Surface strengthening method for improving spinning fatigue strength of shaft steel

Abstract

The invention relates to a surface strengthening method for improving the rotary bending fatigue strength of shaft steel, which comprises the following steps: step 1, pretreatment: carrying out finish machining on the shaft steel, polishing the shaft steel with abrasive paper, then cleaning a sample with acetone and absolute ethyl alcohol solvents to remove oil stains and impurities on the surface, cleaning the sample with an ultrasonic cleaning machine, and air-drying the cleaned sample; and 2, electric pulse auxiliary ultrasonic surface rolling treatment is conducted, the sample is clamped on a lathe, currents are connected to the two ends of the sample through a pulse power source, technological parameters are set, lubricating oil is adopted for lubrication in the rolling process, the surface of the sample is machined for multiple times, and the surface-strengthened shaft steel sample is obtained. According to the method, a gradient plastic deformation layer, a hardened layer and a residual stress layer are introduced by adopting a mode of treating the surface of the shaft steel through electric pulse-assisted ultrasonic surface rolling, so that the expansion of cracks can be obviously inhibited, and the rotary bending fatigue strength of a sample can be effectively improved while the surface hardness and the residual stress are improved.

Description

Technical Field

[0001] The invention relates to the technical field of metal surface strengthening, in particular to a surface strengthening method for improving the rotational bending fatigue strength of shaft steel. Background Art

[0002] In the fields of aerospace, transportation, large-scale infrastructure, etc., shaft parts are used on a large scale and serve for a long time in harsh environments such as high temperature, high pressure and corrosive media. They are affected by factors such as mechanical vibration or dynamic loads, and are subjected to severe alternating loads. Their actual service life is often relatively low.

[0003] Bending fatigue is a common phenomenon in engineering. In the early stage of fatigue, the internal structure will change, followed by the nucleation and expansion of fatigue cracks. When the fatigue cracks expand instability to a certain extent, fatigue fracture will occur. Rotational bending fatigue is one of the main failure forms.

[0004] Therefore, it is of great practical significance to promote the surface strengthening method that can improve the rotational bending fatigue strength of shaft steel. Summary of the invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide a surface strengthening method for improving the rotational bending fatigue strength of shaft steel, which can improve the surface hardness and residual stress and enhance the rotational bending fatigue strength of the sample.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A surface strengthening method for improving the rotational bending fatigue strength of shaft steel comprises the following steps:

[0008] Step 1: Pre-treatment: machine the shaft steel and polish it with 800-grit sandpaper, then clean the machined sample with acetone and anhydrous ethanol solvent to remove surface oil and impurities, and then clean it with an ultrasonic cleaner and air dry it;

[0009] Step 2: Electric pulse assisted ultrasonic surface rolling treatment: the workpiece after finishing and cleaning is clamped on the lathe and the WSM-400T pulse power supply is used to connect the current at both ends of the sample, so that the current root mean square density is 0.04A / mm 2 . Set the ultrasonic rolling process parameters, sample speed V 1 =35r / min, feed speed V 2 =1.2mm / min, ultrasonic vibration is provided by an ultrasonic generator (HK30G), the vibration frequency is 27KHz, the static pressure is 1000N, and the number of sample surface processing times is 7 times. Mobil600XP150 lubricating oil is used for lubrication during rolling, and the surface is strengthened to obtain the shaft steel sample;

[0010] Step 3: Organization and performance testing: The sample organization was observed using a NOVA NANOSEM 430 scanning electron microscope, the rotational bending fatigue strength of the sample was measured using a QBWP-10000X cantilever beam rotational bending fatigue testing machine, a microhardness tester (SCTMC HV-50) was used to measure the cross-sectional hardness gradient of the sample, and a transmission electron microscope (TEM, JEM 2100F-JEOL) was used at a voltage of 200 kV to observe the characteristics of the gradient organization of the cross section of the sample surface layer and measure the residual stress.

[0011] Furthermore, mechanical processing is required before step 1. The shaft steel workpiece is turned into a round rod specimen with a diameter of 9 mm and a length of 200 mm using a lathe. The workpiece surface needs to be finely processed so that the surface roughness of the workpiece reaches Ra0.8 or above. The milled test block is obtained and then polished with 1200-grit sandpaper to remove surface processing marks.

[0012] Furthermore, the cleaning solvent described in step 1 mainly comprises acetone and anhydrous ethanol, and then the cleaning is performed using an ultrasonic cleaning machine and air-dried.

[0013] Furthermore, in order to insulate the ultrasonic equipment described in step 2 from the sample, the rolling tool used is a silicon nitride ball with a diameter of 9 mm and a hardness of 78 HRC.

[0014] In general, the present invention has the following advantages:

[0015] 1. The present invention adopts electric pulse assisted ultrasonic surface rolling to treat the surface of shaft steel, which can effectively improve the rotational bending fatigue strength of the sample while improving the surface hardness and residual stress. The present invention adopts electric pulse assisted ultrasonic surface rolling, introduces a gradient plastic deformation layer, a hardening layer and a residual stress layer, which can significantly inhibit the expansion of cracks.

[0016] 2. The bending fatigue strength of the shaft steel sample prepared by the present invention is 865MPa, which is 53.09% higher than that of the untreated sample; the surface hardness after treatment is 624.5HV 0.2 The plastic deformation layer can reach 160μm, and the maximum residual stress is -849.5MPa. The treated samples are greatly improved. That is, after the electric pulse assisted ultrasonic surface rolling treatment of the present invention, the surface bending fatigue strength, surface hardness and residual stress of the shaft steel sample can be significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a cross-sectional microstructure diagram of the shaft steel before the implementation of the present invention;

[0018] Figure 2 is a cross-sectional microstructure diagram of the shaft steel obtained after the implementation of the present invention;

[0019] Figure 3 This is a comparison diagram of the surface hardness and maximum residual stress of the shaft steel obtained after the implementation of the present invention;

[0020] Figure 4 It is a comparison chart of the rotational bending fatigue life of the shaft steel obtained after the implementation of the present invention. DETAILED DESCRIPTION

[0021] As a new type of material processing surface technology, electric pulse assisted manufacturing technology applies high-density current to the material, allowing high-intensity energy to pass through its interior in a short period of time, producing an "electroplastic" effect, effectively improving the plasticity of the material, and has broad application prospects in actual production. Electric pulse assisted ultrasonic surface rolling strengthening technology is a more efficient surface strengthening technology, which is both efficient and feasible, and the process conditions are easy to achieve. Therefore, this patent proposes a method for electric pulse assisted ultrasonic rolling strengthening of the surface of shaft steel, which can not only improve the rotational bending fatigue strength of shaft steel specimens, but also improve its surface hardness and residual stress. At the same time, the mechanism research and experimental conclusions are applied to the surface strengthening of other metal materials, expanding the field of surface processing of electric pulse assisted ultrasonic surface rolling materials.

[0022] The present invention will be described in further detail below.

[0023] A surface strengthening method for improving the rotational bending fatigue strength of shaft steel comprises the following steps:

[0024] Step 1: Pretreatment: machine the shaft steel and polish it with 800-grit sandpaper, then clean the machined sample with acetone and anhydrous ethanol solvent to remove surface oil and impurities, then clean it with an ultrasonic cleaner and air dry it.

[0025] Step 2: Electric pulse assisted ultrasonic surface rolling treatment: the workpiece after finishing and cleaning is clamped on the lathe and the WSM-400T pulse power supply is used to connect the current at both ends of the sample, so that the current root mean square density is 0.04A / mm 2 . Set the ultrasonic rolling process parameters, sample speed V 1 =35r / min, feed speed V 2 =1.2mm / min, ultrasonic vibration is provided by an ultrasonic generator (HK30G), the vibration frequency is 27KHz, the static pressure is 1000N, and the number of sample surface processing times is 7. Mobil600XP150 lubricating oil is used for lubrication during rolling, and the surface is strengthened to obtain a steel sample for shaft.

[0026] Step 3: Organization and performance testing, the sample organization was observed using a NOVA NANOSEM 430 scanning electron microscope. The QBWP-10000X cantilever beam rotary bending fatigue testing machine was used to measure the sample's rotary bending fatigue strength. The fatigue life reached 107 cycles without damage. The stress intensity is the fatigue strength. A microhardness tester (SCTMC HV-50) was used to measure the cross-sectional hardness gradient of the sample, starting from the sample surface, with a depth gradient of 20μm, a maximum load of 200N, and a holding time of 15s.

[0027] The residual stress on the sample surface and along the layer depth was measured by the sin2ψ method using a transmission electron microscope (TEM, JEM 2100F-JEOL) at a voltage of 200 kV, a diffraction angle of 20°-90°, and an interval step of 0.02° / s. The residual stress can reach a depth and the strength increases significantly.

[0028] The cross-sectional morphology of the untreated shaft steel sample and the sample treated by the method of the present invention was observed under an electron microscope. Figure 1 , Figure 2 As shown. Figure 1 It can be seen that the untreated sample is composed of lath martensite, which contains a small amount of granular carbides. Figure 2 It can be seen that after the electric pulse assisted ultrasonic surface rolling treatment, an amorphous layer appears in the cross section of the shaft steel, and the surface grains are severely refined. In addition, the carbide size is significantly reduced, which is beneficial to improve the bending fatigue performance of the shaft steel.

[0029] The surface hardness and maximum residual stress measurement results are as follows Figure 3 As shown in the figure, it can be seen that the surface hardness and maximum residual stress of the shaft steel alloy treated in Example 1 are significantly improved compared with the control product. After the rotary bending fatigue test, the rotary bending fatigue SN diagram of the control product and the shaft steel sample treated in the embodiment is obtained, as shown in Figure 4 The bending fatigue strength is increased by 53.09% compared with the original sample, indicating that the electric pulse assisted ultrasonic surface rolling treatment can improve the bending fatigue strength, thereby improving the surface properties such as surface hardness and residual stress.

[0030] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A surface strengthening method for improving the bending fatigue strength of shaft steel, characterized in that: The following steps are included: Step 1: Pre-treatment: Finish the shaft steel and polish it with sandpaper, then clean the sample with acetone and anhydrous ethanol solvent to remove surface oil and impurities, and then clean it with an ultrasonic cleaner and air dry it; Step 2: Electric pulse assisted ultrasonic surface rolling treatment, the sample is clamped on the lathe and the current is connected to both ends of the sample with a pulse power supply, the process parameters are set, and lubricating oil is used for lubrication during rolling. The surface of the sample is processed multiple times to obtain a surface-strengthened shaft steel sample.

2. A surface strengthening method for improving the rotational bending fatigue strength of shaft steel according to claim 1, characterized in that: The surface roughness of the sample after fine machining reaches Ra0.8 or above.

3. The surface strengthening method for improving the rotational bending fatigue strength of shaft steel according to claim 1, characterized in that: The sandpaper used for grinding is 800 grit.

4. A surface strengthening method for improving the rotational bending fatigue strength of shaft steel according to claim 1, characterized in that: Process parameters include current root mean square density of 0.03-0.05A / mm 2 , sample rotation speed V1 = 30-40r / min, feed speed V2 = 1.0-1.4mm / min, ultrasonic generator vibration frequency is 25-30KHz, and static pressure is 900-1100N.

5. A surface strengthening method for improving the rotational bending fatigue strength of shaft steel according to claim 4, characterized in that: Process parameters include current RMS density of 0.04A / mm 2 , sample rotation speed V1 = 35r / min, feed speed V2 = 1.2mm / min, ultrasonic generator vibration frequency is 27KHz, and static pressure is 1000N.

6. The surface strengthening method for improving the rotational bending fatigue strength of shaft steel according to claim 1, characterized in that: The rolling tool used in the rolling process is a silicon nitride ball.

7. A surface strengthening method for improving the rotational bending fatigue strength of shaft steel according to claim 6, characterized in that: The diameter of the silicon nitride ball is 9mm and the hardness is 78HRC.

8. The surface strengthening method for improving the rotational bending fatigue strength of shaft steel according to claim 1, characterized in that: After obtaining the surface-strengthened shaft steel sample, the step 3 is also included: organization and performance testing, using a scanning electron microscope to observe the sample structure, using a cantilever beam rotary bending fatigue testing machine to measure the rotational bending fatigue strength of the sample, using a microhardness tester to measure the cross-sectional hardness gradient of the sample, and using a transmission electron microscope to observe the characteristics of the gradient organization of the cross section of the sample surface layer at a voltage of 200 kV and measure the residual stress.

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