A

Project on

BOILER, METALLURGY, MATERIALS & HEAT TREATMENT

Submitted By:

Rakesh Kumar Singh 0210PGD042

Submitted To:

Mr. C. Sidda Raju

April 2010

Post Graduate Diploma in Power Plant Engineering

JSW Energy Centre of Excellence

I

DEPARTMENT OF POWER PLANT ENGINEERING

M.S.R.I.T.

Bangalore

CERTIFICATE

Certified that the project report entitled “BOILER, METALLURGY,

MATERIALS & HEAT TREATMENT” a bonafide work carried out in Partial

fulfilment of the award of Post Graduate Diploma in Power Plant Engineering,

during in the year 2009-10

II

ABSTRACT

Boilers: Consideration of factors concerned with microstructure and strength

has led to the production of a number of martensitic and austenitic development

alloys and subsequent evaluation for the selection of the best alloy from each

group. Testing of these is ongoing.

Metallurgy: In its general, modern sense, metallurgy is the science that studies

the chemical and physical properties of metals, including how they perform

when used for culturally useful industrial purposes. The term often refers to the

procedures used in extracting metals from ore, as well as to the processes

related to metals purification and alloy production.

Boiler Material: The material used for plates, rivets, braces, and all other parts

on which the structural strength of a high-grade boiler depends are made of a

low-carbon open-hearth steel in which is allowable only very small quantities of

phosphorus and sulphur. Nickel is often alloyed with this steel to improve its

tensile and elastic strength.

Heat treatment: Heat treatment is a method used to alter the physical, and

sometimes chemical properties of a material. The most common application is

metallurgical. Heat treatments are also used in the manufacture of many other

materials, such as glass. Heat treatment involves the use of heating or chilling,

normally to extreme temperatures, to achieve a desired result such as hardening

or softening of a material. Heat treatment techniques include annealing, case

hardening, precipitation strengthening, tempering and quenching.

III

OBJECTIVE

The principal aim of this project was to develop and demonstrate the suitability

of advanced materials and components for the power industry. Such materials

and components were aimed at steam temperatures of 620 - 650ºC. Specific

areas covered were:

• Development and assessment of improved alloys for boiler superheaters,

headers, pipework and furnace walls.

• Development of improved alloys for steam turbine high temperature rotor

forgings, castings, and bolting. Prototype component manufacture and

characterisation.

• Development and modelling of welding procedures and consumables for the

above groups.

• Accelerated alloy development and application through improved

understanding and modeling of microstructural evolution and its relationship to

mechanical properties.

• Characterisation of steam oxidation behaviour of new alloys, and modelling of

spallation.

IV

CONTENTS

Chapter Page

TITLE.......................................................................................................... I

CERTIFICATE ...................................................................................................II

ABSTRACT....................................................................................................... III

OBJECTIVE............ ..................................................................................IV

TABLE OF CONTENTS ...................................................................................V

WHAT IS A BOILER........................................................................................1

MATERIALS…....................................................................................................2

FUEL…………....................................................................................................3

CONFIGURATIONS…………….......................................................................4

SUPERHEATED STEAM BOILERS..................................................................5

SUPERCRITICAL STEAM GENERATORS......................................................7

HYDRONIC BOILERS........................................................................................8

BOILER FEED WATER....................................................................................10

METALLURGY...............................................................................................11

HISTORY...........................................................................................................13

EXTRACTION...................................................................................................14

ALLOYS.............................................................................................................15

PRODUCTION...................................................................................................16

MICROSTRUCTURE…………………………………………………………17

BOILER MATERIAL ……………………………………………………….18

HEAT TREATMENT……………………………………………….……….20

BOILER WATER TREATMENT……………………………………………..21

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WHAT IS A BOILER?

A boiler is defined as "a closed vessel in which water or other liquid is heated,

steam or vapor is generated, steam is superheated, or any combination thereof,

under pressure or vacuum, for use external to itself, by the direct application of

energy from the combustion of fuels, from electricity or nuclear energy."

Also included are fired units for heating or vaporizing liquids other than water

where these units are separate from processing systems and are complete within

themselves. This definition includes water heaters that exceed 200,000 Btu/hr

heat input, 200 degrees Fahrenheit at the outlet, or 120 gallons nominal water

containing capacity.

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MATERIALS:

The pressure vessel in a boiler is usually made of steel (or alloy steel), or

historically of wrought iron. Stainless steel is virtually prohibited (by the ASME

Boiler Code) for use in wetted parts of modern boilers, but is used often in super

heater sections that will not be exposed to liquid boiler water. In live steam

models, copper or brass is often used because it is more easily fabricated in

smaller size boilers. Historically, copper was often used for fireboxes

(particularly for steam locomotives), because of its better formability and higher

thermal conductivity; however, in more recent times, the high price of copper

often makes this an uneconomic choice and cheaper substitutes (such as steel)

are used instead.

For much of the Victorian "age of steam", the only material used for boiler

making was the highest grade of wrought iron, with assembly by riveting. This

iron was often obtained from specialist ironworks, such as at Cleator Moor

(UK), noted for the high quality of their rolled plate and its suitability for high-

reliability use in critical applications, such as high-pressure boilers. In the 20th

century, design practice instead moved towards the use of steel, which is

stronger and cheaper, with welded construction, which is quicker and requires

less labor.

Cast iron may be used for the heating vessel of domestic water heaters.

Although such heaters are usually termed "boilers", their purpose is usually to

produce hot water, not steam, and so they run at low pressure and try to avoid

actual boiling. The brittleness of cast iron makes it impractical for high pressure

steam boilers.

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FUEL:

The source of heat for a boiler is combustion of any of several fuels, such as

wood, coal, oil, or natural gas. Electric steam boilers use resistance- or

immersion-type heating elements. Nuclear fission is also used as a heat source

for generating steam. Heat recovery steam generators (HRSGs) use the heat

rejected from other processes such as gas turbines.

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CONFIGURATIONS:

Boilers can be classified into the following configurations:

"Pot boiler" or "Haycock boiler": a primitive "kettle" where a fire heats a

partially-filled water container from below. 18th century Haycock boilers

generally produced and stored large volumes of very low-pressure steam,

often hardly above that of the atmosphere. These could burn wood or

most often, coal. Efficiency was very low.

Fire-tube boiler. Here, water partially fills a boiler barrel with a small

volume left above to accommodate the steam (steam space). This is the

type of boiler used in nearly all steam locomotives. The heat source is

inside a furnace or firebox that has to be kept permanently surrounded by

the water in order to maintain the temperature of the heating surface just

below boiling point.

Water-tube boiler. In this type, the water tubes are arranged inside a

furnace in a number of possible configurations: often the water tubes

connect large drums, the lower ones containing water and the upper ones,

steam and water; in other cases, such as a mono tube boiler, water is

circulated by a pump through a succession of coils. This type generally

gives high steam production rates, but less storage capacity than the

above.

Flash boiler. A specialized type of water-tube boiler.

Fire-tube boiler with Water-tube firebox. Sometimes the two above types

have been combined in the following manner: the firebox contains an

assembly of water tubes, called thermic syphons.

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Sectional boiler. In a cast iron sectional boiler, sometimes called a "pork

chop boiler" the water is contained inside cast iron sections. These

sections are assembled on site to create the finished boiler.

SUPERHEATED STEAM BOILERS:

Most boilers heat water until it boils, and then the steam is used at saturation

temperature (i.e., saturated steam). Superheated steam boilers boil the water and

then further heat the steam in a super heater. This provides steam at much

higher temperature, but can decrease the overall thermal efficiency of the steam

generating plant due to the fact that the higher steam temperature requires a

higher flue gas exhaust temperature. There are advantages to superheated steam

and this may (and usually will) increase overall efficiency of both steam

generation and its utilisation considered together: gains in input temperature to a

turbine should outweigh any cost in additional boiler complication and expense.

Superheated steam presents unique safety concerns because, if there is a leak in

the steam piping, steam at such high pressure/temperature can cause serious,

instantaneous harm to anyone entering its flow. Since the escaping steam will

initially be completely superheated vapor, it is not easy to see the leak, although

the intense heat and sound from such a leak clearly indicates its presence.

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The super heater works like coils on an air conditioning unit, however to a

different end. The steam piping (with steam flowing through it) is directed

through the flue gas path in the boiler furnace. Some super heaters are radiant

type (absorb heat by radiation), others are convection type (absorb heat via a

fluid i.e. gas) and some are a combination of the two. So whether by convection

or radiation the extreme heat in the boiler furnace/flue gas path will also heat

the super heater steam piping and the steam within as well. It is important to

note that while the temperature of the steam in the super heater is raised, the

pressure of the steam is not: the turbine or moving pistons offer a "continuously

expanding space" and the pressure remains the same as that of the boiler. [6] The

process of superheating steam is most importantly designed to remove all

droplets entrained in the steam to prevent damage to the turbine blading and/or

associated piping.

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SUPERCRITICAL STEAM GENERATORS:

Supercritical steam generators (also known as Benson boilers) are frequently

used for the production of electric power. They operate at "supercritical

pressure". In contrast to a "subcritical boiler", a supercritical steam generator

operates at such a high pressure (over 3,200 psi/22.06 MPa or 220.6 bar) that

actual boiling ceases to occur, and the boiler has no water - steam separation.

There is no generation of steam bubbles within the water, because the pressure

is above the "critical pressure" at which steam bubbles can form. It passes

below the critical point as it does work in the high pressure turbine and enters

the generator's condenser. This is more efficient, resulting in slightly less fuel

use. The term "boiler" should not be used for a supercritical pressure steam

generator, as no "boiling" actually occurs in this device.

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HYDRONIC BOILERS:

Hydronic boilers are used in generating heat for residential and industrial

purposes. They are the typical power plant for central heating systems fitted to

houses in northern Europe (where they are commonly combined with domestic

water heating), as opposed to the forced-air furnaces or wood burning stoves

more common in North America. The hydronic boiler operates by way of

heating water/fluid to a preset temperature (or sometimes in the case of single

pipe systems, until it boils and turns to steam) and circulating that fluid

throughout the home typically by way of radiators, baseboard heaters or through

the floors. The fluid can be heated by any means...gas, wood, fuel oil, etc, but in

built-up areas where piped gas is available, natural gas is currently the most

economical and therefore the usual choice.

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Hydronic systems are being used more and more in new construction in North

America for several reasons. Among the reasons are:

They are more efficient and more economical than forced-air systems

(although initial installation can be more expensive, because of the cost of

the copper and aluminum).

The baseboard copper pipes and aluminum fins take up less room and use

less metal than the bulky steel ductwork required for forced-air systems.

They provide more even, less fluctuating temperatures than forced-air

systems. The copper baseboard pipes hold and release heat over a longer

period of time than air does, so the furnace does not have to switch off

and on as much.

They do not dry out the interior air as much.

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They do not introduce any dust, allergens, mold, or (in the case of a faulty

heat exchanger) combustion byproducts into the living space.

Forced-air heating does have some advantages, however. See forced-air heating.

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BOILER FEED WATER:

A boiler is a device for generating steam, which consists of two principal parts:

the furnace, which provides heat, usually by burning a fuel, and the boiler

proper, a device in which the heat changes water into steam. The steam or hot

fluid is then re-circulated out of the boiler for use in various processes in

heating applications.

The boiler receives the feed water,

which consists of varying proportion of

recovered condensed water (return

water) and fresh water, which has been

purified in varying degrees (make up

water). The make-up water is usually

natural water either in its raw state, or

treated by some process before use.

Feed-water composition therefore

depends on the quality of the make-up

water and the amount of condensate

returned to the boiler.

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METALLURGY:

Metallurgy is a domain of materials science that studies the physical and

chemical behavior of metallic elements, their intermetallic compounds, and

their mixtures, which are called alloys. It is also the technology of metals: the

way in which science is applied to their practical use. Metallurgy is commonly

used in the craft of metalworking.

In its general, modern sense, metallurgy is the science that studies the chemical

and physical properties of metals, including how they perform when used for

culturally useful industrial purposes. The term often refers to the procedures

used in extracting metals from ore, as well as to the processes related to metals

purification and alloy production. It also refers to the craft of making culturally

useful objects out of metal, or metalworking. The practice of metalworking has

been carried out over thousands of centuries.

Evidence of this science and craft dates back roughly 6,500 years. Copper, tin,

silver, and meteoric iron, which was used by the Egyptians to make weapons,

all underwent some form of metalworking process in various ancient cultures.

The first evidence of a standard metallurgy technology appeared during the

Bronze Age, which started around 3,500 BC, when it was discovered that by

heating and combining copper and tin, a bronze alloy could be created. The Iron

Age began around 1,200 BC when the Hittites discovered how to extract iron

from ore and work it to advance their cultural aims. Georg Agricola, considered

to be the father of metallurgy, detailed ore mining and metal extraction

procedures, as well as other aspects of the science, in his 16th century book, De

re metallica.

Modern metallurgy is divided into two subtypes. Process metallurgy refers to

the steps involved in producing metals, in most cases, from sulfides or oxides,

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and then refining them in their reduced form through electrolysis or selective

oxidation of impurities. Physical metallurgy studies the structure of metals,

based on their composition and treatment, and how this structure is related to

their properties. It is also concerned with the scientific principles and

engineering applications employed in metals fabrication and treatments, and

how metal products hold up under their industrial usages.

Metallurgical engineers employ different forms of metals testing. In that way,

they can make quantified assumptions about a metal's strength. These tests are

meant to determine such properties as metal hardness, impact toughness, and

tensile strength, to name of few.

In general, elemental metals, in their pure native form, are too soft for industrial

uses. That is why the science of metallurgy tends to focus on the manufacture of

alloys, in which metals are combined together or with non-metals. Steel and cast

irons are examples of iron-carbon alloys. Aluminum, copper, iron, magnesium,

and zinc are the metals that are used most, usually in their alloy forms.

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HISTORY:

The first evidence of human metallurgy dates from the 5th and 6th millennium

BC, and was found in the archaeological sites of Majdanpek, Yarmovac and

Plocnik, Serbia. These examples include a copper axe from 5,500BC belonging

to the Vincha culture.[1] Other signs of human metallurgy are found from the

third millennium BC in places like Palmela (Portugal), Cortes de Navarra

(Spain), and Stonehenge (United Kingdom). However, as often happens with

the study of prehistoric times, the ultimate beginnings cannot be clearly defined

and new discoveries are continuous and ongoing.

Mining areas of the ancient Middle East. Boxes colors: arsenic is in brown,

copper in red, tin in grey, iron in reddish brown, gold in yellow, silver in white

and lead in black. Yellow area stands for arsenic bronze, while grey area stands

for tin bronze.

Silver, copper, tin and meteoric iron can also be found native, allowing a limited

amount of metalworking in early cultures. Egyptian weapons made from

meteoric iron in about 3000 B.C. were highly prized as "Daggers from Heaven".[2]. However, by learning to get copper and tin by heating rocks and combining

those two metals to make an alloy called bronze, the technology of metallurgy

began about 3500 B.C. with the Bronze Age.

The extraction of iron from its ore into a workable metal is much more difficult.

It appears to have been invented by the Hittites in about 1200 B.C., beginning

the Iron Age. The secret of extracting and working iron was a key factor in the

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EXTRACTION:

Extractive metallurgy is the practice of removing valuable metals from an ore

and refining the extracted raw metals into a purer form. In order to convert a

metal oxide or sulfide to a purer metal, the ore must be reduced physically,

chemically, or electrolytically.

Extractive metallurgists are interested in three primary streams: feed,

concentrate (valuable metal oxide/sulfide), and tailings (waste). After mining,

large pieces of the ore feed are broken through crushing and/or grinding in order

to obtain particles small enough where each particle is either mostly valuable or

mostly waste. Concentrating the particles of value in a form supporting

separation enables the desired metal to be removed from waste products.

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ALLOYS:

Common engineering metals include aluminium, chromium, copper, iron,

magnesium, nickel, titanium and zinc. These are most often used as alloys.

Much effort has been placed on understanding the iron-carbon alloy system,

which includes steels and cast irons. Plain carbon steels are used in low cost,

high strength applications where weight and corrosion are not a problem. Cast

irons, including ductile iron are also part of the iron-carbon system.

Stainless steel or galvanized steel are used where resistance to corrosion is

important. Aluminium alloys and magnesium alloys are used for applications

where strength and lightness are required.

Copper-nickel alloys (such as Monel) are used in highly corrosive environments

and for non-magnetic applications. Nickel-based superalloys like Inconel are

used in high temperature applications such as turbochargers, pressure vessel,

and heat exchangers. For extremely high temperatures, single crystal alloys are

used to minimize creep.

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PRODUCTION:

In production engineering, metallurgy is concerned with the production of

metallic components for use in consumer or engineering products. This involves

the production of alloys, the shaping, the heat treatment and the surface

treatment of the product. The task of the metallurgist is to achieve balance

between material properties such as cost, weight, strength, toughness, hardness,

corrosion and fatigue resistance, and performance in temperature extremes. To

achieve this goal, the operating environment must be carefully considered. In a

saltwater environment, ferrous metals and some aluminium alloys corrode

quickly. Metals exposed to cold or cryogenic conditions may endure a ductile to

brittle transition and lose their toughness, becoming more brittle and prone to

cracking. Metals under continual cyclic loading can suffer from metal fatigue.

Metals under constant stress at elevated temperatures can creep.

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MICROSTRUCTURE:

Metallurgists study the microscopic and macroscopic properties using

metallography, a technique invented by Henry Clifton Sorby. In metallography,

an alloy of interest is ground flat and polished to a mirror finish. The sample can

then be etched to reveal the microstructure and macrostructure of the metal. The

sample is then examined in an optical or electron microscope, and the image

contrast provides details on the composition, mechanical properties, and

processing history.

Crystallography, often using diffraction of x-rays or electrons, is another

valuable tool available to the modern metallurgist. Crystallography allows

identification of unknown materials and reveals the crystal structure of the

sample. Quantitative crystallography can be used to calculate the amount of

phases present as well as the degree of strain to which a sample has been

subjected.

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BOILER MATERIAL:

The material used for plates, rivets, braces, and all other parts on which the

structural strength of a high-grade boiler depends are made of a low-carbon

open-hearth steel in which is allowable only very small quantities of phosphorus

and sulphur. Nickel is often alloyed with this steel to improve its tensile and

elastic strength.

The elastic strength, rather than the tensile strength of the material, is of first

importance, as the permanent safety of the boiler depends upon all stresses

remaining within the elastic limit. A good margin between the elastic and the

final strength of the material provides a ductility or elongation which will many

times save actual and disastrous disruption under pressure by allowing the

material to bulge out or otherwise stretch greatly before it breaks.

Specifications for high-grade boiler plate require an elastic strength of about 1/2

the tensile strength, a tensile strength of about 70,000 pounds, and an

elongation, when pulled apart, of about 25% in a test bar 8 inches long. Rivets,

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bolts, and material for boiler braces are required to exceed slightly the

requirements specified for plates.

HEAT TREATMENT:

Heat treatment is a method used to alter the physical, and sometimes chemical

properties of a material. The most common application is metallurgical. Heat

treatments are also used in the manufacture of many other materials, such as

glass. Heat treatment involves the use of heating or chilling, normally to

extreme temperatures, to achieve a desired result such as hardening or softening

of a material. Heat treatment techniques include annealing, case hardening,

precipitation strengthening, tempering and quenching. It is noteworthy that

while the term heat treatment applies only to processes where the heating and

cooling are done for the specific purpose of altering properties intentionally,

heating and cooling often occur incidentally during other manufacturing

processes such as hot forming or welding.

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Metals can be heat treated to alter the properties of strength, ductility,

toughness, hardness or resistance to corrosion. Common heat treatment

processes include annealing, precipitation strengthening, quenching, and

tempering. The annealing process softens the metal by allowing recovery of

cold work and grain growth. Quenching can be used to harden alloy steels, or

in precipitation hardenable alloys, to trap dissolved solute atoms in solution.

Tempering will cause the dissolved alloying elements to precipitate, or in the

case of quenched steels, improve impact strength and ductile properties.

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BOILER WATER TREATMENT:

The treatment and conditioning of boiler feed water must satisfy three main

objectives:

Continuous heat exchange

Corrosion protection

Production of high quality steam

External treatment is the reduction or removal of impurities from water outside

the boiler. In general, external treatment is used when the amount of one or

more of the feed water impurities is too high to be tolerated by the boiler

system in question. There are many types of external treatment (softening,

evaporation, deaeration, membrane contractors etc.) which can be used to tailor

make feed-water for a particular system. Internal treatment is the conditioning

of impurities within the boiler system. The reactions occur either in the feed

lines or in the boiler proper. Internal treatment may be used alone or in

conjunction with external treatment. Its purpose is to properly react with feed

water hardness, condition sludge, scavenge oxygen and prevent boiler water

foaming.

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External treatment

The water treatment facilities purify and deaerate

make-up water or feed water. Water is sometimes

pretreated by evaporation to produce relatively pure

vapor, which is then condensed and used for boiler

feed purposes. Evaporators are of several different

types, the simplest being a tank of water through

which steam coils are passed to heat the water to

the boiling point. Sometimes to increase the

efficiency the vapor from the first tank is passed

through coils in a second tank of water to produce

additional heating and evaporation. Evaporators are

suitable where steam as a source of heat is readily

available. They have particular advantages over

demineralization, for example, when the dissolved

solids in the raw water are very high.

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Certain natural and synthetic materials have the ability to remove mineral ions

from water in exchange for others. For example, in passing water through a

simple cation exchange softener all of calcium and magnesium ions are

removed and replaced with sodium ions. Since simple cation exchange does not

reduce the total solids of the water supply, it is sometimes used in conjunction

with precipitation type softening. One of the most common and efficient

combination treatments is the hot lime-zeolite process. This system of treatment

accomplishes several functions: softening, alkalinity and silica reduction, some

oxygen reduction, and removal of suspended matter and turbidity.

Internal treatment

Internal treatment can constitute the unique treatment when boilers operate at

low or moderate pressure, when large amounts of condensed steam are used for

feed water, or when good quality raw water is available. The purpose of an

internal treatment is to

1) react with any feed-water hardness and prevent it from precipitating on the

boiler metal as scale;

2) condition any suspended matter such as hardness sludge or iron oxide in the

boiler and make it non-adherent to the boiler metal;

3) provide anti-foam protection to allow a reasonable concentration of dissolved

and suspended solids in the boiler water without foam carry-over;

4) eliminate oxygen from the water and provide enough alkalinity to prevent

boiler corrosion.

In addition, as supplementary measures an internal treatment should prevent

corrosion and scaling of the feed-water system and protect against corrosion in

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the steam condensate systems.

 

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