Question Set 91

1.Explain the difference between ferrite, austenite and graphite ?

Ans: Ferrite is the solid solution of carbon and other constituents in alpha-iron. It is soft, ductile and relatively weak. Austenite is the solid solution of carbon and other constituents in gamma-iron. It exists in ordinary steels at elevated temperatures, but it is also found at ordinary temperatures in some stainless steels. Graphite has a hexagonal layer lattice. ‘

2.Explain the terms solid solution, eutectic, eutectoid and peritectic.

Ans: Solid Solution. When a homogeneous mixture of two (or more) atomic forms exists in solid state, it is known as solid solution.Eutectic. A mixture of two (or more) phases which solidify simultaneously from the liquid alloy is called an eutectic. Alloys in which the components solidify simultaneously at a constant tem¬perature the lowest for

the given system, are called eutectic alloys.

Eutectoid. Eutectoid alloys are the alloys for which two solid phases which are completely soluble become completely insoluble on cooling before a certain temperature called eutectoid temperature. Peritectic. A peritectic transformation involves a reaction between a solid and liquid that form a different and new solid phase. This three phase transformation occurs at a point called peritectic point.

3.What do you understand by critical points in iron, iron-carbide diagram ?

Ans: The temperatures at which the phase changes occur are called critical points (or tem¬peratures).

4.Why PERT is preferred over CPM for evaluation of project ?

Ans: PERT is based on the approach of multiple time estimates for each activity.

5.What is the percentage of chromium in 18 : 4 : 1 IISS ?

Ans: 4%.

6.What is stellite ?

Ans: It is a non-ferrous cast alloy containing cobalt, chromium and tungsten.

7.Which rays are produced by cobalt-60 in industrial radiography ?

Ans: Gamma rays.

8.What are killed steels and what for these are used ?

Ans: Killed steels are deoxidised in the ladle with silicon and aluminium. On solidification no gas evolution occurs in these steels because they are free from oxygen.

9.What is critical temperature in metals ?

Ans: It is the temperature at which the phase change occurs in metals.

10.Car tyres are usually made of ?

Ans: Styrene-butadine rubber.