Rushman developed an approach to predict the as-quenched hardness of a steel bar of known hardenability by using the Jominy diagram for the steel of interest, a two-diameter test bar shown in Figure 27, and a modified Grossmann chart shown in Figure 28 (74). These materials can act as solvents for some of the components used in insulating materials with consequent failure of the equipment due to blocked tubes and valves in the refrigerator circuit. High-alloy tool steels and jet engine turbines are common examples of parts often quenched in gas. The concept is relatively simple: Heat a metal and then rapidly cool it to make it harder. From these data, the Jominy distance from the curve of the desired position on the selected round bar size is determined. Gas or air quenching will provide the least distortion and may be used if the steel has sufficient hardenability to provide the desired properties. The Jominy distance that produces the highest hardness is typically taken at the shoulder of the Jominy curve of interest. The results obtained were as follows. (The carbide-forming elements chromium, molybdenum, tungsten, and vanadium tend to alter the shape of the isothermal transformation curve in addition to shifting it to longer times.). Because salts are not flammable, they pose no risk of fire. I almost never use water other than for low carbon or non-hardenable material. Because quenching and the heat treatment process is so important to the production of strong, long-lasting metal parts, it pays to be diligent when choosing a heat treatment provider. The application temperatures range from about 40 to 200 °C. It describes the methods of quenchant characterization using hardening-power and cooling-power tests. Figure 1 shows an overview of the most common quenchants and their cooling capacities (1,2). For example, assume that the hardness of the steel at 5/16 inch from the quenched end of a Jominy bar is Rc = 45. Silicon is frequently used with manganese to obtain a strong, tough steel. Reproduced from Rushman, W. F. How to Determine Quench Severity of Oil and Salt Baths. Air is a popular quenching media used to cool metals for quenching. Conversely, sometimes cooling needs to be slowed. u. Werkstofftech. Figure 2 shows cooling curves for different quenchants (see also Section The Grossmann H-Factor is defined as the ratio of the heat transfer coefficient at the surface of the steel specimen divided by twice the heat conductivity of the steel. This problem can be overcome partly by fitting heaters or by using admixtures with a more volatile gas (such as nitrogen). Specific recommendations for quench media selection for use with various steel alloys is provided by standards, such as AMS 2779. To improve weldability, they are welded using the furnace induction method. If the quenching conditions remain the same, this H-value (H = 2.0) may be used to determine the depth of hardening for a different section size or even a different steel. Reproduced from Surm, H.; Frerichs, F.; Lübben, Th. Table 7.7. Its electric strength is of the order of 2.3 times that of air or nitrogen, and at a pressure of 3−4 atm it has an electric strength similar to that of transformer oil at atmospheric pressure. Strong carbide formers like vanadium or tungsten can tie up carbon so effectively that it resists solution in the austenite during heating for hardening. Modern technology consists of closed systems in which salt is reused and no wastewater is released. The gas sublimes at about − 64°C and it may be used at temperatures up to about 150°C. The severity of a quench refers to how quickly heat can be drawn out of a part. The two-diameter test bar is used to evaluate the quenchant under actual production conditions. Figure 23 illustrates the correlation of half-temperature time at the surface of a Jominy bar with respect to the distance from the quenched end (74). But a variation of this process was only possible by use of different furnaces. A major advantage of gas quenching is that the parts leave the process clean, eliminating the need for a subsequent washing operation. Mohammad A.S. Masoum, Ewald F. Fuchs, in Power Quality in Power Systems and Electrical Machines (Second Edition), 2015. Assume that the bar diameter is 4 inch, as illustrated in Figure 25(b) (R = 2). In this case, only the rings quenched with the used high-pressure gas quenching device fall below this threshold. Quenching oils, normally used are mineral oils and have viscosity around 100 SUS (Saybolt Universal Seconds) at 40°C. Chart correlating Dcrit, DI, and quench severity (H). Illustration of Jominy H-bands for AISI 1020 and 1040 steels. For more details one may refer to the manufacturers' catalogues and literature available on the subject. Experiment: Heat Treatment - Quenching & Tempering Objectives 1) To investigate the conventional heat treatment procedures, such as quenching and annealing, used to alter the properties of steels. Alloy steels are generally more adaptable to marquenching. Some illustrative values are provided in Table 29.9.9,33 Typically, the greater the quench severity, the greater the propensity of a given quenching medium to cause increased distortion or cracking. Therefore, the potential presence of such residual elements must be monitored carefully. Affordability is one of the main benefits of air; its affordability is a result of its profusion on earth. The value of r/R = 1.4/2.0 = 0.7, which corresponds to Figure 26(h). The question is: What is the maximum bar diameter that will produce the maximum hardness throughout when the steel is quenched in a petroleum oil with an H-value = 0.35? at a surge frequency of 5–100 kHz or more, depending upon the closing circuit constants L and C. They may exist in the system for a very short duration of much less than even one half of a cycle, i.e. Reproduced from Surm, H.; Frerichs, F.; Lübben, Th. The details of the heat treatments are given in Table 5. Quench hardening is a mechanical process in which steel and cast-iron alloys are strengthened and hardened. Let us know how we can help with your next heat treatment job, and view the guide below to learn more about the role of quenching in heat treating. Excellent distortion control is typically obtained with austempering, quenching into a medium just above the Ms temperature and then holding until the material transforms completely to bainite. Therefore, for each grade of steel, there exists an acceptable range of hardenabilities exemplified by a so-called ‘Hardenability (H-) Band’. Within the aluminium alloy, there is a special category of alloys which require special attention, the cast aluminium alloy. Chart for predicting approximate as-quenched cross-sectional hardness from Jominy curve data. Table 13. These kinds of alloy are used in many different Although Table 29.7 is useful to obtain a relative measure of the quench severity offered by different quench media, it is difficult to apply in practice because the actual flow rates for ‘moderate’, ‘good’, ‘strong’ and ‘violent’ agitation are unknown. Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels. Lamont developed a methodology based on the Jominy hardenability test and a fractionally hardened round bar (75,76). used a simpler definition of half-temperature time, which was “the time occupied in the quench, in cooling from the quenching temperature (austenitizing temperature) to a temperature halfway down to that of the quenching medium” (74). In steels of high alloy content, it is the usual practice to control the carbon of the case to a somewhat lower level than in low alloy and plain carbon steels. Otherwise they are difficult to … This well-known diagram may be modified by extending the lines indicating quench severity (H-values) to a common point as shown in Figure 28 (77). ; Hoffmann, F.; Zoch, H.-W. Distortion of Rings due to Inhomogeneous Temperature Distributions. This means cooling is more controlled and uniform compared to colder, faster and more severe quenches. Illustration of Jominy H-bands for AISI 4130 and 4140 steels. Keeping in mind that these rings were austenitized at 870 °C, it is clear that more carbon was dissolved and therefore a larger volume change is expected. The application temperature is about 30–60 °C (or even 70 °C). In particular, a large standard deviation of the mean angle is noticeable. In fact, any material that is heated and then allowed to cool to room temperature simply by being left alone is considered to have been air quenched. Retained austenite is most pronounced where Mn and Ni are major components. Some are just…, One of the most critical parts in the heat treatment of a metal part is the quench, or the rapid…, Process is important. The Grossmann diagram relates the DI value to Jominy distances (74). Different mixtures of salts have different melting points and working ranges, offering added versatility as a quenching option. Quenching in hot oil is a slower process compared to quenching in fast oil. It is for the user to choose the most appropriate circuit breaker to suit requirements, application and cost. High-alloy parts with intricate designs quench well in hot oils, as the method reduces the risk of warping and cracking associated with differences in surface and core temperatures. Figure 23. Table 6. The cooling curves have been recorded using a test probe in Inconel 600, ø12.5 × 60 mm, according to ISO 9950. Typical speeds of interrupters may fall in the range of 1–1.5 mm/ms. What quenching media is used for special alloy steels? However, even with a severe quench, undesired microstructures such as ferrite, pearlite or bainite can form. This process is limited to selected alloy-containing steels and suitable section sizes. Some specs contain too little information. SUGGESTED CARBON CONTENT LIMITS FOR WATER, BRINE AND CAUSTIC QUENCHING, L.C.F. The higher percentage carbon alloys can achieve a greater degree of hardness. Although the test bar may be machined from the same alloy as the parts being produced, more typically, to keep the size of the test bar small, it is machined from a low-hardenability steel. Fast quenching oils have viscosity around 50 SUS at 40°C and are blended mineral oils and approach water-quenching power only in the initial stage of cooling. Therefore, many steels are also available as H-grade, which possesses tighter compositional control and the control is specified by the maximum and minimum allowable hardenability. As an example, consider the steel represented by the Jominy curve in Figure 25(a). Cooling capacity can be increased by quenching in a separate cold chamber instead of in the furnace. Careful selection of materials for resistance to these fluids is essential. Quenching metal via gas in vacuum furnaces has become more popular for parts that require high hardness and specific finishes with significantly reduced risk of distortion. Through-hardening is determined from Figure 26(a), where the fractional depth of hardening r/R = 0.0. - 26068648 Water is one of the most efficient quenching media where maximum hardness is desired, but there is a small chance that it may cause distortion and tiny cracking. Most of the fluorinated gases have an electric strength between two and five times that of air or nitrogen under the same conditions but, as with sulphur hexafluoride, care must be taken to prevent high-voltage discharges or arcs in the gas because of the dangers of producing decomposition products. Carbon is the most potent of the alloying elements for lowering the Ms temperature and increasing retained austenite. In vacuum furnaces, gas quenching is normally applied using nitrogen at pressures up to 20 bars, but higher pressures are also used. Aqueous polymer quenchants may often be used to replace quench oils but quench severity is still of primary importance. In this chapter we discuss the types of insulating and quenching mediums, their switching characteristics and merits and demerits of their use. In such cases, the austenite may be lower in carbon content than the corresponding unalloyed steel and the isothermal transformation curve is shifted to shorter rather than longer times. The higher the temperature, the lower the viscosity. One objective in conducting the Jominy end-quench test is to determine the adequacy of using a particular quenchant to harden a steel with a specific hardenability. Download: The role of quenching in heat treating, The salt bath nitriding process and its safer alternative, Understanding heat treatment specifications, The basics of oil quenching in heat treating. Figure 36. Reproduced from Surm, H.; Frerichs, F.; Lübben, Th. The rings quenched in a gas nozzle field show the maximum volume change. Comparison of the Fourier coefficients regarding the change in out-of-roundness of the outer surface for different quenching conditions. Austenitizing temperatures and quenching media for direct hardening 8620 low-alloy steel: AISI: Austenitizing temperature, °C (°F) Quenching medium: Temper, °C (°F) 8620 (UNS G86200) 845 (1550) Oil or Austenitize at 830 °C (1525 °F) for water quench: 200-650 (390-1200) For this distortion criterion, quenching in a gas nozzle field seems to be the best choice to avoid a second order in tilting. However, the Jominy end-quench test involves quenching only one end of a bar of steel with standardized dimensions and heating conditions with a stream of water at a specified temperature and flow conditions. Some typical quenching cycles are direct hardening, martempering, and austempering, which are illustrated schematically in Figure 3. Gas insulated high voltage lines are now in operation using these gas mixtures. The series connection of an FIS with a fault-current limiter either reduces the interrupting conditions or makes the substitution of SF6 with a less effective quenching gas possible. Quenching metal is one of the critical stages in the heat treatment of a metal part because it’s during that process that added hardness is locked in. Surges that may appear on an LT system as a result of transference from the HV side of a transformer (Section 18.5.2) are different and not related to switching. Therefore, the carbon content of the steel is one of the determining factors for quenchant selection. Here we discuss briefly the philosophy of circuit interruption and the effect of insulating and quenching mediums on the arc extinction of these breakers. While these quenchants can pull heat out of parts more quickly than other quenching media, faster isn’t always better. These oils cannot be used for shallow- hardening steels. Thus, the maximum bar diameter of the steel represented by the Jominy curve in Figure 25(a) that will be through-hardened when quenched in a petroleum oil with an H-value of 0.35 is 1.5 inch. Switching surges will also develop when all the three poles of a switching device do not make at the same instant. . Making an improper selection of steel for the process. The depth of hardening is the ratio (r/R) of the unhardened core (r) to the total radius (R) of the bar being quenched. Comparison of the Fourier coefficients regarding the change in angle of outer face (conicity) for different quenching conditions. Table 29.10 summarises the mean carbon content limits for water, brine or caustic quenching of some steels.33,35, Table 29.10. Mat.-wiss. In gas quenching, parts are sealed in a vacuum chamber before being blasted with gases. The most severe quenches are executed with water, brines and caustic sodas. the dielectric) between the contacts becomes incapable of withstanding the impressed voltage and breaks down. ; Hoffmann, F.; Zoch, H. -W. Distortion of Rings due to Inhomogeneous Temperature Distributions. Agrawal, in Industrial Power Engineering Handbook, 2001. The following types of breakers have been developed for the purpose of switching and they mostly relate to the HT systems, except where noted: Minimum Oil or Low Oil Content Circuit Breakers (MOCBs or LOCBs), Air Circuit Breakers (ACBs) – generally for an LT system only, Sulphur Hexafluoride Circuit Breakers (SF6 breakers). By continuing you agree to the use of cookies. Low alloy steels contain a few percent (typically between 1 and 7%) of elements such as Cr, Ni, Mo and V. This category includes chromium steels (containing up to 5% Cr and 1% Mo) and nickel steels … Most machined parts made from alloy steels are oil quenched to minimize distortion. To obtain the desired result of martensite from the surface to a specific depth, quenching must be rapid enough to transform austenite to martensite. Although the gas is considered to be non-toxic, non-flammable and chemically inert, under the influence of arcs or high-voltage discharges, there may be some decomposition with consequent attack on certain insulating materials and metals, and more importantly some recent environmental concerns. In order to clarify the quenching crack phenomenon in cast steel, a disk shaped low alloy cast steel specimen with an opening was quenched in water at 1123 K. The behavior of quenching crack was checked by microstructure observation and analyzed by heat treatment simulation. Half-temperature time versus distance from the water-cooled end of the Jominy bar. After austenitizing and quenching, the two-diameter bar is sectioned, and the hardnesses at the center are determined at three points: at the center of each diameter and at the ‘common point’.

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