Study On The Multi Physical Field Prediction And Fatigue Properties Of 18CrNiMo7-6 Carburizing And Quenching Gear

Carburizing and quenching is the key process in high load-bearing gear manufacturing.At present,the process development and quality evaluation are still based on experience and testing,which is of high cost,low efficiency,poor controllability.Moreover,it is hardly to conduct nondestructive evaluation of the microstructure field,hardness field,residual stress field and other performances of the workpiece.Although there has been some numerical simulation studies on the carburizing and quenching process,there are still many deficiencies in the mathematical model construction and material parameter evaluation,which in combine with other factors restrict the application in engineering.Therefore,focusing on the multi physical field prediction and fatigue test of carburizing and quenching gear,this paper studied the numerical model of carburizing and quenching for microstructure,hardness distribution and residual stress.In the study,the thermophysical characteristics of a typical gear material of18CrNiMo7-6 steel was determined.Using the established model,the evolution law of microstructure field and formation process of hardness field and stress field was analyzed.At the same time,the research of carburizing and quenching process optimization was carried out for the subway gear with high transmission ratio and high load,and the integrated design method of cold and hot processing in engineering was proposed.The fatigue strength of specimens under the optimized process were studied through the rotary bending fatigue test,gear contact fatigue test,tooth bending fatigue test and bench test of the subway gear box,respectively.The failure modes and the crack initiation and propagation mechanisms were also analyzed.The main work and conclusions of this paper are as follows:1)Through the systematic construction of material thermophysical properties,the specific measurement of key parameters and the influence analysis of secondary process,the mathematical models of temperature field,carburizing field and microstructure field predicition were established.Test analysis indicated that the prediction error of carbon content was less than ±5% at each position along the layer depth for 1.2mm shallow and2.5mm deep carburizing,and the predicted retained austenite of 6.23% was also close to the measured value of 7.5%.At the same time,the diachronic evolution of carbon distribution and microstructure on numerical simulation were also consistent with the diffusion theory and phase transformation laws.2)The variation of the hardness of single-phase martensite with different carbon contents for 18CrNiMo7-6 steel was studied,and the hardness drop of high-carbon martensite before and after low-temperature tempering was determined.Based on the volume ratio method,the hardness field prediction model related to the microstructure field was constructed,and the multi-dimensional analysis method of the diachronic evolution for the hardness field was established.Through experimental verification,for the prediction of the quenching hardness and tempering hardness of carburized parts,the volume ratio method could reduce the error from 15.4% and 17.7% to 2.2% and 7.0%,respectively,compared to the end quenching curve method.3)Considering the mutual effect of multiple factors coupling on elastic-plastic stress-strain of high temperature phase transformation,the coupling mechanism of thermal stress and microstructure stress in quenching was analyzed.The prediction method and the two-stage characteristics of internal stress development for carburized parts in quenching was studied.Through numerical analysis,the main compositions of internal stress generated in oil bath and water bath quenching were determined for the18CrNiMo7-6 steel.The maximum instantaneous stress in water bath quenching was up to 1050 mpa,which is easy to induce quenching cracking as what happened in the actual application.4)For subway gears with high transmission ratio and high load bearing,through the comparative analysis of the parameters influence in the carburizing process,the three-stage carburizing method with outstanding stability(carburizing potential: strong carburizing 1.2% C,diffusion 1.0% C and 0.8% C;time ratio: 2-1-1)was developed and optimized.Based on the C-ring sample,the opening distortion of oil bath quenching and salt bath quenching was-59μm and +39μm respectively.Based on the predicted microstructure composition,hardness gradient,stress distribution and distortion law of gear teeth,the co-optimization of the machining allowance in the integrated design for cold and hot machining process was studied and analyzed.5)For the optimized carburizing and quenching process of subway gears,the improvement in the fatigue strength was determined through three kinds of tests.When the centers of parts were not fully quenched and contained a large amount of bainite,the rotational bending fatigue limit under 99% reliability was 480 MPa.For the contact fatigue of tooth surface,when the surface was composed of more than 90% fine needle martensite and 6~9% residual austenite,the contact fatigue limit with 50% reliability was about 1732 MPa.For the bending fatigue of tooth root,the bending fatigue limits of carburizing quenching and additional shot peening with 50% reliability were 666 MPa and 804 MPa respectively.6)For the characteristics and mechanism of fatigue damage of carburized gear,combined with the meshing mechanical behavior analysis,the single pitting damage was further subdivided into the pitch circle pitting caused by alternating shear stress,the shallow spalling caused by the connection of wear pits at the gear tooth tip and root,and the deep spalling caused by a coupling effect of external load stress,residual stress,material defect and local micro-area strength.For the tooth root bending fatigue,cracks generally originated from the surface and propagated unidirectionally for the carburizing quenching gear.With the addition of the residual compressive stress induced by shot peening,the crack initiation location could be moved to a certain depth from the surface and propagated synchronously to both sides of the tooth profile.7)Through the developed carburizing and quenching process of subway gear,the compressive residual stress of-192 MPa and-158 MPa were formed on the tooth root surface of pinion and wheel respectively,and the depths of the negative compressive stress layer were 1.37 mm and 0.95 mm.Combined with additional shot peening process,the overload test of subway gear box was completed with load of 1.8 times than the rated torque.The bench test and engineering operation showed that the developed carburizing and quenching process met the requirement of bearing capacity for products.