| Based on the urgent requirement of high-end equipment manufacturing to develop lightweight,highly reliable and long-life advanced structural materials,additive manufacturing technology integrating structure design and digital manufacturing provides a method for the research and development of advance high-strength stainless steel.In this paper,the advanced high-performance highstrength stainless steel was prepared by laser powder bed fusion(PBF-LB)through optimization of alloy compositions and printing parameters.The internal correlation between microstructure and mechanical properties,corrosion mechanism,hydrogen embrittlement sensitivity was studied by microstructure characterization,electrochemical testing,pitting corrosion evaluation,hydrogen distribution and embrittlement analysis.The results provided theoretical basis for the safety service and ideas for development of high-performance high-strength stainless steel by additive manufacturing.The main conclusions of this study are as following:The Schaeffler phase diagram of the alloy composition and phase composition of PBF-LB stainless steel was established.The advanced high-strength martensite stainless steel with high content of austenite was designed.Then,the manufacturing parameters was optimized for high-density samples.When the laser energy density increased from 82 J·mm-3 to 130 J·mm-3,the density of samples increased from 96.43%to 99.84%.The tensile strength increased from 1250 MPa to 1440 MPa and the elongation increased significantly from 3.3%to 19.2%.The cracks were more likely to occur in crevice defects with large aspect ratios.The pitting potential improved and the passivation phenomenon was gradually obvious.The correlation between microstructure and mechanical properties of PBF-LB high-strength stainless steel after different heat treatment processes was studied and the strengthening and deformation mechanisms were explored.There was a high content of austenite in martensite matrix for the heterostructure high-strength stainless steel after directly aging treatment,which contained 17%dislocation cell decorated bulk austenite distributed in the molten pool boundaries and about 8%thin austenite formed at the martensite lath interfaces.Meanwhile,there were highdensity(3.85×1024 m-3)nanoscale(1.5 ± 0.2 nm)Cu-rich and(Nb-rich,NbN)precipitates in the martensitic lath.Therefore,we accomplished a coupled improvement of the elongation to failure(16.3%)and tensile strength(1.44 GPa)for MSS.The strength mechanisms were mainly contributed to the high density of dislocations and nano-scale precipitates,while,the better deformation gave the credit to phase transformation/coordinated deformation mechanisms of high content austenite.Meanwhile,the correlation between microstructure and corrosion behavior of PBF-LB high-strength stainless steel after different heat treatment processes was explored and its corrosion resistance was optimized by hot isostatic pressing(HIP).After direct aging treatment,the surface potential of the austenite was approximately 15 mV higher than that of martensite by scanning Kelvin probe force microscopy.The pitting potential of the polarization curve was higher and the transient polarization current density was smaller for sample after directly aging treatment.Thus,the corrosion resistance after aging treatment was relatively high mainly due to the high content of austenite.After hot isostatic pressing,the pitting potential of the high-density(99.77%)sample increased from 0.265 VSCE to 0.289 VSCE and the electrochemical impedance value increased.The low-density(97.8%)sample was passivated and the passivation trend was more obvious.The environmental sensitivity was significantly reduced and the density of the passive film was increased after HIP.So that the corrosion resistance of additively manufactured high-strength stainless steels was significantly improved after HIP.In addition,the correlation between diverse austenite morphology and hydrogen trapping/hydrogen embrittlement(HE)is complicated and challenge for martensite stainless steel.We quantified systematically hydrogen traps and analyzed synergistic action of multi-scale austenite on the HE mechanisms for the PBF-LB high strength steel.The thermal desorption spectrometry(TDS)revealed dominant hydrogen trapping sites at martensite lath boundaries(EA=22-29 kJ·mol1),dislocations(EA=25-32 kJ·mol-1)and regions of residual stress(EA=30-44 kJ·mol-1),coupled with a fourth trapping site at the austenite or interface between matrix and austenite.The bulk austenite could be considered as an obvious sink hydrogen trap,which was more stable and impeded crack propagation.In contrast,the thin austenite seemed to release hydrogen much faster and was rather a shallower trap than the bulk austenite.And the thin austenite film to martensite transformation near the crack accelerated hydrogen-induced cracking.The probable crack initiation sites were martensite lath or phase boundaries between transformation martensite/matrix and propagated as transgranular fracture along low angle grain boundary,which generated higher HE susceptibility,explained by hydrogen-enhanced decohesion.Finally,the new PBF-LB high-strength stainless steel is designed by Mo alloying(2.5 wt%).The adjustment between the microstructure of ferrite and martensite realized the comprehensive adjustment of tensile strength between 10621240 MPa and elongation after fracture between 10.8%-26.4%,which was contributed to high content of precipitates and deformation twins.At the same time,there is a Mo-rich layer in the inner layer of the passive film after Mo alloying,and the content of Cr2O3 and Fe2O3 in the passive film was relatively high.The pitting potential is about 421-499 mVSCE,which is significantly higher than that of the heterostructure high-strength stainless steel without Mo addition(336 mVSCE).Therefore,the new PBF-LB high-strength stainless steel with with better mechanical behaviors and better corrosion resistance was manufactured. |
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