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Introduction to The Phase Structure and Properties of Fe-Fe3C Alloy

Although industrial pure iron has better plasticity, but lower strength, it is rarely used to make mechanical parts, and iron-carbon alloys are commonly used. The phase structures in iron-carbon alloys are as follows.

1. Ferrite

Pure iron has a body-centered cubic lattice below 912°C. The interstitial solid solution in which carbon is dissolved in α-Fe is called ferrite and is represented by the symbol F. Because the diameter of the body-centered cubic lattice gap is very small, the ability to dissolve carbon is poor. The amount of dissolved carbon is the largest at 727°C (wc=0.0218%); as the temperature drops, the amount of dissolved carbon gradually decreases, and it dissolves carbon at 600°C The amount is about wc=0.0057%, so its performance at room temperature is almost the same as that of pure iron. The value is as follows:

Tensile strength (σb) — 180~280MPa

Yield point (σs)— 100~170MPa

Elongation (δ) — 30%~50%

Section shrinkage (ψ) — 70%～80%

Impact toughness (αK) — 160~200J/cm²

Hardness — 50~80HBW

It can be seen that the strength and hardness of ferrite are not high, but it has good plasticity and toughness.

Ferrite has ferromagnetism below 770°C, and loses ferromagnetism above 770°C.

2. Austenite

The interstitial solid solution in which carbon is dissolved in γ-Fe is called austenite and is represented by the symbol A. Because γ-Fe is a face-centered cubic lattice, although its density is higher than that of α-Fe in a body-centered cubic lattice, because the diameter of its lattice gap is larger than that of α-Fe, its carbon dissolving ability is also greater. . At 1148℃, the ability to dissolve carbon is the largest (wc=2.11%); as the temperature drops, the amount of dissolved carbon gradually decreases, and the amount of dissolved carbon at 727°C is wc=0.77%.

The performance of austenite is related to the amount of dissolved carbon and grain size. Generally, the hardness of austenite is 170~220HBW, and the elongation rate is 40%~50%. Therefore, the hardness of austenite is lower and the plasticity is higher. Easy to forging and forming.

Austenite exists in the high temperature range above 727°C. Austenite is a non-ferromagnetic phase.

3. Cementite

The molecular formula of cementite is Fe3C, which is an interstitial compound with complex character.
The wc of cementite is 6.69%, and the melting point is 1227°C; no isomeric transformation occurs; but there is magnetic transformation. It has weak ferromagnetism below 230°C, but loses ferromagnetism above 230°C; the hardness is very high (950～1050HV), while the plasticity and toughness are almost zero, and the brittleness is extremely high.

Carbon atoms in cementite can be replaced by small-sized atoms such as nitrogen, while iron atoms can be replaced by other metal atoms (Cr, Mn, etc.). This kind of solid solution with cementite as solvent is called alloy cementite, such as (Fe, Mn) 3C, (Fe, Cr) 3C and so on.

When cementite coexists with other phases in steel and cast iron, it is in the form of flakes, balls, nets or plates. Cementite is the main strengthening phase in carbon steel, and its shape and distribution have a great influence on the properties of steel. At the same time, cementite will decompose under certain conditions to form graphite-like free carbon.