Micro-structure And Properties Of Nitrogenous Austenitic Stainless Steel By Laser Cladding

In order to obtain the laser cladding layer of nitrogen stainless steel with excellent comprehensive performance,Three austenite powders with different nitrogen content were produced by AISI304powder with solution nitriding in super pure nitrogen flow.And owing to nitrogen-bearing metastable austenite promoting the formation of martensite phase under deformation,we report a novel TRIP nitrogen stainless steel design.The influence of microstructure and precipitation phase on the mechanical properties and corrosion resistance of these laser cladding layers are explored by scanning electron microscopy（SEM）,Energy Spectrometer（EDS）,X-ray diffraction（XRD）,material universal testing machine,Universal friction and wear tester,electrochemical workstation and so on.Some findings are described as follows:（1）there were nitrogen segregation and pores in the laser cladding layer of nitrogen-bearing austenite during the process of laser cladding.As the nitrogen content of powder increased,with an more uniform and fine grain size is the nitrogen-bearing austenite and the nitrogen content in the cladding layer gradually increased,the porosity first increased and then decreased and the mechanical properties with an isotropy in some degree,the anisotropy percentage in a range of 3.69%-7.23%,were significantly enhanced,while the elongation and corrosion resistance gradually decreased.The nitrogen content of powder was close to 0.41%,the nitrogen content of the cladding layer sample reached 0.25%and the grain size is around 3.0μm.the tensile strength,yield strength,elongation and micro-hardness of the cladding layer sample was in the range of856±20MPa,747±15MPa,22±3%,280.68HV_0.2.2 respectively.The nitrogen content of powder was 0.16%,the corrosion resistance about its cladding layer whose self-corrosion potential reached 0.0777v and whose corrosion rate was 1.6198×10^-5g/m²h was much better than that of the AISI304 cladding layer.（2）In this novel nickel-saving TRIP nitrogen stainless steel design,the volume fraction of austenite in the cladding layer can be estimated by the mass fraction of austenitic stainless steel powder in the mixed powder.a novel nitrogen bearing low-nickel TRIP nitrogen stainless steel（0.12C-20Cr-2.39Ni-2.52Mn-0.49Cu-0.60Si-0.12N,Wt.%）produced by laser cladding,whose tensile strength is 983±40MPa,yield strength is 685±20MPa and percentage elongation after fracture is 38±2%,far better than the traditional UNSS32304 duplex stainless steel with similar composition.And the TRIP effect does not appear strikingly when the austenite volume fraction in the cladding layer is above 33.5%.Secondly,the TRIP effect under a unidirectional stress and strain exhibits a significant hysteresis as the nitrogen content of the cladding layer increases.In addition,the corrosion resistance of the laser cladding layer(E_corr=-0.3252,E_pitting=0.4530),of which the nitrogen content is0.12%,is slightly worse than that of UNSS32304 duplex stainless steel.as the nitrogen content of the cladding layer increases,its pitting potential and self-corrosion potential gradually decrease,its corrosion resistance decreases,and the traditional calculation formula for the pitting resistance equivalent is partly no longer applicable to nickel-saving TRIP nitrogen stainless steel cladding layer in some degree.