The Iron–Carbon System
1. The Iron–Iron Carbide (Fe –Fe3C) Phase Diagram This is a binary (two-element) diagram, it is essentially a map of the phases that exist in iron at various carbon contents and temperatures under equilibrium conditions.
The iron–carbon system may be divided into two parts: an iron- rich portion, as in Figure below and the other (not shown) for compositions between 6.70 and 100 wt% C (pure graphite). The composition axis extends only to 6.70 wt% C which it compound iron carbide, or cementite (Fe3C).
2. Solid Solutions Carbon is an interstitial impurity in iron and forms a solid solution (-ferrites and -ferrites, and -austenite) single phase fields in Figure. - -ferrite: has a BCC crystal structure, maximum solubility is 0.022 wt% at 727C. This phase is relatively soft, may be made magnetic at temperatures below 768C, and has a density of 7.88 g/cm3. - -austenite has a FCC, The maximum solubility of carbon in austenite, 2.14 wt%, occurs at 1147C. Austenite is nonmagnetic. - -ferrites is the same as -ferrite, has a BCC, except for the range of temperatures over which each exists. Figure 1: (a) -ferrite . (b) - austenite Fe3C (cementite) contains 6.70 wt% C, is extremely hard and brittle (like a ceramic material).
3. Eutectic Reaction
There are one eutectic exists for the iron–iron carbide system, at 4.30 wt% C and 1147C:
cooling L + Fe C heating 3
4. Eutectoid Reaction It occurs for the iron-carbon system at 727C and 0.76 wt% C. Upon cooling, a solid phase transforms into -iron and cementite, according to the reaction:
cooling (0.76 wt% C) (0.022 wt% C) + Fe C(6.7 wt% C) heating 3
The feature distinguishing eutectoid from eutectic is that one solid phase instead of a liquid transforms into two other solid phases at a single temperature. A eutectoid reaction is very important in the heat treating of steels.
5. Peritectic Reaction A peritectic exists for the iron–carbon system at 1493C and 0.18 wt% C: cooling + L heating
One solid phase transforms into a liquid phase and another solid phase. There are three types of ferrous alloys : iron, steel, and cast iron. Iron: (less than 0.008 wt% C) and is composed ferrite phase at room temperature.
Steel: (0.008 — 2.14 wt% C) consists of both and Fe3C phases. Cast iron: (2.14 — 6.70 wt% C). 6. Development of Microstructures in Iron-Carbon Alloys
Phase changes that occur in the case of a steel with eutectoid composition (0.76 wt% C) as it solidifies and cools to room temperature. region into the + Fe3C phase, beginning at point a and moving down the vertical line x푥 to point b, is called pearlite. The thick light layers are the ferrite phase, and the cementite phase appears as thin lamellae most of which appear dark.
Mechanically, pearlite has properties intermediate between the soft, ductile ferrite and the hard, brittle cementite.
Figure 2: The evolution of the microstructure for an iron–carbon alloy of eutectoid composition (0.76 wt% C) during cooling. Figure 3: Photomicrograph of a eutectoid steel showing the pearlite microstructure consisting of alternating layers of -ferrite
(the light phase) and Fe3C (thin layers most of which appear dark).
Figure 4: Schematic representation of the formation of pearlite from austenite; direction of carbon diffusion indicated by arrows. Example 1:
Calculate the amounts of ferrite and cementite present in pearlite. Solution
Since pearlite must contain 0.76 wt% C, using the lever rule:
6.7 – 0.76 W = × 100 = 88.95% 6.7 – 0.022
0.76 – 0.022 푊 = × 100 = 11.05% Fe3C 6.7 – 0.022 Example 2: Calculate the amounts and compositions of phases and in a Fe-0.60 wt% C alloy at temperature just below the eutectoid. Solution
The phases are ferrite and cementite. Using a lever law and working the tie line that extends all the way across the + Fe3C phase field. Thus, C0 is 0.60 wt% C, we find:
6.70 − 0.60 (0.022 wt% C) W = × 100 = 91.35% 6.70 − 0.022
0.60 – 0.022 Fe C (6.70 wt% C) 푊 = × 100 = 8.65% 3 Fe3C 6.70 − 0.022