Metallurgy 1 (Heat Treatment Process)

 Heat treatment may be defined as operation or combination of operations involving heating and cooling of metal / alloy in solid steels to obtain desired conditions ( Relieve stress ) and properties ( like Machinability, ductility etc. )

Purpose of Heat Treatment:

1. To relieve stress created during cold working, welding, casting etc.
2. Improve Machinability.
3. Change grain size.
4. Improve ductility and
5. Homogenous structure.

Cooling rates also plays a important role. Slow cooling produces pearlitic structure and rapid cooling produces a Martensitic ( hard ) structure.

Different heat treatment process:

Annealing:

Metals are annealed to relieve internal stresses, soften them,make them more ductile, and refine their grain structures.Metal is annealed by heating it to a prescribed temperature,
holding it at that temperature for the required time, and then cooling it back to room temperature. The rate at which metal is cooled from the annealing temperature varies greatly. Steel must be cooled very slowly to produce maximum softness,This can be done by burying the hot part in sand, ashes, or some other substance that does not conduct heat readily (packing), or by shutting off the furnace and allowing the furnace and part to cool together (furnace cooling).
 Normalizing:
 

Ferrous metals are normalized to relieve the internal stresses produced by machining, forging, or welding. Normalized steels are harder and stronger than annealed steels. Steel is much tougher in the normalized condition than in any other condition. Parts that will be subjected to impact and parts that require maximum toughness and resistance to external stresses are usually normalized. Normalizing prior to hardening is beneficial in obtaining the desired hardness, provided the hardening operation is performed correctly. Low carbon steels do not usually require normalizing, but no harmful effects result if these steels are normalized. Normalizing is achieved by heating the metal to a specified temperature (which is higher than either the hardening or annealing temperatures), soaking the metal until it is uniformly heated, and cooling it in still air.

Hardening:

A ferrous metal is normally hardened by heating the metal to the required temperature and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil, water, or brine. Most steels must be cooled rapidly to harden them. The hardening process increases the hardness and strength of metal, but also increases its brittleness.

Tempering:

Steel is usually harder than necessary and too brittle for practical use after being hardened. Severe internal stresses are set up during the rapid cooling of the metal. Steel is tempered after being hardened to relieve the internal stresses and reduce its brittleness. Tempering consists of heating the metal to a specified temperature and then permitting the metal to cool. The rate of cooling usually has no effect on the metal structure during tempering. Therefore, the metal is usually permitted to cool in still air. Temperatures used for tempering are normally much lower than the hardening temperatures. The higher the

tempering temperature used, the softer the metal becomes. High-speed steel is one of the few metals that becomes harder instead of softer after it is tempered.

Case Hardening:

Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and a tough core, such as gears, cams, cylinder sleeves, and so forth. The most common

case-hardening processes are carburizing and nitriding.During the case-hardening process, a low-carbon steel (either straight carbon steel or low-carbon alloy steel) is heated to a specific temperature in the presence of a material (solid,liquid, or gas) which decomposes and deposits more carbon into the surface of a steel. Then, when the part is cooled rapidly, the outer surface or case becomes hard, leaving the,inside of the piece soft but very tough.
For many applications, there is a need for a hard case and a soft tough core, which is shock resistant. No carbon can possess both these properties at the same time. Hence low carbon steel with desired core properties are chosen and Carbon / Nitrogen is added to the surface to provide a hardened case to a specified depth by using the following process

• Carburising.
• Nitriding,
• Cyaniding and
• Carbon Nitriding.

Also medium Carbon steel could be taken in normalized condition and case hardened by Induction and Flame Hardening.Carburizing:

This process is also called as cementation. Low carbon steel ( 0.2 % C ) is heated to 870 - 925 C in contact with gases or carbon for several hours. There are three types. They are Pack Carburizing, Gas Carburizing and Liquid Carburizing. This method is used for case hardening Gears, Camshafts and Bearing.

Nitriding:

It involves the addition of Nitrogen on certain types of steels and heating them and holding at a suitable temperature, in contact with ammonia or any other suitable medium. The steel should contain Aluminum or chromium to form hard nitrides.

In this the component to be case hardened is heat resistant container along with ammonia. It is then heated to a temperature of about 500^o C.

Cyaniding:

Both Carbon and nitrogen are introduced on the surface of steel by heating to a suitable temperature and holding the component in molten cyanide. Sodium cyanide is mostly used. This results in the formation of hardened Carbide - Nitride case. In this process Nitrogen provides hardening, but carbon responds to quenching process.

Carbo - Nitriding:

Both Carbon and Nitrogen are added to the surface of steel by using Gas atmosphere and not Molten Cyanide. The gaseous atmosphere contains the following

1. Carrier gas ( H₂, N₂ or CO )
2. Enriching gas ( Natural Gas )
3. Ammonia.

Flame Hardening:

The material is heated on the surface with flame. This is followed by quenching. Thus creating a hardened case and a soft core. Oxy acetylene flame is used and the steel should contain 0.3 % to 0.6 % of carbon.

Induction Hardening:

The material is heated in a alternative magnetic field followed by quenching.

 

 

Basic Terms of Mechanical Engineering

Torque or Turning Force:
It is the total amount of force which is required to create acceleration on moving substance.

Couple:Two forces those acts on equally,parallely & oppositely on two separate points of same material.

Moment:It is the amount of moving effect which is gained for action of turning force.

Stress:
It is the force that can prevent equal & opposite force. That means, it is the preventing force. If one force acts on outside of a material, then a reactive force automatically acts to protest that force. The amount of reactive force per unit area is called stress. e.g. Tensile Stress, Compressive Stress, Thermal Stress.

Strain:If a force acts on a substance, then in that case if the substance would deform. Then the amount of deformation per unit length of that substance is called strain.

Spring:It is one type of device which is being distorted under certain amount of load & also can also go to its original face after the removal of that load.
Its function:
To store energy.
To absorb energy.
To control motion of two elements.

Stiffness:
Load per unit deflection. The amount of load required to resist the deflection.

Specific Weight:Weight per unit volume of the fluid.

Specific Volume:
Volume per unit mass of the fluid.

Specific Gravity:It is the ratio of specific weight of required substance to specific weight of pure water at 4 degree centigrade temperature.

Specific heat:The amount of heat required to increase 1 unit temperature of 1 unit mass.

Viscosity:Dynamic Viscosity:
The amount of resistance of one layer of fluid over other layer of fluid.

Kinematic Viscosity:
It is the ratio of dynamic viscosity to density.

Buoyancy:When a body is immersed in a liquid, it is lifted up by a force equal to weight of liquid displaced by the body. The tendency of liquid to lift up an immersed body is buoyancy. The upward thrust of liquid to lift up the body is called buoyancy force.

Bernoulli's Equation:P/γ +V²/2g +Z = Constant
Where, P = pressure,V = velocity,Z = Datumn Head

Devices for fluid:
Venturimeter:
It measures discharge of fluid.
Notches :
It measures discharge of fluid.
Orifice meter:
It measures discharge of fluid.
Pitot tube :
It measures velocity of fluid.


Mach Number:
It is the ratio of the velocity of fluid to the velocity of sound.
M=1 ----------------- Sonic flow
M> (1-6) ----------- Super-Sonic flow
M>6 ---------------- Hyper-Sonic flow

Fluid discharge/Fluid flow:Quantity of fluid flowing per second.
(through a section of pipe/ through a section of channel)
Q=AV
where, V= velocity of fluid,A= cross-sectional area of pipe/channel
Note: 1m³ = 1000 L1 cusec = 1 ft³/sec1 ft = 0.3048 m

Hydraulic Machine:
Turbine,Pump,Compressor etc.

Draft tube:
It attaches with reaction turbine . Its function is to reduce energy loss from reaction turbine & it also reduce pressure at outlet which is must blow the atmospheric pressure.

Themodynamics Law:
Zeroth Law
First Law of Thermodynamic
Second Law of thermodynamic

Zeroth Law:
If two body are in thermal equilibrium with a third body then these two body are also in thermal equilibrium with each other.

First Law of Thermodynamics:
In a closed system, work deliver to the surrounding is directly proportonal to the heat taken from the surrounding.And also, In a closed system, work done on a system is directly proportonal to the heat deliver to the surrounding.

Second Law of Thermodynamics:
It is impossible to make a system or an engine which can change 100 percent input energy to 100 percent output.

Entropy:
It is a thermodynamic property.
ds = dq/T
where, ds = change of entropy, dq = change of heat, T = Temperature.
In adiabatic process, entropy can not change. Actually,lacking or mal-adroitness of tranfering energy of a system is entropy.

Calorific Value of fuel:
It us the total amount of heat obtained from burning 1 kg solid or liquid fuel.

Boiler/Steam
Generator:It is a clossed vessel which is made of steel. Its function is to transfer heat to water to generate steam.

Economiser:
It is a part of boiler. Its function is to heat feed water which is supplied to boiler.

Superheater:
It is a part of boiler. Its function is to increase temperature of steam into boiler.

Air-Preheater:
It is a part of boiler. Its funtion is to preheats the air to be supplied to furnace and it recover heat from exhaust gas.

Boler Draught:
It is an important term for boiler. It is the difference of pressure above and below the fire grate. This pressure difference have to maintain very carefully inside the bolier. It actually maintaind the rate of steam generation. This depends on rate of fuel burning. Inside the boiler rate of fuel burning is maintained with rate of entry fresh air. If proper amount of fresh air never entered into the boiler, then proper amount of fuel inside the boiler never be burnt. So, proper fresh air enters into the boiler only by maintaining boiler draught.

Nozzle:Nozzle is a duct of varying cros-sectional area. Actually, it is a passage of varying cross-sectional area. It converts steam's heat energy into mechanical energy. It is one type of pipe or tube that carrying liquid or gas.

Scavenging:
It is the process of removing burnt gas from combustion chamber of engine cylinder.

Supercharging:
Actually, power output of engine depends on what amount of air enter into the engine through intake manifold. Amount of entry aiy if increased, then must be engine speed will increased. Amount of air will be increased by increasing inlet air density. The process of increasing inlet air density is supercharging. The device which is used for supercharging is called supercharger.Superchargeris driven by a belt from engine crakshaft. It is installed in intake system.

Turbocharging:Turbocharging is similar to the supercharging. But in that case tubocharger is installed in exhaust system whereas supercharger is installed in intake system. Turbocharger is driven by force of exhaust gas. Generally, turbocharger is used for 2-stroke engine by utilizing exhaust energy of the engine, it recovers energy otherwise which would go waste.

Governeor:Its function id to regulate mean speed of engine when there are variation in the load. If load incrases on the engine, then engine's speed must decrease. In that case supply of working fluid have to increase. In the otherway, if load decrease on the engine, then engine' speed must increase. In that case supply of working fluid have to decrease.Governor automatcally, controls the supply of working fluid to the engine with varying load condition.

Flywheel:It is the one of the main parts of the I.C. engine. Its main function id to store energy in the time of working stroke or expansion stroke. And, it releasesenergy to the crankshaft in the time of suction stroke, compression stroke & exhaust stroke. Because, engine has only one power producing stroke.

Rating of fuel:
S.I. Engine:
Octane number. Octane number indicates ability of fuel to resist knock.

C.I. Engine:
Cetane Number. Cetane number indicates ability of ignition of diesel fuel. That means, how much fast ignites diesel fuel.

Stoichiometric ratio:It is the chemically correct air-fuel ratio by volume. By which theoratically sufficient oxygen will be gotten to burn all combustible elements in fuel completely.

Heat Transfer:It is a science which deals with energy transfer between material bodies as a result of temperature difference.There are three way to heat transfer such as-ConductionConvectionRadiation

Thermal Conductivity:
It is the quantity of heat flows between two parts of solid material by conduction. In this case following consideration will be important fact-
Time------ 1 sec
Area of that solid material-------- 1 m²
Thickness of that solid material------ 1m
Temperature difference between two parts of that material------ 1k

Heat Exchanger:It is one type of device which can transfer heat from one fluid to another fluid. Example- Radiator, intercooler, preheater, condenser, boiler etc.

Refrigeration:
It is the process of removing heat from a substance. Actually, extraction of heat from a body whose temperature is already below the temperature of its surroundings.

1 tonne of refrigeration:It is amount of refrigeration effect or cooling effect which is produced by uniform melting of 1 tonne ice in 24 hours from or at 0 degree centigrade or freezing 1 tonne water in 24 hours from or at 0 degree centigrade.

Humidification:It is the addition of moisture to the air without change dry bulb temperatur.

Dehumidification:
It is the removal of moisture from the air without change dry bulb temperature.

Gear Train:Meshing of two or more gear. It can transmit power from one shaft to another shaft.

Heat Treatment:Operation involving heating and cooling of a metal in solid state for obtaining desirable condition without being changed chemical composition.Its object-increase hardness of metal.increase quality of metal ( heat, corrosion,wear resistance quality )improve machinability.

Ferrous Metal:1. Cast Iron - (2-6.67)%C, Si, Mn, P, S
2. Steel - (0-2)%C
3. Wrought Iron - 99.5% Fe

Non-Ferrous Metal:
1. Brass - (Cu+Zn)
2. Bronze -
(Sn+Cu) ------ Tin Bronze
(Si+Cu) ------- Silicon Bronze
(Al+Cu) ------- Aluminium Bronze

Allowance:
It is the difference between basic dimension of mating parts. That means, minimum clearance between mating parts that can be allowed.

Tolerance:
It is the difference between upper limit of dimension. It is also the permissible variation above and below the basic size. That means maximum permissible variation in dimensions.

Clearance:It is the difference in size between mating parts. That means, in that case the outside dimension of the shaft is less than internal dimension of the hole.

Stiffness:
It is the ability to resist deformation.

Toughness:It is the property to resist fracture.

Fatigue:When a material is subjected to repeated stress below yield point stress, such type of failure is fatigue failure.

Nuclear Fission:It is a nuclear reaction by which one big nucleous divided into two or more nucleous.

Nuclear Fussion:It is also a nuclear reaction by which one big nucleous will produced by adding two small nucleous.

Welding:It is the process of joining two similar or dissimilar metal by fusion.

Arc Welding -
* need D.C current
* produced (6000-7000) Degree Centegrade Temperature
Gas Welding -
* Oxy - acetylene flame join metals
* Oxygen & acetylene gas works
* produced 3200 Degree Centegrade Temperature

Machine Tool:It is the power driven tool. It cut & form all kinds of metal parts.
Example - 1. Lathe2. Drill Press3. Shaper4. Planer5. Grinding6. Miling7. Broaching8. Boring

Cutting Tool:

Tool Materials for Cutting Tool:
1. High Carbon Steel
2. High Speed Steel (W+Cr+V)
3. Carbide (W Carbide+Ti Carbide+Co Carbide)

Indexing:
It is the method of dividing periphery of job into equal number of division. Actually, it is the process of dividing circular or other shape of workpiece into equal space, division or angle.

Jig:
It is one type of device which hold & locate workpiece and also guide & control cutting tool. It uses in drilling, reaming and tapping.

Fixture:
It is one type of device which hold and locate workpiece. It uses in miling, grinding, planning & turning.