718 H Pre-ha rdened mold steels with diffe rent Rare Earth(RE) contents were prepared to investigate the influence of RE on microstructure evolution and mechanical properties through a series of experiments and theore...718 H Pre-ha rdened mold steels with diffe rent Rare Earth(RE) contents were prepared to investigate the influence of RE on microstructure evolution and mechanical properties through a series of experiments and theoretical analysis.The results indicated that the toal oxygen(T.O) content decreased from 15 ppm to 6 ppm with 0.022 wt% RE addition,which is attributed to the active chemical properties of RE elements.For test steels,RE additions of 0.012 wt% and 0.022 wt% were significantly effective for refining inclusions by eliminating 11.5% large-sized inclusions with diameter exceeding 10 μm compared with that of ORE steel.RE addition contributed to modify irregular MnS and Al2 O3 inclusions into ellipsoidal RE-inclusions(RE2 O,RES,RE2 O2 S and REAlO3).The purification of molten steel and the modification of inclusions by RE treatment have significant effects on improvement of the fatigue crack growth tests(FCG) inhibition ability and impact energy as well as the isotropy.However,excessive addition of RE elements(0.07 wt%)seriously reduced the impact energy,ultimate tensile strength and FCG inhibition ability due to rapidly increase of the volume fraction of large-sized inclusions.In addition to the inclusions formed by RE treatment,trace solid solution RE atoms improve the stability of undercooled austenite,resulting in the transformation region of bainite and perlite of 0.07 RE steel shifting to the bottom right and prolonging the incubation period compared with that of ORE steel.展开更多
The effects of different contents of vanadium(V)(0.1,0.2,and 0.3 wt%)on the microstructure evolution and mechanical properties of 718H steel were investigated.The precipitate was characterized by means of atom probe t...The effects of different contents of vanadium(V)(0.1,0.2,and 0.3 wt%)on the microstructure evolution and mechanical properties of 718H steel were investigated.The precipitate was characterized by means of atom probe tomography(APT)and bright-field transmission electron microscopy(TEM).The increase in V content has great benefits for strength,but has an adverse effect on impact toughness.The strength increase can be attributed to the influence of V addition on dislocation density,misorientation gradient,and fine scale MC precipitates.Precipitation strengthening mainly contributes to the V-added steel by analyzing various strengthening mechanisms.However,fine scale MC precipitates can pin dislocation leading to a decrease in its mobility.A large number of immovable dislocations will increase the dislocation accumulation,internal stress and brittle cracking,resulting in a gradual decrease in impact toughness with the V addition.In addition,compared with V-free steel,the dissolved V content in austenite decreases the grain boundary energy and inhibits the diffusion of the C atoms,ultimately reducing the transformation range of pearlite(P).展开更多
A homogenization treatment(1250 °C + 12 h) was carried out to minimize the micro-segregation of bulk 718H martensitic mold steel, as verified by advanced experimental characterization and kinetic model of diffusi...A homogenization treatment(1250 °C + 12 h) was carried out to minimize the micro-segregation of bulk 718H martensitic mold steel, as verified by advanced experimental characterization and kinetic model of diffusion. However, new research found that there are still limitations in the use of the homogenization process. The result indicates that the chemical heterogeneity can be significantly reduced after homogenization. The segregation ratio of Cr and Mo elements of sample decreased by 40.9% and 35.6% of the original level, respectively. Simultaneously, the test steel with higher strength and toughness is produced by controlling micro-segregation tempered from 540 °C to 650 °C. Importantly, it reveals that the impact energy is increased by up to 27.3%. The isotropy of impact energy in different directions can reach 0.89,resulting in an overall improvement in the isotropy. Toughness mainly depends on the orientation relationship between the crack propagation direction and the band segregation region. The chain carbides formed due to the decomposition of the micro-segregated region during tempering are considered the main source of cracks. The more evenly distributed the subsequent tempered carbides after homogenization, resulting in an increase in toughness. However, an abnormal phenomenon is found in which the yield strength after homogenization is lower than that of the untreated sample tempered at 700 °C. This result can be attributed to the combined influences of precipitation strengthening and fine grain strengthening by analyzing various strengthening mechanisms. The mutually restrictive strengthening effect leads to the limitations of the homogenization process of bulk martensitic mold steel.展开更多
Solidification is an important branch of material science.By model calculation and in-situ observation in this work,distinct from traditional solidification of continuous solid growth,the evolution of discrete so-lidi...Solidification is an important branch of material science.By model calculation and in-situ observation in this work,distinct from traditional solidification of continuous solid growth,the evolution of discrete so-lidification was investigated,and a new principle for discrete solidification is established based on segre-gation evolution in a semi-solid matrix.The solidification evolution of Al-2 wt.%Cu alloy was investigated by model calculation under different initial undercooling and cooling rates,under superimposed multi-scale temperature fluctuations,and under variable temperature fluctuations.The initiation and propa-gation of segregation fluctuations in semi-solid matrix were verified within a traditional dendritic arm.The alternate solid elements evolved from semi-solid matrix act as periodic dams inhibiting serious seg-regation.Based on the new solidification principle,a multi-scale dendritic pattern was reproduced in a two-dimensional calculation.The new solidification principle reveals the essence of multi-scale mi-crostructures as multi-scale segregation patterns and highlights the feasibility of controlling multi-scale microstructures and segregations.展开更多
基金financially supported by China Postdoctoral Science Foundation (No. 2019M661153)National Natural Science Foundation of China (No. 51701225)Project to Strengthen Industrial Development at the Grass-roots Level (No. TC190A4DA/35)。
文摘718 H Pre-ha rdened mold steels with diffe rent Rare Earth(RE) contents were prepared to investigate the influence of RE on microstructure evolution and mechanical properties through a series of experiments and theoretical analysis.The results indicated that the toal oxygen(T.O) content decreased from 15 ppm to 6 ppm with 0.022 wt% RE addition,which is attributed to the active chemical properties of RE elements.For test steels,RE additions of 0.012 wt% and 0.022 wt% were significantly effective for refining inclusions by eliminating 11.5% large-sized inclusions with diameter exceeding 10 μm compared with that of ORE steel.RE addition contributed to modify irregular MnS and Al2 O3 inclusions into ellipsoidal RE-inclusions(RE2 O,RES,RE2 O2 S and REAlO3).The purification of molten steel and the modification of inclusions by RE treatment have significant effects on improvement of the fatigue crack growth tests(FCG) inhibition ability and impact energy as well as the isotropy.However,excessive addition of RE elements(0.07 wt%)seriously reduced the impact energy,ultimate tensile strength and FCG inhibition ability due to rapidly increase of the volume fraction of large-sized inclusions.In addition to the inclusions formed by RE treatment,trace solid solution RE atoms improve the stability of undercooled austenite,resulting in the transformation region of bainite and perlite of 0.07 RE steel shifting to the bottom right and prolonging the incubation period compared with that of ORE steel.
基金financially supported by the National Key Research and Development Program of China (No. 2016YFB0300401)supported by Cooperation Program of Hubei province and Chinese Academy of Sciences (The Research and Development of Key Technologies for Special Steel of Homogeneous High Performance)
文摘The effects of different contents of vanadium(V)(0.1,0.2,and 0.3 wt%)on the microstructure evolution and mechanical properties of 718H steel were investigated.The precipitate was characterized by means of atom probe tomography(APT)and bright-field transmission electron microscopy(TEM).The increase in V content has great benefits for strength,but has an adverse effect on impact toughness.The strength increase can be attributed to the influence of V addition on dislocation density,misorientation gradient,and fine scale MC precipitates.Precipitation strengthening mainly contributes to the V-added steel by analyzing various strengthening mechanisms.However,fine scale MC precipitates can pin dislocation leading to a decrease in its mobility.A large number of immovable dislocations will increase the dislocation accumulation,internal stress and brittle cracking,resulting in a gradual decrease in impact toughness with the V addition.In addition,compared with V-free steel,the dissolved V content in austenite decreases the grain boundary energy and inhibits the diffusion of the C atoms,ultimately reducing the transformation range of pearlite(P).
基金The work was financially supported by the China Postdoctoral Science Foundation(No.2019M661153)The authors also appreciate the financial support by Young Talent Project by Shenyang National Laboratory for Materials Science(No.2020000358)+2 种基金Doctoral Research Startup Fund Guidance Program Project of Liaoning Province(No.2020-BS-004)Project to Strengthen Industrial Development at the Grass-roots Level(No.TC190A4DA/35)Guangdong Basic and Applied Basic Research Foundation(No.2019A1515110886).
文摘A homogenization treatment(1250 °C + 12 h) was carried out to minimize the micro-segregation of bulk 718H martensitic mold steel, as verified by advanced experimental characterization and kinetic model of diffusion. However, new research found that there are still limitations in the use of the homogenization process. The result indicates that the chemical heterogeneity can be significantly reduced after homogenization. The segregation ratio of Cr and Mo elements of sample decreased by 40.9% and 35.6% of the original level, respectively. Simultaneously, the test steel with higher strength and toughness is produced by controlling micro-segregation tempered from 540 °C to 650 °C. Importantly, it reveals that the impact energy is increased by up to 27.3%. The isotropy of impact energy in different directions can reach 0.89,resulting in an overall improvement in the isotropy. Toughness mainly depends on the orientation relationship between the crack propagation direction and the band segregation region. The chain carbides formed due to the decomposition of the micro-segregated region during tempering are considered the main source of cracks. The more evenly distributed the subsequent tempered carbides after homogenization, resulting in an increase in toughness. However, an abnormal phenomenon is found in which the yield strength after homogenization is lower than that of the untreated sample tempered at 700 °C. This result can be attributed to the combined influences of precipitation strengthening and fine grain strengthening by analyzing various strengthening mechanisms. The mutually restrictive strengthening effect leads to the limitations of the homogenization process of bulk martensitic mold steel.
基金supported by the General Program of the National Natural Science Foundation of China(Grant No.52271041)the Key Program of the National Natural Science Foundation of China(Grant No.52031013).
文摘Solidification is an important branch of material science.By model calculation and in-situ observation in this work,distinct from traditional solidification of continuous solid growth,the evolution of discrete so-lidification was investigated,and a new principle for discrete solidification is established based on segre-gation evolution in a semi-solid matrix.The solidification evolution of Al-2 wt.%Cu alloy was investigated by model calculation under different initial undercooling and cooling rates,under superimposed multi-scale temperature fluctuations,and under variable temperature fluctuations.The initiation and propa-gation of segregation fluctuations in semi-solid matrix were verified within a traditional dendritic arm.The alternate solid elements evolved from semi-solid matrix act as periodic dams inhibiting serious seg-regation.Based on the new solidification principle,a multi-scale dendritic pattern was reproduced in a two-dimensional calculation.The new solidification principle reveals the essence of multi-scale mi-crostructures as multi-scale segregation patterns and highlights the feasibility of controlling multi-scale microstructures and segregations.
