## 3 Mechanical Properties of 40 CrNiMo Alloy Steel for All-Metal Screw Motors  
In the selection of materials for the stator and rotor of the all-metal screw motor, 40 CrNiM was chosen as the material for the stator after accounting for the extremely high temperature and pressure and very corrosive drilling environment of ultra-deep wells. Furthermore, during the drilling process, the stator and rotor are expanded by heat as the temperature continues rising. To make the stator and rotor undergo the same deformation during the heating process, the rotor material was selected to be the same as that of the stator. To further understand the material mechanical properties of 40 CrNiMo, a uniaxial tensile test was conducted on heat-treated 40 CrNiMo. Tensile testing is an important quantitative method to test the physical and mechanical properties of metals and one of the important metal test methods in the field of engineering research. By using a tensile testing machine for metals combined with a displacement measurement sensor, the mechanical index of the metal in the material elastic stage can be obtained accurately. At present, the displacement measurement sensor widely used in tensile testing is the elongator, and the accuracy of the measurement can be greatly improved by choosing a suitable elongator that matches the length of the specimen. For the alloy steel applied to the drilling tools for ultra-deep wells, the tensile and compressive strengths in high-temperature and high-pressure environments are the focus of research. Thus, in this experiment, a high-temperature ceramic axial extensometer was used. This type of extensometer is widely used in metal tensile tests at present. Photographs of the overall arrangement of the extensometer as well as some details are shown in Figure 3.  
The mechanical properties of 40 CrNiMo high-quality alloy structural steel were obtained according to the indoor test method in the specification \"GB228.1-2021 Tensile test of metal materials\". Steel was cut and machined into bar specimens with a diameter D\({}_{0}\) of 5 mm and a scale distance L\({}_{0}\) of 32 mm. A total of three sets of control tests were designed, named #1, #2 and #3, and the schematic diagram is shown in Figure 4. The tests were conducted on the MTS multifunctional tensile testing machine at the Institute  
Figure 3: Arrangement of the extensometer. of Water Resources and Hydropower, Sichuan University, using quasi-static, uniform displacement-controlled loading with a loading rate of 0.375 mm/min. In this study, the modulus of elasticity, yield strength and ultimate strength of the alloy steel material were calculated by combining the tensile values of the testing machine and the deformation measured by the extensometer. Since there were many unstable factors in the initial stage of the actual tensile test, a smooth section of the curve was generally selected for calculations, and the final results are shown in Table 1. From the tensile and deformation data from the tensile process, the load displacement curve and stress-strain curve of the alloy steel were obtained by combining the dimensions of the specimens, as shown in Figures 5 and 6. From Figures 5 and 6, it can be seen that the load displacement curve and stress-strain of 40 CrNiMo are typical corresponding curves of metals after stretching tensile deformation curves with elastic and yielding stages; the yielding stage was longer. By calculation, the final average modulus of elasticity of alloy steel used for all-metal screw motors at room temperature obtained from three sets of test data was 225,765.3 MPa, the yield strength was 825.690 MPa, and the ultimate strength was 927.650 MPa. In comparison with conventional stators made of rubber, which are subject to aging, large deformation and carbonization in high temperature and pressure environments [11; 38], it was clear that the alloy steel had more stable mechanical properties and was a good material choice for all-metal screw motors.