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7. what are the five classifications of corrosion? Discuss!

Answer :

Five classifications of corrosion for metal exist :

a. Atmospheric.

Atmospheric corrosion is the deterioration and destruction of a material and its vital

properties due to electrochemical as well as the other reactions of its surface with the

constituents of the atmosphere surrounding the material.

The most important factor in atmospheric corrosion, overriding pollution or lack of it,

is moisture, either in the form of rain, dew, condensation, or high relative humidity

(RH). In the absence of moisture, most contaminants would have little or no corrosive

effect. Rain also may have a beneficial effect in washing away atmospheric pollutants

that have settled on exposure surfaces. This effect has been particularly noticeable in

marine atmospheres. On the other hand, if the rain collects in pockets or crevices, it

may accelerate corrosion by supplying continued wetness in such areas.

Dew and condensation are undesirable from a corrosion standpoint if not

accompanied by frequent rain washing which dilutes or eliminates contamination. A

film of dew, saturated with sea salt or acid sulfates, and acid chlorides of an industrial

atmosphere provides an aggressive electrolyte for the promotion of corrosion. Also, in

the humid Tropics where nightly condensation appears on many surfaces, the stagnant

moisture film either becomes alkaline from reaction with metal surfaces, or picks up

carbon dioxide and becomes aggressive as a dilute acid.

Atmospheric Corrosion

Temperature plays an important role in atmospheric corrosion in two ways. First,

there is the normal increase in corrosion activity which can theoretically double for

each ten-degree increase in temperature. Secondly, a little-recognized effect is the

temperature lag of metallic objects, due to their heat capacity, behind changes in the

ambient temperature.

As the ambient temperature drops during the evening, metallic surfaces tend to remain

warmer than the humid air surrounding them and do not begin to collect condensation

until some time after the dew point has been reached. As the temperature begins to

rise in the surrounding air, the lagging temperature of the metal structures will tend to

make them act as condensers, maintaining a film of moisture on their surfaces.

The period of wetness is often much longer than the time the ambient air is at or

below the dew point and varies with the section thickness of the metal structure, air

currents, RH, and direct radiation from the sun.

Cycling temperature has produced severe corrosion on metal objects in the Tropics, in

unheated warehouses, and on metal tools or other objects stored in plastic bags. Since

the dew point of an atmosphere indicates the equilibrium condition of condensation

and evaporation from a surface, it is advisable to maintain the temperature some 10 to

15oC above the dew point to ensure that no corrosion will occur by condensation on a

surface that could be colder than the ambient environment.

b. Water Immersion.

When metal are immersed in water, the amount of oxygen dissolved in the water is an

important factor. If the water does not contain any dissolved oxygen, the water that is

alkaline has very little corrosion activity unless the solution is highly concentrated.

Corrosion Due To Metal Immersed In Water

c. Soil

Soil corrosion is a geologic hazard that affects buried metals and concrete that is in

direct contact with soil or bedrock. Soil corrosion is a complex phenomenon, with a

multitude of variables involved. Pitting corrosion and stress-corrosion cracking (SCC)

are a result of soil corrosion, which leads to underground oil and gas transmission

pipeline failures.

In some respects, corrosion in soils resembles atmospheric corrosion in that observed

rates, although usually higher than in the atmosphere, vary to a marked degree with

the type of soil. For example, a cast iron water pipe may last 50 years in New England

soil, but only 20 years in the more corrosive soil of Southern California.

Pipeline damaged by external corrosion

Corrosive soils contain chemical constituents that can react with construction

materials, such as concrete and ferrous metals, which may damage foundations and

buried pipelines. The electrochemical corrosion processes that take place on metal

surfaces in soils occur in the groundwater that is in contact with the corroding

structure. Both the soil and the climate influence the groundwater composition.

Factors that influence soil corrosion are:

Porosity (aeration)

Electrical conductivity or resistivity

Dissolved salts, including depolarizers or inhibitors

Moisture

pH

Each of these variables may affect the anodic and cathodic polarization characteristics

of a metal in soil. The most corrosive soils have high content of:

Moisture

Electrical conductivity

Acidity

Dissolved salts

Corrosion rates underground have been measured by using:

Stern-Geary linear polarization method

Weight loss method

The former method has been useful in assessing the corrosion rates of footings of

galvanized steel towers used to support power lines. The corrosivity of soils can be

estimated by measuring soil resistivity. Sandy soils are high on the resistivity scale

and therefore considered the least corrosive. Clay soils, especially those contaminated

with saline water are on the opposite end of the spectrum.

Soil corrosion can be controlled by:

Using organic and inorganic coatings

Applying metallic coatings

Alteration of soil

Cathodic protection

d. Chemical Other than water

Rusting, the formation of iron oxides, is a well-known example of electrochemical

corrosion. This type of damage typically produces oxide(s) or salt(s) of the original

metal, and results in a distinctive orange colouration. Rust is another name for iron

oxide, which occurs when iron or an alloy that contains iron, like steel, is exposed to

oxygen and moisture for a long period of time. Over time, the oxygen combines with

the metal at an atomic level, forming a new compound called an oxide and weakening

the bonds of the metal itself. Although some people refer to rust generally as

"oxidation", that term is much more general; although rust forms when iron undergoes

oxidation, not all oxidation forms rust. Only iron or alloys that contain iron can rust,

but other metals can corrode in similar ways.

Heavy rust on the links of a chain near the Golden Gate Bridge in San Francisco; it was

continuously exposed to moisture and salt spray, causing surface breakdown, cracking,

and flaking of the metal.

e. Electrolytic.

Galvanic corrosion occurs when two different metals have physical or electrical

contact with each other and are immersed in a common electrolyte, or when the same

metal is exposed to electrolyte with different concentrations. In a galvanic couple, the

more active metal (the anode) corrodes at an accelerated rate and the more noble

metal (the cathode) corrodes at a slower rate. When immersed separately, each metal

corrodes at its own rate. What type of metal(s) to use is readily determined by

following the galvanic series. For example, zinc is often used as a sacrificial anode for

steel structures. Galvanic corrosion is of major interest to the marine industry and also

anywhere water (containing salts) contacts pipes or metal structures.

Factors such as relative size of anode, types of metal, and operating conditions

(temperature, humidity, salinity, etc.) affect galvanic corrosion. The surface area ratio

of the anode and cathode directly affects the corrosion rates of the materials. Galvanic

corrosion is often prevented by the use of sacrificial anodes.

Galvanic corrosion of aluminium. Galvanic corrosion occurred on the aluminium plate

along the joint with the steel. Perforation of aluminium plate occurred within 2 years.

8. Explain by use of your own diagram the tension testing steel. Show all physical and

mechanical properties and explain each!

Answer :

Tension Testing

Tensile Strength Ductility Yield

PointOffset Yield

StrengthProportional Limit

Elastic and Inelastic Limit

Modulus of Rigidity

Modulus of Toughness

Modulus of Resilience

General Stress-Strain Curve Diagrams

a. Tensile Strength. Tensile strength of metals is the maximum axial load observed in a

tension test divided by the original cros-sectional area. The strength increases and

reaches a maximum in mild steel, after extensive elongation and nexing.

b. Ductility. Is the ability of a material to undergo large deformations without fracture.

Ductility measured by the elongation and reduction, expressed as :

PercentElongation=FinalLength−OriginalLengthOriginalLength

x100

Percent ReductionArea=OriginalArea−AreaAfterFractureOriginalLength

x100

c. Modulus of Elasticity. Modulus of Elasticity (E) is given by the slope of the straight-

line portion of the stress-strain curve. Modulus of Elasticity (young’s modulus) is the

ratio of unit stress to unit strain in the elastic range of the stress-strain curve as

follows :

E=Stress( psi )

Strain( in . /in . )x100

Or in SI units,

E=Stress(Mpa )

Strain(cm /cm )x100

d. Yield Point. Is the first load at wich there is a marked increase a strain without an

increase in stress.

e. Offset Yield Strength. Is defined as the stress corresponding to a permanent

deformation.

f. Proportional Limit. The proportional limit is the gteatest stress that a material is

capable of without deviating from the law of proportionally of stress to strain

(Hooke’s Law) :

f=E∈Where :

f = unit stress

∈ = unit strain

E = Young’s moulud of elasticity

g. Elastic limit and inelastic limit. The elastic limit is the largest unit stress that can be

developed without a permanent set remaining after the load is removed.

h. Modulus of rigidity. Or shearing modulus of elasticity, defined as :

G= vγ

Where :

G = modulus of rigidity

v = unit shearing stress

γ = unit shearing strain

The modulus of rigidity may also be rewritten and related to the modulud of elasticity

by :

G= E2(1+μ )

i. Modulus of toughness. Is the ability of a material to absorb large amounts of energy.

The modulus of toughness can be related to the area under the entire stress-strain

curve.

j. Modulus of Resilience. The resilience of a material is that property of an elastic

body by which energy can be stored up in the body by loads applied to it and given up

in recovering its original shape when the load are removed. The modulus of resilience

is equal to the area under the straight-line portion of the stress-strain curve.

Example of stress-strain curve diagram :

From the diagram above, we can conclude :

a. Tensile Strength = 68.000 psi

b. Ductility

Percentage Elongation = 31.1 %

Percentage Reduction Area = 62,5 %

c. Modulus of Elasticity = 30.000.000 psi

d. Yield Point = 41.000 psi

e. Offset Yield Strength = 42.000 psi

f. Proportional Limit = 33.000 psi

Page 285

1. Explain the purpose and use of the blast furnance !

Answer :

The blast furnace is the first step in producing steel from iron oxides. The first blast furnaces

appeared in the 14th Century and produced one ton per day. Blast furnace equipment is in

continuous evolution and modern, giant furnaces produce 13,000 tons per day. Even though

equipment is improved and higher production rates can be achieved, the processes inside the

blast furnace remain the same. Blast furnaces will survive into the next millenium because

the larger, efficient furnaces can produce hot metal at costs competitive with other iron

making technologies.

Definition:

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial

metals, generally iron, but also others such as lead or copper.

In a blast furnace, fuel, ores, and flux (limestone) are continuously supplied through the top

of the furnace, while a hot blast of air (sometimes with oxygen enrichment) is blown into the

lower section of the furnace through a series of pipes called tuyeres, so that the chemical

reactions take place throughout the furnace as the material moves downward. The end

products are usually molten metal and slag phases tapped from the bottom, and flue gases

exiting from the top of the furnace. The downward flow of the ore and flux in contact with an

upflow of hot, carbon monoxide-rich combustion gases is a countercurrent exchange process.

In contrast, air furnaces (such as reverberatory furnaces) are naturally aspirated, usually by

the convection of hot gases in a chimney flue. According to this broad definition, bloomeries

for iron, blowing houses for tin, and smelt mills for lead would be classified as blast furnaces.

However, the term has usually been limited to those used for smelting iron ore to produce pig

iron, an intermediate material used in the production of commercial iron and steel, and the

shaft furnaces used in combination with sinter plants in base metals smelting.

Blast furnace in Sestao, Spain. The furnace itself is inside the central girderwork

History :

Blast furnaces existed in China from about 1st century AD and in the West from the High

Middle Ages. They spread from the region around Namur in Wallonia (Belgium) in the late

15th century, being introduced to England in 1491. The fuel used in these was invariably

charcoal. The successful substitution of coke for charcoal is widely attributed to Abraham

Darby in 1709. The efficiency of the process was further enhanced by the practice of

preheating the combustion air (hot blast), patented by James Beaumont Neilson in 1828.

Steel-making and the use of the “blast furnace,” developed in 1855 by Henry Bessemer,

allowed for the large-scale production of strong and cheap steel, which became the material

of choice for artillery, guns, and warships.

Types :

Conventional

Coke blast furnaces

Hot blast

Modern Furnances

Iron blast furnaces

Lead blast furnaces

Zinc blast furnaces (Imperial Smelting Furnaces)

Modern Processes :

Modern furnaces are equipped with an array of supporting facilities to increase efficiency,

such as ore storage yards where barges are unloaded. The raw materials are transferred to the

stockhouse complex by ore bridges, or rail hoppers and ore transfer cars. Rail-mounted scale

cars or computer controlled weight hoppers weigh out the various raw materials to yield the

desired hot metal and slag chemistry. The raw materials are brought to the top of the blast

furnace via a skip car powered by winches or conveyor belts.

Blast furnace placed in an installation :

1. Iron ore + limestone sinter2. Coke3. Elevator4. Feedstock inlet5. Layer of coke6. Layer of sinter pellets of ore and limestone7. Hot blast (around 1200 °C)

11. Torpedo car for pig iron12. Dust cyclone for separation of solid particles13. Cowper stoves for hot blast14. Smoke outlet (can be redirected to carbon

capture & storage (CCS) tank)15: Feed air for Cowper stoves (air pre-heaters)16. Powdered coal

8. Removal of slag9. Tapping of molten pig iron10. Slag pot

17. Coke oven18. Coke19. Blast furnace gas downcomer

There are different ways in which the raw materials are charged into the blast furnace. Some

blast furnaces use a "double bell" system where two "bells" are used to control the entry of

raw material into the blast furnace. The purpose of the two bells is to minimize the loss of hot

gases in the blast furnace. First, the raw materials are emptied into the upper or small bell

which then opens to empty the charge into the large bell. The small bell then closes, to seal

the blast furnace, while the large bell rotates to provide specific distribution of materials

before dispensing the charge into the blast furnace. A more recent design is to use a "bell-

less" system. These systems use multiple hoppers to contain each raw material, which is then

discharged into the blast furnace through valves. These valves are more accurate at

controlling how much of each constituent is added, as compared to the skip or conveyor

system, thereby increasing the efficiency of the furnace. Some of these bell-less systems also

implement a discharge chute in the throat of the furnace (as with the Paul Wurth top) in order

to precisely control where the charge is placed.

The iron making blast furnace itself is built in the form of a tall structure, lined with

refractory brick, and profiled to allow for expansion of the charged materials as they heat

during their descent, and subsequent reduction in size as melting starts to occur. Coke,

limestone flux, and iron ore (iron oxide) are charged into the top of the furnace in a precise

filling order which helps control gas flow and the chemical reactions inside the furnace. Four

"uptakes" allow the hot, dirty gas high in carbon monoxide content to exit the furnace throat,

while "bleeder valves" protect the top of the furnace from sudden gas pressure surges. The

coarse particles in the exhaust gas settle in the "dust catcher" and are dumped into a railroad

car or truck for disposal, while the gas itself flows through a venturi scrubber and/or

electrostatic precipitators and a gas cooler to reduce the temperature of the cleaned gas.

The "casthouse" at the bottom half of the furnace contains the bustle pipe, water cooled

copper tuyeres and the equipment for casting the liquid iron and slag. Once a "taphole" is

drilled through the refractory clay plug, liquid iron and slag flow down a trough through a

"skimmer" opening, separating the iron and slag. Modern, larger blast furnaces may have as

many as four tapholes and two casthouses. Once the pig iron and slag has been tapped, the

taphole is again plugged with refractory clay.

The tuyeres are used to implement a hot blast, which is used to increase the efficiency of the

blast furnace. The hot blast is directed into the furnace through water-cooled copper nozzles

called tuyeres near the base. The hot blast temperature can be from 900 °C to 1300 °C (1600

°F to 2300 °F) depending on the stove design and condition. The temperatures they deal with

may be 2000 °C to 2300 °C (3600 °F to 4200 °F). Oil, tar, natural gas, powdered coal and

oxygen can also be injected into the furnace at tuyere level to combine with the coke to

release additional energy and increase the percentage of reducing gases present which is

necessary to increase productivity.

Process Engineering and Chemistry :

Blast furnaces operate on the principle of chemical reduction whereby carbon monoxide,

having a stronger affinity for the oxygen in iron ore than iron does, reduces the iron to its

elemental form. Blast furnaces differ from bloomeries andreverberatory furnaces in that in a

blast furnace, flue gas is in direct contact with the ore and iron, allowing carbon monoxide to

diffuse into the ore and reduce the iron oxide to elemental iron mixed with carbon. The blast

furnaces operates as a countercurrent exchange process whereas a bloomery does not.

Another difference is that bloomeries operate as a batch process while blast furnaces operate

continuously for long periods because they are difficult to start up and shut down. Also, the

carbon in pig iron lowers the melting point below that of steel or pure iron; in contrast, iron

does not melt in a bloomery.

Blast furnace diagram :

1. Hot blast from Cowper stoves2. Melting zone (bosh)3. Reduction zone of ferrous oxide (barrel)4. Reduction zone of ferric oxide (stack)5. Pre-heating zone (throat)6. Feed of ore, limestone, and coke

7. Exhaust gases8. Column of ore, coke and limestone9. Removal of slag10. Tapping of molten pig iron11. Collection of waste gases

Carbon monoxide also reduces silica which has to be removed from the pig iron. The silica is

reacted with calcium oxide(burned limestone) and forms a slag which floats to the surface of

the molten pig iron. The direct contact of flue gas with the iron causes contamination with

sulfur if it is present in the fuel. Historically, to prevent contamination from sulfur, the best

quality iron was produced with charcoal.

The downward moving column of ore, flux, coke or charcoal and reaction products must be

porous enough for the flue gas to pass through. This requires the coke or charcoal to be in

large enough particles to be permeable, meaning there cannot be an excess of fine particles.

Therefore, the coke must be strong enough so it will not be crushed by the weight of the

material above it. Besides physical strength of the coke, it must also be low in sulfur,

phosphorus, and ash. This necessitates the use of metallurgical coal, which is a premium

grade due to its relative scarcity.

The main chemical reaction producing the molten iron is:

Fe2O3 + 3CO → 2Fe + 3CO2

This reaction might be divided into multiple steps, with the first being that preheated blast air

blown into the furnace reacts with the carbon in the form of coke to produce carbon

monoxide and heat:

2 C(s) + O2(g) → 2 CO(g)

The hot carbon monoxide is the reducing agent for the iron ore and reacts with the iron

oxide to produce molten iron and carbon dioxide. Depending on the temperature in the

different parts of the furnace (warmest at the bottom) the iron is reduced in several steps. At

the top, where the temperature usually is in the range between 200 °C and 700 °C, the iron

oxide is partially reduced to iron(II,III) oxide, Fe3O4.

3 Fe2O3(s) + CO(g) → 2 Fe3O4(s) + CO2(g)

At temperatures around 850 °C, further down in the furnace, the iron(II,III) is reduced further

to iron(II) oxide:

Fe3O4(s) + CO(g) → 3 FeO(s) + CO2(g)

Hot carbon dioxide, unreacted carbon monoxide, and nitrogen from the air pass up through

the furnace as fresh feed material travels down into the reaction zone. As the material travels

downward, the counter-current gases both preheat the feed charge and decompose the

limestone to calcium oxide and carbon dioxide:

CaCO3(s) → CaO(s) + CO2(g)

The calcium oxide formed by decomposition reacts with various acidic impurities in the iron

(notably silica), to form a fayalitic slag which is essentially calcium silicate, Ca Si O 3:[57]

SiO2 + CaO → CaSiO3

As the iron(II) oxide moves down to the area with higher temperatures, ranging up to

1200 °C degrees, it is reduced further to iron metal:

FeO(s) + CO(g) → Fe(s) + CO2(g)

The carbon dioxide formed in this process is re-reduced to carbon monoxide by the coke:

C(s) + CO2(g) → 2 CO(g)

The temperature-dependent equilibrium controlling the gas atmosphere in the furnace is

called the Boudouard reaction:

2CO   CO2 + C

The "pig iron" produced by the blast furnace has a relatively high carbon content of around

4–5%, making it very brittle, and of limited immediate commercial use. Some pig iron is used

to make cast iron. The majority of pig iron produced by blast furnaces undergoes further

processing to reduce the carbon content and produce various grades of steel used for

construction materials, automobiles, ships and machinery.

Although the efficiency of blast furnaces is constantly evolving, the chemical process inside

the blast furnace remains the same. According to the American Iron and Steel Institute: "Blast

furnaces will survive into the next millennium because the larger, efficient furnaces can

produce hot metal at costs competitive with other iron making technologies." One of the

biggest drawbacks of the blast furnaces is the inevitable carbon dioxide production as iron is

reduced from iron oxides by carbon and there is no economical substitute – steelmaking is

one of the unavoidable industrial contributors of the CO2 emissions in the world

(see greenhouse gases).

The challenge set by the greenhouse gas emissions of the blast furnace is being addressed in

an ongoing European Program called ULCOS (Ultra Low CO2Steelmaking). Several new

process routes have been proposed and investigated in depth to cut specific emissions

(CO2 per ton of steel) by at least 50%. Some rely on the capture and further storage (CCS) of

CO2, while others choose decarbonizing iron and steel production, by turning to hydrogen,

electricity and biomass. In the nearer term, a technology that incorporates CCS into the blast

furnace process itself and is called the Top-Gas Recycling Blast Furnace is under

development, with a scale-up to a commercial size blast furnace under way. The technology

should be fully demonstrated by the end of the 2010s, in line with the timeline set, for

example, by the EU to cut emissions significantly. Broad deployment could take place from

2020 on.

9. List and discuss the various alloy steels

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0%

and 50% by weight to improve its mechanical properties. Alloy steels are broken down into

two groups: low-alloy steels and high-alloy steels. The difference between the two is

somewhat arbitrary: Smith and Hashemi define the difference at 4.0%, while Degarmo, et al.,

define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.

Strictly speaking, every steel is an alloy, but not all steels are called "alloy steels". The

simplest steels are iron (Fe) alloyed with carbon (C) (about 0.1% to 1%, depending on type).

However, the term "alloy steel" is the standard term referring to steels with other alloying

elements added deliberately in addition to the carbon. Common alloyants include manganese

(the most common one), nickel, chromium, molybdenum, vanadium, silicon, and boron. Less

common alloyants include aluminum, cobalt, copper, cerium, niobium, titanium, tungsten,

tin, zinc, lead, and zirconium.

The following is a range of improved properties in alloy steels (as compared to carbon

steels): strength, hardness, toughness, wear resistance, corrosion resistance, hardenability,

and hot hardness. To achieve some of these improved properties the metal may require heat

treating.

Some of these find uses in exotic and highly-demanding applications, such as in the turbine

blades of jet engines, in spacecraft, and in nuclear reactors. Because of the ferromagnetic

properties of iron, some steel alloys find important applications where their responses to

magnetism are very important, including in electric motors and in transformers.

Chromium

Chromium is primaliry a hardening agent and is generally added to steel in amounts

of 0.7 to 1.20 percent, with a variation in carbon content of 0.17 to 0.55 percent. Its

value is due to principally to its property of combining intense hardness after

quenching with very high strength and elastic limit. Thus it is well suited to withstand

abrasion, cutting, or shock. It does lack ductility, but this is unimportant in view of its

high elastic limit. Chromium steels corrode less rapidly than do carbon steel.

Nickel-Chromium

Nickel-Chromium steels, when properly heat-treated, have a very high tensile strength

and elastic limit, with considerable toughness and ductility. The nickel content is

usually 3.5 percent, with a carbon content ranging from 0.15 to 0.50 percent.

One very important properly of nickel-chromium steels is that by adding aluminium,

cobalt, copper, manganese, silicon, silver, or tungsten, stainless steel result.

Manganese

as previously indicated, manganese is present in all steels as a result of the

manufacturing process. When the manganese is 1.0 percent or greater in solution with

steel, it is considered an alloy. Manganese will add hardness to steel if used within the

proper range.

Molybdenum

Molybdenum provides strength and hardness in steel. It inhibits grain growth on

heating as a result of its slow solubility of austenite. When in solution in the austenite

it decrease the cooling rate and, therefore, increase the depth of hardening.

Silicon

Silicon is added to carbon steel for the purpose of deoxidizing. For this reason, silicon

may be added in amounts of up to 0.25 percent. Silicon does not form carbides but

does dissolve in the ferrite up to about 15 percent. Silicon decrease hysteresis and

eddy-current losses, and thus is valuable for electrical machinery.

Vanadium

Vanadium is a powerful element for alloying in steel. It forms stable carbides and

improves the hardenability of steels. Vanadium promotes a fine-grained structure and

promotes hardness at high temperatures. The amount of vanadium present is 0.1 to 0.3

percent when used.

Copper

Copper increases the yiels strength, tensile strength, and hardness of steel. However

ductility may be increases by about 2 percent. The most important use of copper is to

increase the resistance of steel to atmospheric corrosion.

Tungsten

Tungsten increasses the strength, hardness, and toughness of steel. After moderately

rapid cooling from high temperatures, tungsten steel exhibits remarkable hardness,

which is still retained upon heating to temperatures considerably above the ordinary

tempering heats of carbon steels. It is this property of tungsten that makes it a

valuable alloy, in conjuction with chromium or manganese, for the production of

high-speed tool steel.

Principal effects of major alloying elements for steel

Element Percentage Primary functionAluminium 0.95–1.30 Alloying element in nitriding steelsBismuth - Improves machinabilityBoron 0.001–0.003 A powerful hardenability agentChromium 0.5–2 Increases hardenability

4–18 Increases corrosion resistanceCopper 0.1–0.4 Corrosion resistance

Lead - Improved machinabilityManganese 0.25–0.40 Combines with sulfur and with phosphorus to reduce

the brittleness. Also helps to remove excess oxygen from molten steel.

>1 Increases hardenability by lowering transformation points and causing transformations to be sluggish

Molybdenum

0.2–5 Stable carbides; inhibits grain growth. Increases the toughness of steel, thus making molybdenum a very valuable alloy metal for making the cutting parts of machine tools and also the turbine blades of turbojet engines. Also used in rocket motors.

Nickel 2–5 Toughener12–20 Increases corrosion resistance

Silicon 0.2–0.7 Increases strength2.0 Spring steelsHigher percentages

Improves magnetic properties

Sulfur 0.08–0.15 Free-machining propertiesTitanium - Fixes carbon in inert particles; reduces martensitic

hardness in chromium steelsTungsten - Also increases the melting point.Vanadium 0.15 Stable carbides; increases strength while retaining

ductility; promotes fine grain structure. Increases the toughness at high temperatures