Page 117 - Gear Technology Solutions

The forging scale is another undesirable side effect of die forging. In free form
forging (Figure 2 left side) the scale flakes off during the forming process. In
the case of die forging, the exposure to oxygen is minimized and a high
surface pressure is applied. As a result, a very thin scale is formed and
becomes a permanent part of the surface structure. The thickness of the scale
is between 0.001 to 0.006mm and is dominated by its ferrite concentration.
The forging scale does not form oil pockets like the generating marks of a cut
gear. The low relative speeds of differential gears require a certain surface
structure in order to retain some oil film between the contacting tooth flanks
which is ideally given by generating marks rather than by a smooth surface.

Figure 1: Cross sectional print of forged differential gears

Frequently the argument of the optimal grain flow in the steel structure due to
forging the teeth rather than cutting them is made. The grain flow would
represent the flow of the deforming steel during a billet being pressed in the
forging die. The advantage of the grain flow however disappears after the
recrystallisation during the heat treatment process [1, 2].

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