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Heat Treatment, , ', Introducti., ‘ uction, 2. oO,, t Ss a by}, onp), Yeat Treatment Bioko, of Heat Treatment, 3, nt. 3. Proces, ri ot SS, , . Hardenabilj;, , ab ty. 10, femenability, U2 iT i, , 4. Carburising 15. Nitrigin 13:, , '9, Major Defects in q, , riding. 16 Surface H ., - 16. Cyanidi ardenin :, Metal or A i ene. 17. Induction Hones asa tiage, oy due to Faulty Heat een roe a,, 2 nt., , Hardening., Hardening., , , , , , , , , , , , , , , , INTRODUCTION, , term heat treatment may be fined i, oases erin of o tions, if, y d s, as an Operation or a combination of pera i, , facturing process of :, : : machine, alloy # ed out first by oe tools. As a matter of fact, the heat tre:, ed or desirable mechanical g it in solid state and then cooling it. Iti |, cal properties to steel or alloys for normal ae, 1ons by, , ES OF HEAT TREATMENT, , ), > y', are innumerable objectives which are achieved by heat treatment, yet the, , n structure., chanical properties like tensile strength, ductility and shock resistance etc., ical and magnetic properties., e resistance to wear, tear, heat and corrosion etc., , HEAT TREATMENT, din Art. 11.1 that the process of h, then cooling it. In fact, this process consists of:, , to a specified temperature., , - increased temperature for a specified period., , : inching) according to specified process. ou, , ‘nvolve any chemica, e heat treatment does not invo, , ee ycle, which means the range and rate of, , eat treatment is carried out first, , now that the, time-temperature ©, , , , 223

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7 steels. The process of normali., , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ity Sing j 225, , aloy steels may alse normalised. Ba: they ety applied t, , a re and the: : ys © casti ;, temperatu m cooled in the furnace, Ould be held for two re pant LOrBings ete., , © hours, at a Specified, , cal, magneti i, a ; etic pe other physical properties,, ie gases, trapped in the metal, during initial casting., , nn is of the following two types:, , ; eis seer is to soften the metal, to refine its gain, pee sion: pped in the metal. This process consists, te 50°C above the upper critical temperature for hypoeutectoid steel and, above the lower critical temperature for hypereutectoid steels. The steel, for sometime to enable the internal changes to take place. The time, , 4 minutes for each millimeter of thickness of the largest section,, , aries from 30°C to 200°C per hour, depending upon the composition, ally, carried out in the furnace. The objects may also be taken out of the, as to prolong the cooling time., isati i ing, the steel is packed in a, ation of the steel, during full annealing,, pe of cast iron borings, charcoal, lime, sand or ground mica. The, generally allowed to cool slowly in the furnace after the proper, * ; an: , ealing is to relieve the internal stresses, The ee. obec ie i al the Head In this process, the steel is, increasing the machinability Fee eel Ani, low or close ojthe ene recrystallisation in steels, which have, slowly. This cause ure is formed. The process of annealing 1s, ‘and a new grain nt, ety S., ‘and wire industrie Pe, 2s for annealing depending PO, , n the carbon content in steel, are, , re in °C, , , , nealing remperan

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1.7 SPHEROIDISING, It is a particular type of annealing in which ceme!, , ructure of steel. This is, usually, applied to high, achine. The operation consists of heating steel upto a temperature slightly above the lower, , itical temperature (730°C to 770°C). It is held at this temperature for some time and then cooled, owly to temperature of 600°C. The rate of cooling is from 25°C to 30°C per hour., , The spheroidising improves the machinal, 1. These steels have better elongation properties, , HARDENING, The main object of hardening are:, 1. To increase the hardness of the metal, so that it can resist wear., . To enable it to cut other metals, i.¢., to make it suitable for cutting tools., The process of hardening consists of heating the metal upto a temperature of 30°C to 50°C, pper critical temperature for the hypoeutectoid steels and by the same temperature, , e lower critical point for hypereutectoid steels., , metal is held at this temperature for a considerable time, depending upon its thickness, quenched (cooled suddenly) in a suitable cooling medium., hardness obtained from a given treatment depends upon the rate of cooling, the carbon, nd the work size. A very rapid cooling is necessary to harden low and medium plain, , The quenching in a water or brine solution in a method of rapid cooling, which is, used. For high carbon and alloy steels, mineral oil is generally used as the quenching, its action is not so severe as that of water. Certain alloy steels can be harde, g. But for ordinary steel, such a cooling rate is too slow to give an appreciable, effect. Large parts are, usually, quenched in an oil bath. The temperature of the, ‘hing medium must be kept uniform to achieve uniform results. Any quenching bath, w, work, should be provided with some means for cooling., apid cooling from the hardening temperature cases the austenite to be transformed into, nstituent called martensite, which is very hard and brittle. The hardening of steel, itirely upon the formation ‘of martensite, because austenite is comparatively soft am, , se of the, , ntite in the granular form is produced in the, carbon tool steels, which are difficult to, , bility of steels, but lowers the hardness and tensile, than normally annealed steel., , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ‘oted that the low carbon steels cannot be hardened appreciably, becal, e which is soft and is not changed by heat treatment. As the carbon content, , e, the possible obtainable hardness also increases. The process of hardening is of, , ning. It is the process of hardening a metal, while working on it., ng. It is also called precipitation hardening. The age hardening is 2 P ff, metal when allowed to remain or age after heat treatment. It is m8, - s metals such as alloys of aluminium, magnesiu, ffect of age hardening shows a marked increase in ‘strength an, The duralumin is an alloy of aluminium with 4% copper, alloying elements. The process of age-hardening consists!" toy, treatment and precipitation treatment. In the solution treatment, rane, ingle phase region, held there long enough to dissolve ° as, and is then rapidly quenched into the two phase region. This P a, ‘lid solution. In the precipitation treatment, the alloy is owed, ‘room temperature for a specified time. This produces 4 ¥, hich increase the strength and hardness of the alloy., , n-ferro

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ir hardening. It is the proce:, “The effect of air hardening i, alloys, : ing by heating and, me indicates), which is, , d, , om the, , ly, , Ei, , Ching a, S, Usually, seh an, , quenchin 8. I, 8enerally em, , furnace to an apparatus in w!, jet of water as shown in Fig. 11, , __ water pipe free, "height of jet = 63 mm, , 1, Jominy end-4, , a, cold, a 0., , EEE, SS of har 227, high et itis coo), , Slowly in aj, Steels and Y in air blast,, , Some of the tungsten, , Cis the, , t, Ploy, common, , €d for iron base ae hardening (as the, , alloys having a low carbon, , ions as shown in Fig 11.1(@) is first austenized. Then it is, , hich it is supported by one end and, 1 (6). The water can be supplied, the figure., , the specimen as shown in, , , , 7 <, pecimen, , support, 12.5 Bore, , , , , , i, , 12.5 mm, , mt, , , , (b) Specimen mounted in quenching rig, , uench test., , , , , , , , he length of the, ion is made along ¢ e ‘, 38 mm deep ce areged ‘These are present in a grap!, is, made along, , ed end., , , , RDENABILITY i esasel, ich it is able to resis! a, of whic! d can be measured by, hes material can be stag, ae tc.) or by age hardening: ted, nee ‘al to get hardened. aa e its, teri ition of an alloy,, position, by he co, , HA