Aging Precipitation Behavior And Its Effect On Mechanical Properties Of Mn18Cr18N High Nitrogen Austenitic Stainless Steel

High nitrogen austenitic stainless steel is widely used in energy,petrochemical and medical fields because of its strong toughness,strong resistance to creep resistance,strong corrosion resistance and good cold and hot processing.However,for the production of retaining ring forgings,we need to undergo many times of hot processing.In the production process,there are many problems such as coarse grain structure and cracking on the surface of forging.This paper mainly studied the different nitrogen content Mn18Cr18N（N=0.5,0.6 and 0.5）of high nitrogen austenitic stainless steel in the process of aging could produce species number and distribution of precipitated phase,at the same time through the room temperature tensile and fracture observation to determine the deposition effect on the mechanical properties,its purpose is for Mn18Cr18N thermal deformation of ring forgings production process/provide evidence for the selection of heat treatment parameters,to avoid the deterioration of mechanical properties of large forgings and practical production of forging cracks.First of all,the Jmat Pro 8.0 software is adopted to Mn18Cr18N equilibrium phase diagram of austenitic stainless steel and isothermal precipitation curves are thermodynamic calculation.The types of precipitates that may be produced in the aging process are determined,and the time and temperature of the precipitation in the isothermal precipitation curve are determined.Using OM and SEM to observe the amount and morphology of the precipitates,the results showed that the most sensitive precipitation temperature of Mn18Cr18N0.5 steel was 700℃and incubation period was 4h.The most sensitive precipitation temperature of Mn18Cr18N0.6 was raised to 750℃,and the incubation period was shortened to 2h.With the increase of nitrogen content,the precipitation behavior of Mn18Cr18N steel is more sensitive.The most sensitive precipitation temperature of Mn18Cr18N0.7 steel is 750℃,but the incubation period is shortened to 10min.As the aging process progresses,the morphology of the precipitates is transformed from the original edge to the internal growth in a discontinuous cell.Through the morphology observation of TEM and the electron diffraction analysis of the selection,it was determined that the precipitates were mainly composed of Cr₂N with a six-sided structure and M₂₃C₆ with a small number of face-centered cubic structures.The mechanical properties of Mn18Cr18N austenitic stainless steels depend on the precipitation of Cr2N.After aging treatment,the number of precipitates in Mn18Cr18N0.5 and Mn18Cr18N0.6 steel was small,and the strength and plasticity were slightly reduced.Mn18Cr18N0.7 has a large number of precipitates after aging,and the precipitation of nitrogen weakens the solid solution strengthening effect of nitrogen.With the increase of aging time,the strength did not decrease obviously,but the plasticity obviously deteriorated.The elongation of Mn18Cr18N0.7 was reduced from 52.9%of the solid solution state to 22.7%after the treatment of 750℃+8h,and the fracture mode also changed from the toughness fracture of the solid solution state to the brittle fracture along the crystal and the transgranular fracture.Therefore,in order to avoid the influence of the precipitation of Cr₂N on mechanical properties in the process of thermal deformation,the thermal deformation/heat treatment temperature should be controlled above 900,and the heating/cooling rate is greater than 20℃/min.