Cast duplex stainless steel (CDSS) components suffer embrittlement after long-term thermal aging. The deformation and fracture behaviors of un-aged and thermally aged (at 400 °C for 20,000 h) CDSS were investigated using in situ scanning electron microscopy (SEM). The tensile strength of CDSS had a small increase, and the tensile fracture changed from ductile to brittle after thermal aging. Observations using in situ SEM indicated that the initial cracks appeared in the ferrite perpendicular to the loading direction after the macroscopic stress exceeded a critical value. The premature fracture of ferrite grains caused stress on the phase boundaries, leading the cracks to grow into austenite. The cleavage fracture of ferrite accelerated the shearing of austenite and reduced the plasticity of the thermally aged CDSS.

The cast duplex stainless steels were investigated in order to understand the plastic deformation behaviors and fracture mechanisms after thermal aging at 400°C for up to 10 000 h. Tensile strength has an obvious improvement, but the yield strength has a slightly increase after long-term thermal aging. Microcracks initiate in the ferrite and extend to phase boundaries, leading to the fracture of ferrite before the tensile failure of specimen. Nanoindentation tests indicate that hardness in ferrite continuously increases with aging time, but not strongly affected by deformation degree. Electron backscattered diffraction observations reveales that high-stress concentration on the phase boundaries causes the phase boundary separating. The dislocations pile-up at spinodal decomposition precipitates in ferrite phases and along G-phase boundaries, leading to the reduction of mechanical property of the ferrite.