Transcript Metal Alloys: Their Structure and Strengthening by Heat

Metal Alloys: Their Structure
and Strengthening by Heat
Treatment
Chapter 4
Heat Treatment
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Improves properties of metal alloys
Modifies the microstructure
Improves formability
Improves machinability
Increases strength & hardness
Service performance improved such as in
gears (figure 4.1)
FIGURE 4.1 Cross section of gear teeth showing induction-hardened surfaces. Source:
TOCCO Div., Park-Ohio Industries, Inc.
FIGURE 4.2 Outline of topics described in Chapter 4.
Pure Metals & Alloys
• In PURE METALS, atoms are all the
same type, except for rare impurity
atoms
• ALLOYS are composed of 2 or more
chemical elements, at least one of
which is a metal
Solid Solutions
• Solute: the minor element that is added to
the solvent
• Solvent: the major element
• Substitutional solid solutions: the size of the
solute atom is similar to the solvent atom
(example: brass alloy of zinc & copper)
• Interstitial solid solutions: the size of the
solute atom is much smaller than that of the
solvent (example: steel alloy iron & carbon)
Two phase systems
• A system with 2 or more solid phases (most
alloys)
• PHASE-a physically distinct and homogeneous
portion of the material
• Example: water and sand mixture has two
phases
• Example: lead added to copper in the molten
state (4.3a) lead particles dispersed
throughout
FIGURE 4.3 (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout
the structure of a two-phase system, such as a lead–copper alloy. The grains represent lead in solid
solution in copper, and the particles are lead as a second phase. (b) Schematic illustration of a two
phase system consisting of two sets of grains: dark and light. The green and white grains have
separate compositions and properties.
Phase Diagrams
• Pure metals have distinct melting or
freezing points
• Solidification takes place at a
constant temperature
• Latent heat of solidification is given
off while the temperature remains
constant
FIGURE 4.4 (a) Cooling curve for the solidification of pure metals. Note that freezing
takes place at a constant temperature; during freezing, the latent heat of solidification is
given off. (b) Change in density during cooling of pure metals.
Phase Diagrams
• Alloys solidify over a range of
temperatures
• Liquidus-solidification occurs when
the temperature drops below
• Solidus-solidification is complete
• Between liquidus and solidus the
alloy is in a mushy or pasty state
Phase Diagram
• Equilibrium or Constitutional Diagram
• Shows the relationships among temperature,
composition, and phases present in a
particular alloy at equilibrium
• Equilibrium-the state of a system does not
vary over time
• Binary Phase Diagram (two elements)
FIGURE 4.5 Phase diagram for nickel–copper alloy system obtained at a slow rate of solidification.
Note that pure nickel and pure copper each have one freezing or melting temperature. The top circle
on the right depicts the nucleation of crystals. The second circle shows the formation of dendrites (see
Section 10.2). The bottom circle shows the solidified alloy, with grain boundaries.
Lever Rule
• Used to determine the composition of various
phases in the phase diagram
• Example: Copper Nickel figure 4.5
– At 1288 degrees C, a mixture of solid/liquid
– Solid is 42% Cu, 58% Ni
– Liquid is 58% Cu, 42 % Ni
• The completely solidified alloy is a solid solution
because Cu completely dissolves in Ni and each
grain has the same composition
Mechanical properties of Cu-Ni solid
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Yield strength
Hardness
Percent elongation
These improve up to a point as Ni is
alloyed with pure copper
• Zinc also can be alloyed with pure copper
but has a maximum 40% solid solubility
FIGURE 4.6 Mechanical properties of copper–nickel and copper–zinc alloys as a function of their
composition. The curves for zinc are short, because zinc has a maximum solid solubility of 40% in
copper.
Lead-Tin System (fig.4.7)
• Eutectic Point-the point at which the liquid
solution decomposes into the components
alpha and beta (Greek Eutektos=easily melted)
• Single phases alpha and beta
• Two-two phase regions
– Alpha + liquid
– Beta + liquid
• Low temperature eutectic points important for
soldering so as to prevent thermal damage to
parts during joining.
FIGURE 4.7 The lead–tin phase diagram. Note that the composition of the eutectic point for this
alloy is 61.9% Sn–38.1% Pb. A composition either lower or higher than this ratio will have a higher
liquidus temperature.
Iron-Carbon System
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Steel (up to 2.11% C)
Commercially pure iron (up to .008% C)
Cast iron (up to 6.67% C) most (<4.5% C)
Iron-iron carbide phase diagram up to
6.67% C because Fe3C is a stable phase
Ferrite
• Ferrite. Alpha ferrite. α-ferrite BCC iron
• Delta ferrite. δ-ferrite. Stable only at high
temperatures thus no practical engineering
use
• Soft and ductile. Magnetic at room
temperature
FIGURE 4.8 The iron–iron-carbide phase diagram. Because of the importance of steel as an
engineering material, this diagram is one of the most important of all phase diagrams.
Austenite
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Gamma iron, γ-iron
Polymorphic transformation from BCC to FCC
W.R. Austen, 1843-1902
More solid solubility than ferrite
Denser than ferrite
Its single phase FCC structure is ductile at high
temperatures (good formability)
• Large amounts of Ni and Mangenese can be
dissolved in FCC iron
Cementite
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Carbide is another name
100% iron carbide, Fe3C
6.67% Carbon
Hard
Brittle
Can include other alloying elements such
as chromium, molybdenum, mangenese.
FIGURE 4.9 The unit cells for (a) austenite, (b) ferrite, and (c) martensite. The effect of
percentage of carbon (by weight) on the lattice dimensions for martensite is shown in (d).
Note the interstitial position of the carbon atoms. (See Fig. 1.7.) Note also the increase in
dimension c with increasing carbon content; this effect causes the unit cell of martensite
to be in the shape of a rectangular prism.
Pearlite
• At 727 ⁰C, austenite transforms into
alpha ferrite and cementite
• Eutectoid reaction-single solid phase is
transformed into two other solid phases
• This eutectoid steel is called pearlite
because at low magnification it
resembles mother of pearl
FIGURE 4.10 Schematic illustration of the microstructures for an iron–carbon alloy of
eutectoid composition (0.77% carbon), above and below the eutectoid temperature of
727°C (1341°F).
FIGURE 4.11 Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid
composition. In this lamellar structure, the lighter regions are ferrite and the darker
regions are carbide. Magnification: 2500.
Cast Irons
• A family of ferrous alloys composed of iron,
carbon (ranging 2.11% to 4.5%), and Silicon
(up to 3.5%)
• Gray cast iron or gray iron
• Ductile cast iron, nodular cast iron, or
spheroidal graphite cast iron
• White cast iron
• Malleable iron
• Compacted Graphite iron
FIGURE 4.12 Phase diagram for the iron–carbon system with graphite (instead of
cementite) as the stable phase. Note that this figure is an extended version of Fig. 4.8.
Gray Iron
• Graphite exits largely in the form of flakes.
When broken the fracture path along the
graphite flakes has a gray, sooty appearance
• Negligible ductility
• Weak in tension/strong in compression
• Dampens vibrations
• Used for machine tool bases & machinery
structures
3 types of gray iron
• Ferritic (fully gray iron) consists of
graphite flakes in the alpha-ferrite matrix
• Pearlitic- has a structure of graphite in a
matrix of pearlite. Still brittle but
stronger than ferritic iron
• Martensitic-obtained by austenitizing a
pearilitic gray iron and rapidly quenching
to produce a structure of graphite in a
martensite matrix. Very hard
Ductile (Nodular) Iron
• Graphite is in a nodular or spheroid
form (fig.4.13b)
• Small additions of magnesium
and/or cerium to the molten metal
before pouring
• Somewhat ductile and shock
resistant
Malleable Iron
• Obtained by annealing white cast iron in an
atmosphere of carbon monoxide and carbon
dioxide at between 800⁰ and 900⁰ C for up to
several hours.
• Cementite decomposes into iron and graphite
• Rosettes or clusters of graphite (fig.4.13c) in a
pearlite or ferrite matrix
• Ductility, strength, shock resistance similar to
nodular iron
• Malleable comes from Latin malleus “it can be
hammered”
Compacted-graphite iron
• Graphite is in the form of short,
thick, interconnected flakes
• Mechanical and physical properties
intermediate between flake-graphite
and nodular-graphite cast irons
FIGURE 4.13 Microstructure for cast irons. Magnification: 100. (a) Ferritic gray iron
with graphite flakes. (b) Ferritic ductile iron (nodular iron), with graphite in nodular form.
(c) Ferritic malleable iron; this cast iron solidified as white cast iron, with the carbon
present as cementite, and was heat treated to graphitize the carbon.