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HEAT-TREATING THEORY. The various types of heat-treating processes are similar because they all involve the heating and cooling of metals; they differ in the. properties and identification of metal and heat-treating procedures used for .. are five basic heat treating processes: hardening, case hardening, annealing. are in a high-energy state and are thermodynamically unstable. Annealing is the heat treatment process that softens a metal that has been hardened by cold.
The microstructure of a hardened steel part is ferrite, martensite, or cementite.
Tempering Tempering involves heating steel that has been quenched and hardened for an adequate period of time so that the metal can be equilibrated. The hardness and strength obtained depend upon the temperature at which tempering is carried out. Higher temperatures will result into high ductility, but low strength and hardness.
Low tempering temperatures will produce low ductility, but high strength and hardness.
Heat treatment report.pdf
In practice, appropriate tempering temperatures are selected that will produce the desired level of hardness and strength. This operation is performed on all carbon steels that have been hardened, in order to reduce their brittleness, so that they can be used effectively in desired applications. Annealing Annealing involves treating steel up to a high temperature, and then cooling it very slowly to room temperature, so that the resulting microstructure will possess high ductility and toughness, but low hardness.
Annealing is performed by heating a component to the appropriate temperature, soaking it at that temperature, and then shutting off the furnace while the piece is in it. Steel is annealed before being processed by cold forming, to reduce the requirements of load and energy, and to enable the metal to undergo large strains without failure.
Normalizing Normalizing involves heating steel, and then keeping it at that temperature for a period of time, and then cooling it in air. The resulting microstructure is a mixture of ferrite and cementite which has a higher strength and hardness, but lower ductility. Normalizing is performed on structures and structural components that will be subjected to machining, because it improves the machinability of carbon steels.
Heat treatment provides an efficient way to manipulate the properties of the metal by controlling the rate of diffusion and the rate of cooling within the microstructure. Heat treating is often used to alter the mechanical properties of a metallic alloy , manipulating properties such as the hardness , strength , toughness ,  ductility , and elasticity. There are two mechanisms that may change an alloy's properties during heat treatment: the formation of martensite causes the crystals to deform intrinsically, and the diffusion mechanism causes changes in the homogeneity of the alloy.
In most elements, this order will rearrange itself, depending on conditions like temperature and pressure. This rearrangement, called allotropy or polymorphism, may occur several times, at many different temperatures for a particular metal.
In alloys, this rearrangement may cause an element that will not normally dissolve into the base metal to suddenly become soluble , while a reversal of the allotropy will make the elements either partially or completely insoluble. If the alloy is cooled to an insoluble state, the atoms of the dissolved constituents solutes may migrate out of the solution. This type of diffusion, called precipitation , leads to nucleation , where the migrating atoms group together at the grain-boundaries.
This forms a microstructure generally consisting of two or more distinct phases. This is due to the fact that the steel will change from the austenite phase to the martensite phase after quenching. Some pearlite or ferrite may be present if the quench did not rapidly cool off all the steel. In these alloys, the nucleation at the grain-boundaries often reinforces the structure of the crystal matrix.
These metals harden by precipitation. Typically a slow process, depending on temperature, this is often referred to as "age hardening". When a metal is cooled very quickly, the insoluble atoms may not be able to migrate out of the solution in time.
This is called a " diffusionless transformation. The trapped atoms prevent the crystal matrix from completely changing into its low temperature allotrope, creating shearing stresses within the lattice. When some alloys are cooled quickly, such as steel, the martensite transformation hardens the metal, while in others, like aluminum, the alloy becomes softer. Phase changes occur at different temperatures vertical axis for different compositions horizontal axis.
The dotted lines mark the eutectoid A and eutectic B compositions. The specific composition of an alloy system will usually have a great effect on the results of heat treating. If the percentage of each constituent is just right, the alloy will form a single, continuous microstructure upon cooling. Such a mixture is said to be eutectoid. However, If the percentage of the solutes varies from the eutectoid mixture, two or more different microstructures will usually form simultaneously.
A hypoeutectoid solution contains less of the solute than the eutectoid mix, while a hypereutectoid solution contains more. A eutectic alloy is characterized by having a single melting point.
This melting point is lower than that of any of the constituents, and no change in the mixture will lower the melting point any further.
When a molten eutectic alloy is cooled, all of the constituents will crystallize into their respective phases at the same temperature. A eutectoid alloy is similar, but the phase change occurs, not from a liquid, but from a solid solution.
Upon cooling a eutectoid alloy from the solution temperature, the constituents will separate into different crystal phases , forming a single microstructure. A eutectoid steel, for example, contains 0.
Upon cooling slowly, the solution of iron and carbon a single phase called austenite will separate into platelets of the phases ferrite and cementite. This forms a layered microstructure called pearlite. Since pearlite is harder than iron, the degree of softness achievable is typically limited to that produced by the pearlite. Similarly, the hardenability is limited by the continuous martensitic microstructure formed when cooled very fast.
Both are above the eutectic melting point for the system, but are below the melting points of any constituent forming the system. Between these two melting points, the alloy will exist as part solid and part liquid. The constituent with the lower melting point will solidify first.
When completely solidified, a hypoeutectic alloy will often be in solid solution. Similarly, a hypoeutectoid alloy has two critical temperatures, called "arrests.
As the solution cools from the upper transformation temperature toward an insoluble state, the excess base metal will often be forced to "crystallize-out," becoming the proeutectoid.
This will occur until the remaining concentration of solutes reaches the eutectoid level, which will then crystallize as a separate microstructure. A hypoeutectoid steel contains less than 0. Upon cooling a hypoeutectoid steel from the austenite transformation temperature, small islands of proeutectoid-ferrite will form. These will continue to grow and the carbon will recede until the eutectoid concentration in the rest of the steel is reached.
This eutectoid mixture will then crystallize as a microstructure of pearlite. Since ferrite is softer than pearlite, the two microstructures combine to increase the ductility of the alloy. Consequently, the hardenability of the alloy is lowered. However, between these points, it is the constituent with the higher melting point that will be solid.
Similarly, a hypereutectoid alloy has two critical temperatures. When cooling a hypereutectoid alloy from the upper transformation temperature, it will usually be the excess solutes that crystallize-out first, forming the proeutectoid. This continues until the concentration in the remaining alloy becomes eutectoid, which then crystallizes into a separate microstructure. A hypereutectoid steel contains more than 0. When slowly cooling a hypereutectoid steel, the cementite will begin to crystallize first.
When the remaining steel becomes eutectoid in composition, it will crystallize into pearlite. Since cementite is much harder than pearlite, the alloy has greater hardenability at a cost in the ductility.
The red curves represent different cooling rates velocity when cooled from the upper critical A3 temperature. V1 produces martensite. V2 has pearlite mixed with martensite, V3 produces bainite, along with pearlite and matensite. Proper heat treating requires precise control over temperature, time held at a certain temperature and cooling rate. This temperature is referred to as an "arrest" because at the A temperature the metal experiences a period of hysteresis.
At this point, all of the heat energy is used to cause the crystal change, so the temperature stops rising for a short time arrests and then continues climbing once the change is complete. The alloy will usually be held at this temperature long enough for the heat to completely penetrate the alloy, thereby bringing it into a complete solid solution. Iron, for example, has four critical-temperatures, depending on carbon content. Pure iron in its alpha room temperature state changes to nonmagnetic gamma-iron at its A2 temperature, and weldable delta-iron at its A4 temperature.
However, as carbon is added, becoming steel, the A2 temperature splits into the A3 temperature, also called the austenizing temperature all phases become austenite, a solution of gamma iron and carbon and its A1 temperature austenite changes into pearlite upon cooling. Between these upper and lower temperatures the proeutectoid phase forms upon cooling. Because a smaller grain size usually enhances mechanical properties, such as toughness , shear strength and tensile strength , these metals are often heated to a temperature that is just above the upper critical-temperature, in order to prevent the grains of solution from growing too large.
For instance, when steel is heated above the upper critical-temperature, small grains of austenite form. These grow larger as temperature is increased.
When cooled very quickly, during a martensite transformation, the austenite grain-size directly affects the martensitic grain-size. Larger grains have large grain-boundaries, which serve as weak spots in the structure.
The grain size is usually controlled to reduce the probability of breakage. Cooling a metal will usually suppress the precipitation to a much lower temperature. Austenite, for example, usually only exists above the upper critical temperature. However, if the austenite is cooled quickly enough, the transformation may be suppressed for hundreds of degrees below the lower critical temperature. Such austenite is highly unstable and, if given enough time, will precipitate into various microstructures of ferrite and cementite.
The cooling rate can be used to control the rate of grain growth or can even be used to produce partially martensitic microstructures. If the alloy is cooled to the martensite transformation Ms temperature before other microstructures can fully form, the transformation will usually occur at just under the speed of sound.
When austenite is cooled extremely slow, it will form large ferrite crystals filled with spherical inclusions of cementite. This microstructure is referred to as "sphereoidite. Even faster, and fine pearlite will form. If cooled even faster, bainite will form. Similarly, these microstructures will also form if cooled to a specific temperature and then held there for a certain time. Most often, these are then cooled very quickly to produce a martensite transformation, putting the solution into a supersaturated state.
The alloy, being in a much softer state, may then be cold worked. This causes work hardening that increases the strength and hardness of the alloy. Moreover, the defects caused by plastic deformation tend to speed up precipitation, increasing the hardness beyond what is normal for the alloy.
Even if not cold worked, the solutes in these alloys will usually precipitate, although the process may take much longer. Sometimes these metals are then heated to a temperature that is below the lower critical A1 temperature, preventing recrystallization, in order to speed-up the precipitation. Complex heat treating schedules, or "cycles," are often devised by metallurgists to optimize an alloy's mechanical properties.Popular in Industrial Processes.
In table one, the hardness strength arranged regarding to the more strength as It is a heat treatment process in which steel or iron is heated to a temperature, below the melting point, in the presence of a liquid,. The process of heat treatment involves the use of heating or cooling, usually to extreme temperatures to achieve the wanted result.
But specimen has not obtained good microstructure.
A technique used to recover cold work and relax stresses within a metal. Thermal conductivity of materials is temperature dependent.
Annealing involves recovery, recrystallization and grain growth. Take the parts to hardness tester and calculate its hardness values. Table 5: