Bo Lv

Transformation of onion-like carbon from nanodiamond by annealing

Onion-like carbon (OLC) was synthesized by annealing nanodiamond in low vacuum (1 Pa) at the temperatures from 500 to 11400°C. The high-resolution transmission electron microscope images, X-ray diffraction patterns and Raman spectra showed that, when the annealing temperatures were lower than 900°C, there was no OLC fabricated. The amorphous carbon and the nanodiamond coexisted. The graphitization started from the surfaces of the nanodiamond particles. When the annealing temperatures were higher than 900°C, the OLC was fabricated. At 900°C, OLC began appearing and the size of the OLC particles was smaller than 5 nm. At the annealing temperature of 1400°C all the nanodiamond particles were transformed into OLC. The OLC particles exhibited similarity to the original nanodiamond particles in shape. Based on these results, a mechanism for the OLC synthesis by the method of annealing in vacuum was provided.

HRTEM and Raman characterisation of the onion-like carbon synthesised by annealing detonation nanodiamond at lower temperature and vacuum

Onion-like carbon (OLC) was synthesised by annealing detonation nanodiamond for 1.5 h at temperatures from 500 to 1400°C and at a vacuum of 1 Pa. The results showed that the nanodiamond was transformed into the amorphous carbon (a-C) at first and then the a-C was transformed into the OLC gradually with the increase in annealing temperature. Moreover, at the annealing temperature of 600°C, the nanodiamond started transforming into a-C from the edge of the nanodiamond particle (1 1 1) crystal plane. At the annealing temperature of 750°C, the nanodiamond was transformed into the a-C completely. At the annealing temperature of 850°C, the a-C began transforming into the OLC at the edge area. At the annealing temperature of 1000°C, the OLC particle with a size smaller than 5 nm was synthesised. However, in the centre of the OLC particle, untransformed a-C coexisted. At the annealing temperature of 1100°C, the microstructure of the OLC particle with a size smaller than 5 nm became optimised. At the annealing temperature of 1200°C, the OLC particle with a size larger than 5 nm was fabricated. There was also untransformed a-C coexisting in the centre of the OLC particle. At the annealing temperature of 1350°C, all the a-C was transformed into the OLC. The average size of the OLC was approximately 5 nm, which was the same as that of the nanodiamond. The layers of the OLC were varied from several to 12.

Explosion hardening of Hadfield steel crossing

Abstract In the present paper the effects of explosion hardening on the microstructure and the mechanical properties as well as the lifetime of Hadfield steel (high manganese steel) crossing have been studied. The optimum explosion hardening technology of the high manganese steel crossing was proposed. That is twice explosion by using cyclonite explosive in thickness of 3 mm. The new technology emphasises the formation of a 25 mm deep hardened layer with surface hardness of 370 HB. Upon the explosion impact, the deformation mechanism of the material is found to follow in situ plastic deformation. The explosion hardening mechanisms of the high manganese steel crossing are dislocation and nanoscale deformation twin hardenings in the surface layer which is subjected to large deformation, and dislocation hardening in the subsurface layer which is subjected to small deformation. The explosion hardening enhances the mechanical properties of the material, included the deformation resistance, wear resistance and fatigue resistance, therefore, the lifetime of the high manganese steel crossing can be increased by ∼35% through the explosion hardening treatment.

Rolling Contact Fatigue Performances of Carburized and High-C Nanostructured Bainitic Steels

In the present work, the nanostructured bainitic microstructures were obtained at the surfaces of a carburized steel and a high-C steel. The rolling contact fatigue (RCF) performances of the two alloy steels with the same volume fraction of undissolved carbide were studied under lubrication. Results show that the RCF life of the carburized nanostructured bainitic steel is superior to that of the high-C nanostructured bainitic steel in spite of the chemical composition, phase constituent, plate thickness of bainitic ferrite, hardness, and residual compressive stress value of the contact surfaces of the two steels under roughly similar conditions. The excellent RCF performance of the carburized nanostructured bainitic steel is mainly attributed to the following reasons: finer carbide dispersion distribution in the top surface, the higher residual compressive stress values in the carburized layer, the deeper residual compressive stress layer, the higher work hardening ability, the larger amount of retained austenite transforming into martensite at the surface and the more stable untransformed retained austenite left in the top surface of the steel.

Wear resistance of bainite steels that contain aluminium

Four steels containing Si + Al = 2 wt-% were investigated in this study. Three mixed microstructures, namely, bainite + martensite + retained austenite (MB+Ar), martensite + retained austenite (M+Ar) and low-temperature bainite + retained austenite (LB+Ar) were obtained. Wear resistance was determined by dry sliding friction testing under different conditions. Results showed that Al promoted the formation of oxide films during wear process, resulting in the improvement of wear resistance, especially under a high-load condition. Under the same hardness, the wear resistance of MB+Ar was improved by 65 and 100% compared with that of LB+Ar and M+Ar microstructures, respectively. At high temperature, the worn surface was covered by oxide films and the samples exhibited abrasive and adhesive wear mechanisms. However, at low temperature, the oxide films were difficult to generate on the worn surface; the adhesive wear mode was dominant and resulted in severe wear.

Forging of High-Manganese Steel Crossing

The thermoplastic characteristics of high-manganese steels with different phosphorus and sulfur contents were investigated. The zero-ductility temperature and plasticity-temperature range of the high-manganese steels were reduced significantly with increase in phosphorus or sulfur content. The relationship between the initial forging temperature of the high-manganese steel and the content of phosphorus and sulfur can be expressed as Tift = 1200 – 1530wS – 1650wP. The final forging temperature is 900°C, and the heating rate during the forging of the high-manganese steel crossing must be slow. The lifetime of the forged high-manganese steel crossing was almost doubled compared with conventional cast high-manganese steel crossing.

Colored Metallography Study of Bainite Steel Used for High Speed Railway Crossing

Using the technique of colored metallography, the phase compositions of two medium-/low-carbon low-alloy carbide-free bainite steel used for railway crossings are obtained. The etching of the samples was carried out using the Lepera reagent and a mixed reagent of hyposulphite and picric acid respectively. The proportion of the bainite phase was calculated using an image processing system with an optical microscope and the residual austenite content was measured using XRD method. The martensite phase content can then be determined. It is found that the reagents can effectively and quantifiably analyze the phase composition of bainite steel and produce approximately the same results.

Failure Mechanism of Hadfield Steel Crossing

In this paper failed Hadfield (high manganese austenite) steel crossing in railway system was acted as the research object, the microstructure changes in the worn surface layer of the crossing were investigated by using optical microscopy, X-ray diffraction (XRD) and transmission electron microscope (TEM). The micro-properties of the worn surface and subsurface layers of the crossing were tested by means of nanoindenter equipment. In the mean time, the worn failure mechanism of the crossing was discussed. The results indicated that the microstructure in the worn surface of the Hadfield steel crossing changed to nanocrystalline. The nanocrystalline layer improved the wear resistance of the Hadfield steel. The failure of the Hadfield steel crossing included three stages, that was plastic deformation and wear in the first served stage before the passing trough loads of 1∼2×107 tons, wear in the medial stage among the passing trough loads from 1×107 to 9×107 tons, and fatigue spalling in the final served stage after the passing trough loads of 8∼9×107 tons. Accordingly, some new methods were put forward, which leaded to increase the lifetime of the crossing and enhance the safety of the railway system.