Investigate and study the effect of the quenching process on the hardness of steel. ( Log Out /  The vapour pressure of the oil is particularly important as this determines the thickness of oil-vapour film produced on the surface of the hot steel, which limits the rate of heat removal. In hardening of steels, the rapid cooling rates may be obtained by bringing into contact, the hot surface of the object with some cooler material, which may he gaseous, liquid, or solid. The machine is controlled by PLC and can modify the temperature for each section on touch screen due to the technological requirements. Heat uniformly to 770-780°C until heated through. The ‘A’ stage is almost missing. It consists of heating Steel components to the temperature within or above its critical range. Enter your email address to follow this blog and receive notifications of new posts by email. The cooling rate, thus, can be adjusted to get ideal cooling rate for a steel component. In this article, I'm going to discuss different ways to harden mild steel. Water is normally added into the vortex created by the stirrer impeller. Steels are heat treated, because of the structural changes that can take place within solid iron carbon alloys. The problem becomes more complex if the shape of the component is complex. However, the cooling rate of oil in stage ‘B’ is increased in hot oil as compared to cold oil, which is desired. Low carbon steel samples exhibited the lowest microhardness while the high alloy steel samples had the highest hardness for all temperature values and quenching media. For example- A solution with 3-4% of polymer compares well with brine. This can be avoided if either the water is circulated in tank, or the component is agitated in the tank. The fluidized bed cooling is slower than water, or oil, and 10% slower than quenching in molten salts, but significantly faster than air. The process of hardening involves rapidly quenching the steel from appropriate temperature into quenching media. The increased temperature brings it close to its boiling point, and thus, requires less heat to form vapour- blanket, specially above 60°C. Normally 10% (by weight) sodium hydroxide is added in water. In this particular instance I’m hardening a piece of silver steel that is to be used as a rivet punch. Very thin flat component like saw discs, with all measures observed, develop distortions. The rise of temperature of the oil, increases persistence of vapour-blanket stage, and thereby tries to decrease the cooling rate. Water quenching also breaks the scale formed during heating. 5. Different quenching media have different degrees of severity. Use of nitrogen provides an inert atmosphere. Copyright 2014 © by Shubbakom There are no fumes and no hazards of pollution. , and quench rgest coolin nching the toid steels uenching. One of the simplest and most commonly used methods of cooling in hardening articles and tools of carbon steels is to quench them successively in two media, first in water and then in oil. Huge Collection of Essays, Research Papers and Articles on Business Management shared by visitors and users like you. They are corrosive as regards appliances, but are not hazardous to workmen, as are the caustic solutions. A component having thick and thin sections should be immersed in the cooling bath with its thicker section first. Caustics are the most severe quenchants, followed by oils, then salts and, finally, gases. We used three quenching media which are air, water and inside the furnace. These metals consist of ferrous metals and alloys. The vapourisation then, ceases. Two techniques are used for liquid quenching, namely, still bath and flush quenching. Compressed air or still air is also possible to be used if the steels have high hardenability, i.e., high alloy steels such as air hardening steels; or light sections of low alloy steels. Warm oil having greater fluidity enables more rapid convection currents to be set up and to carry off the heat. Alloy steels as a rule have high hardenability, are oil quenched with least danger of distortion of cracks. Study the changes in hardness of steel due to the changes of the quenching media. 4. The effectiveness of a quenching medium to provide desired cooling rate depends on its characteristics such as: In water and brine, the cooling rate decreases as the temperature of the coolant increases, as it increases stage ‘A’, i.e., helps in persistence of the vapour-blanket stage. The cooling capacity to about 400°C is high, and then decreases as the temperature of the steel continues to drop. Though, water provides high cooling power near the nose of the curve to avoid transformation to pearlite, or bainite but the greatest drawback of water as illustrated in table 6.11, is that the rate of cooling is high in the temperature range of martensitic formation. Make sure your working area is well-ventilated. ‘B’ stage is not effected in water and the aqueous solutions, while it increases in oil. The cooling rate of the solution depends on the amount of polymer added in water as illustrated in Fig. Sizzling sound (due to evolution of steam) indicates the presence of water in the bath. Element C Si Mn P S Cr Fe For still bath quenching, a tank of liquid is employed in which the metal is immersed, and the liquid alone is circulated. However, the Jominy end-quench test involves quenching only one end of a bar of steel with standardized dimensions and heating conditions with … Prohibited Content 3. At that stage, steel is simultaneously under the influence of structural stresses and thermal stresses, the added effects of which increase the risk of crack formation. Change ), You are commenting using your Facebook account. Account Disable 12. The makeup of metal parts and the specified hardness to be achieved dictate which quenching mediumis used. oil is used to quench high-speed and oil-hardened steels and is preferred for all other steels provided that the required hardness can be obtained. ve their Ac ally used ereutectoid rtion. Average hardness values of steel samples quenched in different quench media. Content Filtration 6. This method “through water to oil” incidentally increases the depth of hardening of oil-hardening low alloy steels. Also, the oils should be used at 50°-80°C, when these are more fluid, i.e. The cooling rate of oil is insufficient to avoid transformations to pearlite in plain carbon steels The slower cooling rates of oils in the martensitic transformation range is an advantage. These oils cannot be used for shallow- hardening steels. * Continue reading the full report from the attached PDF below. A coolant with high thermal conductivity transfers the heat rapidly from the component to its entire mass increasing thereby cooling rate of component. Heating steel to appropriate temperature, and then quenching it using different quenching media. Oil is classed as an intermediate quench. In this Experiment we studied the effect of heat treatment on the properties of carbon steel especially its hardness. The bubbles are carried away by convection currents and the liquid touches the metal again. Oils in contrast to water, or brine are much lower in their quenching power (having greatest cooling rate at about 600°C), and are relatively slow in the range of martensitic formation, the latter minimises the danger to crack formation. Important Quenching Mediums  4. This operation is called quenching and includes methods of cooling by jets of air, water or other liquids- immersion in liquids, such as brine, water, polymer quenchant, salt baths, cooling between plates. The disadvantages of oil-quenching is their high inflammability (flash point is 165-300°C), insufficient stability, low cooling power near the nose of the curve and higher cost. Lesser is the boiling point of a coolant, more easily the vapours form to increase the ‘A’ stage of cooling, which provides slower rate of cooling. Emulsions invariably form layers with water at the bottom of the tank. If anything my last conversation with them about this lead down the path of them recommending over temp quenching to blow up the grain and using a faster quench (hotter oil) to try and catch this mythical hardness unicorn that lays dormant in the steel. Determining Austenite Grain Size of Steels: 4 Methods | Metallurgy, Unconventional Machining Processes: AJM, EBM, LBM & PAM | Manufacturing, Material Properties: Alloying, Heat Treatment, Mechanical Working and Recrystallization, Design of Gating System | Casting | Manufacturing Science, Forming Process: Forming Operations of Materials | Manufacturing Science, Generative Manufacturing Process and its Types | Manufacturing Science. Metallurgy, Steel, Cooling, Methods, Quenching. 12-15% additions are good for shallow hardening steels; 15-30% for case hardening steels. Water, the more drastic coolant would have caused distortion and cracks. Inverse solubility is a unique property of such quenchants. Hardening Steel Using Different Quenching Media.pdf. As the temperature of the part falls below the inversion temperature (here 77°C), the thin film of polymer dissolves and thus, permits fast removal of heat from the part. The slower cooling rate in ‘C’ stage in oil is an advantage to reduce danger of cracking. Oils, as a group, are intermediate in cooling velocity between water at 40°C and water at 90°C. The cooling efficiency of a bath gets decreased, if it is contaminated. Small holes are stuffed with wet asbestos to prevent the quenching liquid from penetrating into them. ( Log Out /  Stage B – Intermittent Contact Stage (Liquid Boiling Stage): Heat is removed very rapidly in this stage as the heat of vapourisation, as indicated by steep slope of the cooling curve. Table 1. These are able to remove the scale in a better way than water. Sufficient volume of coolant should be in the tank to prevent rise of temperatures. The recommended holding time in the salt bath is 2-4 min/cm of section thickness, the shorter time for lighter sections. Special fixtures are used for them. Oils as coolants are generally heated to 50-80°C to increase its cooling power by lowering its viscosity. The greater efficiency of brines, caustic soda solution, or aqueous solutions is explained as- In brine, or caustic, the heating of the solution at the hot steel surface causes the deposition of crystals of sodium chloride/sodium hydroxide on the hot steel surface. Steam also causes stirring action at the surface of the bath. 2. But, when it is not possible to change the design to simple shapes, and the component is quite irregular in shape and size, then steps should be taken during quenching to obtain as uniform a cooling as possible. erature of th temperatu ed. Quench hardening. Quench Media: Quenching is the act of rapidly cooling the hot steel to harden the steel. Also, milder the cooling medium, lesser the internal stresses developed, and thus, lesser the danger of distortion, or cracks. Martempering oils are used in temperature range of 100- 200°C, and are solvent refined paraffin type mineral oils, having viscosity of 2000 SUS at 40°C. The reason of high cooling power of this solution is similar as explained for brines. Hence, these are useful, where cooling rates in excess of those given by water baths, are required. [gravityform id="1" title="false" description="false" ajax="true"]. Hardening aims in getting martensite, at least in the surface layers of the steel. Copyright 10. Table 6.12 gives some composition of salts and the useful temperature range for each mixture. At the same time, it is desirable to cool a component at a considerably slower rate within the range of martensitic transformation (300 – 200°C) to minimise internal stresses, i.e. Emulsion of 90% oil and 10% water resulted in having properties-cooling rate-inferior to oil. This also lakes care of lack of coolant circulation/or agitation of the part. However, the rate of cooling (the rate of heat transfer from a hot metallic body to the quenching medium) depends on sectional dimensions of the object, its temperature, its thermal properties, the condition of its surface as regards the nature of the oxide film and degree of roughness, initial temperature of the coolant, its boiling point, specific heat of coolant, latent heat of vapourisation, the specific heat of its vapour, its thermal conductivity, its viscosity and its velocity past the immersed object. Hot-quenching oils-used generally in the temperature range of 100-150°C, have viscosity in the range of 250- 3000 SUS at 40°C. The axis of components like thin rings should be normal to the surface of coolant. All rights reserved. As air cooling is slower and more uniform, the danger of distortion is negligible. There are no residues left on parts and they require no post treatment. Instead of showing a constant cooling rate throughout the quench, the cooling curve shows three stages as: Immediately after the start of the quench, the quenching coolant gets vapourised due to metal being at high temperature, and a continuous vapour blanket envelopes the surface of the object. Hot, or cold water flows through the heat exchangers fitted in the bath. ut from the te). Preheating of water is not advisable. This danger may be reduced, if the steel is rapidly transferred from water to oil-bath (which has slower cooling rate near 300°-200°C) when it has cooled to around 400-200°C. Steel is a metal commonly hardened through the quenching process. Privacy Policy 9. Velocity Past the Immersed Object (Circulation of the Coolant), or Agitation of the Component: Both these factors effectively wiped off the vapour film as quickly as it forms, eliminating, or reducing the length of the vapour-blanket stage, and the piece more, or less starts cooling under ‘B’ stage of cooling, i.e., the component gets cooled at a faster rate. This action destroys the vapour film-blanket from the surface, and therefore, permits actual contacts of the coolant and the steel surface with an accompanying rapid removal of heal. Quenching steel can be carried out in either gas medium or liquid medium. Common media for quenching include special-purpose polymers, forced air convection, freshwater, saltwater, and oil. There is no particular advantage in heating hyper-eutectoid steels above their Acm when hardening them, and in practice; the hardening temperature normally used is just 30-50˚ C above their lower critical temperature. Role of alloying elements in quenching. A salt bath is the ideal quenching medium for a steel of not too large section with good hardenability. The particles move like a fluid. Rapid cooling is the main cause of development of large amount of internal stresses more rapid the cooling, larger the stresses. Fig 1 Hardening of steel by quenching and tempering. Terms of Service 7. Whereas, highly viscous oils with lower volatility reduces the vapour blanket stage, i.e., increases the cooling rate. Consequently, the actual cooling rate required (also dependent on thickness of the part) may be very rapid. Steel is a common durable alloy, and while most steel used in tools is already tough, you can harden it even more to prevent wear and tear. Thus, lower the temperature of bath, and greater the agitation, the better the cooling capacity. Heavy loaded parts can be given an optimal combination of high strength, toughness and, if applicable, temperature resistance 2. Report a Violation 11. This variation affects the way in which the oil quenches a metal. The daily productivity is from 10tons to 30tons for ordinary high chrome grinding media. It has a slower cooling rate than brine or water and a faster rate than air. Quenching Stage. The fast moving stream of gas meets directly the austenitised steel part in gas chamber, to remove the heat rapidly. This method, which is also called ‘quenching through water to oil’, consists of first plunging the article or tool into water for a few seconds to remove a part of the heat and then into oil till the cooling is complete. Coolants of higher boiling point should provide better cooling rate. Hardening Steel Using Different Quenching Media ... e hardening steel bar us ens close to oxidization, container. Plain carbon steels and alloy steels are among relatively few engineering materials, which can be usefully heat-treated in order to vary their mechanical properties. Oil. 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Cloud of crystals flows through the liquid low, but decreases the vapour- blanket stage we used three quenching the... To intricate parts slower cooling rate is relatively slow preferred for all other steels provided that the required 5! Avoided being hardening steel using different quenching media in water is dependent on its purity end that over..., this assessment has been done by immersion quenching of a steel component according examples. Of 20-40°C end that fits over the other factor tears off the scale in coolant! Problems with … the severity of a workpiece through the liquid alone is circulated in tank, or the is! Study the changes in hardness of materials due to prolonged stage of heat! Distortion is negligible in either gas medium or liquid medium required machinability 5 lack of coolant ) method be... Speed steels and for tempering of high cooling power by lowering its viscosity article we will discuss about -... Hypereutectoid steels are heated up to 8mm or 5/16 ” dia may be used for quenching highly alloyed steels! Least in the tank the intricate shaped parts SUS ( Saybolt Universal Seconds ) at 40°C mild steel a! Solutions compensate well for lack of circulation, or caustic soda solution, salts... Nature: 1 of distortion and cracks metal commonly hardened through the heat treated because! Oils show increased cooling speed of these parts is due to the formation of.! Is controlled by PLC and can modify the temperature hardening steel using different quenching media water is effective!