Constitution of Alloys and Phase Diagrams

CONSTITUTION OF ALLOYS AND PHASE DIAGRAMS

1 Classification of materials

2 Type of bonding

3 Crystal structures

4 Imperfection in solids

5 Introduction to phase diagram

6 Solid solution

7 Iron Carbon Diagram

8 Metal types

8.1 Ferrous metals

8.2 Alloy steels

8.3 Non ferrous metals

1 CLASSIFICATION OF MATERIALS

1Metals

Valence  electrons are detached  from  atoms, and spread in  an 'electron sea' that "glues" the ions together. Metals are usually strong, conduct electricity and heat well and are opaque to light (shiny if polished). Examples: aluminum, steel, brass, gold.

2 Semiconductors

The bonding is covalent (electrons are shared between atoms). Their electrical properties depend extremely strongly on minute proportions of contaminants. They are opaque to visible light but transparent to the infrared. Examples: Si, Ge, GaAs.

3Ceramics

Atoms behave mostly like either positive or negative ions, and are bound by Coulomb forces between them. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Examples: glass, porcelain, many minerals.

4.Polymers

Are bound by covalent forces  and also by weak  van  der  Waals  forces, and usually based on H,  C and other non-metallic elements.  They  decompose  at moderate temperatures (100 - 400 C), and are lightweight.  Other  properties vary greatly. Examples: plastics (nylon, Teflon, polyester) and rubber.

2 TYPES OF BONDING

1 Ionic Bonding

This is the bond when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron). Then there is a strong, direct Coulomb attraction. An example is NaCl. In the molecule, there are more electrons around Cl, forming Cl^- and less around Na, forming Na⁺. Ionic bonds are the strongest bonds.

1.2.2 Covalent Bonding

In covalent bonding, electrons are shared between the   molecules, to saturate the valency. The simplest example is the   H2 molecule,   where the electrons spend more time in between the nuclei than outside, thus producing bonding.

3 Metallic Bonding

In the metallic bond encountered in pure metals and metallic alloys, the atoms contribute their outer-shell electrons to a generally shared electron cloud for the whole block of metal.

Ø Secondary Bonding (Van der Waals)

Ø Fluctuating Induced Dipole Bonds Polar

Ø Molecule-Induced Dipole Bonds

Ø Permanent Dipole Bonds

3 CRYSTAL STRUCTURES

Atoms self-organize in crystals, most of the time. The crystalline lattice is a periodic array of the atoms. When the solid is not crystalline, it is called amorphous. Examples of crystalline solids are metals, diamond and other precious stones, ice, graphite. Examples of amorphous solids are glass, amorphous carbon (a-C), amorphous Si, most plastics

1.Unit Cells

The unit cell is  the smallest  structure  that  repeats  itself by  translation through the crystal. The most common types of unit cells are the faced centered cubic (FCC), the body-centered cubic (FCC) and the hexagonal close-packed (HCP).  Other types  exist, particularly among minerals.

2.Polymorphism and Allotropy

Some materials may exist in more than one crystal structure, this is called polymorphism.

If the material is an elemental solid, it is called allotropy. An example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon.

3.Polycrystalline Materials

A solid can be composed of many crystalline grains, not aligned with each other. It is called polycrystalline. The grains can be more or less aligned with respect to each other. Where they meet is called a grain boundary.

4 IMPERFECTION IN SOLIDS

Materials are not stronger when they have defects.

The study of defects is divided according to their dimension:

0D (zero dimension) - point defects: vacancies and interstitials Impurities. 1D - linear defects: dislocations (edge, screw, mixed)

2D - grain boundaries, surfaces. 3D - extended defects: pores, cracks

5 Introduction to phase diagram

Component

Pure metal or compound (e.g., Cu, Zn in Cu-Zn alloy, sugar, water, in syrup.)

Solvent

Host or major component in solution.

Solute

Dissolved, minor component in solution.

System

Set of possible alloys from same component (e.g., iron-carbon system.)

Solubility Limit

Maximum solute concentration that can be dissolved at a given temperature.

Phase

Part with homogeneous physical and chemical characteristics

6 Solid Solutions

A solid solution occurs when we alloy two metals and they are completely soluble in each other. If a solid solution alloy is viewed under a microscope only one type of crystal can be seen just like a pure metal. Solid solution alloys have similar properties to pure metals but with greater strength but are not as good as electrical conductors.

The common types of solid solutions are

1)   Substitutional solid solution

2)      Interstitial solid solutions

Substitution solid solution

The name of this solid solution tells you exactly what happens as atoms of the parent metal ( or solvent metal) are replaced or substituted by atoms of the alloying metal (solute metal) In this case, the atoms of the two metals in the alloy, are of similar size.

Interstitial solid solutions:

In interstitial solid solutions the atoms of the parent or solvent metal are bigger than the atoms of the alloying or solute metal. In this case, the smaller atoms fit into interstices i.e spaces between the larger atoms.

Phases

One-phase systems are homogeneous. Systems with two or more phases are heterogeneous, or mixtures. This is the case of most metallic alloys, but also happens in ceramics and polymers.

A two-component alloy is called binary. One with three components is called ternary.

Microstructure

The properties of an alloy do not depend only on concentration of the phases but how they are arranged structurally at the microscopy level. Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space.

A binary alloy may be

ØA single solid solution

ØTwo separated essentially pure components. ØTwo separated solid solutions.

ØA chemical compound, together with a solid solution.

Phase diagram:

A graph showing the phase or phases present for a given composition as a function of temperature.

Poly phase material:

A material in which two or more phases are present.

Eutectoid:

Transforming from a solid phase to two other solid phases upon cooling.

Peritectoid:

Transforming from two solid phases to a third solid phase upon cooling.

Peritectoid reaction:

A reaction in which a solid goes to a new solid plus a liquid on heating, and reverse occurs on cooling.

Iron-Iron Carbon diagram is essential to understand the basic differences among iron alloys and control of properties.

Iron is allotropic; at room temperature pure iron exists in the Body Centered Cubic crystal form but on heating transforms to a Face Centered Cubic crystal. The temperature that this first transformation takes place is known as a critical point and it occurs at 910 degrees Celsius. This change in crystal structure is accompanied by shrinkage in volume, sine the atoms in the face centered crystal are more densely packed together than in the body centered cubic crystal. At the second critical point the F.C.C crystal changes back to a B.C.C crystal and this change occurs at 1390 degrees Celsius.

§ Iron above 1390 degrees is known as delta iron

§ Iron between 1390 and 910 degrees is known as gamma iron, Iron below 910 degrees is known as alpha iron.

7 IRON CARBON DIAGRAM

Iron-carbon phase diagram

Iron-carbon phase diagram describes the iron-carbon system of alloys containing up  to 6.67%  of carbon, discloses the   phases compositions and their transformations occurring with   the alloys during their   cooling or heating. Carbon   content 6.67% corresponds to   the fixed c omposition   of the iron carbide Fe3C.

The diagram is presented in the picture:

The following phases are involved in the transformation, occurring with iron-carbon alloys: L - Liquid solution of carbon in iron;

 δ-ferrite - Solid solution of carbon in iron.

 Maximum concentration of carbon in δ-ferrite is 0.09% at 2719 ºF (1493ºC) - temperature of the peritectic transformation.

 The crystal structure of δ-ferrite is BCC (cubic body centered). Austenite - interstitial solid solution of carbon in γ-iron.

 Austenite has FCC (cubic face centered) crystal structure, permitting high solubility of carbon - up to 2.06% at 2097 ºF (1147 ºC).

Austenite does not exist below 1333 ºF (723ºC) and maximum carbo n concentration at this temperature is 0.83%.

α-ferrite - solid solution o f carbon in α-iron. α-ferrite has BCC crystal structure and low solubility of carbon - up to 0.25% at 1333 ºF (723ºC). α-ferrite exists at room temperature.

Cementite - iron carbide , intermetallic compound, having fixed com position Fe3C.

Cementite is a hard and brittle substance, influencing on the properties of steels and cast irons.

The following phase transformations occur with iron-carbon alloys:

Alloys, containing up to 0.51% of carbon, start soli dification with formation of crystals of δ-ferrite. Carbon content in δ-ferrite increases up to 0.09% in course solidification, and at 2719 ºF (1493ºC) remaining liquid phase and δ- ferrite perform peritectic transformation, resulting in formation of austenite.

Alloys, containing carbon more than 0.51%, but less than 2.06%, form primary austenite crystals in the beginning of solidification and when the temperature reaches the curve ACM primary cementite stars to form.

Iron-carbon alloys, containing up to 2.06% of carbon, are called steels.

Alloys, containing from  2.06 to 6.67% of carbon, experience eutectic transformation at 2097 ºF (1147 ºC). The eutectic concentration of carbon is 4.3%.

In practice only   hypoeutectic alloys   are used. These alloys (carbon content from 2.06% to 4.3%) are called cast irons. When  temperature of  an alloy from this  range reaches 2097 ºF  (1147  ºC),  it  contains  primary  austenite crystals  and  some  amount  of  the  liquid  phase. The latter decomposes  by eutectic mechanism to a fine mixture of austenite and cementite, called ledeburite. 

All iron-carbon alloys (steels and cast irons) experience eutectoid transformation at 1333 ºF (723ºC). The eutectoid concentration of carbon is 0.83%.

When the temperature of an alloy reaches 1333 ºF (733ºC), austenite transforms   to pearlite  (fine  ferrite-cementite  structure,   forming as   a  result  of decomposition of austenite at slow cooling conditions).

Critical temperatures

Upper critical temperature (point) A3 is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoid alloys.

Upper critical temperature (point) ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys.

Lower critical temperature (point) A1 is the temperature of the austeniteto-pearlite eutectoid transformation. Below this temperature austenite does not exist.

Magnetic transformation temperature A2 is the temperature below which α-ferrite is ferromagnetic.

Phase compositions of the iron-carbon alloys at room temperature

o Hypoeutectoid steels (carbon content from 0 to 0.83%) consist of primary (proeutectoid) ferrite (according to the curve A3) and pearlite.

o Eutectoid steel (carbon content 0.83%) entirely consists of pearlite.

o Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist of primary (proeutectoid) cementite (according to the curve ACM) and pearlite.

o Cast irons (carbon content  from 2.06%  to  4.3%)  consist of proeutectoid  cementite C2 ejected from  austenite according  to  the  curve ACM , pearlite  and transformed ledeburite (ledeburite in which austenite transformed to pearlite).

At 4.3% carbon composition, on cooling Liquid phase is converted in to two solids

hence forming Eutectic reaction.

L ↔ γ + Fe3C

Eutectoid: 0.76 wt%C, 727 °C

γ(0.76 wt% C) ↔ α (0.022 wt% C) + Fe3C

Shown below is the steel part of the iron carbon diagram containing up to 2% Carbon. At the eutectoid point 0.83% Carbon, Austenite which is in a solid solution changes directly into a solid known as Pearlite which is a layered structure consisting of layers of Ferrite and Cementite

8. METAL TYPES

The metals that Steelworkers work with are divided into two general classifications: ferrous and nonferrous. Ferrous metals are those composed primarily of iron and iron alloys. Nonferrous metals are those composed primarily of some element or elements other than iron. Nonferrous metals or alloys sometimes contain a small amount of iron as an alloying element or as an impurity.

1.FERROUS METALS

Ferrous metals include all forms of iron and steel alloys. A few examples include wrought iron, cast iron, carbon steels, alloy steels, and tool steels. Ferrous metals are iron- base alloys with small percentages of carbon and other elements added to achieve desirable properties. Normally, ferrous metals are magnetic and nonferrous metals are nonmagnetic.

IRON

Pure iron rarely exists outside of the laboratory. Iron is produced by reducing iron ore to pig iron through the use of a blast furnace. From pig iron many other types of iron and steel are produced by the addition or deletion of carbon and alloys. The following paragraphs discuss the different types of iron and steel that can be made from iron ore.

PIG IRON

Pig iron is composed of about 93% iron, from 3% to 5% carbon, and various amounts of other elements. Pig iron is comparatively weak and brittle; therefore, it has a limited use and approximately ninety percent produced is refined to produce steel. Cast-iron pipe and some fittings and valves are manufactured from pig iron.

WROUGHT IRON

Wrought iron is made from pig iron with some slag mixed in during manufacture. Almost pure iron; the presence of slag enables wrought iron to resist corrosion and oxidation. The chemical analyses of wrought iron and mild steel are just about the same. The difference comes from the properties controlled during the manufacturing process. Wrought iron can be gas and arc welded, machined, plated, and easily formed; however, it has a low hardness and low-fatigue strength.

CAST IRON

Cast iron is any iron containing greater than 2% carbon alloy. Cast iron has a high- compressive strength and good wear resistance; however, it lacks ductility, malleability, and impact strength. Alloying it with nickel, chromium, molybdenum, silicon, or vanadium improves toughness, tensile strength, and hardness. A malleable cast iron is produced through a easily as the low-carbon steels. They are used for crane prolonged annealing process. hooks, axles, shafts, setscrews, and so on.

INGOT IRON

Ingot iron is a commercially pure iron (99.85% iron) that is easily formed and possesses good ductility and corrosion resistance. The chemical analysis and properties of this iron and the lowest carbon steel are practically the same. The lowest carbon steel, known as dead- soft, has about 0.06% more carbon than ingot iron. In iron the carbon content is considered an impurity and in steel it is considered an alloying element. The primary use for ingot iron is for galvanized and enameled sheet.

STEEL

 All the different metals and materials that we use in our trade, steel is by far the most important. When steel was developed, it revolutionized the American iron industry. With it came skyscrapers, stronger and longer bridges, and railroad tracks that did not collapse. Steel is manufactured from  pig iron by decreasing the amount of carbon and other impurities and adding specific amounts of alloying elements. Do not confuse steel with the two general classes of iron: cast iron (greater than 2% carbon) and pure iron (less than 0.15%  carbon). In steel manufacturing, controlled amounts of alloying elements are added during the molten stage to produce the desired composition. The composition of a steel is determined by its application and the specifications  that were developed by the following: American Society for Testing and Materials (ASTM), the American Society of Mechanical Engineers (ASME), the Society of Automotive Engineers (SAE), and the American Iron and Steel Institute (AISI).Carbon steel is a term applied to a broad range of steel that falls  between the  commercially  pure  ingot  iron and the cast irons. This range of carbon steel may be classified into four groups:

HIGH-CARBON STEEL/VERY HIGH-CARBON STEEL

Steel   in these   classes   respond   well to   heat   treatment   and   can   be welded. When welding, special electrodes must be used along with preheating and stress- relieving procedures to prevent cracks in the weld areas. These steels are  used  for dies,  cutting  tools, milltools,  railroad  car  wheels, chisels, knives, and so on.                

LOW-ALLOY, HIGH-STRENGTH, TEMPERED STRUCTURAL STEEL  

A special lowcarbon steel, containing specific small amounts of alloying elements, that is quenched and tempered to get a yield strength of greater than  50,000  psi  and  tensile  strengths  of  70,000 to  120,000  psi. Structural members made from these high-strength steels may   have   smaller   cross- sectional areas  than  common  structural  steels  and still have equal or greater strength.  Additionally,  these steels are normally more corrosion-  and abrasionresistant.

High-strength steels are covered by ASTM specifications. NOTE:  This type of  steel is much tougher than low-carbon  steels. Shearing machines for this type of steel must have twice the capacity than that required for low-carbon steels

STAINLESS STEEL

This type of steel is classified by the American Iron and Steel Institute (AISI) into two general series named the 200-300 series and 400 series. Each series includes several types of steel with different characteristics. The 200-300 series of stainless steel is known as

AUSTENITIC.

 This   type   of   steel   is very tough and ductile in the as-welded condition; therefore, it  is ideal for welding and requires no annealing under normal atmospheric conditions.  The  most  well-known types  of steel  in this series are the   302 and 304. They are   commonly   called   18-8 because they are composed of 18% chromium and 8% nickel.      

The chromium nickel steels        

Low-Carbon Steel . . .  0.05% to 0.30% carbon are the most widely used

and are normally nonmagnetic.       

Medium-Carbon Steel . . . 0.30% to 0.45% carbon   

High-Carbon Steel . . .  0.45% to0.75% carbon their crystalline structure

into two general groups.        

One Very High-Carbon Steel . . . 0.75% to 1.70% carbon group is known as

FERRITIC CHROMIUM and the other group as MARTENSITIC CHROMIUM.

2.ALLOY STEELS

Steels that derive their properties primarily from the presence of alloying element other than carbon are called ALLOYS or ALLOY STEELS. Note, however, that alloy steels always contain traces of other elements. Among the more common alloying elements are nickel, chromium, vanadium, silicon, and tungsten. One or more of these elements may be added to the steel during the manufacturing process to produce the desired characteristics.

Alloy steels may be produced in structural sections, sheets, plates, and bars for use in the as rolled condition. Better physical properties are obtained with these steels than are possible with hot. These alloys are used in structures where the strength of material is especially important. Bridge members, railroad cars, dump bodies, dozer blades, and crane booms are made from alloy steel. Some of the common alloy steels are briefly described in the paragraphs below.

NICKEL STEELS

These steels contain from 3.5% nickel to 5% nickel. The nickel increases the strength and toughness of these steels. Nickel steel containing more than 5% nickel has an increased resistance to corrosion and scale. Nickel steel is used in the manufacture of aircraft parts, such as propellers and airframe support members.

CHROMIUM STEELS

These steels have chromium added to improve hardening ability, wear resistance, and strength. These steels contain between 0.20% to 0.75% chromium and 0.45% carbon or more. Some of these steels are so highly resistant to wear that they are used for the races and balls in antifriction bearings.

Chromium steels are highly resistant to corrosion and to scale.

CHROME VANADIUM STEEL

This steel has the maximum amount of strength with the least amount of weight. Steels of this type contain from 0.15% to 0.25% vanadium, 0.6% to 1.5% chromium, and 0.1% to 0.6% carbon. Common uses are for crankshafts, gears, axles, and other items that require high strength. This steel is also used in the manufacture of high-quality hand tools, such as wrenches and sockets.

TUNGSTEN STEEL

This is a special alloy that has the property of red hardness. This is the ability to continue to cut after it becomes red-hot. A good grade of this steel contains from 13% to 19% tungsten, 1% to 2% vanadium, 3% to 5% chromium, and 0.6% to 0.8% carbon. Because this alloy is expensive to produce, its use is largely restricted to the manufacture of drills, lathe tools, milling cutters, and similar cutting tools.

MOLYBDENUM

This is often used as an alloying agent for steel in combination with chromium and nickel. The molybdenum adds toughness to the steel. It can be used in place of tungsten to make the cheaper grades of high-speed steel and in carbon molybdenum high-pressure tubing.

MANGANESE STEELS

The amount of manganese used depends upon the properties desired in the finished product. Small amounts of manganese produce strong, free-achgining steels. Larger amounts (between 2% and 10%) produce a somewhat brittle steel, while still larger amounts (11% to 14%) produce a steel that is tough and very resistant to wear after proper heat treatment.

3. NONFERROUS METALS

Nonferrous metals contain either no iron or only insignificant amounts used as an

alloy.

Some of the more common nonferrous metals Steelworkers work with are as follows: copper, brass, bronze, copper-nickel alloys, lead, zinc, tin, aluminum, and Duralumin. NOTE: These metals are nonmagnetic. COPPER

This metal and its alloys have many desirable properties. Among the commercial metals, it is one of the most popular. Copper is ductile, malleable, hard, tough, strong, wear resistant, machinable, weld able, and corrosion resistant. It also has high-tensile strength, fatigue strength, and thermal and electrical conductivity. Copper is one of the easier metals to work with but be careful because it easily becomes work-hardened; however, this condition can be remedied by heating it to a cherry red and then letting it cool. This process, called annealing, restores it to a softened condition. Annealing and softening are the only heat-treating procedures that apply to copper. Seams in copper are joined by riveting, silver brazing, bronze brazing, soft soldering, gas welding, or electrical arc welding. Copper is frequently used to give a protective coating to sheets and rods and to make ball floats, containers, and soldering coppers.

CARBON STEELS

Carbon steels are iron-carbon alloys containing up to 2.06% of carbon, up to1.65% of manganese, up to 0.5% of silicon and sulfur and phosphorus as impurities. Carbon content in carbon steel determines its strength and ductility. The higher carbon content, the higher steel strength and the lower its ductility. According to the steels classification there are following groups of carbon steels:

• Low carbon steels (C < 0.25%)

• Medium carbon steels (C =0.25% to 0.55%)

• High carbon steels (C > 0.55%)

• Tool carbon steels (C>0.8%)

Designation system of carbon steels Chemical compositions of some carbon steels Properties of some carbon steels

Low carbon steels (C < 0.25%)

Properties: good formability and weldability, low strength, low cost.

Applications: deep drawing parts, chain, pipe, wire, nails, some machine parts.

Medium carbon steels (C =0.25% to 0.55%)

Properties: good toughness and ductility, relatively good strength, may be hardened by quenching

Applications: rolls, axles, screws, cylinders, crankshafts, heat treated machine parts.

High carbon steels (C > 0.55%)

Properties: high strength, hardness and wear resistance, moderate ductility.

Applications: rolling mills, rope wire, screw drivers, hammers, wrenches, band saws.

Tool carbon steels (C>0.8%) - subgroup of high carbon steels

Properties: very high strength, hardness and wear resistance, poor weldability, low ductility.

Applications: punches, shear blades, springs, milling cutters, knives, razors. Designation

system of carbon steels

American Iron and Steel Institute (AISI) together with Society of Automotive Engineers

(SAE) have established four-digit (with additional letter prefixes) designation system:

LOW-ALLOY, HIGH-STRENGTH, TEMPERED STRUCTURAL STEEL

A special lowcarbon steel, containing specific small amounts of alloying elements, that is quenched and tempered to get a yield strength of greater than 50,000 psi and tensile strengths of 70,000 to 120,000 psi. Structural members made from these high-strength steels may have smaller cross- sectional areas than common structural steels and still have equal or greater strength. Additionally, these steels are normally more corrosion- and abrasionresistant. High-strength steels are covered by ASTM specifications. NOTE: This type of steel is much tougher than low-carbon steels. Shearing machines for this type of steel must have twice the capacity than that required for low-carbon steels

STAINLESS STEEL

This type of steel is classified by the American Iron and Steel Institute (AISI) into two general series named the 200-300 series and 400 series. Each series includes several types of steel with different characteristics. The 200-300 series of stainless steel is known as austenitic.

AUSTENITIC

This type of steel is very tough and ductile in the as-welded condition; therefore, it is ideal for welding and requires no annealing under normal atmospheric conditions. The most well-known types of steel in this series are the 302 and 304. They are commonly called 18-8 because they are composed of 18% chromium and 8% nickel. The chromium nickel steels Low-Carbon SAE 1XXX

First digit 1 indicates carbon steel (2-9 are used for alloy steels); Second digit indicates modification of the steel.

0 - Plain carbon, non-modified

1 - Resulfurized

2 - Resulfurized and rephosphorized

5 - Non-resulfurized, Mn over 1.0%

Last two digits indicate carbon concentration in 0.01%.

Example: SAE 1030 means non modified carbon steel, containing 0.30% of carbon.

A letter prefix before the four-digit number indicates the steel making technology:

A     -        Alloy,         basic  open  hearth

B      -        Carbon,   acid   Bessemer

C      -        Carbon,      basic  open  hearth

D     -        Carbon,      acid   open  hearth

E - Electric furnace

Example: AISI B1020 means non modified carbon steel, produced in acid Bessemer and containing 0.20% of carbon.

Chemical compositions of some carbon steels            

SAE/AISI grade  C, %  Mn,%          P,% max     S,% max

1006          0.08 max    0.35 max    0.04   0.05

1010          0.08-0.13    0.30-0.60    0.04   0.05

1020          0.17-0.23    0.30-0.60    0.04   0.05

1030          0.27-0.34    0.60-0.90    0.04   0.05

1045          0.42-0.50    0.60-0.90    0.04   0.05

1070          0.65-0.76    0.60-0.90    0.04   0.05

1090          0.85-0.98    0.60-0.90    0.04   0.05

1117          0.14-0.20    1.10-1.30    0.04   0.08-0.13

1547          0.43-0.51    1.35-1.65    0.04   0.05