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(a) Describe the steps usually considered in the process of material selection.
(b) How material selection is done using decision theory.
Ans. (a) Like any other aspect of engineering design, material selection is also equally important aspect. Material of part depends on the requirements of the part, number of parts needed, and manufacturing process to be adopted to make the part. The material selection primarily depends upon its properties and several other factors like availa
Temperature has pronounced effect on the properties of zinc and zinc alloys. The creep resistance, in particular, decreases rapidly with increasing temperature. Ductility and general fabricating characteristics increase with temperature. Drawing and forming operation should never be attempted below 20°С. More severe operations can be performed readily at temperatures above 50°C. Zinc alloys become somewhat brittle below 0°C, depending on the particular composition, but recover their normal prope
Season cracking is spontaneous cracking which occurs on exposure to atmospheric corrosion in brass objects with high residual tensile stresses at the surface. It occurs with high-zinc brasses but rarely with 15 per cent zinc or below. Alloys susceptible to spontaneous season cracking will crack when exposed to corrosive conditions under high service stress, even if they are free from internal strains. Season cracking can be prevented by avoiding the production of internal microstresses or by rem
State, with reasons, suitable material for the manufacture of each of the following, giving the approximate composition :
(a) a brass for deep pressed containers; (b) a brass for small machined bolts ; (c) a bronze for the impeller of a sea-water pump.
(a) The suitable brass for deep pressed containers is the admiralty brass containing copper and zinc in the proportion of 7 to 3, with 1% of tin.
The addition of tin to the basic cartridge brass (70% copper, 30% zinc), gives
(a) What is the main difference between a brass and a bronze ?
(b) State the composition, and two common uses of a bronze commonly used in engineering.
(c) What is the general effect of adding a small proportion of (1) phosphorus, (ii) lead, to a bronze ?
(a) A brass is an alloy of copper and zinc, the major constituent being copper. A bronze is an alloy of copper and tin, the major constituent being copper.
(6) A bronze commonly used in engineering and commonly known
(a) Which two elements are alloyed to form a brass ?
(b) What are the differences (i) in composition, (ii) in uses, of an 'alpha' and an 'alpha-beta' brass ?
(a) A brass is essentially an alloy having major constituents as copper and zinc. Minor proportions of elements such as lead and tin may be included to promote special properties.
(b) An alpha brass is an alloy in which zinc does not exceed by 38%. This brass is ductile and, therefore, used for cold-rolled sheets, wire
What do you understand by a free-cutting steel? What elements are usually added to make a steel free cutting, and how they make the steel free. cutting?
A free-cutting steel is one in which an element like lead or sulphur is deliberately added to promote rapid machining
When sulphur is added to steel, it forms a brittle constituent with manganese, known as manganese sulphide, which being brittle, allows chip cracks to propagate and breaks chips into easily handled lengths.
(i) Permalloy, (ii) Invar, (iii) Elinvar, (iv) Platinite, (v) Dumet wire. Ans. (i) Permalloy has 78.5% nickel. It has a high permeability and low hysteresis loss.
(i) Invar contains 36% nickel and has an exceedingly low coefficient of linear expansion. Since its expansion is proportional to the temperature within limits of atmospheric temperature change, it is used for secondary standards of length.
(ii) Elinvar contains 32% nickel with small percentage of Cr, W, Mn, Si and C. In addit
Music wire is a previously treated steel wire used for small helical springs. It requires no heat-treatment except a low-temperature anneal to relieve the forming strains. Such a wire is given a special heat treatment called patenting and then cold-rolled to develop a high yield strength.
Since plain carbon steels are cheapest and easiest to fabricate; first attempt should be to select the material from this category. The next choice should be for oil-or air-hardening steels which have better wear resistance than plain carbon steel but toughness is not good. These offer maximum safety in hardening and minimum dimensional change after heat treatment. These are preferred for dies with adjacent thin and thick sections, sharp corners, or numerous holes. These are, however, not suited
(a) What is the difference between sheet and strip? (b) What are the characteristics of cold rolled sheets ?
(c) What do you understand, by (i) orange-peel effect, (ii) stretcher strains or Luders' lines ?
(d) Define (i) temper rolling, and (ii) aging in steel.
(a) The difference between sheet and strip is based on width and is arbitrary.
(b) Cold working produces a better surface finish, improves the mechanical properties, and permits the rolling of thinner gauge mat
Grey cast iron can be welded with oxyacetylene gas or with electric arc with special precautions (preheating in case of gas welding and post heating in arc welding) to avoid cracking in welding area. Welding of gray iron is usually restricted for repair work. Braze welding with bronze or nickel copper, is however, used for fabrication also.
Malleable irons are not considered weldable. It is repaired by brazing, preferably silver brazing.
Ductile cast iron needs special considerations i
Ans. It has been observed that hydrogen (even as low as 5 ppm) causes internal flakes in large steel sections, and by pouring the liquid steel in vacuum chamber, it is possible to reduce hydrogen content to 1 ppm. Such vacuum castings are used extensively for large forgings such as electrical rotors.
Sometimes to reduce oxygen content to 0.001%, carbon deoxidation is carried out by removal of oxygen as carbon monoxide. This treatment substantially decreases the number of non-metallic inclus
Hot-rolled steels contain an oxide scale over their surface. It is formed during the heating process. Accordingly dimensional tolerances are not tight. In cold-rolled steels, the scale is removed before cold rolling process and thus the surface finish is very good in this case. Thus its dimensions are held within closer tolerances. Low-carbon cold rolled steels are easier to machine.