Mechnical Engineering Solved Papers

1.  What do you understand by points  Ac₁, Ac₃, Ac₄, Ar₁, Ar₃ and  Ar₄.

Ans.  Ac₁ represents the beginning of transformation of ferrite to austenite on heating.  Ac₃ the end of transformation of ferrite to austenite on heating and  Ac₄ the change from austenite to delta iron on heating. On cooling the critical points are referred to as Ar₄, Ar₃ and Ar₁ respectively.

 2.  What is the percentage of chromium in 18 : 4 : 1 HSS ?

Ans.  4%

 3.  What is stellite ?

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

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

Ans.  Gamma rays.

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

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

 6.  What is critical temperature in metals ?

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

 7.  Car tyres are usually made of ________?

Ans.  Styrene-butadine rubber.

 8.  What is the structure of pure iron and whether it is soft or hard ?

Ans.  Ferrite and it is soft.

9.  Which elements increase the corrosion resistance of steel ?

Ans.  Chromium and nickel.

10.  What causes hardness in steel ? How heat treatment alters properties of steel ?

Ans.  The shape and distribution of the carbides in the iron determines the hardness of the steel.  Carbides can be dissolved in austenite is the basis of the heat treatment of steel.  If steel is heated above the A₁ critical temperature to dissolve all the carbides, and then cooled, suitable cooling thought the cooling range will produce the desired size and distribution of carbides in the ferrite, imparting different properties.

11.  Explain the formation of microstructures of pearlite, bainite and martensite in steel.

Ans.  If austenite containing about 0.80 percent carbon is slowly cooled through the critical temperature, ferrite and cementite are rejected simultaneously, forming alternate plates or lamellae.  This microstructure is called pearlite.  At temperatures just belot the A₁ the transformation from austenite to pearlite may take an appreciable time to initiate and complete, but the product will be lameller pearlite.  As the transformation temperature is lowered, the time to initiate transformation shortens but the product is pearlite of increasing fineness, and at temperatures approaching 550⁰ C it cannot be resolved into its lamellar constituents.  Further decrease in transformation temperature causes a lengthening of the incubation period and a change in structure of the product to a form known as "bainite". 

        If the temperature is lowered sufficiently, the diffusion controlled nucleation and growth modes of transformation are suppressed completely and the austenite transforms by a diffusionless process in which the crystal lattice effectively shears to a new crystallographic configuration known as  "martensite".  This phase has a tetragonal crystal structure and contains carbon in supersaturated solid solution.

 12.  How with alloying of steel it is possible to a achieve properties which can not be achieved with heat treatment ? 

Ans.   A prerequisite to the hardening of steels is that martensite should be formed on cooling, but this can only be achieved if the rate of cooling is great enough to suppress the formation of pearlite or bainite and in plain carbon steels this can be achieved by quenching relatively small specimens in water.  Larger specimens, however, cannot be cooled sufficiently rapidly though the whole section because of the limited rate at which heat can be extracted through the surface of the piece. However, this difficulty can be overcome by alloying the steels to modify their transformation characteristics.  For example, some alloying additions such as nickel and molybdenum reduce the rate of diffusion so that pearlite formation is  suppressed in favour of the martensitic transformation.

13.  What are the major effects of alloying elements ?

Ans.   (1)  To alter the transformation temperatures and times

(2)  To modify the room temperature and elevated temperature strengths of given structures by (a) stiffening the crystals and (b) introducing complex precipitates which tend to harden the steel.

(3)  To modify the type of oxide film formed on the surface of the steel and thereby affect its corrosion resistance.

 14.  What is the difference between austenite stabilisers and ferrite stabilisers ?

Ans.   Austenite stabilisers  have the effect of extending the temperature range over which austenite is formed.  Such elements are carbon, manganese, nickel, copper and cobalt.

Ferrite stabilisers  have the effect of extending the temperature range over which alpha and delta ferrite are formed, which consequently reduces temperature range over which austenite is formed.  Such elements are silicon, chromium, molybdenum, tungsten, titanium and niobium.

 15.  What are the effects of carbon on the properties of steel.

Ans.   In general, an increase in carbon content produces higher ultimate strength and hardness but lowers ductility and toughness of steel alloys.  Carbon also increases air-hardening tendencies and weld hardness, especially in the presence of chromium.  In low-alloy steel for high-temperature applications, the carbon content is usually restricted to a maximum of about 0.15% in order to assure optimum ductility for welding, expanding, and bending operations.  To minimize intergranular corrosion caused by carbide precipitation, the carbon content of austenitic (18-8 type) alloys is limited in commercial specifications to a maximum of 0.08%, or even less, i.e. 0.03% in the extremely low - carbon grades used in certain corrosion-resistant applications.

        In plain carbon steels in the normalised condition, the resistance to creep at temperatures be low 440⁰ C appears to increase with carbon content up to 0.4% carbon, at higher temperatures there is but little variation of creep properties with carbon content.

        An increase in carbon content lessens the thermal and electrical conductivities of steel and increases its hardness on quenching.

16.  What is the role of silicon as alloying element in steels ?

Ans.  Silicon contributes greatly to the production of sound steel because of its deoxidizing and degasifying properties.  When added in amounts up to 2.5%, the ultimate strength of the steel is increased without loss in ductility.  Silicon in excess of 2.5% causes brittleness, and amounts higher than 5% make the steel non-malleable.

        Resistance to oxidation and surface stability of steel are increased by the addition of silicon.  These desirable effects partially compensate for the tendency of silicon to lower the creep properties of steel.  Silicon  increases the electrical resistivity of steel and decrease hysteresis losses.