Laser cladding (also known as laser surfacing) refers to the process of placing the selected coating material on the cladding substrate by different adding methods, after laser irradiation, it melts with the substrate surface, and then rapidly solidifies to form the surface coating with low dilution and metallurgical combination with the substrate. Compared with the traditional surface treatment technology, such as electroplating and thermal spray welding, laser cladding technology has the advantages of fine grain, small heat affected zone and hot deformation zone. In this paper, the effect of carbon content on the properties of laser hardfacing layer of Fe based alloy is studied, and a high hardness and crack free cladding layer is obtained. Because alloy elements also play an important role in the properties of iron and steel materials, this paper further studies the influence of alloy elements on the properties of laser cladding layer of iron-based alloy, so as to obtain cladding layers with different properties (such as different hardness) to meet different production needs.
1 Test materials and methods
In the laser cladding test, rofin tr050 fast axial flow CO2 laser manufactured by rofin company of Germany was used. During the experiment, the laser power is 3.5kw, and the laser beam wavelength is 10.6 μ m; the spot diameter is measured by ablation method, and the spot diameter is 4mm by adjusting the defocusing amount; the preset powder thickness is 1.5mm by single-layer preset laser cladding; the laser scanning speed is 6mm / s, and the scanning length is 30mm; the AR is used as the protective gas, which is mainly used to prevent alloy The powder is oxidized in the process of cladding, and the Ar gas flow rate is 3.6l/min. The base material is made of A3 steel, with the dimension of 100mm × 40mm × 5mm. The surface is pickled and derusted, cleaned with acetone, and then dried for standby. In the experiment, fe-ni-cr-b-si-c alloy with good properties was used as the base alloy. The chemical composition (mass fraction,%) of fe-ni-cr-b-si-c alloy was 92.7fe, 3Ni, 2Cr, 1b, 1si, 0.3C. The results of laser cladding test show that Fe 202 alloy powder can be used as cladding powder to obtain high hardness and crack free Fe based laser cladding layer. Different contents of Ni, Cr and mo were added to fe202 to study the effect of various alloying elements and their contents (mass fraction) on the properties of the cladding layer.
In order to make the added pure alloy elements and the original alloy powder mix evenly and reach a certain degree of alloying, the mixed alloy powder is first mixed by mechanical ball mill. The treatment process is as follows: the milling time is 10h, the grinding ball is about Φ 10 mm cemented carbide ball, the mass ratio of grinding ball and alloy powder is 5:1, and the diluent is a proper amount of anhydrous alcohol. After ball milling, the powder is dried at 100 ℃ for 2h, and then sieved for use.
The surface hardness of various alloy cladding layers is tested by Rockwell hardness tester; the amount of residual austenite (AR) in each cladding layer is measured by X-ray diffraction; the number and length of cracks are observed and measured by stereo microscope with magnification of 10 times; the cracking sensitivity of cladding layer is obtained by measuring the crack length in unit area, α = ∑ Li / A, Li is the i-th crack The length of grain (mm), a is the area of cladding layer (cm2).
2 Test results and analysis
2.1 Effect of alloy elements and content on hardness of cladding layer
The effect of alloy elements and their contents on the surface hardness of cladding layer is shown in Figure 1. It can be seen that Ni can reduce the hardness of the cladding layer. With the increase of Ni content, the hardness of the cladding layer begins to decrease steadily; when the Ni content exceeds 7%, the hardness of the cladding layer decreases sharply; when a small amount of Cr is added, the hardness of the cladding layer increases slightly, but when the Cr content exceeds 4.6%, the hardness of the H + cladding layer decreases slightly. When the Cr content increases further, the hardness of the cladding layer decreases obviously, but the hardness of the cladding layer decreases slightly With the increase of Mo content, the hardness of the cladding layer increases, but when Mo content exceeds 5.75, the hardness of the cladding layer decreases. The influence of alloy elements on the hardness of cladding layer is mainly caused by the change of AR content in cladding layer. Because of the low ar hardness, if the AR content in cladding layer increases more, the hardness of cladding layer will decrease. The influence of the change of alloy element content on the AR content (volume fraction) of cladding layer is shown in Figure 2 (actual measurement).
2.2 Effect of alloy elements and content on cracking sensitivity of cladding layer
The effect of alloy elements on the cracking sensitivity of the cladding layer is shown in Figure 3. As the cladding layer prepared with fe202 alloy powder has no cracks, it can be seen that (1) no matter how much Ni element is added, the cladding layer cannot produce cracks. As Ni element can increase the amount of AR in the cladding layer (see Figure 3), it should reduce the cracking sensitivity of the cladding layer, which is also proved by the research of song Wulin. Although in this test, because the specimen size is relatively small, the cladding layer without Ni element has no crack, so it does not reflect the effective effect of Ni element, but if the cladding condition is worse, for example, when the cooling rate of cladding layer is higher or the surrounding constraint stress is more complex, it can reflect the effect of Ni element. (2) With the increase of Cr content, the cracking sensitivity of the cladding layer increases, but when Cr content reaches or exceeds 9.8%, the cracking sensitivity of the cladding layer is reduced due to the sharp increase of AR content (see Figure 2), and the cladding layer does not produce cracks.
2.3 Effect of alloy elements and content on AR content of cladding layer
The existence and content of AR undoubtedly play an important role in the hardness and cracking sensitivity of the cladding layer. It can be seen from the figure that (1) with the increase of Ni content, the amount of AR in the cladding layer increases, almost in a positive proportion; (2) with the increase of Cr content, the amount of AR in the cladding layer increases slowly at first, then rapidly; (3) with the increase of Mo content, the amount of AR in the cladding layer increases slowly. The amount of AR in the cladding layer increases with the increase of Ni, Cr and Mo content, because these alloy elements reduce the end temperature of martensitic transformation MF, and the level of MF point determines the amount of AR when the steel is quenched to room temperature.
With the increase of Ni content, the amount of AR in the cladding layer increases, the hardness decreases, and the cracking sensitivity remains unchanged; with the increase of Cr content, the amount of AR in the cladding layer increases, the hardness first increases slightly, then decreases, the cracking sensitivity of the cladding layer first increases, then decreases, with the increase of Mo content, the amount of AR in the cladding layer increases slowly, the hardness of the cladding layer first increases, then decreases, and finally the base The cracking sensitivity of the cladding layer increases continuously with the increase of the temperature.
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