1. Requirements for materials
It can be known from the extrusion process that the screw works under high temperature, certain corrosion, strong wear, and high torque. Therefore, the screw must:
1) High temperature resistance, no deformation under high temperature;
2) Wear resistance and long life;
3) Corrosion resistance, the material is corrosive;
4) High strength, can withstand large torque and high speed;
5) Has good cutting performance;
6) After heat treatment, the residual stress is small, and the thermal deformation is small.
2. Common materials and heat treatment
At present, the commonly used screw materials in my country are No. 45 steel, 40Cr, ammoniated steel, 38CrMOAl and so on.
1) No. 45 steel is cheap and has good processing performance, but its wear resistance and corrosion resistance are poor.
Heat treatment: quenching and tempering HB220-270, high-frequency quenching HRC45-48
2) The performance of 40Cr is better than that of No. 45 steel, but it is often plated with a layer of chromium to improve its corrosion resistance and wear resistance. However, the requirements for the chromium plating layer are relatively high. If the plating layer is too thin, it is easy to wear, and if it is too thick, it is easy to peel off. After peeling, it will accelerate the corrosion. At present, it is rarely used.
Heat treatment: tempered HB220-270, hard chrome plating HRC>55
3) Nitrided steel and 38CrMoAl have excellent comprehensive properties and are widely used. Generally, the nitride layer reaches 0.4-0.6 mm. However, this material has low resistance to hydrogen chloride corrosion and is relatively expensive.
Heat treatment: tempering HB220-270, nitriding HRC>65
In foreign countries, a titanium carbide coating method is used to improve the corrosion resistance of the screw surface, but it is reported that its wear resistance is not good enough. In recent years, foreign countries have taken a series of measures to improve the wear resistance and corrosion resistance of the screw. One way is to use highly wear-resistant and corrosion-resistant alloy steel. Such as 34CrAlNi, 31CrMo12, etc. There is also a method of spraying Xaloy alloy on the surface of the screw. This Xaloy alloy has high wear and corrosion resistance (see the barrel section for details).