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Vibration aging to eliminate the residual stress of tailor welded stainless steel plate

Abstract: in order to eliminate the residual stress of super large stainless steel welding base plate, the method of eliminating the welding residual stress by vibration aging (VSR) was studied. JB/t5926 - 91 standard is applied to qualitatively evaluate the vibration aging process. Through the comparative measurement of the residual stress on the base plate weld after welding and vibration aging, the influence of vibration aging process on the change of residual stress and the final stress state is comprehensively and quantitatively understood, and the feasibility and effectiveness of VSR process are understood, so as to achieve the goal of replacing thermal aging process

key words: vibratory stress relief, stainless steel, residual stress

0 Introduction

metal components in forging, cutting, casting, welding ② R & D requirements are increasing 4 Customers can also use cannon equipment with the same configuration for spraying raw materials with different formulas (normal or with mineral filler) and other processing processes. Due to uneven stress or heating, uneven plastic deformation occurs inside, and there is residual stress after processing. Residual stress is an important cause of cracking or deformation of metal components, which greatly affects the fatigue strength and dimensional accuracy stability of metal components. Eliminating residual stress is a very important task in machining industry. The traditional stress relief process is mainly thermal aging (heat treatment). For large components, thermal aging requires a huge stewing furnace, burning coal or electricity, and it takes 2 to 7 days to process a batch of metal parts. Therefore, the investment is large, the energy consumption is large, the efficiency is low, it is easy to produce new deformation, and the material strength is reduced. Vibration stress relief (VSR) is to reduce or homogenize the residual stress in the component by applying vibration, so as to improve the service strength of the component, reduce the deformation and stabilize the accuracy of the size. Compared with the traditional thermal aging method, it can reduce the residual stress of the component in a very short time, without moving the workpiece, nor producing oxide skin or rust skin. Vibration aging is widely used because of its simple and convenient process, strong applicability and other outstanding characteristics. Vibration aging is a normal temperature aging process, which can reduce the peak value of welding residual stress of metal structure, homogenize the distribution, and improve the dimensional stability. Therefore, vibration aging can replace thermal aging aiming at dimensional stability. Vibration aging has incomparable advantages over heat treatment for materials with oxidation resistance requirements, low-temperature phase change and super large components that are prone to heat treatment deformation

the tailor welded stainless steel plate studied has oxidation resistance requirements and large volume. If the traditional thermal aging (heat treatment) process is used to eliminate the welding residual stress, it needs to be carried out in a super large heat treatment furnace and gas protection, which is expensive. Therefore, we used Jin min as an excellent supplier of spray free materials, vibration aging process to replace the traditional heat treatment aging process, and studied the elimination of welding residual stress on the stainless steel base plate

1 aging member

the treated member is the bottom plate of a large experimental device, which is made of ultra-low carbon stainless steel 304L, and the diameter of the entire circular bottom plate is 7 6m, which is made of five stainless steel plates by submerged arc welding (see Figure 5). The thickness of the plates is 75mm. There are 27 installation holes on the plate, including a central hole. See Table 1 for the mechanical properties and chemical composition of the base plate material

2 vibration aging process

2 1 vibration mode

the excitation of the base plate is generated by the rotation of an eccentric motor. The motor is clamped and fixed on the base plate. Combined with the shape and on-site working conditions, the following two vibration modes are determined:

(a) drum vibration: clamp the exciter on the edge of the center hole of the base plate and elastically support it on the circumference of the base plate. The whole system is similar to the vibration mode of knocking the drum surface, as shown in Figure 1

(b) bending vibration: clamp the exciter on the edge of the circumference of the base plate, and the elastic support is still on the circumference of the base plate. The whole system makes wave bending vibration, as shown in Figure 2

2. 2. Analysis of vibration aging process curve

according to the vibration aging theory, under the joint action of vibration stress and residual stress, the aged components will produce local plastic deformation and release the residual stress, the damping of vibration will be reduced and tend to be stable, and the acceleration of vibration will tend to be stable. The a - t (acceleration time) curve and a - n (acceleration speed/frequency) curve are mainly considered in the vibration aging process parameter curve; According to the vibration aging JB/t5926 - 91 standard, when the A-T (acceleration time) curve changes from the initial unstable state to the stable state after several minutes of treatment, it is considered that the vibration process is effective; By comparing the two a - n (acceleration - speed/frequency) sweep curves before and after vibration, it is considered that the vibration process is effective when the formant frequency changes, the peak rises, and the peak shape narrows in the curve after vibration. Figures 3 and 4 are the process curves of two vibrations of 304L large base plate. The current in the figure is the working current of the motor, and the vibration acceleration is measured by the acceleration sensor placed on the base plate. Each aging is carried out with two different frequencies (motor speed), corresponding to two vibration peaks respectively. The results show that both vibrations achieve the effect of vibration aging

3 residual stress measurement

since the parameter curve in the vibration aging Standard cannot quantitatively demonstrate the effectiveness of vibration aging, we also conduct residual stress analysis according to the requirements of JB/t5926 -91 standard. The blind hole method was used to measure the residual stress. According to the shape of 304L large base plate and the determined vibration scheme, taking into account the structural symmetry, 12 typical measuring points (6 before vibration and 6 after vibration) are selected on the weld for measurement. See Figure 5 for the layout of measuring points. Drill holes respectively before and after the vibration, so that the residual stress in the measuring point can be relaxed, so as to record the anisotropic strain through the strain gauge

figures 6 ~ 8 are the maximum principal stresses respectively σ Max, shear stress τ、 Longitudinal stress σ The distribution changes before and after 0 ° vibration directly reflect the effect of stress relief and homogenization of vibration aging

the measurement results show that the maximum principal stress of the weld before vibration σ Max is 83mpa ~ 316mpa; The average value is 174mpa, shear stress τ 4 ~ 155mpa; The average value is 49MPa, longitudinal stress σ 0 ° is 73 ~ 193mpa; The average value is "133mpa in addition". After vibration aging, the maximum principal stress of 304L large base plate weld σ Max is 74mpa ~ 164mpa; The average value is 120MPa, the decrease is 31%, and the shear stress τ 1 ~ 61mpa; The average value is 28Mpa, the decrease is 43%, and the longitudinal stress σ 0 ° is 73mpa ~ 117mpa; The average value is 94mpa, with a decrease of 29%

this shows that the residual stress after VSR process is significantly reduced, the stress distribution is homogenized to a certain extent, and the stability and deformation resistance of the large base plate are improved. The measurement results quantitatively show that the VSR process has reached the technical requirements of reducing the residual stress level by more than 20%

4 conclusion

for eliminating the welding residual stress of super large stainless steel base plate, according to JB/t5926 - 91 standard, the effect of vibration aging process is evaluated by using process curve and sweep frequency parameters, which shows that the vibration aging technology can meet the process requirements. The residual stress was further measured by blind hole method. The results showed that the maximum residual stress of 304L large base plate after vibration aging σ The average value of Max decreased from 175Mpa to 120MPa, with a decrease of 31%, which also shows that the process is effective. (end)

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