Due to the economic efficiency of nitrided steel screws, many countries still use them. For example, the most commonly used in West Germany are nitrided steels 8550 (34CrAlNi7) and 8519 (31CrMoV9), which are mostly medium carbon steels with carbon content of 0.30-0.40%. It also contains alloying elements such as Si, Mn, Cr, Ni, Mo, V, Al, etc. In the United States, about 80% of screws use (42CrMo4) steel and flame-hardened steel, and the remaining 20% use chlorinated steel and spray alloy materials on medium carbon steel. Most of the small and medium-sized screws in Japan use nitrided steel SCM2-4, while the large-scale screws are surface quenched with medium carbon steel S45C.
In China, most screws use nitrided steel 38CrMoAlA, and some product plants also use 40Cr or 45# steel.
In recent years, due to the increase in screw speed and the increase in pressure in the barrel, it is mainly due to the addition of a large amount of fillers during extrusion, which makes the wear resistance of the screw and barrel made of nitride steel appear to be relatively insufficient. Its failure is mainly caused by the following reasons:
Since the depth of the nitrided layer is generally around 0.5mm, and the internal collective hardness is only about HV280, once the nitrided layer is worn away, the wear resistance is very poor. During nitriding, the nitriding layer becomes brittle due to improper nitriding process, and it is easy to peel off under high pressure, which destroys the wear resistance of the screw; in addition, because the collective hardness is too low, the thin nitride layer is compressed under high pressure. The sinking also destroys the stable work of the screw.
Although the corrosion resistance of nitrided steel has been strengthened due to nitriding, in general, the corrosion resistance is insufficient, especially when extruding rigid polyvinyl chloride and some engineering plastics.
Due to the above situation, we can think that nitrided steel is no longer an advanced material for the screw and barrel. The actual need for production is that people begin to seek better materials and processing techniques to meet the requirements of extrusion production. At present, the international community is about to develop in the following directions:
Hard chrome plating
This is an older chemical treatment method. The screw plated with hard chromium has good surface finish and strong corrosion resistance, and the ability to resist wear is also good. However, this method has higher requirements for the chromium plating process.
Surface alloy surfacing
The surfacing welding on the screw can be carried out by arc welding, plasma welding and other methods. The alloy for surfacing can be cobalt-based alloy rod (stellite alloy) or nickel alloy electrode. The hardness of the former can reach HRC36-54, and the hardness of the latter can reach HRC52-58. When higher hardness is required, tungsten carbide can also be directly surfacing. This process can be used to repair the worn screw, and the surfacing welding can reach about 2mm according to the experience of HY-Industry.
This is a method that is currently being vigorously researched. The screw material can be made of nitrided steel, and then a layer of special alloy material of 0.5-0.8mm is sprayed on the surface of the screw edge or even on the entire screw. In order to further improve the wear resistance, tungsten carbide can also be added to the material.
In order to increase the adhesion between the alloy material and the screw matrix, the screw is sandblasted with Al2O3 before spraying. The dispersed alloy powder is completely melted or surface melted during spraying, and sprayed to the screw in an oxygen-acetylene flame at 1750°C surface. When using boron-containing materials, since the melting point is only 1050 – 1070°C, it can be further sintered after spraying. Such a screw can withstand important loads. When the process of sintering after spraying is used, the screw parent material cannot be selected from chromium steel that has a tendency of cold air quenching, because this kind of steel is likely to cause cracks in the nominal sprayed alloy during the cooling process.
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