This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Cast Irons”.
1. How much carbon is present in cast irons?
a) Less than 0.05%
b) Up to 1.5%
c) 1.5% to 2%
d) More than 2%
Explanation: Cast Irons are ferrous alloys containing more than 2% of carbon, along with silicon, sulphur, manganese, and phosphorus. Maraging steels contain less than 0.05% carbon, while plain carbon steels contain up to 1.5% carbon.
2. Cast iron is a _____ alloy.
Explanation: Cast irons are eutectic alloys made of iron and carbon. Since iron is available in abundance, they are easy to find and are less expensive. Therefore cast irons are the cheapest of all metals.
3. Iron obtained from broken ______ is known as white iron.
Explanation: Upon, cementite becomes a silvery-white compound. This is due to the brittle nature of cementite. This fracture results in white cast iron.
4. If the iron surface contains graphite, it is known as ________
a) Alloy cast iron
b) White iron
c) Grey iron
d) Spheroidal graphite
Explanation: When an iron containing graphite is fractured, the surface becomes grey. This is due to the presence of graphite flakes in the iron structure. Hence, it is known as grey iron.
5. Which element causes cementite to behave in a stable manner?
Explanation: Sulphur has a stabilizing effect in cementite. This helps it easily break the cementite and produce white iron.
6. An iron with high-silicon content is a _______
a) White iron
b) Grey iron
c) Malleable iron
d) Pig iron
Explanation: Silicon causes cementite to behave in an unstable manner, which decomposes and releases graphite. If there is a high-silicon presence, more graphite is released and makes it a grey iron. If low-silicon is available, then it becomes a white iron.
7. What is the effect of phosphorus and sulphur in cast irons?
a) Induces brittleness
b) Increases strength
c) Destabilizes cementite
d) No effect
Explanation: Although sulphur stabilizes cementite, it causes brittleness. The same is true for phosphorus, due to which both elements are limited to a minimum quantity. Manganese is used to increase the strength of an iron, which also controls the harmful effects of sulphur.
8. Decomposition of cementite to form ferrite and graphite is known as _______
a) Decomposition of cast irons
b) Production of cast irons
c) Growth of cast irons
d) Prevention of growth of cast irons
Explanation: When cementite is heated at 700C it decomposes into ferrite and graphite. This causes it to occupy more space than the original structure. This phenomenon is known as the growth of cast irons.
9. Which of these are applications of grey cast iron?
a) Camshafts, engine blocks
b) Wear plates, pump linings
c) Brake shoes, pedals
d) Gears, rocker arms
Explanation: Grey cast irons have good strength and corrosion resistance. Therefore, they are used in camshafts and engine blocks. White cast irons are used for pump linings, whereas malleable cast iron is used for pedals and levers. Spheroidal graphite is used in gears and rocker arms.
10. Which of the following cast irons cannot be machined?
a) White cast iron
b) Grey cast iron
c) Malleable cast iron
d) Spheroidal graphite cast iron
Explanation: White cast irons are used where hardness and wear resistance matter, like grinding and crushing. Since it is hard, it cannot be machined. Grey and malleable cast irons, on the other hand, have excellent machinability. Spheroidal graphite cast irons also possess good machinability.
11. How are malleable cast irons designated for different grades?
a) By tensile strength
b) By six or seven-digit numbers
c) By five-digit numbers
d) By alphabets
Explanation: ASTM has designated the grading system for malleable cast irons in five-digit numbers. A grade 32510 has a yield strength of 32500 psi. Six or seven-digit numbers are used to designate spheroidal graphite cast irons (60-40-18 with ferritic structure). Grey cast irons are designated by their tensile strength (grade 20 denotes a tensile strength of 20000 psi).
12. What is the effect of Nickel on cast irons?
a) Stabilizes carbides
b) Increases hardness
c) Refines grain structure
d) Improves corrosion resistance
Explanation: Addition of nickel in cast irons refines the grain structure, and has a graphitizing effect on cementite. Molybdenum and Vanadium are used to increase the hardness of cast irons, while Chromium stabilizes carbides. Copper improves the resistance to corrosion.
13. What is the defining property of Wrought Irons?
a) High carbon
b) Low carbon
c) No carbon
d) Completely carbon-filled
Explanation: Wrought irons are iron alloys contain a very low amount of carbon (less than 0.08%). They were used for producing nails, chains, and bolts initially, but aren’t used these days.
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