What impact does the heat treatment process of high precision brass strips have on its grain size?
Release Time : 2025-11-17
The grain size of high-precision brass strips has a decisive impact on their mechanical properties, machinability, and surface quality, and heat treatment is the core means of controlling grain size. By precisely controlling the annealing temperature, time, and cooling method, grain refinement or coarsening can be achieved, thereby optimizing material properties to meet the needs of different applications.
Recrystallization annealing is a key process for controlling grain size. After high-precision brass strips undergo cold working, their internal grains are elongated and work hardened. Recrystallization annealing can eliminate internal stress and restore plasticity. The annealing temperature needs to be higher than the recrystallization initiation temperature but lower than the abnormal grain growth temperature range. For example, the recrystallization temperature of ordinary brass is usually between 550-700℃. Within this temperature range, the original deformed grains nucleate and grow to form new equiaxed grains. If the annealing temperature is too low or the holding time is insufficient, the recrystallization process is incomplete, resulting in uneven grain size; if the temperature is too high or the holding time is too long, the grains will become excessively coarsened, leading to a decrease in material strength. For example, when H65 brass is annealed at 600℃, the grain size gradually increases with the extension of the holding time, but after exceeding the critical time, the grains coarsen rapidly, forming a coarse structure that is unfavorable for deep drawing.
The effect of cooling method on grain size is particularly significant in two-phase brasses. Two-phase brasses (such as H62 and H59) consist of an α phase (copper-rich solid solution) and a β phase (CuZn ordered body). The cooling rate after recrystallization annealing changes the phase composition and grain morphology. Rapid cooling (such as water cooling) can suppress the transformation of the β phase to the α phase, retaining more of the hard and brittle β phase, while refining the α phase grains; slow cooling (such as furnace cooling) promotes the decomposition of the β phase, forming a structure where coarse α phase grains coexist with soft and tough β phase grains. For example, HSn70-1 brass, after annealing at 560-580℃ and then furnace cooling, can obtain an α+β dual-phase microstructure, with the α phase accounting for 65-70% and the β phase accounting for 30-35%. This microstructure allows the material to maintain tensile strength while possessing high elongation, making it suitable for applications such as marine condenser tubes where both strength and toughness are required.
The impact of low-temperature annealing on grain size is mainly reflected in stress relief and improved dimensional stability. For brass with a zinc content exceeding 20%, residual tensile stress is easily generated after cold working, which may lead to stress corrosion cracking (seasonal cracking) in humid environments. Low-temperature annealing at 260-300℃ can eliminate residual stress without inducing recrystallization, while also preventing grain coarsening. For example, after H68 brass was treated at 300℃ for 2 hours, the residual stress decreased from 120MPa to below 30MPa, and the grain size remained stable, effectively extending its service life in marine environments.
The initial state of grain size is decisively affected by the hot working temperature. During casting or extrusion, high-temperature hot working (e.g., 950-970℃) enables brass bars to form uniform equiaxed grains, providing a good foundation for subsequent cold working. If the hot working temperature is too low, insufficient grain growth can easily lead to edge cracks during cold working; if the temperature is too high, it may cause abnormal grain coarsening, reducing material properties. For example, after annealing C36000 leaded brass at 700-750℃, the grains are refined and the microstructure is uniform, providing ideal conditions for precision machining.
The combination of processes is crucial for precise control of grain size. For brass bars requiring multiple deep drawing passes, a combination of "intermediate annealing + final annealing" can optimize grain size. For example, H62 brass waveguides, through "540℃×2h intermediate annealing + 400℃×1h final annealing," control the grain size within the range of 0.025-0.035mm, avoiding orange peel defects during deep drawing and preventing earing.
Grain size has a multidimensional impact on the performance of high-precision brass strips. Fine grain size (e.g., 0.015-0.045 mm) significantly improves material strength and toughness, making it suitable for parts requiring light stamping or high surface quality. Coarse grain size (e.g., 0.100 mm) is suitable for deep drawing of thick plates, but may sacrifice some surface finish. For example, in deep drawing of H65 brass, if the grain size exceeds 0.04 mm, a "rough surface" phenomenon easily occurs, resulting in the inability to achieve the required gloss after electroplating.
The heat treatment process for high-precision brass strips will develop towards gradient and intelligent methods. By developing gradient heat treatment processes, a fine-grained strengthening layer can be formed on the material's surface, while a coarse-grained toughening layer is retained in the core, enabling differentiated performance design. Combining numerical simulation and artificial intelligence technologies, a quantitative relationship model between heat treatment parameters, grain size, and performance can be established, providing precise material solutions for high-end fields such as marine engineering and aerospace.