Vacuum sintering of tensile screw surface

---

The working temperature of the stretching screw for injection molding machine is generally above 200 C. It not only has to withstand the high pressure during injection, but also bear the abrasive effect of the melt and the frequent load starting during pre-molding. Improving the surface properties of the screw, especially the surface wear resistance, plays an important role in the development and progress of the plastics industry. In recent years, with the continuous progress of surface technology, the technology to prolong the service life of screw has developed rapidly. Sintering and oxyacetylene spray welding, etc. Due to different processes, the structure and properties of the coating layer and the combination of the coating and the substrate will be different. When the tensile screw is in service, it is not only required that its surface has high strength and wear resistance, but also needs to have good resistance. Corrosion properties, and nickel-based alloys can meet the performance requirements, so a nickel-based alloy layer was obtained on the surface of the tensile screw by vacuum sintering. In this paper, the vacuum sintering process of the obtained nickel-based alloy layer was analyzed, and the changes of the interface structure and microhardness between the alloy layer and the substrate were mainly studied.

1 Materials and methods

1.1 Materials

The tensile screw used in the injection molding machine is 40Cr steel, which has good comprehensive mechanical properties, low temperature impact toughness, low notch sensitivity and good hardenability. The tensile screw used in the test is shown in Figure 1, the chemical composition of the raw material is listed in Table 1, and its microstructure is ferrite plus pearlite. The coating adopts Ni60 alloy powder, and its composition is listed in Table 2.

1.2 Methods

1.2.1 Powder coating before vacuum sintering

In order to ensure the uniform coating of the coating, the alloy coating was first prepared on the outer surface of the stretching screw of the injection molding machine by thermal spraying method before vacuum sintering. Using an oxyacetylene flame as a heat source (SH-2000 high-energy flame spray gun, the oxygen pressure is 0.5 MPa, the acetylene pressure is 0.1 MPa, the gun aperture is 1 mm, and the gun moving speed is 10 mm/s) to prepare the nickel-based alloy powder. It was evenly sprayed on the surface of the stretching screw with a coating thickness of 1 mm.

1.2.2 Vacuum sintering equipment and vacuum sintering process

Vacuum sintering uses RGL04 protective atmosphere medium temperature tubular heating furnace. After drying, the sample was placed in a heating furnace, protected by hydrogen, the vacuum degree was 0.1 Pa, the vacuum sintering temperature was 1 020 °C, and the temperature was kept at 1020 °C for 5 min. Its vacuum sintering process curve is shown in Figure 2.

The sample is cut along the transverse direction on the tensile screw, its size is 10mmX10mm, after cutting, grinding, polishing and corrosion for observation and analysis.

1.2.3 Test analysis method

MM-6 metallographic microscope and Nissan JSM-6490LV scanning electron microscope (SEM) were used to observe the structure and morphology of the obtained sample coating respectively; the energy dispersive spectrometer on the SEM was used to analyze the structure of the coating surface. ; The longitudinal microhardness distribution of the nickel-based alloy coating samples was measured by the MH-3 microhardness tester, the hardness tester measurement time was set to 15 s, and the load was 200 g.

2 Test results and analysis

2.1 Microstructure and morphology

Figure 3 shows the comparison of the microstructure of the interface between the Ni60 alloy and the steel matrix before and after vacuum sintering.

In Figure 3a, I is the matrix structure, II is the interface structure, and III is the nickel-based alloy layer before vacuum sintering. The spraying method is adopted, and the spraying material heated to a molten or semi-melted state is atomized by the airflow and then impacts on the surface of the part at high speed to form a coating before vacuum sintering. In this process, while forming a fairly dense and flat alloy coating, the coating also forms a good metallurgical bond with the substrate. Since the spraying is carried out in layers, the adjacent layers cannot be solidified at the same time due to the difference in spraying time, resulting in the phenomenon of delamination. Before vacuum sintering, it can be seen that the microstructure of the Ni60 sprayed layer is a stacked lamellar structure, and there are relatively obvious interfaces between the layers. These interfaces are formed by the molten or semi-molten Ni60 particles during the spraying process. Oxide film. A large number of white granular phases with high dispersion degree are distributed on the interlayer matrix structure. The average composition of the sprayed coating substantially maintained the chemical composition of the original powder.

The arrows in Figure 3b point to the obtained interface area, there are no micro-cracks and voids in the structure, the coating and the substrate are well combined, and the interface is clean, neat and dense. Since the coating alloy contains high alloying elements such as Cr and Ni, the hardenability is improved. After vacuum sintering and melting coating, it is rapidly cooled in the air, which is equivalent to the normalizing treatment of high-chromium cast iron at more than 1000 C. The obtained matrix The organization is martensite plus retained austenite.

A fusion band parallel to the interface was formed between the coating and the substrate, and the solid solution mainly consisted of Ni and Fc groups. This is mainly because the chemical composition of the alloy coating and 40Cr steel is quite different. There is a large concentration gradient of B, Si, Ni, Cr, C, Fe and other elements during vacuum sintering, and the nickel-based alloy is liquid, which promotes mutual The diffusion between the two makes B, Si, C and other elements diffuse into the 40Cr steel, and the Fe element also begins to partially integrate into the Ni60 alloy.

It can also be seen from Figure 3b that a complex structure is formed on the surface of the coating, and the surface has needles and arrows indicated by arrow 1 in the figure.

2 refers to the block compound. The formation process of needle-like and massive compounds is roughly as follows: in the cooling stage, due to the uneven composition of the alloy, it is mainly two-phase austenite and liquid phase. The content of chromium is greater than that in austenite, so a large amount of chromium segregates from the alloy and dissolves into the boundary of the liquid phase.

The scanning photo of the nickel alloy coating and the energy spectrum analysis of the joint between the coating and the substrate are shown in Figure 4. As can be seen from Figure 4, the internal junction of the coating

It is dense and contains only a small amount of closed pores, without cracks and large defects; there is a 20-40 um thick bonding layer between the coating and the substrate. The energy spectrum analysis shows that the bonding layer is the place where the metallurgical bonding of the coating and the substrate occurs, and it is also the place where the atomic diffusion is most intense. The bonding layer should be a solid solution formed by Fe and Ni.

The hardness distribution on both sides of the surface has a large change, the hardness of the matrix near the interface is greatly improved, and the cushioning effect is strengthened, so that the coating can bear a larger load; the hardness distribution of the matrix tends to be stable at a distance of about 1mm from the surface. The phenomenon of hardness fluctuation occurs again. From the perspective of the structure, this is because the number of needle-like and block-like carbides in the structure increases with the movement from the interface to the coating direction. Somewhere near the surface, the accumulation of the hard phase reaches the maximum, at this time The maximum hardness value will appear, and then the hardness will gradually decrease; with the movement from the interface to the inside of the matrix, the pearlite gradually decreases, and the ferrite gradually increases. Due to the low hardness of ferrite, the overall hardness of the matrix is ​​caused. Gradually decreases.

From the perspective of element diffusion, this is due to the diffusion of Fe element in the matrix into the lattice of the coating structure to produce solid solution strengthening,

Therefore, the hardness of the matrix is ​​improved, but when the content of Fe element is high, an intermediate phase with Fe element as the matrix will be locally formed, which

The existence of these phases makes the overall hardness distribution of the coating show a downward trend. The Cr and C elements in the coating diffuse toward the matrix, and the Cr element diffuses into the matrix lattice to produce solid solution strengthening. The diffusion of C atoms into the matrix structure can increase the content of cementite in the matrix structure and the hardness of the cementite. higher than ferrite. Due to the gradient of diffusion, the closer the distance to the bonding layer, the more obvious the diffusion effect, so the microhardness value has a downward trend from outside to inside near the bonding interface. The hardness distribution tends to be stable at a distance of about 1 mm from the surface, indicating that the diffusion of elements has basically terminated at this time, and the structure has basically no longer changed and is close to the original structure of the matrix.

3 Conclusions

On the surface of the tensile screw sample used in the injection molding machine, the nickel alloy powder is firstly coated by thermal spraying, and then the sample is protected by hydrogen.

Vacuum sintering to obtain a nickel-based alloy coating. The bonding layer between the coating and the substrate forms a good metallurgical bond, so that the interface strength is high and the coating is dense. The elements between the sample matrix and the nickel alloy coating diffused to each other, the Ni and Cr elements in the nickel alloy coating diffused into the matrix, and the Fe element in the matrix diffused into the coating. The martensite plus retained austenite structure is obtained in the matrix, and the Ni-based solid solution-based strengthening phase is formed on the surface of the coating, thereby ensuring the high hardness of the coating and the uniform distribution of hardness values, making it suitable for Get high wear resistance. The coating surface of the samples after vacuum sintering has a high microhardness. From the coating surface to the inside of the substrate, the hardness distribution generally follows the law of “first increase, then decrease”, and the hardness reaches the maximum value at a distance of about 100 μm from the surface. , the hardness changes tend to balance at 1 mm from the surface.

Selection of vacuum sintering equipment: The RVS vacuum sintering furnace provided by SIMUWU is an excellent product for handling this kind of process. It has the characteristics of good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.

Learn More:

Vacuum Sintering of Alumina Ceramics

Which process should be used for quenching after vacuum carburizing?

Improvement of Soft Magnetic Properties of FeCo Alloys by Vacuum Heat Treatment