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just keep some basic in mind, its give u enough information about this topic.

just keep some basic in mind, its give u enough information about this topic.

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    hardening hardening Presentation Transcript

    • HARDENING Dr. H. K. Khaira Professor in MSME MANIT, Bhopal 1
    • Introduction • Steels can be heat treated to high hardness and strength levels. • The reasons for doing this are obvious. Structural components subjected to high operating stress need the high strength of a hardened structure. • Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation. 2
    • Hardening • In hardening, the steel is heated 30 to 50oC above Ae3 temperature in case of hypoeutectoid steels and 30 to 50oC above A1 temperature in case of hyper-eutectoid steel, kept at that temperature for some time, followed by quenching at a rate faster than the critical cooling rate of the steel. 3
    • Heat Treatment Temperature for hardening and other heat treatments In hardening, the steel is heated 30 to 50oC above Ae3 temperature in case of hypo-eutectoid steels and 30 to 50oC above A1 temperature in case of hypereutectoid steel …...... ←Acm A3 → 4
    • Critical Cooling Rate …. followed by quenching at a rate faster than the critical cooling rate of the steel. The critical cooling rate is the cooling rate which is tangential to the TTT diagram at the nose of the curve. Critical cooling rate Cooling at or faster than critical cooling rate will transform the austenite to martensite and not to pearlite or bainite. 5
    • Hardening Procedure 1. The steel is heated 30 to 50oC above Ae3 temperature in case of hypo-eutectoid steels and 30 to 50oC above A1 temperature in case of hypereutectoid steel ←Acm A3 → 2. Kept at that temperature for some time 6
    • Hardening Procedure 3. Followed by quenching at a rate faster than the critical cooling rate of the steel. Critical cooling rate 7
    • Ms and Mf Temperatures 4. The martensite starts forming at Ms temperature 5. At Mf temperature, the martensite formation gets completed. 6. Therefore, the steel has to be quenched to a temperature below Mf. Critical cooling rate 8
    • Retained Austenite If the quenching is stopped before Mf temperature, proportionately less austenite will transform to martensite. The remaining austenite will also be present in the microstructure. This remaining austenite is known as retained austenite. Critical cooling rate 9
    • Sub-Zero Treatment The presence of retained austenite will result in decreased hardness after quenching. Thefore, retained austenite is not desirable in the hardened steel. In some steels, the Mf is below room temperature and, therefore, the steel is quenched to sub-zero temperatures . It is known as sub-zero treatment. Critical cooling rate 10
    • Comparison of Cooling Rates for Different heat treatments Different heat treatments result in different phases in steel with different properties. 11
    • Martensitic Transformation • Because Martensite transformation is almost instantaneous, the Martensite has the identical composition of the parent phase, unlike ferrite and pearlite which result from a slower chemical diffusion process, so each have different chemical compositions than the parent austenite. • Formation of Martensite involves a transformation from a facecentered cubic (FCC) structure to body - centered tetragonal (BCT) structure. • Martensite is super-saturated solid solution of carbon in iron having BCT structure. • The large increase in volume that results creates a highly stressed structure. • This is why Martensite has a higher hardness than Austenite for the same composition. 12
    • ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure (a) The unit cell of BCT martensite is related to the FCC austenite unit cell. (b) As the percentage of carbon increases, more interstitial sites are filled by the carbon atoms and the tetragonal structure of the martensite becomes more pronounced resulting in increase in hardness. 13
    • Hardness of Martensite • The hardness of martensite depends on its carbon content. 14
    • Effects of Carbon Content on Hardness and Distortion in Martensite The hardness of martensite increases with carbon content because of the increasing supersaturation and distortion of the lattice The distortion results in brittleness also. More carbon → More distortion → More internal stresses → More brittleness 15
    • Comparison of Hardness of Different Phases in steels Hardening produces martensite whereas normalizing results in fine pearlite. 16
    • Martensite Some Important Points • Cooling at a rate faster than the critical cooling rate produces martensite in the steel. • Martensite is formed by diffusion less transformation. • Because Martensite transformation is almost instantaneous, the Martensite has the composition identical to that of the austenite. • Martensite is a super saturated solid solution of carbon in alpha iron. • Martensite has BCT (body centered tetragonal) structure. • Martensite , therefore, is hard. • Hardness of martensite increases with increase in its carbon content. • Formation of Martensite from austenite involves large increase in volume that results in a highly stressed structure. It increases the hardness of martensite. • Martensite makes steel hard and strong. • But martensite is very brittle also. • Therefore, hardened steels are hard and strong but brittle. 17
    • Microstructure of Martensite 18
    • Increase in carbon content decreases Ms and Mf temperatures ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Effect of Carbon Content on Ms and Mf Temperatures 19
    • Effect of Alloying Elements on Ms and Mf 20
    • Quenching Media Four commonly used quenching media are: – 1. Brine – the fastest cooling rate – 2. Water – moderate cooling rate – 3. Oil – slowest cooling rate – 4. Liquid Nitrogen – used in automatic furnaces, can be very fast cooling. Too rapid cooling can cause cracking in complex and heavy sections. 21
    • Quenching Media • Many quenchants can be used for hardening of steels – Each quenchant has its own thermal properties • Thermal conductivity • Specific heat • Heat of vaporization – These properties cause rate of cooling differences – Different quenching medium have different cooling or quenching capacity. – The severity of quenching obtained from the quenching medium is indicated by H Value. 22
    • Severity of Quenching • • • Cooling capacities (Severity of quench) of quenching medium is known as H value. H values of some of the quenchants are given below. Cooling rates are at the center of a 2.5 cm bar. H Value – – – – – – – – – Ideal Quench Agitated brine Brine (No agitation) Agitated Water Still water Agitated Oil Still oil Cold gas Still air ∞ 5 2 4 1 1 0.25 0.1 0.02 Cooling Rate (0C/s) ∞ 230 90 190 45 45 18 - 23
    • Ideal Quenchant • Ideal quenchant is one which brings down the steel temperature to room temperature instantaneously and keeps it at that temperature thereafter. 24
    • Influence of quench medium and sample size on the cooling rates at different locations. • Effect of quenching medium – Severity of quench: • Water > Oil > Air, e.g. for a 50 mm diameter bar, the cooling rate at center is about 27°C/s in water, but, 13.5 °C/s in oil. – Effect of size • For a particular medium, the cooling rate at center is lower when the diameter is larger. For example, 75mm vs. 50mm. 25
    • The Grossman chart used to determine the hardenability of a steel bar for different quenchants. This diagram can be used to determine bar diameters having equal cooling rates when quenched in different quenching media. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. 26
    • Quenching • Water is one of the most efficient quenching media where maximum hardness is required, but it may cause distortion and cracking of the article. • Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidise and form sludge with consequent lowering of efficiency. • The quenching velocity of oil is much less than water. Ferrite and bainite are formed even in small sections. • Intermediate rates between water and oil can be obtained with water containing 10-30 % Ucon, a substance with an inverse solubility which therefore deposits on the object to slow rate of cooling. • To minimise distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. • To prevent steam bubbles forming soft spots, a water quenching bath should be agitated. 27
    • Important Points • Hardening of steel is obtained by a suitable quench from within or above the critical range. The temperatures are the same as those given for full annealing. • The soaking time in air furnaces should be 1.2 min for each mm of cross-section or 0.6 min in salt or lead baths. • Uneven heating, overheating and excessive scaling should be avoided. • The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite, and to cause a decomposition at a low temperature to produce martensite. • To obtain this, steel requires cooling faster than the critical cooling rate. • Critical cooling rate is greatly reduced by the presence of alloying elements, which therefore cause hardening with mild quenching (e.g. oil and air hardening steels). 28
    • Effect of Alloying Elements on Critical Cooling Rate Alloying elements shift TTT diagram towards right thereby decreasing the critical cooling rate.
    • Quench cracks • The volume changes, which occur when austenite is quenched, are: – a) expansion when austenite transforms to martensite – b) normal thermal contraction. • When steel is quenched these volume changes occur very rapidly and unevenly throughout the specimen. • The outside cools most quickly to martensite. • Stresses are set up which may cause the metal either to distort or to crack if the ductility is insufficient for plastic flow to occur. • Such cracks may also occur some time after the quenching or in the early stages of tempering. 30
    • - The outside cools most quickly to martensite. - Stresses are set up which may cause the metal either to distort or to crack if the ductility is insufficient for plastic flow to occur. 31
    • Quench cracks • Quench cracks are liable to occur: 1) due to presence of non-metallic inclusions, ; 2) when austenite is coarse grained due to high quenching temperature; 3) owing to uneven quenching; 4) in pieces of irregular section and when sharp re-entrant angles are present in the design. 32
    • Precautions against Quench Cracks • The relation of design to heat-treatment is very important. Articles of irregular section need special care. • When steel has been chosen which needs a water-quench, then the designer must use generous fillets in the corners and a uniform section should be aimed at. • This can sometimes be obtained by boring out metal from bulky parts without materially affecting the design. 33
    • The CCT diagram for a low-alloy, 0.2% C Steel Mild steels can not be hardened because the TTT diagram touches the Y axis 34
    • Protective Atmospheres • Steel is heated to high temperatures in hardening. • At such high temperatures, decarburisation at the surface may take place in the presence of oxygen • Decarburisation will result in decrease in carbon content in the martensite formed at the surface • It will give rise to low hardness in the surface after hardening • Hence protective atmospheres may be used in heat treatment to avoid oxidation or decarburisation • Innert gases may be used as protective atmosphere • Heat treatments may also be carried out in vacuum 35
    • Summery • 1) Martensite is the hardest and most brittle microstructure obtainable in a given steel. • 2) Martensite hardness of the steel is a function of the carbon content in that steel. • 3) Martensite results from cooling from austenitic temperatures faster than the critical cooling rate by pulling the heat out using a quenchant before pearlite can form. • 4) As quenched Martensitic structures are too brittle for economic use. They must be tempered. • 5) Tempering a hardened steel results in the best combination of strength and toughness. 36
    • Need of Tempering • As-quenched hardened steels are so brittle that even slight impacts may cause fracture. • Tempering is a heat treatment that reduces the brittleness of a steel without significantly lowering its hardness and strength. • All hardened steels must be tempered before use. 37
    • Hardened and Tempered Steels • Fully hardened and tempered steels develop the best combination of strength and notchductility. 38
    • THANKS 39