Q.5 Write short note on wrought iron. (Imp.)
Related Questions -
Q. Explain the properties and application of wrongh iron. (AKTU - 2009-10)
Ans. Wrough Iron: -
Wrought iron is an iron alloy with a very low carbon content, in comparison to steel, and has fibrous inclusions, known as slag. This is what gives it a “grain” resembling wood, which is visible when it is etched or bent to the point of failure. wrought iron is a variety of iron, with additives that make it twistable with a low corrosion rate. Wrought iron is tough, malleable, ductile and easily welded. Historically, it was known as “commercially pure iron”, however it no longer qualifies because current standards for commercially pure iron require a carbon content of less than 0.008 wt%.
Before the development of effective methods of steelmaking and the availability of large quantities of steel, wrought iron was the most common form of malleable iron. A modest amount of wrought iron was used as a raw material for manufacturing of steel, which was mainly used to produce swords, cutlery and other blades. Demand for wrought iron reached its peak in the 1860s with the adaptation of ironclad warships and railways, but then declined as mild steel became more available.
Items traditionally produced from wrought iron include rivets, nails, chains, railway couplings, water and steam pipes, nuts, bolts, horseshoes, handrails, straps for timber roof trusses, and ornamental ironwork.
Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture and gates, are made of mild steel.
Wrought iron is so named because it is worked from a bloom of porous iron mixed with slag and other impurities. Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith or other smith. It was used in this narrower sense in British Customs records, such manufactured iron being subject to a higher rate of duty than what might be called “unwrought” iron.
Related Questions -
Q. Explain the properties and application of wrongh iron. (AKTU - 2009-10)
Ans. Wrough Iron: -
Wrought iron is an iron alloy with a very low carbon content, in comparison to steel, and has fibrous inclusions, known as slag. This is what gives it a “grain” resembling wood, which is visible when it is etched or bent to the point of failure. wrought iron is a variety of iron, with additives that make it twistable with a low corrosion rate. Wrought iron is tough, malleable, ductile and easily welded. Historically, it was known as “commercially pure iron”, however it no longer qualifies because current standards for commercially pure iron require a carbon content of less than 0.008 wt%.
Before the development of effective methods of steelmaking and the availability of large quantities of steel, wrought iron was the most common form of malleable iron. A modest amount of wrought iron was used as a raw material for manufacturing of steel, which was mainly used to produce swords, cutlery and other blades. Demand for wrought iron reached its peak in the 1860s with the adaptation of ironclad warships and railways, but then declined as mild steel became more available.
Items traditionally produced from wrought iron include rivets, nails, chains, railway couplings, water and steam pipes, nuts, bolts, horseshoes, handrails, straps for timber roof trusses, and ornamental ironwork.
Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture and gates, are made of mild steel.
Wrought iron is so named because it is worked from a bloom of porous iron mixed with slag and other impurities. Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith or other smith. It was used in this narrower sense in British Customs records, such manufactured iron being subject to a higher rate of duty than what might be called “unwrought” iron.
Q.6 What are various classifications of steel ? (AKTU - 2012 - 13)
Sol. The steel can be classified according to the following methods
(i) Classification of steel on the basis of carbon percentage.
(ii) Classification based on degree of deoxidation.
(iii) Classification based on alloy elements.
Q.7 What is effect of alloying elements on the property of steel.
Related Questions -
Q. Discuss the importance of any five alloying elements used in steel. (AKTU - 2012 - 13)
Ans. Effects of Alloying Elements: -
The various alloying element affect the properties of steels as follows:
Nickel: -
It improves toughness, tensile strength, ductility and corrosion resistance.
Chromium: -
It is added in varying proportions upto 18%. Below 1.5 % addition increases tensile strength and 12% addition imparts high corrosion resistance. In general, chromium addition improves hardenability and toughness simultaneously.
Cobalt: -
It improves hardness, toughness, tensile strength, thermal resistance and magnetic properties. It also acts as a grain refiner.
Manganese: -
In lower proportions, say from 1.0 to 1.5 percent, its addition increases strength and toughness. Higher proportions upto 5 percent impart hardness accompained by brittleness. Still higher proportions, say between 11 to 14 percent, provide very high degree of hardness.
Silicon: -
It acts as a ferrite strengthener and improves elastic limit. It improves magnetic permeability and decreases hysterisis losses. Higher percentage of silicon gives rise to corrosion resisting alloys.
Molybdenum: -
Its addition increases wear resistance, thermal resistance, hardness, ability to retain mechanical properties at elevated temperatures and helps to inhibit temper brittleness. When added with nickel, it also improves corrosion resistance.
Tungsten: -
It increases hardness, toughness, wear resistance, shock resistance, magnetic reluctance and ability to retain mechanical properties at elevated temperatures.
Vanadium: -
It improves tensile strength, elastic limit, ductility, shock resistance and also acts as a degaser when added to molten steel.
Boron: -
It increases hardenability and is, therefore, very useful when alloyed with low carbon steels.
Aluminium: -
It is basically used as a deoxidiser. It promotes the growth of fine grains helps in providing a high degree of hardness through nitriding by forming aluminium nitrides.
Titanium: -
It is a fairly good deoxidiser and promotes grain growth. Also, it readily forms titanium carbides but has no marked effect on the hardenability of the material.
Copper: -
It increases strength and improves resistance to corrosion. Its proportion normally varies from 0.2 percent to 0.5 percent.
Niobium: -
It improves ductility, decreases hardenability and substantially increases the impact strength. Also, it promotes fine grain growth. It is also known as ‘columbium’.
Q.8 Explain different type of steel?
Related Questions -
Q. Write short note on High Speed Steel. (AKTU - 2011 - 12, 15 - 16)
Ans. These steels from a very important group of alloy steels which have been developed to meet specific requirements in respect of properties under specific situations and special applications. The most common varieties of these steels are described below:
(i) Stainless Steels: -
They are also known as corrosion resistant steels. Their principal alloying element is chromium while some other elements like nickel, manganese, etc. can also be present in small amounts. Since substantial amount of chromium is present in them they can not be considered as low alloy steels. While it is seen that an addition of just 4 to 6 percent chromium to low carbon steels render them fairly good corrosion resistant for most of the common uses, but if they are required to be highly corrosion resistant with very superior appearance a very high percentage of chromium ( usually > 12%) is added. The chromium reacts with the oxygen to form a strong layer of chromium oxide on the surface of the metal which is responsible for offering the resistance to corrosion. Stainless steels carrying more than 12% chromium are known as true stainless steels. Classification of stainless steels is generally done on the basis of their structures as follows:
(a) Ferritic Stainless Steels: -
It is well known that chromium is an effective ferrite stabilizer. Its addition, therefore, widens the temperature range through which ferrite will be a stable structure. As such, with the addition of sufficient amount of chromium to a low carbon steel an alloy is produced which carries a stable ferritic structure at all temperatures below its solidification temperature. Such alloys are called ferritic stainless steels. This group of stinless steels carries chromium content in the ranges of 11 to 27 percent, usually without any other alloying element. Sometimes , of course, manganese (1 to 1.5%) and silicon (upto 1 %) are added. They possess bcc crystal structure and, therefore, their ductility and formability are poor. However, they possess good weldability. They can be made good heat resistant by the addition of about 3% silicon. They exhibit fairly good strength even at elevated temperatures can be not worked, but can not be hardened through heat treatment. These steels are widely used in dairy equipments, food processing plants, chemical industries, heat exchangers, various types of household utensils, cutlery, surgical instruments, neuclear plants, etc.
(b) Martensitic Stainless Steels: -
This group of stainless steels carries chromium between 12 to 18 percent but contains a higher percentage of carbon usually (0.15 to 1.2%). The carbon dissolves in austenite which, when quenced, provides a martensitic structure to the alloy. Hence, the name. Due to formation of chromium carbides the corrosion resistance of this alloy is decreased. Different amounts of carbon are used to vary the strengths of these alloys. They are costlier than ferritic stainless steels and can be hardened by heat treatment. Their main applications are in the manufacture of items like springs, bolts, nuts, screws, valves, cutlery, etc.
(c) Austenitic Stainless Steels: -
Indeed the most important, and at the same time costliest, is this group of stainless steels. The main idea behind the development of this alloy steel is to stabilize the austenite structure, for which nickel is added in sufficient quantity in addition to chromium. This provides a stable austenite structure at room temperature. Manganese and nitrogen are sometimes added to reduce the cost, but that result in slight deterioration in quality as well . This group of stainless steels may contain 0.03 to 0.25% carbon, 16 to 26 percent chromium, 3.5 to 22% nickel, 2% manganese, 1 to 2% silicon and in some cases small amounts of molybdenum, titanium, etc. A very widely used variety of this type of steel, called 18-8 stainless steel, carries 18% chromium and 8% nickel. It responds well to cold working and its strength and hardness can be increased through cold working. It can also be cold drawn into wires.
These steels are non-magnetic and highly corrosion resistant, However, they may be corroded in salt media and halide acids surroundings. They possess excellent formability and good weldability. They offer the best corrosion resistance out of all the three classes of stainless steels. They however, can not be hardened by heat treatment. Titanium or niobium is sometimes added to these alloys to stabilize carbon and molybdenum to improve corrosion resistance. These steels have wide applications where high corrosion resistance and attractive appearance are vital requirements.
(ii) Magnetic Steels: -
These steels are rich in cobalt and tungsten contents and carry varying percentages of other elements like carbon, chromium, nickel, etc. A typical magnetic steel composition shows 15 to 40% cobalt, upto 10% tungsten; 1.5 to 9% chromium and upto 1.0% carbon. These steels are mainly used to make permanent magnets for electrical measuring instruments, loud speakers, magnetos, etc.
(iii) Heat Resistant Steels: -
With alround developments in high-technologies in modern era a continued need has been to develop such metals which can resist the influence of such parameters that can lead to the failures of common metals at elevated temperatures. Such conditions commonly arise in the operations of nuclear power plants, structure and parts of high temperature furnaces, supersonic aircrafts, missiles, etc. The metals required for use in such equipments should have high corrosion resistance, good strength and good creep resistance at high temperatures. These requirements are satisfactorily met by heat resisting alloy steels, although non-ferrous alloys have also been developed which meet these requirements equally successfully.
Some typical compositions of such ferrous alloys are given below:
Sol. The steel can be classified according to the following methods
(i) Classification of steel on the basis of carbon percentage.
(ii) Classification based on degree of deoxidation.
(iii) Classification based on alloy elements.
Q.7 What is effect of alloying elements on the property of steel.
Related Questions -
Q. Discuss the importance of any five alloying elements used in steel. (AKTU - 2012 - 13)
Ans. Effects of Alloying Elements: -
The various alloying element affect the properties of steels as follows:
Nickel: -
It improves toughness, tensile strength, ductility and corrosion resistance.
Chromium: -
It is added in varying proportions upto 18%. Below 1.5 % addition increases tensile strength and 12% addition imparts high corrosion resistance. In general, chromium addition improves hardenability and toughness simultaneously.
Cobalt: -
It improves hardness, toughness, tensile strength, thermal resistance and magnetic properties. It also acts as a grain refiner.
Manganese: -
In lower proportions, say from 1.0 to 1.5 percent, its addition increases strength and toughness. Higher proportions upto 5 percent impart hardness accompained by brittleness. Still higher proportions, say between 11 to 14 percent, provide very high degree of hardness.
Silicon: -
It acts as a ferrite strengthener and improves elastic limit. It improves magnetic permeability and decreases hysterisis losses. Higher percentage of silicon gives rise to corrosion resisting alloys.
Molybdenum: -
Its addition increases wear resistance, thermal resistance, hardness, ability to retain mechanical properties at elevated temperatures and helps to inhibit temper brittleness. When added with nickel, it also improves corrosion resistance.
Tungsten: -
It increases hardness, toughness, wear resistance, shock resistance, magnetic reluctance and ability to retain mechanical properties at elevated temperatures.
Vanadium: -
It improves tensile strength, elastic limit, ductility, shock resistance and also acts as a degaser when added to molten steel.
Boron: -
It increases hardenability and is, therefore, very useful when alloyed with low carbon steels.
Aluminium: -
It is basically used as a deoxidiser. It promotes the growth of fine grains helps in providing a high degree of hardness through nitriding by forming aluminium nitrides.
Titanium: -
It is a fairly good deoxidiser and promotes grain growth. Also, it readily forms titanium carbides but has no marked effect on the hardenability of the material.
Copper: -
It increases strength and improves resistance to corrosion. Its proportion normally varies from 0.2 percent to 0.5 percent.
Niobium: -
It improves ductility, decreases hardenability and substantially increases the impact strength. Also, it promotes fine grain growth. It is also known as ‘columbium’.
Q.8 Explain different type of steel?
Related Questions -
Q. Write short note on High Speed Steel. (AKTU - 2011 - 12, 15 - 16)
Ans. These steels from a very important group of alloy steels which have been developed to meet specific requirements in respect of properties under specific situations and special applications. The most common varieties of these steels are described below:
(i) Stainless Steels: -
They are also known as corrosion resistant steels. Their principal alloying element is chromium while some other elements like nickel, manganese, etc. can also be present in small amounts. Since substantial amount of chromium is present in them they can not be considered as low alloy steels. While it is seen that an addition of just 4 to 6 percent chromium to low carbon steels render them fairly good corrosion resistant for most of the common uses, but if they are required to be highly corrosion resistant with very superior appearance a very high percentage of chromium ( usually > 12%) is added. The chromium reacts with the oxygen to form a strong layer of chromium oxide on the surface of the metal which is responsible for offering the resistance to corrosion. Stainless steels carrying more than 12% chromium are known as true stainless steels. Classification of stainless steels is generally done on the basis of their structures as follows:
(a) Ferritic Stainless Steels: -
It is well known that chromium is an effective ferrite stabilizer. Its addition, therefore, widens the temperature range through which ferrite will be a stable structure. As such, with the addition of sufficient amount of chromium to a low carbon steel an alloy is produced which carries a stable ferritic structure at all temperatures below its solidification temperature. Such alloys are called ferritic stainless steels. This group of stinless steels carries chromium content in the ranges of 11 to 27 percent, usually without any other alloying element. Sometimes , of course, manganese (1 to 1.5%) and silicon (upto 1 %) are added. They possess bcc crystal structure and, therefore, their ductility and formability are poor. However, they possess good weldability. They can be made good heat resistant by the addition of about 3% silicon. They exhibit fairly good strength even at elevated temperatures can be not worked, but can not be hardened through heat treatment. These steels are widely used in dairy equipments, food processing plants, chemical industries, heat exchangers, various types of household utensils, cutlery, surgical instruments, neuclear plants, etc.
(b) Martensitic Stainless Steels: -
This group of stainless steels carries chromium between 12 to 18 percent but contains a higher percentage of carbon usually (0.15 to 1.2%). The carbon dissolves in austenite which, when quenced, provides a martensitic structure to the alloy. Hence, the name. Due to formation of chromium carbides the corrosion resistance of this alloy is decreased. Different amounts of carbon are used to vary the strengths of these alloys. They are costlier than ferritic stainless steels and can be hardened by heat treatment. Their main applications are in the manufacture of items like springs, bolts, nuts, screws, valves, cutlery, etc.
(c) Austenitic Stainless Steels: -
Indeed the most important, and at the same time costliest, is this group of stainless steels. The main idea behind the development of this alloy steel is to stabilize the austenite structure, for which nickel is added in sufficient quantity in addition to chromium. This provides a stable austenite structure at room temperature. Manganese and nitrogen are sometimes added to reduce the cost, but that result in slight deterioration in quality as well . This group of stainless steels may contain 0.03 to 0.25% carbon, 16 to 26 percent chromium, 3.5 to 22% nickel, 2% manganese, 1 to 2% silicon and in some cases small amounts of molybdenum, titanium, etc. A very widely used variety of this type of steel, called 18-8 stainless steel, carries 18% chromium and 8% nickel. It responds well to cold working and its strength and hardness can be increased through cold working. It can also be cold drawn into wires.
These steels are non-magnetic and highly corrosion resistant, However, they may be corroded in salt media and halide acids surroundings. They possess excellent formability and good weldability. They offer the best corrosion resistance out of all the three classes of stainless steels. They however, can not be hardened by heat treatment. Titanium or niobium is sometimes added to these alloys to stabilize carbon and molybdenum to improve corrosion resistance. These steels have wide applications where high corrosion resistance and attractive appearance are vital requirements.
(ii) Magnetic Steels: -
These steels are rich in cobalt and tungsten contents and carry varying percentages of other elements like carbon, chromium, nickel, etc. A typical magnetic steel composition shows 15 to 40% cobalt, upto 10% tungsten; 1.5 to 9% chromium and upto 1.0% carbon. These steels are mainly used to make permanent magnets for electrical measuring instruments, loud speakers, magnetos, etc.
(iii) Heat Resistant Steels: -
With alround developments in high-technologies in modern era a continued need has been to develop such metals which can resist the influence of such parameters that can lead to the failures of common metals at elevated temperatures. Such conditions commonly arise in the operations of nuclear power plants, structure and parts of high temperature furnaces, supersonic aircrafts, missiles, etc. The metals required for use in such equipments should have high corrosion resistance, good strength and good creep resistance at high temperatures. These requirements are satisfactorily met by heat resisting alloy steels, although non-ferrous alloys have also been developed which meet these requirements equally successfully.
Some typical compositions of such ferrous alloys are given below:
(iv) Maraging Steels: -
These are ferrous alloy developed by adding 15 to 25 percent nickel, fairly high proportions of cobalt and molybdenum and small quantities of other elements to lower grades of steel, like dead mild steels. Such a chemical composition leads to the development of an alloy of which the structure will be changed to martensite when air cooled from a temperature of 815°C. Its yield strength and elongation properties can be substantially enhanced by age-hardening at 480°C. Such alloys are known as maraging steels and are widely favoured when extremely high strength and good tougness are the main requirements.
These steels have good machinability and respond well to both hot and cold working. They can also be welded, but ageing is necessary after welding.
(v) High Speed Steels (HSS): -
These steels are meant for the manufacture of cutting tools, specially those used in metal machining, and other similar applications where the amount of heat developed during the operation is very high and the tools used are required to retain their hardness at elevated temperatures. The factors responsible for high heat generation are the application of higher cutting speeds, heavy cuts, hardness of material being machined, high friction at tool and job interface, etc. All such factors contribute to heat generation and raising the temperature to such an extent that the cutting edge of the tool may become red hot. If the tool material is unable to retain its hardness at that time it will fail to perform the cutting operation. A high carbon steel tool fails to meet this requirement and that necessitated the development of these alloys (H.S.S.).It is reckoned that tools made of these alloys can safely operate at 2 –3 times higher speeds than those possible with high-carbon steel tools and retain their hardness upto a temperature of 620°C.
The most commonly used form of these alloys is the 18 – 4 – 1 high speed steel, which carries 18% tungsten, 4% chromium, 1% vanadium, 0.7% carbon and the rest iron. It carries a balanced combination of good red hardness, wear resistance and shock resistance and is, therefore, widely used for making cutting tools for lathes, shapers, planers, slotters, milling cutter, drill bits, broaches, etc.
Another popular variety of high-speed steels is cobalt high-speed steel. Addition of cobalt improves red hardness and wear resistance. A typical composition of cobalt high speed steel contains 12% cobalt, 20% tungsten, 4% chromium, 2% vanadium, 0.8% carbon and the rest iron. This ensures better red hardness and can safely operate upto 620°C. These steels are also known as super high-speed steels.
Another variety of high-speed steels, called vanadium high-speed steel, carry higher proportions of vanadium and shows better abrasive resistance than 18 – 4 – 1 HSS. It is therefore, preferred for machining difficult- to machine materials.
Yet another variety of this category of alloys called molybdenum high-speed steel, having 6% molybdenum, 4% chromium, 6% tungsten, 2% vanadium and higher percentage of carbon possesses very high toughness and excellent cutting properties. It is now a very widely used high-speed steel.
These are ferrous alloy developed by adding 15 to 25 percent nickel, fairly high proportions of cobalt and molybdenum and small quantities of other elements to lower grades of steel, like dead mild steels. Such a chemical composition leads to the development of an alloy of which the structure will be changed to martensite when air cooled from a temperature of 815°C. Its yield strength and elongation properties can be substantially enhanced by age-hardening at 480°C. Such alloys are known as maraging steels and are widely favoured when extremely high strength and good tougness are the main requirements.
These steels have good machinability and respond well to both hot and cold working. They can also be welded, but ageing is necessary after welding.
(v) High Speed Steels (HSS): -
These steels are meant for the manufacture of cutting tools, specially those used in metal machining, and other similar applications where the amount of heat developed during the operation is very high and the tools used are required to retain their hardness at elevated temperatures. The factors responsible for high heat generation are the application of higher cutting speeds, heavy cuts, hardness of material being machined, high friction at tool and job interface, etc. All such factors contribute to heat generation and raising the temperature to such an extent that the cutting edge of the tool may become red hot. If the tool material is unable to retain its hardness at that time it will fail to perform the cutting operation. A high carbon steel tool fails to meet this requirement and that necessitated the development of these alloys (H.S.S.).It is reckoned that tools made of these alloys can safely operate at 2 –3 times higher speeds than those possible with high-carbon steel tools and retain their hardness upto a temperature of 620°C.
The most commonly used form of these alloys is the 18 – 4 – 1 high speed steel, which carries 18% tungsten, 4% chromium, 1% vanadium, 0.7% carbon and the rest iron. It carries a balanced combination of good red hardness, wear resistance and shock resistance and is, therefore, widely used for making cutting tools for lathes, shapers, planers, slotters, milling cutter, drill bits, broaches, etc.
Another popular variety of high-speed steels is cobalt high-speed steel. Addition of cobalt improves red hardness and wear resistance. A typical composition of cobalt high speed steel contains 12% cobalt, 20% tungsten, 4% chromium, 2% vanadium, 0.8% carbon and the rest iron. This ensures better red hardness and can safely operate upto 620°C. These steels are also known as super high-speed steels.
Another variety of high-speed steels, called vanadium high-speed steel, carry higher proportions of vanadium and shows better abrasive resistance than 18 – 4 – 1 HSS. It is therefore, preferred for machining difficult- to machine materials.
Yet another variety of this category of alloys called molybdenum high-speed steel, having 6% molybdenum, 4% chromium, 6% tungsten, 2% vanadium and higher percentage of carbon possesses very high toughness and excellent cutting properties. It is now a very widely used high-speed steel.