high speed steel, heat resisting steel, silicon steel, spring steel and stainless steel etc.

 

Give the composition, properties and uses of the following alloy steels: high speed steel, heat resisting steel, silicon steel, spring steel and stainless steel etc.

A. High Speed Steel:

High Speed Steel:

1. High speed steels are widely used for cutting of metals where hardness must be retained at elevated temperatures.
2. These steels are obtained by alloying 18 % tungsten, 4 % chromium and 1% vanadium with a carbon content of 0.6 to 0.7 %. This alloy is termed as 18:4:1 while an increase of vanadium to 2 % produces 18: 4:2 steels. 
3. In addition to heat resistance high speed steels have the desirable properties of high hardness, high compressive strength and outstanding wear resistance.

Uses: This steel is used for high-speed cutting tools.

B. Heat Resisting Steel :

Heat Resisting Steel :

1. Steels which must be resistance to creep at high temperatures must contain molybdenum. Silicon and chromium impart resistance to oxidation and scaling.
2. Steels which are satisfactory upto about 700°C operating temperature are C 0.15 %, Si = 0.5 to 20 %, Mn = 0.5 % maximum, Ni = none, Cr= 1.0 to 6%, Mo = 0.5%

Uses: These are used in valves of internal combustion engines in rolled or in forged condition. For higher temperature upto 1000 °C, steels containing upto 22 % nickel and 26% chromium are used.

C. Silicon Steel:

Silicon Steel:

1. Improves the electrical properties of steel.
2. Silicon imparts fatigue strength and resistance to steel.
3. Steel containing silicon is more ductile than plain carbon steel.
4. Steel containing 3 to 5% silicon has very low magnetic hysteresis.

Uses:

• i. Steel with Mn = 1%, Si = 2%, C = 0.4 to 0.6 % has very high elastic limit and is used for springs.
• ii. With Cr 5 to 7 %, Si = 2 to 4 %, C = 0.4 to 0.5 % steel retain its hardness and resistance to oxidation even at red heat. Such steels are used for internal combustion engines.
• iii. 13% silicon content steel has very high corrosion resistance so it is used in chemical industries.

D. Stainless Steel :

Stainless Steel 

a. Plain Chromium and High Chromium Low Nickel Steel:

1. Out of this group the former has C = 0.8%, Cr = 12 to 20 % and latter has C=0.1 to 0.2 %, Cr= 12 to 20 % and Ni = 2%.
2. These steels can be heat-treated.

Uses: For dies, valves and cutlery.

b. Chromium-Nickel Steel:

1. These steels are non-magnetic and cannot be hardened.
2. They have varieties due to varying contents of chromium and nickel respectively e.g., small quantity of copper, tungsten and molybdenum is also added to these steels.
3. They have high resistance to corrosion and may be cold or hot worked, pressed welded, brazed or soldered.
4. These steels are poor conductors of heat and electricity. 

Uses: It is used in making utensils.

 Question : With the help of neat sketch explain different types of  crystal structure.

Answer: 

A. Crystal Structure : 

 Crystal Structure

1. The crystal structure is formed by associating every lattice point with an assembly of atoms or molecules or ions, which are identical in composition, arrangement and orientation.
2. Crystals have the three types of unit cells which are mostly being used: 

a .Body Centred Cubic Structure (BCC):

Body Centred Cubic Structure (BCC)

1. BCC structure has atoms at its each corner and one atom in its center. So, the BCC consists of net total of two atoms.   Total atoms in BCC = (8/1)*8+1 =2 atoms
2. The coordination number of BCC arrangement is 8 and packing factor is 0.68.
3. The BCC structure can be generally seen in Lithium, Potassium, Sodium etc.

b. Face Centered Cubic Structure (FCC):

1. It consists of atoms at its each corner and one atom at center of each face. 

Classify the crystal imperfections and give their brief introduction.

Que . Classify the crystal imperfections and give their brief introduction.

Answer : 

All defects and imperfections in crystals can be classified in following types:

a.       Point Imperfections:

Point Imperfections:

1. 1 . Point defects are where an atom is missing or is in an irregular place in the lattice structure.
2. 2 . These defects are completely local in effect.
3. 3. Point imperfections are always present in crystals and their presence results in a decrease in the free energy.
4. 4. The number of defects at equilibrium concentration at a certain temperature can be computed as,

Where     N=Ne-EdikT

                                         n = Number of imperfections,

                                                     N = Number of atomic sites per mole,

                                   k = Boltzmann's constant,

                                                                             Ed = The free energy required to form the defect, and

                                   T  = Absolute temperature.

b.      Line Imperfection or Dislocations:

Line Imperfection or Dislocations

1. 1.   A linear disturbance of the atomic arrangement, which can very easily occur on the slip plane       through the crystal, is known as dislocation.
2. 2.   Dislocation is a two-dimensional line defect and is a very important crystal imperfection. It is        responsible for the deformation of metals by
3.       slip phenomenon.
4. 3.   It may also be concluded that it is region of localized lattice disturbances separating the                  slipped and unslipped regions of a crystal.
5. 4.   These are formed in the process of solidification of metals and mainly in their plastic                    deformation of strain hardening, yield point, creep and fatigue and brittle fractures.
6. 5. Causes of dislocation are:

•     a. Thermal stresses or external stresses causing plastic flow.
•     b. Crystal growth.
•     c . Phase transformation.
• d. Segregation of solute atoms causing mismatches.

1. 6. There are two types of dislocation:
2.      a. Edge dislocations, and
3.      b. Screw dislocations.
4. 
   
5. 7.    These dislocations are responsible for the useful property of ductility in metals, ceramic and        polymers.

C. Surface and Grain Boundary Imperfections:

Surface and Grain Boundary Imperfections:

1. 1.   These defects are two-dimensional and are due to a change in the stacking of atomic planes            on  or across a boundary, twin boundary, stacking faults,
2.       etc.
3. 2.   The external surface of the material is an imperfection itself because the atomic bonds do not        extend beyond it.
4. 3.  Since these surface atoms are not entirely surrounded by other , they possess higher energy           than internal atoms.
5. 4.  Surface atoms have neighbours on only one side while atoms inside the crystal have neighbour on both sides.

Explain the properties and use of types of stainless steels commonly used in industries. Explain how the "stainless" properties are obtained in these materials.

 Que 1.35. Explain the properties and use of types of stainless steels commonly used in industries. Explain how the "stainless" properties are obtained in these materials. 

Define refractoriness , and type of refractories

 A.  Refractoriness :

1. It is defined as a quality of materials to withstand at high temperatures without decomposing or melting . 
2. The materials which have this property are called refractory materials 
3. Bricks are the most common refractory material .
4. Refractory ceramics are the mostly used ceramics in the industry .
5. Refractory materials have important properties which are :

• 1. To withstand at high temperatures without melting .
• 2. To remain unreactive and inert when exposed to harsh environment .
• 3. To provide thermal insulation . 

B. Classification of Refractories : 

1. On the basis of their compositions refractories can be classified as :

a.   Fire Clay Refractories :

• Fire clay refractories are the mixture of alumina and silica having 25 to 45 % alumina and 70 to 50 % silica and remaining part consists of other oxides of Fe , Ca and Ti .
•  It can withstand at high temperature upto 1860 K. As alumina content increases , it will increase the maximum working temperature with formation of small amount of liquid .

 Application : Fire clay refractories are used to cover the high temperature region as it is used for making the wall of furnaces .

b . Silica Refractories : 

• It is also known as acid refractories . These materials show very good load - bearing capacity at high temperatures . 
• It consists of very small amount of alumina about 0.2 % .
• Addition of small amount of alumina results into decrease in its melting point.
• It can withstand at high temperature upto 1650 ° C . Silica refractories show good resistant towards slag formation . 
• These refractories are used to make arched roofs of steel and glass - making furnace . 

2. On the basis of their chemical behaviour the refractories may be classified as : 

a. acidic refractories : They are reactive towards bases and having silica as main constituent in it

Example : silica, quartz and sand etc. 

Explain Normalization or Hardening

 A.  Normalization 

1. This process consists of heating steel to a point 40 to 50 ° C above its upper critical temperature , holding at that temperature for a short duration and subsequently cooling in still air at room temperature . This is also known as air quenching .
2. This process is suggested for manufacturing operations like hot rolling and forging which are carried out on steels in the austenite range .
3. It is also useful for eliminating coarse - grained structure in castings , removing internal stresses that may have been caused by hot or cold working and improving the mechanical properties of the steel by eliminating the carbide network at the grain boundaries of the steels .
4. Normalizing produces microstructures consisting of ferrite and pearlite for hypoeutectoid steels and pearlite and cementite for hypereutectoid steels .

Explain the FBC (Fluidized Bed combustion) and neat diagram

• Principal of FBC :

1. A fluidized bed may be defined as the bed of solid particles behaving as a fluid.
2.  When a gas is passed through a packet bed of finely divided solid particles, it experiences a pressure drop across the bed.
3. At low gas velocities, this pressure drop is small and does not disturb the particles. But if the gas velocity is increased further, a stage is reached, when particles are suspended in the gas stream and the packet bed becomes a 'fluidized bed'.
4. With further increase in gas velocity, the bed becomes turbulent and rapid mixing of particles occurs.
5. In general, the behavior of this mixture of solid particles and gas is like a fluid. Burning of a fuel in such a state is known as fluidized bed combustion (FBC).

• Working :

1. Fig. given below shows the arrangement of the FBC system.
2. The fuel and inert material dolomite are fed on the distributor plate and from its bottom air is supplied.
3. The high velocity of air keeps the solid feed material in suspending condition during burning.
4. The generated heat is rapidly transferred to the water passing through the tubes immersed in the bed and generated steam is taken out.
5.  During the burning sulphur dioxide formed is absorbed by the dolomite and prevents its escape with the exhaust gases. The molten slag is tapped from the top surface of the bed.
6. The primary object of using the inert material is to control the bed temperature, it accounts for 90% of the bed volume.

• Advantages :

1. As a result of better heat transfer, the unit size and hence the capital costs are reduced.
2. Since combustion temperatures are low, the fouling and corrosion of tubes is reduced considerably.
3. Pollution is controlled and combustion of high-sulphur coal is possible.

• Disadvantages :

1. The major drawback is that the air has to be supplied at a high pressure so as to support the bed, the fan power is increased sufficiently so controls are difficult and plant life is low. 
2. 
   Fluidized bed systems are generally smaller for a given thermal output than conventional equipment.

what do you mean by "supercritical boiler" and "Super charged boiler" ?

A.  Supercritical Boiler :

1. The steam generator in which steam is produced above critical pressure of 221.2 Bar are know as super critical boiler.
2. It operates on Rankine cycle and drum less boiler. 
3. Usually , a sub-critical boiler consist of three distinct section an preheat (economiser), evaporator and super heater.






4. The condensate water from the condenser is compressed from the condenser pressure to super critical pressure in the feed pump. The operation theoretically is shown by process 1-2.
5.  this water is heated at super critical pressure shown by process 2-3. It may be observed that water is in sub-cooled state at point 2. Its temperature goes on increasing with heat addition.
6. No distinction is observed between the liquid and gaseous state.
7. The liquid water become gas after its temperature is rise above critical temperature of 374.15 degree Celcius.
8. thus it is a continuous tube which is heated along its length with water going in at one end super heated steam leaving at the other end.
9. Due to large frictional resistance the feed pump pressure is about 40% higher then boiler pressure.

B.  Supercharged boiler :

1. In a super charged boiler , the combustion is carried out under pressure in the combustion chamber by supplying the compressed air. 
2. The exhaust gases from the combustion chamber are used to run the gas turbine as they are exhaust to higher pressure.
3. The gas turbine runs the air compressor to supply the compressed air to the combustion chamber.
4. Followings are the advantage of the supercharged boiler : 

a. Owing the very high overall heat transfer coefficient the heat transfer surface required to hardly 20-25% of the heat transfer surface of a conventional boiler. 

b. The part of the gas turbine output can be used to drive other auxiliaries.

c. The small heat capacity storage of the boiler gives better response to control.

d. Rapid start of the boiler is possible.

e. The comparatively less number of operators is required.
 

 

Working of Bobcock and wilcox Boiler

 working principal

D = Drum                                  PG = pressure gauge 

     DTH = Down take header              ST =  super heater tube 

       WT = water tube                          SV =  Safety valve 

       BP  = Baffle plates                    MSV =  Main stop valve

          D = Doors                                APP =  Anti priming Pipe

          G = Grate                                      L = Lower junction box

       FD = Fire Door                               U = Upper junction box

      MC = Mud collector                     FV = Feed valve 

     WLI = Water level indicator 

1.  Fig.. shown the Bobcock and Wilcox  water tube boiler. It concist of a large number of parallel tubes inclined at angle which varies from (5 to 15 degree) to the horizontal.
2.  These tubes connect the up take header with the down take header.
3.  Both of these are connected to the shell having a substantial quantity of water in it.
4.  The up take header is connected to the shell through a short tube.
5.  A long tube is employed is connected the down take header with shell.
6.  The coal is fed through the fir hole on to the chain grate stoker.
7.  The velocity of the chain is so adjusted as to ensure complete combustion of coal by the time it reaches the other end of the great.
8.  The flue gasses first rise up then move down and once again rises up due to presence of baffles.
9.  The hot water and steam moisture rise up through the up take header into the boiler shell.
10.  The steam separates from water in the shell and collects in the steam space
11.  The cold water flows down into the tubes through the down take header.
12.  Thus a continuous circulation of water is maintained by the connection current set up.

 

 

Layout of a steam power plant and explain it

The layout of steam power plant comprises of the following four circuits: 

• a. Coal and Ash circuit: 

1. Coal arrives at storage yard and after necessary handling, passes on to the furnaces through the fuel feeding device.
2. Ash resulting from combustion of coal collect at back of the boiler and is and is removed to the ash storage yard through ash handling equipment.

• b. Air and gas circuit:

1. Air is taken from the atmosphere through the action of a forced or induced format fan and passes on the furnace through  the air preheat, where it has been heated by heat of flu gases which pass to the chimney via the preheat .

• c. Feed water and Flow circuit:

1. In the water and steam circuit condensate leaving the condenser is first heated in a closed feed water  heater through extracted steam from the lowest pressure extraction point of turbine.
2. It then passes through the and a few more water heaters before going into the boiler through economizer

• d. Cooling water Circuit:

1. The cooling water supply to the condenser helps in maintaining a low pressure in it.
2. The water may be taken from a natural source such as river, Lake or Sea or the same water may be cooled and circulated over again.

Renewable and non-renewable energy resources

A.   Renewable Energy Sources :  These are the source of energy which are inexhaustible i.e., can be used the produce energy again  and again. Example : Sun. Water. Animal dunk. etc.

a. Solar Energy : Solar energy is a clean, cheap and abundantly available renewable energy and it also that most important of the non-conventional source of energy because it is non-polluting and, therefore help in decreasing the green house effect.
b. Biogas : 

1. Biogas is formed due to the decomposition of organic waste matter.
2. During the decomposition of organic matter the gases, such as carbon dioxide, hydrogen and hydrogen sulphide are formed.
3. the organic waste is generally animal dunk, plant waste etc.

c.  Hydro Energy :  

1. it is renewable energy source, which is used to generate electricity.
2. hydro power is obtained from water flow falling water from a height.
3. Water store behind dam and at height has lot of potential energy which is converted into mechanical and electrical energy.

d. Wind Energy : Wind energy is a renewable energy of non-polluting energy and it has tremendous potential which if harnessed, can easily satisfy the energy demand of a country.

e. Tidal Energy : 

1. Gravitational pull by sun and moon result in the tides.
2. This type of energy can be harnessed by constructing the tidal barrage.
3. energy can be harnessed from high as well as from low tides.

B. Non-renewable energy resources :  These are the sources of energy which are exhaustible i.e., cannot be replaced if once they are used.

Example: coal,petroleum products, natural gas etc.

a. coal energy : 

1. coal is a conventional energy sources 
2. it is prepared by carbonisation of coal.
3. it is composition of mainly carbon and hydrocarbons.

b. Natural Gas : 

1. it is a one of the fossil fuels.
2. Natural gas is the cleanest fossil fuel.
3. it is mainly composed of methane with small amount of propane and ethane.