Research On Cryogenic Treatment Process Of Metal Material And Its Micro Mechanism

The development trend of China's modern manufacturing industry is to create a green and environmentally friendly manufacturing system.Cryogenic treatment is an extension of the metal heat treatment process.It is a clean,environmentally friendly and inexpensive metal processing process.It is gradually recognized by the manufacturing industry and plays an important role in the field of machinery manufacturing.effect.Cryogenic treatment can control the microstructure of the metal by controlling the temperature,so as to achieve the purpose of improving the comprehensive performance of the metal material.The research work of metal heat treatment process has become more and more complete,but for the effect of cryogenic treatment on the mechanical properties of materials,the strengthening mechanism of cryogenic treatment on different material systems?such as different crystal structures,single/multiphase structure,and different stress states?Research in other areas is inconclusive,which greatly limits the development and application of cryogenic treatment.In order not to change the shape of the material,to further tap the potential of the material,improve the service life and life of the material,explore the influence of cryogenic treatment on the mechanical properties of metal materials,find the optimal cryogenic treatment process parameters,which attracts a large number of researchers'attention.Enabled the cryogenic treatment to be further applied in the industrial field.In this paper,fcc structure,single-phase structure of pure copper,bcc structure,combine martensite and austenite,two-phase structure of boron-molybdenum high-strength steel and hcp structure,with?-Mg as the matrix,the second phase is composed of?-Mg₁₇Al₁₂ AZ31B magnesium alloy is the research object.Cryogenic treatments with different cooling rates,different holding times,and different cooling methods were carried out.Subsequently,the macroscopic mechanical properties of the specimens after different cryogenic treatments were tested.Using three-dimensional optical full-field strain measurement and numerical simulation to explore the evolution characteristics of the strain distribution of the material during the cryogenic treatment and the relationship between the temperature field and the strain field during the cryogenic treatment,combined with the macroscopic mechanical properties,from the mechanical point of view,explain the cryogenic treatment Reasons for the influence of mechanical properties of metal materials.The following main conclusions are obtained:?1?Cryogenic cooling rate,holding time and stress state of annealed pure copper have an effect on the mechanical properties after cryogenic treatment.At the same holding time,when the cooling rate is 15?/min,the yield strength of the sample is increased by 59.03MPa and the tensile strength is increased by 4.5MPa;with the extension of the cooling time,the yield strength is increased by 72.92MPa and by14.68MPa.After cryogenic treatment of 5%pre-deformed samples,the yield strength increased by 73.53MPa;after cryogenic treatment of 10%pre-deformed samples,the yield strength increased by 155.1MPa.The results of numerical simulation show that the cryogenic treatment will significantly change the stress/strain distribution of the material.After cryogenic treatment,the stress shows the"external tensile and internal pressure"distribution,and the residual stress difference between the surface layer and the core becomes more obvious,which leads to the trial The intensity of the sample is significantly improved.?2?Cryogenic treatment has an effect on the mechanical properties and deformation behavior of boron-molybdenum high-strength steel.After cryogenic treatment,the strength of the upper yield point of the specimen increased by 65.3MPa,the lower yield point increased by 15.4MPa,and the tensile strength increased On the other hand,the"yield drop"phenomenon of boron-molybdenum high-strength steel after cryogenic treatment is significantly enhanced,which is shown by the difference between the upper and lower yield points from 11MPa to 160.9MPa;there is basically no change in uniform plasticity.According to the DIC test results before and after cryogenic cooling,in the"yield drop"stage,the strain evolution characteristics of the boron-molybdenum high-strength steel after cryogenic treatment evolved from a uniform strain state to a local stress concentration state,and the local stress level increased significantly;at the yield platform stage The strain distribution and evolution characteristics before and after cryogenic treatment were basically unchanged,and the deformation coordination ability was reduced.Combined with the micro-mechanism of cryogenic treatment of steel materials,it can be seen that after cryogenic treatment,the residual austenite in the material changes to martensite,and carbon atoms segregate and precipitate to form carbide particles,which leads to the increase of the strength of boron-molybdenum high-strength steel,"yield drop"is significantly enhanced.?3?Cryogenic treatment has no significant effect on the mechanical properties and deformation behavior of AZ31B magnesium alloy.The deformation behavior of the AZ31B magnesium alloy samples before and after cryogenic treatment was continuous yielding;the mechanical properties of the samples that were immersed in liquid nitrogen for 1.5h in the RD direction were the most obvious,and the yield strength was increased by 2.95MPa;the samples that were immersed in liquid nitrogen for 45min in the TD direction The strength improvement is most obvious,and the yield strength is increased by 1.2MPa.From the results collected by DIC,after cryogenic treatment,whether in the RD or TD direction,the stretching process is basically a uniform distribution of strain.Due to the short cryogenic treatment time,the temperature distribution of the plate sample is uniform,the strength of the AZ31B magnesium alloy is high,and there is no second phase precipitation within the material,so the cryogenic process used in this paper has not produced mechanical properties and deformation behavior of the AZ31B magnesium alloy Significantly affected.