MSE 4131
Physical Metallurgy and Heat Treatment
Lecture 03
Physical Metallurgy of Ferrous Alloys and Their Basic
Heat Treatment Options
Ratul Islam Antor
Lecturer
Department of MSE, RUET
Ref: Heat Treatment of Metallic Alloys by Md. Aminul Islam;
A Textbook of Material Science and Metallurgy by O.P. Khanna;
Introduction to Physical Metallurgy by Sydney H. Avner
Contents
• Alloy steel and effect of alloying elements on steels
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Alloy
• An alloy is a mixture of two or more chemical elements, with at
least one (primary component) being a metal.
• Typically, one base metal (large amount) + smaller amounts of
other metals / non-metals.
Some common examples of alloys
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Objectives of adding alloying elements
• Improve tensile strength & ductility
• Enhance toughness
• Increase hardenability
• Reduce distortion & quench cracking
• Retain strength at high temperatures
• Boost corrosion resistance
• Improve wear resistance
• Impart fine grain structure
• Enhance abrasion resistance & fatigue behavior
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Alloy Steel
• Alloy steels are made by adding small percentages of alloying
elements to liquid steel. This process alters properties such as
hardness, toughness, elasticity, and durability.
• Common Alloying Elements:
• Carbon
• Manganese
• Silicon
• Copper
• Chromium
• Molybdenum
• Vanadium
• Nickel
• Aluminum
• Boron
• Titanium
• Zirconium
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Effect of alloying elements on steels
Carbon
• Carbon is the most crucial alloying element in steel, significantly
influencing its response to heat treatment, hardening potential and
the specific microstructures that can be achieved.
Carbon & Key Microstructures
Austenite High-T phase. Max carbon solubility upto 2.14%. Most heat
treatments begin by heating steel to the austenitic phase.
Ferrite Soft, ductile. Max carbon solubility only 0.022% at 727°C.
Cementite Iron carbide, a hard and brittle intermetallic compound.
Significantly increases hardness.
Pearlite Lamellar (layered) structure of ferrite and cementite. Forms during
slow cooling from austenite.
Martensite Hard, needle-like structure formed by rapid cooling of austenite.
Bainite Feathery microstructure of ferrite and cementite, formed at
temperatures between pearlite and martensite.
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The strengthening effect of C in steels
consists of solid solution strengthening
and carbide dispersion strengthening.
Fig: Effect of carbon on tensile
and elongation of steels
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• Carbon shows moderate macrosegregation during solidification
—more than most alloying elements. It strongly segregates at defects
like grain boundaries, dislocations.
(a) (b)
Macro-segregation of carbon using numerical modelling in the ProCAST simulation
software in (a) 90 ton steel ingot (b) 57 ton hollow steel ingot
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Manganese
• Like C, Mn also enhances strength and hardness of steel, though
to a lesser extent than carbon, with its degree of enhancement
dependent on the carbon content.
Hardenability of steels By critical quenching speed
• Thus, can be quenched in oil rather than water, and minimizing the
cracking susceptibility of steels.
• However, more than 2% of Mn results in increased tendency
toward cracking and distortion during quenching.
• Reduces the risk of hot shortness in steel by reacting with sulfur to
form manganese sulfide (MnS) as discrete and randomly
distributed globules, rather than low melting FeS.
• A Mn/S ratio of at least 8/1 is crucial to avoid hot shortness.
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Manganese
• The presence of Mn enhances the impurities such as P, Sn, Sb and
As segregating to GBs, causing temper embrittlement in steels.
• Higher Mn content in tool and die steels ensures deep hardening.
This is important because they contain strong carbide forming
alloying elements, which can withdraw carbon from solid solution,
and reduce hardenability.
• Manganese is an austenite stabilizer that lowers transformation
temperatures and expands the austenite phase field in steels.
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Copper
• Enhances corrosion resistance of steels.
• Increases strength via precipitation hardening (when >0.75% Cu)
• The copper and iron have only very limited mutual solubility in
both liquid and solid states. Maximum solubility in austenite is
2.3% at 850°C, rising to about 10% at 1450°C.
• Over 0.2% Cu can cause surface checking during forging,
especially with high Cu/C content and oxidizing atmospheres.
This is due to preferential iron oxidation, leaving a low-melting,
copper-rich phase at grain boundaries, leading to hot shortness.
• To avoid this, preheat in non-oxidizing conditions, work below
1090°C, the melting point of the copper-rich phase. Adding Ni or
Co (1/3 to 1/2 of Cu content) raises the melting point of the copper
phase. This is the most common solution.
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Chromium
Corrosion and oxidation resistance, hardness, wear
resistance, scaling at elevated temperatures.
Critical quenching speed, thus oil/air hardening is
possible without the risk of cracking
• At least 12% Cr is a must – to be considered stainless. Cr creates a
thin but tightly adherent protective chromium oxide layer.
• Tool steels, superalloys and other specialty metals contain high
chromium content. Constructional alloy steels have less than 3%.
• At low Cr/C ratios → forms alloyed cementite (Fe,Cr)₃C.
• At higher Cr/C ratios → forms chromium carbides: (Cr,Fe)₇C₃,
(Cr,Fe)₂₃C₆.
• Complex Cr-Fe carbides dissolve slowly in austenite → Requires
longer heating time before quenching.
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Nickel
• Nickel is a non-carbide-forming element in steels.
• Austenite stabilizer (same as Mn) → expands the gamma (γ) phase
field. Also works synergistically with Cr and Mo to expand the
austenite region in Fe-C diagram.
• 304 (Austenitic SS) → 18% Cr and 8% Ni
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Nickel
• Lowers A3 transformation temperature and in presence of carbon,
depresses the A1 line as well, especially with carbon present, thus
retards pearlite and bainite transformations.
• Increases strength without sacrificing ductility or toughness.
• Enhances corrosion resistance (especially in stainless steels).
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Vanadium
• Vanadium (V) is a very strong carbide former.
• Slightly dissolves in cementite. Strongly dissolves in austenite and
increases hardenability but undissolved vanadium carbides decrease
hardenability. Though excessive V can reduce hardenability in
medium carbon steels because of excessive free V in the matrix.
• Refines grains; improves strength and hardness.
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Vanadium
• Vanadium is a strong nitride former as well. Fine vanadium
carbides/nitrides cause strong dispersion hardening in microalloyed
steels after controlled rolling and cooling.
• Provides strong secondary hardening during tempering; raises hot
hardness and cutting ability.
• Enhances fatigue strength, weldability, wear resistance, and
high-temperature strength.
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Molybdenum
• Strong carbide former; slightly dissolves in cementite.
• Induces secondary hardening during the tempering of quenched
steels by the formation of molybdenum carbide precipitates.
• Enhances creep strength at high temperatures, and increases
hardenability, fatigue strength, and promotes fine grains.
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Molybdenum
• Improves corrosion and pitting resistance in stainless steels when
used in high content.
• Strong solid solution strengthener in austenitic alloys at high
temperatures.
Mo increases the austenitizing
temperature of steels by
delaying the recrystallization. So,
higher temperatures are needed to
fully transform into austenite
during heat treatment.
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Tungsten
• Strong carbide former; forms hard, abrasion-resistant carbides.
• Key element in high-speed and tool steels (~30% of global W use).
• Induces secondary hardening during tempering of quenched steel.
It promotes hot strength and red hardness and thus cutting ability.
• Prevents grain growth at high temperatures.
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Niobium and Tantalum
• Strong carbide and nitride formers.
• Small amounts of Nb and Ta form fine nitrides/carbonitrides →
grain refinement → boosts yield strength, toughness, and fatigue
resistance.
• ~0.03% Nb can raise yield strength by 150 MPa in medium carbon
steels.
• Nb stabilizes Cr–Ni austenitic steels → prevents intergranular
corrosion, by preferentially forming niobium carbides, especially
in high-temperature service.
• Nb-stabilized grades (like 347 stainless steel) are used where
welded or heat-exposed austenitic steels must retain their
corrosion resistance.
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Niobium and Tantalum
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Titanium
• Strong carbide and nitride former; more stable than Nb and V
compounds.
• Used in austenitic stainless steels for stabilization against
intergranular corrosion, similar to Nb.
• Enhances creep rupture strength via stable nitrides.
• Ti, Nb, V act as grain growth inhibitors because their nitrides and
carbides are quite stable and difficult to dissolve in austenite.
• If dissolved in austenite, hardenability increases (especially with
Mn and Cr) as can reduce carbide stability.
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THE END
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