Phase Diagrams and Structure of Alloys. System of Iron-Carbon Alloys

 

1.5.1. Essence and Plotting of Phase Diagrams

 

The solidification (crystallization) of metallic alloys and many laws concerning their structure can be described by means of the equilibrium diagrams discussed below. These diagrams, also called constitutional and phase diagrams, are convenient graphical representations of the phase content and structure of an alloy at any temperature and composition. Equilibrium or phase diagrams as their name implies are plotted for equilibrium conditions or for conditions sufficiently close to equilibrium.

The method of thermal analysis is the most frequently used for the phase diagrams plotting. The idea of method is the plotting the cooling curves of different alloys by special device and a thermocouple (Fig. 1.30).

Plotting of lead-antimony (Pb-Sb) diagram is frequently discussed as an example.

The group of Pb-Sb alloys is taken, where Sb content increases from 0 to 100 %:
(table 1.1).

Fig. 1.30. Method of thermal analysis in plotting of phase diagrams: 1-alloy; 2-crucible;

3-thermocouple; 4-device, recording the change of the temperature in the time

 

Table 1.1 - Group of Pb-Sb alloys for the thermal analysis method

No of alloys	1	2	3	4	5	6	...	11	12
Pb, %	100	95	90	87	80	70	...	10	0
Sb, %	0	5	10	13	20	30	...	90	100

 

Every alloy is melted in the crucible; and the cooling curve is plotted by the device
(Fig. 1.31).

 

Fig. 1.31. Plotting of Pb-Sb phase diagram

 

Upon cooling the temperature of pure lead (Pb) drops uniformly to temperature 327°C or melting point, at which lead solidifies. A horizontal step appears on the curve because the latent heat of solidification evaluate. When lead has completely solidified, the temperature again drops uniformly. Antimony (Sb) solidifies in a similar manner (alloy №12).

Other alloys, except № 4 (table 1.1), solidify in interval of temperatures, i.e. they have temperatures of solidification beginning and temperatures of solidification finish. For example, when alloy № 2 is cooled, the temperature drops uniformly to the temperature of 285ºС. At this temperature solidification begins and an inflection (critical point) is observed on the cooling curve. This inflection is due to a reduction in the cooling rate as a result of the evolution of the latent heat of solidification. Solidification of the alloys begins at the temperatures lying on the liquidus line ABC. Crystallization ends at the temperatures lying on the solidus line DBF.

Liquid lead-antimony solution exits at temperatures above the liquidus line. Two phases: liquid alloy and solid Pb exist between the lines AB and DB; and liquid alloy and solid Sb exist between lines BC and BF. Alloy № 4 differs from other alloys because it solidifies at constant temperature, like pure metals. Similar structure consisting of definite combinations of two (or more) solid phases, simultaneously freezing out of the liquid alloy, is called a eutectic. The eutectic is a mechanical mixture with a definite composition (in our case 13 %Sb+87%Pb).

During solidification the liquid solution of all Pb-Sn alloys is converted into eutectic at temperature 245°C (solidus line DBF).

 

1.5.2. Components and Phases of the Iron-Carbon System

 

The iron alloys are the most extensively used in industry. The most widely used ones are steel and cast iron, which are alloys of iron with carbon. Various alloying elements are added to obtain steel and cast iron with the required properties.

Iron is a metal of silvery-whitish color. Its atomic number is 26, the atomic mass is 55.85, and the atomic radius is 1,27 . The melting point is 1539 °C. The density of a-iron at room temperature is 7,68 g/cm. Iron is polymorphic, the crystal lattice of a-Fe is of the body-centered cubic type with a lattice constant of 2.8606 .

The g-iron exits at temperatures above 911°C to 1392°C. It has face-centered cubic lattice with constant of 3,645  at the temperature 911°C. The density of g-Fe is from 8.0 to 8.1 g/cm³.

Carbon is nonmetallic element. Its atomic number is 6, the density is 2.5g/cm³, the atomic mass is 12.011, the melting point is 3500°C, and atomic radius is 0,77 . Carbon is also polymorphic. It is brittle, and has low mechanical properties, when it has the graphite allotropic form. But carbon also occurs in the metastable diamond form.

The following phases and structural components are distinguished in the Fe-C system of alloys: liquid alloy, solid solutions (ferrite and austenite), the compound (cementite), mechanical mixtures (pearlite and ledeburite) and free graphite.

Ferrite (Fig.1.32a) is solid solution of carbon and other constituents in a-iron. Depending upon temperature, the carbon content in ferrite ranges from 0.006% at 20°C to 0.02 % at 727°C. Under a microscope ferrite is seen as homogeneous polyhedral grains. It has the following approximate mechanical properties:

.

Austenite (Fig.1.32 b) is the solid solution of carbon and other constituents in g-iron. The maximum solubility of carbon in g-iron is 2.14 %. The microstructure is made up of polyhedral grains. Austenite possesses the following approximate mechanical properties:

 

a                           b                                c

 

d                                      e                                 f

g                                        h

Fig. 1.32. Phases in steel: a – ferrite; b – austenite; c – cementite (white); d – lamellar pearlite;

e – globular pearlite; f – ledeburite; g – lamellar graphite; h – spheroidal (globular) graphite. x100

 

Cementite (Fig.1.32 c) is a chemical compound of iron and carbon, iron carbide Fe₃C. It has a carbon content of 6.67%. Owing to the ability of its decomposition at high temperature the melting point of cementite has not been precisely determined, but is taken equal to about 1250°C. It has high hardness, about 10000 MPa, and low, near zero, ductility. Cementite is a metastable phase. Under equilibrium conditions (holding at high temperature) cementite disintegrates into iron and carbon: Fe₃C®3Fe + C with formation of graphite.

Pearlite (Fig 1.32 d, e) is the mechanical mixture of ferrite and cementite with constant carbon content equal to 0.8 %. Pearlite is a structural component, which may be considered as two phases: ferrite and cementite. Pearlite has the following approximate mechanical properties: . It is named eutectoid.

Ledeburite, or eutectic (Fig. 1.32 f) is the mechanical mixture of pearlite and cementite at t£727°C or austenite and cementite at t>727°C. It has carbon content equal to 4.3 % C. The ledeburite has low strength and ductility, but high hardness, near 6500 MPa.

Graphite (Fig. 1.32 g, h) is soft, and has low strength. Its hardness is HB 100MPa. But carbon graphite fibers of high strength can be obtained in the decomposition of certain organic compounds (natural gas). They are used e.g. for reinforcing of aluminium alloys.

 

1.5.3. Iron-Cementite Equilibrium Diagram (Metastable Equilibrium)

 

The iron-cementite diagram shows the phase composition and structure of alloys in the carbon content range from pure iron to cementite (6.67 %C) (Fig. 1.33). The Fe-Fe₃C system is metastable. The formation of cementite in place of graphite takes place in case of rapid cooling of alloys.

Point A (1539°C) on the Fe-Fe₃C diagram is the melting point of pure iron. Point G (911°C) corresponds to the allotropic transformations a«g.

 

Fig. 1.33. The iron-cementite equilibrium diagram

 

The carbon content (by mass) for the characteristic points of the diagram is as follows:

- point E (2.14 %C) is the maximum carbon content in austenite at the eutectic melting temperature 1147°C;

- point S (0.8 %C) is the point of eutectoid formation;

- point C (4.3 %C) is the point of eutectic formation;

- point P (0.02 %C) is the maximum carbon content in ferrite at the eutectoid temperature 727°C.

Line ABC indicates the temperatures at which austenite (or ferrite 5) begins to freeze out of the liquid alloy, it is the liquidus line; line CD (liquidus line) - the temperature at which primary cementite (Fe₃C) begins to precipitate out of the liquid alloy.

Line AHJECF indicates the temperatures at which the solidification of alloys is finished. It is solidus line. Line ECF corresponds to the crystallization of the eutectic, which is called ledeburite. Thus:

                                     (1.19)

 

Phase and structural changes in Fe-Fe₃C alloys following solidification are due to the allotropy of iron and the change in solubility of carbon in austenite and in ferrite as the temperature is lowered.

In cooling line SE (the maximum solubility of carbon in austenite) is Acm. It represents the temperatures at which secondary cementite starts separating out of the austenite:

 

                                      (1.20)

 

The horizontal line PSK at the eutectoid temperature 727ºC, conventionally labeled A₁ (in cooling Ar₁, in heating Ac₁), is termed the lower critical temperature, below which, under equilibrium conditions, all austenite will have transformed into ferrite and cementite phases:

 

                                            (1.21)

 

The phase boundaries denoted by A₃ (line GS, in cooling Ar₃, in heating Ac₃) and by A_cm (line SE, in cooling and in heating is denoted by the same A_cm) represent the upper critical temperature lines, for hypoeutectoid and hypereutectoid steels, respectively.

The critical point of the g«a transformation at 1392ºC is denoted by Ac₄ (in heating) and by Ar₄ (in cooling).

Variation of the solubility of carbon in ferrite with the temperature corresponds to line PQ. In cooling under equilibrium conditions, this line corresponds to the temperatures at which tertiary cementite begins to precipitate out of the solid alloy; in heating it is completely dissolved at these temperatures. When the temperature drops to 727°C the austenite containing 0.8 %C (point S), is transformed into pearlite. Line PSK is named the austenite-pearlite transformation line.

Alloys, containing 0,02 %C or less are called ingot irons or simply irons.

Alloys, containing from 0,02 to 0,8 %C are called hypoeutectoid steels; steel with 0.8 %C is eutectoid steel; alloys, containing from 0.8 to 2.14% C are called hypereutectoid steels.

Alloys, containing from 2.14 to 6.67 %C are called cast irons: they are hypoeutectic (C=2.14…4.3 %), eutectic (4.3 %C) and hypereutectic (from 4.3 to 6.67 %C) alloys.

 

1.5.4. Effect of Carbon and Minor Constituents on the Properties of Steel

 

Steel is a multiple-component alloy containing carbon, alloying elements (Mn, Si, Ni, Cr, W, etc.), a number of constant (Mn, Si, Cr, Cu, etc.) and unavoidable impurities (S, P, O, N, H and others), which influence its properties. The presence of these impurities is due to the difficulty of removing them in smelting of the steel (S, P, O, N, H), or their transfer to steel in deoxidation (Mn, Si, Al), or from the charge of scrap metal (Cr, Ni, Cu, etc.)

Effect of Carbon. The more carbon is in steel the more cementite and less ferrite is in it
(Fig 1.34). Hard and brittle particles of cementite impede the motion of dislocations, thereby increasing the resistance to deformation and also reduce the ductility and toughness. Consequently, an increase in carbon content in steel increases its hardness, tensile strength and yield point, and reduces the percent elongation, reduction in area and impact strength. The carbon content in excess of 1.0 or 1.2 % increases the hardness of steel in the annealed state, but reduces its tensile strength (Fig. 1.35). The latter is due to the precipitation of secondary cementite along the boundaries of grains, forming a continuous network.

Effect of Silicon and Manganese. The silicon content in carbon steel as an impurity does not usually exceed 0.35 or 0.4 %. The manganese content ranges from 0.5 to 0.8 %.

Silicon and manganese are introduced in the deoxidization period in steelmaking. Silicon and manganese deoxide liquid steel, i.e. they combine with the oxygen of the ferrous oxide (FeO) and go over into the slag in the form of oxides SiO₂ and MnO. Deoxidation improves the properties of steel.

The silicon that remains after deoxidation in the solid solution (in the ferrite) greatly increases the yield point s_y. This, in turn, reduces the drawing capacity of the steel and, especially, its capacity to be efficiently cold-headed. Therefore, the silicon content must be kept low in steels intended for cold press working and cold heading.

Manganese appreciably raises the strength of steel without practically reducing its ductility. It sharply reduces the read-shortness, i.e. brittleness at high temperatures due to the effect of sulphur.

 

 

Fig. 1.34. Percent change in ferrite and cementite volumetric content in steel and cast iron

 

 

Fig. 1.35. Effect of carbon on the mechanical properties of steel

 

Effect of Sulphur. Sulphur is a harmful impurity in steel. It forms the chemical compound FeS with iron, which is practically insoluble in iron in the solid state, but is soluble in the liquid metal. This compound forms a eutectic with the iron with the low melting point (988°C). The eutectic is formed even with slight sulphur content, separating out of the liquid at the very end of solidification, and locates mainly along the grain boundaries. When the steel is heated to the rolling or fording temperature (1000° to 1200°C) the eutectic melts violating the bonds between the metal grains. As a result, tears and cracks are developed when the metal is hot-worked. This phenomenon is known as hot-shortness, or red-shortness. The presence of manganese in steel, which has a greater affinity to sulphur than iron and forms the high-melting compound MnS, practically excludes red-shortness.

Sulphur inclusions unfavorably affect the mechanical properties: impact strength (KCU), ductility and cold resistance. For these reasons, the sulphur content in steel is strictly limited. Depending upon the quality of the steel, the maximum permissible content ranges from 0.015 to 0.055 %S.

Effect of Phosphorus. Phosphorus dissolves in both a-Fe and g-Fe. When it dissolves in ferrite, phosphorus distorts the crystal lattice and increases tensile strength and yield point, but greatly reduces ductility, toughness and cold-resistance. The maximum phosphorus content may range from 0.015 to 0.060 %.

Effect of Nitrogen, Oxygen and Hydrogen. Nitrogen and oxygen are present in steel in the form of brittle nonmetallic inclusions (oxides SiO₂, MnO, Al₂O₃, nitrides Fe₄N, TiN, VN etc.). They reduce ductility, toughness, impact strength and cold resistance.

The hydrogen absorbed in smelting steel not only embrittles steel, but also promotes the formation of flakes (cavities) in rolled stock and large forgings.

Vacuum smelting and blowing by inert gases processes, widely used in steelmaking in recent years, considerably reduce the gas content in steel.

 

Heat-Treatment of Steel

 

1.6.1. Phase Transformations in Iron Alloys

 

The heat-treatment of steel is based on:

- polymorphism of iron;

- different solubility of carbon in a-Fe and g-Fe;

- ability of cementite Fe₃C to decompose by the reaction Fe₃CÛFe+3C;

- high diffusion ability of carbon.

Let us consider the transformation of a ferrite - cementite mixture (pearlite) into austenite, using eutectoid steel (0.8 %C) as an example. According to the diagram Fe-Fe₃C, when the steel is heated above line PSK (or point S), pearlite transforms in austenite:

                                              

                               (1.22)

 

As we can see the polymorphic transformation Fe_a®Fe_g and carbon diffusion take place. According to scientific investigations, process of pearlite-to-austenite transformation consists of some steps (Fig. 1.36):

- beginning of austenite formation at temperature higher than 727°C (point 1);

- end of ferrite-to-austenite transformation (point 2);

- complete dissolution of the carbides and receipt of non-homogeneous austenite
(point 3);

- homogenization of austenite (point 4).

When steel with an austenite structure, obtained by heating to temperature above the line GSE, is supercooled to temperatures below the line PSK (A₁), the austenite is in a metastable state and undergoes the transformation.

Fig. 1.36. Pearlite-to-austenite transformation in heating

 

The kinetics of supercooled austenite transformations may be comprehensively described by means of an experimentally plotted time-temperature-decomposition diagram, or isothermal (constant-temperature) austenite transformation diagram (more commonly called TTT diagrams, because they relate the transformations of austenite to the pertinent time and temperature conditions). Such diagrams are also called S-curves and C-curves because of their shapes
(Fig. 1.37). Curve 1 of diagram represents the beginning of austenite decomposition and curve 2 indicates the time required for complete decomposition. The area to the left of the curve 1 showing the start of austenite decomposition determines the length of the incubation period. In the temperature and time ranges, specified by this area, supercooled austenite exists in which no appreciable decomposition has yet occurred.

If austenite is cooled with low speed (V₁), decomposition of the austenite occurs with the formation of a lamellar structure of ferrite and cementite (pearlite). This process is diffusive. This follows from the fact that austenite, which is practically homogeneous in its carbon concentration, decomposes with the formation of ferrite (almost pure iron) and cementite containing 6.67 % C, i.e. into two phases with greatly differing carbon content.

The transformation Austenite®Pearlite has the same steps as transformation Pearlite®Austenite, but the process goes in reverse order. At low degrees of supercooling (V₁) a clearly differentiated ferrite-cementite aggregate (pearlite) is formed. At a higher degree of supercooling (V₂) a finer mixture called sorbite is obtained. At still greater supercooling of the austenite (V₃) an even more dispersed mixture, called troostite, is obtained. Pearlite, sorbite and troostite are ferrite-cementite mixtures differing from each other by dispersion only.

Fig. 1.37. TTT diagram for carbon steel

 

If austenite is supercooled with high speed (V₅), diffusion is completely suppressed and the formation of phase consisting of ferrite and cementite becomes impossible. This leads to the diffusionless transformation of austenite into the structure of hardened steel, called martensite, which is a supersaturated interstitial solid solution of carbon in a -iron. For this reason, the carbon content in the martensite in the general case is the same as in the supercooled austenite (ferrite contains no more than 0.02 %C).

An intermediate transformation takes place with intermediate cooling speed (V₄). This transformation has many features inherent in both the pearlite (diffusion) and the martensite (diffusionless) transformations. As a result of this supercooled austenite transformation, a structure is formed which consists of the a-phase (ferrite), oversaturated with carbon and particles of cementite of a typical needlelike shape. This kind of structure is called bainite.

 

1.6.2. Practice of Heat-Treatment of Steel

 

The term heat-treatment means a change in the structure and, consequently, in the properties of an alloy, accomplished by heating it to a definite temperature, holding at this temperature and subsequent cooling at a special rate.

There are several kinds of heat-treatment used in practice (annealing, normalizing, hardening and tempering) which differently affect the structure and properties of steel, and which are assigned to meet the requirements to the semifabricated materials (castings, forgings, rolled stock etc.) and finished articles.

The scheme of main kinds of heat-treatment is given below (Fig. 1.38).

First type annealing includes homogenization, recrystallization and residual stress-relief annealing (Fig. 1.39). A distinctive feature of this kind of annealing is that mentioned processes are performed regardless of whether or not phase transformations take place in the alloys during the treatment.

Homogenization (Diffusion Annealing) is applied to alloy steel ingots to reduce dendrite and intracrystalline segregation, which increase the susceptibility of steel to brittle failure, anisotropy of properties, etc. Dendritic segregation reduces the ductility and toughness of steel. Steel should be heated to a high temperature, equal to (0.8…0.9). T melting, K, or 1000…1200°C, in diffusion annealing because the diffusion processes required to equalize the composition throughout the steel are more fully completed at these temperatures.

 

                                             Homogenization (Diffusion annealing)

                     First type Recrystallization Annealing

Annealing                    Stress Relief Annealing

                                            Full Annealing

                     Second type       Partial Annealing

                                            Isothermal Annealing

    Normalizing              Full Hardening

    Hardening                 Partial Hardening

                                            Low-temperature Tempering

    Tempering                Medium-temperature Tempering

                                            High-temperature Tempering

 

Fig. 1.38. Scheme of main kinds of heat-treatment

 

Fig. 1.39. Temperature regions for heating of steel for heat-treatments:

1- diffusion annealing; 2- recrystallization annealing; 3- stress-relief annealing; 4- full annealing;

5-partial annealing; 6 – full quenching; 7 – partial quenching; 8 – low – temperature tempering;

9 – medium - temperature tempering; 10 – high - temperature tempering

 

The total time required for diffusion annealing may reach from 50 to 100 hours and even more. A coarse grain is produced by diffusion annealing. After homogenization metal undergoes full annealing or normalizing to refine the grain and improve the properties.

Recrystallization annealing consists of heating of cold-worked steel to a temperature above that of recrystallization (0.4…0.6) T_melt., K, holding at this temperature and subsequent cooling. The temperature of recrystallization annealing depends upon the composition of the steel and is usually in the range from 650 to 750°C. The heating time is from 0.5 to 1.5 hours. Recrystallization annealing removes (eliminates) strain hardening, decreases strength and increases plastic properties.

Stress Relief Annealing is applied to castings, weldments and work pieces, following machining and other operations, in which residual stresses have developed in previous processing as a result of non-uniform cooling, non-uniform plastic deformation, etc. The annealing temperature is usually from 350 to 600°C, the holding time is several hours. Residual stresses are also relieved in performing other kinds of heat-treatment, for example, recrystallization annealing, annealing with phase recrystallization (second type), tempering hardened steel, etc.

Second-type annealing (Phase recrystallization). Annealing of this type consists in heating steel to temperatures above point Ac₃ or Ac₁ (Fig. 1.39), holding at these temperatures and slow cooling. As a result of the phase transformations in the annealing process a state of practically equilibrium structure is reached.

Full annealing is the heating of steel to temperatures above point Ac₃ or Ac_m (line GSE) (Fig. 1.40).

Partial annealing is the heating of steel to temperatures above point Ac₁ (line PSK).

In isothermal annealing alloy steel is heated as for ordinary annealing and then is cooled relatively rapidly (by putting the steel into another furnace) to a temperature lower than Ac₁ (usually by 50 to 100°C) (Fig. 1.33). The steel is hold isothermally at this temperature during a certain time sufficient for complete austenite decomposition. This is followed by comparatively rapid cooling in air. The main advantage of isothermal annealing is that it reduces the time, required for the process, especially for alloy steel, which must be cooled very slowly to obtain the required reduction in hardness.

Normalizing of steel. Normalizing consists in heating hypoeutectoid steel to a temperature exceeding Ac₃ (lines GS) and hypereutectoid steel to one exceeding Acm (lines SE) by 50 or 60°C, holding and then cooling in air.

Fig. 40. Diagram of the full annealing of hypoeutectoid alloy steel

 

Fig. 1.41. TTT curve for the isothermal annealing of alloy steel

 

Normalizing causes recrystallization of the steel and, consequently, refines the coarse-grained structure obtained in casting or rolling.

More rapid cooling (in air), used in normalizing, causes the austenite to decompose at lower temperatures. This increases the dispersity of the ferrite-cementite aggregate and improves the mechanical properties of steel.

Hardening of steel. Hardening (quenching) consists in heating to a temperature from 30 to 70°C above point Ac₃ (line GS), or above point A_cm (line SK), holding until the phase transformations are completed and then cooling at a rate above the critical (Fig. 1.34). Such cooling is called quenching (carbon steels are usually quenched in water, alloy steels in oil or other media). The austenite is transformed into martensite during quenching.

Fig. 1.42. Diagram of hardening of hypoeutectoid alloy steel

 

Hardening or quenching is not final heat-treatment. It is followed by tempering to reduce brittleness and stresses due to hardening and to obtain the required mechanical properties.

Tool steels are hardened and tempered mainly to increase their hardness, wear resistance and strength; structural steel, to increase its strength (s_u, s_y) and hardness and to obtain a sufficiently high ductility (d and j) and impact strength (KCU).

Tempering of Steel. Tempering consists in heating hardened steel to a temperature not above Ac₁, holding at given temperature (from 1.0 to 2.5 hours) and subsequent cooling at specified rate. Tempering is a final operation in heat-treatment. Steel acquires the required mechanical properties (Fig. 1.43) as a result of tempering. Besides, tempering completely or partly relieves the internal stresses developed in quenching. The higher the tempering temperature the more completely are the stresses relieved.

Low-temperature tempering is performed by heating to temperatures from 150 to 250°C. It decreases the internal stresses and transformers martensite, produced by quenching, into tempered martensite. Mechanical properties retain without any appreciable changes.

Medium-temperature tempering at 350 to 500°C provides improvement of elastic limit and toughness and some decrease in strength and hardness. The tempered steel has structure containing temper troostite.

High-temperature tempering is performed in the range from 500 to 680°C. The steel has a structure consisting of temper sorbite. This heat-treatment almost completely relieves internal stresses, increases the plasticity and toughness and reduces the strength and hardness.

Fig. 1.43. Effect of tempering temperature on mechanical properties of steel

containing 0.4 % C (a) and 0.4 % C, 1.5 % Cr and 3.0 % Ni (b)