Research On Optimization Of Forming And Cooling Process Of The Non-Quenched And Tempered Steel Crankshaft Forgings And Its Grain Size Control Mechanism

The crankshaft is one of the important components of the engine,its working environment is more complex and variable,which requires good comprehensive mechanical properties such as rigidity,strength and toughness.At present,the non-quenched and tempered steel with the name of "green steel grades" has gradually replaced the quenched and tempered steel material which requires high energy consumption heat treatment process for the matrix material of passenger car engine crankshaft forgings.However,in order to meet the normal operation of crankshafts used in non-road vehicles of construction machinery under extreme operating conditions such as large torque and forced vibration,the existing mechanical improvement technology of the non-quenched and tempered steel crankshaft forgings still needs further study.Based on this,this topic is aimed at the defects such as excessive grain sizes and flow of core impurities to the journal surface after hot forging and forming of non-quenched and tempered steel crankshaft forgings(the main cause of cracks),by regulating the controlled cooling rate of forgings and optimizing the structure of preforming dies.The purpose is to improve the grain size grade(average grain sizes effect)and control the flow characteristics of the forming core of the matrix material,and to provide support for the in-depth research of key technologies for improving the performance of non-quenched and tempered steel crankshaft forgings.The specific contents and results are as follows:1.Taking the given type of S43 CMV non-quenched and tempered steel three-cylinder crankshaft forgings as the research object,the forging process of the crankshaft is simulated by Deform finite element software,and the internal correlation between the controlled cooling rate after forging and the evolution of crankshaft whole-local(key positions)grain sizes of the crankshaft is systematically analyzed.The results show that the grain sizes can be refined by increasing the overall cooling rate.When the cooling rate is controlled at 60°C/min,the microstructures of the key positions of the crankshaft(the outside of the first connecting rod journal,the main journal and the flange end)are pearlite and ferrite,and the grain size grades are improved.At the same time,by comparing the grain size grades of the corresponding positions of the three-cylinder and four-cylinder crankshafts,it is learned that the structure and forming methods of the crankshaft forgings will affect the forging flow characteristics of the core material and its grain sizes effect.2.Combining with the finite element simulation analysis data and experimental results of forging and cooling processes of three-cylinder and four-cylinder crankshaft forgings of the same material,the forming flow characteristics of the forging core material,the grain size grades of key positions and the origins of surface cracks on the journals are correlated.The results show that due to the difference between the three-cylinder crankshaft(the cross section of connecting rod journals are distributed at 120°)and the four-cylinder crankshaft structure(the connecting rod journals are distributed at 180°),the preforming method of the three-cylinder crankshaft needs to adopt the bending process,which leads to the accumulation of more core impurity materials at the inside forming area of the first and third connecting rod journals of the three-cylinder crankshaft.At the same time,combined with the streamline experiments and SEM microscopic morphology analysis,it can be known that the porosity of this inside area is larger and the grain sizes uniformity are poor,which means that the excessive volume of the initial core bar material is the key origin of the crack defects.3.Based on the structural characteristics of the three-cylinder crankshaft forgings,the preforming method which can control the flow of the core material to the journal surface is optimized.Combined with the finite element analysis results,it can be seen that after the improvement of the structure of the pre-bending die,the sections of the core bar are flat and elliptical after pre-forging,which makes it difficult to flow to the surface area of the journal during the pre-forging and final-forging,and reduces the adverse influence of the core material on the grain size grades of the key positions of the crankshaft.In addition,the inside sampling positions of each connecting rod journal of the three-cylinder crankshaft show the trend of decreasing grain sizes and uniform distribution,which verifies the effectiveness of controlling the flow of the core material to improve the grain size grades and reduce the probability of fatigue cracks at the journals.Through the study on the regulation mechanism of grain size grades and forming flow characteristics of core materials,it is expected to provide theoretical and experimental support for the key technologies to improve the comprehensive mechanical properties of crankshaft forgings with non-quenched and tempered steel.