Effect Of Nickel Content On The Microstructure And Properties Of Medium Silicon Molybdenum Heat Resistant Ductile Iron

In recent years,with the continuous development of the transportation,mining machinery and power industries,the structure and performance of large castings,such as automobile engine castings,medium-pressure outer cylinders of steam turbines and nuclear spent fuel containers,have also put forward higher requirements.Although medium silicon molybdenum ductile iron has good heat resistance,due to its high silicon content and carbon equivalent,serious spheroidization inoculation recession occurs when the wall thickness is large,resulting in poor comprehensive mechanical properties at room temperature.In this paper,medium silicon molybdenum ductile iron was taken as the research object.By optimizing the pretreatment process,adding Ni alloy elements and heat treatment,the effects of Ni on the microstructure,mechanical properties and high-temperature heat resistance of medium silicon molybdenum ductile iron were compared and analyzed by metallographic microscopy,scanning electron microscopy,electronic universal tensile testing and X-ray diffraction.In this paper,Y-shaped test blocks of medium silicon molybdenum ductile iron with wall thicknesses of 75 mm and 25 mm were cast by different pretreatment processes.The test results show that after pretreatment with Si C before furnacing,the spheroidization rate of the sample with a wall thickness of 25 mm reaches 90%,and the graphite grade is grade 6.The spheroidization rate of the edge position of the sample with 75 mm wall thickness reaches 95%,and the graphite grade is 7.The spheroidization rate at the center of the specimen with a wall thickness of 75 mm also reached 80%.Graphite grade is 5.There is no significant difference in the pearlite content of the samples.The Si C pretreatment process has little effect on the matrix structure of medium silicon molybdenum ductile iron.After Ni alloying of medium silicon molybdenum ductile iron,the results show that with the increase of Ni content,the graphite roundness of the sample increases slightly,the pearlite content in the microstructure increases gradually,the tensile strength and hardness increase,and the elongation decreases as a whole.When Ni is not added,the spheroidization rate of 75 mm wall thickness sample is 80%,the graphite grade is 5,the pearlite content is 11%,the tensile strength and hardness are525 MPa,203 HBW,and the elongation is 18.5%.The spheroidization rate of 25 mm wall thickness sample is 80%,the graphite grade is 6,the pearlite content is 9%,the tensile strength and hardness are 539 MPa,207 HBW,and the elongation is 16.2%.When the Ni content was 1.9%,the spheroidization rate of the specimen with 75 mm wall thickness was 80%,the graphite grade was 5,the pearlite content reached 33%,the tensile strength and hardness reached 618 MPa and 218 HBW,and the elongation reached 12.0%.The spheroidization rate of the specimen with 25 mm wall thickness was 90%,the graphite grade was 6,the pearlite content reached 29%,the tensile strength and hardness reached 640 MPa and 236 HBW,and the elongation reached11.4%.After low-temperature graphitization annealing at 760°C,most of the pearlite in the matrix decomposed,but the sample with Ni contents of 1.1%-1.9%still contained approximately 10%pearlite.At the same time,a large number of Mo-containing carbides dominated by M₆C are dispersed in the matrix and the crystal boundary.Compared with the as-cast state,an obvious strengthening effect appeared after annealing,and the tensile strength of medium silicon molybdenum ductile iron was significantly improved.When the Ni content was 0.7%,the strengthening effect was the most obvious,and the tensile strength of the specimen with a wall thickness of 75 mm was 783 MPa,which was increased by approximately 47%,while the elongation did not significantly reach 13.6%.The tensile strength of the specimen with a 25 mm wall thickness is 831 MPa.When the cyclic oxidation temperature is 700°C,the oxidation products of medium silicon molybdenum ductile iron are mainly Si O₂,Fe O,Fe₃O₄ and Fe₂O₃,and Fe₂Si O₄ with short rod-like parallel to the interface distribution in Fe O.Some clusters of oxide whiskers appear on the surface of the oxide film.At 800°C,the whisker size on the oxide film surface increased and covered the oxide film surface.At 900°C,the surface oxide film showed a short bulge and a large number of holes,while the oxide film exhibited a cracking phenomenon.At 800°C and 900°C,a small amount ofNi Fe₂O₄ appeared in the oxide film.With increasing Ni content and oxidation temperature,the average thickness of the oxide film increases.With increasing oxidation temperature,the oxidation kinetics curve of the sample changed from a parabolic law to a linear law,and the oxidation resistance decreased gradually.When the oxidation temperature is 900°C,the graphite below the oxide layer decreases,irregular and less.In the thermal fatigue test,with the increase in Ni content and thermal fatigue temperature,the number of thermal cycles required for crack initiation decreases gradually.At 700°C,the crack initiation of the sample without Ni begins at approximately 15 thermal cycles,and the crack initiation of the sample with 1.9%Ni starts at approximately 5 thermal cycles.At 900°C,microcracks were observed in the samples after five thermal cycles.During the thermal fatigue process,the crack initiation and propagation rates of the specimens after heat treatment were reduced,and the thermal fatigue performance was improved.