| The21-6-9cold drawn pipe, which has good combination of strength andtoughness, outstanding formation property and corrosion resistance, has the potential forapplication in aerospace. However, multiple cold drawn processes make it easy to creatework-hardening and residual stress. In order to complete the following cold drawingforming successfully, proper heat treatment is required to improve its comprehensivemechanical properties.The main objective of this work focused on the investigation of effect of solidsolution and aging on microstructure of domestic and imported21-6-9cold drawn pipes.In addition, the relationship between microstructure and mechanical properties of colddrawn pipes after annealed was comprehensively investigated. Analytical facilities, suchas thermodynamic software, OM and XRD were used to detect the influence of heattreatment on grain size, twins, and secondary phases.The influence of different solution and aging temperature on microstructureshowed that: both the domestic and imported21-6-9cold drawn pipes were composedof austenite after solid solution at1100℃for60min, twin content increases comparedwith the untreated specimens. The secondary phase, which existed in the untreateddomestic specimens disappeared after solid solution at1100℃. For the domestic pipes,twin content increased after aging at650℃compared with that after solid solution, butlower twin content than that of aging at650℃was found after aging at850℃. For theimported pipes, twin content decreased after aging at650℃compared with that aftersolid solution, yet the twin content after aging at850℃was very close to that of agingat650℃. Besides, precipitation was found in the two kinds of specimens after aging at650℃and the content of precipitation increased with the increase of aging temperature.The effect of different annealing temperature on microstructure and mechanicalproperties showed that: Twin content of domestic pipes increased with the increase ofannealing temperature as annealed above550℃and for the imported pipes, twincontent increased with the increase of annealing temperature as annealed between550℃and650℃, and then decreased above650℃. When annealed at650℃, muchmore CrN particles among grain boundaries of domestic21-6-9cold drawn pipe thanthat of the untreated specimens were found and disappeared over850℃. For theimported pipes M23C6particles participated at650℃and lasted until900℃. However, both CrN particles and M23C6particles participated intensely at800℃. From studyingthe anneal temperature on mechanical properties of the cold drawn pipes, it was foundthat the tensile strength reached1062MPa at500℃and yield strength reached942MPaat600℃for the domestic one. At500℃, the tensile strength reached1041MPa andyield strength reached924MPa for the imported one.The influence of holding time on microstructure at700℃showed that, twincontent of domestic pipe reached slump after holding for60min, and when holding for60~360min, twins gradually increased with the time increasing. For the importedspecimens, the twins content increased while holding for60min compared with that ofholding for40min, after holding for120min lower twin content than that of holding for60min was seen and while holding for60~120min, twins gradually decreased with thetime increasing. In addition, after holding for10min secondary phase was found amonggrains in both domestic and imported pipes. |
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