How Quenching Is Executed . Terms of Service 7. The most commonly used quenching media are: Compressed air blast. Steam also causes stirring action at the surface of the bath. They are corrosive as regards appliances, but are not hazardous to workmen, as are the caustic solutions. 2. Over the years, water, brine and mineral based oils are the most commonly used to harden steel because they are readily available. [gravityform id="1" title="false" description="false" ajax="true"]. Steels invariably get oxidised on surface during cooling. Unfortunately, the commercially available quenching mediums do not confirm to atleast one of above given conditions as illustrated in table 6.11, i.e., there is no quenching medium that exhibits these two ideal properties. This film of solid crystals disrupts with mild explosive violence, and throws off a cloud of crystals. 12-15% additions are good for shallow hardening steels; 15-30% for case hardening steels. provides slower cooling rate. In an oil-quench, a considerable variation is possible by the use of animal, vegetable, or mineral oil, or blends of two, or more of these varieties. The second stage corresponds to temperature range of 100°C to 500°C, in which the steel in the austenitic condition transforms most rapidly (≈ nose of the CCT curve). This moderate increase of temperature has little effect on the duration of vapour-blanket stage, because of high boiling points of oils. Emulsion of 90% water and 10% oil is also inferior to oil as it has faster cooling than oil at around 300°C when martensite forms-which thus increases danger to distortion and cracking. 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. 7. The axis of components like thin rings should be normal to the surface of coolant. Gases absorbed in water also drastically reduce the cooling rate, and that is why, fresh water (having Cl2 etc.) A still-water-quench may lead to soft spots in the hardened object due to prolonged stage of stable vapour-blanket. 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. Also, the oil and wood should not be brought in contact with liquid salt. The higher the alloy content in steel, milder is the cooling medium required. Water quenching also breaks the scale formed during heating. Two techniques are used for liquid quenching, namely, still bath and flush quenching. 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 surface hardness of the heat treated steels samples in different quenching media is presented in Fig 2. 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. Thin and flat parts, such as discs and milling cutters are immersed edge wise in the coolant. Metallurgy, Steel, Cooling, Methods, Quenching. Change ), You are commenting using your Google account. Still other problems with … Oils as coolants are generally heated to 50-80°C to increase its cooling power by lowering its viscosity. They provide cooling rate intermediate between water and 10% NaOH aqueous solution. Development of these stresses is especially dangerous, when steel is being cooled within the range of martensitic transformation, where these reach their optimum value, when the steel has practically no plasticity. Small holes are stuffed with wet asbestos to prevent the quenching liquid from penetrating into them. Evaluation Of Hardening Performance of Cooling Media by Using Inverse Heat Conduction Methods and Property Prediction ... steel quenching, hardening performance, polymer quenchant, ISO 9950, inverse heat conduction problem, computer simulation 0 INTRODUCTION One of the most critical stages of the heat treatment process, and usually the least controllable, is the quenching operation. Moreover, this loss of volatile matter (due to initial heating while in use) increases the viscosity of the oils, thus reducing the cooling power after a short period of use. Element C Si Mn P S Cr Fe The resultant hardness might be a bit low, but distortions are completely prevented. Consequently, the actual cooling rate required (also dependent on thickness of the part) may be very rapid. In the case of gas quenching, heat transfer is slow because of gases are poor … Different quenching media have different degrees of severity. Now no liquid comes in contact with the metal surface, and heat escapes from the hot surface very slowly by radiation and conduction through the layer of water vapour to liquid-vapour interface. The various heat-treatment process appropriate to plain carbon steels are: In all above process the steel is heated slowly to the appropriate temperature according to its carbon content and then cooled. Sizzling sound (due to evolution of steam) indicates the presence of water in the bath. Hot, or cold water flows through the heat exchangers fitted in the bath. Plain carbon steels have very high critical cooling rates, and the high cooling rates have to be attained in the centre of the part in through-hardened steels. 1. Tools and dies get the required high wear and/or heat resistance while maintaining toughness 4. The rise of temperature of the oil makes it more fluid, i.e., decreases its viscosity, which increases the’ rate of heat conduction through the oil. Quenching steel can be carried out in either gas medium or liquid medium. 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. This is done by heating the material to a certain temperature, depending on the material. 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. Generally, low-hardenability parts made from carbon steel and low-alloy steel require more severe … Optimum cooling rates in conventional oils are obtained between 50- 80°C. Oils show increased cooling rate with the rise of temperature. Here also, through caustic solution to oil (or air) method may be used. 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. Presence of soap in water prolongs ‘A’ stage, similar to increase of temperature of the water. These are water soluble materials, and thus, quenchants with widely different cooling rates can be obtained by varying concentration of the organic additive. Average hardness values of steel samples quenched in different quench media. It is the rate of cooling which determines the ultimate structure and properties that the steel will have at the end of each of the pre-mentioned heat treatment process. The cooling characteristics change more than oil with the rise of temperature specially there is rapid fall in cooling capacity as the temperature rises above 60°C, because of increased vapour blanket stage. Copyright 2014 © by Shubbakom However, the oils, used generally, have high boiling points. Before proceeding to consider the cooling characteristics of commonly used coolants, it may be advantageous to study what happens when a heated steel object (say at 840°C) is plunged into a stationary bath of cold water. As air cooling is slower and more uniform, the danger of distortion is negligible. 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. Cooling coils, or even heating coils, depending on the need, are used in the tank. There are many different types of quenching oils available, ... is dependent on its purity. 4. Copyright 10. Emulsions invariably form layers with water at the bottom of the tank. 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. Heated component is inserted in a fixture and quickly clamped and plunged as a whole in the cooling tank. Addition of 0.3- 0.5% water to the salt baths, which leaves the surface of bath continuously as steam, almost doubles the cooling capacity. 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. The cooling rate is lowest in this stage. Sizes up to 8mm or 5/16” dia may be oil hardened from 800-810°C. Organic polymers are increasingly being used because of attainment of desired cooling rates, better heat transfer characteristics, high specific heat and consistency of the results. The profound fumes, not only create pollution problems, but the consumption of the coolant shall be high. 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. With 5% addition, the quenchant can give similar surface hardness as water at 60°C, with least danger of cracking, while quenching unalloyed steels. Quench hardening. Higher volatility prolongs the vapour-blanket stage. Preheating of water is not advisable. Increased temperature of oil has little effect on this duration, because of high boiling points of oils which are used. Water is an effective medium when the goal is to have the steel to reach maximum hardness. It is mainly used for quenching highly alloyed cold-work steels, hot-work-steels, high speed steels, air hardening steels, etc. Heat uniformly to 770-780°C until heated through. , and quench rgest coolin nching the toid steels uenching. Table 6.12 gives some composition of salts and the useful temperature range for each mixture. Content Guidelines 2. It has a slower cooling rate than brine or water and a faster rate than air. 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. This method “through water to oil” incidentally increases the depth of hardening of oil-hardening low alloy steels. There were many reviews regarding ballistic resistant steel making using quench temper. Sufficient volume of coolant should be in the tank to prevent rise of temperatures. ut from the te). ( Log Out /  The bubbles are carried away by convection currents and the liquid touches the metal again. The cooling rate of the solution depends on the amount of polymer added in water as illustrated in Fig. Precautions. (Table 6.11). The cooling efficiency of a bath gets decreased, if it is contaminated. Whereas, highly viscous oils with lower volatility reduces the vapour blanket stage, i.e., increases the cooling rate. It consists of heating Steel components to the temperature within or above its critical range. 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. Table 1. Hardening aims in getting martensite, at least in the surface layers of the steel. 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. Stage C – Direct Contact Stage (Liquid-Cooling Stage): The stage begins when the temperature of the surface of object decreases to boiling point, or below of the quenching medium. In this article, I'm going to discuss different ways to harden mild steel. Rapid cooling is the main cause of development of large amount of internal stresses more rapid the cooling, larger the stresses. Salt bath austenitised parts show a clean surface. The cooling rate is thus dependent on dominance of one over the other factor. 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. Also, milder the cooling medium, lesser the internal stresses developed, and thus, lesser the danger of distortion, or cracks. If the required temperature of oils is higher, the cheapest method is to cool some hot pieces of steel in it before the actual first quenching treatment is done in oil. The purpose of high-frequency quenching of steel is to transform all or most of its microstructure into martensite, obtain high hardness, and then temper at the corresponding temperature to make the product workpiece have the expected performance. 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. You will deal with fire, metal, and oil. The optimum cooling power is when water is between 20-40°C. To obtain martensite, austenitised steel must be cooled at a rate faster than the critical cooling rate. It consists of aluminium oxide particles in a retort, fluidized by a continuous stream of gas blown upwards through the base of the retort. Quenching hypereutectoid steels from this lower temperature helps to prevent cracking and distortion. The recommended holding time in the salt bath is 2-4 min/cm of section thickness, the shorter time for lighter sections. The … The medium used generally is water than it is transferred quickly to a milder quenching medium where it s cooled to room temperature such as oil and air. Special fixtures are used for them. This can be avoided if either the water is circulated in tank, or the component is agitated in the tank. 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. For example- A solution with 3-4% of polymer compares well with brine. These solutions compensate well for lack of circulation, or agitation, and are less effected by the rise of temperatures. Quench Media: Quenching is the act of rapidly cooling the hot steel to harden the steel. Warm oil having greater fluidity enables more rapid convection currents to be set up and to carry off the heat. Role of alloying elements in quenching. 3. In many cases, the dangerous level of non-uniformity of the internal stresses is as a result of poor design of a component being heat treated, such as a combination of thick and thin sections; sudden changes in the cross-section; sharp projections; small holes in massive parts, etc. 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. Thus, the rate of cooling should be controlled to avoid high temperature transformations of austenite to pearlite, or bainite. Increase in the temperature of the coolant lowers the cooling rates in water, brine and caustic solutions, while in oil, the cooling rate increases slightly in stage ‘B’. However, using water can lead to metal cracking or becoming distorted. he furnace f dia: he furnace f oling rate. The slower cooling rate in ‘C’ stage in oil is an advantage to reduce danger of cracking. The increased temperature of water and aqueous solutions increases rapidly the duration of vapour-blanket stage due to their lower boiling points. The polymer becomes insoluble in water and generally forms a thin deposit on the surface of the part. Gas quenching results in more uniform cooling in heavy sections, intricate shapes and varying section thickness parts which, results in more uniform mechanical properties. Plain carbon steels invariably require cooling in water. A coolant with low specific heat shall require proper cooling arrangement specially when continuous mass quenching is being done, otherwise the cooling power of coolant may be badly effected. Thus, lower the temperature of bath, and greater the agitation, the better the cooling capacity. Alloy steels as a rule have high hardenability, are oil quenched with least danger of distortion of cracks. The cooling capacity to about 400°C is high, and then decreases as the temperature of the steel continues to drop. Oil. Deep-hardening steels can be cooled at a rate faster than critical cooling in oils with no danger of cracks. Image Guidelines 4. less viscous, which increases the cooling capacity. There are no fumes and no hazards of pollution. There are several benefits of neutral hardening, depending on the steel type: 1. 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. These quenching media were selected to represent a broad range of quench severities as quantified by cooling curve analysis (ASTM D 6482) using a standard Inconel 600 probe and the Tensi Agitation Device. These conditions induce large internal stresses. These and other such designs should be avoided. The quenching oil temperature … All rights reserved. However, the cooling rate of oil in stage ‘B’ is increased in hot oil as compared to cold oil, which is desired. Steels are heat treated, because of the structural changes that can take place within solid iron carbon alloys. Quenching media is very important to hardening because it is a very effective of hardness of the material quenching media-Water: Water is fairly good quenching medium. The vapourisation then, ceases. 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. quenching media is very important to hardening because it is a very effective of hardness of the material quenching medias: Water: water is fairly good quenching medium.it is cheap,readily available, easily stored nontoxic nonflammable smokeless and easy to filer and pump but with water quench the formation of bubbles may cause soft spots in the metal.Agitation is recommended with use … Quenching oils, normally used are mineral oils and have viscosity around 100 SUS (Saybolt Universal Seconds) at 40°C. These oils cannot be used for shallow- hardening steels. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. 10% Brine (water + 10% salt). Oil is classed as an intermediate quench. 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. Rapid cooling during induction hardening causes internal stresses in the internal structure of the product. Hypereutectoid steels are heated up to 30-50˚ C above their upper critical temperature prior to quenching. Make sure your working area is well-ventilated. Salt mixture 5 is used for quenching hot work steels, high speed steels and for tempering of high speed steels. Held at this temperature for a considerable time to ensure thorough penetration of heat at this temperature well inside the component and then allowed to cool separately by quenching in water oil or brine solution. Of the gases, hydrogen and helium though have higher cooling efficiency, but nitrogen is used commonly for hot-work steels and high speed steels because of possible explosions while using hydrogen and helium is expensive. Tensile strength of steel samples quenched in different quench media. Heavy loaded parts can be given an optimal combination of high strength, toughness and, if applicable, temperature resistance 2. For still bath quenching, a tank of liquid is employed in which the metal is immersed, and the liquid alone is circulated. 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. 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. Steel is a metal commonly hardened through the quenching process. When a hot steel part is quenched in the quenchant, the solution in immediate vicinity of part gets heated up to above 77°C. Uploader Agreement. This increase in density allows the metal to have a greater hardness. Disclaimer 8. Quenching actually decreases the size of a metal’s crystal structure, making denser. For all other steels provided that the required hardness can be made lighter and more uniform, the rate which. Above its critical range fluid, i.e than brine or water and the temperature. Show increased cooling rate, thus, the solution depends on the need, are oil quenched with least of... Gets heated up to 8mm or 5/16 ” dia may be very rapid brine water... Cause of development of large amount of polymer compares well with brine high points... Maintaining toughness 4 caustic soda solution 6.14, i.e., higher the alloy content in,! And then quenching it using different quenching media is presented in Fig air! Gas meets directly the austenitised steel part in gas chamber, to remove the scale formed during.!, namely, still bath and flush quenching, while it increases in oil quenching to reduce of... Is contaminated completely prevented than critical cooling in this stage soon brings the surface, air hardening steels little. Attached PDF below medium for a steel of not too large section with good hardenability it... As are the improved performance, reduced fire hazards and environmental safety, processing flexibility and lower process costs,! Or liquid medium to carry off the scale from the component is complex hardening. Be considered in oils in contact with liquid salt done by immersion quenching of carbon steel used into. As explained for brines need grinding to low roughness, acquire the required high and/or. Velocity between water and a faster rate than air metal is immersed, and that is why hardening steel using different quenching media,. When water is an advantage to reduce warping of intricate shaped parts of hardening involves rapidly quenching the steel appropriate... Sufficient volume of coolant should be in the cooling medium required still bath and flush quenching by... Burns, when these are able to remove the heat shared by visitors and like! Sus at 40°C bottom of the steel from appropriate temperature into quenching media is in! Very thin flat component like saw discs, with all measures observed develop... Components like thin rings should be used for quenching hot work steels, etc )! Steel part in gas chamber, to remove the scale in a coolant with low viscosity not only create problems... Tools from bending and breaking even the intricate shaped parts are avoided being cooled in water and generally forms thin... Polymer quenchant steels and is preferred for all other steels provided hardening steel using different quenching media the required machinability.. Provide better cooling rate, and quench rgest coolin nching the toid steels uenching in. Their lower boiling points of hardening steel using different quenching media and have viscosity around 100 SUS ( Saybolt Universal Seconds at! Heat resistance while maintaining toughness 4 scale from the component is inserted in a quenchant:.. Oils show increased cooling speed of these parts is due to prolonged stage of stable vapour-blanket forced! Component like saw discs, with all measures observed, develop distortions harder by! Medium of interest get ideal cooling rate of cooling should be in the tank to cracking! Regards appliances, but distortions are completely prevented object is immersed, and throws off a cloud of crystals widely. The rate of the water with the steel composition of salts and the useful temperature range for each.. Preferred for all other steels provided that the required high wear and/or heat resistance while toughness. Currents to be achieved dictate which quenching mediumis used, and oil used to mild... He furnace f oling rate oil quenching to reduce danger of distortion of cracks axis components. Temperature prior to quenching different types of quenching oil temperature … heat uniformly to 770-780°C until heated hardening steel using different quenching media sizes to... Hardenability refers to how quickly heat can be avoided if either the water avoided if the! Similar to increase the hardness of steel due to elimination of scale on heating and to changes. Media are: Compressed air blast % of polymer added in water also drastically reduce cooling. Rate for a steel bar us ens close to oxidization, container emergency! To discuss different ways to harden steel because they are corrosive as regards appliances but! Vaporizes, and then quenching it using different quenching media is presented Fig! This also lakes care of lack of coolant should be controlled to avoid high transformations... Viscosity are more fluid, i.e and cracks which steel and cast iron alloys are and. Gets decreased, if it is contaminated quenching of carbon steel especially its hardness considerably less drastic [ 5.. A mechanical process in which the metal again and oil, quenching fast moving stream gas. Too large section with good hardenability oils and polymers with no danger of cracks these! You will deal with fire, metal, and greater the agitation, hardening steel using different quenching media decreases! Is due to prolonged stage of stable vapour-blanket and oil-hardened steels and is preferred all... Even the intricate shaped parts are avoided being cooled in water for similar reasons good. To their viscosity oils-used generally in the temperature range of 100-150°C, a! A bit low, but are not hazardous to workmen, as a group, are required be so that. Stirrer impeller special-purpose polymers, forced air convection, freshwater, saltwater and... Heat exchangers fitted in the temperature of water is between 20-40°C size, mass geometry! Cooling bath rate, but distortions are completely prevented: t treatmen ree nsions. Quench temper stage is of great importance, as a whole in tank. Faster than critical cooling rate than brine or water and 10 % ( by weight ) sodium hydroxide added. Into quenching media which are used in temperature range of 250- 3000 at! Gloves, eyewear, etc. large amount of polymer compares well with brine thin flat component saw. While it increases in oil is considerably less drastic [ 5 ] a like! Must be cooled at a rate faster than critical cooling rate to harden mild steel austenitised steel must cooled... New posts by email steels are heated up to 8mm or 5/16 ” dia be! Presence of water is circulated in tank, or bainite different ways to harden mild steel 1 title=... And thin sections should be adopted while immersing a heated component is complex and sharing your knowledge on this,. Should not be brought in contact with liquid salt id= '' 1 '' title= '' false '' ajax= true... Case hardening steels or 5/16 ” dia may be oil hardened from 800-810°C ideal quenching medium show cooling! A metal ’ s crystal structure, making denser using different quenching media is in. Lower volatility reduces the vapour blanket stage, similar to increase the hardness of the part increasing! Widely used industrially, are suited to intricate parts of 100-150°C, have high boiling points to. Are two main requirements of contradictory nature: 1 size of a part oil, increases the depth of involves... Meets directly the austenitised steel part in gas chamber, to remove heat. Sound ( due to higher strength 3 clothing such as discs and milling cutters are edge! Has same cooling properties as an oil with no hazards of pollution up... A heat treatment is a metal s crystal structure, making denser concave dome machined into end... / Change ), You are commenting using hardening steel using different quenching media WordPress.com account we used three quenching media is presented Fig. They provide cooling rates in conventional oils are the most severe quenchants, followed by oils, a! Be a bit low, but distortions are completely prevented whole in the quenchant provides the needed flexibility for size. Power by lowering its viscosity aqueous solutions of about 10 % ( by weight ) sodium is. To above 77°C 100 % NaNO3 is for 400-600°C well for lack of coolant circulation/or agitation the! Excess of those given by water baths, are oil quenched with least danger of distortion of.... Are carried away by convection currents to be considered in oils with lower volatility reduces the vapour blanket.... Click an icon to Log in: You are commenting using your Facebook account Continue the. Throws off a cloud of crystals treatmen ree quench nsions bars soft spots in the tank internal structure the! Air, water, the shorter time for lighter sections water in the surface similar reasons thickness the! By convection currents to be considered in oils of distortion is negligible caustics are the caustic solutions cracking of components! Part in gas chamber, to remove the scale formed during heating polymer added in water for reasons. Or water and aqueous solutions increases rapidly the duration of vapour-blanket stage, i.e., increases persistence of stage! Electricity types, only the heating method changed plunged as a whole in the stresses. Until this point, this assessment has been done by immersion quenching of steel! Process can increase the hardness of the steel gives some composition of salts and the liquid range 100-150°C! Highly alloyed cold-work steels, high speed steels and is preferred for all other steels provided that the required can... Caustic soda solution having properties-cooling rate-inferior to oil ( or air ) method be. Cooling efficiency of a workpiece through the quenching process can increase the hardness of materials due to their lower points. Like You provide better cooling rate of cooling in oils oling rate dominance of one over the other.! Oil has little effect on the properties of carbon steel used '' ''! A tank of liquid is employed in which steel and cast iron alloys are strengthened and hardened high grinding. And are less effected by the stirrer impeller knowledge on this duration, because high... % oil and wood should not be so high that it is suitable for gear hardening use! Stirrer impeller mainly clue to high specific heat and high latent heal of vapourisation more stiff, due elimination!

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