Again the steel is cooled in free air and the austenite transforms into fine

Again the steel is cooled in free air and the

This preview shows page 33 - 37 out of 73 pages.

Again, the steel is cooled in free air and the austenite transforms into fine grain pearlite and cementite. The fine grain structure resulting from the more rapid cooling associated with normalising gives improved strength and toughness to the steel but reduces its ductility and malleability. The increased hardness and reduced ductility allows a better surface finish to be achieved when machining. (The excessive softness and ductility of full annealing leads to local tearing of the machined surface.) However, the level of ductility and malleability achieved by normalising is not sufficient for more than limited clod working.
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~ Page 34 of 73 ~ Normalising is frequently used for stress relieving between rough machining and the finish machining of large castings and forgings to avoid subsequent ‘movement’ due to the slow release of internal stresses and loss of accuracy. At one time large castings and forgings were left outside to ‘weather’ for up to a year or more after rough machining to ensure that the workpieces became stabilized. Although highly successful, this procedure tied up an excessive amount of working capital and space and nowadays heat-treatment is preferred as the work in progress is turned round more quickly.
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~ Page 35 of 73 ~ QUENCH HARDENING Fig. 1 below shows the temperature band from which plain carbon steels are cooled when they are quench hardened. As can be seen the band is similar to that for full annealing. The band is not continued below 0.4% carbon for, although some grain refinement and toughening occurs, no appreciable hardening takes place. If a plain carbon steel with a carbon content above 0.4% is quenched (cooled very rapidly) from the appropriate temperature for its carbon content as shown in Fig. 1, there is not sufficient time for the equilibrium transformation to take place and the steel becomes appreciably harder. The final hardness will depend solely upon the carbon content and the rate of cooling. Fig. 2 below helps explain the reason for the increase in hardness. In the annealed condition metals can be formed by bending, stretching or squeezing them to shape. This is possible because the orderly arrangement of atoms in the crystals allows individual layers of atoms (slip planes) to slide over each other as shown in Fig. 2(a). However, if distortion of the lattice occurs, as shown in Fig. 2(b), or particles of another material are introduced, as shown in Fig. 2(c), then slip cannot occur so easily and the metal becomes hard and brittle. It is like the difference between trying to slide two pieces of smooth paper over each other compared with trying to slide two sheets of glass paper over each other.
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~ Page 36 of 73 ~ When steel is heated to its hardening temperature it becomes austenitic. If it is cooled quickly, the equilibrium transformations into pearlite and ferrite or pearlite and cementite do not have time to take place.
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