Tratamientos Isotérmicos PDF

# CAPÍTULO X # TRATAMIENTOS ISOTÉRMICOS ## 109 - Como consecuencia de los estudios derivados de la curva de la S, se ha incrementado mucho el uso los baños calientes (plomo y sales fundidas) para el tratamiento de piezas y herramientas, debido principalmente a los siguientes motivos: - El conoc...

# CAPÍTULO X # TRATAMIENTOS ISOTÉRMICOS ## 109 - Como consecuencia de los estudios derivados de la curva de la S, se ha incrementado mucho el uso los baños calientes (plomo y sales fundidas) para el tratamiento de piezas y herramientas, debido principalmente a los siguientes motivos: - El conocimiento de las valiosas propiedades del nuevo constituyente “bainita”, cuya presencia es interesante en gran número de piezas y perfiles. - La gran reducción de grietas y deformaciones que se consigue al emplear baños de sales para el enfriamiento del acero desde la temperatura de temple. Estos defectos se presentan con más frecuencia cuando se templa el acero en agua o en aceite. The periphery of the pieces cool rapidly while the center is still very hot. When the structural transformations take place, this occurs first in the periphery and then in the center. This creates large tensions between these zones, which can lead to deformations, cracks, and failures. In contrast, when cooling in a hot bath, these problems are avoided because the temperatures of the center and periphery equalize before the transformation of the austenite begins. The transformation can then be completed at the same time throughout the piece. - When using salts of appropriate composition in these treatments, surface oxidation of the steel can often be avoided. - The possibility of softening certain steels in far less time than is needed to achieve the same results by conventional annealing treatments. ## Ventajas del enfriamiento en baños de sales - The influence of the cooling method on the appearance of cracks and deformation, as observed in Figure 268, is fairly clear. - Figure 268 graphically represents the cooling process for four identically made pieces that were quenched in water, oil, a salt bath, and air. It shows the transformation of an alloyed steel when cooled from a high temperature in any of these media. - The piece always remains hardened. The cooling rates for the four cases are all greater than the critical hardening rate. Lines M_S and M_R indicate the beginning and end of the transformation of austenite into marten-site. - When looking at these lines, we know the time at which the transformation of austenite into martensite begins and ends in the center and periphery of the pieces. - It is clear that the more energetic the cooling medium is, the greater the temperature difference between the periphery and the center, resulting in greater internal stresses. This is what causes cracking and deformation of the steel. - When quenching in water, as the cooling curve rises above point Ms on the austenite-to-martensite line, the temperature difference from the center to the periphery is large. When the periphery of the piece hardened in water has completely transformed to martensite, point C, the transformation of the austenite in the center has not begun, point D. The process leaves a lot of austenite in the core. - This difference is smaller in oil, but the austenite in the center has not completely transformed when the periphery has completely hardened. - In air, the difference is even smaller. All parts transform at nearly the same time, so there is minimal residual stress and the risk of cracking and deformation is minimal. - The most beneficial method is to cool a piece in a salt bath at a temperature slightly above M_S and hold it there for a while. This ensures that the temperatures of the periphery and center are equalized before any austenite transforms. Then, when air-cooled, the austenitic transformation of the periphery and center occur nearly simultaneously. Because the temperature difference between the two zones is small, the resulting stresses and deformations are minimal. This almost eliminates the risk of cracking. ## 110 Recocido isotérmico - This type of treatment, which has become very popular in industry in recent years, is much faster than conventional slow-cooling annealing treatments. It is noteworthy, that in the latter treatment, despite the long duration, the transformation time of austenite is relatively short in comparison to the total treatment duration. For this reason, the special importance of the austenite transformation curve (the S-shaped curve) in modern annealing treatment, is particularly significant. This curve helps in identifying the temperature range to reach, for a desired microstructure and hardness. This curve also allows the prediction of the transformation events. - Essentially, isothermal annealing involves heating steel to a temperature slightly above the critical point A_C1 (sometimes above A_C3), then cooling to just below point A₁. It is then held at this temperature for the time needed for the austenitic transformation to occur, then cooled in air (Figure 270). - In Figure 269, the S-curve of an austenitization treatment is shown, and because it is a continuous cooling curve, it is generally displaced to the right of Figure 270. The 270 curve more closely represents what is happening during an isothermal treatment. - The S-curve can be used to estimate how much time can be saved by implementing isothermal annealing. For example, the isothermal annealing cycle for tool steel containing high alloying elements, such as carbon tool steel, is described in Table XXII. - For conventional annealing, the steel is heated and held at 1472°F (800°C) for over 12 hours, slowly reducing the temperature until it reaches 1112°F (600°C), and then the steel is air cooled. This can take anywhere from 12 to 24 hours. - For isothermal annealing, the steel is heated and held at 1472°F (800°C) for one hour, cooled to 1292°F (700°C) and held at that temperature for another two hours. Then it is air cooled. This completes the entire process in 6 to 12 hours. - Overall, this indicates that the isothermal annealing treatment is far faster than conventional annealing for obtaining the same microstructure and hardness. ## 11. Recocido isotérmico del mismo acero se hace en la siguiente forma: - Heat to 1427°F (780°C) for one hour. - Cool to 1292°F (700°C) for two hours. - Air-cool. - The total cycle time is 6 to 12 hours. - In Figure 271, the results obtained by Peter Payson in laboratory tests on small samples of 18-4-1 high-speed steel are presented. The samples were austenized at 1607°F (875°C) for 1 and 12 hours. Then the steel was held at 1420°F (780°) for different time periods, from 10 minutes to 6 hours, and then quenched in water. These results are quite noteworthy because, even with very short holding times of 3.5 hours, it was possible to soften the steel, achieving a hardness of 260 Brinell. This is quite good for a high-speed steel. - The isothermal annealing of an 12% chrome, non-deforming steel is performed as follows: - Heat to 1700°F (925°C) and hold for 2 hours - Cool to 1436°F (780°C) and hold for 4 hours. - Air-cool. - The final hardness will be approximately 225 Brinell. - It is essential for the cooling process to take place quickly from the temperature of the austenite. This will ensure that the process will short and result in a soft steel. - The faster the heat is removed, the harder the material will become. - The isothermal annealing treatment has become widely used for forging. - A classic method for annealing forgings was to reheat the piece above the critical point A_C1, hold the temperature for a while, and then cool slowly. This process was time-consuming and prone to cracking. - By implementing isothermal annealing of forgings, it is possible to achieve an acceptable hardness in a much shorter time. This is shown in the example of a high-speed steel 18-4-1, shown in Figure 271. - This method takes advantage of several things: - The piece does not need to be reheated for a specific time when being transferred from the press or forging hammer to the salt bath. Temperatures can be above 1340°F (725°C). - The bath is then held at a specific temperature that is dependent on the type of steel used and the hardness desired. - The most commonly used temperature for such treatments is 1260°F (685°C). It is then air-cooled. - This method is particularly beneficial for forgings and reduces the time in comparison to conventional annealing treatment. ## 111 Austempering - This treatment developed in the United States, and was widely adopted in the early years. It is particularly useful for small forgings, tools, or parts that need to have a hardness of 40 to 55 Rockwell-C. - The pieces subjected to this treatment have bainitic structures. These structures offer higher toughness than the structures of similarly hard pieces treated by other methods that were available at the time. - The process of austempering the piece, which is diagrammatically shown in Figure 272, involves heating to a temperature that is slightly above the critical point A_C1, and then holding the temperature for the appropriate time to fully transform the austenite. The temperature of the bath should be above M_S, to avoid the formation of martensite. It is also important to maintain this temperature. - This treatment is very promising because it eliminates the risk of cracks associated with conventional hardening and tempering treatments. - It is also easily implemented and automated. - The treatment is most effective, and the results are very good when applied to carbon or alloyed steels with 0.50% to 1.20% of carbon. - Unfortunately, the results are not always as good for construction steels containing 0.20% to 0.40% carbon. It's difficult to make generalized recommendations about the advantages and disadvantages of this method for these types of steels. This is because the results are inconsistent in these low-carbon steels. - Table XXIII presents a comparison of the properties of a 0.74% carbon steel quenched in water and those of a 0.74% carbon steels that have been austempered for 15 minutes at 572°F (300°C) for a Hardness of 50 Rockwell-C. - The comparison shows that the austempered steel: -is stronger, -has a higher yield strength, -has a higher elongation, -is more ductile (has higher reduction in area) -and has higher impact strength. - In Figure 274, a comparison of the results obtained from a 0.90% carbon steel after austempering and conventional hardening and tempering is presented. The specimens were all 8 mm (0.31 inch) round, and the round, and the round that was subjected to austempering had a Rockwell-C hardness of 50. - The results show that the austempered steel: - is more ductile, - has greater impact strength, - has greater bending strength. - This Austempering treatment is more easily implemented for smaller parts because they reheat to the austenitizing temperature in a furnace of any type and then can be transferred into the molten metal or salt bath held at the proper temperature. - However, the austempering process can also be implemented for larger parts. First, you need to overcome the challenge of cooling larger items quickly. High-alloy steels, and advanced mechanical methods have made this possible. - A recent innovation in the austempering process utilizes a variation of the process for larger items, similar to the austempering process for the smaller pieces. - In this variation, the steel is quickly cooled in a bath that is below M_S after the austenitization process. This ensures that a portion of the austenite transforms to martensite. The piece is then immediately transferred to a bath that is at the correct temperature for isothermal transformation. This approach ensures that the rest of the steel, which is still austenitic, is subject to isothermal transformation. - Overall. this allows for the austempering process of larger items even though some of the piece will transform to martensite. ## 112 Martempering - This treatment involves heating the steel to a temperature that is above the critical point A_C1 and then holding the temperature in a salt bath, heated between 390°F (200 °C) and 570°F (300°C) for enough time to achieve temperature uniformity within the piece. The salt bath temperature must be above M_S. - The salt bath is heated this temperature to prevent martensite formation even though the piece is cooling. - The piece is then air-cooled. - This process ensures a martensitic structure with minimal residual stresses. If the piece is too hard, it can be tempered later in the process. - The martempering process requires a relatively rapid initial cooling rate to prevent the austenite transformation curve (the S-curve) from being breached. - If the S-curve is breached, some of the austenite will transform into other phases, which makes the transformation of the remaining austenite difficult and possibly prevents attaining the desired microstructure and hardness. - Martempering is often used for larger pieces because it is difficult to achieve the desired cooling rates in thicker items. - Figure 275 shows how the martempering process is implemented. - Unfortunately, the use of martempering is limited. This is particularly true when using larger pieces, because achieving the necessary cooling rates in these cases is challenging. - Figure 276 shows a martempering process being used for 450-mm (18-inch) diameter bearings. The piece is heated to 1562°F (850°C) in the furnace on the right, then transferred to a 260°F (130°C) bath in the furnace on the left. - The martempering treatment is ideally suited for manufacturing smaller items. ## 113 Patenting - Patenting is a special process for treating wire, which is used to increase its strength. It involves a combination of heat treating and plastic deformation. - Patenting is used in two basic ways: - First, it acts as an intermediate treatment that is performed on wire between multiple drawing operations. This treatment is designed to eliminate the effects of prior drawing and ensure that the material is ready for further drawing. - Second, it can act as the final treatment for wire. This treatment is used to ensure that the wire has the desired ductility and strength. - Patenting usually involves the following steps: - Heat the wire to a temperature that is above its upper critical point A_c3. - Slowly cool the wire in a salt bath near 390°F (200°C). - After cooling, the wire is drawn. - The drawing process can be repeated several times, depending on the desired properties of the wire. - The wire, which has been deformed and cooled, will have bainitic or sorbitic structures. The properties created by the patenting process are superior to those obtained from conventional annealing. The process also eliminates residual stresses. - Additionally, the patenting process makes it easier to further draw the wire, which can be challenging with large, high-carbon-content wire. - Wire with a high carbon content is often drawn several times, before fully finishing the wire. Due to its high carbon content, drawing is often difficult. This treatment avoids this problem. - The final draw should be performed at a temperature above the critical temperature A_C3. This ensures that the structure is fully austenitic and that the wire has the desired ductility. - During this final draw, the wire is quickly cooled in water, then passed through a series of dies. The dies gradually reduce the diameter of the wire and significantly improve its strength. - The patenting process helps create the desired microstructure in the wire, which optimizes its strength and toughness. - The patenting process is also beneficial for other reasons: - It reduces the amount of labor required in the production process. - It increases the quality of the final product. - It lowers the cost of production. ## 114 Tratamiento subcero -This treatment, which is sometimes abbreviated as cryogenic treatment or low-temperature treatment, involves cooling steel to very low temperatures below 0°C (32°F) (typically below -75°C (-103°F)). - The treatment was developed in the same period as the isothermal annealing treatments, and it's an outgrowth of the research that had been conducted on the austenite transformation curve (the S-curve) at lower temperatures. - This innovative treatment has created significant interest in the metallurgical community. Many metallurgists see the treatment as a game changer, offering a way to significantly extend the lifetime of tools. Others disagree, offering no evidence of a real improvement. - Subzero treatment is primarily applied to tool steels that have already been hardened and tempered in a conventional manner. These tool steels often contain some residual austenite after the conventional treatment, and this residual austenite is the cause of a decrease in hardness. - The amount of residual austenite in a treated piece varies significantly and depends on factors including the composition of the steel, the heat treatment, the size of the part, the cooling method, and the temperature it is held at in the salt bath. - When comparing different types of steels; the percentage of residual austenite that is found in hardened and tempered steel is generally: - between 25% and 35% in low-carbon steels with 1% carbon and 5% chromium; - between 15% and 30% in low-carbon steels with 1.5% carbon and 12% chromium; -between 15% and 25% in high-speed steels; -between 3% and 15% in low-alloy tool steels; -between 5% and 10% in plain carbon steels. - In the past, the residual austenite of steel usually transformed as the tool underwent the tempering process. However, this transformation was gradual and often resulted in a decrease in the hardness of the tool over time. - The subzero treatment, however, offers a method for transforming this residual austenite into martensite, which also increases the hardness of the tool. - Figure 229 shows the transformation of austenite in a steel as it is tempered. The curve for the S-curve shows that the transformation of austenite to martensite does not occur below -75°C (-103°F). - The subzero treatment is especially effective for high-speed steels, cold-work tool steels, and cemented steels. However, it is not always as effective in steels that are typically used for standard construction applications. This is because the steel is also subject to a conventional quenching and tempering process, and the results of the two treatments can be inconsistent. - Figure 281 shows the transformation of an 18-4-1 high-speed steel. This steel is austenized, tempered, and then held at 0°C (32°F), -50°C (-58°F), and -100°C (-148°F), then cooled to even lower temperatures. - The chart shows that: - 80% of the austenite transforms when the steel is held at 0°C (32°F). - 92% of the austenite transforms when the steel is held at -100°C (-148°F). - The transformation of austenite is almost complete after the steel is held at -100°C (-148°F). - The graph also shows that if you hold the piece at 0°C (32°F) for a long period of time, the amount of austenite that transforms is significantly higher than if you continue to cool without interruption. The figure shows that if the cooling is continuous, nearly 98.5% of the austenite transforms, whereas 98.5% of the austenite transforms when the piece is held at room temperature (0°C (32°F)) for a certain time before continuing the cooling. - Additional studies have also confirmed these results, including those of R.H. Hays as seen in Figures 283 and 284. - These figures show the surface of a 1% carbon, 5% chromium, 1% molybdenum, and 0.80% manganese steel that has been conventional tempered and then subjected to a subzero treatment at -75°C (-103°F) for a specific period of time. - Additionally, Figure 283 shows an 18-4-1 high-speed steel after quenching and tempering that has been subjected to a subzero treatment at -100°C (-148°F). - These studies show that when the piece is held at room temperature (0°C (32°F)) for 1, 10, and 50 hours then treated with subzero treatment, 92%, 92.5%, and 93% of the austenite transforms. By contrast, when the piece is continuously cooled, the austenite is fully transformed. - By understanding the influence of the subzero treatment, it is possible to improve the properties of tools. This is achieved by: - Combining the subzero treatment with multiple tempering operations at 160°C (320°F) and 200°C (392°F). - Then, cooling to below 0°C (32°F). - This approach produces a martensitic structure, which offers high toughness and hardness. ## 115 Temple en agua y en aceite. - This is a very useful for treating complex tools made of water-hardening steels. - These tools tend to crack or deform during the quenching process. - Therefore, it's often advisable to first cool the parts in water, then immerse them in oil. - The process works like this: - The piece is quickly cooled in water to ensure its complete martensitic transformation. - The piece is then immediately transferred to oil to prevent excessive cooling rates. - This process: -prevents the transformation curve from being compromised. -effectively prevents cracks and deformation. - The amount of time a tool is held in water varies greatly and depends on its size and form. - As a rule of thumb, tools about 0.197 inches (5mm) thick are immersed for 3 seconds. - Tools about 0.394 inches (10mm) thick are immersed for 5 seconds. - Tools about 0.787 inches (20mm) thick are immersed for 10 seconds. -Tools thicker than 1.969 inches (50 mm) are immersed for 12 seconds. - After the water immersion is complete, the tool is immediately moved to the oil. - Another technique for quenching involves removing parts from the quench bath (water or oil) before the piece has cooled completely. - This method is widely used. - The piece is removed from the bath when it reaches approximately 392°F (200°C) or 572°F (300°C). Then the cooling process is continued more slowly to equalize the temperature throughout the piece. - This method is beneficial because it minimizes the temperature differences within the piece, reducing internal stress and significantly reducing the risk of cracking or deformation. - The use of this process is ideal for many forging applications, where the risk of deformation and cracking is very high. This technique provides better results, as it minimizes these issues. - This treatment helps to obtain the desired microstructure and hardness without any cracks or deformations. It also avoids additional labor and ensures a higher product quality, all while reducing the cost of production. - The process is often used for bearings, where the production of bearings is a highly complex process that typically results in a high reject rate. - The use of the cooling method in this process has significantly improved the quality and quantity of bearings produced. - It has also dramatically improved the quality of bearings due to its ability to reduce cracking. It has reduced the amount of rejecting, and it has significantly reduced the amount of finishing work needed. - It has significantly improved the quality of bearings by reducing overall costs. - The use of this method has successfully automated production, making it easier and faster to complete the production of bearings in large quantities. - The martempering process was then implemented. The process was used to preheat the bearings to 1562°F (850°C). The bearings were then very quickly cooled in a salt bath at 260°F (130°C). - This method is particularly useful for roller bearing rings, which are 18 inches (450mm) in diameter. The process ensures that the piece is evenly cooled, which prevents the formation of cracks or deformation. - The martempering process is shown in Figure 276, where it illustrates the process for heating a bearing in a furnace and then transferring it to the salt bath. - The martempering process is highly recommended for use in the production of bearings due to its effectiveness in minimizing deformation, enhancing toughness, and reducing the risk of cracking. - Overall, these treatments are essential to the efficient production of high-quality steel products, such as high-speed tools and bearings. They are essential for meeting the increasing demands of a modern industrial world. This chapter provides an in-depth look at treatments, explaining their advantages, disadvantages, and applications.