Metals and Engineering Alloys II

Question 1

What is the carbon content of Steel

Answer 1

0.04% - 1.7%

Question 2

What is the carbon content of Cast Iron

Answer 2

More than 1.7%

Question 3

What is the carbon content of Low Carbon Steel, Medium Carbon Steel and High Carbon Steel.

Answer 3

Low 0.04% - 0.3%
Medium 0.3% - 0.7%
High 0.7% - 1.7%

Question 4

What is the composition of the additional alloy metals in stainless steel?

Answer 4

18% Cr and 8% Ni
(in high carbon steel)
Molybdenum may also be added for corrosion resistance

Question 5

What are the three forms of pure iron

Answer 5

Ferrite, Austenite and δ-Fe

Question 6

What is the third phase that can form in Fe-C alloys

Answer 6

This is the compound Fe3C, known as cementite. It has a fixed content of 6.7% C by weight and is a hard, brittle compound. The mechanical properties of steel depend to a large extent on how much cementite exists in the microstructure.

Question 7

What is a eutectic point?

Answer 7

The point in a phase diagram indicating the chemical composition and temperature corresponding to the lowest melting point of a mixture of components. For a eutectic point, a liquid is in equilibrium with two solid phases..

Question 8

Where is the eutectic point in a Fe-C diagram

Answer 8

4.3% C

Question 9

Why is the eutectic point of Fe-C not suitable for engineering use?

Answer 9

It is too brittle

Question 10

What is the eutectoid point?

Answer 10

The point in a phase diagram indicating a solid is in equilibrium with two other solid phases. For the Fe-C system this is at 0.76 % C.

Question 11

What are the properties of Cementite (Fe3C) and Ferrite (α)

Answer 11

Cementite (Fe3C) is hard and brittle but by comparison, ferrite (α) is relatively soft and ductile.

Question 12

What is pearlite

Answer 12

Pearlite is the eutectoid structure, an equilibrium structure of alternating ferrite and cementite layers

Question 13

What is a hypo-eutectoid alloy

Answer 13

An alloy containing less than the eutectoid composition of carbon is known as hypo-eutectoid

Question 14

What is a hyper-eutectoid alloy

Answer 14

An alloy with more than the eutectoid composition of carbon is known as hyper-eutectoid.

Question 15

On cooling through the eutectoid temperature, what will a hypo-eutectoid alloy form?

Answer 15

A hypo-eutectoid alloy will cool to form a microstructure of ferrite and pearlite

Question 16

On cooling through the eutectoid temperature, what will a hyper-eutectoid alloy form?

Answer 16

A hyper-eutectoid alloy will cool to form a microstructure of cementite and pearlite.

Question 17

What is proeutectoid ferrite and proeutectoid cementite.

Answer 17

Ferrite (αp) and cementite (Fe3C)p formed before the formation of pearlite as the metal cools past the eutectoid temperature.

Question 18

What are typical applications of stainless steel

Answer 18

Typical applications where stainless steels are employed include: gas turbines, high-temperature steam boilers, heat-treating furnaces, aircraft, missiles, and nuclear power generating units.

Question 19

What are the five main types of cast iron?

Answer 19

Grey, nodular, white, malleable and compacted graphite iron

Question 20

What is the composition and formation of Grey Iron?

Answer 20

2.5-4% C and 1-3% Si
The graphite exists in the form of flakes, which are normally surrounded by an α-ferrite or pearlite matrix.
Mechanically, grey iron is comparatively weak and brittle in tension as a consequence of its microstructure; the tips of the graphite flakes are sharp and pointed, and may serve as points of stress concentration when an external tensile stress is applied.

Question 21

What are benefits and uses of Grey Iron?

Answer 21

Strength and ductility are much higher under compressive loads. Grey irons do have some desirable characteristics and, in fact, are utilized extensively. They are very effective in damping vibrational energy. Base structures for machines and heavy equipment that are exposed to vibrations are frequently constructed of this material. In addition, grey irons exhibit a high resistance to wear. Furthermore, in the molten state they have a high fluidity at casting temperature, which permits casting pieces having intricate shapes; also, casting shrinkage is low. Finally, and perhaps most important, gray cast irons are among the least expensive of all metallic materials.

Question 22

What is ductile (or nodular) iron?

Answer 22

Adding a small amount of magnesium and/or cerium to the grey iron before casting produces a distinctly different microstructure and set of mechanical properties. Graphite still forms, but as nodules or sphere-like particles instead of flakes. The resulting alloy is called nodular or ductile iron. The matrix phase surrounding these particles is either pearlite or ferrite, depending on the heat treatment.

Question 23

What is the composition and structure of White Iron?

Answer 23

For low-silicon cast irons (containing less than 1% Si) and rapid cooling rates, most of the carbon exists as cementite instead of graphite. A fracture surface of this alloy has a white appearance, and thus it is termed white cast iron. Thick sections may have only a surface layer of white iron that was “chilled” during the casting process; grey iron forms at interior regions, which cool more slowly.

Question 24

What are the mechanical properties of white iron?

Answer 24

As a consequence of large amounts of the cementite phase, white iron is extremely hard but also very brittle, to the point of being virtually unmachinable. Its use is limited to applications that necessitate a very hard and wear-resistant surface, without a high degree of ductility-for example, as rollers in rolling mills. Generally, white iron is used as an intermediary in the production of yet another cast iron, malleable iron.

Question 25

What is the composition of Compacted Graphite Iron?

Answer 25

As with grey, ductile, and malleable irons, carbon exists as graphite, which formation is promoted by the presence of silicon. Silicon content ranges between 1.7 and 3%, whereas carbon concentration is normally between 3.1 and 4%.

Question 26

What is the microstructure and characteristics of Compacted Graphite Iron?

Answer 26

Microstructurally, the graphite in CGI alloys has a worm-like shape. Compared to the other cast iron types, desirable characteristics of CGIs include the following:
• Higher thermal conductivity
• Better resistance to thermal shock (i.e., fracture resulting from rapid temperature changes)
• Lower oxidation at elevated temperatures Compacted graphite irons are now being used in a number of important applications-these include: diesel engine blocks, exhaust manifolds, gearbox housings, brake discs for high-speed trains, and flywheels.

Question 27

What are limitations of ferrous alloys?

Answer 27

(1) a relatively high density, (2) a comparatively low electrical conductivity, and (3) an inherent susceptibility to corrosion in some common environments

Question 28

What is the difference between cast and wrought alloys?

Answer 28

On occasion, a distinction is made between cast and wrought alloys. Alloys that are so brittle that forming or shaping by appreciable deformation is not possible ordinarily are cast; these are classified as cast alloys. On the other hand, those that are amenable to mechanical deformation are termed wrought alloys.

Question 29

What are advantages of copper and its alloys.

Answer 29

Unalloyed copper has an almost unlimited capacity to be cold worked. Furthermore, it is highly resistant to corrosion in diverse environments including the ambient atmosphere, seawater, and some industrial chemicals. The mechanical and corrosion-resistance properties of copper may be improved by alloying. Most copper alloys cannot be hardened or strengthened by heat-treating procedures; consequently, cold working and/or solid-solution alloying must be utilized to improve these mechanical properties. The most common copper alloys are the brasses for which zinc, as a substitutional impurity, is the predominant alloying element. Some of the common brasses are yellow, naval, and cartridge brass, muntz metal, and gilding metal. Some of the common uses for brass alloys include costume jewelry, cartridge casings, automotive radiators, musical instruments, electronic packaging, and coins. The bronzes are alloys of copper and several other elements, including tin, aluminum, silicon, and nickel. These alloys are somewhat stronger than the brasses

Question 30

What are the advantages and limitations of Aluminium and its alloys.

Answer 30

Aluminum and its alloys are characterized by a relatively low density (2.7g/cm3 as compared to 7.9 g/cm3 for steel), high electrical and thermal conductivities, and a resistance to corrosion in some common environments, including the ambient atmosphere. Many of these alloys are easily formed by virtue of high ductility; this is evidenced by the thin aluminium foil sheet into which the relatively pure material may be rolled. Since aluminum has an FCC crystal structure, its ductility is retained even at very low temperatures. The chief limitation of aluminum is its low melting temperature (660◦C),which restricts the maximum temperature at which it can be used. The mechanical strength of aluminum may be enhanced by cold work and by alloying; however, both processes tend to diminish resistance to corrosion.

Question 31

What are the advantages and limitations of magnesium and its alloys.

Answer 31

Perhaps the most outstanding characteristic of magnesium is its density, 1.7 g/cm3, which is the lowest of all the structural metals; therefore, its alloys are used where light weight is an important consideration (e.g., in aircraft components). Magnesium has an HCP crystal structure, is relatively soft, and has a low elastic modulus: 45 GPa. At room temperature magnesium and its alloys are difficult to deform; in fact, only small degrees of cold work may be imposed without annealing. Consequently, most fabrication is by casting or hot working at temperatures between 200 and 350◦C. Magnesium, like aluminum, has a moderately low melting temperature (651 ◦C).

Question 32

Describe the micro-structure of bainite.

Answer 32

Bainite forms via the transformation from austenite. Like pearlite, it consists of the two phases, ferrite and cementite (Fe3C). However, the structure of Bainite is very fine, being resolved only by an electron microscope. Bainite forms as needles or plates, depending on the transformation temperature. It forms at lower temperatures than pearlite through more rapid cooling.

Question 33

Describe the micro structure of martensite.

Answer 33

If quenched,then there will be no time for atoms to diffuse through the structure. If pure iron is cooled from within the γ field very fast, there is no time for the Fe atoms to diffuse to change from γFe (FCC) to α-Fe (BCC), so γ-Fe (FCC) persists to 550 °C. Below 550 °C, γFe is so unstable that small regions within crystals change by displacement, not diffusion. This single phase structure is called Martensite. The structure consists of needles of Martensite surrounded by the lighter γ that failed to transform. Martensite is very hard and strong but very brittle.