Spheroidization Annealing

Spheroidization Annealing: 
  • Spheroidization annealing consists of heating, soaking and cooling, invariably very slowly to produce spheroidal pearlite or globular form of carbides in steels. 
  • To improve the machinability of the annealed hypereutectoid steel spheroidize annealing is applied.
  • Hypereutectoid steels consists of perlite and cementite. T he cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels.
  • This process will process a spheroidal or globular form of a carbide in a ferritic matrix which makes machining easy.
  • Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network. The structure is called spheroidite.
Spheroidising Process: 
  • Heat the part to a temperature just below the Ferrite-Austenite line, line A1 or below the Austenite-Cementite line, essentially below the 7270C (13400F) line. Hold the temperature for a prolonged time and follow by fairly slow cooling. Or 
  • Cycle multiple times between temperatures slightly above and slightly below t he 7270C (13400F) line, say for example between 700 and 7500C (1292 - 13820F), and slow cool. Or 
  • For tool and alloy steels heat to 750 to 8000C (1382 - 14720F) and hold for several hours followed by slow cooling. 
All these methods result in a structure in which all the cementite is in the form of small globules (spheroids) dispersed throughout the ferrite matrix. This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.

Aim of Spheroidization Annealing:
  • minimum hardness
  • maximum ductility
  • maximum machinability
  • maximum softness
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