material assignment 2

8/9/2019 Material Assignment 2

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Material Science 1Assignment 2

Ans. 1) a) TTT Curve Diagrams: When plain carbon steel in the austenitic state is quenched to roomtemperature, it changes very quickly to martensite. However quenching to temperatures in the 250 o

550 oC range, give structures intermediate between martensite and pearlite since there isinsufficient atomic mobility to form the cementite laminations of pearlite. The carbide is thenprecipitated as fine platelets.

In each case there is a time lag or incubation period before the austenite starts to transform, and thetransformation itself takes a definite time. This is shown by an TTT Curve diagrams which is thecritical cooling rate. The slower the critical cooling rate the better is the hardenability of the steel.

b) Advantages of Titanium alloys: Such alloys have very high tensile strength and toughness (even atextreme temperatures), light weight, extraordinary corrosion resistance, and ability to withstandextreme temperatures.For most applications titanium is alloyed with small amounts of aluminium and vanadium, typically6% and 4% respectively, by weight. This mixture has a solid solubility which varies dramatically withtemperature, allowing it to undergo precipitation strengthening

Applications: Used for military , aircraft, spacecraft, medical devices, connecting rods onexpensive sports cars and some premium sports equipment and consumer electronics

c) Surface hardening of plain low carbon steel: Since such steels have low carbon content, they musthave a hard surface or skin induced around them by a method known as case hardening .

The steel to be case hardened is packed in a box containing a carbon rich compound and heated in afurnace. After soaking, the part is quenched with the resulting structure having a very tough coreand a hard outer case.

d) Nitriding: It is a process which introduces nitrogen into the surface of a material which requires theuse of special steels- nitralloy steels eg. Al, Cr, V.

Processes are named by the medium used to donate nitrogen. The three main methods used are:

Gas nitriding

Liquid or salt bath nitriding


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Ion or plasma nitriding

Gas nitriding

In gas nitriding the donor is a nitrogen rich gas usually ammonia (NH 3). When ammonia comes into

contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then

diffuses from the surface into the core of the material.

Salt bath nitriding

In salt bath nitriding the nitrogen donating medium is a nitrogen containing salt such as cyanide salt.

The salts used also donate carbon to the workpiece surface making salt bath a nitrocarburising

process. The temperature used is typical of all nitrocarburising processes; 550-590 oC.

Plasma nitriding

For this process an ionized gas like nitrogen is used in a vacuum chamber at a low pressure when

the electronic characteristics of the gas come into act making it penetrate the surface of the materialbeing treated.

e) Advantage of using brine: Brine is the result of dissolving common rock salt in water. This mixture

reduces the absorption of atmospheric gases that, in turn, reduces the amount of bubbles. As aresult, brine wets the metal surface and cools it more rapidly than water. In addition to rapid anduniform cooling, the brine removes a large percentage of any scale that may be present.

f) Modification with respect to aluminium and silicon alloys: It means to impart the desiredproperties in aluminium and silicon alloys with the addition of compounds such as p-bis(dimethylsilyl) benzene, tris(dimethylsilyl) amine, triethylsilane, etc. They increases the tensile

strength of the alloys.

They reduce the scrap rate and help increase the productivity by:-

y Improving mechanical propertiesy Improving machinabilityy Reducing hot tearingy Reducing the time required for heat treatment processesy Controlling porosity distributiony Reducing the standard deviationy Reducing die stickingy Suppressing primary silicon formation


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g) Interstitial as applied to metallic lattice structure: is related to a crystalline compound in whichusually small atoms or ions of a non-metal occupy holes between the larger metal atoms or ions inthe crystal lattice. So it is the space available for the impurities between the crystalline latticestructures.

h) Unsuitability of tough pitch copper for welding: Since it contains 0.02-0.05% oxygen (as cuprousoxide Cu 2O), using it in reducing atmosphere above 400

oC as in gas welding, makes the oxygenpresent to give steam which cannot escape (molecules too large for diffusion) and therefore causesporosity known as gassing .

i) Advantages of grey cast iron:

y G ood compression strength and damping capacity (ability to absorb vibrations).y Easily machinable and is easier and cheaper to cast than steel (MP about 1150 oC as against

1550 oC for steel).

Applications:

y Machines bases and framesy Engine blocksy Manholes and drain covers

j) S-N Curves: S-N curves are derived from tests on samples of the material to be characterised,where a regular sinusoidal stress is applied by a testing machine which also counts the number of cycles to failure. These curves show that how the stress levels in the test specimen decrease with theincreases in the number of cycles.

Ans. 2) a) ii) Phase changes for a 80% Cu, 20% Zn composition alloy: A 80% Cu, 20% Zn compositionalloy starts its transformation from a liquid state into a liquid + alpha region at around 1050 oC whichcompletes at 1000 oC where it completely transforms into an alpha state.

iii) Unsuitability of gamma phase for commercial purposes:

y Brass undergoes maximum elongation at 30% Zn concentration.y The ultimate tensile strength is maximum at about 46% Zn concentration.y A good combination of strength and elongation is achieved at 40% Zn concentration.


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But, the gamma phase in brass is present when the concentration of Zn exceeds beyond 49% makingit loose both its strength and the ability to withstand elongation which makes it unsuitable forcommercial applications.

iv) Addition of lead to improve the machinability: The addition of lead upto 5% helps the chips tobreak easily during the machining of brass as it precipitates into soft inclusions which providelubrication and weak points in the chips.

v) Alloying elements that are added to Cu Zn alloys to form high tensile brasses:

y Manganese y Aluminium y Tiny Irony Nickel

material assignment 2

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