Fundamental Theory And Experimental On Hydraulic Necking And Bulging Repeated Deformation Strengthening Technology Of Large Generator Retaining Ring

With the development of energy and power demand is increasingly urgent,coupled with the instability of the international situation,the localization and stabilization of generator retaining ring has become an urgent problem to be solved.As a large-scale high-end forging,the production process of retaining ring is very complex,and there are many difficulties.In the cold deformation strengthening process,the yield strength ratio of the retaining ring is close to 1.In order to reduce the yield ratio of retaining ring products and maximize the strengthening potential of retaining ring materials,Mn18Cr18 N retaining rings steel was taken as the research object in this paper.The mechanical properties and microstructure evolution of Mn18Cr18 N steel under complex loading paths were analyzed based on the deformation strengthening mechanism of tension,compression and rolling.Then,the hydraulic necking and bulging process of retaining ring were studied by the combination of numerical simulations and physical tests.The deformation laws of retaining ring with different die structures and initial sizes of ring blanks were obtained.The load instability and geometric instability critical conditions of retaining ring during hydraulic bulging were studied.Finally,on this basis,a new technology of hydraulic necking an bulging repeated deformation strengthening of large generator retaining ring was first proposed,which provides a new idea for cold deformation strengthening method of retaining ring.Firstly,the simple path loading tests were carried out on Mn18Cr18 N steel.The results showed that the initial yield strength of Mn18Cr18 N steel was about604 MPa at room temperature.The maximum flow stress before necking was1154 MPa.The elongation and reduction of area were 61.7% and 61.4%,respectively.The fracture modes were ductile fracture.In the case of compression or rolling at room temperature,the deformation was mainly plane slip when the deformation was small.When the deformation was greater than30%,twinning was activated and became an important deformation mechanism of Mn18Cr18 N steel.Then,the complex path loading tests were carried out on Mn18Cr18 N steel.It was found that in the process of uniaxial compression tension continuous loading,after different degrees of compression deformation,the strength and plastic of the second stage of tension increased first and then decreased with the increase of the first stage compression amounts.When the compression amount was 25%,both of strength and plasticity reached the extreme value,were 1040.0MPa and 1439.2 MPa respectively.The elongation and reduction of area reached the extreme values when the compression were 25% and 20%,which were 73.8 % and 68.9 % respectively.The tensile yield strength and maximum flow stress of Mn18Cr18 N steel increased with the increase of rolling amount in the rolling and tension discontinuous biaxial loading process.When the rolling amounts were more than 30%,the increase of maximum flow stress decreased gradually.In the process of uniaxial cyclic loading,the cyclic strengthening and softening behavior of Mn18Cr18 N steel mainly depended on the internal stress components in the matrix rather than the equivalent stress components.When the cyclic strain amplitude was small,Mn18Cr18 N steel was mainly characterized by cyclic stress softening.When the cyclic strain amplitude was large,Mn18Cr18 N steel was mainly characterized by cyclic stress strengthening.Loading path was an important factor affecting the mechanical behavior of Mn18Cr18 N steel.Under uniaxial cyclic loading,every alternation of strain direction will lead to a certain degree of strength loss.Under the same cumulative plastic strain,the smaller the cyclic strain amplitude and the more alternating times of strain direction,the more unfavorable the strengthening of the material.The distribution of equivalent strain in retaining ring is highly similar in the process of hydraulic bulging and necking.With the increase of hydraulic pressure,the inner wall of ring blanks entered the plastic deformation state first and then expanded to the outer wall gradually.The equivalent strain of inner wall was always greater than that of outer wall,and the equivalent stress of inner wall was always greater than that of outer wall.Die cone angle was an important factor affecting the shape of retaining ring after hydraulic necking and bulging.When the die with small angle cone angle was used in hydraulic necking,the ring blank often presented a bulging shape after necking.When the die with large angle cone angle was used in hydraulic necking,the ring blank was trumpet shaped.In hydraulic bulging,with the increase of die cone angles,the ring billets transited from trumpet shape to bulging shape.The optimal die cone angles are 50°and 60°respectively for hydraulic necking and bulging.Then,the instability of cylinder-shaped retaining ring parts during hydraulic bulging is analyzed.The results showed that the global force instability of the cylinder was closely related to the initial radio of outer and inner diameter of the cylinder and the strain hardening exponent of the material.With the increase of the initial radio of outer and inner diameter and the strain hardening exponent of the material,the ability of the cylinder to resist force instability is stronger.The local load instability of the cylinder always occurred at the inner wall of the cylinder first,and then extended to the outer wall of the cylinder.The middle diameter layer of cylinder can be regarded as the equivalent force instability layer.When the local force instability occurs at the equivalent force instability layer of the cylinder,the cylinder has global force instability.The cylinder is in metastable state after the force instability.With the increase of deformation,geometric instability of cylinder parts will occur.The geometric instability of the cylinder also occurred at the inner wall first and then extended to the outer wall.When the equivalent strain of the external wall reaches 2n,the cylinder becomes fully geometric instability.Finally,the loading path ratios of repeated hydraulic bulging and necking process for Mn18Cr18 N steel retaining ring were analyzed and optimized by numerical simulation.When the cone angle of hydraulic necking die and bulging die are 45° and 60° respectively,the final shapes of retaining ring change from trumpet shape to bulging shape with the increase of first stage necking amount.The results show that the forming effect of repeated hydraulic bulging and necking process of retaining ring is relatively good when the deformation amounts difference between the necking and bulging is small.The feasibility of repeated hydraulic bulging and necking process was verified by shrinkage test.The test results show that the strength of the retaining ring is obviously strengthened after the repeated hydraulic bulging and necking process,and its strength and plasticity are better than those of the retaining ring after direct hydraulic bulging.The yield strength and tensile strength of the retaining ring are 1168.1 MPa and 1292.2 MPa respectively when the middle diameter deformation of the first stage of hydraulic necking and the second stage of hydraulic bulging are 11.0% and 11.3%,respectively,which are 49.2 MPa and144.0 MPa higher than those of direct bulging.The yield strength ratio is 0.9,which is 7.2 % lower than that of direct bulging retaining ring.In terms of plasticity index,the elongation and area reduction of retaining ring are 28.2 %and 68.3 % respectively,which are 48.4 % and 16.4 % higher than that of directly bulging retaining ring.