| High chrome cast steels are used successfully for the production of hot rollers. These steels possess a high mechanical strength at the working temperature and a high wear resistance. However, the primary carbides along grain boundaries constitute a favorable propagation path for the mechanical and thermal fatigue cracks. Moreover, during rolling the poor cooperation of eutectic carbides and matrix cause oxidation wear, deteriorating the high-temperature wear resistance. In this paper laser surface melting (LSM) were performed on the surface of high chrome cast steels, and the microstructure of the melted layer was analysed as well as the influence of overlapping parameter on the structure and property. High-temperature oxidation, wear resistance and the internal relationship were studied.High chrome cast steel is composed of tempered martensite and primary eutectic M7C3 carbides. The surface layer obtained after LSM with a power of 2.7kW and a traverse speed of 300mm/min was relatively smooth, morphological homogenous without presence of porosity. The results after electrochemical test show that for high chrome cast steel, corrosion attack at the phase boundaries between the tempering martensite matrix and the carbides was observed. Moreover, LSM leads to the enhancement in the corrosion resistance due to the combined effects of dissolution and refinement of large carbides and the increase of the alloying elements in the ultrafine solid solution of austenite. Using the overlapped ratio of 33.3% gives a more uniform hardened-depth, and uniform corrosion was observed in the laser-melted steel.After LSM the structure of high chrome cast steel changed dramatically, the eutectic carbides were completely dissoluted, and the microstructure in the melted layer is obviously refined. The bottom and the central region show cellular/dendritic structures while the upper region consists of equiaxed dendrites.The microstructure is austenite and the interdendritic carbides of type M23C6, and the hardness of the austenite can reach 473.1HV0.2 due to the combined effects of solid solution, dislocations and ultrafine grains. The HAZ consists of cryptyo-crytal martensite, retained austenite and dispersed carbides, while the microhardness of the HAZ is higher than that of the substrate, and reaches 759HV0.2. The laser overlapped zone is divided into the overlapped melted zone and the overlapped HAZ, and the overlapped melted zone is composed of austenitic dendrites while the structure at the overlapped HAZ is same as that of HAZ by the single track.Heat resource model, which is agreement with the real laser spot, was built by SYSWELD software, and the temperature and stress fields were studied by numerical simulation. The results show that during LSM the heating rate and the cooling rate at the center of the laser spot can reach 3.2×104℃/s and 1.5×104℃/s, respectively. After the single-track LSM, the laser melted layer bears tensile stress, and at the HAZ which is 2.5mm away from the center of the beam spot, the Mises stress reaches the maximum value of 713MPa, resulting in the high cracking susceptibiltiy. The structure in the laser melted layer possesses high strength and ductility without cracks. However, the cracks initiate along the interface between the carbides and the matrix because of the tensile stress at the HAZ as well as the low ductility of the structures. Moreover, overlapping can effectively decrease the residual stress and the cracking sensitivity. Testing proved that preheating at 150℃for 1h can avoid cracking at the HAZ.The hardness of the laser melted layer changes little and keeps at about 430HV0.2 when tempering below 400℃, which is slightly lower than that of the untemperte laser melted layer. The secondary hardening phenomenon appears, beginning at 450℃, at this time a large amount of the dislocation and slip bands substructures still exist in the austenite tempered at 450℃, and the precipitation of fine M23C6 carbides and the formation of martensite contribute to the slight increase in the hardness. When tempering at 560℃, the secondary hardening resulted simultaneously from the martensite phase transformation and the precipitation of secondary carbides as well as the dislocation strengthening within a refined microstructure. The matrix transformed to ferrite completely after tempering at 650℃, and the decrease of hardness could be caused by the coagulation of carbides.High-temperature oxidation at 650℃of high chrome cast steel before and after LSM is slow, following an oxidation kinetic with a logarithmic trend, while the oxidation increases at 800℃with a parabolic trend. The oxide scale of high chrome cast steel is composed of Fe and Fe2O3, oxidation nucleates at the carbide-matrix interface, even cracks at 800℃giving rise to an uneven oxide scale. For the laser melted steel, the oxide scales consist of Fe, Fe2O3 and (Fe0.6Cr0.4)2O3 after LSM. The oxide scale covers the laser melted layer evenly due to the refined microstructure and grows as a result of the electrical transport of electrons or ions across the oxide film. Compared with the as-received steel, the oxide scales of the laser melted specimens is thicker.The high temperature wear resistance of high chrome cast steel at 560℃and 650℃has been improver obviously by LSM and the weight gains at 800℃, but the gain in weight is lower than that of the as-received high chrome cast steel. At 560℃wear mechanism of high chrome cast steel is abrasive wear, while grooves and carbide particles appear on the worn surface. Adhesive wear dominates at 650℃with some scoring marks on the worn surface, while at 800℃sever adhesion occurs accompanied by some deep grooves. The wear resistance of the laser melted steel is improved at 560℃and 650℃resuting from the high strength and ductility and the wear mechanism is mainly slight abrasive wear which is adhesive wear at 650℃. The improved adhesion resistance and 800℃is attributed to the formation of the oxide protective layer formed on the worn surface due to even oxidation by way of tribochemical reactions.
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