Analysis of high temperature wear resistance test

Analysis of high temperature wear resistance test of spray welding layer

Abstract: under the condition of determining the spray welding process parameters, high temperature wear tests were carried out on three oxygen acetylene flame powder spray welding layers with different composition ratios, and compared with salt bath quenched parts. Through the analysis of the test results and the metallographic microstructure of the specimen, the best material composition of the high temperature wear resistant spray welding layer is obtained. The research results can provide a basis for the correct application of these strengthening processes

key words: thermal spraying; High temperature cladding; Adhesive wear; Test

Introduction

the wear resistance or wear reduction performance of spray welding layer depends not only on the inherent characteristics of spray welding materials, but also on the working parameters of the whole friction system. Under certain working conditions, the composition, microstructure and properties of spray welding materials have an important impact on the friction and wear behavior. The effect of temperature on wear is indirect. For example, when the temperature increases, the hardness decreases, the miscibility increases, and the wear intensifies; As the temperature rises, the oxidation rate intensifies, which also affects the wear performance. The research on the surface of materials to improve their high-temperature wear resistance generally starts from two aspects: one is to have good mechanical properties, and the other is to try to form a friction surface with non-metallic properties. For some rigid structural parts in construction machinery and agricultural machinery, various surface technologies, such as thermal spraying technology, can generally be used to form a protective layer on the material surface that can inhibit the adhesion, fusion and metal transfer of the friction pair in the friction process, so as to improve the wear resistance. At the same time, the red hardness of these surface layers greatly improves the high-temperature wear resistance [1]

this test is based on the previous research on the adhesive wear, thermal life and thermal shock resistance of the spray welding layer [2]. The high-temperature wear resistance of the spray welding layer is further analyzed and evaluated, providing a reference for practical application

1 experimental design

oxygen acetylene flame powder spray welding process is adopted, and self fusible alloy powder, composite powder and a small amount of ceramic powder are selected to form by mechanical mixing

1.1 spray welding process

this test adopts the "two-step" spray welding process. The spray gun model is sph2/h. See Table 1 for the spray welding process parameters. Table 1 spray welding process parameters

oxygen

pressure

P1/MPA oxygen

flow

Q1/l.h-1 acetylene

pressure

P2/MPA acetylene

flow

Q2/l.h-1

powder feeding

Q/kg. H-1 powder

melting point

t/℃ powder spraying

distance

d/mm workpiece

speed

n/r.min-1 preheating

temperature

T1/℃ working

temperature

T2/℃ coating

thickness

δ/mm0.4310～

3600.05～

0.07430～

5601.～

~

270700 ~

8000.4 ~

0.5

1.2 design of spray welding layer

the so-called self fusible alloy refers to a kind of alloy with low melting point, which can deoxidize and slag by itself in the melting process, and can "wet" the surface of the substrate, showing metallurgical bonding. Most self fluxing alloys are made by adding an appropriate amount of boron and silicon to Ni based, Co based and Fe based alloys. Among them, B, Si, Ni, Fe and CO can form eutectic alloys with low melting points and significantly reduce their melting points, which are between 950 and 1200 ℃; When the alloy is in the molten state, the oxides generated by B and Si compound into silicate glass slag with low melting point. This slag has low density, low viscosity and good fluidity. It is easy to float out of the molten pool and cover the surface to protect the welding layer alloy from oxidation [3]

g112 powder is NiCrBSi self fusible alloy powder. During spray welding, Cr can dissolve in Ni to form solid solution and increase the oxidation resistance of spray welding layer. CR can also form hard intermetallic compounds such as cr-2b, CRB and cr-7c-3 with B and C, so as to improve the hardness and wear resistance of the coating. This kind of alloy still maintains high hardness (hrc48 ~ 53) at 500 ~ 600 ℃ and has oxidation resistance of 760 ℃ [4]

various hard wear-resistant composite powders can be made by using various carbide hard particles as core materials, metals or alloys as coating materials, and different components and proportions. The spray welding layer of this material is hard spots with carbide dispersed in the matrix of strong and tough metal or alloy coating. The wear-resistant hard phase WC has the characteristics of high melting point, high hardness and chemical stability. At the same time, the solid solution of Ni based alloy after G112 powder spray welding has excellent wetting ability to WC particles, so that the hard spots in the coating are evenly distributed and form a strong binding force, so that WC particles will not collapse from the coating during wear. Considering that too much WC content will affect the processability and expansion coefficient of spray welding, the content of f5010 is controlled at about 10%

white alumina (Al2O3) is a metal oxide ceramic powder. The spray welding layer formed by it has good wear resistance, and its thermal stability is also quite good at high temperature. Their melting points are generally high, γ Al2O3 becomes α The Al2O3 test piece shall be heated to 750 ~ 1200 ℃. According to the practice of the author, a better ceramic coating can also be obtained by using oxygen acetylene flame spray welding process, but its content must be controlled within 1%. These trace amounts of Al2O3 can make the boride, carbide and other hard points in the spray welding layer more evenly distributed, and make the dispersion strengthening and plastic deformation strengthening of the spray welding layer reach a better state, so that the spray welding layer has better wear resistance and high temperature resistance

see Table 2 for the proportion scheme of spray welding powder in this test, and compare it with non spray quenched specimens

Table 2 spray welding powder proportioning scheme%

specimen group number G112 powder f5010 powder Al2O3 powder 187.5012.50289.7510.00.25394.255.00.7544 is salt bath quenched specimen

2 high temperature wear test

2.1 specimen preparation

the test is carried out on mg2000 high speed high temperature wear testing machine, which is a spindle pin disc wear testing machine (Figure 1)

Figure 1 Schematic diagram of high temperature wear test

1 Fixture 2 Test piece 3 Metal disc

prepare two specimens for each group. The test piece is made of 45 steel, and the size is  φ one ×δ 1＝8mm × 12mm。 Spray weld or quench each group of test pieces according to the spray welding parameters. The metal disc with grinding parts is also numbered for opposite grinding. Its material is 3Cr2W8V, and its size is  φ two ×δ 2＝70mm × 10mm, the heat treatment process is salt bath quenching and low temperature tempering, and the surface hardness is hrc47 ~ 49.5. 2.2 test piece parameters

relative speed of the test piece n=600r/min, test temperature t=400 ℃, test load and test time distribution are t=1h when p=30n, t=3h when p=50n

2.3 test data processing

2.3.1 weight loss method is used to weigh and calculate the wear amount of the test piece with TG328A electro-optic analysis balance. Before the test, stick metallographic sandpaper on the metal disc to grind the test piece to ensure good contact. Then remove the test piece, wash it, weigh it and reassemble it in situ for high-temperature wear test. See Table 3 for test data. Table 3 high temperature wear test data

test piece number

mass before test m0/g4.84024.84024.83984.84044.83964.84004.84024.8394 mass after test m1/g4.81924.81954.82264. In addition, it can also be equipped with ordinary computers 82344.82144.82164.81704.8164 wear amount w/mg21.020.717.217.018.218.423.223.0 surface hardness hrc465253 Note: the HRC data in the table is the average value of 3 points

2.3.2 according to the formula of relative wear resistance ε= W standard/w test, taking 45 steel quenching as the standard test piece, calculate and draw the relative wear resistance ε The change diagram is shown in Figure 2

Figure 2 Schematic diagram of relative wear resistance

3 test results and discussion

3.1 analysis of high temperature wear test

high temperature wear resistance is closely related to the proportion of powder components, especially if the content of Al2O3 is too high or too low, it will reduce the high temperature wear resistance of spray welded layer. Relatively speaking, the change of G112 and f5010 powder content has little effect on it. When the Al2O3 content is controlled at about 0.25%, its high-temperature wear resistance is the best. Although the surface hardness of quenched parts is high, its high-temperature wear resistance is poor. This is due to the plastic deformation of the contact surface of the quenched parts under high temperature, the increase of the real contact area and the rupture of the oxide film, resulting in the increase of adhesion and the intensification of wear [3]. From the data results (Fig. 2), the high temperature wear resistance of No. 2 specimen is the best

3.2 metallographic microstructure analysis

the surface morphology, interface bonding and internal structure of the spray welding layer of each group of specimens were analyzed and compared by electron microscope photos, and it was concluded that the spray welding layer of No. 2 specimen had the best wear resistance (see Figure 3). It can be seen from the figure that the high-temperature wear surface is smooth, with micro indentation, uniform wear, and light adhesive wear (Figure 3a); The spray welding layer is well combined with the base surface, with few and uniform pores, diffusion of Cr, Si and Ni, and obvious diffusion of Al (Fig. 3b); In the spray welding layer, the white and bright WC is dissolved better, the black Al2O3 is dissolved less, the hard phase WC and Al2O3 are evenly distributed, and the dark area is dominated by NiCr phase (Fig. 3C)

Fig. 32 electron micrograph of specimen

(a) surface wear morphology（ × 40) (b) interface combination（ × 1000) (c) spray welding layer structure（ × 150)

Figure 4 is the EDS energy spectrum of test piece 2. It can be seen from the figure that the hard phase contains high W and Cr, w mainly exists in the form of WC, Cr in the form of compounds, and a small amount of Fe and Ni. W is dispersed near the hard phase, indicating that w diffuses well and distributes evenly in the spray welding dissolution process, and the spray welding layer is dense

Fig. 42 EDS energy spectrum of test piece

(a) energy spectrum of hard phase a (b) energy spectrum of hard phase B

3.3 discussion of test results

in the spray welding layer, Cr is dissolved in the Ni matrix, but B is insoluble in the Ni matrix or the dissolution amount is very small. Different boride hard spots have different wear resistance. There is no Si intermetallic compound in the Ni rich phase of NiCrBSi, only boride. These alloys have high red hardness, mainly due to the existence of boride. It can be seen from the electron microscope pictures that the interface bonding of spray welding layer is metallurgical bonding, with obvious diffusion of Ni and Cr and high bonding strength. B has an active gradient from the interface into the matrix, and the diffusion rate begins to be very high and decreases rapidly, so the outer boride region has a high hardness; The boron content of the inner layer is low, and the hardness value is small. During remelting, Al2O3 turns into stable α Al2O3 (corundum), which is a non reversible transformation, will not change phase due to heating and cooling, which is conducive to improving the bonding strength between the spray welding layer and the substrate and the ability to withstand the thermal cycle. When the content of Al2O3 is 0.25%, the effect of micro cutting and furrow on the surface of spray welding layer is small, and the peeling is less. The worn surface is smooth and flat, with uniform dissolution and diffusion, and good wear resistance. However, the specimen with Al2O3 content greater than 0.25% is accompanied by grain boundary fracture and local brittle fracture and spalling of grains during high-temperature wear, and micro cracks will appear on the wear surface. Its wear is mainly spalling and brittle cracking, so its high-temperature wear resistance is poor. The distribution of WC in the spray welding layer structure is related to the f5010 content. Too much WC is not completely dissolved, and the diffusion is poor, which affects the quality of the spray welding layer; Too little hard phase distribution is uneven, and less hard particles affect its wear resistance. The friction surface of quenched parts peels off greatly, with obvious furrows, indentations and serious wear

4 conclusion

under the condition of determining the spray welding process parameters, the high temperature wear tests of spray welding layers with different components were carried out, and the influence of spray welding materials on high temperature wear resistance was analyzed

(1) it is mainly self fusible alloy powder, and adding an appropriate amount of composite powder and ceramic powder will help to improve the high-temperature wear resistance of spray welding layer

(2) the test shows that the content of WC and Al2O3 in the spray welding material, which is mainly composed of chromides and borides, affects the high-temperature wear resistance of the spray welding layer

(3) the analysis results show that the ideal formula of high temperature wear-resistant spray welding layer in this test is 89.75% G112 + 10% f5010 + 0.25% Al2O3