This residual stress acts simultaneously with the stress that occurs when the spring is in use. In the case of stretching and compressing the spring, the wire is subjected to twisting and shearing by an axial load. However, under normal circumstances, the value of the shear force is small, so usually only the torque can be considered. The maximum value of the shear stress generated by the torsion occurs at the outermost circumference of the wire, but the maximum value of the residual stress is as shown in b), which occurs near the center of the wire. Therefore, the maximum value of the residual stress should be the most direct problem, and the future discussion is based on this maximum value.
Removal of Low Temperature Annealing Residual Stress The small spring is fabricated by appropriate cold working and heat treatment to impart appropriate characteristics to the spring material before winding it. Subsequent low temperature annealing removes residual stress without compromising its pre-process characteristics. The current heat treatment conditions are: 190350e, 2040min for phosphor bronze and 350450e, 1h for stainless steel.
The purpose of this study was to shorten this processing time. At present, there are few reports on this issue, and only reports on improved furnaces <6> have been retrieved. In addition, the stress changes during annealing and the extent of their removal are not well understood. The only analysis report <7> only discusses the temperature distribution during annealing.
The investigation of low-temperature annealing technology summarizes the main points of the literature [814], and it can be considered that the removal of residual stress is related to the stress relaxation of metals. That is, in order to retain the previously imparted characteristics and only remove stress, it should be annealed below the recrystallization temperature. In the process of removing stress, no change in metal structure occurs. In addition, this stress relaxation is also a phenomenon of metal recovery. The so-called recovery is a process in which the grain shape and the crystal direction do not change, but the physical and mechanical properties change, which is a phenomenon that is performed before crystallization. Furthermore, if the theory of metal atomic dislocation is used, the reply can be understood as the disappearance of the movement of the dislocation or the disappearance of the point defect, but there is no consistent conclusion.
Low-temperature annealing and creeping shorten the time of low-temperature annealing, and it is desirable to determine the formula for the rate of stress reduction during annealing. However, there is a lack of literature on low temperature annealing and no research data on the formula for stress reduction. Therefore, this study focuses on the formation of creep in a large area of ​​engineering. Creep is a phenomenon in which deformation increases with time when the stress is maintained at a high temperature. Stress relaxation is one of the categories of creep, that is, the phenomenon that stress decreases with time when the deformation is constant. This relaxation of stress can also be considered as the process of recovery <15>.
The so-called stress relaxation and annealing differ only in that the latter is limited to after cold working. For creep, it can be explored from various phenomena of phenomena, experience and physics according to the high temperature strength of the material and the high temperature plasticity of crystallization <1516>. Creep is divided into three stages: transient creep, uniform creep, and third creep. Among them, the uniform creep is dominant in the creep phenomenon, and various physical models have been proposed. This paper intends to explain the dislocation motion as the center, and give the creep speed in a quantitative way. It is used to guide the low temperature annealing treatment.
Quantification of the low temperature annealing process Here, it is intended to set the low temperature annealing speed formula by the extension of the creep velocity formula. The creep rate formula relates to the relationship between creep speed and temperature and is subject to the following heat activated formulas <12, 1516>. Here, RT is the average vibrational energy of an atom, Q is the energy necessary to cause a creep phenomenon by rearrangement of atoms, and exp(-Q/RT) is the probability that an atom with a larger energy than Q exists. The relationship between stress and uniform creep can be expressed by the following Dorn velocity formula.
In summary, the general creep speed formula is <1518>. dEdt=A2nexp(-QRT)(3) where A is a constant. Low Temperature Annealing Velocity Formula First, it is assumed that the above creep velocity formula can also be applied to low temperature annealing. Further, assuming that the process of removing the residual stress obeys Hooke's law of the formula (4), and the longitudinal elastic modulus E has no correlation with the temperature, the low-temperature annealing is a phenomenon that can be explained by the reply.
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