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MODULE – 5
ALLOY & SPECIAL STEELS
Rahul Kumar
KIIT UNIVERSITY

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General Categories of Alloy Steels
 Carbon and Alloy Steels
 Stainless Steel
 Tool and Die Steels
 Cast Steels (Crucible Steels)

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Effects of Alloying Elements in Steels
• Boron: improves hardenability without the loss of (or even with some
improvement in) machinability and formability
• Calcium: deoxidizes steels, improves toughness and improve formability
and machinability
• Carbon: improves hardenability, strength, hardness and wear
resistance, as well as reduces ductility, weldability and toughness
• Cerium: controls shape of inclusions and improves toughness in high-
strength low alloy steels, as well as deoxidizes steels

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• Chromium: improves toughness, hardenability, wear and corrosion
resistance and high-temperature strength. It also increases depth of
hardness penetration resulting from heat treatment by promoting
carburization
• Cobalt: improves strength and hardness at elevated temperatures
• Copper: improves resistance to atmospheric corrosion and to lesser
extent increases strength with little loss in ductility, as well as also
adversely affects hot-working characteristics and surface quality
• Lead: improves machinability, as well as causes liquid-metal
embrittlement

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• Magnesium: has the same effects as cerium
• Manganese: improves hardenability, strength, abrasion resistance and
machinability, as well as deoxidizes molten steel, reduce shot shortness,
and decreases weldability
• Molybdenum: improves hardenability, wear resistance, toughness,
elevated-temperature strength, creep resistance and hardness, as well as
minimizes temper embrittlement
• Nickel: improves strength, toughness and corrosion resistance, as well
as improves hardenability
• Niobium (columbium): imparts fineness of grain size and improves
strength and impact toughness, as well as lowers transition temperature
and decrease hardenability

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• Phosphorus: improves strength, hardenability, corrosion resistance and
machinability, as well as severely reduces ductility and toughness
• Selenium: improves machinability
• Silicon: improves strength, hardness, corrosion resistance, and
electrical conductivity; it decreases magnetic-hysteresis loss,
machinability and cold formability
• Sulfur: improves machinability when combined with manganese, as
well as lowers impact strength and ductility and impairs surface
quality and weldability

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• Tantalum: has effects similar to those of niobium
• Tellurium: improves machinability, formability and toughness
• Titanium: improves hardenability; it deoxidizes steels
• Tungsten: has the same effects as cobalt
• Vanadium: improves strength, toughness, abrasion resistance and
hardness at elevated temperatures, as well as inhibits grain growth
during heat treatment
• Zirconium: has same effects as cerium

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1. Carbon and Alloy Steels
• carbon and alloying steels are the most commonly used metals
• structural makeup and controlled processing of these steels make
them suitable for many different functions
• basic product shapes include plate, sheet, bar, wire, tube, castings,
and forgings
• increasing % of alloying elements in steels, increases properties they
impart (different elements are added to give different properties
• elements pass on properties such as hardenability, strength, hardness,
toughness, wear resistance, etc
• some properties are beneficial while others are detrimental

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Carbon Steels
• also known as plain carbon steels
• group by % of carbon content (weight basis)
• higher the carbon content greater the hardness, strength and wear
resistance after heat treatment
• soft, tough, easily machined, welded & case hardened
• designation: e.g. 1040 steel - 0.40 wt % C
• types:
Low-carbon steel (mild steels)
Medium-carbon steel
High-carbon steel

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Low-carbon steel (mild steels)
• has less than 0.30 % carbon
• used in everyday industrial products like bolts, nuts, sheet, plate and
tubes
Medium-carbon steel
• has 0.30% to 0.60 % carbon
• used for jobs requiring higher strength such as machinery, automotive
equipment parts, and metalworking equipment
High-carbon steel
• has more than 0.60 % carbon
• used parts that require the highest strength, hardness and wear
resistance
• once manufactured they are heat treated and tempered

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High carbon steel nails
Low carbon steel wires
Medium carbon steel nuts

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Alloy Steels
• contain significant amounts of alloying elements
• expensive
Types of Alloy Steels
• High strength low alloy steels (HSLA)
• Microalloyed steels
• Nanoalloyed steels
• Bearing steels
• Cold forming steels
• Chained steels

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High-Strength, Low-Alloy Steels (HSLA Steels)
• developed to improve the ratio of strength to weight
• commonly used in automobile bodies and in transportation industry
(reduced weight makes for better fuel economy)
Microalloyed Steels
• provide superior properties without the use of heat treating
• when cooled carefully these steels develop enhanced and consistent
strength
Nanoalloyed Steels
• have extremely small grain size (10-100 nm)
• Since their synthesis is done at atomic level their properties can be
controlled specifically

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High strength low alloy
steel sheets
Microalloyed steel
connecting rods
Nanoalloyed steel
bicycle hub

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Bearing steel pipes
Cold forming steel
front
Chained steel chain

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2. Stainless Steels
• primarily know for their corrosion resistance, high strength, and
ductility and chromium content
• reason for name stainless is due to the fact that in presence of oxygen,
steel develops a thin, hard, adherent film of chromium
• Even if surface is scratched, protective film is rebuilt through
passivation
• for passivation to occur, there needs to be minimum chromium content
of 10 to 12 % by weight

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• tend to have lower carbon content since increased carbon content
lowers the corrosion resistance of stainless steels
• since carbon reacts with chromium, it decreases the available
chromium content which is needed for developing protective film
• using stainless steel as reinforcing bars, has become a new trend in
concrete structures such as highways buildings and bridges
• more beneficial than carbon steels because it is resistant to corrosion
from road salts and the concrete itself

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Applications Products of Stainless Steels

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3. Tool and Die Steels
• generally alloyed steels
• medium to high carbon
• up to 25% total alloying elements
• design for high strength, impact toughness and wear resistance at
normal and elevated temperatures
• used at temperatures up to 600°C
• specialty steels – very expensive
• quench and tempered
• very clean steels
• applications like dies, drills, cutting blades, hot working dies, etc

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Desirable Properties of Tool Steels
• Hardness - Resistance to Deforming & Flattening
• Toughness - Resistance to Breakage & Chipping
• Wear - Resistance to Abrasion & Erosion
• Corrosion - Resistance to Rusting and Pitting

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Alloying Elements in Tool Steels & their Effects
Carbon (C) +Strength, +Hardenability, -Toughness
Chromium (Cr) +Strength, +Hardenability, +Corrosion Resistance - Toughness
Molybdenum (Mo) +Strength, +Hardenability, +Toughness, +Hot Hardness
Vanadium (V) +Hardenability, +Toughness, +Hot Hardness, +Wear
Tungsten (W) +Strength, +Hardenability, +Hot Hardness, -Toughness
Cobalt (Co) +Hot Hardness, +Wear, -Toughness
Manganese (Mn) +Strength, +Hardenability, +Toughness
Nickel (Ni) +Hardenability, +Toughness, +Corrosion Resistance
+ increases
- decreases

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Classification of Tool Steels
Tool Steel Class of Alloys Examples (industrial names)
Cold Work W (water hardening)
O (oil hardening)
A (air hardening)
D (high C & Cr)
W1, W2, W5
O1, O2, O6, O7
A2, A4, A6, A7, A8, A9, A10, A11
D2, D3, D4, D5, D7
Shock
Resisting
S S1, S2, S4, S5, S6, S7
Hot Work H Chromium types: H10-H19
Tungsten types: H20-H39
Molybdenum types: H40-H59
High Speed M
T
Molybdenum types: M1, M2, M3-1, M3-2,
M4, M6, M7, M10, M33, M34, M36, M41
Tungsten types: T1, T4, T5, T6, T8, T15
Mold P P6, P20, P21

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Classification of Tool Steels

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Cold Work Tool Steels
• used at low temperature-sharpness
• include all W, O, A & D class of alloys
• typical applications include cold working operations such as stamping
dies, draw dies, burnishing tools, coining tools and shear blades

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Shock Resisting Tool Steels
• used at low temperature-toughness & impact toughness
• include all S class alloys
• toughest tool steels
• typically applications include screw driver blades, shear blades, chisels,
knockout pins, punches, and riveting tools

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Hot Work Tool Steels
• used at high temperature-toughness, high resistance to wear & cracking
• include all H class alloys
• typical applications include dies for forging, die casting, heading,
piercing, trimming, extrusion and hot-shear and punching blades

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High Speed Tool Steels
• used at high temperature-sharpness
• Include M1 to M52, T1 to T15 class of alloys
• can be hardened to 62-67 RC and can maintain it in service temperatures
as high as 540°C, making them very useful in high-speed machinery
• typical applications include end mills, drills, lathe tools, planar tools,
punches, reamers, routers, taps, saws, broaches, chasers and hobs

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4. Cast Steels (Crucible Steels)
• term originally applied to crucible steel (sometimes used to describe tool
steels, which is misleading)
• fine variety of steel, originally made by smelting blister or cementation
steel & pouring molten steel into moulds
• manufacture is essentially a refining process, which is dependent on pre-
existing furnace products
• cannot not subjected to further forging or rolling