Precipitation Hardening Behavior Of Nano-precipitates In 15-5PH Stainless Steel

15-5PH stainless steel is a typical martensite precipitation hardening stainless steel,which is extensively used in aerospace,marine and nuclear industries because of its high strength,good toughness,excellent corrosion resistance and simple heat treatment process.Its high strength is mainly achieved by the precipitation hardening of nanoscale Cu-rich precipitates in the aging process.Besides,G phase,Nb(C,N)and Cr-rich region caused by spinodal decomposition also appear during aging.These precipitates have an important impact on the mechanical properties of 15-5PH stainless steel,which is of great significance to research.In this paper,the changes of mechanical properties of 15-5PH stainless steel at three different aging temperatures of 420 ℃,500 ℃ and 580 ℃ were studied.The strengthening behavior and evolution of nanoscale precipitates such as Cu-rich precipitates,G phase,Cr-rich region and carbonitride during aging were systematically studied by TEM and APT,and the following conclusions were obtained:1.With the increase of aging temperature,the time for 15-5PH stainless steel to reach the peak hardness decreases and the peak hardness reduces.2.In the aging process of 420 ℃,the hardness rises to the peak value after aging to 8 h,and then remains at a relatively high level for a long time.After aging to1024 h,the hardness reaches the peak value again.The first hardness peak is mainly due to the precipitation strengthening of Cu-rich precipitates.Subsequently,the precipitation of Nb(C,N)and the Cr-rich region formed by spinodal decomposition makes up for the weakening of the strengthening effect caused by the coarsening of Cu-rich precipitates,resulting in little change in hardness.After aging to 1024 h,due to the further increase of the density of Cr-rich region,the strengthening effect is remarkable,and a second hardness peak appears.3.There are three hardness peaks in the hardness curve during aging at 500 ℃,and the first hardness peak is obvious,which is mainly caused by the precipitation of Cu-rich precipitates.The second and third hardness peaks are relatively weak,which are caused by the precipitation of Nb(C,N)and the Cr-rich region formed by spinodal decomposition,respectively.4.G phase is found in the later stage of aging at 420 ℃ and 500 ℃,and which is adjacent to Cu-rich precipitates.It is considered that the evolution process of G phase is as follows: At the initial stage of aging,Ni and Mn elements are segregated around the Cu-rich precipitates.With the extension of aging time,Si and Nb elements are also gradually segregated around the Cu-rich precipitates.When the content of Ni,Mn,Si and Nb elements at the Cu-rich precipitates/matrix interface reaches a certain proportion,G phase is formed.5.In the sample aged at 500 ℃ for 128 h,the coexistence of martensite,reverted austenite and retained austenite is found.The size of Cu-rich precipitates in martensite is the largest,which has been coarsened into long rod shape with FCC structure.Due to the same FCC structure as the matrix,the Cu-rich precipitates in retained austenite grow slowly,have the smallest size and the highest density.The growth rate of the Cu-rich precipitates in the reverted austenite is faster when the matrix is still martensite.After the matrix transform into austenite,the growth rate of the Cu-rich precipitates slows down.Therefore,the size and density of the Cu-rich precipitates are between that of martensite and retained austenite.