Learning Objectives:

• Describe the major features of a phase diagram.

Gibbs Phase Rule:

In a system under a set of conditions, number of phases (P) exist can be related to the number of components (C) and degrees of freedom (F) by Gibbs phase rule.

\[P + F = C+ 2\]

Degrees of freedom refers to the number of independent variables (e.g.: pressure, temperature) that can be varied individually to effect changes in a system.

Information Obtained from Phase Diagram:

Important information, useful in materials development and selection, obtainable from a phase diagram:

• It shows phases present at different compositions and temperatures under slow cooling (equilibrium) conditions.

• It indicates equilibrium solid solubility of one element/compound in another.

• It suggests temperature at which an alloy starts to solidify and the range of solidification.

• It signals the temperature at which different phases start to melt.

• Amount of each phase in a two-phase mixture can be obtained.

Types of Phase Diagram:

1. Unary Phase Diagram: If a system consists of just one component (e.g.: water), equilibrium of phases exist is depicted by unary phase diagram. The component may exist in different forms, thus variables here are – temperature and pressure.

2. Binary phase diagram: If a system consists of two components, equilibrium of phases exist is depicted by binary phase diagram. For most systems, pressure is constant, thus independently variable parameters are – temperature and composition. Two components can be either two metals (Cu and Ni), or a metal and a compound (Fe and Fe\(_3\)C), or two compounds (Al\(_2\)O\(_3\) and Si\(_2\)O\(_3\)), etc. Two component systems are classified based on extent of mutual solid solubility

   • completely soluble in both liquid and solid phases (isomorphous system) and

   • completely soluble in liquid phase whereas solubility is limited in solid state.

   For isomorphous system - E.g.: Cu-Ni, Ag-Au, Ge-Si, Al\(_2\)O\(_3\)-Cr\(_2\)O\(_3\).

Learning Objectives:

• Understand the critical point and the meaning of the terms saturated liquid, saturated vapor and quality of mixture.

The liquid-vapour equilibrium curve has a top limit which is known as the critical point. The temperature and pressure corresponding to this are known as the critical temperature and critical pressure.

Learning Objectives:

• Identify the different points (triple point, normal boiling point, freezing point) and regions (solid, liquid, vapor) of a phase diagram.

Where all three curves, solid, liquid and gas, meet, there is a unique combination of temperature and pressure where all three phases are in equilibrium together. This point is called a triple point.

Learning Objectives:

• Introduce students to the basic understanding of the interpretations of iron-carbon diagrams and isothermal transformation diagrams.

• provide students with the skills needed to determine the temperatures to which steel must be heated to cause it to harden.

AG Caesar, CC BY-SA 4.0, via Wikimedia Commons

There are two iron-carbon equilibrium diagrams:

1. Stable iron-graphite Fe-Gr

2. Metastable iron-cementite Fe-Fe\(_3\)C

The stable condition usually takes a very long time to develop. The metastable diagram is of more interest. Fe\(_3\)C iron carbide called cementite because it is hard. Following phases exist on Fe-Fe\(_3\)C diagram: - Liquid solution of iron and carbon (L) - Ferrite (\(\alpha\)) – an interstitial solid solution of carbon in Fe \(\alpha\) (BCC). At room temperature ferrite is ductile but not very strong. - Austenite - an interstitial solid solution of carbon in Fe \(\gamma\) (FCC). - Cementite (Fe\(_3\)C) hard and brittle compound with chemical formula Fe\(_3\)C. It has metallic properties. On a base of Fe-Fe\(_3\)C diagram we can divide iron-carbon alloys into:

• Steels,

• Cast steels,

• Cast irons.

Steel is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content up to 2% intended for wrought products or semi products. Cast iron is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content over 2% intended for castings. Now, we consider only a part of Fe-Fe\(_3\)C diagram referring to steel. Perlite is a structure (i.e. consists of two phases) consists of alternate layers of Ferrite and Cementite in the proportion 87:13 by weight. Perlite is formed from Austenite at eutectoid temperature (A\(_1\)) 727\(^\circ\)C upon slow cooling. There are three groups of steels according to carbon content:

• Hypoeutectoid steels containing less than 0.76% C

• Eutectoid steel with carbon content about 0.76%

• Hypereutectoid steels contain more than 0.76% C (up to 2% C).

Learning Objectives:

• Interpret an iron-carbon diagram.

• Evaluate several industrial isothermal transformation diagrams

• Recognize that different steels have very different isothermal transformation diagrams.

• Cite the value of an isothermal transformation diagram and how it can be used.

• Differentiate between the different heat treatment procedures and the structural changes that accompany them.

• Identify different structures of alloy;

• Understand the different structural changes that accompany structure modifications.

• Eutectic: 4.30 wt%C, 1147 deg C.
  
  Eutectic Point refers to the point in a phase diagram indicating the chemical composition and temperature corresponding to the lowest melting point of a mixture of components.

• Eutectoid: 0.76 wt%C, 727 Deg C.
  
  A eutectoid reaction is a three-phase reaction by which, on cooling, a solid transforms into two other solid phases at the same time. If the bottom of a single-phase solid field closes (and provided the adjacent two-phase fields are solid also), it does so with a eutectoid point.
  
  The eutectoid (eutectic-like) reaction is similar to the eutectic reaction but occurs from one solid phase to two new solid phases. It also shows as V on top of a horizontal line in the phase diagram. There are associated eutectoid temperature (or temperature), eutectoid phase, eutectoid and proeutectoid microstructures.

Learning Objectives:

• Describe important structural forms of steel and iron.

There are three allotropic forms of iron:

1. Alpha iron (\(\alpha\)) or ferrite:
   As molten iron cools down, it solidifies at 1,538\(^\circ\)C into its \(\delta\) allotrope, which has a body-centered cubic (BCC) crystal structure.

2. Gamma iron (\(\gamma\)) or austenite:
   As the iron cools further to 1,394\(^\circ\)C its crystal structure changes to a face centered cubic (FCC) crystalline structure. In this form it is called gamma iron (\(\gamma\)-Fe) or Austenite. \(\gamma\)-iron can dissolve considerably more carbon (as much as 2.04% by mass at 1,146\(^\circ\)C).

3. Delta iron (\(\delta\)):
   At 912\(^\circ\)C the crystal structure again becomes BCC as \(\alpha\)-iron is formed. \(\alpha\)-iron can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910\(^\circ\)C).

Learning Objectives:

• Given a binary phase diagram, the composition of an alloy, its temperature, and assuming that the alloy is at equilibrium, determine:

  • what phase(s) is (are) present,

  • the composition(s) of the phase(s), and

  • the mass fraction(s) of the phase(s)

The Lever Rule is a way in which to calculate the proportions of each phase present on a phase diagram in a two phase field at a given temperature and composition.
The lever rule is a mechanical analogy to the mass balance calculation. The tie line in the two-phase region is analogous to a lever balanced on a fulcrum. Finding the amounts of phases in a two phase region:

1. Locate composition and temperature in diagram

2. In two phase region draw the tie line or isotherm

3. Fraction of a phase is determined by taking the length of the tie line to the phase boundary for the other phase, and dividing by the total length of tie line

Learning Objectives:

• Basic knowledge on thermodynamics and kinetic factors controlling solid state phase transformations in metals and alloys.

• Practical applications and procedures of heat treatments of metals and alloys.

Solidification of Pure Metal: Solidification is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. This process is a first order transformation which involves nucleation and growth. Nucleation is the process whereby nuclei (seeds) act as templates for crystal growth. Solid phase will nucleate till liquid phase is exhausted.

Precipitation Hardening: The strength and hardness of some metal alloys may be improved by the formation of extremely small, uniformly dispersed particles (precipitates) of a second phase within the original phase matrix.

Isothermal Transformation Diagrams or Time-Temperature-Transformation (TTT): Isothermal transformation diagrams are also known as Time-Temperature-Transformation (TTT) diagrams) are plots of temperature versus time (usually on a logarithmic scale). A typical TTT diagram shows what structures can be expected after various rates of cooling. It graphically describes the cooling rate required for the transformation of austenite to pearlite, bainite or martensite. TTT diagram also gives the temperature at which such transformation takes place.

Learning Objectives:

• Appreciate the distinction between equilibrium phase, TTT and CCT diagrams (Fe-Fe\(_3\)C phase diagram example)

• Understand how cooling rate influences steel hardness and gain familiarity with heat treatment regimes for steels and the microstructures that are developed

• Learn about hardenability and how quench severity, carbon content, grain size and other alloying elements influence it.

• Understand the basic principles of carburising/carbonitriding and nitriding/nitrocarburising and the effect these treatments have on surface structure and properties

Annealing

The annealing process is intended to optimize the steel’s machinability and formability. In manufacturing steel products, machining and forming are often employed. Quenched and tempered steel may not machine or bend very easily and annealing is often necessary to manufacture steel components economically. Annealing is used after cold forming operations, since during forming, the deformed areas of the steel may become work-hardened and susceptible to fracture.

Full Annealing

Steel is heated 50 to 100\(^\circ\)F (10 to 38\(^\circ\)C) above the A3 for hypoeutectoid steels, and above the Acm for hypereutectoid steels, with slow controlled cooling, resulting in soft andductile microstructures that have commercially maximized machinability and formability. In full annealing, cooling must take place very slowly so that a coarse pearlite is formed. When the term annealing is applied to steels it is assumed that full annealing was performed.

Normalizing

The process of normalizing consists of heating to a temperature 50 to 100\(^\circ\)F (10 to 38\(^\circ\)C) above the A3 and allowing the part to cool in still room temperature air. Normalizing can be described as a homogenizing or grain-refining treatment. Within any piece of steel, the composition is usually not uniform throughout. That is, one area may have more carbon than the area adjacent to it. These compositional differences affect the way in which the steel will respond to heat treatment. If the steel is austenitized, the carbon can readily diffuse throughout, and the result is a reasonably uniform composition from one area to the next. The steel is then more homogeneous and will respond to the heat treatment is a more uniform way.

Tempering

Tempering is generally applied to hardened or quenched steel to improve mechanical properties, for the most part, tensile strength, ductility and toughness. Tempering, formerly called drawing, is performed by heating a quenched part to some point below the lower critical transformation temperature for sufficient time depending on its size, commonly 2 hours or more. Most steels are tempered between 400 to 1100\(^\circ\)F (205 to 595\(^\circ\)C). As higher temperatures or longer periods of time are employed, toughness and ductility are increased, but at the expense of reduced hardness and tensile strength. The microstructure of quenched and tempered steel is referred to as tempered martensite.

Stress Relieving

When a metal is heated, expansion occurs which is proportional to the temperature rise. Similarly, upon cooling a metal, contraction occurs. When a steel component is heated at one point more than at another, as in welding or during forging, internal stresses are set up. During heating, expansion of the heated area do not occur uniformly, and the component tends to distort. On cooling, contraction is restricted from occurring by the unyielding cold metal surrounding the heated area, as in a weldment. The forces attempting to contract the metal are not relieved, and when the metal is cold again, the forces remain as internal stresses, also called residual stresses. Stresses also result from volume changes which accompany phase transformations, such as the transformation of austenite to martensite.
Residual stresses are harmful because they may cause distortion of steel parts and/or may render the part susceptible to brittle fracture and stress corrosion cracking mechanisms. To relieve these stresses, plain carbon steel is typically heated to between 940 to 1100\(^\circ\)F (482 to 595\(^\circ\)C), assuring that the entire part is heated uniformly, then slowly cooled back to room temperature. This procedure is called relief annealing or, more commonly, stress relieving.

Learning Objectives:

• Demonstrate a critical understanding of the importance of heat treatment in achieving fit for purpose in metals and alloys.

• Apply knowledge of thermodynamics and heat transfer knowledge theories to produce specific heat treatment outcomes.

The process of heat treatment consists of the following or steps:

1. Controlled Heating of Metal/Alloy (Steel) Heating temperature of metal depends upon its grade, grain size, type and shape of a metal or alloy. Generally, the metal is heated upto its upper critical temperature.

2. Holding or Soaking period Metal or alloy to be given heat treatment, is held at the specified temperature for a specified period so that there is a uniformity of temperature throughout the mass. The period of heating depends upon the size and shape of the component.

3. Controlled Cooling of the Metal (Steel) Main changes in the properties of metal or alloy take place in the cooling process. Change in properties or transformation depends mostly on the rate at which the cooling takes place. Cooling of metal is also known as quenching. Generally five quenching media adopted for quenching are: caustic soda solution, brine, water, oil and air.