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Attention to precipitation or aging hardening of stainless steel plates

Several methods can be used to increase the strength of stainless steel plates, which are alloying, quenching, working hardening, and a very common form of heat treatment, namely, precipitation or aging hardening. The so-called age hardening is in fact dependent on the formation of precipitates. In order to achieve the best combination of mechanical properties, the heat treatment cycle of stainless steel plates must be strictly controlled.
In order to understand how sediment affects mechanical properties, some basic metallurgy needs to be understood. The mechanism of precipitation hardening requires that the alloy element, that is, the solute is in the metal, and the solubility in the solvent increases with the increase of temperature, and the solid solution line shows that the alloy element B decreases with the temperature of the solvent A. A metaphor is a salt in the water. As the temperature rises, more salt can be dissolved, but when the salt crystal begins to form or precipitate, it allows the solution to cool and vice versa.
In addition to the dissolution and precipitation process that occurs in the solid and thus the atom is much slower because the atom is more difficult to move beyond the solid than the liquid solution, the same process occurs in the suitable alloying metal. As a result, once the deposit has been dissolved by a high enough temperature, that is, the wire is above the line line, it can be prevented by rapid cooling or quenching to prevent it from re forming.
This heat treatment is called solid solution heat treatment and is formed to form an unstable supercooled solid solution. If it is reheated to a lower aging or precipitation hardening temperature, the precipitates will begin to be reformed. These precipitates are carried out with heat treatment.
In a solid solution heat treated stainless steel plate, the atoms of the alloying elements are randomly distributed in the whole matrix, but once the temperature rises, the precipitation begins to form through the nucleation and growth process. At relatively low temperatures and shorter timescales, the solute atoms begin to gather together to form a very small, very small sediment called Guinier-Preston (GP), the two regions named after the first two metallurgists. The GP region is very small, which is not visible by ordinary optical microscope, but can be observed by electron microscope at about 100 thousand magnification.
The GP region is described as coherent. In other words, they have the same crystal structure as solvent metals. However, they distort the lattice, that is, the framework of atoms. This makes the dislocation more difficult to move in the lattice and is the dislocation movement of the metal deformation; therefore, the tensile strength and hardness are increased, but the ductility and toughness are reduced. As the aging treatment continues or the temperature increases, the tensile strength continues to increase with the growth and thickening of the sediment. However, to some extent, the sediment began to lose consistency. Just before that, the alloy has the highest tensile strength. As the formation and size of these particles increase, the tensile strength decreases. It is said that the alloy is surplus, although the precipitates still contribute to the tensile strength of the alloy. High strength low alloy (HSLA) steel is a good example. In this case, incoherent, over aging precipitates are used to substantially increase the tensile strength.
In order to achieve the best combination of properties, precipitates need to be uniformly distributed throughout the alloy grains and have the best size. The aging temperature and / or time can be significantly changed to adapt to the distribution and size of the sediment, and longer time and / or higher temperature usually lead to a decrease in strength, but the ductility increases, and the overused structure provides the lowest tensile strength but the highest ductility.
Ferritic and nickel based alloys are usually used for over aging to ensure reasonable ductility. It can be seen that for some alloys, such as 17 / 4PH stainless steel, the precipitation mechanism is low enough to be able to cool the components in the stationary air or, like A286 stainless steel, requires long aging time.
On the other hand, Al Cu alloy 2219 can be aged for several days at room temperature. Some of the 6000 (Al-Si-Mg) and 7000 series (Al-Zn-Mg) alloys will be similar to aging at ambient temperature. This is called natural aging, and high temperature aging is called artificial aging. In general, strict control of heat treatment time, temperature and cooling rate is essential if desired performance is required.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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