1. Preface
The definition of ferrite: Pure iron has body-centered cubic lattice α-Fe below 912 ℃, soluble carbon in α-Fe, and the interstitial solid solution is called ferrite, which is represented by the symbol F.
Because the gap diameter of the body-centered cubic lattice of α-Fe is very small, the solubility of carbon in it is very small.
At 600℃, the solubility is only 0.0057%, and at 727℃, the maximum solubility is only 0.0218%.
Therefore, the mechanical properties of ferrite are almost the same as pure iron, with good plasticity and toughness (A ≈ 30% – 50%, KU ≈ 128-160J) and low strength and hardness (Rm ≈ 180-280MPa, 50 -80HBW ).
Ferrite has ferromagnetism below 770℃ and loses magnetism above 770℃.
Fig. 1 Ferrite Microstructure
As shown in Figure 1, the microstructure of ferrite consists of bright polygonal grains, and the grain boundary is easy to be corroded, showing irregular black lines.
However, due to the different heat treatment processes selected, ferrite often presents different shapes in materials.
Without enough experience, it's easy to get confused.
This article mainly shares the difference between “undissolved ferrite and proeutectoid ferrite”.
2. Undissolved ferrite and proeutectoid ferrite
Undissolved ferrite is the microstructure obtained after hypoeutectoid steel is heated to Ac1-Ac3 and quenched (i.e. subcooled quenching).
It is white polygonal and has obvious grain boundaries.
The retained martensite and austenite matrix is slightly dark.
Fine tuning focus will find that the undissolved white ferrite and martensite are in the same plane.
As shown in Figure 2, the microstructure morphology of 45 steel after quenching in water at 760 ℃ for 30 min is shown.
The white polygonal phase is insoluble ferrite, the grayish-white phase is quenched martensite and residual austenite, and the dark phase is lath martensite.
Fig. 2 Undissolved white polygonal ferrite
Proeutectoid ferrite is the white network structure that precipitates at the grain boundary when hypoeutectoid steel is heated above A c3 and quenched, due to the slow cooling rate.
The microstructure generally contains extinct black globular troostite.
As shown in Fig. 3, the microstructure of 45 steel heated to 880 ℃ for 30 min and quenched in oil is composed of white fine cross-linked proeutectoid ferrite, quenched dark troostite, grayish-white martensite and a small amount of feathered upper bainite along grain boundary precipitated during quenching because the cooling rate is less than the critical cooling rate.
Fig. 3 Fine white reticular proeutectoids
3. Ferrite Atlas
4. Conclusion
This article shared with you the identification of undissolved ferrite and proeutectoid ferrite. Although there is only one point of knowledge, if you can fully grasp one point of knowledge every day, you can also achieve the effect of “little makes a noise”.
