Effect Of Precipitate Phases On Mechanical Properties Of Mn18Cr18N Steel For Retaining Ring

Mn18Cr18N high-nitrogen austenitic stainless steel has many advantages such as good toughness,good corrosion resistance,no magnetism and so on,and it becomes a choice of reactor coolant pumps flywheel retaining ring material.The shrink fit process of the retaining ring requires that the retaining ring material possess excellent thermal expansion properties;the integrity of the retaining ring requires that the retaining ring material possess excellent mechanical properties.This article mainly investigates the micro-structure evolution of air-cooled Mn18Cr18N steels after insulation at 300?to 900?for 1 h?2 h?4 h and 6 h,and the effect of precipitates on properties of Mn18Cr18N steel?micro-hardness,thermal expansion,strength,fracture toughness?,the effect of precipitates on the retaining ring shrink fit and the flywheel integrity.The investigation shows that the recovery will only occur at 300°C⁸00°C,and recrystallization will occur at 900?to refine the grains.At 600?and 700?,carbide precipitates were occured along the grain boundaries.At 800?,a large amount of nitrides precipitates within the grains.There were some short rod-like?phase dispersion at 900?in the grain and grain boundary,and the amount of precipitation of the second phase increases with the holding time.The longer the holding time,the worse the thermal expansion performance of Mn18Cr18N steel,the continuous precipitation of M₂₃C₆ in the thermal expansion test heating process makes the expansion coefficient smaller,and the continuous precipitation of Cr2N makes the expansion coefficient larger.The micro-hardness of the sample at 300?⁸00?decreases with the increasing of heat treatment temperature.The precipitation of carbides,nitrides,and?phase will cause the strength of Mn18Cr18N austenitic stainless steel to decrease,The Mn18Cr18N steel which was heated at 900?for 2 hours has the lowest yield strength,and the value is 711.73 MPa.The higher the heat treatment temperature,the smaller the yield ratio of the material and the greater the elongation after breaking.The precipitation of the second phase causes the fracture toughness to deteriorate,The fracture toughness of the sample heated at 700?for 2 hours is 88.53 MPa·m^1/2,which is only 1/2 of the as-received material sample.The sample held at 600?for 2 h were brittle fractures along the crystal.The samples held at 700?and 800?for 2 h were quasi-dissociation faults along the phase boundary.The sample held at 900?for 2 h were ductile fractures.The outer diameter of the retaining ring is 902 mm,At 350?,the radial expansion of the retaining rings which were heated at 700??800?for 2 hours are about 2 mm smaller than those heated for 1h.For the same holding time,at 350?,the retaining rings treated at700?are always larger than the retaining rings treated at 800?.The structural strength of the retaining ring gradually decreases as the heating temperature increases.A small amount of precipitated phase will increase the structural strength of the retaining ring,and recrystallization will reduce its structural strength.The precipitation phase causes the stress intensity factor of the retaining ring to increase,while the safety factor,critical failure speed and critical crack size decrease.Among them,the sample which hated at 700?for 2 h has the lowest safety factor,critical failure speed and critical crack size.