Structural change in carbon steel of eutectoid composition during tempering

Conclusions

• 1. Study of the structure of the carbide phase by the electron diffraction method shows that as a result of tempering at 200° for up to 100 hours, at 300° for up to 20 minutes and at 400° for up to 10 seconds, no reflections suggesting orthorhombic cementite lattice are detected. Tempering for more than 20 seconds at 300° and between 20 seconds and 5 minutes at 400° leads to weaker reflections, similar to those forε-carbide, in addition to reflections indicating cementite. After tempering at 400° for more than 5 minutes and at 500° for any length of time only cementite is detected.

• 2. The carbide phase after tempering at 200, 300 and 400° (up to 5 minutes) is present in the steel in the form of plates (or films) consisting of intergrown plane particles. We observe orientation of the carbide segregations within the martensite needles or plates in either one or two directions. The carbide-phase particles lying along the edges of the martensite plates grow more rapidly than those within the body. The thickness of the blocks of the matrix is about 440 Å after tempering for one hour at 300°.

• 3. At 400° the spheroidization of the carbide-phase particles becomes appreciable, but their original orientation is still preserved. At 500° there is recrystallization of the ferrite matrix at an appreciable rate; the carbide particles spheroidize and the orientation of their position is lost.

• 4. As the duration of the tempering is increased at 600 and 700°, the carbide particles which after recrystallization of the matrix were found on the ferrite grain boundaries now grow, whereas the particles within the ferrite grains are dissolved. Tempering up to 20 hours at 700° leads to the concentration of all carbide particles on the ferrite grain boundaries.