Question Set 94

1.What do you understand by eutectic ?

Ans: It is mechanical mixture of two or more phases which solidify simultaneously from the liquid alloy.

2.Explain the difference between grey iron and white iron. What is mottled iron ?

Ans: The carbon in cast iron could exist at room temperature as either iron carbide, or as graphite which is the more stable form. Irons containing carbon as graphite are soft, easily machinable and are called “grey irons”. Irons with carbon present as iron carbide are extremely hard, difficult to machine and are called “white” irons. Irons with fairly equal proportions of graphite and iron carbide have intermediate hardness and are called “mottled” irons.

3.The graphite in grey irons exists in the form of flakes which act as stress-raisers under tensile loading and consequently grey irons have relatively low tensile strength and ductility. Still grey iron is extensively used in engineering. Why ?

Ans: Grey iron is extensively used in engineering because of following characteristics.

(a) Cheapness.

(B) Low melting point and high fluidity making it suitable for castings of intricate shape.

© Relatively good erosion and corrosion resistance.

(d) High damping capacity, with respect to vibration.

(e) Relatively good mechanical properties under compressive loading.

4.Under what condition a convergent divergent nozzle required ?

Ans: When pressure ratio is greater than critical pressure ratio.

5.What is endurance limit and what is its value for steel ?

Ans: Endurance limit is the maximum level of fluctuating stress which can be tolerated indefinitely. In most steels this stress is approximately 50% of the ultimate tensile strength and it is defined as the stress which can be endured for ten million reversals of stress.

6.How the net work to drive a compressor and its volumetric efficiency behave with increase in clearance volume ?

Ans: Work remains unaltered and volumetric efficiency decreases.

7.What do you understand by sulphur print ?

Ans: Sulphides, when attached with dilute acid, evolve hydrogen sulphide gas which stains bromide paper and therefore can be readily detected in ordinary steels and cast irons. While sulphur is not always as harmful as is sometimes supposed, a sulphur print is a ready guide to the distribution of segregated impurities in general.

8.What is the different between brass and bronze ?

Ans: Brass is an alloy of copper with zinc; and bronze is alloy of copper with tin.

9.What is the effect of addition of zinc in copper? What is the use of 70/30 brass ?

Ans: By addition of zinc in copper, both tensile strength and elongation increases. The 70/30 brass has excellent deep drawing property and is used for making radiator fins.

10.What for admirality brass used ?

Ans: Admirality brass with 29% zinc and 1% tin has good corrosion resistance and is used for condenser and feed heater tubes. Aluminium is also added to brass to improve corrosion resistance.