ISRO Mechanical-Engineering Contributed by Sunil updated on May 2019

 ISRO Practice Questions with answers for Mechanical Engineering 

 ISRO Mechanical engineering questions for practice

1.  Which is the common element between brass and bronze ?

Ans. Copper

 

2.  What does following alloy designation indicate FG 250 ?

Ans. Grey cast iron with tensile strength of 250 MPa.

 

3.  How is ceramic defined ?

Ans.  It is a solid formed by combination of metallic and non-metallic elements.

 

4.  Give one example of metal classified as per structure as BCC, FCC, HCP and CCP.

Ans.  BCC (body centred cubic) structure — Molybdenum

FCC (face centred cubic) structure — Aluminium

HCP (hexagonal closed packed) structure — Zinc

CCP (cubic closed packed) structure — Copper.

 

5.  What is the name of solid solution of carbon in alpha iron and delta iron ?

Ans.  Ferrite and austenite respectively.

 

6.  Explain the difference between pearlite and cementile ?

Ans.  Pearlite is eutectoid mixture of ferrite and cementile.  Cementite is chemical compound of iron and carbon.

 

7.   Give one example each of the following proportion of materials-dimensional, physical, technological and mechanical.

Ans.  Roughness, enthalpy, toughness, and hardness respectively.

 

8.  For which parts the Wahl factor and Lewis form factor used ?

Ans.  For springs and gears respectively.

 

9.  How oxygen can be removed from steel during melting ? What are fully killed steels ?

Ans.  Oxygen can be removed by adding elements such as manganese, silicon or aluminium which, because of their high affinity for oxygen, react with it to form non-metallic oxides which rise into the slag.  Steels which have had most of their dissolved oxygen removed are called "fully killed steels".

 

10.  Hydrogen cannot be removed easily from molten steel. What harm hydrogen has on property of steel ?

Ans.  Execessive hydrogen results in the formation of small fissures often described as hairline cracks or flakes in the steel. Large forgings in alloy steel are particularly sensitive to this phenomenon.

 

11.  What is allotrope ?  In what forms of cubic pattern, iron exists ?

Ans.  Some elements exist in more than one crystalline form.  Each form is known as "allotrope".  Iron exists in two forms of cubic pattern, namely body centered cubic (bcc) and face-centered cubic (fcc).

 

12.  What is the difference between alpha iron, delta iron and gamma iron ?

Ans.  The bcc form of iron exists between room temperature and 9100 C, and between 14000 C and the melting point at 15390 C. The lower temperature form is know as "alpha"-iron and the higher temperature form as "delta"-iron.  The face-centered cubic form existing between 910 C and 1400 C is referred to as "gamma-iron".

 

13.  Metals, in general are of low strength and do not possess required physio-chemical and technological properties for a definite purpose. Alloys are therefore more than metals alone.  Discuss the arrangement of atoms and structures of alloys.

Ans.  Alloys are produced by melting or sintering two ore more metals, or metals and a non metal, together.  Alloys possess typical properties inherent in the metallic state. The chemical elements that make up an alloy are called its components.  An alloy can consist of two or more components.  The phase and structures of alloys describe the constitution, transformations and properties of metals and alloys.  A combination of phases in a state of equilibrium is called a system. A phase is a homogeneous portion of a system having the same composition and the same state of aggregation throughout its volume, and separated from the other portions of the system by interfaces.  For instance, a homogeneous pure metal or alloy is a single-phase system.  A state in which a liquid alloy (or metal) coexists with its crystals is a two-phase system. Structure refers to the shape, size or the mutual arrangement of the corresponding phases in metals or alloys.  The structural components of an alloy are its individual portions, each having a single structure with its characteristic features.

 

14.  What is the difference between isotropic material and homogeneous material ?

Ans.  In homogeneous material the composition is same throughout and in isotropic material the elastic constants are same in all directions.

 

15.  Explain the difference between the points of inflexion and contra flexure.

Ans.  At points of inflexion in a loaded beam the bending moment is zero and at points of contra flexure in loaded beam the bending moment changes sign from increasing to decreasing.

 

16.  What is the difference between proof resilience and modulus of resilience ?

Ans.  Proof resilience is the maximum strain energy that can be stored in a material without permanent deformation. Modulus of resilience is the maximum  strain energy stored in a material per unit volume.

 

17.  What is the difference between column and strut ?

Ans.  Both column and strut carry compressive load. Column is always vertical but strut as member of structure could carry axial compressive load in any direction.

 

18.  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.

 

19.  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 temperature 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 periectic 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.

 

20.  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 temperatures).

 

21.  Draw iron/iron carbide equilibrium diagram.

 

 22.  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.

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

Ans.  4%

 24.  What is stellite ?

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

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

Ans.  Gamma rays.

 26.  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.

 27.  What is critical temperature in metals ?

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

 28.  Car tyres are usually made of ________?

Ans.  Styrene-butadine rubber.

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

Ans.  Ferrite and it is soft.

 30.  Which elements increase the corrosion resistance of steel ?

Ans.  Chromium and nickel.

 31.  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.

 32.  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.

 33.  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.

 34.  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.

 35.  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.

 36.  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.

 37.  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.

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