Issues regarding the annealing of stainless steel composite panels

The effects of the composition, microstructure, and properties of stainless copper composite plates. As a result of explosive welding, many physical and chemical changes occur in the matrix of these metal materials, especially in their joint areas. For example, there is explosive strengthening and hardening of the matrix metal, plastic deformation of a thin layer of metal in the joint area, melting (to form solid solutions or intermetallic compounds), as well as diffusion. Some of these changes in composition, microstructure, and properties have no impact on the use of such composite materials, while others have a significant effect. To eliminate such effects, heat treatment is an effective method: stress-relief annealing can remove residual stresses within the material; recrystallization annealing can restore and recrystallize the metal structure that has undergone plastic deformation. By selecting appropriate heat treatment processes, explosive composite materials can be given physical and chemical properties that meet technical requirements. Therefore, heat treatment is an important post-processing step for explosive composite materials. Based on experiments, this paper examines the effects of heat treatment on the composition, microstructure, and properties of nickel-stainless steel explosive composite plates, thereby providing experimental data for the development of appropriate annealing procedures. It also discusses the useful application of alloy phase diagrams in this regard. Annealing is carried out in a non-vacuum furnace with air cooling. After the experiments, the sample blanks are processed into specimens for metallographic and mechanical property testing, so that various compositions, microstructures, and properties can be examined. All the results are then analyzed to identify patterns. 1. Effects of annealing on the microstructure of the joint area in composite plates During annealing, due to the influence of heat, many changes occur in the joint area of these composite plates. These changes become more apparent at lower temperatures; their microstructure is shown in Figure 1. At higher temperatures, regardless of the processing conditions, the wavy pattern in the joint area remains after annealing, as does the swirling region at its edges. When held at a certain temperature for 30 minutes, even at 800°C, the plastic-deformed structure on one side is still clearly visible, while the stainless steel side shows little signs of deformation, and its metal structure is not evident. After annealing, recrystallization occurs on that side. After 1000°C, the grains on that side grow larger and undergo aggregate recrystallization. Under annealing conditions, the grains on that side form a regular polygonal structure, while the stainless steel side exhibits a fine-grained structure. During annealing at 900°C–1100°C, a band-like substance with gradually increasing thickness appears at the interface between the stainless steel and copper. Under mild etching, it appears white (Figure 1), while under intense etching, it appears black (Figure 1). It can be inferred from these observations that this band-like structure does not significantly affect the bonding strength between the two materials. 2. Analysis of the elements in the joint area of the composite plate Using a 500-type scanning electron microscope, the amounts and distribution of nickel and iron in the joint area of the annealed nickel-stainless steel composite plate were analyzed. The results are shown in Figure 2. The analysis indicates that the original diffusion zones of nickel and iron in the joint area during the explosive welding process still exist after annealing, and they widen as the annealing temperature increases (see Figure 2). This is because, at high temperatures, metal atoms become more active and can penetrate and diffuse across the interface. The higher the temperature, the stronger this effect. However, since nickel and iron only form solid solutions at high temperatures, their diffusion does not have as severe an impact as that of other elements. 3.3 Effects of annealing on the mechanical properties of the composite plate After annealing, the composition distribution and microstructure of the joint area in this composite plate change as described above, and these changes inevitably lead to changes in its mechanical properties. 3.3.1 Microhardness distribution across the cross-section of the composite plate The microhardness distribution across the cross-section of this composite plate after annealing under different conditions is shown in Figure 3. It can be seen that as the temperature increases, the hardness of both nickel and stainless steel decreases. However, after annealing at 600°C and 800°C, the hardness of stainless steel remains above its original value, and it only drops below the original value after annealing at 1000°C. This is likely related to the presence or absence of the original solid solution structure in the stainless steel. Even after annealing at 1000°C, the hardness of the copper layer remains mostly above its original value, probably because its explosive hardening effect has not yet dissipated. The hardness distribution on the surfaces of the composite plate at 1000°C is caused by severe oxidation of the surface metals during annealing, as well as the formation of nitrides. Due to the formation of a layer with lower hardness at the interface, the highest hardness values are not located there. From the data in the table, it can be seen that as the heating temperature increases and the holding time prolongs, the bonding strength of this composite plate decreases, and the rate of decrease is greater than that of nickel-stainless steel composite plates. The differences in shear strength and bonding strength among large and medium-sized composite plates are related to the parameters of the explosive welding process and the instantaneous nature of that process. They are also related to the uneven distribution of properties throughout the large composite plates, as well as the differences between the various testing methods used. According to the data, the bonding strength of this composite plate, as indicated by this metric, remains quite high. 3.4 Comparison of bending properties Different annealing processes were tested to evaluate the bending properties of this composite plate. The data show that, under all conditions, the bending angle of the composite plate specimens could reach 180° (with bending inward, with the coating on the inside). ] This post was last edited by jyszyx on April 8, 2009, at 13:14