Effect Of Solution And Aging Treatment On Microstructure And Mechnical Properties Of High Nitrogen Stainless Steel

High nitrogen austenitic stainless steels,developed by partial or total replacement of nickel(Ni)through nitrogen(N),have been widely applied in manufacturing and various fields because of a good combination of mechanical properties,high resistance to corrosion and low cost.However,the microstructure of these steels may contain some secondary phases except for austenite,such as ? ferrite,nitrides(Cr2N),and the ? phase(?-Fe-Cr)due to the segregation of Cr,Mo,N and Mn during casting and forging,which worsens the mechanical performance and causes premature failure.In this paper,the effect of solution and aging treatment on microstructure and mechanical properties of Cr19Mn19Mo2N0.7 stainless steel is investigated by using optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM)with energy dispersive X-ray spectroscopy(EDS),microhardness testing,wear testing and 3-D optical profiler observation.Solution temperature and time have a great influence on the content of nitride and ferrite in high nitrogen stainless steel.When the solution temperature is lower than 960?,the nitrides inside the ferrite strips and along the austenite grain boundaries will not dissolve,and has the tendency to be coarsen.As the solution temperature increases,the nitride in the ferrite strip is prior to dissolve.When the solution temperature exceeds 1060?,only austenite and ferrite are found in the sample,and the nitride is completely dissolved in the matrix.With the increase of the solution temperature,the smooth boundary of the ferrite strips will gradually be polygonal,and the zigzag boundary will formed,and the ferrite content will gradually decrease with the increase of the solution temperature.Solution at 1200?,with the increase of the solution time,the number of ferrite strips decrease,the length becomes shorter,and the width becomes narrower.But when the solution time reaches 240min,the width of some ferrite strips will increase.When the solution temperature exceeds 1010?,a white single-phase ferrite layer is formed on the surface of the high nitrogen stainless steel.When the solution temperature exceeds 1200?,a multi-layer structure is obtained,which consists of a white surface layer with fully ferrite and a grey subsurface layer with 100%austenite.With increasing solution treatment temperature and time,the thickness of either the surface or the subsurface layer increases.After aging at 900? for 3h,it is found from OM and SEM,that some small and fine nitrides are re-precipitated along the austenite grains boundaries and within ferrite grains.After aging for 12h,the fingerprint nitrides nucleate at the grain boundaries has grown into the interiors of austenite.After 24h,more coarse fingerprint nitrides formed within austenite grains.It is indicated from microhardness testing that a maximum microhardness value can be obtained after 12h during aging,which is due to moderate amount and fine fingerprint nitrides.The optimum wear resistance is obtained at 3h during wearing test.Nitride as a brittle phase,is easy to peel off from the surface during wearing.The nitrides amount is increased with the increasing time.Thus,a abrasive wearing with high wear rate can be formed.Although there is non nitride after the solid solution,low wear resistance is obtained due to the low matrix hardness.