| Maraging hardening stainless steel with its excellent comprehensive performance is widely used in aviation,aerospace and marine areas,however,when its strength increases,its plastic toughness decreases significantly.Therefore,the Co-Cu complex maraging hardening stainless steel used in this paper is based on a typical 15-5PH stainless steel alloy composition designed by controlling the carbon at ultra-low levels of 0.03%,adding cobalt at 9.7% and keeping the micro-alloy composition unchanged,it relies on the aging strengthening of nano ?-Cu phase and other trace elements,and vias the ultra-low carbon lath martensite,revert austenite and precipitated phases to achieve the purpose of improving the toughness and strength.In this paper,the microstructures of Co-Cu complex maraging hardening stainless steel were analyzed by metallographic microscope,scanning electron microscope,transmission electron microscopy and X-ray diffractometer,and the mechanical properties were measured by tensile and hardness tests.The effects of cobalt-copper alloying and aging system(425? ~ 600 ?,1h ~ 8h)to the martensite,revert austenite,the ?-Cu phase and the strengthen-toughness mechanism was studied in order to provide theoretical guidance and basis for practical application.Through the experimental study,we get the following conclusions:The results showed that the composite structure of Co-Cu complex maraging hardening stainless steel were martensite,invert austenite and nano ?-Cu phase.Cobalt-copper alloying made the brittle ?-Fe and ? phase disappear,Co element was mainly dissolved in the matrix and due to solid solution strengthening to improve the strength.Compared with the tested steel without Co,the size of the original austenite grains and martensite slab were smaller,the content of retained austenite and the residual austenite were higher under the same aging conditions.The content of copper in the matrix was higher,so the spherical ?-Cu phase was finer.In addition,the hardness of Co-Cu complex maraging hardening stainless steel was lower,while the elongation was higher,which indicated that cobalt-copper alloying can improve the plastic toughness by fine-rain strengthening,aging strengthening and austenite toughening.The aging results showed that with the increase of aging temperature,the microhardness of test steel decreased gradually,the tensile strength and yield strength rised first and then decreased,the elongation rate increased and the yield ratio increased first and then reduced.In the matrix,the martensite slabs were broadened and the volume fraction of the austenite increased,and the martensite with the inversion austenite was in accord with the K-S relationship which was [11—0]??[111]M,(111)??(011)M.The spherical ?-Cu phase grew significantly,and rodlike ?-Cu phase was observed.With the extension of time,the hardness and strength of the test steel rised rapidly and then reduced,and they had the same change trend.The elongation rate increased first and then decreased.a large number of small size spherical ?-Cu phase in the matrix were observed,and it's maximum diameter reached 17 nm.The optimum aging treatment conditions of Co-Cu complex maraging hardening stainless steel was aging at 475? for 5h,the microhardness,tensile strength,yield strength and elongation were 426.4 HV,1255 MPa,1122 MPa and 15.5% respectively,and it had good multiphase structure.The lath martensite size was about 0.1 ~ 0.5?m,the density of internal dislocations was extremely large;the thin film revert austenite was distributed in the martensite boundaries,and it's amount was 1.4%.The lamellar structure of martensite and revert austenite can effectively improve the plastic toughness through the dislocation strengthening and austenite toughening,and it will not lose too much strength.The average diameter of ?-Cu phase was 4 nm,and the small ?-Cu phase distributed along the martensite matrix and effectively pinned dislocations to ensure the effect of aging hardening. |
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