Engineering Materials MCQ Questions & Answers | Mechanical Engineering

The lower critical point for all steels is at 723°C

Tensile strength of steel can be safely increased by Adding carbon up to 0.83%

Monel is a group of nickel alloys, primarily composed of nickel and copper, with small amounts of iron, manganese, carbon, and silicon.

Hypo-eutectoid steel has less than 0,8% of Carbon in its composition. It is composed by pearlite and α-ferrite

Spheroidizing of high carbon steel is a method of prolonged heating at a temperature below the eutectoid temperature.
By heating at this temperature pearlite, which is the lowest energy arrangement of steel, gets converted to ferrite and cementite. .
However, the process of spheroidizing does consume a lot of energy.

Lead is poured into the joint between two pipes. These pipes may be made of Cast iron
Cast iron has a much higher melting point than lead.

a copper–nickel alloy used in electrical work for its high resistance. It usually consists of 55% copper and 45% nickel

The correct answer is B: Martensite.

Let's break down why:

Hardness refers to a material's resistance to indentation or scratching. A harder material is more difficult to scratch or dent.

Now, let's look at each option:

A: Austenite
Austenite is a phase of steel that exists at high temperatures. It's relatively soft and ductile (easily shaped). So, it *decreases* hardness.

B: Martensite
Martensite is formed when austenite is rapidly cooled (quenched). This rapid cooling traps carbon atoms within the iron crystal structure, causing significant distortion. This distortion makes it very hard and brittle.

C: Pearlite
Pearlite is a mixture of ferrite (soft iron) and cementite (iron carbide). While it's harder than ferrite, it's not as hard as martensite.

D: Cementite
Cementite (Fe3C) is a very hard and brittle compound of iron and carbon. However, in steel, its *dispersion* and *form* play a crucial role in overall hardness. While cementite itself is hard, it's the *martensitic transformation* that leads to a significant and more effective increase in steel's hardness.

Therefore, Martensite contributes the most to increasing the hardness of steel due to its unique crystal structure and the way it traps carbon atoms.

As the iron cools further to 1,394°C its crystal structure changes to a face centered cubic (FCC) crystalline structure. In this form it is called gamma iron (γ-Fe) or Austenite. γ-iron can dissolve considerably more carbon (as much as 2.04% by mass at 1,146°C). This γ form of carbon saturation is exhibited in stainless steel.

As molten iron cools down, it solidifies at 1,538°C (2,800°F) into its δ allotrope, which has a body-centered cubic (BCC) crystal structure. δ-iron can dissolve as much as 0.08% of carbon by mass at 1,475°C.