Fig 1a shows the temperature bands for the annealing processes superimposed on

Fig 1a shows the temperature bands for the annealing

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Fig. 1(a) shows the temperature bands for the annealing processes superimposed on the iron-carbon phase equilibrium diagram. In all annealing processes the cooling rate is as slow as possible. STRESS-RELIEF ANNEALING This process is reserved for steels below 0.4% carbon content. Such steels will not satisfactorily quench harden but, as they are relatively ductile, they are frequently cold worked and become work hardened. Since the grain structure will have been severely distorted by the cold working, recrystallisation can commence at 500 °C but, in practice, annealing is usually carried out between 630 and 700 °C to speed up the process and limit grain growth. The rate of cooling and the length of time for which the steel is heated depends upon the subsequent processing and use to which the material is going to be put. If further cold working is to take place then increased ductility and malleability will be required. This is achieved by prolonging the heating and slowing the cooling to encourage grain growth. However, if grain refinement and strength and toughness are of more importance, then heating and cooling should be more rapid. SPHEROIDISING ANNEALING Crystals of pearlite have a laminated structure consisting of alternate layers of cementite and ferrite. When steels containing more than 0.4% carbon are heated to just below the critical temperature (650- 700 °C) the cementite in the crystals tend to ‘ball up’. This is referred to as the aspheroidisation of pearlitic cementite and the process is shown diagrammatically in Fig. 1(b) above. Since the temperatures involved are subcritical, no phase changes take place and spheroidisation of the cementite is purely a surface a surface tension effect. If the layers of cementite are relatively coarse prior to annealing, they take too long to break down and tend to form coarse globules (spheroids) of cementite. This, in turn, leads to impaired physical properties and machined surfaces with a poor finish. Thus, grain refinement by a quench treatment prior to aspheroidisation is recommended to produce fine globules of cementite. The process is most effective when it is used to soften plain carbon steels containing more than 0.4% carbon and which have been either work hardened or quench hardened. After spheroidising annealing the steel can be cold worked and it will machine freely to a good surface finish. Furthermore, steel which has been subjected to spheroidising annealing will re-harden more uniformly and with less chance of cracking. As with any other annealing process, slow cooling is required after the heating cycle. It is usual to turn off the furnace and allow the furnace and the charge to cool down slowly together. FULL ANNEALING Plain carbon steels solidify at temperatures well above the temperatures with which heat-treatment processes are concerned.
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