| This dissertation originally designed a new type of low-carbon high-silicon MnSiCrMoNi martensitic steel suitable for fixed type assembled heavy-load railway frogs.This type of steel was studied and compared with the traditional 00Ni18Co9Mo4 Ti maraging steel.Firstly,the heat treatment process of the new low-carbon high-silicon martensitic steel was optimized.Then,research methods such as metallographic,XRD,SEM,TEM,and tensile,impact,friction and wear and strain fatigue were utilized to systematically study the macroscopic microstructure,phase composition,conventional mechanical properties,fracture toughness,fatigue properties and wear properties of the two test steel.The main conclusions are as follows:22SiMnCrNiMo steel had optimal mechanical properties after being austenitised at 900 °C followed by water quenched and then tempered at 320 °C.The strength and plasticity of 22 SiMnCrNiMo steel are comparable to those of maraging steel,and its impact toughness is 14.3% higher than that of maraging steel.The comprehensive mechanical properties of 22 SiMnCrNiMo steel reach the level of maraging steel,which is suitable for the manufacture of fixed type heavy-load railway frogs.The high-density dislocations and the diffuse distribution of fine ?-carbide ensure its ultra-high strength.At the same time,the precipitation of ?-carbide reduces the C content in martensite and adjusts the lattice distortion to ensure its high toughness.The fatigue performance of 22MnSi2 CrMoNi steel is comparable to that of maraging steel,but its wear resistance is slightly lower than that of maraging steel.22MnSi2 CrMoNi steel has excellent crack propagation resistance and fatigue performance,which is due to its high strength,plasticity and toughness.It prolonged the formation period of fatigue cracks and strong plastic deformation resistance,reducing crack tip stress.The film retained austenite between the lath martensite also effectively suppresses fatigue crack growth.The difference in wear resistance between the two steels mainly depends on its wear mechanism.22MnSi2 CrMoNi steel is mainly abrasive wear,and the surface adsorbs less oxide,which is not conducive to lubrication.At the same time,the friction heating is equivalent to tempering,reducing the hardness of the surface and accelerating Wear.While maraging steel is mainly adhesive wear,the large amount of oxide adsorbed on the surface is equivalent to a solid lubricant,which simultaneously acts as a heat insulator to lower the surface temperature and thus weaken the wear.The transformation kinetics,microstructure and mechanical properties of the 18Mn3Si2 CrMo and 18Mn3Si2 CrMoNi steels are the same as the austempering temperature increase.Partially martensite is formed first in the two test steel,and a bainite transformation occurs in the subsequent austempering process to form a mixed microstructure composed of low carbon martensite,low temperature bainite and high carbon retained austenite.At lower austempering temperatures,the small size martensite is fully tempered during the austempering process,and a certain amount of high strength and toughness bainite and high toughness retained austenite make the material had optimal mechanical properties.With the increase of austempering temperature,the bainite and martensite laths are obviously coarsened,and the unstable massive austenite is greatly increased,which reduces the impact toughness of the material.When the austempering temperature was 315 °C,the unstable large block austenite transform to martensite reduces the transition fatigue life of the material.The total fatigue life of the test steel is affected by the coordination of the phases,so that the fatigue life under different total strain amplitudes is different;The main effect on the total fatigue life of the material is the content of high strength and toughness martensite and the morphology of retained austenite,The content of high strength and toughness bainite and retained austenite is the secondary reason. |
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