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Materials processing and joining

When carbon steel parts of the same shape are made by casting and by hot forging, the forged part generally has better toughness and fatigue strength. Which reason is correct?

  1. AThe cast structure is broken up and refined, small internal voids are pressed shut, and grain flow lines form along the shape
  2. BHot forging suppresses recovery and recrystallization, leaving heavy work hardening that makes the part several times harder than a casting
  3. CThe large strain of forging stabilizes austenite, so the crystal structure of carbon steel stays FCC all the way down to room temperature
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Answer: A. The cast structure is broken up and refined, small internal voids are pressed shut, and grain flow lines form along the shape

In hot forging, the coarse columnar and dendritic cast structure is broken up by large deformation and recrystallization into fine equiaxed grains. Small internal shrinkage cavities and pores are pressed shut and disappear, as long as they are not connected to the surface. In addition, inclusions and segregation are stretched in the direction of deformation, forming streaky grain flow lines (fiber flow) that follow the shape of the part, and the toughness and fatigue strength along them are higher (across the flow lines they are actually lower, so dies are designed to make the flow follow the shape). In hot working, recovery and recrystallization take place during working, so almost no work hardening remains, and the option citing large residual work hardening is wrong. Nor does forging leave carbon steel as FCC austenite down to room temperature, so the option saying it stays FCC is also wrong.

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