Synergistic Precipitation Mechanism And Strengthening Behavior Of Nano-Precipitates In 17-4 PH Stainless Steel

Due to its high strength,excellent corrosion resistance and ease of thermal treatment,17-4 precipitation hardening stainless steel(17-4 PH SS)has been extensively used in the latest-stage blade of steam turbine,the lead screw and the valve stem of the driving mechanism for reactor's control and so on.During the tempering process,the high strength of 17-4 PH stainless steel is caused by the precipitation-hardening effects of the nano-scale Cu-rich precipitates(CRPs).In addition to Cu atom,Ni,Mn and the elements that can form carbide/nitride(C,N,V,Nb and Mo)are also contained in the steel.Meanwhile,the nano-scale Cr-rich regions and Fe-rich regions are also formed during the spinodal decomposition at the intermediate temperature tempering(350 ??480 ?),which also leads to the precipitation-hardening effects.As can be seen from the results,the synergistic strengthening behavior of multiple nano-precipitates will be more complex.Therefore,the study on the strengthening mechanism and behavior of multiple nano-precipitates of of practical importance and is valuable for the development of new generation of high-strength stainless steel.In the thesis,Transmission Electron Microscope(TEM),Atom Probe Tomography(APT),Micro-hardness tester,Tensile testing machine,Impact testing machine and Optical Microscope(OM)were utilized to investigate the precipitation mechanism of multiple nano-precipitates and their strengthening behavior.The main conclusions are drawn as follows:(1)The microstructures in 17-4 PH stainless steel are lath martensite after 4h tempering at different temperatures.As the tempering temperature increases,the hardness and strength increase,reach peak value at 450? and then decrease while the impact toughness decreases to the minimum value at 450? then increases.As the tempering temperature increases,Cu atoms gradually transform from forming the clusters to the CRPs with large size.The number density firstly increases,reachs peak value at 450? and then decreases;the peak in number density corresponds to the strongest strengthening effect.As the temperature reaches 570?,NiMn phase is formed at the CRPs/matrix interface.The CRPs is a coreshell structure with short bar shape or spherical morphology.Compared with that at 450?,the number density of CRPs are less and its size are larger,resulting in lower strength and hardness.(2)During the tempering process of 17-4 PH stainless steel at 450?,the strength and hardness firstly rose to the "peak" and decreased to the "platform".Meawhile,the impact toughness firstly falls to the "trough" and rises to the "platform",then the toughness decreases again.The trends of increasing first then decreasing for strength and hardness were caused by precipitation,growth and coarsening of the CRPs.The"platform" stage is due to that the strengthening effects by Cr-rich ?' domains compensated for the decline of strengthening effects of CRPs.The final increases in strength and hardness are mainly caused by precipitation-strengthening effect of Cr-rich domains.The precipitation sequence of multiple nanoscale phases and its evolution mechanism as follows:Cu clusters(1 h)? Cu-rich clusters(Cu-rich in the core and Ni-Mn-Si-rich in the shell)/Nb-rich clusters(8 h)?untwinned 9R CRPs/NiSiMnNb-rich clusters/Cr-rich ?' domains(32 h)? twinned 9R CRPs/NiSiMnNb-rich clusters/Cr-rich ?' domains(100 h)?twinned 9R CRPs/bcc G phase/Cr-rich ?' domains(200 h).The untwinned CRPs were identified as the precursor of G precipitates and CRPs.The structure orientation relationship among twinned 9R CRPs,matrix and G obeyed the following rule:(009)9R-CRPs//(101)BCC-Matrix//(202)BCC-G.(3)During the tempering at 540?,the strength and hardness of 17-4 PH stainless steel first increase to the peak value and decrease,then rise and decrease again,that is the strength and hardness reached the first and second peak,respectively,at tempering for 30 min and 120 min.APT result indicates that the combined strengthening effect of CRPs and MoNb(C,N)led to variation in mechanical properties.The first peak means that the number density of CRPs reached the peak and leads to the best strengthening effect,corresponding to high the strength and hardness.The second peak means that the number density of MoNb(C,N)reaches the peak and leads to the best strengthening effect while the precipitation-strengthening effect of CRPs significantly weakens,thereby resulting in relatively lower value of the strength and hardness.(4)The segregation of alloying elements with different extents occur at grain boundary and martensite lath interfaces in 17-4 PH stainless steel is different.The segregation tendency of elements at grain boundary from strong to weak is C>Cu>P>Nb>N/Mn(?N=?Mn)>Mo>Ni>Cr>V(480? for 2 h)and that at martensite lath interfaces(600? for 15 min)is C>Cu>Nb>N>V>Mo>P/Mn(?P=?Mn)>Cr>Ni.The radius of CRPs at the grain boundary is smaller than that in matrix,the contents and distribution of all elements in different size of CRPs are similar.M23(CN)6 distributed adjacent to the flat sphere CRPs at lath interfaces.While,no carbonitride distributed adjacent to the ellipsoid CRPs in matrix.The lattice distortion energy(?G)generated by the segregation of P atoms in the grain boundary is higher than in the martensite lath interfaces.