Q.9 How do stainless steel become stainless. (AKTU - 2008-09)
Ans. Stainless Steel: -
Stainless steels have sufficient amounts of chromium present so that a passive film of chromium oxide forms which prevents further surface corrosion and blocks corrosion from spreading into the metal’s internal structure. Due to this oxide film it is called stainless.
Q.10 Explain the properties & application of tool steel. (AKTU - 2009-10)
Ans. Tool Steel: -
Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). Tool steel is generally used in a heat-treated state.
With a carbon content between 0.7% and 1.4%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The manganese content is often kept low to minimize the possibility of cracking during water quenching. However, proper heat treating of these steels is important for adequate performance, and there are many suppliers who provide tooling blanks intended for oil quenching.
Tool steels are made to a number of grades for different applications. Choice of grade depends on, among other things, whether a keen cutting edge is necessary, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements. In general, the edge temperature under expected use is an important determinant of both composition and required heat treatment. The higher carbon grades are typically used for such applications as stamping dies, metal cutting tools, etc.
Tool steels are also used for special applications like injection molding because the resistance to abrasion is an important criterion for a mold that will be used to produce hundreds of thousands of parts.
Q.11 Write short note on Annealing and normalizing of carbon steel. (AKTU - 2009-10)
Related Questions -
Q. What do you understand by heat treatment of carbon steels ? Explain any process in detail. (AKTU - 2010-11)
Q. Explain full annealing and process annealing. (AKTU - 2012 - 13)
Q. Describe the process of steel hardening. Why steels are required to be tempered after it has been hardened? (AKTU - 2012 - 13)
Ans. Heat treatment: -
The heat treatment of carbon steel is necessary for changing the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. But the electrical and thermal conductivity are slightly altered.
The process of heat treatment involves heating of solid metals to specified temperatures holding them at that temperature and then cooling them at suitable rates in order to enable the metals to acquire the desired properties to the required extents. All these takes place because of the changes in size, form, nature and the distribution of different constituents in the micro-structures of these metals. All heat treatment processes, therefore, comprise the following three stages or components :
1. Heating the metal to a pre-decided temperature.
2. Holding it at that temperature for sufficient time so that the structure of the metal becomes uniform throughout.
3. Cooling the metal at a predetermined rate in a suitable media so as to force the metal to acquire a desired internal structure and thus, obtain the desired properties to the required extent.
Here is a list of the types of heat treatments possible:
Annealing: –
It is done to soften the steel. The process involves, heating the metal slowly above the critical temperature then holding at that temperature for about 1 hour and finally cooling it in furnace at about 300 to 150 C/hour.
Annealing serves the following purposes-
Ans. Stainless Steel: -
Stainless steels have sufficient amounts of chromium present so that a passive film of chromium oxide forms which prevents further surface corrosion and blocks corrosion from spreading into the metal’s internal structure. Due to this oxide film it is called stainless.
Q.10 Explain the properties & application of tool steel. (AKTU - 2009-10)
Ans. Tool Steel: -
Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). Tool steel is generally used in a heat-treated state.
With a carbon content between 0.7% and 1.4%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The manganese content is often kept low to minimize the possibility of cracking during water quenching. However, proper heat treating of these steels is important for adequate performance, and there are many suppliers who provide tooling blanks intended for oil quenching.
Tool steels are made to a number of grades for different applications. Choice of grade depends on, among other things, whether a keen cutting edge is necessary, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements. In general, the edge temperature under expected use is an important determinant of both composition and required heat treatment. The higher carbon grades are typically used for such applications as stamping dies, metal cutting tools, etc.
Tool steels are also used for special applications like injection molding because the resistance to abrasion is an important criterion for a mold that will be used to produce hundreds of thousands of parts.
Q.11 Write short note on Annealing and normalizing of carbon steel. (AKTU - 2009-10)
Related Questions -
Q. What do you understand by heat treatment of carbon steels ? Explain any process in detail. (AKTU - 2010-11)
Q. Explain full annealing and process annealing. (AKTU - 2012 - 13)
Q. Describe the process of steel hardening. Why steels are required to be tempered after it has been hardened? (AKTU - 2012 - 13)
Ans. Heat treatment: -
The heat treatment of carbon steel is necessary for changing the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. But the electrical and thermal conductivity are slightly altered.
The process of heat treatment involves heating of solid metals to specified temperatures holding them at that temperature and then cooling them at suitable rates in order to enable the metals to acquire the desired properties to the required extents. All these takes place because of the changes in size, form, nature and the distribution of different constituents in the micro-structures of these metals. All heat treatment processes, therefore, comprise the following three stages or components :
1. Heating the metal to a pre-decided temperature.
2. Holding it at that temperature for sufficient time so that the structure of the metal becomes uniform throughout.
3. Cooling the metal at a predetermined rate in a suitable media so as to force the metal to acquire a desired internal structure and thus, obtain the desired properties to the required extent.
Here is a list of the types of heat treatments possible:
Annealing: –
It is done to soften the steel. The process involves, heating the metal slowly above the critical temperature then holding at that temperature for about 1 hour and finally cooling it in furnace at about 300 to 150 C/hour.
Annealing serves the following purposes-
- Softens the metal for easy machinability
- Relieves internal stresses
- Refines and removes structural inhomogeneity
- Removes gases trapped during casting of metals
- Change physical and mechanical properties
- Prepares the steel for further treatment Produces the desired structure
The various types of annealing operations are given as follows -
Full Annealing: -
Carbon steel is heated to approximately 40 °C for 1 hour; this assures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the “bands” of pearlite are thick. Fully-annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming.
Process Annealing: -
A process annealing is used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. This process is very useful in mild steels and low carbon steels.
Isothermal Annealing: -
It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature.
Normalizing: -
Carbon steel is heated to approximately 55 °C for 1 hour; this assures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.
Hardening: -
It is the process of heating steel to a temperature with in the hardening range which is 300 to 500C above the higher critical point for steels and by the same amount above the lower critical point for hypereutectoid same steels holding at that temperature for sufficient time to allow it to attain austenitic structure and the cooling it rapidly by quenching in a suitable medium like water, oil or salt both. This process is widely applied to all cutting tools all machine parts made from alloy steels, dies and some selected machine part subjected to heavy duty work.
Tempering and quenching: -
A hardened steel piece due to martensitic structure is extremely hard and brittle due to which it is found unsuitable for most practice purposes as a subsequent treatment is require to obtain a desired degree of toughness at the cost of some strength and hardness to make it suitable for use. It is specially true in case of the tools. This is exactly what is mainly aimed at through tempering of steel. This process is enables transformation of some of martensite into ferrite and cementite. The exact amount of martensite transformed into fersite plus cementite will depend upon the temperature to which the metal is reheated and the time allowed for the transformation.
Case Hardening: - (AKTU - 2010 -11)
It is the process through which a hard wear resistant and shock resistant surface is produced on steel having a tough core inside. The main theory behind this processes is to heat the steel to red heat and then force the carbon content into its surface structure so that a certain depth all along its surface becomes rich in carbon. It is then hardened as usual. For the above reason only this process is also frequently known as carbusising or carbonizing.
The process of case hardening is carried out in two stages : (i) carburising i.e., adding carbon to the outer skin to make it rich in carbon content so that it can be hardened, (ii) hardening this skin and refining the structure of the core to make it tough.
There are three common methods of carburising : -
1. Pack carburising. 2. Liquid carburising. 3. Gas carburising.
PACK CARBURISING: -
We have seen in the previous articles that iron has a definite affinity for carbon when it is at high temperature, particularly above the critical range. So what we do in the first stage is that we enclose the metal in a cast iron or steel box, containing a material rich in carbon, such as small pieces of carcoal, wood, charred leather, tar, etc. arid then heat it to a temperature slightly above the critical range, usually between 900°C to 950°C, and allow it to remain at that temperature for sufficient time so that the carbon is absorbed to the required depth. Thus, the outer skin is converted into high carbon steel containing about 0.8 to 1.2 per cent carbon. The depth to which this carbon ensichment takes place depends upon the time allowed to the metal to remain at the elevated temperature, which may vary from 3 to 20 hours depending upon the size of the box carrying the articles for pack carburizing. The usual depth attained in 3 to 4 hours time varies from 0.8 to mm to 1.6 mm. The metal is then allowed to cool slowly within the box and then removed. The second stage consists of reheating the metal to about 900°C and quenching in oil so that its structure is refined, brittleness removed and the coin becomes soft and tough. The metal is then reheated to about 700°C and quenched in water, or oil, depending upon whether it is carbon steel or alloy steel, the outer casing, which had been rendered soft during the proceeling operation, is again hardened.
LIQUID CARBURISING: -
Liquid carburising, as the name suggests, is done in a liquid medium and no carbonaceous materials are required to be packed around the workpiece. For this process, liquid salt baths of cyanities, chlorides and carbonates (sometimes, carbides too) are prepared and the parts to be case carburised immersed in them, Heating of the bath is done by means of electrodes immersed in the bath. Stirring of the bath is necessary to ensure uniform temperature. If the entire component is to be carburised then no additional treatment is necessary, but if only some selected portions of the component are to be carburised the remaining portions are covered by copper plating.
Temperatures for liquid carburing vary from 843°C to 927°C. However, it is observed that a 2 hours heating of components with the bath temperature at 900°C a depth penetration of about 0.5 mm can be achieved. Deeper the penetration desired longer will be the heating time. After carburising is over the components are removed from the bath and quenched in water, oil or brine, depending upon the operational requirements,
Some major advantages of this process are uniform heating, very little deformation of article, ease of carburising a wider range of parts, greater depth of penetration possible, selective carburing (if needed) and a faster process.
GAS CARBURISING: -
It is a relatively more recent process in which the components are heated in an atmosphere of hydro-carbon rich gases like natural gas (methane), propane, butane, hydrogen, etc. mixed with carbon monoxide or carbon-di-oxide. The components are either suspended in the gas furnace from hooks or are heated in a horizontal rotary type gas carburiser. The mixture of the gases is continuously fed into the furnace retort and a controlled atmosphere maintained inside. There is a provision for exhausting the spent gases into atmosphere.
After the parts have been soaked in the carburising chamber they are directly quenched without being exposed to atmospheric air. This enables a very superior surface finish, which can not be attained by any other method. It is found that a 4 hour heating in a gas carburising temperature of about 927°C enables a depth penetration of about 0.5 mm to 0.75 mm.
The main advantages of this process are a superior surface finish on the component controlled heating, accuracy of desired case depth, speedy process, requirement of less floor area and relatively more reliable results. Also, the process can be mechanised, if desired.
Choose Correct Answers
Full Annealing: -
Carbon steel is heated to approximately 40 °C for 1 hour; this assures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the “bands” of pearlite are thick. Fully-annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming.
Process Annealing: -
A process annealing is used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. This process is very useful in mild steels and low carbon steels.
Isothermal Annealing: -
It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature.
Normalizing: -
Carbon steel is heated to approximately 55 °C for 1 hour; this assures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.
Hardening: -
It is the process of heating steel to a temperature with in the hardening range which is 300 to 500C above the higher critical point for steels and by the same amount above the lower critical point for hypereutectoid same steels holding at that temperature for sufficient time to allow it to attain austenitic structure and the cooling it rapidly by quenching in a suitable medium like water, oil or salt both. This process is widely applied to all cutting tools all machine parts made from alloy steels, dies and some selected machine part subjected to heavy duty work.
Tempering and quenching: -
A hardened steel piece due to martensitic structure is extremely hard and brittle due to which it is found unsuitable for most practice purposes as a subsequent treatment is require to obtain a desired degree of toughness at the cost of some strength and hardness to make it suitable for use. It is specially true in case of the tools. This is exactly what is mainly aimed at through tempering of steel. This process is enables transformation of some of martensite into ferrite and cementite. The exact amount of martensite transformed into fersite plus cementite will depend upon the temperature to which the metal is reheated and the time allowed for the transformation.
Case Hardening: - (AKTU - 2010 -11)
It is the process through which a hard wear resistant and shock resistant surface is produced on steel having a tough core inside. The main theory behind this processes is to heat the steel to red heat and then force the carbon content into its surface structure so that a certain depth all along its surface becomes rich in carbon. It is then hardened as usual. For the above reason only this process is also frequently known as carbusising or carbonizing.
The process of case hardening is carried out in two stages : (i) carburising i.e., adding carbon to the outer skin to make it rich in carbon content so that it can be hardened, (ii) hardening this skin and refining the structure of the core to make it tough.
There are three common methods of carburising : -
1. Pack carburising. 2. Liquid carburising. 3. Gas carburising.
PACK CARBURISING: -
We have seen in the previous articles that iron has a definite affinity for carbon when it is at high temperature, particularly above the critical range. So what we do in the first stage is that we enclose the metal in a cast iron or steel box, containing a material rich in carbon, such as small pieces of carcoal, wood, charred leather, tar, etc. arid then heat it to a temperature slightly above the critical range, usually between 900°C to 950°C, and allow it to remain at that temperature for sufficient time so that the carbon is absorbed to the required depth. Thus, the outer skin is converted into high carbon steel containing about 0.8 to 1.2 per cent carbon. The depth to which this carbon ensichment takes place depends upon the time allowed to the metal to remain at the elevated temperature, which may vary from 3 to 20 hours depending upon the size of the box carrying the articles for pack carburizing. The usual depth attained in 3 to 4 hours time varies from 0.8 to mm to 1.6 mm. The metal is then allowed to cool slowly within the box and then removed. The second stage consists of reheating the metal to about 900°C and quenching in oil so that its structure is refined, brittleness removed and the coin becomes soft and tough. The metal is then reheated to about 700°C and quenched in water, or oil, depending upon whether it is carbon steel or alloy steel, the outer casing, which had been rendered soft during the proceeling operation, is again hardened.
LIQUID CARBURISING: -
Liquid carburising, as the name suggests, is done in a liquid medium and no carbonaceous materials are required to be packed around the workpiece. For this process, liquid salt baths of cyanities, chlorides and carbonates (sometimes, carbides too) are prepared and the parts to be case carburised immersed in them, Heating of the bath is done by means of electrodes immersed in the bath. Stirring of the bath is necessary to ensure uniform temperature. If the entire component is to be carburised then no additional treatment is necessary, but if only some selected portions of the component are to be carburised the remaining portions are covered by copper plating.
Temperatures for liquid carburing vary from 843°C to 927°C. However, it is observed that a 2 hours heating of components with the bath temperature at 900°C a depth penetration of about 0.5 mm can be achieved. Deeper the penetration desired longer will be the heating time. After carburising is over the components are removed from the bath and quenched in water, oil or brine, depending upon the operational requirements,
Some major advantages of this process are uniform heating, very little deformation of article, ease of carburising a wider range of parts, greater depth of penetration possible, selective carburing (if needed) and a faster process.
GAS CARBURISING: -
It is a relatively more recent process in which the components are heated in an atmosphere of hydro-carbon rich gases like natural gas (methane), propane, butane, hydrogen, etc. mixed with carbon monoxide or carbon-di-oxide. The components are either suspended in the gas furnace from hooks or are heated in a horizontal rotary type gas carburiser. The mixture of the gases is continuously fed into the furnace retort and a controlled atmosphere maintained inside. There is a provision for exhausting the spent gases into atmosphere.
After the parts have been soaked in the carburising chamber they are directly quenched without being exposed to atmospheric air. This enables a very superior surface finish, which can not be attained by any other method. It is found that a 4 hour heating in a gas carburising temperature of about 927°C enables a depth penetration of about 0.5 mm to 0.75 mm.
The main advantages of this process are a superior surface finish on the component controlled heating, accuracy of desired case depth, speedy process, requirement of less floor area and relatively more reliable results. Also, the process can be mechanised, if desired.
Choose Correct Answers
1. The % of carbon in medium carbon steel varies (AKTU - 2008-09)
(a) Less than 0.20 (b) 0.10 to 0.30
(c) 0.30 to 0.60 (d) More than 0.60.
2. Stainless steel comes under the categorty of (AKTU - 2008-09)
(a) Non ferrous metal and alloy (b) Ferrous metal and alloy
(c) Polymer (d) Composite.
3. Heat treatment in carbon steel is necessary for changing which of the following property
(a) physical (b) mechanical (c) electrical (d) none of the above
4. Wrought iron is an iron alloy with a very ....... carbon content, in comparison to steel.
(a) low (b) high (c) zero (d) none of the above
5. Grey cast iron has
(a) carbon in the form of free graphite (b) high tensile strength
(c) low compressive strength (d) none of the above
6. The heat treatment process used for casting is
(a) quenching (b) normalizing
(c) tempering (d) carburising
7. Quenching is not necessary when hardening is done by
(a) case hardening (b) flame hardening
(c) nitriding (d) any one of these
8. The small quantity of sulpher in low carbon steel can improve
(a) formability (b) machinability
(c) hardenability (d) weldability
9. The connecting rod is generally made of
(a) high speed steel (b) high carbon steel
(c) medium carbon steel (d) low carbon steel
(a) case hardening (b) flame hardening
(c) nitriding (d) any one of these
8. The small quantity of sulpher in low carbon steel can improve
(a) formability (b) machinability
(c) hardenability (d) weldability
9. The connecting rod is generally made of
(a) high speed steel (b) high carbon steel
(c) medium carbon steel (d) low carbon steel
10. The material generally used for cutting tool is
(a) high speed steel (b) chrome steel
(c) nickel-chrome steel (d) silicon steel
(a) high speed steel (b) chrome steel
(c) nickel-chrome steel (d) silicon steel
Answer -
1. (c) 2. (b) 3. (b) 4. (a) 5. (a) 6. (c) 7. (c) 8. (b)
9. (a) 10. (a)
1. (c) 2. (b) 3. (b) 4. (a) 5. (a) 6. (c) 7. (c) 8. (b)
9. (a) 10. (a)
Fill in the blanks -
1. Cast iron fails showing ............... facture. (AKTU - 2010 -11)
Ans. ductile
Ans. ductile
2. In galvanizing process the surface is coated with ............... (AKTU - 2010 -11)
Ans. zinc.
Ans. zinc.