Question Set 92

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

Ans: Ferrite and it is soft.

2.Which elements increase the corrosion resistance of steel ?

Ans: Chromium and nickel.

3.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 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.

4.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 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 ncubation 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.

5.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

6.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.

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

Ans: 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 areformed, which consequently reduces temperature range over which austenite is formed. Such elements are silicon, chromium, molybdenum, tungsten, titanium and niobium.

8.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 corro¬sion 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.

9.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 inductility. 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.

10.Discuss the role of manganese in alloying steels.

Ans: 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.

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