Microstructural evolution under ausforming and aging conditions in 17-4 PH stainless steel

In this investigation the effect of a deformation process, called ausforming, and the subsequent aging treatment on the microstructural evolution and mechanical properties of 17-4 PH stainless steel are studied in the aging temperature range of 300 degrees C to 600 degrees C. Results obtained showed that increasing the ausforming strain increased the strength and hardness of the investigated steel, particularly at the 400 degrees C aging temperature. The optimum mechanical properties can be obtained by aging at 400 degrees C for few hours due mainly to the precipitation of coherent nano size copper precipitates. Long time aging at 400 degrees C also did not change the coherency characteristics of these precipitates and instead added to the strength. For aging temperature higher than 400 degrees C, i.e. 500 degrees C, a peak hardness value was obtained in less than 2 h. Longer time aging at these temperatures reduced the hardness and strength due mainly to the loss of coherency of the copper precipitates as well as the formation of ferritic bars. Formation of reversed austenite at 600 degrees C was also another reason for the loss of strength together with some restoration.