Designation of metallurgical states of aluminum
'T' – Heat treatment (i.e. for alloys hardened by maturation or aging) the 'T' will always be followed by one or more digits.
F - Raw state for the implementation of advanced needs of the manufactured.
T1 - Cooled from a high temperature during the forming process and naturally aged.
T2 - Cooled from a high temperature during the forming process, cold worked and naturally aged.
T3 - Solution heat treated, cold worked and naturally aged.
T4 - Solution heat treatment and naturally aged.
T5 - Cooled from a high temperature during the forming process and artificially aged.
T6 - Solution heat treatment and then artificially aged.
T7 - Solution heat treatment and then artificially aged.
T8 - Solution heat treatment, cold worked and artificially aged.
T9 - Solution heat treatment, artificially aged and cold worked.
Annealing
Annealing is the heat treatment that, in general, has the main purpose of softening steel or other metals, regenerating the structure of superheated steels or simply eliminating the internal stresses that follow cold working. (Cooling in the oven). That is, eliminate residual stresses produced during cold working without affecting the mechanical properties of the finished part, or annealing can be used to completely eliminate strain hardening. In this case, the final part is soft and ductile but still has good surface finish and dimensional accuracy. After annealing, additional cold working can be performed as ductility is restored; By combining repeat cycles of cold work and annealing, large total strains can be achieved.
The term "annealing" is also used to describe other heat treatments. For example, glass can be heat treated or annealed to eliminate residual stresses present in it. Irons and steels can be annealed to maximize their properties, in this case ductility, even when the material has not been worked cold.
There are three stages considered the most important in the annealing process:
The original microstructure worked at low temperatures is composed of deformed grains that contain a large number of dislocations intertwined with each other. When the metal is first heated, the additional thermal energy allows the dislocations to move and form the boundaries of a polygonized subgranular structure. This means that, as the material heats up, the dislocations disappear and in turn the grains become larger. However, the density of the dislocations remains virtually unchanged. This low temperature treatment eliminates the residual stresses due to cold working, without causing a change in the density of the dislocations, and is called recovery.
The mechanical properties of the metal remain relatively unchanged, since the total number of dislocations that occur during this stage is not reduced. Because residual stresses are reduced or even eliminated when dislocations are rearranged, the recovery is often referred to as stress relief annealing. In addition, recovery restores the high electrical conductivity of the material, which would allow the manufacture of wires that could be used to transmit electrical energy, because they would also be highly resistant. Finally, recovery often improves the corrosion resistance of materials.
When a previously cold-worked metal is subjected to very high temperatures, rapid recovery eliminates residual stresses and produces the structure of polygonized dislocations. During this instant, the formation of nuclei of small grains occurs at the boundaries of the cells of the polygonized structure, eliminating most of the dislocations. Because the number of dislocations is reduced on a large scale, the recrystallized metal has low strength but high ductility. The temperature at which a microstructure of new grains that have few dislocations appears is called the recrystallization temperature. Recrystallization is the process during which new grains are formed through heat treatment of a cold-worked material. The recrystallization temperature depends on several variables, therefore it is not a fixed temperature.
When the temperatures applied in annealing are very high, the recovery and recrystallization stages occur more quickly, thus producing a finer grain structure. If the temperature is high enough, grains begin to grow, with favored grains crowding out grains that are smaller. This phenomenon, which can be called grain growth, is carried out through the reduction in the area of grain boundaries. Grain growth will occur in most materials if they are kept at a high enough temperature, which is not related to cold working. This means that recrystallization or recovery are not essential for the grains to grow within the structure of the materials.
Ceramic materials that exhibit almost no hardening show a considerable amount of grain growth. Likewise, abnormal grain growth can occur in some materials as a result of liquid phase formation.
In homogenization annealing, typical of hypoeutectoid steels, the heating temperature is that corresponding to A+200 °C without reaching the solids curve in any case, with subsequent slow cooling carried out in the furnace itself, its main objective being to eliminate the heterogeneities produced during solidification.
Also called normalized, its function is to regenerate the structure of the material produced by tempering or forging. It is generally applied to steels with more than 0.6% C, while it is only applied to steels with a lower percentage of C to refine and organize their structure.
Example.
After cold rolling, where the grain is elongated and subjected to tension, this treatment returns the microstructure to its initial state.
Generally, it is desired to obtain globulization in pieces such as thin plates that must have high drawing and low hardness.
The highest drawing values are generally associated with the globulized microstructure that is only obtained in a range between 650 and 700 °C. Temperatures above the critical level produce the formation of austenite which, during cooling, generates pearlite, causing an undesirable increase in hardness.
Generally, pieces such as plates for protective boots must be globulized in order to obtain the necessary bends for use and avoid breaking or cracking. Finally they are tempered to guarantee hardness.
It is used for hypereutectoid steels, that is, with a percentage greater than 0.89% of C, to achieve the lowest possible hardness than in any other treatment, improving the machinability of the part. The annealing temperature is between AC and AC.
Softening of alloy tool steels of more than 0.8% C.
For a hypoeutectoid carbon steel:
The microstructure obtained in this treatment varies depending on the annealing temperature. Generally, those that do not exceed 600°C will release stresses in the material and cause some grain growth (if the material was not previously tempered). Generally showing Ferrite-Pearlite. Above 600 and below 723 we speak of globulization annealing since it does not exceed the critical temperature. In this case there is no pearlite grain, the carbides are spheroidized and the matrix is completely ferritic.
It is used for forging or rolling steels, for which an annealing temperature lower than AC1, but very close, is used. Through this procedure, the internal stresses produced by its molding and machining are destroyed. It is commonly used for high-strength alloy steels, Cr-Ni, Cr-Mo, etc. This procedure is much faster and simpler than the aforementioned, its cooling is slow.
Surface hardening
Surface hardening is the process of hardening the surface of a metal object while allowing the deeper metal to remain soft, thus forming a thin layer of hard metal on the wear-resistant surface, but maintaining a soft core such that it can absorb stresses without cracking.
Also known as carburizing, it consists of hardening the external surface of low carbon steel, leaving the core soft and ductile. Since carbon is what generates hardness in steels, the casehardening method has the possibility of increasing the amount of carbon in low-carbon steels before being hardened. Carbon is added by heating steel to its critical temperature while in contact with a carbonaceous material. The three most common carburizing methods are: carburizing packing, liquid bath, and gas.
This procedure consists of placing the low carbon steel material in a closed box with carbon material and heating it to 900 to 927 °C for 4 to 6 hours. During this time the carbon found in the box penetrates the surface of the piece to be hardened. The longer the piece is left in the box with carbon, the deeper the hard layer will be. Once the piece to be hardened is heated to the appropriate temperature, it is quickly cooled in water or brine. To avoid deformations and reduce surface tension, it is recommended to let the piece cool in the box and then remove it and heat it again between 800 and 845 °C (cherry red) and proceed to cooling by immersion. The most commonly used hardened layer has a thickness of 0.38 mm, however thicknesses of up to 0.4 mm can be had.
The steel to be cemented is immersed in a bath of liquid sodium cyanide. Potassium cyanide can also be used but its vapors are very dangerous. The temperature is maintained at 845 °C for 15 minutes to 1 hour, depending on the depth required. At this temperature the steel will absorb the carbon and nitrogen from the cyanide. The steel must then be quickly cooled in water or brine. With this procedure, layers with thicknesses of 0.75 mm are achieved.
In this procedure, carburizing gases are used for cementation. The low carbon steel piece is placed in a drum into which gas is introduced to carburize, such as hydrocarbon derivatives or natural gas. The procedure consists of keeping the oven, the gas and the piece between 900 and 927 °C. After a predetermined time, the carburizing gas is cut off and the oven is allowed to cool. The part is then removed and reheated to 760°C and rapidly cooled in water or brine. With this procedure, pieces are achieved whose hard layer has a thickness of up to 0.6 mm, but usually does not exceed 0.7 mm.
There are several surface hardening procedures using nitrogen (nitriding) and cyanide (cyanidation), which are usually known as carbonitriding or cyaniding. In all these processes, with the help of cyanide and ammonia salts, hard surfaces are achieved as in the previous methods.