Part I: Thermal Processing of Metal AlloysHeat TreatmentPrecipitation HardeningPart II: Metal Alloys and Fabrication of Metals

OutlineHeat Treatment of Steels HardenabilityInfluence of quenching medium, specimen size, and geometryAnnealing ProcessesAnnealing of ferrous alloysFull annealingNormalizingProcess annealingStress reliefPrecipitation Hardening

The complete isothermal transformation diagram for an iron-carbon alloy of eutectoid composition.

A: austeniteB: bainiteM: martensiteP: pearlite

Heat Treatment of SteelsConventional heat treatment procedures for producing martensitic steels involvescontinuous and rapid cooling of an austenitized specimen in some type of quenching medium, such as water, oil, or airThe optimum properties of a steel that has been quenched and then tempered can be realized only if,during the quenching heat treatment, the specimen has been converted to a high content of martensite

The cooling rate varies with positionAdapted from Fig. 11.12, Callister 6e. Why hardness changes with position?

Effect of Quenching MediumThe severity of quench: water > oil > air

FIG. 11.17 Radial hardness profiles for (a) 50 mm (2 in.) diameter cylindrical 1040 and 4140 steel specimens quenched in mildly agitated water, and (b) 50 and 100 mm (2 and 4 in.) diameter cylindrical specimens of 4140 steel quenched in mildly agitated water.Effect of Part Size

When surface-to-volume ratio increasescooling rate increaseshardness increasesEffect of Part Geometry

Annealing ProcessesAnnealing: a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled.Three stages of annealing Heating to the desired temperatureHolding or soaking at that temperatureCooling, usually to room temperature

Purposes for AnnealingRelieve Internal StressesInternal stresses can build up in metal as a result of processing. Stresses may be caused by previous processing operations such as welding, cold working, casting, forging, or machining.If internal stresses are allowed to remain in a metal, the part may eventually distort or crack.Annealing helps relieve internal stresses and reduce the chances for distortion and cracking.

Purposes for Annealing (Contd)Increasing Softness, Machinability, and FormabilityA softer and more ductile material is easier to machine in the machine shop.An annealed part will respond better to forming operations.Refinement of Grain StructuresAfter some types of metalworking (particularly cold working), the crystal structures are elongated. Annealing can change the shape of the grains back to the desired form.

The IronIron Carbide Phase Diagram

Temperature Regime of Steel Heat TreatmentMost heat treating operations begin with heating the alloy into the austenitic phase field to dissolve the carbide in the ironSteel heat treating practice rarely involves the use of temperatures above 1040C (1900F)FIG. 11.9 The iron-iron carbide phase diagram in the vicinity of the eutectoid, indicating heat treating temperature ranges for the plain carbon steels.

Full AnnealingFull annealing is the most basic of the annealing processes and is often simply referred to as annealing.Utilized for low- and medium-carbon steels that will be machined or will experience extensive plastic deformation during a forming operation

Full AnnealingThe alloy austenitized by heating to 15 to 40C above the A3 or A1 lines until equilibrium is achieved (i.e., the alloy changes to austenite), and then furnace cooledThe soaking time: to soak the material for 1h at the annealing temperature for every inch of thickness (a rule of thumb)A cooling rate of 100F/hr is typical for full annealing.FIG. 11.9 The iron-iron carbide phase diagram in the vicinity of the eutectoid, indicating heat treating temperature ranges for the plain carbon steels.

Microstructure after Full AnnealingMicrostructure product: coarse pearlite in addition to any proeutectoid phase soft and ductileFIG. 11.9 The iron-iron carbide phase diagram in the vicinity of the eutectoid, indicating heat treating temperature ranges for the plain carbon steels.

Process Annealing (or Intermediate Annealing)A heat treatment used to negate the effects of cold work, i.e., to soften and increase the ductility of a previously strain-hardened metalIn process annealing, parts are not as completely softened as they are in full annealing, but the time required is considerably lessened.

Process Annealing (or Intermediate Annealing)Process annealing is frequently used as an intermediate heat-treating step during the manufacture of a part. A part that is stretched considerably during manufacture may be sent to the annealing oven three or four times before all of the stretching is completed.ForgingRolling

Process Annealing (Contd)Recovery and recrystallization processes are allowed to occur1.RecoverySome of the stored internal strain energy is relieved by virtue of dislocation motion, as a result of enhanced atomic diffusion at the elevated temperature. 2.RecrystallizationRecrystallization is the formation of a new set of strain free and equiaxed grains that have low dislocation densities and are characteristic of the precold-worked condition.Ordinarily a fine-grained microstructure is desired; the heat treatment is terminated before appreciable grain growth has occurred.

FIG. 7.11 Alteration of the grain structure of a polycrystalline metal as a result of plastic deformation. (a) Before deformation the grains are equiaxed. (b) The deformation has produced elongated grains.Alteration of Grain Structure as a Result of Plastic Deformation

New crystals are formed that:have a small dislocation densityare smallconsume cold-worked crystals33% coldworkedbrassNew crystalsnucleate after3 sec. at 580C.Adapted from Fig. 7.19 (a),(b), Callister 6e. 0.6 mm0.6 mmRecrystallization

All cold-worked crystals are consumed.After 4secondsAfter 8secondsAdapted from Fig. 7.19 (c),(d), Callister 6e. 0.6 mm0.6 mmFurther Recrystallization

NormalizingThe name normalizing comes from the original intended purpose of the process to return steel to the normal condition it was in before it was altered by cold working or other processing.Heating the alloy to 55 to 85C above the A3 or Acm and holding for sufficient time so that the alloy completely transforms to austenite, followed by air coolingFIG. 11.9 The iron-iron carbide phase diagram in the vicinity of the eutectoid, indicating heat treating temperature ranges for the plain carbon steels.

Normalizing (Contd)To refine the grains and produce a more uniform and desirable size distribution for steels that have been plastically deformedNormalizing does not soften the material as much as full annealing does. The cooling process does not leave the material as ductile or as internally stress-free.A normalized part will usually be a little stronger, harder, and more brittle than a full-annealed part.

Stress Relief AnnealingInternal residual stresses may develop in metal pieces:Plastic deformation processes (machining and grinding)Nonuniform cooling of a piece that was processed or fabricated at an elevated temperature (welding or casting)A phase transformation that is induced upon cooling wherein parent and product phases have different densitiesDistortion and warpage may result if these residual stresses are not removed.

Stress Relief Annealing (Contd)The work piece is heated to the recommended temperature, held long enough to attain a uniform temperature, and finally cooled to room temperature slowlyThe annealing temperature is ordinarily a relatively low one such that effects resulting from cold work and other heat treatments are not affected

This electrical heat-treating furnace is used to heat treat strip steels (Fig. 11-1 in Metallurgy Fundamentals, by D. A. Brandt and J. C. Warner)

Furnaces Widely Used in Heat Treatment of Steels

The interior of this roller hearth-treating furnace has cast heating elements on the top, bottom, and side walls. (Fig. 11-1 in Metallurgy Fundamentals, by D. A. Brandt and J. C. Warner) Furnaces Widely Used in Heat Treatment of Steels

Furnace FixturesExamples of furnace baskets. (Practical Heat Treating, H. E. Boyer, ASM, 1984, pp. 74-75)Fabricated by welding wrought components of a 35%Ni-18%Cr alloy

Temperature Control SystemsProcess temperature should be controlled to within ~ 5F (2.5C). Although this close range is sometimes possible, a more practical control range is nearer 10F (5C). Temperature SensorsTemperature MeasurementTemperature Control

Three Major Components in A Temperature Control SystemTemperature SensorsThermocouples are the most widely used sensors for measuring temperatures of heat treating furnaces.Type K is, by far, the most widely used.

Three Major Components in A Temperature Control SystemTemperature ControlA temperature controller must provide sufficient energy to satisfy process requirements, even though operating conditions vary.The controller set point (that represents the desired temperature) is compared with the actual temperature. Based on this comparison, the controller regulates the energy flow to the process.Temperature MeasurementMeasurement instruments measure the output signal of the temperature sensor & convert it to a temperature indication.

Comparison of Annealing, Normalizing, & Quenching

Annealing & Normalizing

Quenching

Slow cooling process

Rapid cooling process

Softens and weakens metal

Hardens and strengthens metal

Produces ductility

Produces brittleness

Reduces internal stresses

Causes internal stresses

Helps prevent cracking and distortion

Increases chances of cracking and distortion

Effects of Annealing, Normalizing, and Quenching

Annealing

Normalizing

Air Quenching

Oil Quenching

Water Quenching

( Softer, less strong

Harder and stronger (

( More ductile

More brittle (

( Less internal stress

More internal stress (

( Less distortion, cracking

More distortion, cracking (