Mechinopedia For Mechanical Engineers

 The Simplest and most reliable of these methods is the Jominy or end quench test. The standard Jominy specimen consists of a cylindrical rod 100 mm long and 25 mm in diameter. In making a test, the specimen is first heated to a suitable  austenitic temperature and held there long enough to uniform austenitic structure. It is then placed in a jig and a stream of water is allowed to strike one end of the specimen. The experimental arrangement is shown in the image.  The advantage of the Jominy test is that in a single specimen one is able to obtain a range of cooling rates varying from a very rapid water quench at one end to slow air quench at the other end. Following the complete transformation of the austenite in the bar, two shallow flat surfaces are ground on opposite sides of the bar and Rockwell hardness values are taken at 1.5 mm Intervals along the bar. The results of the test are expressed by the hardenability number lc, in which l is the distance from the quenched end to the p

 Hardenability is that property of a steel which determines the depth and distribution of hardness obtained by the quenching. It is not an indication of the hardness. Hardenability is usually interpreted as the ability to become uniformly hard or to harden in depth. The depth of hardening is usually taken as distance from the surface to semi-martensitic zone, i.e., 50 per cent martensite plus 50 percent pearlite.  Full hardening of carbon steels is observed in articles of a diameter or thickness up to 25mm. Alloy steels harden to considerably larger depth due to the high stability of the super cooled austenite and correspondingly lower critical cooling rate. Therefore, alloy steels may be effectively hardened by quenching in oil instead of water. DETERMINATION OF HARDENABILITY : Several methods are used to determine the hardenability of steels, 1. By appearance of fracture. 2. By the distribution of the hardness along the cross-section and  3. By the end quench test.

 In austempering or Isothermal quenching the steel part is heated to the required hardening temperature and then quenched in a molten salt or lead bath usually at a high temperature, i.e, from 300*C to 350*C, than that is prescribed for martempering. The steel is held in the bath for as long a time as is needed for isothermal transformation of  the austenite , i.e, until transformation to bainite (acicular troosite) and not martensite is complete.  Although the steel is of same hardness as that of martensite, it is tougher and more ductile than other quenched and temper steels. Tempering is rarely needed after austempering. The foundation for the austempering process in based upon the austenitic transformation at constant temperature and is represented diagrammatically by TTT - curves. 

 Martempering or interrupted quenching is a hardening operation that produces martensite. It is not tempering. In this method the steel is heated to the hardening temperature and then quenched in a medium, usually in a salt bath, having a temperature, just above that where martensite starts to form (usually from 150*C to 300*C). The article is held until it reaches the temperature of the medium but not long enough to austenite decomposition. It is then cooled further to room temperature in air and sometimes in oil. The austenite is transformed to martensite during the last period of cooling to room temperature. This treatment will provide a structure of martensite and retained austenite in the hardened steel. Martempering has the following advantages over the conventional quenching : 1. Less volume changes occur due to the presence of a large amount of retained austenite. 2. Less warping since the transformations occur almost simultaneously in all parts of the article. 3. Less danger o

  Ferrite :   It is the grain or crystal of solid solution of carbon in alpha - iron. It is relatively soft (50-100 Brinell)., Ductile and strongly magnetic. Cementite : Cementite is a chemical compound of 93.33 per cent iron and 6.67 per cent carbon. It is identified as round particles in the structure. Being extremely hard ( about 1400 Brinell ) and Brittle. It has no ductility. Cementite becomes ferro - magnetic below 210*C. Pearlite : It is readily recognized by its pearly lustrous appearance and its structure of thin alternating plates of 13 per cent cementite in a matrix  of 87 per cent ferrite. Pearlite is a strong metal having 180 Brinell. Austenite : It is a solid solution of carbon in gamma-iron. It is generally soft and ductile than ferrite, but denser than ferrite , and is non- magnetic.  Sorbite : It is a finely dispersed pearlite, and the properties are intermediate between those of pearlite and troostite. The Brinell Hardness is 350. Troostite or Bainite : It is the most

This has a high melting point 1950*C and good strength, ductility and corrosion resistance especially to liquid sodium coolants, and excellent capability of coexistence with uranium. Its oxidation resistance above 400*C is indifferent, but is greatly improved by alloying.  The metal niobium, titanium, hafnium, tantalium and zirconium alloys are all used in jet aircraft, reactors, missiles, and for nuclear reactor. 

Beryllium is a light metal and melts at about 1280*C. The metal is used as a moderator, reflector and neutron source. This is very reactive and forms compounds with the furnace atmospheres and refractories. Vacuum or inert gas are necessary during melting. The cast metal is usually coarse grained and brittle, and powder metallurgy methods are employed in the fabrication of beryllium components. Beryllium is a toxic metal. Its compounds are also very toxic. Safety measures are to be taken when handling this metal. 

Zirconium minerals contain 0.5 to 2 percent hafnium which is a strong absorber of neutrons and must therefore be removed. The main use of zirconium is for cladding fuel elements and for structural components in water-cooled systems. So it must have increased corrosion resistance. Zircaloy-2 containing 1.5% Zn , 0.1 % Fe , 0.05 % Ni and 0.1 % Cr provides better corrosion resistance and is generally used in water-cooled reactors. Zirconium has a relatively poor resistance to Co2 at elevated temperatures, but this is improved by the addition of 0.5 % Cu , 0.5% Mo with an increase of tensile strength to 51 kg-f / mm2. (510 M pa) and improved creep resistance at 450*C. This zirconium is especially useful in gas cooled reactors. The melting point of Zirconium is 1750*C.

This is a synthetic element and does not occur in nature. It is produced through neutron absorption by fertile U238 and subsequent beta decays. Plutonium melts at 640*C. It is extremely toxic and emits alpha rays. Due to its naturally fissionable , plutonium continues to emit high energy gamma and other radiation. The metal is chemically more reactive than uranium and has a poor resistance to corrosion. It has six allotropic forms. The chief application of plutonium is in the production of atomic weapons and also in breeder reactors. Since some plutonium alloys and compounds emit neutrons, adequate health protection is necessary while using these materials.

Thorium is another possible fuel and is free from phase changes below 1480*C. Thorium (90Th24) is also a radioactive metal like uranium and this can be converted into Uranium (92U234) by beta decay. The mechanical properties of thorium which is soft and weak when pure , are drastically changed by small addition of impurities. Only 0.2 per cent of carbon raises its tensile strength from 14 to 38 kg-f / mm2 (140 - 380 M pa). Small additions of  titanium, zirconium, and niobium decreases the strength and hardness of the metal. Uranium addition increases the strength of thorium. Thorium like Uranium is an emitter of alpha rays and releases considerable quantity of the radioactive products during processing, but this being a cubic metal (fcc) is less susceptible to irradiation damage. The melting point of thorium is 1845*C.

The most Important metal found in nature and used for nuclear energy is Uranium. This is used as a nuclear fuel and is radioactive, easily oxidized and exists in three allotropic forms. This has a poor resistance to corrosion and needs to be protected for use as fuel elements by roll cladding a thin aluminum or zirconium jacket. The metal in the pure condition is weak and is susceptible to severe irradiation damage and growth in the reactor environments. Addition of some alloying elements such as chromium, molybdenum, plutonium, zirconium, etc. are added to make the material highly suitable for nuclear power. Uranium compounds, such as UO2 as a dispersion in cermet or as ceramic slugs, have been found to give better service. Uranium oxide is highly refractory, shows no phase change in an inert atmosphere , possesses a good strength and higher corrosion resistance. But it as a low thermal shock resistance, poor thermal conductivity and a high coefficient of expansion. The melting point

Recent Nuclear Power developments have created a demand for materials to withstand stringent condition imposed, and as a result some metals, previously considered "rare" are now coming in the use in a wide perspective. The various metals for producing nuclear energy are used as raw materials, moderators reflectors, fuel elements , fuel canning materials , control elements, and pressure vessel materials. Thus, uranium, thorium, plutonium, zirconium, beryllium, niobium, and their alloys are primarily used for nuclear engineering purposes. 1. Uranium  2. Thorium 3. Plutonium 4. Zirconium 5. Beryllium 6. Niobium (Columbium) 

 Many components in Jet and Rocket Engines , and in Nuclear Equipment have to withstand temperatures in excess of 1100*C. This has made to develop a number of highly specialized alloys. Nickel or Cobalt forms the base metals in this range of alloys. Most of these alloys possess yield strength in excess of 70 MN / mm2. or 700 kg-f / mm2 and 250 to 370 Brinell hardness number at room temperature.  Alloys used for High Temperature and Its Composition : 1.) Nimonic 80 A : Chromium -  21% ,Titanium -2.5 % , Aluminum - 1.2 %  ,Carbon - 0.04% ,Nickel - 75.26 % 2.) Inconel 713C : Chromium - 12% ,Titanium -0.5 % ,Tantalum - 2.0% ,Aluminum - 6% ,Carbon - 4.5 % ,Nickel - 75% 3.) Incoloy 910 : Nickel - 42% ,Chromium -13 % ,Titanium - 2.4 % ,Carbon - 0.04 % ,Molybdenum - 6% ,Iron - 36.56% 4.) Hastelloy : Nickel - 45% ,Chromium - 22% ,Cobalt - 1.5% ,Tungsten - 0.5% ,Carbon - 0.15% ,Molybdenum - 9% 5.) Vitallium : Nickel - 2.5% ,Chromium - 28% ,Cobalt - 62% , Iron - 1.7% ,Carbon - 0.28% ,Molybdenum -

  Copper - Aluminum Alloy or Aluminum Bronze : Copper Alloys with aluminum gives aluminum bronze and the chief alloys are those containing 6 per cent and 10 per cent of aluminum and copper respectively. The aluminum gives the alloy lightness, while the addition of  copper to pure aluminum increases its strength. The 6 per cent aluminum alloy has a fine gold color, being used for Imitation Jewellery and decorative Purposes.

 Although some nickel is obtained commercially from oxide ores, arsenical ores , and the ores of copper, manganese and iron, at least 85 percent of all nickel production is obtained from sulphide ores. Pure Nickel is tough, silver-colored metal, rather harder than copper, and of about the same strength, but possessing somewhat less ductility. It closely resembles iron in several of its properties, being malleable and weldable, and perceptibly magnetic, but unlike iron it is little affected by dilute acids, is far less readily oxidisable, and deteriorates much less rapidly under atmospheric influences. For this reason articles of iron and steel are frequently nickel - plated  to protect them from rusting. Nickel is much used for cooking utensils, and other vessels for heating and boiling. Nickel enters as a constituent into a large number of ferrous and non ferrous alloys, and frequently finds application as a catalyst in important Industrial processes.  Nickel Alloys which are particul

The most widely used Copper - zinc Alloys are Brass and Muntz Metal. BRASSES :  This is fundamentally a binary alloy of copper with as much as 50 percent Zinc. Various Classes of Brass , depending on the proportion of Copper and Zinc, are available for various uses. Suitable types of Brass lend themselves to the following processes : Casting, Hot Forging , Cold forging , cold rolling into sheets , drawing into wire and being extruded through dies to give special shaped bars. The melting point of brass ranges from 800*C to 1000*C.  The alloy is non corrosive and air, water and some acids do not appreciably affect it. It is soft, ductile and has tensile strength with good fusibility and surface - finish characteristic. It is non - magnetic and is poor conductor of electricity. By adding small quantities of other elements the properties of brass may be greatly changed, as for example, the addition of 1 or 2 percent of  lead Improves the machining quality of  brass. Small amounts of  tin a

 The chief ores of  zinc are zinc blende ( Zinc sulphide ) and calamine ( Zinc carbonate ). In the extraction of the metal, the ore is first roasted in a reverberatory furnace to convert the sulphide to oxide, and in the case of calamine, to drive off carbonic acid and water. The roasted ore is then reduced either in a furnace or electrolytically.  Zinc is a fairly heavy , bluish - white metal used principally because of its low cost, corrosion resistance, and alloying properties. The melting point of zinc is 419 *C. The protection of Iron and steel from corrosion is done more often with zinc than with any other metal coating. The oldest and most important methods of applying the zinc coating are known as galvanizing. When rolled into sheets, zinc is used for roof covering and for providing a damp-proof non corrosive lining to containers , etc. Zinc casts well and forms the base of various die-casting alloys.  A typical high strength die - casting alloy would have , Cu = 1.25 %  , Fe

 GUN METAL : Gun Metal Contains 10 per cent tin , 88 per cent copper , and 2 per cent zinc. The zinc is added to clean the metal and increase its fluidity.  It is not suitable for being worked in the cold state but may be forged when at about 600*C. The metal is very strong and resistant to corrosion by water and atmosphere. originally , it was made for casting boiler fittings , bushes , bearings , glands etc. BELL METAL : It contains 20 per cent tin and the rest is copper. It is hard and resistant to surface wear. Bell metal is used for making bells , gongs , utensils, etc. 

Manganese bronze is an alloy of copper , zinc , lead and a little percentage of manganese. The metal is highly resistant to corrosion. It is stronger and harder than phosphor bronze. It is generally used for preparing bushes, plungers and feed pumps, rods etc. Worm gears are frequently made from this bronze. 

Silicon Bronze has an average composition of 96 percent copper, 3 per cent silicon, and      1 per cent manganese or zinc. It has the good general corrosion resistance of copper, combined with higher strength, and in addition can be cast , rolled, stamped , forged , and pressed either hot or cold and can be welded by all the usual methods.  Silicon Bronze finds application in parts for boilers , tanks , stoves , or wherever high strength and good corrosion resistance are required.