Study On Q&P Process And Microstructures-properties For CrMnN Martensitic Stainless Steels

CrMnN martensitic stainless steel is new steel developed on the base of CrNiMo stainless steels by replacing Ni with Mn and N.It can not only reduce the production cost,but also save the scarce resources Ni,and is expected to replace the turbine runner and blade steels such as 00Cr13Ni4-6Mo and 00Cr16Ni5Mo.However,there are still some problems during production and operation,such as processing difficulties,cavitation resistance to be further improved and so on.The key to solve these problems are to obtain a high combination of strength and toughness.The diffusion of carbon atoms from martensite to austenite by Q&P treatment has been used to regulate the mechanical properties and the strength-ductility of the carbon steels.At present,this treatment has been widely used in the field of medium and low carbon steels and martensitic stainless steel,but it is not so clear whether interstitial atom nitrogen,as an austenite stabilizer and former,plays a role in the Q&P process,as well as how influence N partition may have on the microstructure and properties of the steels.In this thesis,independently designed ultra low carbon CrMnN martensitic stainless steels were investigated,where the behaciors of N partition were introduced into the Q&P process.The pseudo-binary equilibrium phase diagram was calculated by the use of thermodynamic calculation.The effect of Q&P process on microsructure,mechanical properties and resistance to cavitation erosion were analyzed by means of OM,DSC,XRD,SEM and TEM etc.The N atom parititioning behavior was discussed.The obtained main results are as follows.The pseudo-binary equilibrium phase diagram of the CrMnN stainless steel were obtained,the?single phase area of G1 steel is 780-1190?;the phases such as M₂₃C₆,Cr₂N,sigma start to precipitate at 780?,715?and 628?,respectively.The austenite is completely transformed into?phase at 580?.The measured austenitizing temperature and martensite transition start point are 590?and 170?,respectively.The?single phase area of G3 steel is 816-1190?;the phases such as M₂₃C₆,Cr₂N,sigma and M₆C start to precipitate at 763?,816?,645?and 290?,respectively.The austenite is completely transformed into?phase at 530?.The measured austenitizing temperature and martensite transition start point are 540?and 125?,respectively.The relationship between temperature and equilibrium phase mole fraction of the steels,and the alloying element composition distribution in each phase under equilibrium conditions were obtained also.The relationship between the retained austenite content at room temperature and the initial quenching temperature was obtained,and the Q&P process of CrMnN stainless steel was designed.The partitioning of N atoms in the ultra low carbon CrMnN martensitic stainless steesl had been found and confirmed.Compared with C partitioning,N partitioning period required longer time to achieve the best partition process.The optimum paritioning period of the tested steel was 10min.The retained austenite content in the steels increased with the increase of partitioning temperature and increased first and then decreased with the prolongation of the partitioning period.The maximum retained austenite content of G1and G3 steel were 8.1vol%and 19.2 vol%,respectively,when the steels were treated at500?and 400?for 10min,respectively.Compared with the quenching state,the content of retained austenite was increased by 8.1vol%and 11.6vol%respectively.The retained austenite was observed between the lath martensitic spaces as strips and thin layers.The effect of partitioning temperature and time on the mechanical properties of CrMnN martensitic stainless steel was studied.It was found that the tensile strength decreased slightly with increase of the partitioning temperature,and the prolongation of partitioning period.The elongation and strength-ductility first increased and then decreased with the increase of partitioning temperature and time.The increase of Mn content reduced the strength of CrMnN stainless steel,improved the toughness and plasticity,and expanded the temperature range of Q&P process.The hardness decreased with the increase of partitioning temperature and the extension of time.The higher the partition temperature is,the smaller the magnitude of reduction is.The effect of partitioning temperature and time on the resistance to cavitation erosion of CrMnN stainless steel was investigated.It was found that the mass loss and mass loss rate of CrMnN steel decreased with the increase of partitioning temperature,and decreased slightly with the prolongation of time.The partitioning period had little effect on the incubation period of cavitation erosion,but the partitioning temperature had a great influence.