The hardness and tensile strength of the annealed steel are related to bonding forces on the atomic level. It is therefore expected that the hardness and tensile strength of the steel are related.
This relationship can be empirically defined by the relative measure of the amount of correspondence between the material hardness and its tensile properties at different soaking time. This could be expressed by the Pearson product-moment correlation coefficient [19] as:. Figures 8 and 9 show these correlation relationship of the properties.
It could infer that the tensile properties of the material are directly proportional to the material hardness. The R 2 value which is the correlation coefficient of the relationship is shown on the Figures 8 and 9. The results demonstrate the influence of the annealing process on the mechanical properties of the cold drawn low carbon steel.
The steel when annealed at deg. Celcius at different soaking time demonstrate variance in its strength. At soaking time between 10 minutes and 30 minutes, the residual stress due to cold deformation is relieved causing the strength of the steel to reduce slightly as evident in the slow reduction of its yield strength and tensile strength. It could be explained that this residual stress relief stage is a recovery stage of the steel during which new grains begins to nucleate from the highly deformed region of the steel as shown in Figure 7 a at soaking time between 30 minutes and 40 minutes, the new grains begins to recrystallize annihilating the deformed grains Figure 7 b.
This process of recrystallization causes drastic reduction in the strength of the steel as shown in Figures 2 and 3. The recrystallization process ensures the reformation of the grain structure to an equiaxial grain shape desired for improved ductility of the material. It is observed that with increasing annealing soaking time, the tensile properties of the drawn steel reduces. Sharp drop in these properties is observed between the soaking time of 30 minutes and 40 minutes.
It is suggested that recrystallization could possibly be taking place during this period. The hardness of the material increases at a slower rate for the first 30 minutes of soaking after which it dropped sharply between 30 minutes and 40 minutes during which the ductility reduces at a slower rate. Increase of the material ductility after 40 minutes of soaking was observed which suggested grain growth in the material. A linear correlation between the material hardness and the tensile properties is established.
The yield strength, impact toughness and the ultimate tensile strength of the steel decrease with increasing annealing soaking time. The more the yield strength is lowered, the greater the plastic deformation and correspondingly the greater the possibility of reducing the residual stresses. Phelippeau, S. Pommier, T. Tsakalakos, M. Clavel and C. Fuller and R. Humphreys and M. Jia, K. Ramesh and E. Yu, P. Sun, P. Kao and C. Kim, H. Jeong and H. Adegbuyi and A. Hoyos, A.
Ghilarducci, H. Salva, C. Chaves and J. Song and V. Lee, J. Lee, Y. Lee, K. Park and W. Akpari, G. Hasani and M. Olokode, B. Bolaji and O. Stoyka, F. Kovac and B. Schindler, M. Rusz and P. Kang, Y. It is generally carried out by prolonged heating at temperatures just below the A1 temperature, but can be facilitated by alternately heating to temperatures just above the A1 temperature and cooling to just below the A1 temperature.
The final step, however, consists of holding at a temperature just below the critical temperature A1. The rate of cooling is not important after slowly cooling to around deg C. The rate of spheroidization is affected by the initial structure.
The finer the pearlite, the more readily spheroidization takes place. A martensitic structure is very receptive to spheroidization. This treatment is normally used for the high carbon steels 0. The purpose of this treatment is to improve the machinability of the steel.
The process is also used to condition high-carbon steel for cold-drawing into wire. Normalized treatment is frequently applied to the steel in order for the achievement of any one or more of these objectives, namely i to refine the grain structure, ii to obtain uniform structure, iii to decrease residual stresses, and iv to improve the machinability of the steel.
Normalizing is a process in which steel is heated, to a temperature above the A3 or the Acm temperatures and then cooled in atmospheric air. The purpose of the normalizing treatment is to remove the effects of any previous heat treatment including the coarse grained structure sometimes resulting from high forging temperatures or cold-working. The normalizing process is done to ensure a homogeneous austenite on reheating for hardening or full annealing.
The resultant structures are pearlite or pearlite with excess ferrite or cementite, depending upon the composition of the steel. The structures after normalizing are different from the structures resulting after annealing and the steels of the same carbon content in the hypo-eutectoid or hyper-eutectoid ranges, there is less excess ferrite or cementite and the pearlite is finer.
These are the results of the more rapid cooling. Since the type of structure, and, hence, the mechanical properties, are affected by the rate of cooling, substantial variations can take place in normalized steels because of differences in section thickness of the shapes being normalized. Steels can be hardened by the simple means of heating the steel to a temperature higher than the A3 transformation temperature, holding long enough to ensure the achievement of uniform temperature and solution of carbon in the austenite, and then cooling the steel rapidly quenching.
Complete hardening depends on cooling so rapidly that the austenite, which does not decompose on cooling through the A1 temperature and is maintained at relatively low temperatures. When this is accomplished, the austenite start transforming to martensite on cooling below the Ms temperature around deg C and is completely transformed to martensite below Mf temperature. Rapid cooling is necessary only to the extent of lowering the temperature of the steel to well below the nose of the S curve Fig 2.
Once this is achieved then slow cooling from then on, either in oil or in air, is beneficial for avoiding distortion and cracking.
Special treatments, such as time quenching and mar-tempering, are designed to bring about these conditions. As martensite is quite brittle, steel is rarely used in the as-quenched condition, that is, without tempering. The maximum hardness which can be achieved in completely hardened low-alloy steels and plain carbon structural steels depends primarily on the carbon content. Effect of mass — The mass of the steel has its influence on the formation of the martensite. It can be seen that even with a sample of relatively small dimensions, the rate of removal of heat is not uniform.
Heat is always removed from the surface layers at a faster rate than from the inner potion. In a given cooling medium, the cooling rate of both the surface and inner portion decreases as the dimensions of a sample increase and the possibility of exceeding the critical cooling rate becomes less.
To overcome this, the steel is required to be quenched in a medium which is having a very high rate of heat removal, such as iced brine, but, even so, many steels have a physical restriction on the maximum size responsive to complete hardening regardless of the quenching medium.
The marked effect which the mass has upon the hardness of quenched steel can be shown by measuring the hardness distribution of different size rounds of the same steel quenched in the same medium.
Effect of carbon — Content of carbon in the plain carbon and low alloy steels influences the Ms transformation temperature. As the carbon content increases, the Ms temperature is reduced Fig 3. Tempering sometimes called drawing is the process of reheating hardened martensitic or normalized steels to some temperature below the A1 temperature. The rate of cooling is not important except for some steels which are susceptible to temper brittleness.
As the tempering temperature is increased, the martensite of the hardened steel passes through stages of tempered martensite and is gradually changed into a structure consisting of spheroids of cementite in a matrix of ferrite formerly termed as sorbite.
These changes are accompanied by a decreasing hardness and increasing toughness. The tempering temperature depends upon the desired properties and the purpose for which the steel is to be used. If substantial hardness is essential, then the tempering temperature is to be low. On the other hand, if substantial toughness is needed, then the tempering temperature is to be high. Proper tempering of hardened steel needs a certain amount of time. At any selected tempering temperature, the hardness drops rapidly at first, gradually decreasing more slowly as the time is prolonged.
Short tempering periods are normally undesirable and are to be avoided. Good practice needs at least 30 minutes or preferably, 1 to 2 hours at tempering temperature for any hardened steel. The necessity for tempering the steel promptly after hardening cannot be overstressed. If fully hardened steel is allowed to cool to room temperature during hardening there is a danger of the cracking of the steel.
Carbon steels and most of the low alloy steels are required to be tempered as soon as they are cool enough to be held comfortably in the bare hands. Steels are not to be tempered before they cool to this temperature because in some steels the Mf temperature is quite low and untransformed austenite can be present.
Part of all of this residual austenite transforms to martensite on cooling from the tempering temperature so that the final structure consists of both tempered and untempered martensite. The brittle untempered martensite, together with the internal stresses caused by its formation, can easily cause failure of the heat-treated steel part. When it is possible that such a condition exists, a second tempering treatment double tempering is to be given to temper the fresh martensite formed on cooling after the initial tempering treatment.
If structural steels are to be used in the normalized condition, the normalizing operation is frequently followed by heating to a temperature of around deg C to deg C.
The purpose of this treatment, which is also designated as tempering, is to relieve internal stresses resulting on cooling from the normalizing temperature and to improve the ductility of the steel. Case hardening is a process of hardening a ferrous alloy so that the surface layer or case is made considerably harder than the interior or core. The chemical composition of the surface layer is altered during the treatment by the addition of carbon, nitrogen, or both. The most commonly used case-hardening processes are carburizing, cyaniding, carbo-nitriding, and nitriding.
Carburizing — Carburizing is a process which introduces carbon into a solid ferrous alloy by heating the metal in contact with a carbonaceous material to a temperature above the A3 temperature of the steel and holding at that temperature. The depth of penetration of carbon is dependent on the temperature, the time at the temperature, and the composition of the carburizing agent.
As a rough indication, a carburized depth of around 0. Since the primary objective of the carburizing is to get a hard case and a relatively soft, tough core, only low-carbon steels upto a maximum of around 0.
After carburizing, the steel has a high-carbon case graduating into the low-carbon core. A variety of heat treatments can be used subsequent to carburizing, but all of them involve quenching the steel to harden the carburized surface layer.
The simplest treatment consists of quenching the steel directly from the carburizing temperature. This treatment hardens both the case and core in so far as the core is capable of being hardened. Another simple treatment, and perhaps the one most frequently used, consists of slowly cooling from the carburizing temperature, reheating to above the A3 temperature of the case around deg C ,and quenching.
This treatment hardens the case only. A more complex treatment is to double quench consisting of first quenching from above the A3 temperature of the core around deg C for low-carbon steel and then from above the A3 temperature of the case around deg C. This treatment refines the core and hardens the case.
The plain carbon steels are almost always quenched in water or brine while the alloy steels are usually quenched in oil. Although tempering following hardening of carburized steel is sometimes omitted, a low-temperature tempering treatment at around deg C is a good practice.
It is sometimes desirable to carburize only certain parts of the surface. This can be done by covering the surface to be protected against carburizing with some material which prevents the passage of the carburizing agent. The most widely used method is copper plating of the surfaces to be protected. Several proprietary solutions or pastes, which are quite effective in preventing carburization, are also available.
The commercial compounds commonly used for pack solid carburizing contain mixtures of carbonate usually barium carbonate , coke diluent , and hardwood charcoal, with oil, tar, or molasses as a binder. Mixtures of charred leather, bone, and charcoal are also used. The carburizing action of these compounds is diminished during use and it is necessary to add new material before the compound is reused.
Addition of one part of unused to three to five parts of used compound is common practice. The parts to be carburized are packed in boxes or other suitable containers made of heat-resistant alloys, although rolled or cast steel can also be used where long life of the box is not important.
The top cover of the box is to be sealed with fire clay or some other refractory to help prevent escape of the carburizing gas generated at the carburizing temperature. The depth and uniformity of case is affected by the method of packing and design of the container.
As the work comes to a climax, the cooling phase allows the material to return to room temperature. Remember, workpiece dimensions vary. Furnace temperatures are key of course. To avoid trial-and-error work and impossibly intricate engineering equations, soak time tables can be consulted. However, as every furnace technician knows all too well, there are several different structures altering branches.
The temperature in the furnace rises, the alloy diffusion effect accelerates, and then the quench phase rapidly cools and hardens the material.
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