Causes Analysis Of Cracks On The Finish Turning Ring Of Steel 42CrMo Yaw Bearing

Steel 42CrMo has become a commonly used material for large yaw bearing rings due to its good forging shape and overall mechanical properties after heat treatment.In a previous producing process of steel 42CrMo yaw bearing ring in the company,it was found that cracks appeared on the upper end surface near the inner diameter of the finish turming rings,which affected the rate of the qualified products.In this paper,the macroscopic position,chemical composition and distribution,macrostructure,microstructure,non-metallic inclusions,mechanical properties and other inspections for the cracked area of the steel 42CrMo yaw bearing ring are inspected and analyzed.Meanwhile,in order to find out the cause of the crack,and put forward corresponding solutions in combination with the actual production situation to improve the product qualification rate,the DEFORM-3D software is used.The platform performs a finite element simulation of the metal flow direction in the central region of the raw material during the forging process.The results obtained in the paper are as follows:The macroscopic,macrostructure,chemical composition,non-metallic inclusions and mechanical properties of the steel 42CrMo yaw bearing ring are in compliance with relevant national standards and customer requirements.The C,P,S and other elements on the radial section of the steel 42CrMo yaw bearing ring have different degrees of segregation,which is similar to the shape of the center shrinkage of the corrosion sample.The crack is located near the boundary between the inner diameter and the upper end surface of the yaw bearing ring of steel 42CrMo.It is located at the center shrinkage boundary of the raw material.It is a quenching crack.It is due to the serious segregation of the C content in the center shrinkage of the raw material and the local exceeding the national requirement C.The combination of the factors of content causes,and the slightly faster cooling rate at the junction of the inner diameter and the upper end surface have a certain effect on the formation of cracks compared to other parts.Although the forging process can remove most of the center shrinkage defect area of the steel 42CrMo yaw bearing ring material,the small center shrinkage defect area remaining near the intersection of the inner diameter and the upper end surface cannot be completely removed.The DEFORM-3D simulation results are a favorable illustration of the unique shape of the steel 42CrMo yaw bearing ring macrostruture,which further confirmes the correctness of the formation of the quenching crack of the bearing ring proposed in this paper.In combination with the actual production,the following two preventive measures are proposed:First,it is recommended that the steel mill reduce the center shrinkage defect of the continuous casting round billet by controlling the speed of the crystallization process or the casting speed,in order to facilitate the steel 42CrMo yaw bearing ring to be totally removed during the blanking process.Second,under the condition of not changing the overall machining allowance,the method of removing the symmetrical size of the upper and lower end surfaces of the existing steel 42CrMo yaw bearing ring during the rough and finish turning processes is changed to increase the upper end machining size,which is a method of removing the asymmetric size removal of the upper end surface where cracks have occurred and lower one.In the end,the company adopted the continuous casting round billet with the smaller diameter center shrinkage field provided by the steel mill,and there was no quenching crack in the subsequent production process of the 42CrMo steel yaw bearing ring after finish turning.