Steel name: 312
  Diagram No.: 1919
  Type of diagram: CCT, TTT
  AISI designation: O2 Tool Steel
  Chemical composition in weight %: 0.85% C, 1.98% Mn, 0.40% Si, 0.46% Cr, 0.14% V.
  Note: Tool steel. Austenitizing temperature: 790^oC (1454^oF) for
  30 min. Grain size: 11.
  Ac₁: 719^oC, Ac₃: 783^oC.
  Diagram Description: Duree de chauff. - Time to temperature, Duree d'austenit. - Time
  while austenite, Temperature d'austenit. - Austenitizing temperature, Grain austenit.
  - Austenite grain size, Duree du refroidissement entre 800 et 500^oC - min - Cooling time
  between 800 and 500^oC minutes , Durete Vickers (30 kg) - Vickers hardness
  (30 kg load), kg/mm² - Units for HV test.
  The x-axis is not a simple plot of time and, therefore, cooling curves are not
  superimposed over diagrams. Instead, the x-axis plots the time required to cool from
  800 to 500^oC, which is related to the cooling rate. Because of this manner of graphical
  presentation, the transformation is given along vertical lines, rather the usual
  superimposed cooling curves. Also because of this construction, it is possible to plot
  room tmeperature hardness as a function of the 'half-cooling' time with a single curve.
  Source: M. Economopoulos, N. Lambert, L. Habraken, Diagrammes de Transformation
  des Aciers Fabriques dans le Benelux, Centre National de Recherches
  Metallurgiques, Brussels, Belgium, 1967.

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