First of all, if you want to understand the iron-carbon alloy and iron-carbon phase diagram, you need some preparatory knowledge, such as the concepts of alloy, phase, component composition, etc., which are basically as follows:
Alloy: A substance with metallic characteristics formed by combining one metal element with another one or several elements through melting or other methods.
Phase: The uniform components of the alloy with the same chemical composition, the same aggregation state, and separated from each other by the interface.
Solid solution: A solid metal crystal in which the atoms (compounds) of one (or several) elements are dissolved into the lattice of another element while still maintaining the lattice type of the other element. The solid solution is divided into interstitial solid solution and replacement solid solution. Two kinds.
Solid solution strengthening: As the solute atoms enter the gaps or junctions of the solvent crystal lattice, the crystal lattice is distorted and the hardness and strength of the solid solution are increased. This phenomenon is called solid solution strengthening.
Metal compound: A new phase formed by the interaction between the components of the alloy in a certain proportion, and its composition can usually be expressed by a chemical formula.
The iron-carbon alloy phase diagram is actually the Fe-Fe3C phase diagram, and the basic components of the iron-carbon alloy should also be pure iron and Fe3C. Iron has an allotropic transformation, that is, it has a different structure in the solid state. Different structures of iron and carbon can form different solid solutions, and the solid solutions on the Fe-Fe3C phase diagram are all interstitial solid solutions. Due to the different characteristics of the pores in the α-Fe and γ-Fe lattices, the carbon dissolving capabilities of the two are also different.
There are three phases in iron-carbon alloys, namely ferrite, austenite and cementite.
1. Ferrite
Ferrite is an interstitial solid solution of carbon in α-Fe, represented by the symbol "F" (or α), body-centered cubic lattice;
Although the total volume of the gap of BCC is large, the volume of a single gap is small, so its dissolved carbon is very small, at most only 0.0218% (at 727°C), and it is almost 0 at room temperature. Therefore, the performance of ferrite is comparable to that of pure iron. Similar, low hardness and high plasticity, and ferromagnetic.
δ=30%~50%, AKU=128~160J σb=180~280MPa, 50~80HBS.
The microstructure of ferrite is the same as that of pure iron. After being etched with a 4% nitric acid alcohol solution, bright polygonal equiaxed grains appear under the microscope. When the carbon content is close to the eutectoid composition, the ferrite is distributed in an intermittent network around the pearlite due to the small amount.
2. Austenite
Austenite is an interstitial solid solution of carbon in γ-Fe, represented by the symbol "A" (or γ), a face-centered cubic lattice;
Although the total volume of the gap of the FCC is small, the volume of the single gap is larger, so its dissolved carbon amount is larger, up to 2.11% (at 1148°C), and 0.77% at 727°C.
In general, austenite is a high-temperature structure, and the stable temperature range is 727~1394°C. Therefore, austenite has low hardness and high plasticity. It is usually used for hot deformation processing of steel materials, such as forging, When hot rolling, etc., it should be heated to austenite state. The so-called "strike while the iron is hot" means exactly this. σb=400MPa, 170~220HBS, δ=40%~50%.
In addition, austenite has another important property, that is, it has paramagnetism and can be used for parts or components that require no magnetic field.
The structure of austenite is similar to that of ferrite, but the grain boundaries are relatively straight and twins are often present.
3. Cementite
Cementite is a metal compound with a complex structure formed by iron and carbon. It is represented by the chemical formula "Fe3C". Its carbon mass fraction Wc=6.69%, and its melting point is 1227°C.
Hard and brittle, corrosion resistant. After being etched with 4% nitric acid alcohol solution, it will be white under the microscope. If it is etched with 4% picric acid solution, the cementite will be dark black.
Cementite is a strengthening phase in steel. According to different formation conditions, cementite has strip, network, flake, granular and other forms.
Summarize:
There are three phases in iron-carbon alloys, namely ferrite, austenite and cementite. But austenite generally only exists at high temperatures, so there are only two phases in all iron-carbon alloys at room temperature. Ferrite and cementite. Since the carbon content in ferrite is very small, it can be considered that most of the carbon in the iron-carbon alloy is present in the cementite. This is very important.
Iron and carbon can form a series of compounds, such as Fe3C, Fe2C, FeC, etc. The only part that has practical significance and has been studied in depth is the Fe-Fe3C part, which is usually called the Fe-Fe3C phase diagram. At this time, the component of the phase diagram is Fe And Fe3C.
Since the iron-carbon alloy actually used has a carbon content of less than 5%, the composition axis ranges from 0 to 6.69%. The so-called iron-carbon alloy phase diagram is actually the Fe-Fe3C phase diagram.
The alloys on the iron-carbon phase diagram can be divided into three categories according to their composition:
⑴Industrial pure iron (<0.0218% C), whose microstructure is ferrite grains, is rarely used in industry.
⑵Carbon steel (0.0218%-2.11%C), its characteristic is that the high temperature structure is single-phase A, easy to deform, carbon steel is divided into hypoeutectoid steel (0.0218%-0.77%C), eutectoid steel (0.77%C) ) And hypereutectoid steel (0.77%-2.11%C).
⑶White cast iron (2.11%-6.69%C) is characterized by good casting performance, but hard and brittle. White cast iron is divided into hypoeutectic white cast iron (2.11%-4.3%C) and eutectic white cast iron. Cast iron (4.3%C) and hypereutectic white cast iron (4.3-6.69%C)
Phase Diagram Analysis
The Fe-Fe3C phase diagram looks more complicated, but it is still composed of some basic phase diagrams. We can divide the Fe-Fe3C phase diagram into two parts for analysis.
Eutectic transition
At 1148℃, 4.3%C liquid phase undergoes eutectic transformation:
Lc(AE+Fe3C),
The product of transformation is called ledeburite and is represented by the symbol Ld.
The ledeburite that exists between 1148°C and 727°C is called high-temperature ledeburite, which is represented by the symbol Ld, and the structure is composed of austenite and cementite; the ledeburite that exists below 727°C is called metamorphic ledeburite Body or low-temperature ledeburite, represented by the symbol Ldˊ, the organization is composed of cementite and pearlite.
Low-temperature ledeburite is a mechanical mixture composed of pearlite, Fe3CⅡ and eutectic Fe3C. After being etched in 4% nitric acid alcohol solution and observed under a microscope, the pearlite is distributed on the Fe3C matrix in the form of black particles or short rods, Fe3CⅡ Intertwined with eutectic Fe3C, it is generally indistinguishable.
Eutistic transformation
At 727°C, 0.77% of austenite undergoes eutectoid transformation:
AS (F+Fe3C), the product of transformation is called pearlite.
The difference between eutectoid transformation and eutectic transformation is that the transformation product is a solid rather than a liquid.
Feature points
The characteristic points that should be mastered in the phase diagram are: A, D, E, C, G (point A3), S (point A1), and their meaning must be clarified. According to the phase diagram, analyze the following points:
Important points in the phase diagram (14 points):
1. The melting point of the component: A (0, 1538) the melting point of iron; D (6.69, 1227) the melting point of Fe3C
2. Allotrope transformation point: N(0, 1394) δ-Fe γ-Fe; G(0, 912)γ-Fe α-Fe
3. The maximum solubility point of carbon in iron:
P(0.0218,727), the maximum solubility of carbon in α-Fe
E(2.11,1148), the maximum solubility of carbon in γ-Fe
H (0.09,1495), the maximum solubility of carbon in δ-Fe
Q(0.0008,RT), the solubility of carbon in α-Fe at room temperature
Three-phase coexistence point:
S (eutectoid point, 0.77, 727), (A+F +Fe3C)
C (eutectic point, 4.3, 1148), (A+L +Fe3C)
J (peritectic point, 0.17, 1495), (δ+ A+L)
Other points
B (0.53, 1495), the composition of the liquid phase when the peritectic reaction occurs
F(6.69,1148), cementite
K(6.69,727), cementite
Characteristic line
Some lines in the phase diagram should be mastered: ECF line, PSK line (A1 line), GS line (A3 line), ES line (ACM line)
The horizontal line ECF is the eutectic reaction line.
Iron-carbon alloys with a carbon mass fraction between 2.11% and 6.69% have a eutectic reaction during the equilibrium crystallization process.
The horizontal line PSK is the eutectoid reaction line
For iron-carbon alloys with a carbon mass fraction of 0.0218% to 6.69%, eutectoid reactions occur during the equilibrium crystallization process. PSK line is also known as A1 line.
The GS line is the critical temperature line at which F begins to precipitate from A when the alloy is cooled, usually called A3 line.
ES line is the solid solution line of carbon in A, usually called Acm line. Since the maximum amount of carbon dissolved in A at 1148℃ is up to 2.11%, but at 727℃ it is only 0.77%, so the carbon mass fraction is greater than 0.77% When the iron-carbon alloy is cooled from 1148℃ to 727℃, Fe3C will be precipitated from A. The precipitated cementite is called secondary cementite (Fe3CII). The Acm line is also the critical point for Fe3CII precipitation from A Temperature line.
The PQ line is the solid solution line of carbon in F. The maximum amount of carbon dissolved in F can reach 0.0218% at 727°C, and only 0.0008% at room temperature. Therefore, iron-carbon alloys with a carbon mass fraction greater than 0.0008% are cooled from 727°C to room temperature. In the process, Fe3C will be precipitated from F. The precipitated cementite is called tertiary cementite (Fe3CIII). The PQ line is also the critical temperature line at which Fe3CIII is precipitated from F. The amount of Fe3CIII is very small and is often ignored.
Phase region
1. Single-phase area (4 + 1): L, δ, A, F, (+ Fe3C)
2.Two-phase area (7): L + δ, L + Fe3C, L + A, δ + A, A + F, A + Fe3C, F + Fe3C.
