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Of .38 Crmoal Laser Hardening And Nickel-based Alloy Laser Cladding

Posted on:2006-04-02Degree:MasterType:Thesis
Country:ChinaCandidate:Q B WangFull Text:PDF
GTID:2191360155969735Subject:Optics
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Laser quenching is the most usually methods used in laser surface processing. It has the special merit that the distortion is very little. It has successfully used in cast iron , carbon steel, alloy steel, high-speed steel and so on. In the heat treatment of the steels, it potentially can be the effective way to solve aberrance problem when we use it instead of the traditional heat spray methods. Besides this, it can save the more expensive cobalt-based alloy, which can reduce the produce cost.This paper use the continuum wave laser beam to quench the 38CrMoAl steel surface. The hardness measurement in the cross-section of the clad layers. The test of rigidity distributing is performed.The laser cladding of nickel-based alloy doped without and with various concentration of Mo on steel 45 by powder feeding were carried out under different processing parameters. A kind of commercial nickel-based powder named Ni45+WC was employed in present work. The structural observation on the cross-section of laser coatings by an optical microscope was performed. The cracking observation on the surface of the coatings, the hardness measurement in the cross-section of the clad layers. The test of rigidity distributing and friction and abrasion performance of laser cladding is performed.Cracks are easy to generate in laser-clad layers for laser cladding is a process of sharply heating and solidifying, which is one of the main problems hindering the industrial application of laser cladding. Ni-based alloy consisted of Fe-Ni-Cr-B-Si Ni60 is a kind of cheaper material among the widedly-used self-melting alloys, and is of good performance under low stress abrasive condition and weak corrosive environment. But it is more sensitive to internal stress that is produced in the processing of laser cladding. This thesis system analyzes the special property and the mechanism of crack generating. According to the mechanism, we focused on the effect of processing parameters and the addition of several element with the function of refine the structure to cladded alloy.The laser cladding of nickel-based alloy doped without and with various concentration of Mo on steel 45 by powder feeding were carried out under different processing parameters. A kind of commercial nickel-based powder named Ni60 was employed in present work. The structural observation on the cross-section of laser coatings by an optical microscope and a scanning electron microscope (SEM) with energy-dispersive spectrobolometer (EDS) analysis was performed. The cracking observation on the surface of the coatings, the hardness measurement in the cross-section of the clad layers. The test of rigidity distributing and friction and abrasion performance of laser cladding is performed.The main results and conclusions are shown as the following:1 .the surface quenching of 38CrMoAl steel is carried out with a continuous CO2 laser beam firstly. The micro hardness is 3-4 times as high as the unquenching area that reaches 850 Hvo.3. Both the micro-hardness and the microstructure distribution within the hardening layer indict that the hardened layer consist of two sub-layer, uniform phase transformation layer, and transition layer .The corresponding microstructure of uniform phase transformation layer are plaste martensite and a little residual austenite. The microstructures of transition layer are plaste martensite and presentation ferrite. The depth and the width decrease during the scan velocity increase in the condition of the laser power and off-focus fixed.2.The experimental results indicate that both concentration of Mo and processing parameters seriously affected the quality and properties of the Ni45+WC laser coatings and a perfect coating could be produced with 3.0% Mo under 2.0kw laser power of 2mm/s scanning speed.3. The hardness and wear resistance of the Ni45+WC laser coating are improved very much and so is The cracking sensitivity. The improvement is attributed to the reaction between Mo and Si, B, C that can be analyzed from two factors. On the one hand, the amount of phases including Si and B with lower melting points is reduced. On the other hand, the formation of M0B2 and M02C in the eutectics is benefit to the fine of grain sizes and uniform distribution of microstructure.4.The experimental results indicate that both concentration of Mo and processing parameters seriously affected the quality and properties of the Ni60 laser coatings and a perfect coating could be produced with 2.5% Mo under 2.0kw laser power of 3mm/s scanning speed.5.The coating is mainly made up of pseudo eutectics. The cracking sensitivity is improved at the expense of a little decrease in hardness, but the wear resistance does not reduce. .Microstruture analysis proves that the improvement of the structure and performance of the clad layers are due to the formation of M02C at higher temperature and well distribution that fines the microstructures and reduces phases with lower melting points. The amount of phases including Si and B with lower melting points is reduced. On Mo is benefit to the fine of grain sizes and uniform distribution of microstructure.6..Comparing the results above in all, a basic conclusion about adding Mo to alloys including Fe, Ni, Cr , B and Si can be drawn. The conclusion is made up of three items. The first is that the Mo itself can't be the bad phase during the process. The second is that the element will react with Si or B to produce phases with higher melting points than the temperature of the formation of y (Fe-Ni) solution. The third is that the productions of the reaction between the element and other elements can not be bulky and brittle or be of lower melting points than the temperature of the formation of Y (Fe-Ni) solution.
Keywords/Search Tags:Laser cladding, nickel-based alloy, Crack, laser quenching, 38CrMoAl, steel
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