| Ductile iron(DI)is widely used in manufacturing industries owing to its excellent casting performance and machinability,superior wear resistance,good hardenability,high thermal conductivity,and low thermal expansion coefficient.DI has become an indispensable material in industries and is increasingly used in recent decades.The wide range of industrial applications of DI has given rise to some higher-performance requirements.For example,slip is one of the key components used in the exploration of shale gas.The core of slip is required to have high strength and toughness,and not to be ruptured when experiencing high force;further.the teeth of slip are required to have high hardness.so that these can pierce the drivepipe smoothly during operation.Further.good machining properties are required for slips to facilitate chip removal.It is difficult to strengthen the matrix of slips due to its complex performance.DI is one of the materials to prepare slips,but the traditional DI cannot meet the requirements of slips.Again,such as carbon steel wheel used for low speed rail-transit vehicle,that subjected to very high temperature because there’s a lot of heat between the wheel and the rail during the operation of rail-transit vehicle.This leads to welding between the rim and brake rotor,causing the vehicle to shut down.That promptes researcher to look for new materials to improve the wear and heat properties of track wheels.DI with excellent thermal conductivity and mechanical properties is expected to replace carbon steel in the preparation of wheels.DI QT450-10 with good comprehensive performance was chosen as the base test material in the present study.In order to prepare DI with enhanced mechanical properties,a series of experiments was conducted.First,DI QT450-10 was strengthened by heat treatment with different process parameters to prepare a DI specimen to meet different use requirements.Second,high-nitrogen DI was prepared by a new method of injecting nitrogen/ammonia gas into molten iron and nodularizing treatment.This was the preliminary exploration for the preparation of high-nitrogen DI.After comparing the above strengthening methods,the heat treatment process was adopted to strengthen heavy-section DI QT450-10 with complex shape(DI wheel used for rail-transit vehicles).The results are as follows:(1)The DI QT450-10 specimen was austenitized at different temperatures and quenched in water or a 15-17%UCONA aqueous solution.Then,the DI specimen with a better matrix after quenching at 880℃ was selected for tempering at 560,570,and 580℃.The martensite content in DI gradually increased with the quenching temperature.The martensite content in the matrix after quenching in the UCONA aqueous solution was higher than that after quenching in water.Fine acicular martensite was formed in DI after quenching in the UCONA aqueous solution at 880℃.The highest hardness value(579.0 HBW)of DI was obtained at this time.The sorbite content gradually increased with the tempering temperature.The sorbite content in the matrix of DI after quenching in the UCONA aqueous solution and tempering was greater than that after quenching in water and tempering at the same tempering temperature.The investigated DI specimen exhibited the best mechanical properties of 974.0 MPa tensile strength,681.8 MPa yield strength.and 7.5%elongation after quenching in the 15-17%UCONA aqueous solution at 880℃ and tempering at 570℃,because of the formation of granular pearlite in the matrix.(2)The DI 450-10 slip was prepared by sand casting,followed by normalizing at 920℃,and high-frequency surface quenching with a frequency of 80 kHz was then carried out.A large amount of pearlite appeared in the core of the slips after strengthening.with a tensile strength of 900.0 MPa and an elongation of 8.0-10.0%.The tooth of the slip was composed of martensite and massive cementite,and had a hardness of 56.0-60.0 HRC.The detrital chips were obtained after machined of the slip,which meet the use requirement.(3)The DI was strengthened by injecting nitrogen gas and ammonia gas respectively into molten iron for different time periods in the DI smelting process,and then the molten iron was inoculated and spheroidized to prepare DI.The nitrogen content first increased and then decreased with an increase in the nitrogen-gas-injection time;the maximum nitrogen content of 0.0096%was obtained after 20 min of nitrogen gas injection.The nitrogen content first increased,reached the maximum value of 0.0100%at 40 min of ammonia gas injection,and,thereafter,tended to remain stable.The nitrogen content in the DI prepared by ammonia gas injection was higher than that in the DI prepared by nitrogen gas injection.Nitrogen has the effect of solution hardening and grain-refinement strengthening in DI.The injection of nitrogen/ammonia gas in molten iron accelerated the formation of pearlite in the DI matrix.The best mechanical properties were obtained after 20 min of nitrogen/ammonia gas injection.and the corresponding tensile strength and elongation were 18.2%and 22.1%higher than those of the DI prepared without the nitrogen/ammonia gas injection treatment,respectively.The graphite morphology gradually deteriorated,the ferrite content decreased.and the cementite content gradually increased with the increase in the nitrogen/ammonia gas injection time,and as a result,the microstructure and mechanical properties of the investigated DI deteriorated.(4)After austenitizing at 880℃,followed by quenching in the 15-17%UCONA aqueous solution and tempering at 570℃.the pearlite content in the matrix of DI wheel used for rail-transit vehicles was 97.0%.The DI wheel exhibited uniform matrix and performance in each position,and it had a tensile strength of 971.5 MPa,yield strength of 798.0 MPa.elongation of 6.8%,transverse section hardness of 271.7 HBW,and rim surface hardness of 292.0 HBW.Some mechanical properties of the DI wheel after heat treatment were close to those of the carbon steel wheel and meet the performance requirements of chinese national standards.The DI wheel material experienced various wear mechanisms under different friction loads(50,60,and 70 N)and sliding velocities(500,750,and 1000 r/min).The wear mechanism of the DI wheel changed from adhesion to abrasion with an increase in the normal load level.Adhesion was the main wear mechanism at different sliding velocities.Oxidation occurred throughout the wear process.DI wheel can dissipate the friction heat in time during the friction process due to its excellent thermal properties,which ensured that the ductile iron material of wheel tread did not oversoften to be flaked off.The wear rate of ductile iron wheel was 28.7%-52.1%that of carbon steel wheel material under different worn test conditions.Thus,DI wheel may have a longer wear life during operation than carbon steel wheel. |
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