Technology and performance of co2n2 protective sur

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Alloy powder thermal spraying (co2+n2) protective surfacing process and performance

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hebei University of science and technology Yang yuanxiuabstract the paper introduced the co2+n2 arc surfacing procedure after the thermal spraying of alloy powder was performed on the preliminary surfacing region And it also discussed the effect of alloy powder content and nitride of the spraying layer on hardness, microstructure and wear-resistance of the surfacing layer.

key words: thermal spraying, co2+n2 arc surfacing, procedure, property0 preface

surfacing is a simple and cheap method to obtain bimetal materials and improve the wear resistance, corrosion resistance and other properties of workpiece surface. CO2 welding is the most widely used welding method in production with high efficiency, low consumption and low cost. However, the current solid CO2 alloy welding wire in China has few varieties, specifications and high prices, which limits the expansion of the application of CO2 surfacing technology; Although the flame spray melting equipment is simple, the alloy powder has high price, low efficiency, large heating area of workpiece and easy deformation. The plasma spray melting equipment is complex and the investment is large. Nitrogen is a kind of gas with wide sources and low prices that will also be vigorously promoted by virtue of the world cup effect, and nitrogen compounds have a strong reinforcing effect. Whether the flame spraying technology and CO2 surfacing can be combined to give full play to the strengthening effect of nitride, the respective advantages of the two methods and the potential of the existing CO2 welding equipment in the factory, and the alloy composition can be adjusted at will according to the working requirements is the purpose of this research

1 test conditions and methods

1.1 determination of surfacing metal alloy system

in this test, fe-cr-mn-si-c-b-ti-w system multi-element microalloying is used. The purpose is to strengthen the microstructure matrix, disperse and precipitate a large number of hard points with high hardness, but also reduce the crack resistance of surfacing metal and improve its wear resistance. See Table 1 and table 2 for the typical formula of the two test pieces and the chemical composition and hardness of the surfacing metal

1.2 spraying of alloy powder

weigh various alloy powders according to the predetermined alloy formula and mix them evenly. Use qh-2/h dual-purpose torch to spray the alloy powder on the material Q235 and size 100 mm × 50 mm × On the surface of 16mm test piece, the coating thickness shall be controlled within 0.4~0.5 mm to ensure the alloy content of surfacing metal. The particle size of the alloy powder is controlled at 100~150 mesh, and the coarseness of the alloy powder will affect the spraying of the alloy powder; The burning loss of alloy powder increases during surfacing. The spraying process parameters are oxygen pressure of 0.5~0.8 MPa, acetylene pressure of 0.05~0.07 MPa, and the distance between the nozzle and the workpiece surface is 150~200 mm. Before spraying, the surface of the specimen shall be derusted and degreased

1.3 (co2+n2) shielded arc surfacing process (co2+n2) shielded arc surfacing is performed after the specimen surface is sprayed with alloy powder. The surfacing method and process parameters are similar to those of ordinary solid CO2 surfacing. This test adopts φ 1.2 mm H08Mn2SiA welding wire, CO2 gas flow of 11 l/min, nitrogen flow of 1.5 l/min, CO2 and N2 enter the gas mixing chamber through their respective pressure reducing valves and flow meters and are introduced into the air inlet pipe of the welding gun after uniform mixing. The arc voltage is 19 V and the welding current is 135 a. The following points shall be paid attention to during surfacing: ① during welding, the arc shall pass through the center of the powder layer as far as possible to ensure the full fusion of the alloy powder; ② When the weld bead is wide, the welding gun needs to swing laterally to ensure that the alloy powder is fully fused to obtain a wide surfacing layer; ③ The surfacing speed has a great influence on the alloy content of surfacing layer, which can be determined by test. If automatic welding is adopted, it will be better to master; ④ It is better to add arc striking plate and arc extinguishing plate when surfacing, because the defects such as poor fusion of alloy powder, porosity and slag inclusion are easy to occur at the arc striking and arc extinguishing places

2 test results and analysis

2.1 effect of nitrogen content on surfacing process performance

it is found that when the nitrogen content in the mixed gas is high, the arc is unstable, the spatter increases, the weld formation is poor, and a large number of nitrogen pores are often generated in the weld. Of course, if the nitrogen content is too low, the effect of nitrogen cannot be shown, that is, it is not easy to form nitride hard phase, and the strengthening effect is greatly reduced. Generally, the nitrogen content accounting for 10%~15% is appropriate

2.2 effect of alloy elements on hardness of surfacing metal

c, Cr and W not only have strong solid solution strengthening effect and improve the hardenability of materials, but also these new composite materials are the main forming elements of hard phases such as carbides and carbon boron composite compounds with high hardness and excellent wear resistance because of their light weight. Therefore, with the increase of graphite content in the powder layer, the hardness of surfacing metal increases significantly (Fig. 1a), However, if the content of C is too high, the brittleness of the metal increases and the tendency to produce cracks increases, so the content of C is generally controlled below 0.65%. CR can also form CrN hard phase with N, which makes the eutectic point of Fe-B phase diagram shift to the left, resulting in the decrease of iron boron eutectic with low hardness, while the number of composite borides with high hardness increases. Therefore, the hardness of surfacing metal increases with the increase of ferrochromium content in the powder layer (Fig. 1b)

b can greatly improve the hardenability of the metal and form a variety of high hardness boride hard phases with Fe, C, Cr and other elements. Therefore, with the increase of boron and iron content in the powder layer, the hardness of the surfacing metal is greatly improved (Fig. 1c). However, the high boron content leads to the formation of brittle phase at the grain boundary, which increases the brittleness and crack tendency of the metal (Fig. 2)

ti has great affinity with oxygen, and the transition coefficient of Ti is very small under general conditions. However, under the test conditions, the oxidation probability of Ti is greatly reduced because the molten alloy does not pass through the arc space, and there are other strong deoxidizing elements (C, Mn, Si, etc.) in the alloy powder layer. The addition of N2 reduces the oxidation of the welding atmosphere, which is conducive to the transition of Ti. Ti is a very strong carbide and nitride forming element, and plays a strong role in grain refinement. Therefore, the hardness of surfacing metal increases with the increase of titanium and iron content in the powder layer (Fig. 1D)

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