AProject 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 EngineeringJSW Energy Centre of Excellence

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

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.

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 - 650C. 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 tomechanical properties. Characterisation of steam oxidation behaviour of new alloys, and modelling of spallation.

CONTENTS

Chapter Page

TITLE.......................................................................................................... ICERTIFICATE ...................................................................................................IIABSTRACT....................................................................................................... IIIOBJECTIVE............ ..................................................................................IVTABLE OF CONTENTS ...................................................................................VWHAT IS A BOILER........................................................................................1MATERIALS....................................................................................................2FUEL....................................................................................................3CONFIGURATIONS.......................................................................4SUPERHEATED STEAM BOILERS..................................................................5SUPERCRITICAL STEAM GENERATORS......................................................7HYDRONIC BOILERS........................................................................................8BOILER FEED WATER....................................................................................10METALLURGY...............................................................................................11HISTORY...........................................................................................................13EXTRACTION...................................................................................................14ALLOYS.............................................................................................................15PRODUCTION...................................................................................................16MICROSTRUCTURE17BOILER MATERIAL .18HEAT TREATMENT..20BOILER WATER TREATMENT..21

III

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.

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.

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.

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. 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.

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.

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 3,200psi/22.06MPa or 220.6bar) 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.

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.

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. 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.

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.

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, 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.

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

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.

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.

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.

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.

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, 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.

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.

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.

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.

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 treatmentInternal 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 to1) 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 the steam condensate systems.