Study On Long Lifetime Hadfield Steel Crossing

158 groups of failed Hadfield steel crossings used in the railway system of China were analyzed statistically to study the failure mechanism of the crossing. 10 Hadfield steel crossings were undertaken tracking test to research the change processes of the morphologies of cross-section, the hardness and the size of the working crossing surface during their services processes. Thus, the failed Hadfield steel crossings were anatomized to study the hardness distribution, the microstructures and the properties of micro/nano-mechanical behavior of their subsurface. In the laboratory a high-temperature tube furnace was used to study the experimental parameters of pure Hadfield steel, the Hadfield steel liquid was refined by blowing nitrogen and compound modification approach. Based on the results, the manufacturing technology of the pure Hadfield steel crossing was studied. Besides, the pure Hadfield steel crossing was tested in the actual railway. At the same time, explosion hardening experiment was carried out with simulated point Hadfield steel crossing small samples to study the technical parameters of the explosion hardening. Based on the results of the research in laboratory, explosion hardening of the Hadfield steel crossing was undertaken. Furthermore, the Hadfield steel crossing subjected to explosion hardening was used in actual railway system.The results show three stages for the Hadfield steel crossing failure including deformation wear, friction wear, and fatigue wear. There are crack groups existing in the subsurface of the fatigue crossing in parallel with the tread base. The location of the crack groups'formation is in the range of 2~3.5 mm depth of the subsurface. The way to improve the lifetime of the Hadfield steel crossing is to improve its intrinsic quality, to increase its initial hardness and to improve its work-hardening ability. During the service process of the Hadfield steel crossing, the nanocrystalline layer is formed in the surface. Its formation mechanism is that the nanograins are formed directly through the dynamic recrystallization in the deformation structures when the stored deformation energy reaches certain degree in the repeated process of the rolling contact. The difference of the Gibbs free energy is 7.3 kJ/mol for the transformation from coarse-grain to amorphous in the surface of the Hadfield steel crossing. However, the difference of the Gibbs free energy is always positive for the transformation from coarse-grain to nanocrystalline. This means that such transformation can not be formed spontaneously and the external power must be included. The refined process is from coarse-grain to fine-grain and finally to nanocrystalline step by step. The smaller the surface temperature raises the easier formation of the nanostructure during the service process of the Hadfield steel crossing. The deformation structures do not form white etching layer (WEL) in the subsurface of the crossing. The reason is that there is no re-dissolution carbide process during the deformation process of the Hadfield steel.The micro elastic modulus of the Hadfield steel mainly depends on the vacancy density and has little to do with other defects, such as dislocation. Under the rolling contact stress for the Hadfield steel crossing, the formation mechanism for the fatigue crack is a lot of vacancy gathering and combining to form microporous. Then a large number of microporous connect to form cracks. The anti-rolling contact fatigue performance of the steel materials should be higher through alloying process treatment with larger diameter atom heavy metal.Using the special modifier and the refining technology of bottom blowing nitrogen to deal with the Hadfield steel liquid combining with V process of vacuum sealed molding technology, the high-purity-compact Hadfield steel crossing is made, which results in the content of P, S, [H] and [O] decreasing by 2.7, 5.2, 2.5 and 4 times respectively, whereas, the content of [N] increasing by 23 times. The best component(wt%)of the special modifier is CaO 25%,CaF2 25%, and Re-Mg alloy powder mixture 50%. The average particle size of the powder is 50~ 100 meshes with addition of 1wt%. During the process of the purification treatment with nitrogen blowing, the average nitrogen pressure is 0.6 MPa, the flow rate is 1 m3/t?h, and the nitrogen blowing processing time is 15 min. The pure-dense treatment improves the yield strength, tensile strength, impact toughness and other mechanical properties of the Hadfield steel crossing, which increases the lifetime of the Hadfield steel crossing more than 30%.The work surface of the Hadfield steel crossing is treated with two pre-hardening explosions by using a thickness of 3 mm of RDX explosive. Moreover, the point dislocation for the two initial-explosion is above 20 mm, which is available for the 380HB surface hardness, 40mm hardened layer depth, and the ideal explosion hardening effect of the hardness gradient smooth transition. Thus, the actual lifetime of the Hadfield steel crossing is increased by more than 30%. The surface explosion hardening mechanism of the Hadfield steel crossing is that there are the deformation twinning and dislocation strengthening in the region of larger surface deformation, whereas the dislocation strengthening mainly in the internal region of smaller deformation. The explosion hardening surface deformation mechanism of the Hadfield steel is the plastic deformation in-situ.