| Ultra-precision cutting,precision and ultra-precision grinding occupy an irreplaceable important position in the field of mechanical processing.Surface integrity can measure the surface properties of the workpiece,which is of great significance to characterize the surface characteristics of the machined parts in the machining process.In this paper,the influence of high speed cylindrical grinding process parameters such as wheel speed,workpiece speed,wheel feed rate and granularity on the surface integrity of 18 CrNiMo7-6 carburizing steel was studied.The formation mechanism of grinding surface hardness and residual stress was revealed from the perspective of microstructure transformation.At the same time,the machining technology of the funnel-shape rotary bending fatigue sample was studied,the surface quality of the sample was tested,the fatigue test and fracture analysis were carried out,and the technological possibility of “turning instead of grinding” for the funnel-shape sample of 18 CrNiMo7-6 quenched steel was explored.Research indicated:(1)The high speed cylindrical grinding of 18 CrNiMo7-6 carburizing steel: With the increase of wheel speed,the surface hardness of the workpiece first increases and then stabilizes.The effect of workpiece speed on surface hardness is not monotonous.Wheel feed rate has the greatest effect on the surface hardness of the workpiece,and the maximum depth of the work hardening layer is 110 ? 160 ?m.W20 CBN wheel has the largest hardness introduced to the workpiece surface,followed by 120/140# and230/270#.The surface layer microstructure of 18 CrNiMo7-6 carburized steel is highcarbon martensite,retained austenite and granular carbide,the transition zone is lowcarbon martensite and retained austenite,and the the heart zone is pearlite and ferrite.Unreasonable process parameters may lead to microstructure transformation.When wheel feed rate is 0.6 mm/min,the high temperature of grinding causes the surface layer structure to transform from high-carbon martensite to tempered sorbite,and the reduction of phase volume produces residual tensile stress,and the hardness decreases significantly.Optimal process parameters: 120/140# ceramic CBN wheel,wheel speed is 90 ? 105 m/s,workpiece speed is 6.60 m/min,and wheel feed rate is 0.4 mm/min.(2)Machining process of 18 CrNiMo7-6 hardened steel funnel-shape sample: The fatigue samples of CAK4085 hard-turning,MKE1620 A grinding and WS11 grinding all meet the requirements of fatigue test under the studied and developed technological conditions.Among them,hard-turning process can introduce larger residual compressive stress,lower surface roughness,longer fatigue life,and better machining accuracy.The fracture of the sample is divided into crack source zone,crack growth zone and instantaneous fracture zone,and the crack growth zone belongs to brittle fracture and the instantaneous fracture zone belongs to ductile fracture.The funnelshaped samples of 18 CrNiMo7-6 quenched steel are processed by “turning instead of grinding”,which is of great significance for improving the processing efficiency,reducing the processing cost and realizing green manufacturing. |
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