Influence of high temperature exposure on the mechanical behavior and microstructure of 17-4 PH stainless steel

The mechanical behavior and microstructural evolution of 17-4 PH stainless steels in three conditions, i.e. unaged (Condition A), peak-aged (H900) and overaged (H1150), exposed at temperatures ranging from 200 to 700°C were investigated. The high-temperature yield strength of each condition decreased with an increase in temperature from 200 to 400°C except for Condition A at 400°C with a longer hold time where a precipitation-hardening effect occurred. At temperatures from 500–700°C, the decrease in after-exposure hardness of Condition A and H900 at longer exposure times was caused by a coarsening effect of copper-rich precipitates. A Similar microstructural change was also responsible for the hardness of H1150 exposed at 700°C decreasing with increasing exposure time. Scanning electron microscopy (SEM) observations indicated that the matrix structures of Condition A and H900, when exposed at 600°C and above, exhibited lamellar recrystallized α-ferrite in the tempered martensite and the size and quantity of these lamellar ferrite phases increased with exposure time. X-ray diffraction (XRD) analyses showed that the reverted austenite phase in H1150 that formed during the over-aging treatment was stable and hardly affected by deformation at temperatures of 200–400°C.