behavior of the Alloy 709 for different combinations of strain rate, stress, and temperature. Understanding these factors is essential for the safety of structural components for a wide range of applications.