Method for high cycle fatigue strengthening of metals induced by strain induced by rate dependence

Abstract

The present invention provides a method for strengthening metal high-cycle fatigue by inducing rate-dependent strain. The method is applicable to most metals and their components. The appropriate strain rate is determined by a simple specimen tensile test method, and the metal is pre-strained by the strain rate, and the strengthening effect of pre-straining once or multiple cycles of pre-straining can be performed. Compared with the prior art, the present invention has strong applicability and is suitable for metal products or components of different materials and structures, and is suitable for different service environments of metals. The present invention focuses on exploring the mechanism of strain rate in strain strengthening. The strengthening mechanism mainly improves the plane sliding ability of dislocations within metal grains and the uniform deformation ability of the lattice, which helps to delay the initiation of fatigue sources; and forms a complete, closed, and independent dislocation structure at the grain boundary and phase interface, further hindering the expansion of fatigue cracks. The present invention can improve the high-cycle fatigue performance of metal materials and components without changing existing mechanical equipment, which is very convenient for application and promotion, and can still maintain the good plasticity of the target metal while significantly improving fatigue performance.

Description

Technical Field

[0001] The invention relates to a method for improving the high cycle fatigue performance of metal materials and structures through a physical means of rate-dependent strain, and belongs to the technical fields of materials, machinery and the like. Technical Background

[0002] Many metal components in modern engineering work under cyclic loads, such as crankshafts, connecting rods, rotors, blades, bridges, axles, nuclear reactors and power generation equipment. Unlike low-cycle fatigue, the maximum cyclic stress of high-cycle fatigue is lower than the yield strength of the metal. High-cycle fatigue fracture usually occurs suddenly, and no matter whether it is brittle or ductile material, no obvious macroscopic plastic deformation will occur, which often leads to catastrophic consequences. Many research results have shown that metals will break after 10 7 Fracture may still occur in the high cycle fatigue stage above cycles. How to improve the high cycle fatigue performance of metals is of great significance and is a hot topic of research.

[0003] On the other hand, after a period of service, metal components will often experience a significant reduction in residual fatigue life due to the release of internal residual stress. How to quickly improve the residual fatigue performance of metal without changing the original structure is a research difficulty.

[0004] Patent CN106769440A discloses a method for processing strain-hardened tensile specimens, which achieves strain hardening of the material by stretching the specimen to a certain stress and then maintaining pressure for a long time. Patent CN106048412B discloses a method for manufacturing phase-change hardened cold-processed high-strength steel, steel pipe and steel pipe, which provides a new type of steel and steel pipe composition ratio, and in the heat treatment stage, through phase-change hardening generated during quenching and strain hardening generated during cold processing, martensite hardening and strain hardening produce a composite strengthening effect. Patent CN110595744A discloses a strain hardening process equipment, its operation method and test system, which adjusts the mechanical supercharger to keep the hydraulic pressure at a stable value, and can maintain a constant hydraulic pressure to perform cold-stretching hardening treatment on the metal wall material of the container, thereby ensuring the strengthening effect. Patent CN109777936A discloses a method for ultra-low temperature strain hardening of martensitic stainless steel. The yield strength measured in a low temperature environment is used as a unidirectional tensile stress and loaded on the sample according to a loading cycle. The number of cycles is not more than 30 times. Under the premise of not reducing plasticity, the low temperature yield strength of stainless steel is improved. There are many patents on different forms of strain hardening, but there is a lack of research and introduction on the effect of strain rate on strain hardening. Therefore, the strengthening effect of rate-dependent strain on high-cycle fatigue, and the convenient and rapid improvement of the residual fatigue strength of metals without changing the original structure, is an urgent problem to be solved. Summary of the invention

[0005] Technical problem: The purpose of the present invention is to provide a method for high-cycle fatigue strengthening of metals by inducing rate-dependent strain. This method is applicable to most metals and their components. The appropriate strain rate is determined by a simple specimen tensile test method, and the metal is pre-strained at this strain rate. The strengthening effect of pre-straining once or multiple cycles of pre-straining can be performed, and the material can still maintain good plasticity.

[0006] Technical solution: The present invention provides a method for strengthening metal high cycle fatigue by rate-dependent strain induction, comprising the following steps:

[0007] (1) Prepare the parent materials of metal products or components into several standard round bar tensile specimens and several standard round bar fatigue specimens according to GB / T 228.1-2010 and GB / T 3075-2008 respectively;

[0008] (2) The long-term service environment of metal products or components is simulated in a temperature control box. According to the standard GB / T 228.1-2010, several tensile specimens are subjected to tensile tests in this temperature environment to determine the upper yield strength (R eH ) and lower yield strength (R eL );

[0009] (3) Several fatigue specimens of the same material were subjected to pre-strain tensile tests under the same temperature environment, with tensile strain rates of 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 , 10 -2 s -1 , 10 -1 s -1 , the maximum tensile stress is the R eL , maintain pressure for 5s, and then unload immediately;

[0010] (4) The pre-strained specimen is subjected to a cyclic loading test, and the maximum cyclic stress is (R eH +R eL ) / 2, the minimum cyclic stress is -[(R eH +R eL ) / 2-100MPa], each set of cyclic loading until cyclic softening occurs;

[0011] (5) Determine the strain rate of the pre-strain tensile loading under the longest cyclic hardening condition and define it as It is used as the rate-dependent strain hardening standard of the metal product or component at the temperature, and the next step is to perform a single pre-strain or multiple cyclic pre-straining action;

[0012] (6) The strain rate of the metal at this temperature determined by the above steps Pre-strain treatment of metal products or components, the measured R eL The stress magnitude is taken as the maximum loading stress, and the metal product or component is cyclically loaded in a tension-tension loading mode with a stress ratio of 0, and a pre-strain or multiple cyclic pre-strain strengthening effect is performed, and the number of cycles is not higher than that in the step (4) test. Maximum number of hardening cycles under applied prestrain.

[0013] Beneficial effects of the present invention: Compared with the prior art, the present invention has the following advantages:

[0014] 1. It has strong applicability and is suitable for metal products or components of different materials and structures, and is suitable for different service environments of metals.

[0015] 2. This method is applicable to most metals and their components. The appropriate strain rate is determined by simple tensile tests of several base material specimens, and the metal is pre-strained at this strain rate. The strengthening effect of pre-straining once or multiple cycles of pre-straining can be performed, while the material can still maintain good plasticity.

[0016] 3. The present invention provides a method for high-cycle fatigue strengthening of metals by inducing strain-dependent strain. Compared with traditional work hardening, it focuses on exploring the mechanism of the strain rate in strain strengthening and facilitates application and promotion.

[0017] 4. The method is simple to operate and can effectively improve the high-cycle fatigue performance of metal products or components. The appropriate hardening strain rate is determined through experiments, and pre-strain or cyclic pre-strain treatment is performed based on this strain rate. The mechanism of fatigue strengthening is mainly to enhance the plane sliding ability of dislocations within metal grains and the uniform deformation ability of the lattice, which helps to delay the initiation of fatigue sources; and form a complete, closed, and independent dislocation structure at the grain boundary and phase interface, further hindering the expansion of fatigue cracks. Implementation

[0018] The present invention provides a method for strengthening metal high cycle fatigue by inducing rate-dependent strain. After the method of the present invention is used to strengthen metal products or components of different materials and structures under different service environments by rate-dependent pre-strain or cyclic pre-strain, the high cycle fatigue performance of the metal is significantly improved. The specific steps are as follows:

[0019] (1) Prepare the parent materials of metal products or components into several standard round bar tensile specimens and several standard round bar fatigue specimens according to GB / T 228.1-2010 and GB / T 3075-2008 respectively;

[0020] (2) The long-term service environment of metal products or components is simulated in a temperature control box. According to the standard GB / T 228.1-2010, several tensile specimens are subjected to tensile tests in this temperature environment to determine the upper yield strength (R eH ) and lower yield strength (R eL );

[0021] (3) Several fatigue specimens of the same material were subjected to pre-strain tensile tests under the same temperature environment, with tensile strain rates of 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 , 10 -2 s -1 , 10 -1 s -1 , the maximum tensile stress is the R eL , maintain pressure for 5s, and then unload immediately;

[0022] (4) The pre-strained specimen is subjected to a cyclic loading test, and the maximum cyclic stress is (R eH +R eL ) / 2, the minimum cyclic stress is -[(R eH +R eL ) / 2-100MPa], each set of cyclic loading until cyclic softening occurs;

[0023] (5) Determine the strain rate of the pre-strain tensile loading under the longest cyclic hardening condition and define it as It is used as the rate-dependent strain hardening standard of the metal product or component at the temperature, and the next step is to perform a single pre-strain or multiple cyclic pre-straining action;

[0024] (6) The strain rate of the metal at this temperature determined by the above steps Pre-strain treatment of metal products or components, the measured R eL The stress magnitude is taken as the maximum loading stress, and the metal product or component is cyclically loaded in a tension-tension loading mode with a stress ratio of 0, and a pre-strain or multiple cyclic pre-strain strengthening effect is performed, and the number of cycles is not higher than that in the step (4) test. Maximum number of hardening cycles under applied prestrain.

[0025] For a further understanding of the present invention, the present invention is further described below in conjunction with the accompanying drawings and specific embodiments. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A process flow chart of the method for pass rate-related strain-induced metal high cycle fatigue strengthening provided by the present invention;

[0027] Figure 2 It is the tensile stress-strain curve of the original 50# steel sample without pre-strain strengthening;

[0028] Figure 3 This is the tensile stress-strain curve of 50# steel after 100 cycles of 0-285MPa cyclic loading;

[0029] Figure 4 The cyclic strain response curves of 50# steel without pre-strain and with different rate-related pre-strain are -235MPa~335MPa;

[0030] Figure 5 The fatigue life of 50# steel without prestrain and with different rates of prestrain with stress ratio of 0.1 and stress amplitude of 402MPa;

[0031] Figure 6 Transmission electron microscope images of 50# steel after different rates of related pre-strain;

[0032] Figure 7 Transmission electron microscope images of 50# steel without prestrain and after constant amplitude fatigue fracture with different rate-related prestrain;

[0033] Figure 8 Electron backscatter diffraction patterns of the fatigue crack source positions of 50# steel without prestrain and with different rate-related prestrain-constant amplitude. Specific embodiments

[0034] A method for high cycle fatigue strengthening of metal induced by rate-dependent strain, a specific embodiment of the present invention is:

[0035] (1) Cut the 50# steel ingot after hot rolling and prepare several standard round bar tensile specimens and several standard round bar fatigue specimens according to standards GB / T 228.1-2010 and GB / T 3075-2008;

[0036] (2) The long-term service environment of metal products or components is simulated in a temperature control box. According to the standard GB / T 228.1-2010, several tensile specimens are subjected to tensile tests in this temperature environment to determine the upper yield strength (R eH ) is 385MPa, and the lower yield strength (R eL ) is 285MPa;

[0037] (3) Several fatigue specimens of the same material were subjected to pre-strain tensile tests under the same temperature environment, with tensile strain rates of 10 -5 s -1 , 10-4 s -1 , 10 -3 s -1 , 10 -1 s -1 , the maximum tensile stress is the R eL That is, 285MPa, holding pressure for 5s, and then immediately unloading;

[0038] (4) The pre-strained specimen is subjected to a cyclic loading test, and the maximum cyclic stress is (R eH +R eL ) / 2 is 335MPa, and the minimum cyclic stress is -[(R eH +R eL ) / 2-100MPa], i.e. -235MPa, each set of cyclic loading is carried out until cyclic softening occurs. The cyclic strain response is as follows: Figure 4 ;

[0039] (5) Determine the strain rate of the pre-strain tensile loading under the longest cyclic hardening condition and define it as That is 10 -4 s -1 ,like Figure 4 , which is used as the rate-dependent strain hardening standard for the metal product or component at the temperature;

[0040] (6) The strain rate of the metal 50# steel at this temperature determined by the above steps That is 10 -4 s -1 , pre-strain treatment of metal products or components, the measured R eL That is, 285 MPa is used as the maximum loading stress, and the metal product or component is loaded cyclically in a tension-tension loading mode with a stress ratio of 0, and a pre-strain or multiple cyclic pre-strain strengthening effect is performed, and the number of cycles is not higher than that in the step (4) test. That is 10 -4 s -1 The maximum number of hardening cycles under applied prestrain is 100.

[0041] (7) Perform a rate-dependent fatigue strengthening test after a pre-strain, such as Figure 5 Under the fatigue load of stress ratio of 0.1 and stress amplitude of 402MPa, compared with the case without pre-strain, the rate-dependent pre-strain has different degrees of fatigue strengthening on the metal, among which the strain rate That is 10 -4 s -1 The pre-strain strengthening is the largest, which significantly improves the fatigue life of 50# steel.

[0042] (8) After 100 cycles of cyclic strengthening, the tensile test results (such as Figure 3) shows that the metal retains the good plasticity of the parent material.

[0043] (9) Strengthening mechanism: rate-dependent prestrain changes the dislocation characteristics inside the metal (e.g. Figure 6 ), and further evolve in subsequent fatigue loading to form different dislocation structures (such as Figure 7 ) and lattice reconstruction (e.g. Figure 8 ). Strain rate That is 10 -4 s -1 Under the pre-strain, the planar sliding ability of dislocations within the grains and the uniform deformation ability of the lattice are improved during the metal fatigue process, which helps to delay the initiation of fatigue sources; and a complete, closed and independent dislocation structure is formed at the grain boundary and phase interface, further hindering the expansion of fatigue cracks.

[0044] The above description is only a preferred example of the present invention, which is only illustrative and non-restrictive to the present invention. Those skilled in the art will understand that many modifications and adjustments may be made within the spirit and scope defined by the claims of the present invention, but all of them will fall within the scope of protection of the present invention.

Claims

1. A method for strengthening metal high cycle fatigue by rate-dependent strain induction, the method is implemented as follows: (1) Prepare the parent materials of metal products or components into several standard round bar tensile specimens and several standard round bar fatigue specimens according to GB / T 228.1-2010 and GB / T 3075-2008 respectively; (2) Use a temperature control box to simulate the long-term service environment of metal products or components. According to standard GB / T 228.1-2010, several tensile specimens are subjected to tensile tests in this temperature environment to determine the upper yield strength R of the parent material of the metal product or component. eH and lower yield strength R eL ; (3) Several fatigue specimens of the same material were subjected to pre-strain tensile tests under the same temperature environment, with tensile strain rates of 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 , 10 -2 s -1 , 10 -1 s -1 , the maximum tensile stress is the R eL , maintain pressure for 5s, and then unload immediately; (4) The pre-strained specimen is subjected to a cyclic loading test, and the maximum cyclic stress is (R eH +R eL ) / 2, the minimum cyclic stress is -[(R eH +R eL ) / 2-100MPa], each set of cyclic loading until cyclic softening occurs; (5) Determine the strain rate of the pre-strain tensile loading under the longest cyclic hardening condition and define it as It is used as the rate-dependent strain hardening standard of the metal product or component at the temperature, and the next step is to perform a single pre-strain or multiple cyclic pre-straining action; (6) The strain rate of the metal fatigue specimen at this temperature determined by the above steps Pre-strain treatment of metal products or components, the measured R eL The stress magnitude is taken as the maximum loading stress, and the metal product or component is cyclically loaded in a tension-tension loading mode with a stress ratio of 0, and a pre-strain or multiple cyclic pre-strain strengthening effect is performed, and the number of cycles is not higher than that in the step (4) test. Maximum number of hardening cycles under applied prestrain.

2. A method for high cycle fatigue strengthening of metals by rate-dependent strain induction according to claim 1, characterized in that: It has strong applicability and is suitable for metal products or components of different materials and structures, and is suitable for different service environments of metals.

3. A method for high cycle fatigue strengthening of metals by rate-dependent strain induction according to claim 1, characterized in that: This method is applicable to most metals and their components. The appropriate strain rate is determined by simple tensile tests on several base material specimens, and the metal is pre-strained at this strain rate. It can be strengthened by pre-straining once or by multiple cycles of pre-straining, while the material can still maintain good plasticity.

4. A method for high cycle fatigue strengthening of metals by rate-dependent strain induction according to claim 1, characterized in that: Compared with traditional work hardening, the focus is on exploring the mechanism of strain rate in strain hardening, which facilitates application and promotion.

5. A method for high cycle fatigue strengthening of metals by rate-dependent strain induction according to claim 1, characterized in that: The method is simple to operate and can effectively improve the high-cycle fatigue performance of metal products or components. The appropriate hardening strain rate is determined through experiments, and pre-strain or cyclic pre-strain treatment is performed based on this strain rate. The mechanism of fatigue strengthening is mainly to enhance the plane sliding ability of dislocations in metal grains and the uniform deformation ability of the lattice, which helps to delay the initiation of fatigue sources; and to form a complete, closed and independent dislocation structure at the grain boundary and phase interface, which further hinders the extension of fatigue cracks.

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