The three failure modes of deformation, fatigue fracture and excessive wear are almost simultaneously present on the original used bending screws. When used in this state, the service life of the bending screw is not long. For the above reasons, while appropriately changing the shape of the bent screw, the assembly gap is reduced, and the material is re-selected on the basis of this, and a reasonable heat treatment process is established to improve the service life.
In view of this, in the selection of materials, the chemical composition, hardenability, impact toughness and fatigue limit of the material were fully considered. The high-quality alloy structural steel 38CrSi was selected. The basis for the selection was: (1) to ensure sufficient strength, hardness and Wear resistance. The steel has a carbon content of 035% and 043%, and after proper heat treatment, sufficient strength, hardness and wear resistance can be ensured. If the carbon content is too low, the above properties are low; if the carbon content is too high, the impact toughness value is too low.
The alloying elements Cr and Si ensure high hardenability, tempering stability and impact toughness, and further increase the strength. Both Cr and Si improve the hardenability element, wherein the Cr content is 130%160%, the Si content is 100%130%, quenching in oil, and the critical hardening diameter of 38CrSi steel is 33mm57mm, which is enough to ensure the bending screw with a diameter of 25mm. It is completely hardened in the oil. The original design of the bending screw material is 45 steel, its critical hardenable diameter water quenching is only 10mm18mm. 25mm diameter of the bending screws can not be hardened, and due to quenching in water, increasing the possibility of cracking, 38CrSi steel does not exist these problem.
It can be seen that the hardness of the central part of the 38CrSi steel is 4554HRC when it is hardened in the oil, which guarantees the performance requirements. After quenching, the Cr and Si elements existing in martensite further strengthen the martensite, and when tempered, the Si element can also improve the tempering stability, so that the steel remains at moderate temperature tempering. At higher hardness, a higher impact toughness value is obtained.
Ensure that the working state of the bending screw is in the fatigue damage safety zone. Through the tensile test of the material, the tensile strength of the sample after quenching and medium temperature tempering is: b=1670MPa The fatigue limit is obtained according to the Augerg formula: -1=(049013)b =(049013)1670=5811035(MPa) The maximum impact load of the hoist on the bending screw is 20000kg, then the maximum value of the alternating stress is: max=20000(314252)4=410(MPa)38CrSi steel The hardenability curve l is lower than the value of -1 of the material. Therefore, the 38CrSi steel is in the fatigue fracture safety zone with respect to the operating state of the mechanism, and fatigue damage will not occur. It is the N curve of 38CrSi steel. It can be seen from the curve that when the stress value of <38CrSi steel is <581MPa, the number of cycles can reach 106107 or more, which means that when the amplitude of the alternating stress is lower than 581MPa, even if it reaches The high number of fatigue weeks, the sample will not break, if the bending screw has high hardness and wear resistance at this time, the anti-wear ability is high, and the service life can be prolonged.
Heat treatment of bent screws At present, the company has installed and is running 38CrSi steel bending screws in use, and implemented three different solutions. The general principle is that the quenching is combined with the medium temperature tempering, and the structure is tempered toughness, obtaining the highest possible elastic limit and high yield ratio, avoiding the brittle fracture caused by the excessive hardness and the impact toughness, or The associated chain is worn out prematurely. The specific process is: Scheme 1: quenching temperature (88010), tempering temperature (41010), hardness control at 4045HRC.
After tensile test, the tensile strength is MPa and the elongation is 10%. If compared with 45 steel of the same heat treatment method, the strength is increased by 50% or more, and the hardness is increased by 40%. If the hardness is the same, the strength is compared. Increased by 20%, the impact toughness value increased by 40%, meeting or exceeding the assumptions when selecting materials. Option 2: quenching temperature (88010), tempering temperature (36010), hardness control at 4449HRC. Scheme 3: quenching temperature (88010), tempering temperature (45010), hardness control at 3540HRC.
The latter two schemes are not ideal. Although the scheme 2 shows better wear resistance and fatigue limit, it is found that the link connected with it is severely worn during use, resulting in a decrease in the life of the link. Scheme 3 has lower hardness, lower wear and tear, lower fatigue limit and lower service life.
Conclusion The high-efficiency lifting screw on the machine is made of 38CrSi steel. It has been running at full load for two years without any breakage and shutdown. It greatly saves the downtime and maintenance time, reduces the labor intensity of workers and improves labor efficiency. According to preliminary calculations, the direct economic benefits can reach 1.92 million yuan per year and nearly 4 million yuan in two years. The indirect economic benefits are even more impressive.
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