Effect Of Heating Temperature On Microstructure And Properties Of Mn18Cr18N Retaining Ring

High nitrogen austenitic stainless steel is a kind of high quality steel,which hasthe advantages of high strength,good ductility,good work hardening ability and good corrosion resistance.Mn18Cr18 N,a typical representative of Cr-Mn series high nitrogen austenitic stainless steel,is often used in the manufacture of reactor coolant pump flywheel.In this paper,the microstructure evolution of Mn18Cr18 N retaining ring after heat treatment at different temperatures from 300 ? to 1000 ? and the effect of heat treatment temperature on the tensile properties,microhardness,impact toughness,thermal expansion properties and stress relaxation was studied.The relationship between temperature and microstructure evolution was investigated,that is,the evolution of deformed microstructure and the law of second phase precipitation.It was found that the material has a cold deformation microstructure with serious deformation inhomogeneity and texture.Partial recrystallization occured at 900 ?,complete recrystallization occured at a temperature of 1000 ?.The second phase of the carbide was observed to precipitate in the grain boundary at 600 ?,and the lamellar nitrides precipitated at 800 ?.Grain boundary granular precipitation occured at 900 ? and complete solid-solution state at 1000 ?.The yield strength and tensile strength did not change with increasing temperature range from 300 ? to 500 ?,respectively in 1600 MPa and 1670 MPa.There was a dramatic reduction in the yield strength and tensile strength at 600 ?.The yield strength decreased with the increase of temperature,while the tensile strength reached the minimum at 700 ?,and then rised again with the temperature increasing.The microhardness was different at the different grains.The grain hardness decreased with the temperature increasing range from 300 ? to 550 ?.From 600 ? to 700 ?,hardness values fluctuated in a certain range with the temperature increasing at different grains.The impact toughness remained unchanged at 300 ? to 500 ? with the temperature increasing,and the fracture wasmicroporous aggregation mechanism.The impact energy greatly reduced at 600 ?,and the fracture was intergranular fracture and cleavage type.The impact energy continued to decrease at 700 ? and 800 ?,and the fracture was intergranular fracture type.At 1000 ?,the impact energy of the sample increased greatly and the fracture showed microporous aggregation mechanism.The relaxation stability of the sample after the heat treatment at 700 ? for 1 h was the worst,followed by the sample after the heat treatment at 800 ? for 1 h.The relaxation stability of the sample after the heat treatment at 300 ?for 1 h was the best.Below 640 ?,the relationship between the thermal expansion coefficient(?)and the temperature(T)of the Mn18Cr18 N retaining ring can be expressed as ?=3.95288×10-6+1.61401×10-7T-5.65169×10-7T2+1.05651×10-12T3-6.08818×10-16T4.In the process of the second phase precipitation,the change in the volume of the phase would cause the variety of the thermal expansion coefficient.