module last prelim Flashcards by chowchie the dog

Metal alloys, by virtue of composition, are often grouped into two classes they are

classes—ferrous

and non-ferrous.

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those in which iron is the principal constituent include

steels and cast irons.

Ferrous alloys

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all alloys that are not iron based.

The nonferrous

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Another classification is made based on their formability. If materials are hard to
form, components with these materials are fabricated by casting, thus they are called

cast alloys.

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If material can be deformed, they are known as

wrought alloys.

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Materials are usually strengthened by two methods they are

cold work and heat treatment.

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involves either precipitation hardening or martensitic transformation,

Strengthening by

heat treatment

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When a material cannot be

strengthened by heat treatment, it is referred as

non-heat-treatable alloys.

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those in which iron is the prime constituent—are produced in

larger quantities than any other metal type.

Ferrous alloys

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exist in abundant quantities within the Earth’s crust;

iron containing compounds

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may be produced using relatively economical extraction,

refining, alloying, and fabrication techniques;

metallic iron and steel alloys

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may be tailored to have a wide

range of mechanical and physical properties

ferrous alloys are extremely versatile,

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The principal disadvantage of many ferrous alloys is their susceptibility

corrosion.

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are alloys of iron and carbon plus other alloying elements. carbon
present in atomic form, and occupies interstitial sites of Fe microstructure.

Steels

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Alloying

additions are necessary for many reasons including:

improving properties, improving

corrosion resistance, etc.

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Mechanical properties of steels are very sensitive to carbon content. Hence, it is
practical to classify steels based on their carbon content. Thus steels are basically three
kinds:

low-carbon steels medium carbon steels high-carbon steels

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The other parameter available for classification

of steels is amount of alloying additions, and based on this steels are two kinds:

(plain)

carbon steels and alloy-steels.

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These are arguably produced in the greatest quantities than other alloys. These are arguably produced in the greatest quantities than other alloys. Their microstructure
consists of ferrite and pearlite, and these alloys are thus relatively soft, ductile combined
with high toughness. Hence these materials are easily machinable and weldable. Typical
applications of these alloys include: structural shapes, tin cans, automobile body
components, buildings, etc.

Low Carbon Steels

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low carbon steels these alloys are strengthened by

cold work.

These are stronger than low carbon steels. However these are of less ductile than
low carbon steels. These alloys can be heat treated to improve their strength. Usual heat
treatment cycle consists of austenitizing, quenching, and tempering at suitable conditions
to acquire required hardness. They are often used in tempered condition. As hardenability
of these alloys is low, only thin sections can be heat treated using very high quench rates.
Ni, Cr and Mo alloying additions improve their hardenability. Typical applications include:
railway tracks & wheels, gears, other machine parts which may require good combination
of strength and toughness.

Medium Carbon Steels

These are strongest and hardest of carbon steels, and of course their ductility is
very limited. These are heat treatable, and mostly used in hardened and tempered
conditions. They possess very high wear resistance, and capable of holding sharp edges.
Thus these are used for tool application such as knives, razors, hacksaw blades, etc. With
addition of alloying element like Cr, V, Mo, W which forms hard carbides by reacting with
carbon present, wear resistance of high carbon steels can be improved considerably.

High Carbon Steels

A special group of ferrous alloys with noticeable amount of alloying additions Common alloying elements are: Cu, V,
Ni, W, Cr, Mo, etc. These alloys can be strengthened by heat treatment, and yet the same
time they are ductile, formable. Typical applications of these steels include: support
columns, bridges, pressure vessels.

HSLA (High-Strength Low-Alloy) Steels

The name comes from their high resistance to corrosion i.e. they are rust-less Steels are made highly corrosion resistant by addition of special alloying
elements, especially a minimum of 12% Cr along with Ni and Mo. Stainless steels are
mainly three kinds: ferritic & hardenable Cr steels, austenitic and precipitation hardenable
(martensitic, semi-austenitic) steels. This classification is based on prominent constituent
of the microstructure. Typical applications include cutlery, razor blades, surgical knives,
etc.Steels are made highly corrosion resistant by addition of special alloying
elements, especially a minimum of 12% Cr along with Ni and Mo. Stainless steels are
mainly three kinds: ferritic & hardenable Cr steels, austenitic and precipitation hardenable
(martensitic, semi-austenitic) steels. This classification is based on prominent constituent
of the microstructure. Typical applications include cutlery, razor blades, surgical knives,
etc.

Stainless Steels

are principally Fe-Cr-C alloys with 12-14% Cr. They also

contain small additions of Mo, V, Nb, and Ni.

Ferritic Stainless Steels

usually contain 18% Cr and 8% Ni in addition to other minor alloying elements. Ni stabilizes the austenitic phase assisted by C and N. Other alloying additions include Ti, Nb, Mo (prevent weld decay), Mn and Cu (helps in stabilizing austenite).

Austenitic Stainless Steels

is made to be above the room temperature. These alloys are heat treatable. Major alloying elements are: Cr, Mn and Mo.

Martensitic Steels

steels are hardened and strengthened by cold work because | they are not heat treatable.

Ferritic and austenitic

are heat treatable.

martensitic steels

are most corrosion resistant, and they are produced in large quantities. are non-magnetic as against ferritic and martensitic steels, which are magnetic.

Austenitic steels

Though ferrous alloys with more than 2.14 wt.% C. Hard and brittle constituent presented in these alloys, cementite is a meta-stable phase, and can readily decompose to form α-ferrite and graphite. In this way disadvantages of brittle phase can easily be overcome.

CAST IRONS

is the most used fabrication technique for these | alloys.

casting

Tendency of cast irons to form | graphite is usually controlled by their

composition and cooling rate.

Based on the form of | carbon present, cast irons are categorized as

gray, white, nodular and malleable cast | irons.

These alloys consists carbon in form graphite flakes, which are surrounded by either ferrite or pearlite. Because of presence of graphite, fractured surface of these alloys look grayish, and so is the name for them. Alloying addition of Si (1-3wt.%) is responsible for decomposition of cementite, and also high fluidity. Thus castings of intricate shapes can be easily made. Due to graphite flakes, gray cast irons are weak and brittle. However they possess good damping properties, and thus typical applications include: base structures, bed for heavy machines, etc. they also show high resistance to wear.

Gray Cast Iron

When Si content is low (< 1%) in combination with faster cooling rates, there is no time left for cementite to get decomposed, thus most of the brittle cementite retains. Because of presence of cementite, fractured surface appear white, hence the name. They are very brittle and extremely difficult to machine. Hence their use is limited to wear resistant applications such as rollers in rolling mills. Usually white cast iron is heat treated to produce malleable iron.

White Cast Iron

Alloying additions are of prime importance in producing these materials. Small additions of Mg / Ce to the gray cast iron melt before casting can result in graphite to form _______. Matrix surrounding these particles can be either ferrite or pearlite depending on the heat treatment. These are stronger and ductile than gray cast irons. Typical applications include: pump bodies, crank shafts, automotive components, etc.

Nodular (Or Ductile) Cast Iron

These formed after heat treating white cast iron. Heat treatments involve heating the material up to 800-900 deg. C, and keep it for long hours, before cooling it to room temperature. High temperature incubation causes cementite to decompose and form ferrite and graphite. Thus these materials are stronger with appreciable amount of ductility. Typical applications include: railroad, connecting rods, marine and other heavy- duty services.

Malleable Cast Iron

are consumed in exceedingly large quantities because they have such a wide range of mechanical properties, may be fabricated with relative ease, and are economical to produce.

Steel and other ferrous alloys

have specific advantages over ferrous materials. They can be fabricated with ease, high relatively low density, and high electrical and thermal conductivities. However different materials have distinct characteristics, and are used for specific purposes.

Non-ferrous materials

These are characterized by low density, high thermal & electrical conductivities, and good corrosion resistant characteristics. As Al has FCC crystal structure, these alloys are ductile even at low temperatures and can be formed easily.

Aluminum Alloys

One special feature of most of these alloys is their corrosion resistant in diverse atmospheres. Most of these alloys are strengthened by 9 either cold work or solid solution method. Common most Cu alloys: Brass, alloys of Cu and Zn where Zn is substitutional addition (e.g.: yellow brass, catridge brass, muntz metal, gilding metal); Bronze, alloys of Cu and other alloying additions like Sn, Al, Si and Ni. Bronzes are stronger and more corrosion resistant than brasses. Mention has to be made about Beryllium coppers who possess combination of relatively high strength, excellent electrical and corrosion properties, wear resistance, can be cast, hot worked and cold worked. Applications of Cu alloys include: costume jewelry, coins, musical instruments, electronics, springs, bushes, surgical and dental instruments, radiators, etc.

Copper Alloys

is low density among all structural metals. has HCP structure, are difficult to form at room temperatures. are usually fabricated by casting or hot working. As in case of Al, alloys are cast or wrought type, and some of them are heat treatable. Major alloying additions are: Al, Zn, Mn and rare earths. Common applications of Mg alloys include: hand-held devices like saws, tools, automotive parts like steering wheels, seat frames, electronics like casing for laptops, camcorders, cell phones etc.

Magnesium Alloys

are of relatively low density, high strength and have very high melting point. At the same time they are easy to machine and forge. However the major limitation is Ti’s chemical reactivity at high temperatures, which necessitated special techniques to extract. Thus these alloys are expensive. They also possess excellent corrosion resistance in diverse atmospheres, and wear properties. Common applications include: space vehicles, airplane structures, surgical implants, and petroleum & chemical industries.

Titanium Alloys

These are metals of very high melting points. For example: Nb, Mo, W and Ta. They also possess high strength and high elastic modulus. Common applications include: space vehicles, x-ray tubes, welding electrodes, and where there is a need for corrosion resistance.

Refractory Metals

These are eight all together: Ag, Au, Pt, Pa, Rh, Ru, Ir and Os. All these possess some common properties such as: expensive, soft and ductile, oxidation resistant. Ag, Au and Pt are used extensively in jewelry, alloys are Ag and Au are employed as dental restoration materials; Pt is used in chemical reactions as a catalyst and in thermo couples.

Noble Metals