All questions of Engineering Materials for Mechanical Engineering Exam

Brittle fracture is more dangerous than ductile fracture because of several reasons. Let's discuss each reason in detail.

No warning sign:
- Brittle materials do not show any warning signs before fracturing, unlike ductile materials.
- Ductile materials undergo plastic deformation before fracturing, which gives a warning sign to prevent catastrophic failure.
- Brittle fracture can occur suddenly without any warning sign, making it more dangerous.

Crack propagates at a very high speed:
- In brittle materials, the crack propagates at a very high speed once it starts.
- Due to the absence of warning signs, there is no time to detect and control the crack propagation.
- The high-speed crack propagation can cause catastrophic failure in a very short time.

No need for extra stress during crack propagation:
- Brittle fracture can occur even at a low level of stress, which makes it more dangerous.
- In ductile fracture, the crack propagation requires extra stress to continue, which can be controlled to prevent catastrophic failure.

All of these:
- All the above reasons make brittle fracture more dangerous than ductile fracture.
- The absence of warning signs, high-speed crack propagation, and no need for extra stress during crack propagation can cause catastrophic failure in a very short time.

Therefore, it is important to understand the differences between brittle and ductile fracture and take appropriate precautions to prevent catastrophic failure in brittle materials.

Metallic Structure

Metallic structure refers to the arrangement of metal atoms in a solid. The arrangement of metal atoms can be granular, crystalline, or amorphous.

Granular Structure

A granular structure refers to a metal structure that is composed of granules or grains. The grains are usually visible to the naked eye and are formed when molten metal solidifies.

Wrought Structure

Wrought structure refers to the arrangement of metal atoms in a metal that has been worked or shaped by mechanical or thermal means. This type of structure is often found in metals that have been forged, rolled, or drawn.

Amorphous Structure

An amorphous structure refers to a metal structure that lacks long-range order or crystalline structure. This type of structure is usually found in metals that have been rapidly cooled, such as in the case of metallic glasses.

Crystalline Structure

A crystalline structure refers to a metal structure that has a repeating pattern of atoms in three dimensions. This type of structure is often found in metals that have been slowly cooled, allowing the atoms to arrange themselves in an ordered pattern.

Conclusion

In structure, all metals are crystalline, meaning they have a repeating pattern of atoms in three dimensions. This is due to the way in which metals solidify, allowing the atoms to arrange themselves in an ordered pattern as the metal cools.

Ternary stage creep is associated with necking.

Explanation:
Creep is the time-dependent deformation that occurs in a material under a constant load or stress. It is a common phenomenon in materials exposed to high temperatures and constant loading conditions. Creep can be divided into three stages: primary (or transient), secondary (or steady-state), and tertiary (or accelerated) creep.

Ternary stage creep, also known as tertiary creep, is the final stage of creep where the deformation rate increases rapidly. During this stage, the material experiences significant necking.

Necking is the localized reduction in cross-sectional area of a material under tension. It typically occurs in ductile materials and is characterized by the formation of a narrow region of reduced diameter or thickness. Necking is a result of strain localization and can lead to fracture.

During the primary and secondary creep stages, the material undergoes uniform deformation without significant necking. The deformation is distributed evenly across the material. However, as the material enters the tertiary creep stage, the deformation becomes highly localized, and necking occurs. This is often accompanied by a rapid increase in the deformation rate.

Necking in the context of ternary stage creep is an important phenomenon to consider because it indicates that the material is approaching its ultimate failure. It signifies that the material has undergone significant plastic deformation and is getting closer to fracture. Therefore, the occurrence of necking during ternary stage creep is a critical factor in assessing the remaining life and structural integrity of the material.

In conclusion, ternary stage creep is associated with necking, which is the localized reduction in cross-sectional area of a material under tension. Necking occurs during the final stage of creep when the material undergoes rapid deformation and is an important indicator of impending failure.

Understanding Cyaniding
Cyaniding is a surface hardening process that enhances the hardness of steel, making it suitable for applications requiring high wear resistance. Here’s how it works:
Process Overview
- Cyaniding involves the introduction of carbon and nitrogen into the surface of low-carbon steel.
- Temperature Range: The process typically occurs at temperatures between 850°C and 950°C, where the steel is immersed in a cyanide salt bath.
Mechanism of Hardening
- Diffusion: The cyanide compounds diffuse into the steel's surface, enriching it with carbon and nitrogen.
- Formation of Hard Phases: This results in the formation of hard phases such as cyanide compounds (e.g., iron nitrides), which significantly increase surface hardness.
Advantages of Cyaniding
- High Hardness: The surface can achieve hardness levels between 60-70 HRC (Rockwell Hardness Scale).
- Improved Wear Resistance: The enhanced hardness contributes to superior wear resistance, making it ideal for components like gears and shafts.
- Tough Core: While the surface becomes hard, the core remains tough, preventing brittleness.
Comparison with Other Processes
- Annealing: This process softens steel, making it more ductile.
- Normalizing: Focuses on refining grain structure without significantly increasing hardness.
- Tempering: Reduces brittleness of hardened steel, but does not enhance its hardness.
In conclusion, cyaniding is the most effective method among the options for significantly increasing the hardness of steel, making it valuable for engineering applications that require durability and wear resistance.

Understanding Isothermal Hardening
Isothermal hardening is a heat treatment process that enhances the mechanical properties of steel, particularly its hardness and strength. The process involves holding the steel at a specific temperature for a designated time, allowing for transformations in the microstructure.
Microstructures Resulting from Isothermal Hardening
- Martensite:
- Formed by rapid cooling, martensite is a very hard and brittle phase, typically produced by quenching from a high temperature.
- Acicular Troostite:
- This microstructure appears as needle-like formations and is not typically associated with isothermal treatments.
- Sorbite:
- A mixture of ferrite and cementite, sorbite results from tempering martensite but is not a direct product of isothermal hardening.
- Bainite:
- This is the correct answer. Bainite is produced through isothermal transformation at intermediate temperatures. It displays a fine, elongated structure and offers a balance of strength and ductility.
Why Bainite is the Correct Answer
- Temperature Range:
- Isothermal hardening occurs at specific temperatures (around 250-550°C), which favor the formation of bainite rather than martensite.
- Mechanical Properties:
- Bainite provides superior toughness compared to martensite, making it ideal for applications requiring both strength and ductility.
- Transformation Mechanism:
- The isothermal process allows for a slower transformation, leading to a more refined microstructure, characteristic of bainite.
In summary, the grain structure obtained from isothermal hardening is bainite, which is preferred for its improved balance of mechanical properties.

Nitriding:
Nitriding is a surface hardening process in which nitrogen is diffused into the surface of the steel to create a hardened layer. While nitriding can be a suitable method for hardening certain types of steel components, it is not typically used for surface hardening cast steel crankshafts.

Normalizing:
Normalizing is a heat treatment process that involves heating the steel to a specific temperature, holding it at that temperature for a period of time, and then cooling it in air. Normalizing is primarily used to refine the grain structure of the steel and improve its mechanical properties, but it does not provide surface hardening.

Induction Heating:
Induction heating is a process that uses electromagnetic induction to heat a metal object. In the case of a cast steel crankshaft, induction heating can be used to selectively heat the surface of the crankshaft to a high temperature, allowing for rapid quenching and subsequent hardening of the surface.

Carburizing:
Carburizing is a surface hardening process in which carbon is diffused into the surface of the steel to create a hardened layer. This process is commonly used for hardening steel components such as crankshafts, gears, and bearings. In the case of a cast steel crankshaft, carburizing can be an effective method for achieving the desired surface hardness.

The co-ordination number of a crystal structure refers to the number of nearest neighboring atoms surrounding a central atom. In the case of a body-centered cubic (BCC) crystal structure, the co-ordination number is 8.

Explanation:

1. Body-Centered Cubic (BCC) Structure:
- The BCC crystal structure is one of the common arrangements of atoms in a solid material.
- In this structure, the atoms are arranged in a cubic lattice, with an additional atom located in the center of the cube.
- Each corner of the cube is shared by 8 adjacent unit cells, while the center atom is not shared with any other unit cells.

2. Identifying Nearest Neighbors:
- To determine the co-ordination number of a BCC structure, we need to identify the nearest neighboring atoms surrounding a central atom.
- In this case, the central atom is located at the center of the cube.
- The nearest neighboring atoms are the 8 atoms situated at the corners of the cube.
- Each corner atom is shared by 8 unit cells, with one unit cell belonging to the central atom.

3. Co-ordination Number:
- The co-ordination number is the total number of nearest neighboring atoms surrounding a central atom.
- In the BCC crystal structure, there are 8 corner atoms surrounding the central atom.
- Therefore, the co-ordination number of a BCC crystal structure is 8.

In summary, the co-ordination number of a BCC crystal structure is 8 because there are 8 corner atoms surrounding the central atom.

Introduction:
High-speed steels are a class of tool steels that are widely used in cutting tools, such as drills, taps, and milling cutters, due to their excellent combination of hardness, toughness, and wear resistance at high temperatures. Tungsten is one of the key alloying elements in high-speed steels, but there are other elements that can be used as a replacement for tungsten.

Molybdenum as a replacement for tungsten:
Molybdenum (Mo) is an effective alloying element that can replace tungsten in high-speed steels. It has similar properties to tungsten and can provide several benefits.

1. Increased hardenability:
Molybdenum improves the hardenability of high-speed steels, allowing them to be hardened throughout their cross-section. This results in uniform hardness and improved strength and wear resistance.

2. Improved high-temperature strength:
Molybdenum enhances the high-temperature strength of high-speed steels. It forms a stable carbide phase, which helps to retain the hardness and strength of the steel even at elevated temperatures.

3. Increased red hardness:
Red hardness refers to the ability of a material to maintain its hardness at high temperatures. Molybdenum improves the red hardness of high-speed steels, allowing them to retain their cutting ability even at high operating temperatures.

4. Enhanced toughness:
Molybdenum contributes to the toughness of high-speed steels, preventing the formation of cracks and improving their resistance to chipping and breakage during use.

5. Improved machinability:
Molybdenum can also improve the machinability of high-speed steels, making them easier to machine into complex shapes and reducing tool wear during the machining process.

Conclusion:
In summary, molybdenum is an excellent alloying element that can replace tungsten in high-speed steels. It provides increased hardenability, improved high-temperature strength, increased red hardness, enhanced toughness, and improved machinability. These properties make molybdenum a valuable addition to high-speed steels, allowing them to perform effectively in demanding cutting tool applications.

Carbon Equivalent and Weldability of Steel

The carbon equivalent (CE) is a parameter used to measure the weldability of steel. Weldability refers to the ease with which a material can be welded without developing defects or experiencing problems during the welding process. The carbon equivalent value helps to predict the risk of cracking and other issues that may arise during welding.

Definition of Carbon Equivalent (CE)

The carbon equivalent is calculated based on the chemical composition of the steel, particularly the carbon content and the presence of other alloying elements. It is a numerical value that indicates the relative contribution of carbon and other elements to the weldability of the steel.

Range of Carbon Equivalent for Good Weldability

For good weldability, the carbon equivalent (%) of steel should be in the range of 0.2 - 0.4. This means that if the carbon equivalent value falls within this range, the steel is considered to have good weldability.

Explanation of the Correct Answer

The correct answer, option 'A' (0.2 - 0.4), is the range of carbon equivalent values that indicate good weldability. This range is widely accepted and used in various welding codes and standards.

Reason for the Range of 0.2 - 0.4

- Low Carbon Equivalent: A low carbon equivalent value indicates a low risk of cracking and other welding-related issues. This is because a lower carbon equivalent means a lower carbon content in the steel, which reduces the likelihood of hardening and cracking during welding.

- Optimum Carbon Equivalent: The range of 0.2 - 0.4 is considered the optimum range for good weldability. It strikes a balance between reducing the risk of welding defects and maintaining desirable mechanical properties in the welded joint.

- Effect of Alloying Elements: The presence of alloying elements such as manganese, silicon, and other elements affects the carbon equivalent value. These elements can help reduce the carbon equivalent and improve the weldability of the steel.

- Consideration of Welding Process: The carbon equivalent value is also influenced by the specific welding process being used. Some welding processes, such as high heat input processes, may require a lower carbon equivalent to ensure good weldability.

Conclusion

In summary, the carbon equivalent is an important parameter for assessing the weldability of steel. A carbon equivalent value in the range of 0.2 - 0.4 indicates good weldability, as it reduces the risk of welding defects while maintaining desirable mechanical properties in the welded joint. It is crucial to consider the carbon equivalent when selecting steel for welding applications to ensure successful and reliable welds.

Understanding Cold Working of Steel
Cold working refers to the process of deforming metal at temperatures below its recrystallization point. For steel, this generally occurs at room temperature or slightly above.
Definition and Characteristics
- Temperature Range: Cold working is performed below the recrystallization temperature, typically less than 0.6 times the melting temperature of the steel.
- Deformation Mechanism: The process involves plastic deformation, where the steel is shaped or stretched, leading to an increase in dislocation density within the metal’s structure.
Benefits of Cold Working
- Increased Strength: The dislocations created during cold working hinder the movement of other dislocations, resulting in work hardening, which significantly increases the strength and hardness of steel.
- Improved Surface Finish: Cold working processes like rolling and drawing can enhance the surface finish of the steel, making it smoother.
Applications
- Manufacturing: Cold working is widely used in manufacturing components such as wires, sheets, and rods, where precise dimensions and properties are crucial.
- Structural Integrity: The enhanced mechanical properties obtained through cold working make it suitable for applications requiring high strength, such as in aerospace and automotive industries.
Conclusion
Cold working is a vital process in metalworking that allows for the manipulation of steel without elevated temperatures, leading to improved mechanical properties and surface characteristics. Understanding the significance of this process is essential for materials engineers and manufacturers alike.

Hardenability
Hardenability is defined as the ability of a material to be hardened through heat treatment. It specifically refers to the ability of a structure to transform into martensite, a hard and brittle phase, when cooled rapidly from a high temperature.

Explanation
When a metal is heated to a high temperature and then rapidly cooled, it undergoes a phase transformation from austenite to martensite. Austenite is a high-temperature phase with a face-centered cubic (FCC) crystal structure, while martensite is a low-temperature phase with a body-centered tetragonal (BCT) crystal structure.

The formation of martensite is accompanied by a significant increase in hardness and strength. This is because the BCT crystal structure of martensite is highly distorted, leading to a high dislocation density and a high resistance to deformation. Therefore, the ability of a structure to transform into martensite directly affects its hardness and strength.

Factors Affecting Hardenability
The hardenability of a material depends on several factors, including:
1. Alloying elements: Alloying elements such as carbon, chromium, and nickel can significantly affect the hardenability of a material. For example, increasing the carbon content in steel increases its hardenability.
2. Cooling rate: The rate at which a material is cooled from the austenitizing temperature influences its hardenability. Rapid cooling, such as quenching in water or oil, promotes the formation of martensite.
3. Grain size: Fine-grained materials have higher hardenability compared to coarse-grained materials. This is because smaller grains provide more nucleation sites for the formation of martensite.
4. Heat treatment: Heat treatment processes such as quenching and tempering can be used to manipulate the hardenability of a material. Quenching involves rapid cooling to maximize the formation of martensite, while tempering is a subsequent heat treatment to improve toughness and reduce brittleness.

Importance of Hardenability
Hardenability is an important property to consider in the design and selection of materials. It determines the depth and distribution of the hardened layer, which affects the overall mechanical properties of the material. Materials with high hardenability are suitable for applications requiring high strength and hardness, such as cutting tools and gears. On the other hand, materials with low hardenability are preferred for applications that require increased toughness and resistance to fracture, such as structural components.

In conclusion, hardenability is the ability of a structure to transform into martensite, a hard and brittle phase, upon rapid cooling. It is influenced by factors such as alloying elements, cooling rate, grain size, and heat treatment. Hardenability directly affects the hardness and strength of a material, making it an important consideration in material selection and design.

Properties of Aluminium

Aluminium is a lightweight, strong, and durable metal that is widely used in various industries. Some of the properties of aluminium are:

1. Modulus of Elasticity is Fairly Low:
Aluminium has a relatively low modulus of elasticity compared to other metals. This means that it is more flexible and less stiff than materials like steel. However, this property also makes it more susceptible to deformation and bending under stress.

2. Wear Resistance is not Very Good:
Contrary to the statement given in the question, the wear resistance of aluminium is not very good. It has a relatively low hardness and can easily scratch, scuff, or dent under abrasive or impact forces. Therefore, it is often coated or treated with other materials to enhance its wear resistance.

3. Fatigue Strength is not High:
Aluminium has a low fatigue strength, which means that it can fail or crack under cyclic loading or repeated stresses. This property makes it less suitable for applications that require high durability and long-term reliability.

4. Corrosion Resistance is Good:
Aluminium has a natural oxide layer that forms on its surface when exposed to air or water. This layer provides a protective barrier against corrosion, rust, or tarnishing. Therefore, aluminium is often used in applications that require resistance to environmental or chemical exposure.

Conclusion:
In conclusion, the false statement about properties of aluminium is option 'B' that the wear resistance of aluminium is very good. Aluminium has a relatively low wear resistance and is susceptible to scratches, scuffs, or dents under abrasive or impact forces.

2

Puddling Process Overview
The puddling process is a crucial step in the metallurgy of iron, specifically for converting pig iron into wrought iron. This method involves the removal of excess carbon and impurities present in pig iron, resulting in a more malleable and ductile form of iron.
Key Steps in the Puddling Process
- Furnace Setup: The process takes place in a reverberatory furnace, where pig iron is heated to high temperatures.
- Oxidation: The pig iron is stirred to facilitate the oxidation of carbon and other impurities. This is essential for transforming the brittle pig iron into a workable form.
- Formation of Wrought Iron: As carbon is oxidized, it escapes as gas, and the remaining iron becomes purer. The final product is wrought iron, which is characterized by its low carbon content and improved mechanical properties.
Characteristics of Wrought Iron
- High Ductility: Wrought iron is known for its excellent ductility, allowing it to be easily shaped and worked.
- Corrosion Resistance: It exhibits better resistance to corrosion compared to cast iron due to its lower carbon content.
- Applications: Wrought iron is used in various applications, including construction, automotive components, and decorative items, owing to its strength and flexibility.
Conclusion
In summary, the puddling process is significant for converting pig iron into wrought iron, enhancing the material's properties and making it suitable for various applications in engineering and manufacturing industries.

Understanding Polyamides
Polyamides are a class of polymers characterized by the presence of amide linkages (-CONH-). They are known for their strength, durability, and resistance to wear and tear.
Polymers in Question
1. Nylon-6:
- A synthetic polymer made from caprolactam.
- Contains amide linkages, classifying it as a polyamide.
2. Nylon-6,6:
- Formed from hexamethylenediamine and adipic acid.
- Also contains amide linkages, making it another example of a polyamide.
3. Polyvinyl Chloride (PVC):
- A widely used plastic made from vinyl chloride monomer.
- It does not contain amide linkages and is not classified as a polyamide.
4. Polystyrene:
- A polymer made from the monomer styrene.
- Lacks amide linkages and is not classified as a polyamide.
Correct Answer Explanation
- The correct answer is option 'B' (Nylon-6 and Nylon-6,6).
- Both Nylon-6 and Nylon-6,6 are polyamides due to their amide linkages.
- Polyvinyl chloride and polystyrene do not fit into the polyamide category because they lack these linkages.
Summary
In conclusion, the only polymers that are classified as polyamides from the given options are Nylon-6 and Nylon-6,6. PVC and polystyrene do not meet the criteria for polyamides, thus reinforcing that option 'B' is the correct choice.