Experimental study on ultra-fine tungsten carbide

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Experimental study on wide-band laser cladding of ultra-fine tungsten carbide on the surface of hot forging dies

1 introduction

in order to adapt to the harsh environment of hot working dies and improve the service life, in addition to the requirements that the hot working die steel itself should have high toughness and plasticity, small coefficient of thermal expansion and good thermal conductivity and other excellent mechanical and service properties, especially for the die surface directly in contact with the workpiece, it should have high thermal strength and good resistance to ignition, Therefore, the traditional use of all kinds of (no one should put hands and other items into the experimental area, carbon, sulfur, etc.) infiltration and PVD processes to improve the service life of the mold has a good effect. However, under the working conditions of hot mold, the surface of the modified layer is thin, and it is easy to cause oxidation and spalling, resulting in failure. The best solution is to apply a hard film on the surface of the mold material, Laser cladding technology can effectively attract high-temperature cemented carbide coatings such as WC, form metallurgical bond with the substrate, and obtain good wear resistance

h13 (4Cr5MoSiV1) steel is a kind of air cooling hardening hot work die steel widely used in the world. In this paper, the ultra-fine tungsten carbide alloy cladding layer (the average particle size of tungsten carbide powder is about 200nm as shown in Figure 1) was prepared on the H13 surface by wide-band laser cladding process, and the microstructure, hardness, wear resistance and residual stress of the cladding layer were analyzed

Fig. 1 SEM photo of ultrafine WC powder

2 test material and process method

the base material is tempered H13 steel, the sample size is 100mm* 50mm * 20MM, and its chemical composition is shown in Table 1. The sample surface is polished before the experiment, and then cleaned with absolute ethanol

add ultra-fine WC and a certain amount of self-made powder containing Cr, Fe, Si and other elements into the light absorbing coating, homogenize the solution with ultrasonic for one hour, and then apply about 0 A 2mm thick thin layer was placed on the surface of the sample and then dried, which established a benchmark for the domestic plastic machine industry

hydrogen is used as the protective gas, and the gas protection device is fixed on the laser focusing lens. With the lens moving, the gas protection is carried out for the molten pool in real time, using continuous CO with a maximum power of 7 kW. The laser is used to conduct laser treatment on the surface of the test block under the laser process parameters with the spot size of 9*2mm2, the scanning speed of 0.25~0.40m/min and the power of 2~3kw

after preparing the sample, the hardness, wear resistance, surface morphology, element distribution, microstructure, phase and weight loss were detected and analyzed by hxd-1000 microhardness tester, wm-2002 friction and wear tester, Hitachi s-4700 (II) field emission scanning electron microscope, thermo NORAN van tags EIS spectrometer, thermoarl scintagx Trax x x X-ray diffractometer and sartorius-bs21s electronic Tianping (accurate to mg). X-350a X-ray stress tester was used to analyze the residual stress of the cladding layer

3 test results and analysis

3 1. The macro morphology and residual stress

as shown in Figure 2 are the scanning photos of the surface and cross section of ultra-fine WC cladding layer prepared by laser cladding process with the spot size of 9*2mm. From the metallographic abrasive paper grinding in Figure 2a and the original surface morphology after laser treatment in Figure B, it can be seen that the surface of the cladding layer is flat and smooth without cracks and pores. The section of the cladding layer can be divided into four areas: the cladding layer, the transition zone (the boundary between the cladding layer and the heat affected zone), the heat affected zone and the matrix; The thickness of the cladding layer is about 0 4mm.

Figure 2 macro morphology of WC Laser cladding layer

in order to obtain a large area of cladding layer, we adopted the method of lapping in the test process. It can also be seen from Figure 2 that the cladding layer at the lapping place has a natural transition, which is of great significance for the die surface that needs to be strengthened in a large area

use x-350a X-ray stress tester to measure the surface residual stress. The stress measurement method is the lateral fixed dynamic method, the peak determination method is the cross-correlation method, and the CR target K α Characteristic radiation, time constant 1 5S, step angle of step scanning 0 1. The scanning start angle and end angle are 162 and 146 respectively, and the roll angle Ψ Take 0, 20, 35 and 45 respectively, and the stress constant k=-318mpa/degree

as shown in Figure 3, the matrix residual compressive stress values in X direction are 118mpa, Y direction is 127mpa, and the ultra-fine WC cladding layer residual compressive stress values in X direction are 468mpa and Y direction are 566mpa, indicating that the H13 surface residual compressive stress has been significantly improved after laser cladding ultra-fine WC treatment. The existence of large residual compressive stress on the surface is beneficial to improve the fatigue life of hot working dies. This is because under the action of the laser molten pool, the external WC and other elements enter the cladding layer in the form of solid solution or compound, and the phase changes. The rapidly cooled molten pool cannot make the added WC fully fuse with the matrix elements, resulting in a large increase in the dislocation inside the crystal. Therefore, the surface shows residual compressive stress. In addition, it can be seen from the "arc" cladding layer that during the laser scanning process, The size of the cross-section cladding layer in the laser scanning direction (Y direction) (z direction) is the maximum thickness of the cladding layer, while the cladding thickness in the X direction decreases in an arc, so the residual stress in the Y direction is much larger than that in the X direction

Fig. 3 Comparison of residual stress before and after laser treatment

3 2 micromorphology and analysis

this is also the driving force for developing the small panel control system.

from Figure 4 (a), we can see the morphology of the entire cladding layer, which is a combination of equiaxed crystal, columnar dendrite, fine cellular crystal and planar crystal from the surface to the inside; Figure 4 (b) shows the SEM photo of the cladding layer. It can be seen from the figure that the whole cladding layer is mainly composed of grain boundaries and the surrounding grains, with grain size of 6 μ M, relative to the austenite grain boundary (40) shown by the arrow in the matrix of Fig. 4 (c μ M) has been greatly refined; Figure 4 (d) shows the XRD phase results of the cladding layer. From the XRD phase analysis results of the cladding layer in Figure 4 (d), it is known that the phase of the cladding layer is relatively complex, mainly including Fe Cr, W3 C, fe, W3 C, Fe3 C3, W, Cr and c0.14fe1.86. It can be seen from the phase results that under the experimental conditions, WC is fully decomposed in the solution pool. At high temperature, C atoms are relatively active and have strong diffusion ability. During the diffusion process, Fe elements from the matrix are encountered and combined with them to form a stable fe7c3. With the reduction of C atoms, the existence time of the molten pool is short, and there is no carbon atoms from the matrix to supplement, Therefore, the extra W atoms can only combine multiple W atoms with one carbon atom to form another carbon deficient phase W3C, and even share the C atoms with the Fe atoms from the matrix to form a composite highly stable hard phase fe3w3c, until the consumption of C atoms in the molten pool is over, and c: and a small amount of w do not form a compound but exist in the cladding layer as a simple substance; Another c0.34fe1.86. The appearance of phase shows the lack of C atoms in the molten pool. The phase results also show that Fe3 W3C and Fe Cr are the phases that make up the framework of the cladding layer. The hard phase Fe3 W3C particles are displaced by the surface tension of the liquid phase. The atoms on the surface of the particles gradually dissolve in the liquid phase and increase with the increase of temperature. Due to the movement, the particles begin to close and contact to form a solid skeleton. The research results show that the phase structure formed by laser cladding WC is fe-w-c phase, and the resulting fe7 C3, W3C, The Fe Cr phase and Fe liquid are filled in the gaps and cavities between the skeletons together to form the structure shown in Fig. 4 (b). Maintenance of Jinan assay drop tester

Fig. 4 cross section microstructure and analysis of laser WC cladding layer

3 3 hardness and wear resistance analysis

using hxd-1000 microhardness tester, the load is 200g and the loading time is 15s, the measured hardness curve is shown in Figure 5 (a). The four areas of the cross section of the cladding layer: cladding layer, transition zone, heat affected zone and matrix can also be well reflected in the layer depth hardness curve, which reflects the structural differences between different areas. The average value is obtained by measuring five points on the surface and the substrate respectively. The hardness of the surface of the cladding layer after treatment is measured to be 831hv0.2 and the hardness of the substrate is 465hv0.2. It can be seen that the hardness of the cladding layer after laser treatment has been significantly improved, mainly because the stable hard phase Fe3 W3 C of the cladding layer encloses the hard phases Fe3 C3, W3 C and Fe Cr in the gap grains, the existence of the hard phase and the traction of the like Fe3 W3 C phase, The micro deformation of the cladding layer under load is effectively resisted, and the surface hardness of the cladding layer is improved

in order to further analyze the performance of the cladding layer, cut the sample into 8*8mm2 2mm thick rectangular sheet test pieces, and test them with wm-2002 friction and wear tester, with a load of 250g, a rotating speed of 800r/min, and a Si3N4 ceramic ball as the friction pair. The test time is 2 hours. The friction coefficient and wear weight loss results under the two states are shown in Figure 5 (b). The average friction coefficient and weight loss of the laser ultra-fine WC cladding layer are 3/4 and 5/8 of H13 matrix respectively. This is mainly due to the addition of ultra-fine hard WC. Under the action of laser molten pool, the hard phases fe3c3 and W3C are generated, which increases the nucleation rate. At the same time, the interposition of heterogeneous points effectively hinders the grain growth, greatly refines the grain, and forms a large number of dislocation during the rapid cooling process. Under the action of external load, due to the obstruction of dislocation, the support of hard points, and the traction of like skeleton fe3w3c, The bonding between grains is very close, and the cladding layer is not easy to produce deformation and adhesion, and thus has a smooth friction coefficient curve as shown in Figure 5 (b), thus effectively improving the wear resistance

(a) section layer deep hardness curve

(b) cladding layer surface friction and wear curve

figure 5

3 4 treatment and analysis of physical die

the cross forging die is treated by broadband laser with optimized process parameters. The results are shown in Figure 6, as shown by the arrow in the figure: the surface morphology is as smooth as the surface morphology of the sample, without crack and pore defects; The laps transition naturally and keep the original shape of the cavity; The mold surface is measured as 780hv with a portable hardness tester, and the hardness is 0 4mm thick cladding layer (Fig. 2). The metallurgical bonding between the cladding layer and the matrix ensures that the mold is not as thin as the infiltrating layer and PVD layer, and is easy to oxidize and fall off; From the small friction coefficient and wear amount in the friction and wear process, it also shows that the hard and smooth surface can ensure easy demoulding during stamping, so as to improve the number of stamping parts of the die

Fig. 6 cross forging die after laser treatment

4 conclusion

the wide-band laser cladding of ultra-fine WC on H13 surface can obtain an ideal cladding layer that can realize large-area lapping, the surface is flat and smooth, and the surface residual compressive stress is greatly improved

the addition of ultra-fine WC refines the grains. The cladding layer is composed of equiaxed crystals, columnar dendrites and fine cellular crystals. The main phases are fe3c3, W3 C, Fe3 W3 C, F3, C3, W and 1.86, and the main skeleton is Fe3 W3C structure

the microhardness of the cladding layer is 1 8 times, the friction coefficient and wear loss are 3/4 and 5/8 of the matrix respectively. Due to the fine grain strengthening, hard point support, dislocation hindrance and shape skeleton Fe3 W3C traction, the cladding layer shows good hardness and wear resistance

the processing effect of the physical die is good, which shows that the wide-band laser cladding of ultra-fine WC provides an effective and practical surface strengthening process for the hot forging die. (end)

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