Development Of Self-balanced Fabrication Device And Properties Characterization For Gradient Nanostructured Round Bar Samples

Compared with traditional coarse-grained materials,nanostructured materials have higher strength.But the work hardening ability,plasticity and toughness are significantly reduced.The application and development of nanostructured materials have been greatly restricted.It is found that gradient nanostructured materials can effectively overcome the shortcomings of nanostructured materials,improve the strength of materials and maintain good plasticity.The thesis mainly focuses on researching the preparation technology and properties of gradient nanostructured round bar samples,including the following parts:(1)Designing a self-balancing device of producing gradient nanostructured round bar samples based on surface mechanical rolling treatment(SMRT).To refine the grains and get gradient nanostructured metal materials,the self-balancing rolling device is used to produce a high strain rate of strong deformation on the surface of the material.The self-balancing device of manufacturing gradient nanostructured round bar samples mainly includes four parts: ordinary lathe,self-balancing rolling device,rolling pressure system and working fuid circulation system.(2)Producing and characterizing gradient nanostructured 316 L stainless steel samples.The majority of coarse grained 316 L stainless steel is austenite phase.The volume fraction of martensite is ~3.6%.The grain sizes range from 18 ?m to 222 ?m,with an average of 53 ?m in the planes.The outer SMRT layer(depth up to 50 ?m)is composed mostly by a GNS layer with high amount of martensitic volume fraction.The average grain sizes in this region increases from 120 nm to 260 nm.From depths of 50 ?m to 250 ?m,ultra-refined grains with considerable amount of martensitic phase transformation are resolved.In this depth span,the average grain size is 260 nm.With the increase of depth,the content of martensite decreases.The content of martensite is about 33% at 15 ?m and 21% at 148 ?m.The micro-hardness of gradient nanostructured 316 L stainless steel changes from surface to inside.The Vickers hardness of the core coarse-grained material is about 198 HV,while the micro-hardness of 316 L stainless steel can reach 440 HV at 40 ?m away from the surface,an increase of 122 percent.As the depth increases,the nanocrystals,slip bands and twins in the gradient layer decreases gradually,and the hardness also decreases gradually,which is consistent with the core coarse grains.The tensile strength of 316 L stainless steel with gradient nanostructure increased by about 13%(from 591 MPa to 666 MPa)and the elongation at break decreased by about 25%(from 114% to 85%).The surface roughness(Ra)of the sample is less than 0.1 ?m,the residual stress of axial surface compression can reach-1500 MPa,and the residual stress of the circumferential surface compression can reach-700 MPa.(3)Gradient nanostructured 310 S stainless steel samples were prepared.In the experiment,the strain rate is changed by changing the spindle speed and the longitudinal feed speed.After a lot of experiments,the processing parameters of 310 S stainless steel suitable for heated-treatment and unheat-treated were optimized.Gradient nanostructured 310 S stainless steel(heated-treatment)and gradient nanostructured 310 S stainless steel(unheat-treated)were successfully prepared.(4)Characterization of the main properties of gradient nanostructured 310 S stainless steel.Heat-treated samples: The surface grain size of the sample is obviously reduced,and the grains within the range of 30 ?m from the boundary can not be seen by metallographic microscope,and the depth of the affected layer is about 1200 ?m;the tensile strength of the sample is increased by about 27%,the uniform elongation is decreased by about 24%,and the fracture elongation is decreased by about 23%.The microhardness of the specimen changes gradiently in the radial direction of the cross section,and the hardness value increases about 111% at 50 ?m from the surface of the specimen;the surface residual stress of the specimen is compressive residual stress,in which the axial surface compressive residual stress can reach-1700 MPa,the circumferential surface compressive residual stress can reach-700 MPa;the surface roughness Ra of the specimen is as low as 0.052 ?m.Unheat-treated samples: With the increase of SMRT processing times and SMRT working pressure,the fracture shape of samples will change.When the maximum SMRT working pressure reaches 910 N,the fracture shape of samples will change from cup-cone to oblique angle.The maximum working pressure of SMRT is 1200 N,and the tensile strength of 310 S stainless steel samples with gradient nanostructure increases by 9.7%,the uniform elongation decreases by about 5%,the fracture elongation decreases by about 5%,the hardness increases by about 78% at 50 ?m from the surface of the sample,the thickness of gradient layer of the sample is about 1.2 mm,and the surface roughness(Ra)is less than 0.1 ?m.The surface residual stresses of the samples processed by SMRT are all compressive residual stresses,which increase with the increase of the number of SMRT processing and the increase of the working pressure of SMRT.When the number of SMRT processes reaches 12 times and the working pressure of SMRT reaches 2.5 MPa,then the number of SMRT processes and the working pressure of SMRT increase,and the residual stress of surface compression no longer changes significantly.The prepared samples were ground to a certain thickness of gradient layer for residual stress detection and unidirectional tensile test.With the increase of the grinding thickness,the compressive residual stress on the axial surface of the sample decreases,the tensile strength decreases,the uniform elongation increases,and the shape of the fracture changes from oblique to cup-cone.