ENT145 Materials Engineering
Assignment 2 (Answer)
Q1. Briefly explain conditions of atomic motion in diffusion.
Answer:
1)
2)
There must be empty adjacent site.
Must have sufficient energy to break bonds with its neighbour atoms and then
cause some lattice distortion during the displacement.
Q2. Discuss the concept of non-steady state as it applies to diffusion. Give an example in
your answer.
Answer:
Q3. Low-alloy steel can be categorized into low-carbon steel, medium-carbon steel and
high carbon steel. For each of the low-alloy steel, describe the properties and its typical
applications.
Answer:
Low carbon steel
< 0.25wt%C
Medium carbon steel
High carbon steel
0.25-0.6wt% C
0.6 -1.4 wt% C
Bridge, towers, auto
structures sheet
Piston, gears, etc
Turbine, furnace etc
Q4. Briefly explain ductile iron and malleable iron.
Answer:
Q5. Cast irons are very important engineering materials. Briefly explain cast irons and their
basic range of composition. Give their applications and four basic types of cast irons.
Answer:
1. Cast irons are a family of ferrous alloys intended to be cast into a desired shape
rather than worked in the solid state.
2. These alloys typically contain 2 - 4 % C and 1 – 3 % Si. Additional alloying
elements may also be present to control or vary specific properties.
3. Cast irons are easily melted and highly fluid and do not form undesirable surface
films or shrink excessively; consequently, they make excellent casting irons. They
also possess a wide range of strength and hardness values and can be alloyed to
produce superior wear, abrasion, and wear resistance. In general, they are easy to
machine.
4. Their applications: engine cylinder blocks, gear boxes, connecting rods, valve and
pump casings, gears, rollers, and pinions.
5. The 4 basic types: white, gray, ductile and malleable.
Q6. Plain carbon and alloy steels are extensively used in manufacturing of bolts and screws.
Give five reasons for this.
Answer:
1. Bolts and screws must be made of materials that are high strength (to avoid failure)
2. High modulus of elasticity (to allow high pre-loads without yielding)
3. Resist fatigue failure (especially in applications that involve loading and unloading)
even in the presence of threads.
4. Be economical.
5. Have ease of manufacturing steels satisfy all of these conditions.
Q7. For a plain carbon steel having carbon content of 1 wt % at 900 °C. On average, how
many carbon atoms can you find in 100 unit cells? If at room temperature, the carbon
content of ferrite drops to 0.005 wt %, on average, how many unit cells would you have
to search to find one carbon atom? Briefly explain the differences in these two cases.
Answer:
Q8. Describe ductile, brittle and fatigue failures. Explain in terms of failure surfaces.
Answer:
Ductile Fracture:
– Accompanied by
significant plastic
deformation
Brittle Fracture:
– Little or no plastic
deformation
– Catastrophic
Q9. A fatigue test is made with a maximum stress of 120 MPa and a stress amplitude of 165
MPa. Calculate the maximum and minimum stresses, the stress ratio and the stress
range.
Answer: