By By Daniel Herring

Making Parts Stronger

To fully understand the advantages of heat-treating processes to manufacturing it is important to first understand a fundamental principal of metals – structure. As a molten metal solidifies, the atoms orient themselves into a repetitive pattern that we call a crystal structure. Body-centered cubic (BCC) and face-centered cubic (FCC) are two of the more common crystal structures. Elements such as Aluminum (Al), Chromium (Cr), Copper (Cu), Iron (Fe), Molybdenum (Mo), Nickel (Ni) and Silicon (Si) are a few examples of metals having these crystal structures.

As the crystals form, their structures grow in a uniform pattern in all directions. As the metal cools, these crystals meet newly developing crystals forming grains. The line of intersection between grains is called a grain boundary. These grain boundaries are oriented in a variety of directions since the individual grains all formed independently from one another. These newly formed crystalline structures are held together by the electromagnetic force between the atoms.

If a load is applied to a metal it will cause the metal to deform first by elastic deformation and then, if enough force is applied, by plastic deformation. The strength of the electromagnetic force between atoms determines the yield strength as well as the ultimate tensile strength of the material.

Alloying elements help make metals stronger and more resistant to deformation by strengthening their crystal structures. Adding alloying elements – other metals or non-metallic elements – causes the crystal structure to be rearranged, resulting in increased strength. Iron is a good example since in its unalloyed form it is not as strong as most plastics! By alloying with carbon (C) and manganese (Mn) we make steel, however, which is inherently stronger than iron. And we can heat treat steel to make it even stronger still. This is the secret to making a metal a useful engineering material.

Crystal structure

A unique arrangement of atoms in a metal. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice. Motifs are located upon the points of a lattice, which is an array of points repeating periodically in three dimensions. The points can be thought of as forming identical tiny boxes called unit cells that fill the space of the lattice. The lengths of the edges of a unit cell and the angles between them are called the lattice parameters. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure and optical properties.

Elastic deformation

The type of deformation that is reversible. Once the forces are no longer applied, the object returns to its original shape

Electromagnetic force

The force that the electromagnetic field exerts on electrically charged particles. It is the electromagnetic force that holds electrons and protons together in atoms and atoms together to make molecules. The electromagnetic force operates via the exchange of messenger particles called photons and virtual photons. The exchange of messenger particles between bodies acts to create the perceptual force whereby instead of just pushing or pulling particles apart, the exchange changes the character of the particles that swap them.

Grain boundary

The interface between two grains in a polycrystalline material. Grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve strength, as described by the Hall-Petch relationship. Since grain boundaries are defects in the crystal structure, they tend to decrease the electrical and thermal conductivity of the material. The high interfacial energy and relatively weak bonding in most grain boundaries often makes them preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep.

Plastic deformation

The type of deformation that is not reversible. An object in the plastic deformation range will first have undergone elastic deformation.

Figure 1:Simplified Iron-Iron Carbide Phase Diagram

Ultimate tensile strength

The maximum stress a material can withstand when subjected to tension (as opposed to compression or shearing). It is the maximum value on the stress-strain curve at which a material breaks or permanently deforms. Tensile strength is an intensive property and, consequently, does not depend on the size of the test specimen. However, it is dependent on the preparation of the specimen and the temperature of the test environment and material. Tensile strength, along with elastic modulus and corrosion resistance, is an important parameter of engineering materials that are used in structures and mechanical devices.

Yield strength (or yield point)

The stress at which a material begins to deform plastically. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.

Phases of Steel

A phase diagram is a map used by metallurgist and heat treaters to determine the phase or phases that exist in equilibrium as a function of temperature and composition. An abbreviated Iron–Carbon phase diagram is presented in Figure 1. Some of the key features of the phase diagram are described below. For steel, the composition is given in weight percent (wt %) carbon and the temperature is expressed in either degrees Celsius or Fahrenheit. Typically steels contain between 0.10 and 0.80 wt% carbon.

Pure iron (along the y-axis of the diagram) can exist with two crystal structures. At temperatures below 1674°F (912°C), iron is body-centered cubic (BCC) and is called ferrite (a) or a-iron. At temperatures between 1674°F (912°C) and 2541°F (1394°C), iron is face-centered cubic (FCC) and is called austenite (g) or g-iron. Above 2541°F (1394°C) and below the melting point of pure iron at 2800°F (1538°C), iron returns to BCC and is designated as delta (d) ferrite or d-iron. Steel can be heat treated as a result of the austenite to ferrite phase transformations.

Iron can form a solid solution with carbon. The carbon atoms dissolve into the solid by filling the interstitial sites of the FCC and BCC crystals. Due to the size and shape of these interstitial sites, the austenite (FCC iron) can dissolve up to 2.14 wt% C while ferrite (BCC iron) can only contain 0.022 wt% C. This 100-fold difference in carbon solubility in the two iron crystal structures is also necessary for the heat treatment of steel.

Pearlite Formation

The iron-carbon (or iron-iron carbide) phase diagram – Benefits of Heat Treating (Part 2): Phases of Steel; Fig. 1– tells us that steel containing approximately 0.76 wt% carbon is at the eutectoid composition. If this steel is heated to 1472°F (800°C), austenite (g), which is a face-centered-cubic (fcc) structure, exists as a solid solution. If the austenite is cooled to below 1333°F (723°C), however, a phase transformation will take place. Iron carbide (Fe3C), also known as cementite, and ferrite (a), a body-centered-cubic (bcc) solid solution containing less than 0.02 wt% carbon, will form. At 1333°F (723°C) a “three-phase equilibrium” exists as seen on the phase diagram. Austenite is in equilibrium with ferrite and cementite. On cooling:
Austenite (0.76 wt %C) à Ferrite (a) + Fe3C (0.02 wt%C)
If this phase transformation takes place as we are slow cooling, a microstructural component called pearlite is formed (Fig. 2). Pearlite consists of plates of Fe3C in a matrix of ferrite (a). Pearlite is formed from austenite containing the eutectoid composition of 0.76 wt%C. This microstructure develops as a result of the decomposition of austenite by nucleating carbide plates first and then the ferrite on the sides of the carbide plates.

Figure 2: Ferrite (white) and pearlite (black) formation in steel

Steels that contain less than 0.76 wt %C are called hypoeutectoid steels. For example, the phase diagram tells us that when a hypoeutectoid steel containing 0.40 wt%C is heated to 1650°F (900°C) austenite forms. When this steel is slowly cooled, the first (or primary) phase formed is ferrite. On reaching 1333°F (723°C) on cooling, a microstructure of primary ferrite and austenite develops. Upon cooling below 1333°F (723°C), the austenite, which now contains 0.76 wt%C, transforms to pearlite. The microstructure of this alloy will consist of approximately 50% primary ferrite and 50% pearlite.
Steels containing more than 0.76 wt%C are called hypereutectoid steels. For example, the phase diagram tells us that when a hypereutectoid steel containing 0.90 wt%C is heated to 1650°F (900°C), an austenite solid solution forms. Upon slow cooling, the first (or primary) phase formed is iron carbide (Fe3C). Upon cooling below 1333°F (723°C), austenite, which now contains 0.76 wt%C, transforms to pearlite. The microstructure of this alloy will consist of approximately 2% primary carbide and 98% pearlite.

Glossary of Terms

Austenite: Austenite is the name given any solid solution in which gamma (γ) iron is the solvent. Austenite is the structure from which all quenching heat treatments must start.
Cementite: The common name for iron carbide, Fe3C, the chemical combination of iron and carbon. The stoichiometric phase.
Eutectoid: A eutectoid system is one in which a single-phase solid transforms directly to a two-phase solid.
Ferrite: Ferrite is the name given any solid solution in which alpha (α) iron is the solvent. The α-phase has a body centered cubic lattice. If you want to be precise, you call it α- ferrite.
Hypereutectoid steels: Hypereutectoid steels are those whose carbon content greater than 0.76 wt%.
Hypoeutectoid steels: Hypoeutectoid steels are those whose carbon content less than 0.76 wt%.
Pearlite: The two-phase mixture obtained right below the eutectoid point at 0.76 % carbon concentration.

Martensite Formation & Tempering

When austenite is rapidly cooled (i.e. quenched) to room temperature, an unexpected phase transformation can occur. Instead of forming ferrite and pearlite or cementite and pearlite, austenite can transform to martensite, a highly stressed body-centered-tetragonal (BCT) phase. This reaction is not diffusion-controlled but rather takes place by a shear-type transformation and is only a function of temperature and not time. The result is a very hard but brittle microstructure. The hardness of the martensite depends on the carbon content of the austenite. The martensite and the austenite it came from have the same chemical composition.

As-quenched martensite is too hard and brittle for most practical uses, but it can be tempered to recover some toughness and ductility. Tempering consists of reheating the martensite to temperatures typically between 275-750°F (135–400°C) for several hours. During the temper heat treatment, carbides precipitate in the martensite matrix. This transformation increases the toughness of the steel by the carbide precipitates as well decreasing the carbon content of the martensite. In some steels, an increase in strength and toughness occurs.

For tempering, the important process parameters are temperature and time. The temperature must be selected to control the size and distribution of the carbides that precipitate during the tempering process and to convert any retained austenite to tempered martensite. The time must be long enough to heat the entire load to the desired temperature. In addition, time is needed for the nucleation and growth of the carbides to form the tempered martensite. The rate of cooling from tempering temperature, though not critical in many applications, must be rapid enough to avoid temper embrittlement (TE and TME) conditions.

Definition

Martensite: Rapidly cooled austenite. No carbon diffusion - trapped in martensite. Shear transformation involved. Supersaturated solution of carbon in alpha iron (ferrite). Trapped carbon induces built-in stresses. Martensite is very hard and brittle. Must be toughened by tempering. End result is still harder and stronger than typical slow-cooled pearlitic steel.

Figure 1: Time -Temperature-Transformation (TTT) Diagram for a Eutectoid Steel [1]

Time-Temperature-Transformation (TTT) Diagrams

The phase transformations described in "Benefits of Heat Treating" Parts 2, 3, and 4 occur by nucleation and growth and are diffusion-controlled. The kinetics or rates of these transformations are a function of temperature and steel composition and can be described with a TTT diagram for the eutectoid steel as seen in Figure 1.

This particular diagram presents the kinetics for isothermal (constant-temperature) transformations of a eutectoid steel. In these diagrams the ordinate (y-axis) is temperature and the abscissa (x-axis) is time. In this case it is the logarithm of time. As seen on this diagram, at temperature above 1333°F (723°C) the austenite phase is always stable. When this steel is cooled very rapidly to 1112°F (600°C), however, the austenite becomes unstable and transforms to pearlite over time. As seen in Figure 1, after 2-3 seconds the austenite starts to transform to pearlite, after 6–7 seconds the transformation is 50% complete (i.e. 50% austenite and 50% pearlite) and after about 15 seconds the transformation is complete.

For this eutectoid steel, when rapidly cooled to a temperature below the nose of the TTT curve a slightly different type of transformation occurs. The austenite will transform to a structure called bainite. Similar to pearlite, bainite consists of a mixture of ferrite and carbide. However, the morphology is finer and can have a more feathery or acicular appearance.

As seen in Figure 1, after rapid cooling to 752°F (400°C), the austenite is unstable. After about 7 seconds the austenite starts to transform to bainite, after about 80 seconds the transformation is 50% complete and after about 200 seconds the transformation to bainite is complete. These TTT diagrams are available for a wide variety of steel compositions.

As presented in these TTT diagrams, the transformation from austenite to pearlite, bainite and primary ferrite or carbide takes time. These transformations are diffusion controlled, and diffusion takes time.

Daniel H. Herring - Tel: (630) 834-3017)

Dan Herring is president of THE HERRING GROUP, which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.He can be reached at: This e-mail address is being protected from spambots. You need JavaScript enabled to view it