As a relatively common 18-8 austenitic stainless steel, 304 stainless steel is used in one of the most common occasions to make fasteners such as stamping washers. Because the degree of deformation of stainless steel after stamping into various materials is also different, the deformation range is usually in the range of 15%-40%, so the degree of work hardening of stainless steel materials is also different.
After the cold working of the
304 stainless steel sheet, in fact, a large number of dislocations have been formed on the slip plane and grain boundary from a microscopic point of view, resulting in lattice distortion. The higher the distortion value, the higher the dislocation density, and the more serious the internal stress and lattice distortion, which will increase the various deformation degrees of the stainless steel material. When the hardening of stainless steel reaches a certain level, if the deformation continues, there is the possibility of cracking or brittle fracture, and the residual stress after forming is very easy to cause the stainless steel workpiece to explode and rupture. Under the action of the ambient atmosphere, after being placed for a period of time, the stainless steel workpiece will automatically form intergranular cracking. Therefore, in the stamping and forming process of 304 stainless steel, softening annealing between processes is usually performed, that is, intermediate annealing, which can remove residual stress, reduce hardness, and restore material plasticity, so that the workpiece can be processed in the next step.
With the increase of the pre-deformation variable of 304 stainless steel, its yield strength and tensile strength are also significantly increased, the hardness value also increases, and the plasticity decreases, resulting in a significant phenomenon of work hardening. In addition, it can be clearly seen that with the increase of the pre-deformation amount, the yield ratio of the specimen will also increase, which means that the formability of the stainless steel workpiece will decrease with the increase of the cold deformation amount.
Work-hardened 304 stainless steel can restore plasticity by using high temperature and low temperature annealing methods, reduce the degree of hardening, and remove or reduce residual stress. Sensitization temperature range of ℃-850℃. Low temperature annealing has a low effect on the yield strength of 304 stainless steel. When annealed at 500 °C, the change in yield strength value after annealing is relatively small. High temperature annealing has a high impact on the yield strength of 304 stainless steel samples. After annealing at 1050 °C, Re decreased to 260 MPa, and Rm decreased almost linearly with the annealing temperature, but the magnitude of change was much smaller than that of Re. And the Vickers hardness value of the samples decreased with the increase of annealing temperature. With the increase of the annealing temperature, the elongation of the sample increases significantly, especially in the high-temperature annealing state, Re decreases most significantly, reaching a completely softened state. Annealing at 1050°C achieves the best combination of elongation A and hardness HV to soften.
In general, 304 stainless steel sheets with different preforms are annealed at high temperature (1040 ℃-1080 ℃) for a short time (5-10 minutes) and rapidly cooled, and the structure is completely recrystallized, and the grain size is relatively uniform. , the most suitable for the manufacture of gaskets for fasteners, and the softening effect of annealing is also the most significant.
