Optimal Heat Treatment And Fatigue Behavior Analysis Of Low Speed Diesel Engine Bolts

The fatigue strength of diesel engine fatigue bolts is directly related to the safety and stability of low-speed diesel engines.Therefore,it is of great significance to carry out the fatigue strength analysis and optimization research of bolt materials and bolt structures for the design and performance optimization of low-speed diesel engine overall fatigue strength.In the research work of this subject,on the one hand,the structure and conventional mechanical properties of bolt materials and typical parts can be analyzed through experiments,which provide the basic performance parameters required for bolt fatigue strength analysis,optimization and service life prediction.In terms of the heat treatment process from the bolt factory,the best quenching and tempering process?including quenching temperature,tempering temperature and tempering holding time?is explored to obtain the best microstructure and mechanical properties.In addition,the fatigue test of 42CrMo steel for existing diesel engine bolts can be used to investigate the influence of different stress levels on the fatigue life of high-strength bolts and the fatigue fracture mechanism.First,the physical and chemical inspection of the sample bolts is conducted.The chemical composition of the bolts and nuts of the diameter of 42mm are 0.40%C-0.27%Si-0.60%Mn-0.98%Cr-0.19%Mo,0.36%C-0.26%Si-0.56%Mn-0.94%Cr-0.17%Mo.Secondly,metallographic test,SEM scan,XRD detection and other experimental research are used for the microstructure.The M42 high-strength bolts are tempered sorbite-based structures from the surface layer to the core,and a small amount of bainite structure appears from 1/4;the composition of inclusions in steel is Al₂O₃/MgO and SiO₂/Al₂O₃,etc.The hardness test results show that the hardness of the M42 sample gradually increases from the outside to the inside,but the average hardness does not change much,and the average hardness is 333?±16?HV.The tensile test results of the sample showed that the average yield strength,tensile strength and elongation for M42 sample are 964 MPa,the 1062 MPa,and 15.6%,respectively.Compared with the factory value,the yield strength was about 100 MPa increase,the tensile strength was equivalent,and the elongation was slightly higher.The overall tensile test results of the large bolts show that the overall tensile curves of the M42 bolts have no yielding points and are continuous yielding.The yield strength,tensile strength and uniform elongation of M42 are 989MPa,1083MPa,4.3%,respectively.The yield ratios of M42 were 0.91,respectively.According to the twelve different quenching and tempering processes formed by the combination of three quenching temperatures and four tempering processes,the microstructure of M42 high-strength bolts under different quenching and tempering processes was investigated by metallographic test and SEM scanning;The Charpy impact test and tensile test at normal temperature respectively obtain the mechanical properties data such as hardness data,impact toughness and tensile stress-strain curve,and perform macroscopic observation and microscopic analysis on the impact fracture,and determine the optimal strength of the material by combining the data.Comprehensive mechanical properties combined with plasticity.According to the test results,under the quenching and tempering process with quenching temperature of 900°C and tempering temperature of 600°C,the comprehensive mechanical properties of 42CrMo high-strength steel is optimal,the hardness is 325.2HV,the yield strength is 956MPa,and the tensile strength is 1060MPa and the elongation rate is 15.8%.Fatigue tests made of M42 high-strength bolts were selected for fatigue test.The fatigue life changes and SN curves of high-strength bolts under two stress levels and stress amplitudes were investigated.And the results are as follows:the fatigue strength decreased with the increase of average stress.Under the average stress level of 80%yield strength,the fatigue strength?stress value?is 450 MPa,the fatigue strength at 85%yield strength level,the fatigue strength is 350 MPa,By analyzing the fatigue fracture,it can be seen that as the stress level increases,the fatigue source area of the fracture increases gradually,and when a certain fatigue life?cycle number?is reached,the fatigue zone will appear in the termination zone;with the finite element simulation analysis of the high-strength bolt stress,the conclusion is that the maximum stress-strain position of the bolt during the simulation analysis is the first thread of the bolt and the nut.The results of the fatigue analysis also show that after a certain number of cycles after the cyclic load,the portion where the fracture occurs is also the first thread of the bolt and the nut.