Study On Heat Treatment Micro Structure And Dimensional Stability Of 8Cr4Mo4V Steel

8Cr4Mo4V steel is a high-alloy steel with excellent comprehensive properties.It has the advantages of high fatigue life,high temperature hardness,high strength and high toughness.The high temperature hardness can maintain 58HRC at 400?.8Cr4Mo4V steel is the most commonly used aero-engine high-temperature bearing steel.It is widely used in aero engine spindle bearings with the operating temperature below 316? and the DN value below 2.4 ×106.In this paper,the characteristics of dimension,residual stress and microstructure changes were measured and analyzed respectively at various operating temperatures.The main factors causing the dimension change of bearing ring were identified.The effect of blasting on microstructure,surface dimension,residual stress and surface roughness was studied by sandblasting 8Cr4Mo4V steel after heat treatment.The effect of stress relief treatment on the microstructure and residual stress of sandblasted bearing rings was studied.The characteristics of residual stress generated during heat treatment were analyzed by measuring the surface residual stress caused by bearing quenching and tempering.Finally,the bearing ring was subjected to stress and tissue stabilization treatment,and the dimension change law at the working temperature was measured to verify the beneficial effect of the stable phase and stable residual stress on the dimension stability of the bearing ring.Results show that the residual stress release and phase transformation after heat treatment are the main factors that cause the bearing dimension change,and the two factors also affect each other.Under stress-free conditions,the phase transition at the operating temperature is mainly the precipitation of filamentous carbides in the martensite,the volume of the bearing ring is reduced,and the height is reduced.Under the operating temperature,the precipitation of filamentous carbides in steel is caused by the decomposition of martensite.The precipitation of filamentous carbides causes volume shrinkage,dimension increase of the inner diameter,dimension decrease of the outer diameter,but the release of residual compressive stress will cause the inner diameter and outer diameter increase.Under the combined effect of phase change and residual stress release,the range of change in the inner diameter of the bearing ring increases,while the range of change in the outer diameter decreases.The stabilization treatment can stabilize the residual stress generated during the bearing processing.After the phase stability and stress stabilization treatment of the bearing ring,the residual stress of the inner surface and the outer surface of the bearing ring changes little under the operating temperatures.And the dimension changes of the bearing do not exceed 1?m.