Research On Cold Deformation Mechanical Behavior And Strengthening Parameters Of Mn18Cr18N Steel

The higher strength requirement of heavy generator retaining rings made of Mn18Cr18 N austenitic stainless steel is obtained by cold deformation strengthening. However, the yield ratio value of Mn18Cr18 N austenitic stainless steel is close to 1 gradually during the unidirectional tensile deformation, which will limit the requirement unidirectional tensile deformation of cold deformation strengthening. Therefore, research on mechanical behavior and microscopic mechanism of Mn18Cr18 N austenitic stainless steel under cold complex loading conditions has practical significance. The main contents of this paper are as follows:Firstly, Mn18Cr18 N austenitic stainless steel unidirectional loading test are carried out. The stress-strain curves of the tensile and compressive stress-strain curves respectively are obtained. It is basic test, which purpose is to make reference and comparison for the follow-up of other tests.Secondly, The Bauschinger effect of Mn18Cr18 N austenitic stainless steel were investigated when the small strain amplitude of cyclic loading. The effects of pre-strain modes, pre-strain levels and cycles on the Bauschinger effect are analyzed. The result indicate that Bauschinger effect exist during forward and reverse loading. In the pre-strain range 0.005 to 0.08, with the increase of strain amplitude, Bauschinger effect increases generally. The effect of prestrain styles on the Bauschinger effect is not obvious. The Bauschinger stress parameter ??of Mn18Cr18 N austenitic stainless steel increases fastest between the first and second cycles, then increase slows down and tends to be stable.In order to investigate the cold deformation strengthening by complex loading paths of Mn18Cr18 N austenitic stainless steel, compression-tensile deformation behavior of Mn18Cr18 N austenite stainless steel at room temperature has been investigated by compression and tensile consecutive loading deformation experiments with the first compressive reduction range of0%-40% and the second tensile range to fracture. Microstructure evolution,deformation dislocations, fracture behavior and mechanisms have been analyzed by OM, SEM and TEM. The results indicate that the subsequent tensile yield stress and the maximum tensile stress in the uniform plastic deformation stage,the reduction of cross sectional area and elongation increase at first then decrease with increase of the compressive deformation. When the compressive deformation increases up to the critical reduction of 25%, the subsequent tensile yield stress and the maximum tensile stress reach up to the maximum values of1039.97 and 1439.20 MPa respectively, and the reduction of cross sectional area and the elongation also reach up to the maximum values of 68.99% and 73.80%respectively. Macro- and micro- fractures and microstructure were observed by OM and SEM. When the compressive deformation is less than the critical reduction, appearance of fractures characterized by the cup-cone shaped macroscopic fracture profiles, the dimpled microscopic fracture surfaces and the elongated grains. When the compressive deformation is greater than the critical reduction, fractures morphology is distinguished by the flat macroscopic fracture profiles, the crystalline microscopic fracture surfaces and the equiaxed grains with a lot twin structures. The results of TEM indicate that several dislocation configurations with different density formed by dislocation slip when the compressive reduction is lower. Dislocation pile-up can be observed in the subsequent broken tensile specimen. Cross twins emerge in the specimen compressed up to 35% reduction. Twins with high density dislocation tangles arranged in parallel in the subsequent broken tensile specimen.Finally, The cyclic hardening and softening characteristics of Mn18Cr18 N steel were studied. It showed that the cyclic stress is higher than that under the uniaxial loading, Mn18Cr18 N steel can be strengthened by cyclic loading.compensation reinforcement strain increases with the increase of initial strain during different styles of strain alternates. Blindly increasing cycles is less effective to strengthening of Mn18Cr18 N steel. The best repeated deformation process of Mn18Cr18 N steel retaining rings is shrinkage- expansion process.And the 3D trength reference diagram of necking-bulging technology of the retaining ring is obtained.