All questions of Engineering Materials for Mechanical Engineering Exam

Cast iron is a popular material used for machine beds due to its unique properties. The following are the reasons why cast iron is preferred for machine beds:

Damping capacity:
Cast iron has a high damping capacity, making it an ideal material for machine beds. Damping capacity refers to the ability of a material to absorb and dissipate vibrations. This property is important in machine tools because it helps to reduce vibrations and improve accuracy.

Compressive strength:
Cast iron is also known for its high compressive strength, which is the ability to withstand crushing forces without deformation or failure. This property is important in machine tools because it ensures that the machine bed can support the weight of heavy workpieces and resist deformation under load.

Other properties:
Cast iron also has other properties that make it suitable for machine beds, including high wear resistance, good thermal conductivity, and low thermal expansion. These properties help to ensure that the machine bed can withstand the rigors of machining operations and maintain its accuracy over time.

Conclusion:
In conclusion, the high damping capacity and compressive strength of cast iron make it an ideal material for machine beds. Its other properties also contribute to its suitability for this application.

Eutectoid alloy is formed at 0.8% CIf below 0.8% it is Hypo-eutectoid if above than Hyper eutectoid alloy

Understanding Tetragonal Crystal System
The tetragonal crystal system is one of the seven crystal systems in crystallography. It is characterized by specific relationships between the lengths of its axes and the angles between them.
Key Characteristics of Tetragonal System
- Axis Lengths: In a tetragonal crystal system, two axes are of equal length, while the third axis is of a different length. This can be summarized as:
- a = b ≠ c
- Inter-Axial Angles: All angles between the axes are right angles (90 degrees). Therefore, the angles can be defined as:
- α = β = γ = 90°
Analyzing the Options
- Option A: a = b = c; α = β = γ = 90°
- This describes a cubic system, not tetragonal.
- Option B: a = b ≠ c; α = β = γ = 90°
- This correctly represents the tetragonal system, as it has two equal axes and one different.
- Option C: a ≠ b ≠ c; α = β = γ = 90°
- This describes an orthorhombic system, where all axes are of different lengths.
- Option D: a = b = c; α = β = 90°
- This again describes a cubic system.
Conclusion
The correct answer is indeed Option B: a = b ≠ c; α = β = γ = 90°, accurately representing the characteristics of the tetragonal crystal system.

Explanation:

List-I
- A. Nickel
- B. Chromium
- C. Tungsten
- D. Silicon

List-II
- 1. Corrosion resistance
- 2. Magnetic permeability
- 3. Heat resistance
- 4. Hardenability

Matching Alloying Elements with Properties
- Nickel (A) confers corrosion resistance (1) on steel.
- Chromium (B) confers heat resistance (3) on steel.
- Tungsten (C) does not match with any property given in List-II.
- Silicon (D) confers hardenability (4) on steel.
Therefore, the correct match of alloying elements in steel with the properties conferred on steel by those elements is:

(a) 4 1 3 2

Understanding the Manufacturing Sequence of Steel Balls
The manufacture of steel balls for ball bearings involves several critical operations, each serving a specific purpose. The correct sequence of these operations is crucial to achieve the desired mechanical properties and surface finish.
1. Cold Heading
- This is the initial operation where steel wire is cut and shaped into rough ball forms using dies.
- It is essential for forming the basic shape and size of the balls.
2. Annealing
- After cold heading, the balls undergo annealing, which involves heating and then slowly cooling them.
- This process relieves internal stresses and improves the ductility of the material.
3. Rough Grinding
- Following annealing, rough grinding is performed to remove excess material and achieve a more precise size and shape.
- It prepares the balls for further refinement and improves their surface finish.
4. Hardening
- Hardening is a heat treatment process that significantly increases the hardness of the balls.
- This step ensures that the balls can withstand heavy loads and high friction during operation.
5. Oil Lapping
- The final operation is oil lapping, which involves polishing the balls with abrasive materials and oil.
- This step enhances the surface finish and reduces friction, ensuring smooth operation in ball bearings.
Conclusion
The correct sequence, therefore, is: Cold Heading (2), Annealing (3), Rough Grinding (5), Hardening (4), and Oil Lapping (1). This sequence (option 'C') ensures that each process effectively prepares the steel balls for the next, resulting in high-quality ball bearings.

Phenol Formaldehyde as a Thermoset Polymer

Introduction:
Phenol formaldehyde, also known as phenolic resin, is a type of polymer that is formed by the reaction between phenol and formaldehyde. It is a versatile and widely used polymer due to its excellent thermal and mechanical properties. In this response, we will explain why phenol formaldehyde is classified as a thermoset polymer.

Definition of Thermoset Polymer:
Before delving into the classification of phenol formaldehyde, let's understand what a thermoset polymer is. A thermoset polymer is a type of polymer that undergoes a chemical reaction called crosslinking during its curing process. This crosslinking forms a three-dimensional network structure that cannot be melted or re-molded once it is set. Thermoset polymers are known for their high strength, rigidity, and resistance to heat and chemicals.

Explanation:
Phenol formaldehyde falls under the category of thermoset polymers due to the following reasons:

Crosslinking:
During the curing process, the phenol formaldehyde resin undergoes crosslinking reactions. The formaldehyde molecules react with phenol molecules to form methylene bridges. These bridges create strong covalent bonds between the polymer chains, resulting in a three-dimensional network structure. Once the crosslinking is complete, the polymer becomes rigid and cannot be reshaped.

Irreversible Curing:
The curing of phenol formaldehyde is an irreversible process. Once the resin is heated and cured, it solidifies into a hard and infusible material. Unlike thermoplastic polymers, which can be melted and reshaped multiple times, phenol formaldehyde cannot be melted without decomposing. This irreversibility is a characteristic feature of thermoset polymers.

Properties:
Phenol formaldehyde exhibits excellent mechanical properties, such as high strength, stiffness, and dimensional stability. It also possesses good thermal resistance, electrical insulation properties, and resistance to chemicals. These properties are desirable in various applications, including electrical components, adhesives, coatings, and molded products.

Conclusion:
In summary, phenol formaldehyde is classified as a thermoset polymer due to its crosslinking reactions during curing, irreversible curing process, and the resulting three-dimensional network structure. Its unique properties make it suitable for a wide range of industrial applications.

List I (Cutting tool Material) List II (Major characteristic constituent)
A. High speed steel 1. Carbon
B. Stellite 2. Molybdenum
C. Diamond 3. Nitride
D. Coated carbide tool 4. Columbium
5. Cobalt

Codes: A B C D A B C D
(a) 2 1 3 5 (b) 2 5 1 3
(c) 5 2 4 3 (d) 5 4 2 3

The correct answer is option 'B' (2 5 1 3). Let's understand why:

1. High speed steel: High speed steel is a cutting tool material that is known for its high hardness, toughness, and wear resistance. It is mainly composed of iron, carbon, tungsten, chromium, vanadium, and sometimes cobalt. The major characteristic constituent in high speed steel is carbon (Option 1).

2. Stellite: Stellite is a cobalt-based alloy that is commonly used as a cutting tool material. It is known for its excellent wear resistance, high temperature resistance, and corrosion resistance. The major characteristic constituent in Stellite is cobalt (Option 5).

3. Diamond: Diamond is an extremely hard material that is used in cutting tools for machining non-ferrous materials, ceramics, and composites. It has exceptional hardness and thermal conductivity. The major characteristic constituent in diamond is carbon (Option 1).

4. Coated carbide tool: Coated carbide tools are widely used in machining operations due to their high hardness, wear resistance, and thermal stability. The major characteristic constituent in coated carbide tools is tungsten carbide (Option 2). However, none of the given options match with tungsten carbide, so we can eliminate this option.

By eliminating option 4, we are left with option 'B' (2 5 1 3) as the correct answer.

Statement 1: Medium carbon steel can be quench-hardened but not case-hardened.
Quench-hardening is a heat treatment process in which steel is heated to a high temperature and then rapidly cooled (quenched) in a liquid medium, typically oil or water. This process results in a hardened structure with increased strength and hardness. Medium carbon steel, typically containing around 0.3-0.6% carbon, can undergo quench-hardening. The high carbon content allows for the formation of a hard martensitic structure during the quenching process.

On the other hand, case-hardening is a heat treatment process used to increase the hardness of the outer surface of a steel component while retaining a relatively tough and ductile core. This is achieved by introducing carbon or nitrogen into the surface layer of the steel through a diffusion process. Medium carbon steel is not suitable for case-hardening as it already contains a sufficient amount of carbon to achieve the desired hardness through quench-hardening. Therefore, statement 1 is correct.

Statement 2: Medium carbon steel cannot be quench-hardened but case-hardening can be done.
This statement is incorrect. As mentioned earlier, medium carbon steel can be quench-hardened by heating it to a high temperature and then rapidly cooling it. Case-hardening, however, is not suitable for medium carbon steel.

Statement 3: Medium carbon steel exhibits a distinct yield point under tension test.
The yield point is the stress level at which a material begins to deform plastically, meaning it does not return to its original shape after the applied stress is removed. Medium carbon steel, like most steels, typically exhibits a distinct yield point during a tension test. This yield point is characterized by a sudden drop in stress after which the material undergoes plastic deformation. Therefore, statement 3 is correct.

In conclusion, statements 1 and 3 are correct. Medium carbon steel can be quench-hardened and exhibits a distinct yield point under tension test. Statement 2 is incorrect as medium carbon steel is not suitable for case-hardening. Therefore, the correct answer is option C.

Highest possible atomic packing factor (APF) in metals is 0.74.

Explanation:
Atomic packing factor (APF) is a measure of how efficiently atoms are arranged in a crystal structure. It is the ratio of the volume occupied by atoms to the total volume of the unit cell.

In metals, atoms are closely packed together in a regular pattern. The two most common types of arrangements are face-centered cubic (FCC) and body-centered cubic (BCC) structures.

- Face-centered cubic (FCC) structure:
In an FCC structure, each corner of the cube is occupied by an atom, and there is an additional atom at the center of each face. This gives a total of 4 atoms per unit cell.
The volume occupied by atoms in an FCC structure can be calculated as follows:
Volume occupied by atoms = (number of atoms) * (volume of each atom)
= 4 * (4/3 * π * (radius of atom)^3)
= 16/3 * π * (radius of atom)^3

The total volume of the unit cell in an FCC structure is given by:
Total volume of unit cell = (length of unit cell)^3

Therefore, the atomic packing factor (APF) for an FCC structure is:
APF = (volume occupied by atoms) / (total volume of unit cell)
= (16/3 * π * (radius of atom)^3) / ((length of unit cell)^3)

- Body-centered cubic (BCC) structure:
In a BCC structure, each corner of the cube is occupied by an atom, and there is an additional atom at the center of the cube. This gives a total of 2 atoms per unit cell.
The volume occupied by atoms in a BCC structure can be calculated as follows:
Volume occupied by atoms = (number of atoms) * (volume of each atom)
= 2 * (4/3 * π * (radius of atom)^3)
= 8/3 * π * (radius of atom)^3

The total volume of the unit cell in a BCC structure is given by:
Total volume of unit cell = (length of unit cell)^3

Therefore, the atomic packing factor (APF) for a BCC structure is:
APF = (volume occupied by atoms) / (total volume of unit cell)
= (8/3 * π * (radius of atom)^3) / ((length of unit cell)^3)

Comparing the APF values for FCC and BCC structures, it can be observed that the highest possible APF is 0.74, which corresponds to the FCC structure. Therefore, the correct answer is option 'B' (0.74).

Condensation polymerization is the process by which two or more chemically different monomers are polymerized to form a cross-linked polymer along with the production of a by-product such as water or ammonia. This type of polymerization occurs through the formation of covalent bonds between the monomers, resulting in the removal of a small molecule as a by-product.

Here is a detailed explanation of the process of condensation polymerization:

1. Definition:
Condensation polymerization is a reaction where two different monomers react by forming covalent bonds and eliminating a small molecule as a by-product.

2. Monomers:
Condensation polymerization involves two or more chemically different monomers. These monomers have functional groups that are capable of reacting with each other, such as alcohol groups (-OH) and carboxylic acid groups (-COOH).

3. Reaction:
The process begins with the reaction between the functional groups of the monomers. For example, in the case of a polyester, the reaction occurs between an alcohol group of one monomer and a carboxylic acid group of another monomer. This reaction forms an ester linkage, resulting in the elimination of a water molecule as a by-product.

4. Polymerization:
As the reaction proceeds, more monomers join the growing polymer chain through similar condensation reactions. The polymerization continues until all the monomers are consumed or until the desired polymer length is achieved.

5. Cross-linking:
In some cases, the condensation polymerization process can lead to the formation of cross-linked polymers. Cross-linking occurs when the polymer chains are connected through additional covalent bonds, resulting in a three-dimensional network structure. This enhances the mechanical properties of the polymer, such as strength and toughness.

6. By-product:
During condensation polymerization, a small molecule, such as water or ammonia, is produced as a by-product. The formation of this by-product is a result of the elimination of functional groups from the monomers during the reaction.

7. Examples:
Condensation polymerization is commonly used in the production of various polymers, including polyesters, polyamides, and polyurethanes. For instance, the condensation reaction between ethylene glycol and terephthalic acid produces polyethylene terephthalate (PET), which is widely used in the manufacture of bottles and fibers.

In conclusion, condensation polymerization is a process in which two or more chemically different monomers react to form a cross-linked polymer, along with the generation of a by-product such as water or ammonia. This type of polymerization is important in the synthesis of various polymers, offering a wide range of applications in industries such as packaging, textiles, and coatings.

Point defect in crystal lattice : Self interstitials
Linear defect in crystal lattice : Grain boundaries
Planar defect in crystal lattice : External surfaces
Volume defect in crystal lattice : Other phases

The correct answer is option B: Linear defect in crystal lattice - Grain boundaries.

Explanation:

Point Defects:
- Point defects refer to the defects that occur at a single or few lattice sites in a crystal lattice.
- These defects can be classified into two types: vacancy defects and interstitial defects.
- Vacancy defects occur when an atom is missing from its lattice site, leaving a vacant site.
- Interstitial defects occur when extra atoms occupy the interstitial sites between the regular lattice positions.

Linear Defects:
- Linear defects, also known as dislocations, occur when there is a deviation from the regular arrangement of atoms along a line or plane in the crystal lattice.
- They can be classified into two types: edge dislocations and screw dislocations.
- Edge dislocations occur when an extra half-plane of atoms is inserted into the crystal lattice, causing a misalignment of the atoms on either side of the dislocation line.
- Screw dislocations occur when the atoms are arranged in a helical pattern around the dislocation line.

Planar Defects:
- Planar defects refer to the defects that occur on a plane or surface within the crystal lattice.
- These defects can be classified into two types: grain boundaries and external surfaces.
- Grain boundaries occur when there is a discontinuity in the crystal lattice due to the presence of different crystal orientations or grains.
- External surfaces refer to the surfaces of the crystal lattice that are exposed to the external environment.

Volume Defects:
- Volume defects, also known as bulk defects, occur within the bulk of the crystal lattice.
- These defects can include impurities, second phases, or voids within the crystal structure.
- Impurities are foreign atoms or ions that are incorporated into the crystal lattice.
- Second phases refer to the presence of a different crystal phase within the same material.
- Voids are empty spaces or vacancies within the crystal lattice.

In the given options, the pair "Linear defect in crystal lattice: Grain boundaries" is not correctly matched because grain boundaries are planar defects, not linear defects. The correct pair for linear defects would be "Linear defect in crystal lattice: Dislocations" which include edge and screw dislocations.