Study On Carburizing Heat Treatment Process Of 18CrNiMo7-6 Alloy Steel Notch Component

Gears,bearings,shafts,etc.are the key components for transmitting force and motion in machinery,and are widely used in automobiles,ships,aerospace,military equipment and other fields.These components need to withstand complex cyclic loads such as bending,contact and torsion for a long time,thus easily lead to fatigue failure.The surface of the component has grooves,sharp corners and other discontinuous planes,i.e.,notches.The carbon distribution,hardness field,microstructure field and stress field near the notch will change after heat treatment,and the stress concentration caused by the notches can seriously affect the fatigue performance of the component,so the notches are often the source of component failure.In this paper,18CrNiMo7-6 alloy steel was taken as the research object,and smooth specimens and two kinds of notched specimens with Kt=1.86 and 3.46 were prepared,and the specimens were treated with different carburizing processes.The hardness gradient,microstructure and residual stress of the specimens before and after heat treatment were characterized.The carburizing process was simulated by DEFORM-HT heat treatment simulation software.By comparing the hardness gradient and microstructure distribution obtained by experiment and simulation,the feasibility and correctness of numerical simulation for carburizing heat treatment of notched specimens are verified,and the simulation results complement each other.The stress-strain curves and S-N curves of notched specimens treated by different carburizing processes were obtained by tensile test and rotary bending fatigue test.In addition,the effects of carburizing process and notch stress concentration on notch sensitivity and fatigue properties were comprehensively analyzed by observing the fatigue fracture through SEM,and the evolution of organization and hardness during the fatigue process were investigated.Based on the above research,the following conclusions were obtained:(1)The notch shape characteristics affect the carbon distribution,hardness field,microstructure field and stress field of carburized specimens.(2)When subjected to axial tensile load,the tensile strength of Kt=1.86 and 3.46notched specimens under the quenching and tempering process increased by～30%compared with the smooth specimens,showing the’notch strengthening’effect;When the effective case depth of the carburized notched specimen is 0～0.5mm,the strengthening effect gradually weakens;when the effective case depth is 0.5～1.5mm,the effect changes from’notch strengthening’to’notch weakening’.(3)The fatigue limit of the two notched specimens both showed a trend of first increase and then decrease,and the peak value of notched fatigue limit appeared when the relative case depth was within the range of 0.1～0.15.By observing the fatigue fracture of notched specimens,it is found that the fatigue sources of uncarbonized specimens and carburized specimens are all on the surface of notched roots,and both are multi-source fractures.(4)During the fatigue process,the residual austenite on the surface of carburized specimens is gradually consumed,and its transformation has a critical value.As the fatigue process progresses,the surface hardness gradually increases.After 10~7cycles,the surface hardness of the smooth part is higher than that of the notch bottom surface,which indicates that the hardness of the notch surface slightly higher than that of the smooth surface after carburizing is not enough to make the fatigue performance of the notch specimens better than that of the smooth specimens.