Carbon steel and cast iron are the most widely used metal materials. They are alloys of iron and carbon. Carbon steel and cast iron with different compositions have different structures and properties. Iron-carbon phase diagram is needed for the research and use of iron and steel materials, the formulation of hot working and heat treatment processes and the analysis of the causes of process waste. In iron-carbon alloys, according to different crystallization conditions, carbon can be composed of two forms: carbide iron 3C (cementite) and graphite. Cementite is a metastable phase in thermodynamics, while graphite is a stable phase. In general, the iron-carbon alloy is transformed according to the Fe-Fe 3C system, and the iron-carbon phase diagram discussed in this chapter is actually the Fe-Fe 3C phase diagram.

Composition of 4- 1 iron-carbon alloy

First, pure iron

The melting point of pure iron is 1538℃, and its cooling curve is shown in Figure 7. 1.

Temperature (℃)

time

Fig. 7. Cooling curve and crystal structure change of1pure iron.

After pure iron crystallized from liquid to solid, the lattice type changed twice when it was cooled to 1394℃ and 9 12℃. The lattice transformation of metals in solid state is called allotropic transformation. Isomerization transformation is accompanied by thermal effect, so the cooling curve of pure iron has plateaus at 1394℃ and 9 12℃. The allotrope transformation of iron is as follows:

(body-centered cube) δ? Fe？ γ? Fe？ α? Face-centered cube (body-centered cube) 1394O C 9 12O C

Iron with a temperature lower than 9 12℃ is a body-centered cubic lattice called α-Fe; Iron at 9 12~ 1394℃ is a face-centered cubic lattice called γ-Fe; Iron with the temperature of 1394~ 1538℃ is a body-centered cubic lattice called δ-Fe.

The mechanical properties of industrial pure iron are characterized by low strength and hardness and good plasticity, and their mechanical properties are roughly as follows:

Tensile strength σ b18×107 ~ 28×107n/m2

Yield strength σ 0.210x107 ~17x107n/m2.

Elongation δ 30~50%

Area shrinkage ψ 70~80%

Impact value 160~200J/cm2

Brinell hardness HB 50~80

Second, the solid solution of carbon in iron

The atomic radius of carbon is small, and both α-Fe and γ-Fe can enter the gaps between Fe atoms to form interstitial solid solutions.

The interstitial solid solution formed by carbon in α-Fe is called ferrite, which is usually represented by symbol F or α. Its maximum solubility is 0.02 18wt%C, which occurs at 727℃, and most of the carbon exists in the octahedral vacancy of body-centered cubic α structure. Ferrite and α-Fe are ferromagnetic below Curie point 770℃.

The interstitial solid solution formed by carbon in γ-Fe is called austenite, which is usually represented by symbol A or γ. Its maximum solubility is 2. 1 1wt%C, which appears at 1 148℃. Most carbon exists in the octahedral gap of face-centered cubic γ structure. Austenite and γ-Fe are paramagnetic.

Three. Iron-carbon compound

When the carbon content in iron-carbon alloy exceeds its solubility limit in iron, the excess carbon mainly exists in the form of carbide 3C.

Iron 3C, also called cementite, is a kind of interstitial compound with complex structure, with carbon content of 6.69wt%, high hardness and almost zero plasticity.

Analysis of 4-2 Fe-Fe3C phase diagram

The Fe-Fe 3C phase diagram is shown in Figure 7.2.

Fig. 7.2 Fe-Fe3C phase diagram

ABCD is liquidus and AHJECF is solidus. The whole phase diagram mainly consists of peritectic, * * * crystal and * * * analysis three isothermal transformations:

Peritectic transformation occurs at HJB horizontal line (1495℃);

L B +δH →γJ

The conversion product is γ. This transformation only occurs in iron-carbon alloys with carbon content of 0.09~0.53%.

(2) the * * crystal transformation occurs at the ECF horizontal line (1 148℃);

L C →γE +Fe 3C

The transformation product is a mechanical mixture of γ and Fe 3C, called ledeburite, which is represented by the symbol Ld or Le. Iron-carbon alloys with carbon content of 2. 1 1~6.69% all undergo this transformation.

(3) The analytical transition occurs at the PSK horizontal line (727℃):

γ→αP +Fe 3C

The phase transformation product is a mechanical mixture of α and Fe 3C, called pearlite, which is denoted by symbol P. This phase transformation occurs in all iron-carbon alloys with carbon content exceeding 0.02 18%. * * * The precipitation transition temperature is usually called 1 temperature.

In addition, there are three important solid-state transition lines in the Fe-Fe 3C phase diagram:

(1) GS line: the transition line of α or α dissolving in γ begins to precipitate in γ, which is usually called A 3 temperature.

(2) ES line: the solubility line of carbon in γ. This temperature is usually called Acm temperature. Below this temperature, Fe 3C will precipitate in γ, which is called secondary cementite Fe 3C II, which is different from primary cementite Fe 3C I crystallized from liquid through CD filament.

(3) PQ line: the solubility line of carbon in α. When α is cooled down from 727℃, Fe 3C will also precipitate, which is called the third cementite Fe3C3III.

Table 7. 1 also lists the temperature and carbon content of each characteristic point in the phase diagram and their meanings.

Table 7. 1 Fe-Fe3C Phase Diagram Temperature, Carbon Content and Its Meaning Symbol Temperature (℃) Carbon Content (wt%)

A melting point of pure iron of 0 means Q 600.

What is the composition of liquid alloy when peritectic transformation is 0.53 at room temperature? →γE +Fe 3C 4.30 *** Crystal point L C? 6.69 Melting point of Fe3C 2. 1 1 Maximum solubility of carbon in γ-Fe 6.69 Composition of Fe3C 0? Fe-Fe allotropic transition point (A3) Maximum solubility of 0.09 carbon in δ-Fe? →γJ peritectic point L B +δH? Composition of iron 3C 0 γ? Fe？ δ? What is the maximum solubility of carbon in α-Fe at Fe allotropic transition point (A 4)? →αP +Fe 3C *** Analytical point (A 1)γ? Solubility of carbon in α-Fe at 0.0057 0.0008 600℃ (or room temperature)

Equilibrium Solidification of 4-3 Typical Fe-C Alloys

Usually, carbon steel and cast iron are distinguished according to whether there is * * * crystal transformation, that is, carbon steel with carbon content less than 2. 1 1% is greater than.

2. 1 1% is cast iron, which is crystallized according to Fe-Fe 3C system and is called white cast iron.

According to the microstructure characteristics and iron-iron 3C phase diagram (Figure 7.3), iron-carbon alloys can be divided into seven types according to carbon content:

Fig. 7.3 Position of Typical Fe-C Alloys in Fe-Fe 3C Phase Diagram

(1) industrial pure iron

(2) eutectoid steel 0.77% C.

(3) hypoeutectoid steel 0.02 18 ~ 0.77% C

④ hypereutectoid steel 0.77 ~ 2. 1 1% C

(5) *** eutectic white cast iron 4.30% c.

(6) Sub-nodular cast iron 2.11~ 4.30% c.

(7) 4.30 ~ 6.69% C hypereutectic white cast iron.

The transformation process and room temperature structure of each alloy during equilibrium solidification are analyzed below.

I. Industrial pure iron

Figure 7.4 shows the cooling curve and equilibrium solidification process of industrial pure iron.

Figure 7.4

Cooling curve and schematic diagram of equilibrium solidification process of industrial pure iron

The alloy solution crystallizes δ solid solution in the temperature range of1~ 2. When cooled to 3 o'clock, the isomorphic transformation δ→γ of solid solution began to occur. This transformation ends at 4 o'clock, and the alloy is single-phase γ. After cooling to 5~6 o'clock, the isomorphic transformation γ→α occurs again, and all below 6 o'clock are α. When cooling to 7 o'clock, the solubility of carbon in α reaches saturation, and below 7 o'clock, the third cementite Fe _ 3C _ III will precipitate from α. Therefore, the room temperature structure of industrial pure iron is α+Fe3C Ⅲ, as shown in Figure 7.5.

Fig. 7.5 Room temperature equilibrium structure of 250× industrial pure iron

Second, * * * steel analysis.

Fig. 7.6 shows the cooling curve and equilibrium solidification process of * * * steel precipitation.

The alloy solution crystallized γ solid solution in the temperature range of 1~2, solidified at 2, and the alloy was single-phase γ. When it is cooled to 3: 00 (727℃), it will undergo precipitation transformation at constant temperature;

γ→αP +Fe 3C

The transformation product is pearlite, that is, P, which is a flaky and fine mixture of α and Fe 3C, as shown in Figure 7.7. Iron 3C in phosphorus is called * * * cementite. Therefore, the room temperature structure of * * * steel is P, as shown in Figure 7.7. The relative quantities of α and Fe 3C in P can be obtained by lever law:

α(%)=6.69? 0.77× 100%≈88%

6.69

Phen3c (%) =1? 88%= 12%

Figure 7.6

* * * Cooling curve of steel precipitation and schematic diagram of equilibrium solidification process.

Three, points * * steel analysis

Fig. 7.8 is a schematic diagram of cooling curve and equilibrium solidification process of sub-* * steel.

Fig. 7.8 Cooling curve and schematic diagram of the equilibrium solidification process of sub-* * steel.

The alloy solution crystallizes δ solid solution in the temperature range of1~ 2. When cooled to 2 o'clock (1495℃), the carbon content of δ solid solution is 0.09%, and that of liquid phase is 0.53%. At this time, peritectic transformation occurs between liquid phase and δ phase:

L B +δH →γJ

Because the carbon content of the alloy in Figure 7.8 is more than 0. 17%, there is still excess liquid phase after peritectic transformation. Between 2 ′ and 3 ′, γ continues to crystallize in the liquid phase, and the composition of all γ solid solutions changes along the JE line. When cooled to 3 o'clock, the alloy is all composed of γ. When cooling to 4 o'clock, α begins to precipitate from γ, and the carbon content of α changes along GP line, while the carbon content of remaining γ changes along GS line. When cooled to 5 o'clock (727℃), the carbon content of the remaining γ reaches 0.77%, and pearlite is precipitated and transformed at constant temperature. At 5 feet.

However, due to the small number, it can generally be ignored.

Below this point, the third cementite Fe _ 3C _ III will precipitate in ferrite.

Therefore, the room temperature structure of sub-* * steel is P+α, as shown in Figure 7.9. As can be seen from Figures (a), (b) and (c), the higher the carbon content of sub-* * steel, the more the P content in the room temperature structure.

(a)0.20% C 4 10 ×( b)0.45% C 400 ×( C)0.60% C 300×

Fig. 7.9 Room temperature equilibrium structure of sub-* * steel.

Fourthly, the steel products are analyzed by * * *

Fig. 7. 10 is a schematic diagram of the cooling curve and the equilibrium solidification process of the steel.

Fig. 7. 10 cooling curve and schematic diagram of equilibrium solidification process of steel after * * *

The alloy solution crystallized γ solid solution in the temperature range of 1~2, solidified at 2, and the alloy was single-phase γ. After cooling to 3 o'clock, the secondary cementite Fe _ 3C _ II will precipitate from γ until 4 o'clock. This prior analysis

When the amount is large, it is still needle-like in the crystal. The temperature drops to 4: 00 (727Fe 3C is mostly distributed in a network along the γ grain boundary,

C), the carbon content of the remaining γ reaches 0.77%, and pearlite is precipitated and transformed at constant temperature.

Therefore, the room temperature structure of the steel precipitated by * * * is P+Fe3C II, as shown in Figure 7. 1 1.

Fig. 7. The room temperature equilibrium structure of11* * steel is 500×.

The higher the carbon content of the steel after precipitation, the more the Fe _ 3C _ II content in the room temperature structure.

Five, * * * crystal white cast iron

Fig. 7. 12 shows the cooling curve and equilibrium solidification process of * * crystal white cast iron.

Figure 7. 12

* * * Schematic diagram of cooling curve and equilibrium solidification process of crystalline white cast iron

Fig. 7. Equilibrium microstructure of13 * * crystalline white cast iron 100× room temperature.

When the alloy solution is cooled to 1 (1 148℃), * * crystal transformation occurs at constant temperature;

L C →γ

E +Fe 3C

The phase transformation product is a mechanical mixture of γ and Fe 3C, that is, ledeburite Ld, γ is distributed on Fe 3C matrix in the form of short rods. When the temperature is lower than 1, the secondary cementite Fe _ 3cii is continuously precipitated in the * * * crystal γ, and usually adheres to the * * * crystal Fe _ 3c, which is indistinguishable. When the temperature drops to 2: 00 (727℃), the carbon content of * * * crystal γ reaches 0.77%, and it undergoes * * * transformation at constant temperature to form pearlite. The final microstructure consists of P distributed on * * * crystal Fe 3C, as shown in Figure 7. 13. This structure at room temperature retains the morphological characteristics of Ld, the product of crystal transformation at high temperature, but the composition phase γ has changed, so it is called metamorphic ledeburite, which is represented by the symbol Ld'.

Therefore, the room temperature structure of * * * crystal white cast iron is Ld', as shown in Figure 7. 13.

Six, sub * * crystal white cast iron

Fig. 7. 14 is a schematic diagram of cooling curve and equilibrium solidification process of sub-crystalline white cast iron.

Fig. 7. Cooling curve and equilibrium solidification process of14 sub-grained white cast iron.

In the temperature range of1~ 2, γ solid solution crystallizes from alloy solution. At this time, the composition of liquid phase changes along BC line, while the composition of γ solid solution changes along JE line. When cooled to 2: 00 (1 148℃), the composition of the remaining liquid phase reached the * * * crystal composition, and the * * crystal transformation occurred at constant temperature, forming Ld. Below 2 o'clock, primary crystal γ and * * * crystal γ

With the precipitation of Fe _ 3C _ II, the composition of γ solid solution gradually decreases along the es line. They all precipitate secondary cementite Fe _ 3C _ II.

When the temperature drops to 3: 00 (727℃), all γ is transformed into pearlite by precipitation.

Therefore, the room temperature structure of sub-* * crystal white cast iron is Ld'+P+Fe3C II, as shown in Figure 7. 15.

Fig. 7. Room temperature equilibrium structure of15 sub-grained white cast iron 100.

×

Seven, * * * crystal white cast iron

Fig. 7. 16 shows the cooling curve and equilibrium solidification process of * * * crystal white cast iron.

Fig. 7. Cooling curve and schematic diagram of16 * * * crystal white cast iron during equilibrium solidification.

When white cast iron containing * * crystal is in equilibrium solidification, the primary crystal phase is Fe 3C, and other transformations are the same as those of * * * crystal alloy. get through

* * * The room temperature structure of crystalline white cast iron is Ld'+Fe3C I, as shown in Figure 7. 15, and the primary crystal Fe3C I is plate-like.

Fig. 7. Room temperature equilibrium structure of17 white cast iron with * * * crystal 100×

Effect of 4-4 carbon content on microstructure and properties of Fe-C alloy

Generally speaking, the composition of iron-carbon alloy determines its structure, and the structure (including quantity, shape and distribution) determines the properties of iron-carbon alloy.

Effect of 1. Carbon Content on Room Temperature Equilibrium Structure of Fe-C Alloy

According to the analysis of the crystallization process in the previous section and the calculation results by using the lever law, the relationship between the composition and microstructure of iron-carbon alloy can be summarized as shown in Figure 7. 18.

Fig. 7. Relationship between composition, phase composition and microstructure composition of18 iron-carbon alloy.

As can be seen from Figure 7. 18, with the increase of carbon content, the room temperature microstructure of the alloy changes as follows:

When the carbon content increases, not only the amount of Fe 3C in the microstructure increases, but also the existing form of Fe 3C changes from being distributed in α matrix (such as P) to being distributed in γ grain boundary (Fe 3C II). Finally, when Ld was formed, Fe 3C appeared as the matrix. It can be seen that iron-carbon alloys with different carbon contents have different structures, which is why they have different properties.

Second, the influence of carbon content on the mechanical properties of iron-carbon alloy

From the previous analysis, it can be seen that the room temperature equilibrium structure of Fe-C alloy consists of α and Fe 3C phases, in which α is a soft and tough phase and Fe 3C is a hard and brittle phase. Their mechanical properties are roughly as follows:

α:

Tensile strength σb 100~240MN/m2 yield strength σ0.2 100~ 180MN/m2 elongation δ 30~50% area shrinkage ψ 70~80% Brinell hardness HB 50~80.

Philippine 3C:

Brinell hardness HB 800 elongation δ 0

Therefore, iron 3C is a strengthening stage. If the matrix of the alloy is α, the strength of the material will be higher if the number of 3C is more and the distribution is more uniform. However, when the hard and brittle Fe 3C phase is distributed at the grain boundary, especially as the matrix, the plasticity and toughness of the material will be greatly reduced. This is the reason for the high brittleness of high carbon steel and white iron.

Fig. 7. 19 shows the effect of carbon content on the mechanical properties of carbon steel.

Fig. 7. Relationship between mechanical properties and carbon content of19 carbon steel

As can be seen from Figure 7. 19, pure iron with low carbon content is composed of single-phase α, so its properties are α, that is, good plasticity, low hardness and strength.

The microstructure of sub-* * steel is composed of different amounts of α and P. With the increase of carbon content, the amount of P in the microstructure increases correspondingly, and the hardness and strength of steel increase linearly, while the plasticity indexes (δ, ψ, impact value) decrease correspondingly.

The slow cooling structure of * * * steel consists of flake P. Because Fe 3C is a kind of strengthening phase, it is dispersed in the soft and tough α matrix in the form of fine flakes, which plays a strengthening role and makes P have high strength and hardness, but poor plasticity.

The microstructure of * * * precipitation after slow cooling is composed of P and Fe _ 3C _ II. With the increase of carbon content, iron 3C II

The number is gradually increasing. When the carbon content does not exceed 1.0%, the Fe _ 3C _ II precipitated on the grain boundary generally does not form a network, so it has little effect on the properties. When the carbon content is greater than 1.0%, due to the increase of fe3c I content and its continuous network distribution, the steel has high brittleness, low plasticity and reduced strength.

impact value