Study On Microstructure And Properties Of 22Cr Duplex Stainless Steel Based On Laser Selective Melting Technology

Developing excellent performance corrosion resistant steel and its preparation technology is a key development direction in the fields of marine engineering equipment and excellent tech ships,aeronautics and astronautics equipment,and new materials in“Made in China 2025”.Duplex stainless steel（DSS）has an approximately equal phase balance of body-centered cubic（BCC）Ferrite and face centered cubic（FCC）Austenite in its microstructure.DSS have gained popularity and have been widely used due to its good mechanical properties and corrosion resistance,which depend on the presence of both the Austenite and Ferrite phases in the microstructure.In order to promote the development of steel marine engineering in China and meet the increasing demand for complex structural components,the research of the selective laser melting（SLM）forming excellent performance complex structure 22Cr type duplex stainless steel was carried out.Firstly,the effects of different process parameters on the density,porosity and surface roughness were explored.Establishing and optimizing the process parameters of 22Cr type duplex stainless steel.Subsequently,due to the unique,very fast thermal heating and cooling cycles during SLM processing,microstructure of duplex stainless steels was almost fully ferritic.The study of DSS microstructures for different temperature solid solution treatment was carried out,which revealed the correlation mechanism of microstructures,mechanical performance and corrosion resistance.Finally,from the perspective of the design of the alloy component,designing a new duplex stainless steel materials that phase balance of ferrite and austenite in the microstructure by SLM forming,which shortened the preparation time and reduced production costs.The specific research results were as follows:（1）Optimization of laser process parameters of duplex stainless steel.With the increase of laser power,the density of samples increased first and then decreased.With the increase of hatch distance,the porosity of samples showed a trend of decreased first and then increased,the surface roughness showed a similar pattern to porosity.When the laser power was 275 W,the hatch distance was 80μm,the scanning speed was 700 mm/s,and layer thickness was 50μm,the sample had a lower porosity（0.26%）.There were 84.8 wt.%ferrite in the duplex stainless steel.The hardness,tensile strength,elongation and self-current of samples respectively were 323.8 HV_0.2,922.1 MPa,21.17%and 1.62×10^-5 A/cm².（2）Solution annealing restored the duplex microstructure.With the solution temperature（900℃,960℃,1 020℃,and 1 080℃）increased,the ferrite content of the sample increased.The brittle sigma phase was observed at the crystal boundary of samples that heat treated at 900℃and 960℃.A nearly balanced dual-phase structure of ferrite and austenite was obtained through 1020℃solution annealing,which had excellent comprehensive performance.The hardness,tensile strength,elongation and self-current of samples that heat treated at 1020℃respectively were281 HV_0.2,850.7 MPa,32.2%and 1.04×10^-5 A/cm²,which had better corrosion resistant than 304 austenite stainless steel(4.56×10^-5 A/cm²).（3）Development new duplex stainless steel materials of SLM dedicated.With the increased the content of Ni in stainless steel,the austenite content of the samples increased.When Ni content was 9.0 wt.%,The duplex stainless steel phase ratio of ferrite:austenite was 46.3 wt.%:53.7 wt.%.The hardness,tensile strength,elongation and self-current of samples respectively were 308.7 HV_0.2,908.9 MPa,26.65%and1.61×10^-5 A/cm².（4）This article had almost solved the problem of dual phase imbalance after SLM forming DSS.In the future,the attention should be paid to further improving the corrosion resistance of the samples by optimizing the element composition ratio.Based on the microstructures characteristics of SLM formed DSS,microstructures transformation behavior was studied.A control method was established for its comprehensive performance.The relevant results can provide theoretical guidance for the application of SLM formed DSS components in marine engineering.