Mechanical Basic Interview Question and Answers Part 6

Basic of Mechanical Engineering Interview, viva ,Oral Question and Answers Part 6

Mechanical Engineering Interview Technical questions and answers with explanation for Job interview, competitive examinations and entrance tests.

151. Car tyres are usually made of ?

Styrene-butadine rubber.

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

Ferrite and it is soft.

153. Which elements increase the corrosion resistance of steel ?

Chromium and nickel.

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

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 A critical temperature to dissolve all the carbides, and then cooled, suitable cooling through the cooling range will produce the desired size and distribution of carbides in the ferrite, imparting different properties.

155. Explain the formation of micro structures of pearlite, bainite and martensite in steel.

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 be lot 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 550°C it cannot be resolved into its lamellar constituents. Further deerease 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.

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

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

157. What are the major effects of alloying elements?

To alter the transformation temperatures and times
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.
To modify the type of oxide film formed on the surface of the steel and thereby affect its corrosion resistance.

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

Austenite stabilisers have the effect of extending the temperature range overwhich 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.

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

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 below 440°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.

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

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 decreases hysteresis losses.

161. Discuss the role of manganese in alloying steels.

Manganese is an excellent deoxidizer and sulfur neutralizer, and improves the mechanical properties of steel, notably the ratio of yield strength to tensile strength at normal temperatures. As an alloying element, manganese serves as an inexpensive means of preventing “hot shortness”. It improves rolling properties, hardenability, and resistance to wear. However manganese increases the crack sensitivity of weldments, particularly with steels of higher carbon content.

162. Define buckling factor.

It is the ratio of the equivalent length of column to the minimum radius of gyration.

163. What do you understand by catenary cable ?

A cable attached to the supports and carrying its own weight.

164. What is coaxing ?

It is the process of improving fatigue properties by first under-stressing and then increasing the stress in small increments.

165. What is difference between conjugate beam and continuous beam ?

A conjugate beam is an imaginary beam of same size as original beam and carrying a distributed load in accordance with the bending moment diagram.
A continuous beam is one which is resting on more than two supports.

166. What is isotropic material ?

It is a material having same elastic constants in all directions.

167. Explain difference between modulus of resilience and modulus of rigidity ?

Modulus of resilience is the maximum strain energy stored in a material per unit volume and modulus of rigidity is the ratio of shearing stress to the shearing strain within the elastic limit.

168. What is the difference between basic hole and basic shaft ?

A basic hole is one whose lower deviation is zero and in case of basic shaft the upper deviation is zero.

169. What for pyranometer is used ?

It is used to measure the total hemispherical solar radiation.

170. Describe transfer machines in brief.

It is an automatic machine in which workpiece along with fixture is transferred from one station to other automatically and several operation on workpiece are performed at each station.

171. What is burnt-out point ?

It corresponds to maximum heat flux at which transition occurs from nucleate boiling to film boiling.

172. What do you understand by eutectic ?

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

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

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.

173. 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 ?

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
(c) Relatively good erosion and corrosion resistance.
(d) High damping capacity, with respect to vibration.
(e) Relatively good mechanical properties under compressive loading.