Engineering materials and metallurgy -Heat treatment

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HEAT TREATMENT Thermal processing is also known as heat treatment Purpose of heat treatment  T o refine grain structure/size  T o impart phase changes  Improvement in ductility  Relieving internal stresses  Increase of strength  Improvement in machinability toughness etc. Dr.K.RaviKumar Dr.N.G.P .Institute of T echnology

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REASONS FOR HEAT TREATMENT  Softening: Softening is done to reduce strength or hardness remove residual stresses improve toughnesss restore ductility refine grain size or change the electromagnetic properties of the steel.  Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold working is to be performed such as in a cold-rolling operation or wiredrawing.  Hardening: Hardening of steels is done to increase the strength and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the steel can be directly hardened. Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques.

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FACTORS AFFECTING HEAT TREATMENT  T emperature up to which material is heated  Length of time that the material is held at the elevated temperature  Rate of cooling and the surrounding atmosphere under the thermal treatment.  Material  pre-processing of the material’s chemical composition  size and shape of the object  Final properties desired  Material’s melting point/liquidus etc.

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TYPES OF HEAT TREATMENT  Annealing  Normalising  Hardening and quenching  T empering  Martempering  Austempering

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ANNEALING  Heat treatment process in which the material is taken to a high temperaturemeta stable to remove the instability.  kept there for some time and then cooled.  High temperatures allow diffusion processes to occur fast.  The time at the high temperature soaking time must be long enough to allow the desired transformation to occur.  Cooling is done slowlymainly in furnace to avoid the distortion warping of the metal piece or even cracking caused by stresses induced by differential contraction due to thermal in homogeneities .

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BENEFITS OF ANNEALING  Relieves internal stresses  Increase softness ductility and toughness  Produce a specific microstructure  Removing gasses  Improving machinability

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TYPES OF ANNEALING  Process annealing  Stress relief annealing  Full annealing  Spheroidizing Annealing Process annealing  Is primarily applied to cold worked metals to negotiate the effects of cold work.  During this heat treatment material becomes soft and thus its ductility will be increased considerably.  It is commonly sandwiched between two cold work operations. During this recovery and recrystallization are allowed whereas grain growth was restricted.  Normally heated just below the transformation range  Ferrous alloys are heated in the range of 550-650 o C

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Stress relief  Removes the stresses that might have been generated during quenching casting machining welding cold working etc..  Unless removed these stresses may cause distortion of components. T emperature used is normally low such that effects resulting from cold working are not affected. Full annealing  Is normally used for products that are to be machined subsequently such as transmission gear blanks.  After heatingto austenitizing temperature and keeping at an elevated temperature components are cooled in furnace at the rate of about 20 ºC/hr in a furnace to about 50 ºC into the Ferrite-Cementite range. Typically the product receives additional heat treatments after machining to restore hardness and strength.

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Spheroidizing Annealing  Is an annealing process used for high carbon steels Carbon 0.6 that will be machined or cold formed subsequently.  Mainly improves machinability.  Heat the part to a temperature just below the Ferrite- Austenite line or below the Austenite-Cementite line essentially below the 727 ºC . Hold the temperature for a prolonged time and follow by fairly slow cooling for the formation of spheroidal or global form of carbide in steel. For tool and alloy steels heat to 750 to 800 ºC 1382-1472 ºF and hold for several hours followed by slow cooling.

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NORMALIZINGAIR QUENCHING  Normalizing is a type of heat treatment applicable to ferrous metals only.  The purpose of normalizing is to remove the internal stresses induced by heat treating welding casting forging forming or machining  It is the process of raising the temperature to over 60 º C above Austenite range. It is held at this temperature to fully convert the structure into Austenite and then removed from the furnace and cooled at room temperature under natural convection.  It is used to refine the grains and produce a more uniform and desirable size distribution. It involves heating the component to attain single phase e. g : austenite in steels then cooling in open air atmosphere.  This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite.  Cooling rate is faster than full annealing  The resulting material is soft free from residual stress the degree of softness depends on the actual ambient conditions of cooling Produces a uniform structure.

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QUENCHING  Quenching is heat treatment process where material is cooled at a rapid rate from elevated temperature to produce Martensite phase. This process is also known as hardening.  Increases the hardness.  Rapid cooling rates are accomplished by immersing the components in a quench bath that usually contains quench media in form of either water or oil accompanied by stirring mechanism.  Quenching media- water oil salt water BrineNacl

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TEMPERING PROCESSES  T empering is a process done subsequent to quench hardening. Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of Martensite. This brittleness is removed by tempering. T empering results in a desired combination of hardness ductility toughness strength and structural stability.  Tempering is the process of heating martensitic steel at a temperature below the eutectoid transformation temperature to make it softer and more ductile. During the tempering process Martensite transforms to a structure containing iron carbide particles in a matrix of ferrite.  Relieves internal stress  Reduces hardness  Increases strength

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INTERRUPTED QUENCHING  T wo stage quenching process.  The piece is first quenched in a medium which cools rapidly.  After it is furher quenched in a second medium but slowly than the first medium to avoid bainite transformation.  Eg : Martempering Austempering

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AUSTEMPERING  i Heated to above the critical range to make it to austenite.  ii Quenched into salt bath at temperature until formation of bainite.  iii Subsquently cooling the work piece in air.  As the part is held longer at this temperature the Austenite transforms into Bainite. Bainite is tough enough so that further tempering is not necessary and the tendency to crack is severely reduced.  Allowed to cool to room temperature.  The biggest advantage of Austempering over rapid quenching is that there is less distortion and tendency to crack.

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MARTEMPERING  i Heated to above the critical range to make it to austenite.  ii Quenched into salt bath at temperature above Ms.  iii Subsquently cooling the work piece in air.  Martempering is similar to Austempering except that the part is slowly cooled through the martensite transformation. The structure is martensite which needs to tempered just as much as martensite that is formed through rapid quenching.  First quenched in salt bath.  Subsequently cooling the workpiece in air through martensite range.

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BAINITE  Is an acicular microstructure not a phase that forms in steels at temperatures from approximately 250-550°C depending on alloy content.  It is one of the decomposition products that may form when austenite the face centered cubic crystal structure of iron is cooled past a critical temperature of 727 °C.  A fine non-lamellar structure bainite commonly consists of cementite and dislocation-rich ferrite. The high concentration of dislocations in the ferrite present in bainite makes this ferrite harder.  Bainite forms needles or plates.

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CASE HARDENING  Case hardening is a process in which an alloying element most commonly carbon or nitrogen diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material which improves wear resistance without sacrificing toughness.  Case hardening is specified by hardness and case depth.

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CARBURIZING  Is a process of adding Carbon to the surface.  This is done by exposing the part to a Carbon rich atmosphere at an elevated temperature and allows diffusion to transfer the Carbon atoms into steel.  This diffusion will work only if the steel has low carbon content0.1-0.25.  If the steel had high carbon content to begin with and is heated in a carbon free furnace such as air the carbon will tend to diffuse out of the steel resulting in Decarburization.

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PACK CARBURIZING:  Parts are packed in a high carbon medium such as carbon powder or cast iron shavings and heated in a furnace for 12 to 72 hours at 900 ºC  At this temperature CO gas is produced which is a strong reducing agent. The reduction reaction occurs on the surface of the steel releasing Carbon which is then diffused into the surface due to the high temperature.  The Carbon on the surface is 0.7 to 1.2 depending on process conditions. The hardness achieved is 60 - 65 R C . The depth of the case ranges from about 1 mm up to 2 mm .  Some of the problems with pack carburizing is that the process is difficult to control as far as temperature uniformity is concerned and the heating is inefficient.

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GAS CARBURIZING:  Gas Carburizing is conceptually the same as pack carburizing except that Carbon Monoxide CO gas is supplied to a heated furnace .  This processes overcomes most of the problems of pack carburizing. The temperature diffusion is as good as it can be with a furnace.  The only concern is to safely contain the CO gas.  The depth of the case ranges from about 0.2 mm up to 0.5 mm

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LIQUID CARBURIZING:  The steel parts are immersed in a molten carbon rich bath. In the past such baths have cyanide CN as the main component. However safety concerns have led to non-toxic baths that achieve the same result.  Medium will be fused salt bath composed of sodium cyanide sodium chloride barium chloride.  The depth of the case ranges up to 0.08 mm

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NITRIDING  Produces the hardest surface of any of the hardening processes.  It differs from the other methods in that the individual parts have been heat-treated and tempered before nitriding.  The parts are then heated in a furnace that has an ammonia gas atmosphereNH3.  No quenching is required so there is no worry about warping or other types of distortion.  This process is used to case harden items such as gears cylinder sleeves camshafts and other engine parts that need to be wear resistant and operate in high-heat areas.  Heated in a furnace for 40 to 100 hours at 550 ºC

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CYANIDING  Diffusion of both carbon and nitrogen  This process is a type of case hardening that is fast and efficient.  Preheated steel is dipped into a heated cyanide bath and allowed to soak30NaCN40Na2Co330sNaCl.  Used at temperatures 787-898 o C  Upon removal it is quenched and then rinsed to remove any residual cyanide. This process produces a thin hard shell that is harder than the one produced by carburizing and can be completed in 20 to 30 minutes vice several hours.  The major drawback is that cyanide salts are a deadly poison.  Nitrogen increases hardenability

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CARBONITRIDING  Is most suitable for low carbon and low carbon alloy steels. In this process both Carbon and Nitrogen are diffused into the surface.  The parts are heated in an atmosphere of hydrocarbon such as methane or propane mixed with Ammonia NH3 gas. The process is a mix of Carburizing and Nitriding.  Carburizing involves high temperatures around 900 ºC and Nitriding involves much lower temperatures around 600 ºC. Carbonitriding is done at temperatures of 760 - 870 ºC which is higher than the transformation temperatures of steel that is the region of the face-centered Austenite.  After carbotonitriding quenching is done followed by tempering  Applied for nuts bolts and gears etc.

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FLAME HARDENING -  A heat treat method used to harden the surface of some parts where only a small portion of the surface is hardened and where the part might distort in a regular carburizing or heat treating operation.  The operation consists of heating the surface to be hardened by an acetylene torch to the proper quenching temperature followed immediately by a water quench and proper tempering.  Generally wrought or cast steels with carbon contents of .30 to .40 low alloy steels and ductile and malleable cast irons are suitable for flame hardening.  After hardening stress relieve is required Types  Stationary  Progressive- tool moves over work piece.  Spinning- T ool is stationary w/p rotates  Progressive spinning

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INDUCTION HARDENING  Is a form of heat treatment in which a metal part is heated by induction heating and then quenched.  It is done by an alternating magnetic field to a temperature above or within a specific transformation range  The quenched metal undergoes a martensitic transformation increasing the hardness and brittleness of the part.  This procedure of hardening can be specifically applied to both full as well as surface hardening methods. This is the kind of heat treatment process in which the metal part is heated and later it is quenched.  Typical applications are power train suspension engine components and stampings.

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HARDENABILITY  Hardenability is the ease with which the hardness can be attained in the depth direction of an object.  Hardness is the measure of resistance to plastic deformationby indentation.  The ability of the material to harden deeply upon quenching and takes into consideration the size of the part and the method of quenching.  The test used to determine the hardenability of any grade of steel is the Jominy end quench T est.  Hardenability is a material property dependent on chemical composition and grain size but independent of the quenching medium or quenching system cooling rate.

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PRECIPITATION OR AGE HARDENING  Hardening process applicable especially for non ferrous alloys  Objective is to create a dense and fine dispersion of the particles  Step I: Heated to a hightemperature between the solvus and solidus temperaturenear austenite for a short period of a time .  Step II: Quenching is done  StepIII: Lowering of energy is done by heating at a low temperature for a long period of time and cooling at a very slow rateroom temperature .

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CONTINUOUS COOLING TRANSFORMATION CCT TIME TEMPERATURE TRANSFORMATION TTT DIAGRAM S CURVE C CURVE BAINS CURVE Isothermal transformation diagram also known as TTT diagram measures the rate of transformation at a constant temperature i.e. it shows time relationships for the phases during isothermal transformation They are useful for understanding the transformations of an alloy steel that is cooled isothermally An isothermal transformation diagram is only valid for one specific composition of material and only if the temperature is held constant during the transformation and strictly with rapid cooling to that temperature CCT diagrams measure the extent of transformation as a function of time for a continuously decreasing temperature. For continuous cooling the time required for a reaction to begin and end is delayed thus the isothermal curves are shifted to longer times and lower temperatures.

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Partial TTT diagram for a eutectoid Fe-C alloy Te - Eutectoid Temperature

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Complete TTT isothermal transformation diagram for eutectoid steel

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Superimposition of TTT and CCT diagrams for a eutectoid steel Ms-Start of the martensite transformation Mf -completion of transformation

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CONTINUOUS COOLING RATE CCR  It is the slowest rate of cooling at which all the austenite is converted into 100 martensite

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 Austenite Martensite  Pearlite Bainite

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