Heat treatment of steels- I

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Heat Treatment Of Steels

Heat Treatment Of SteelsBy:Nishant KhatodAssistant ProfessorSTC, Latur

Cooling curve of pure ironCooling curve of pure iron [Source: V.D. Kodgire, S.V. Kodgire, 2010]


BCC StructureFCC Structure

Iron-carbon diagram

iron carbon phase diagram.mp4I-C diagram [Source: V.D. Kodgire, S.V. Kodgire, 2010]

Phases existing in iron-carbon diagram-ferrite:Interstitial solid solution of carbon in BCC -iron, at low temperaturesSolubility of carbon in -iron at room temperature is 0.008% and increases with increase in temperature to about 0.025% at 727 CSoft and ductile phase(Austenite):Interstitial solid solution of carbon in FCC -ironPhase is stable above 727CSolubility of carbon in -iron at 1147 C is 2%Soft, ductile, malleable and non-magnetic phase-ferrite:Interstitial solid solution of carbon in BCC -iron, at high temperaturesSimilar to -ferrite except occurrence at high temperaturesCementite:Intermetallic compound with a fixed composition of 6.67%Orthorhomic structureExtremely hard and brittle phase

Three invariant reactions in I-C diagramPeritectic transformation:General reaction:S1 + L S2 [At constant temperature]In I-C diagram(0.1% C) + L(0.55%C) (0.18%C) [At 1492 C]Eutectoid transformation:General reaction:S1 S2 + S3 [At constant temperature]In I-C diagram(0.8% C) (0.025%C) + Fe3C (6.67%C) [At 727 C]Eutectoid mixture of and Fe3C is called pearlite (average carbon content is 0.8%)Eutectic transformation:General reaction:L S1 + S2 [At constant temperature]In I-C diagramL (4.3% C) (2% C) + Fe3C(6.67%) [At 1147 C]Eutectic mixture of and Fe3C is called ledeburite (average carbon content 4.3%)

Critical temperaturesSr. No.Critical Points [Symbols]Temperature [C]Significance during heating1A0 [Curie temperature of cementite]210Cementite becomes paramagnetic2A1 [Lower critical temperature]727Pearlite starts transforming to austenite3A2 [Curie temperature of ferrite]768Ferrite becomes paramagnetic4A3 [Upper critical temperature for hypoeutectoid steels]727-910Completion of ferrite to austenite transformation5Acm [Upper critical temperature for hypereutectoid steels]727-1147Completion of cementite to austenite6A41400-1492Completion of austenite to -ferrite transformation

Termination of I-C diagram @ 5%Beyond 5% C,Carbon sublimes at atmospheric pressures instead of melting and alloys are very difficult to prepareCementite decomposes into long and thick graphite flakes as soon as it forms and detoriates the mechanical properties

Transformation products of austeniteTransformation of austenite to pearlite:

Growth of pearlite colony in austenite [Source: V.D. Kodgoire, S.V. Kodgire, 2010]Nucleation and growth of pearlite colonies [Source: V.D. Kodgoire, S.V. Kodgire, 2010]

Mic rostructures of coarse and fine pearlite [Source: R. N. Ghosh, 2006]

Transformation products of austeniteTransformation of austenite to pearlite:

Transformation products of austeniteTransformation of austenite to bainite:Obtained on rapid cooling below 550CBainite is a extremely fine mixture of ferrite and cementitteTransformation starts with nucleation of ferriteLower temperatures and hence diffusion rate is very lowUpper bainite: Formed at higher temperatures and has feathery appearanceLower bainite: Formed at lower temperature and has acicular(needle) like appearanceFiner distribution of carbides in lower banite than in upper bainite and hence lower bainite is stronger, harder and tougher than upper bainite Properties depend upon temperature and carbon content

Microstructures of lower and upper bainite [Source: R. N. Ghosh, 2006]

Transformation products of austenite

Transformation products of austeniteTransformation of austenite to martensite:Diffusionless transformationTransformation takes place by shear mechanismInstability of austenite at lower temperatures and as there can be no diffusion austenite tries to stabilize by changing its microstructureThus FCC gets transformed to BCTMartensite is very hard strong and brittleProperties depend on carbon contentMs: Martensite tranformation startsMf: Martensite transformation completesMs and Mf depend upon amount of carbon and alloying elementDifference between Ms and Mf is in the range of 150-210 C99% austenite transforms to martensite, 1% remains untransformed and is called as retained austenite

Microstructure of martensite [Source: R. N. Ghosh, 2006]

Transformation products of austenite

Retained austenite99% austenite transforms to martensite, 1% remains untransformed and is called as retained austeniteAustenite is relatively soft phase and its presence detracts from the hardness usually desired in a steel requiring full hardeningMartensite has a BCT structure whereas austenite has FCC structureDue to this, components are like to distort or crack due to the volume changes resulting in increase in residual stressesThus retained austenite is not a useful phase for applications like precision gauges and measuring instrumentsRetained austenite can be eliminated by two methods namely, subzero treatment and tempering

TTT diagrams [I-T diagrams]

TTT diagram [Source: R. N. Ghosh, 2006]

Critical cooling rateIt is a rate which just bypasses the nose of the TTT diagram OR Rate of cooling necessary to just suppress the diffusion transformation Critical cooling rate depends upon amount of carbon and alloying elementsWith increase in carbon content and alloying elements (except Co) the critical cooling rate decreases i.e. shift of nose of TTT curve to the rightShift of nose to the right gives an idea about hardenability; lower the cooling rate higher is the hardenabilityLower cooling rate reduces the tendency of warping and cracking

Critical cooling rate [Source: V.D. Kodgoire, S.V. Kodgire, 2010]

Heat treatmentEvery heat treatment process consists of three steps;Heating: Small grains will combine and form large grainsHeating media:Air Non-uniform and SlowOil Uniform and rapid heating Used upto 200CSalt bath - Uniform and rapid heating Used above 200C to avoid oxidation and decarburizationHolding: All grain will turn into uniform shape and sizeAll carbides dissolve to form austeniteTime of soaking depends uponSize of componentType of steelInitial microstructureCooling: Based on rate of cooling, final grain size of the component (properties) will be decided

Quenching mediaQuenching medium should be in such that it should extract heat rapidly at high temperatures and slowly at lower temperaturesCooling media in decreasing order of cooling rates:Brine (Cold water + 5-10% salt): Reduces distortions and eliminates weak spotsCostlier than water, corossive, service life is reducedCommonly used salts are NaCl, CaCl2, NaOH etcCold water:Low cost, abundantly available and easy handlingUsed for carbon steels, alloy steels and non-ferrous alloysBubbles can be formed which lead to soft/weak spotsWater + soluble oil:Non-uniform hardness, distortion and cracksOil:Minerals oils are usedReduces risk of distortionMore suitable for alloys steels than plain carbon steelsFused salt (Salt containing large number of ions):Used for HSSAdvantages over brine solutions: Uniformity in temperature, uniform heat transfer and no danger of oxidation and decarburisationAir:It should be dryLowest cooling rate, no cracking

Objectives of heat treatmentIncrease hardness, wear and abrasion resistance and cutting ability of steelsRe-softening the hardened steelTo adjust mechanical, physical or chemical properties like hardness, tensile strength, ductility, electrical and magnetic properties, microstructure or corrosion resistanceEliminate internal residual stressInduce controlled residual stressesStabilize the steelRefine grain sizeIncrease machinabilityEliminate gases like hydrogen which embrittles steelChange composition of surface by diffusion of C, N, Si etc so as to increase wear resistance, fatigue life or corrosion resistance

Conventional annealingApplicable: Steels with %C = 0 to 2%Objectives:Relieve residual stresses induced during cold workingTo soften the hardened steelTo refine grain sizeTo increase ductilityTo make steel suitable for subsequent heat treatment Process:Heating: Hypo-eutectoid steel: A3 + 50 C; Hyper-eutectoid steel: A1 + 50 CHolding: Holding at this temperature for definite period (for equalization of temperature and complete austenitization)Cooling: Furnace cooling to room temperature

Conventional annealingHypereutectoid steels are always annealed from above A1 temperature and never annealed from above Acm temperature;Dislocations get blocked which induces brittleness in the steelOxidation and decarburisationGrain coarsening

Schematic representation of blocking of dislocation by cementite region [Source: V.D. Kodgoire, S.V. Kodgire, 2010]

Isothermal annealingApplicable: Medium carbon (0.3-0.6%C), high carbon (0.6-2%C) and some of the alloy steelsObjective: To obtain improved machinabilityProcess:Heating: Hypo-eutectoid steel: A3 + 50 C; Hyper-eutectoid steel: A1 + 50 CHolding: Holding at this temperature for definite periodCooling: Slightly fast cooling than conventional annealing to a constant temperature just below A1, holding at this temperature for completion of transformation and then cooling to room temperature in airAdvantages:Reduced annealing time than conventionally annealing specially for alloy steelsHomogeneity in structureImproved machinability (because of spheroids of cementite), surface finish and less warping during subsequent heat treatmentDisadvantages:Large size components cannot be subjected to this treatment

Isothermal annealing (a) Hypereutectoid steel (b) hypo-eutectoid steel [Source: T. V. Rajan, C. P. Sharma, 2013]

Diffusion anne