Material Science: Ferrous materials

quiz-aug2,2017
August 3, 2017
Communication Technologies
August 6, 2017

1.Introduction

2. Classification of Metals

3. Processing of Iron ore

    3.1 Iron ore

    3.2 Pig iron

4. Classification of ferrous metals

4.1 Steels

   a) Low carbon steels

   b) Medium carbon steels

   c) High carbon steels

   d) HSLA steels

 4.2 Effect of alloying elements on steels

   4.3 Special alloy steels

   f) Stainless steels

   g) Magnetic steels

   h) High speed steels

   i) High temperature resistant steels

5. Cast iron

    a) Gray cast iron

    b) White cast iron

    c) Nodular(Ductile) cast iron

   d) Malleable cast iron

   e) Compacted graphite cast iron

6. Wrought iron

7. FAQs

 

1. Introduction

What are ferrous materials?

Ferrous Metals mostly contain Iron.  They have small amounts of other metals or elements added, to give the required properties.  Ferrous Metals are magnetic and give little resistance to corrosion.

Ferrous materials are produced in larger quantities than any other metallic material. Three factors account for it:

  • availability of abundant raw materials combined with economical extraction,
  • ease of forming and
  • their versatile mechanical and physical properties.

In ferrous materials the main alloying element is carbon (C).

  • Depending on the amount of carbon present, these alloys will have different properties, especially when the carbon content is either less/higher than 2.14%.

 

Material carbon(C) < 2.14% eutectoid transformation steels
Material carbon(C) > 2.14% eutectic transformation Cast irons

 

  • Thus the ferrous alloys with less than 2.14% C are termed as steels, and the ferrous alloys with higher than 2.14% C are termed as cast irons.

One main drawback of ferrous alloys is their environmental degradation i.e. poor corrosion resistance. Other disadvantages include: relatively high density and comparatively low electrical and thermal conductivities.

2. Classification of Metals

3. Processing of iron ore

Iron is not found in free state because of its high reactive nature.Iron is found in combined state in the form of oxide, carbonate and sulphide.

Processes involved to produce final products

3.1 Iron ore

Pure iron is a soft, grayish-white metal. Although iron is a common element, pure iron is almost never found in nature. The only pure iron known to exist naturally comes from fallen meteorites.

            Most iron is found in minerals formed by the combination of iron with other elements. Iron oxides are the most common. Those minerals near the surface of the earth that have the highest iron content are known as iron ores and are mined commercially.

 

Iron ore type Properties Available places
Haematite
  • Reddish
  • best quality
  • 70 per cent metallic content.
  • Found in Dharwad and Cuddapah rock systems of the peninsular India.
  • 80 per cent of haematite reserves are in Odisha, Jharkhand, Chhattisgarh and Andhra Pradesh.
Magnetite
  • Black ore
  • 60 to 70 per cent metallic content.
  • Magnetic quality.
  • Karnataka, Andhra Pradesh, Rajasthan, Tamil Nadu and Kerala.
Limonite
  • Inferior ores
  • yellowish in colour
  • 40 to 60 per cent iron metal.
  • easy and cheap mining because of open cast mining.

 

  • Damuda series in Raniganj coal field, Garhwal in Uttarakhand,
  • Mirzapur in Uttar Pradesh and Kangra valley of Himachal Pradesh
Siderite
  • Iron carbonate
  • inferior quality,mining is not economically variable.
  • less than 40 per cent iron.
  • it is self-fluxing(purifying) due to presence of lime.

 

  • Kolar district in karnataka,jhabua district in Madhya Pradesh etc.

Iron ore is converted into various types of iron through several processes. The most common process is the use of a blast furnace to produce pig iron which is about 92-94% iron and 3-5% carbon with smaller amounts of other elements.

3.2 Pig iron

The raw materials used to produce pig iron in a blast furnace are

  • Iron ore
    • The iron content of these ores ranges from 70% down to 20% or less.
  • Coke
    • Coke is a substance made by heating coal until it becomes almost pure carbon.  
  • Sinter
    • Sinter is made of lesser grade, finely divided iron ore which, is roasted with coke and lime to remove a large amount of the impurities in the ore.
  • Limestone.
    • Iron ore often has silicon dioxide (sand) impurity which will be removed by adding limestone.

Pig iron has only limited uses, and most of this iron goes on to a steel mill where it is converted into various steel alloys by further reducing the carbon content and adding other elements such as manganese and nickel to give the steel specific properties.

4. Classification of Ferrous materials

In ferrous materials the main alloying element is carbon (C). Depending on the amount of carbon present, these alloys will have different properties, especially when the carbon content is either less/higher than 2.14%.

Thus the ferrous alloys with less than 2.14% C are termed as steels, and the ferrous alloys with higher than 2.14% C are termed as cast irons.

4.1 Steels

  • Steels are alloys of iron and carbon plus other alloying elements.
  • In steels, carbon present in atomic form, and occupies interstitial sites of Fe microstructure.

Alloying additions are necessary for many reasons including:

  • Improving properties
  • Improving corrosion resistance, etc.

Arguably steels are well known and most used materials than any other materials.

Classification of steels

Mechanical properties of steels are very sensitive to carbon content. Hence, it is practical to classify steels based on their carbon content.

Thus steels are basically three kinds based on carbon content

  • Low- carbon steels (% wt of C < 0.3)
  • Medium carbon steels (0.3 <% wt of C < 0.6) and
  • High- carbon steels (% wt of C > 0.6).

The other parameter available for classification of steels is amount of alloying additions, and based on this steels are two kinds:

  • Carbon(plain) steels
  • Alloy-steels.

a) Low carbon steels

These are arguably produced in the greatest quantities than other alloys.

Strengthening mechanism:

  • Carbon present in these alloys is limited, and is not enough to strengthen these materials by heat treatment
  • Hence these alloys are strengthened by cold work.

Structure:

  • Their microstructure consists of ferrite and pearlite

Properties:

  • Because of above atomic structure, these alloys are thus relatively soft, ductile combined with high toughness.
  • Hence these materials are easily machinable and weldable.

Applications:

  • Structural shapes, tin cans, automobile body components, buildings, etc.

b) Medium carbon steels

These are stronger than low carbon steels. However these are of less ductile than low carbon steels.

Strengthening mechanism:

  • These alloys can be heat treated to improve their strength.
  • Usual heat treatment cycle consists of austenitizing, quenching, and tempering at suitable conditions to acquire required hardness.

Properties:

  • They are often used in tempered condition. As hardenability of these alloys is low, only thin sections can be heat treated using very high quench rates.

Applications:

  • Railway tracks & wheels, gears, other machine parts which may require good combination of strength and toughness.

Effects of alloys:

  • Ni, Cr and Mo alloying additions improve their hardenability.

c) High carbon steels

These are strongest and hardest of carbon steels, and of course their ductility is very limited.

Strengthening mechanism:

  • These are heat treatable, and mostly used in hardened and tempered conditions.

Properties:

  • They possess very high wear resistance, and capable of holding sharp edges.

Applications:

  • Used as tool and die steels owing to the high hardness and wear resistance property
  • Tool application such as knives, razors, hacksaw blades, etc.

Effect of an alloy:

  • With addition of alloying element like Cr, V, Mo, W which forms hard carbides by reacting with carbon present, wear resistance of high carbon steels can be improved considerably.

d) HSLA (high-strength low-alloy) steels.

A special group of ferrous alloys with noticeable amount of alloying additions are known as HSLA (high-strength low-alloy) steels.

  • Common alloying elements are: Cu, V, Ni, W, Cr, Mo, etc.
  • These alloys can be strengthened by heat treatment, and yet the same time they are ductile, formable.
  • HSLA steels are more resistant to corrosion than the plain carbon steels, which they have replaced in many applications where structural strength is critical.

Applications:

  • Support columns, bridges, pressure vessels.

4.2 Effects of alloying elements on steels

Carbon

Carbon is by far the most important constituent of steel. It combines readily with iron to form iron carbide (Fe3C),which is a compound known as cementite.

  • Within certain limitations, the higher the carbon content of steel is, the greater will be the ultimate strength, the hardness, and the range through which it can be heat treated.
  • At the same time, the ductility, malleability, toughness, impact resistance, and the weld ability will be reduced as the carbon increases.

Manganese

Next to carbon, manganese is the most important ingredient in steel.

  • Its primary purpose is to deoxidize and desulphurize the steel to produce a clean metal.
  • It de-oxidizes by eliminating ferrous oxide, which is a harmful impurity, and it combines with sulphur.
  • This excess manganese exists as manganese carbide (Mn3C), which has characteristics in hardening and toughening the steel similar to those of cementite (Fe3C), although not to as great an extent.

Manganese does possess the property known as “penetration hardness” which means that in heat treatment of large sections, the hardness is not merely on the surfaces but penetrates to the core as well.

 Silicon

Only a very small amount, not exceeding 0.3% of silicon, is present in steel.

  • It is an excellent deoxidizer
  • But it also has the property of combining with iron more readily than carbon. Therefore it must be limited.
  • Its main purpose however, is to produce a sound metal.

Silicon and manganese in large amount are used as alloying elements in the formation of silico-manganese steels. These steels have good impact resistance.

Sulphur

Sulphur is a very undesirable impurity.

  • The presence of sulphur renders steel brittle at rolling or forging temperatures.
  • In this condition the steel is said to be “hot short”.
  • As stated previously, manganese combines with the sulphur to form manganese sulphide, which is harmless in small amount.
  • When too much sulphur is present, an iron sulphide is formed which, breaks up the cohesion of the crystals of the metal.
  • When required amounts of sulphur and manganese presented then, all the sulphur will be in the form of manganese sulphide, which is harmless in such small quantities.

Phosphorus

Phosphorus, like sulphur, is an undesirable impurity.

  • Phosphorus is believed responsible for “cold shortness” or brittleness when the metal is cold.

Nickel

Nickel is a white metal almost as bright as silver. In the pure state it is malleable, ductile, and weldable.

  • The addition of nickel to steels increases the strength, yield point, and hardness without materially affecting the ductility.
  • Nickel increases the corrosion-resistance of the steel.
  • It is one of the principle constituents of the so-called “stainless” or corrosion-resisting steels.

Chromium

Chromium is a hard gray metal with a high melting point.

  • Chromium imparts hardness, strength, wear resistance, and corrosion resistance to steel.
  • It also improves the magnetic qualities to such an extent that chromium steel is used for magnets.
  • Chromium possesses excellent “penetration hardness” characteristics and its alloys heat treat well.

Corrosion-resisting steels contain large amounts of chromium.

  • The most common of these steels is 18-8 steel –approximately 18% chromium and 8% nickel.
  • This metal is very corrosion-resistant.

Some chromium alloys are used where great wear resistance is required.

  • A chrome-vanadium alloy is used for ball bearings, and a tungsten-chromium alloy for high-speed cutting tools.

Molybdenum

A small percentage has as much effect as much larger amounts of other alloying elements.

  • It improves the homogeneity of the metal and reduces the grain size.
  • It also increases the elastic limit, the impact value, wear resistance, and fatigue strength.

Uses in aircraft:

  • An exceptionally important property from the aircraft viewpoint is the improvement in the air-hardening properties of steel containing molybdenum.
  • This property is particularly useful where the steel has been subjected to a welding process, as is very common with chrome-molybdenum steel in airplane construction.

Vanadium

Vanadium is the most expensive of the alloying elements.

It is an intensive deoxidizing agent and improves the grain structure and fatigue strength.

 

Chemical It’s effect on steel
Manganese
  • Increases strength and hardness
  • decreases ductility and weldability
  • increases hardenability of steel.
  • deoxidiser and desulphuriser
Phosphorus
  • increases strength and hardness
  • decreases ductility and notch impact toughness of steel
  • undesired impurity, causes cold shortness
Sulfur
  • decreases ductility and notch impact toughness
  • weldability decreases.
  • Found in the form of sulfide inclusions.
  • undesirable impurity, causes hot short
Silicon
  • one of the principal deoxidizers used in steelmaking.
  • In low-carbon steels, silicon is generally detrimental to surface quality.
Copper
  • detrimental to hot-working steels
  • beneficial to corrosion resistance (Cu>0.20%)
Nickel
  • ferrite strengthener
  • increases the hardenability
  • increases impact strength of steels.
  • corrosion resistance
Molybdenum
  • increases the hardenability
  • enhances the creep resistance of low-alloy steels
  • In aircraft materials
Chromium
  • excellent “penetration hardness” characteristics
  • 18-8 steel –approximately 18% chromium and 8% nickel.
  • This metal is very corrosion-resistant.

4.3 Special alloy steels

By adding alloys, specials alloy steels can be produced to suit specific requirements by modifying the properties

These are

  • Stainless steels
  • Magnetic steels
  • High resistance steels
  • High speed steels

f) Stainless steels

The name comes from their high resistance to corrosion i.e. they are rustless (stain-less).

  • Steels are made highly corrosion resistant by addition of special alloying elements,
    • especially a minimum of 12% Cr along with Ni and Mo.

Stainless steels are mainly three kinds based on prominent constituent of the microstructure

  • Ferritic and austenitic steels are hardened and strengthened by cold work because they are not heat treatable.
  • On the other hand martensitic steels are heat treatable.
  • Austenitic steels are most corrosion resistant, and they are produced in large quantities.
  • Austenitic steels are nonmagnetic as against ferritic and martensitic steels, which are magnetic.

Applications:

  • Typical applications include cutlery, razor blades, surgical knives, etc.

g) Magnetic steels

  • Alloys like carbon.chromium, tungsten and cobalt are used to produce permanent magnetic steels having high magnetic retentive power
  • Typical alloy has 60% iron,20% nickel, 8% cobalt and 20% aluminium

h) High speed steels

For modern industrial production, in particular mass production, machining is one of the most important shaping and forming processes. High Speed Steels are high-performance special steels offering high hardness at temperatures up to 500°C and high wear resistance, thanks to alloying elements like tungsten, molybdenum, vanadium and chromium which are able to form carbides. To improve hot hardness, cobalt may also be added.

High Speed Steels can be produced either by conventional route or by powder metallurgy.

The characteristic properties of all high speed steels grades include.       

  • High working hardness   
  • High wear hardness   
  • Excellent toughness   
  • High retention of hardness and red hardness[FAQ]

i) High temperature resistant alloys

These are high nickel based steels which have high red hot hardness. These are also called as super alloys

5. Cast iron

  • Ferrous alloys with more than 2.14 wt.% C are designated as cast irons
  • Commercially cast irons contain about 3.0-4.5% C along with some alloying additions.
  • Alloys with this carbon content melt at lower temperatures than steels i.e. they are responsive to casting. Hence casting is the most used fabrication technique for these alloys.

Based on the form of carbon present, cast irons are categorized as gray, white, nodular and malleable cast irons.

a) Gray cast iron

Constituents:

  • These alloys consists carbon in form graphite flakes, which are surrounded by either ferrite or pearlite.
  • Because of presence of graphite, fractured surface of these alloys look grayish, and so is the name for them.

Effects of alloy:

  • Alloying addition of silicon(Si) (1- 3wt.%) is responsible for decomposition of cementite, and also high fluidity.
  • Thus castings of intricate shapes can be easily made.

Properties:

  • Due to graphite flakes, gray cast irons are weak and brittle in tension.
  • Stronger in compression
  • However they possess good damping properties
  • They also show high resistance to wear.

Applications:

  • Because of its damping property, these are used as base structures, bed for heavy machines, etc.

Gray iron: the dark graphite flakes are embedded in an alfe-ferrite matrix

b) White cast iron

How name came:

  • When Si content is low (< 1%) in combination with faster cooling rates, there will be no time left for cementite to get decomposed and retains the brittle cementite.
  • Because of presence of this cementite, fractured surface appear white, hence the name.

Properties:

  • They are very hard and brittle because of this, extremely difficult to machine.
  • Hence their use is limited to wear resistant applications.

Applications:

  • These are used as rollers in rolling mills.
  • Usually white cast iron is heat treated to produce malleable iron.

White iron: the light cementite regions are surrounded by pearlite, which has the ferrite–cementite layered structure

c) Nodular (or ductile) cast iron

Alloying additions are of prime importance in producing these materials.

Constituents:

  • Small additions of magnesium or cerium (Mg / Ce) to the gray cast iron melt before casting produces distinctly different microstructure.
  • This result in graphite to form nodules or sphere-like particles.
  • Matrix surrounding these particles can be either ferrite or pearlite depending on the heat treatment.

Properties:

  • These are stronger and ductile than gray cast irons.

Applications:

  • Pump bodies, crank shafts, automotive components, etc.

Nodular (ductile) iron: the dark graphite nodules are surrounded by an alfa-ferrite matrix.

d) Malleable cast iron

  • These formed after heat treating white cast iron.
  • Heat treatment involves heating the material up to 800-900 ْC, and keep it for long hours, before cooling it to room temperature.
  • High temperature incubation causes cementite to decompose and form ferrite and graphite.

Properties:

  • These materials are stronger with appreciable amount of ductility.

Applications:

  • Railroad, connecting rods, marine and other heavy- duty services.

Malleable iron: dark graphite rosettes (temper carbon) in an alfa-ferrite matrix.

e) Compacted Graphite Iron

A relatively recent addition to the family of cast irons is compacted graphite iron (abbreviated CGI). In the gray, ductile, and malleable irons, carbon exists as graphite, which formation is promoted by the presence of silicon.

Structure:

  • Microstructurally, the graphite in CGI alloys has a worm-like (or vermicular) shape.

Properties:

  • An increase in degree of nodularity of the graphite particles leads to enhancements of both strength and ductility.
  • Tensile and yield strengths for compacted graphite irons are comparable to values for ductile and malleable irons, yet are greater than those observed for the higher- strength gray irons  

Compared to the other cast iron types, desirable characteristics of CGIs include the following:

  • Higher thermal conductivity
  • Better resistance to thermal shock (i.e., fracture resulting from rapid temperature changes)
  • Lower oxidation at elevated temperatures

Applications: 

  • diesel engine blocks, exhaust manifolds
  • gearbox housings
  • brake discs for high-speed trains
  • and flywheels.

6. Wrought Iron

Wrought iron is composed primarily of iron with 1 to 2% of added slag, the byproduct of iron ore smelting—generally a mix of silicon, sulfur, phosphorous, and aluminum oxides.

During manufacture, the iron is removed from heat and worked with a hammer while still hot to get it into its intended final form.

Properties:

  • Wrought iron is often characterized by its fibrous appearance, but it’s also softer and more ductile than cast iron.
  • Wrought iron is highly malleable, meaning it can be heated, and reheated, and worked into various shapes.
  • In fact, it gets stronger the more it’s worked.
  • Wrought iron has a much higher tensile strength than cast iron, making it more suitable for horizontal beams in construction.
  • In general, it strongly resists fatigue.

Difference between Cast iron and wrought iron?

Cast iron is iron that has been melted, poured into a mold, and allowed to cool.

Wrought iron is iron that has been heated and then worked with tools. In fact, the term “wrought” derived from the past participle of the word “worked.”

 

Chemical Its effect
Nickel Increases machinability of cast iron by improving carbon structure
Copper Improves machinability,castability and toughness by promoting graphite formation
Chromium Acts as carbon stabilizer thereby improves hardness, strength and corrosion resistance
Silicon It acts as a softener in cast iron by promoting free graphite and decreases cementite
Sulphur Promotes formation of cementite
Phosphorus Improves castability of cast iron
Molybdenum Improves machinability,toughness and fatigue strength
Manganese Reduces effect of sulphur on cast iron there by reduces hardness and brittleness

 7. FAQs

Q: What is red hardness? 

Ans: Red-Hardness. (also called heat resistance), the ability of steel, upon heating to red heat, to retain the great hardness and durability obtained through heat treatment. A high level of red-hardness is characteristic of tool steel.

4 Comments

  1. yogesh sharma says:

    hello sir, i m final year student of CE. I am preparing GS part from your content only.Your content is really great.I just want to be sure if this material is sufficient for scoring 120-140 in GS or i need to follow other material also?

  2. Kamlesh says:

    It is very useful

  3. Kamlesh says:

    Sir please tell about the all types of material structure. Like aluminum copper etc.

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