Gradient distribution of residual austenite in 17-4PH steel constructed by selective laser melting
Martensite precipitation hardening(17-4PH)steel is widely used in compressor impeller,hydrofoil and other over-flow parts with complex structures.Selective laser melting(SLM)is a new near-net-shape forming tech-nology,and its point-by-point and layer-by-layer scanning stacking manufacturing method is helpful to optimize the structure of parts and realize the integrated forming of parts with complex shapes through topological design.The gradient distribution of retained austenite in 17-4PH steel constructed by selective laser melting was studied by alter-nately printing 17-4PH steel with two scanning strategies of"cross bidirectional"and"bidirectional".The results show that the gradient distribution of retained austenite content in 17-4PH steel can be realized by adjusting the scan-ning strategy and changing the heat input and dissipation mode.Cross-bidirectional scanning strategy can effectively refine martensite grains,and the size of martensite grains is related to construction height,while the size of retained austenite grains has little to do with scanning mode and construction height.The fine martensite formed by cross bidi-rectional scanning will produce additional stress on the surrounding austenite,which will lead to the stability of residual austenite and make the bottom of the sample more austenite.The cooling speed of bidirectional scanning is fast,the undercooling degree is high,the driving force of martensite transformation is increased,and the elimination phase is stable,resulting in more martensite at the top and less retained austenite.With the increase of stacking lay-ers,the cooling rate of microstructure changes from slow to fast,the transformation degree of martensite increases and the content of retained austenite decreases.The control of austenite gradient structure in 17-4PH steel is real-ized,and the preparation of this gradient structure provides a new way for optimizing material properties and realizing multifunctional application.
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