Metal Alloys: Metal Alloys: Their Structure & Their Structure & Strengthening by Strengthening by

Heat TreatmentHeat TreatmentPart 2

Chapter 4

Heat treatment of Heat treatment of ferrous alloys (4.7)ferrous alloys (4.7)

• Heat-treatment techniques: the controlled heating and cooling of alloys at various rates

• Phase transformations: greatly influence the mechanical propertieso StrengthoHardnessoDuctilityo ToughnessoWear resistance

Iron-carbon system Iron-carbon system microstructural microstructural

changeschanges•Pearlite•Spheroidite•Bainite•Martensite

PearlitePearlite• Course pearlite• Slow rate of

cooling• As in a furnace

• Fine pearlite• High rate of cooling• As in air (fig.4.11)

SpheroiditeSpheroidite• Higher toughness• Lower hardness• Can be cold

worked• Spheroidites less

conductive to stress concentration

• Pearlite is heated to just below the eutectoid temperature for a period of time

• Example: a day at 700oC

• Cementite lamellae transform into roughly spherical shapes

FIGURE 4.14 FIGURE 4.14 Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700°C (1292°F). Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700°C (1292°F).

Magnification: 1000Magnification: 1000..

BainiteBainite• Very fine microstructure

consisting of ferrite and cementite

• Bainitic steel is stronger and more ductile than pearlitic steels at the same hardness levels

MartensiteMartensite• When austenite is cooled at a high

rate such as by quenching in water its FCC structure is transformed to BCT (body-centered tetragonal)

• Hard• Brittle• Lacks toughness so limited in

usefulness

FIGURE 4.15 FIGURE 4.15 (a) Hardness of martensite as a function of carbon content. (b) Micrograph of (a) Hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same

composition as the original austenite (white regions). Magnification: 1000composition as the original austenite (white regions). Magnification: 1000..

Quench crackingQuench cracking• Internal stresses cause parts to

undergo distortion or even crack during heat treatment

• Distortion is the irreversible dimensional change of the part during heat treatment

FIGURE 4.15 FIGURE 4.15 (a) Hardness of martensite as a function of carbon content. (b) Micrograph of (a) Hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same

composition as the original austenite (white regions). Magnification: 1000composition as the original austenite (white regions). Magnification: 1000..

TTT (time-TTT (time-temperature-temperature-

transformation) transformation) diagramsdiagrams

• Allow metallurgists to design heat treatment schedules to obtain desirable microstructures

• Fig.4.17a The higher the temperature or the longer the time, the more austenite that transforms into pearlite

FIGURE 4.17 FIGURE 4.17 (a) Austenite-to-pearlite transformation of iron–carbon alloy as a function of time and (a) Austenite-to-pearlite transformation of iron–carbon alloy as a function of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation

temperature of 675°C (1247°F). (c) Microstructures obtained for a eutectoid iron–carbon alloy as a temperature of 675°C (1247°F). (c) Microstructures obtained for a eutectoid iron–carbon alloy as a function of cooling rate.function of cooling rate.

HardenabilityHardenability• Hardenability is the capability of an

alloy to be hardened by heat treatment

• Measures the depth of hardness obtained by heat treatment/quenching

• Hardenability is not the same as hardness

FIGURE 4.19 FIGURE 4.19 Mechanical properties of annealed steels as a function of composition and Mechanical properties of annealed steels as a function of composition and microstructure. Note in (a) the increase in hardness and strength, and in (b), the decrease in ductility microstructure. Note in (a) the increase in hardness and strength, and in (b), the decrease in ductility

and toughness, with increasing amounts of pearlite and iron carbideand toughness, with increasing amounts of pearlite and iron carbide ..

End-Quench End-Quench hardenability test (Jominy hardenability test (Jominy

Test)Test)• Quenching media• Water• Brine• Oil• Molten salts• Air• Caustic solutions• Polymer solutions• gases

• Round test bar is austenized (heated to the proper temperature to form 100% austenite)

• Bar then quenched at one end

• Hardness decreases away from the quenched end of the bar

FIGURE 4.20 FIGURE 4.20 (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels, (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels, as obtained from the end-quench test. Small variations in composition can change the shape of these as obtained from the end-quench test. Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment curves. Each curve is actually a band, and its exact determination is important in the heat treatment

of metals, for better control of properties.of metals, for better control of properties.

Precipitation Precipitation hardeninghardening

• Small particles of a different phase called precipitates are uniformly dispersed in the matrix of the original phase

• Precipitates form because the solid solubility of one element in the other is exceeded

• The alloy is reheated to an intermediate temperature and held there for a long time during which time precipitation takes place

Aging or Age Aging or Age HardeningHardening

• Because the precipitation process is one of time and temperature, it is also called AGING.

• Age hardening is the property improvement of the material

• Artificial aging is carried out above room temperature

• Natural aging: some aluminum alloys harden and become stronger over time at room temperature

FIGURE 4.22 FIGURE 4.22 The effect of aging time and temperature on the yield stress of 2014-T4 aluminum The effect of aging time and temperature on the yield stress of 2014-T4 aluminum alloy. Note that, for each temperature, there is an optimal aging time for maximum strength.alloy. Note that, for each temperature, there is an optimal aging time for maximum strength.

Case hardeningCase hardening• Hardening of the surface• Improves resistance to surface indentation, fatigue,

wear• Gear teeth• Cams• Shafts• Bearings• Fasteners• Pins• Automotive clutch plates• Tools and dies

TABLE 4.1 TABLE 4.1 Outline of Heat-treatment Processes for Surface Outline of Heat-treatment Processes for Surface HardeningHardening

TABLE 4.1 (continued) TABLE 4.1 (continued) Outline of Outline of Heat-treatmentHeat-treatment Processes for Surface Hardening Processes for Surface Hardening

AnnealingAnnealing• Steps1.Heat to a specific

temperature range in a furnace

2.Hold at that temperature (soaking)

3.Cooling in air or in a furnace

• The restoration of a cold-worked or heat-treated alloy to its original properties

• Increase ductility• Reduce hardness and

strength• Modify the

microstructure• Relieve residual

stresses• Improve machinability

More about annealingMore about annealing• Normalizing-the cooling cycle is

completed in still air to avoid excessive softness

• Process annealing • Stress-relief annealing• Tempering• Austempering• Martempering• Ausforming