Heat Treating Archives - Industrial Metallurgists

More about Annealing Metal and Metal Ductility
Abstract: Annealing Metal is a heat treating process used to modify the properties of cold-worked metal. This article discusses the reasons for annealing, the metallurgical changes that take place within a metal during cold working and annealing, the effects of these metallurgical changes on the properties of metals, and the effects of annealing temperature and time on the final microstructure and properties of annealed metals.

Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing. Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases. There comes a point when additional cold working is not possible without causing the metal to crack. At this point, it is necessary to anneal the metal if continued cold-working is required.

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During an anneal, metallurgical changes occur that returns the metal to its pre-cold-worked state. These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working. In order for these changes to occur, the metal must be heated above its recrystallization temperature. The recrystallization temperature for a particular metal depends on the metal’s composition. This specific annealing process is sometimes called a recrystallization anneal, though other names like process anneal are also used.

Metallurgical effects of cold working

During cold-working there is an increase in the number of dislocations in a metal compared to its pre-cold-worked condition. Dislocations are defects in the arrangement of atoms in a metal (discussed in Principles of Metallurgy).

The increase in the number of dislocations causes a metal’s yield and tensile strength to increase and its ductility to decrease.  After a certain amount of cold work, a metal cannot be cold worked further without cracking.  The amount of cold working that a particular metal can withstand before cracking depends on its composition and microstructure.

Metallurgical effects of recrystallization anneal

During a recrystallization anneal, new grains form in a cold-worked metal.  These new grains have a greatly reduced number of dislocations compared to the cold-worked metal.  This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

With continued time at the annealing temperature, some of the newly formed grains grow at the expense of neighboring grains.  There is some further decrease in strength and increase in ductility as the average grain size increases during the grain growth phase of the annealing process.

The final grain size depends on the annealing temperature and annealing time.  For a particular annealing temperature, as the time at the temperature increases the grain size increases.  For a particular annealing time, as the temperature increases the grain size increases.  A piece of metal with large grains has lower strength and more ductility than a piece of metal of the same alloy with smaller grains.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed at two different temperatures.  Figure (a) shows the microstructure of the cold rolled sample.  Figure (b) shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour.  Figure (c) shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour.

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The sample that was annealed at 1022 °F (550 °C) for 1 hour had yield strength of 11 ksi (75 MPa).  Many small grains are present in this sample.  The sample that was annealed at 1202 °F (650 °C) for 1 hour had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the sample in Figure (b).

Other reason for recrystallization anneal

In addition to enabling additional cold-working, recrystallization annealing is also used as a final processing step to produce metal sheet, plate, wire, or bar with specific mechanical properties.  Control of the annealing temperature and time, heating rate up to the annealing temperature, and amount of cold-working prior to anneal is important for obtaining the desired grain size, and therefore the desired mechanical properties.

Learn more with the Principles of Metallurgy Course

Tempering Steel

Imagine you’re a warrior during the middle ages and it’s time to get a new sword. So, you go to a blacksmith to buy a sharp, shiny long sword. A few weeks later you’re in a battle, fighting at the front of the shield wall. You take a huge swing at the enemy, who meets your blow with his sword, and your sword shatters into several pieces. Unfortunately for you, your blacksmith outsourced a batch of swords to a blacksmith on the other side of town who didn’t have time to temper the swords. As a result, the swords were strong, but brittle. Their lack of toughness meant that they could not absorb much of an impact before fracturing.

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Purpose of tempering

Tempering is used to improve toughness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite. During the tempering process the steel is heated to a temperature between 125 °C (255°F) and 700 °C (1,292 °F). At these temperatures the martensite decomposes to form iron carbide particles. The higher the temperature, the faster the decomposition for any given period of time. The micrograph shows a steel after substantial tempering. The black particles are iron carbide.

Tempered martensite

Untempered martensite

Untempered martensite is a strong, hard, brittle material. The stronger and harder it is, the more brittle it is. The strength and hardness is a due to elastic strain within the martensite, which is a result of too many carbon atoms being in the spaces between the iron atoms in the martensite. As the amount of carbon in a steel increases (up to about 0.8 weight percent carbon) the martensite strength and hardness increases.

What happens during tempering

During the tempering process, the carbon atoms move out of the spaces between the iron atoms in the martensite to form the iron carbide particles. The strain within the martensite is relieved as the carbon atoms move out from between the iron atoms in the martensite. This results in an improvement in the steel toughness, at the expense of reduced strength.

Amount of tempering required

The amount of tempering required depends on the particular application in which the steel will be used. In some cases, toughness is not important, so tempering at a low temperature for a short period of time is acceptable. In cases where very strong and tough steel is required a high carbon steel tempered at a high temperature might be used.

Learn more about about steel heat treating is in our Steel Metallurgy and Steel Through Hardening courses. The book Steels: Processing, Structure, and Performance by George Krauss provides a comprehensive discussion of steel heat treating.

Abstract: Recrystallization annealing is a heat treating process used to modify the properties of cold-worked metal.  This article discusses the reasons for preforming a recrystallization anneal, the metallurgical changes that take place within a metal during cold working and annealing, the effects of these metallurgical changes on the properties of metals, and the effects of annealing temperature and time on the final microstructure and properties of annealed metals.

Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing.  Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases.  There comes a point when additional cold working is not possible without causing the metal to crack.  At this point, it is necessary to anneal the metal if continued cold-working is required.

The specific annealing process used is called recrystallization anneal.  During this annealing process, metallurgical changes occur that returns the metal to its pre-cold-worked state.  These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working.  In order for these changes to occur, the metal must be heated above its recrystallization temperature.  The recrystallization temperature for a particular metal depends on its composition.

Metallurgical effects of cold working

During cold-working there is an increase in the number of dislocations in a metal compared to its pre-cold-worked condition.  Dislocations are defects in the arrangement of atoms in a metal (discussed in Principles of Metallurgy).  The increase in the number of dislocations causes a metal’s yield and tensile strength to increase and its ductility to decrease.  After a certain amount of cold work, a metal cannot be cold worked further without cracking.  The amount of cold working that a particular metal can withstand before cracking depends on its composition and microstructure.

Metallurgical effects of recrystallization anneal

During a recrystallization anneal, new grains form in a cold-worked metal.  These new grains have a greatly reduced number of dislocations compared to the cold-worked metal.  This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

With continued time at the annealing temperature, some of the newly formed grains grow at the expense of neighboring grains.  There is some further decrease in strength and increase in ductility as the average grain size increases during the grain growth phase of the annealing process.

The final grain size depends on the annealing temperature and annealing time.  For a particular annealing temperature, as the time at the temperature increases the grain size increases.  For a particular annealing time, as the temperature increases the grain size increases.  A piece of metal with large grains has lower strength and more ductility than a piece of metal of the same alloy with smaller grains.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed at two different temperatures.  The figure on the left shows the microstructure of the cold rolled sample.  The center figure shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour.  The figure on the right shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour.

Annealed brass grains

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The sample that was annealed at 1022 °F (550 °C) for 1 hour had yield strength of 11 ksi (75 MPa).  Many small grains are present in this sample.  The sample that was annealed at 1202 °F (650 °C) for 1 hour had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the center sample.

Other reason for recrystallization anneal

In addition to enabling additional cold-working, recrystallization annealing is also used as a final processing step to produce metal sheet, plate, wire, or bar with specific mechanical properties.  Control of the annealing temperature and time, heating rate up to the annealing temperature, and amount of cold-working prior to anneal is important for obtaining the desired grain size, and therefore the desired mechanical properties.

Learn more

Take out Principles of Metallurgy course to learn more about the recrystallization anneal and fundamental metallurgy concepts that are important to understand to understand metallurgy.

Whether you are through hardening steel, annealing a cold-worked metal, or solution treating an aluminum alloy, controlling heat treatment temperature is critical for obtaining the desired microstructure, and therefore, the desired metal properties.  Microstructure refers to such things as the metallurgical phases present in a metal and the grain size.

Using a temperature that is too hot can result in a metallurgical transformation that proceeds too quickly or the formation of undesired phases.  Using a temperature that is too low can result in incomplete metallurgical phase transformations, cold worked metals that do not soften sufficiently, or insufficient stress relief.

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Steel heat treating

For example, during the through hardening heat treatment of a carbon steel, the steel is heated to transform all the ferrite and cementite to austenite and then quenched to form martensite.  If the steel is not heated to a high enough temperature, then there is the risk that all the ferrite and cementite does not transform to austenite.  If this occurs, then when the steel is quenched, the remaining ferrite and cementite will be present along with the martensite.  These ferrite and pearlite remnants can weaken the steel.

Annealing

Another example is cold-rolled sheet metal that is annealed to improve its ductility, and reduce its strength and hardness.  If the annealing temperature is too high, then excessive grain growth will occur.  This will result in the metal having lower strength and hardness than intended.  Also, if the metal is to be formed, there is the risk of orange peel, a cosmetic defect in heavily formed metals with grains that are too large.

Why temperature might be too high or low

So, why might a heat treater use a heat treating temperature that is too high or too low?  To save money, to save time, or just sloppy.  To reduce energy costs a heat treater might try to run its furnaces at the low end of the required temperature range.  However, normal temperature variations throughout a load and normal composition variations within the metal can result in the temperature being too low to cause the desired metallurgical transformations.

To save time, a heat treater might operate a furnace at the high end of the specified temperature range to try to move the metallurgical transformations along as fast as possible.  Again, with normal temperature and composition variations, the temperature may end up being too high, resulting in excessive or undesired changes in the metal’s microstructure.

As for a sloppy heat treater, who knows what you will get from batch to batch of metal stock or components.

To learn more about the effects of temperature control on steel microstructure and properties, take our Steel Metallurgy course or read Practical Heat Treating by J.L. Dorsett and H.E. Boyer or Steels: Processing, Structure,and Performance  by George Krauss.  Also, the two courses mentioned in the introduction above will discuss the effects of temperature control on precipitation strengthening and annealing cold-worked metals.

The strength of metals is improved by impeding the motion of dislocations through metals. One approach to achieving this improvement is precipitation strengthening - forming a uniform distribution of closely spaced sub-micron sized particles throughout an alloy. The particles, which are called precipitates, impede dislocation motion through the alloy.

Not every alloy can be precipitation strengthened. Alloys that can be precipitation strengthened include Al-Cu, Al-Mg-Si, Cu-Be, and 17-8 PH steel. The figure shows precipitates in a Al-Cu alloy.

precipitation strengthening
Al2Cu precipitates in a Al-4% Cu alloy. © DoITPoMS Micrograph Library, Univ. of Cambridge

An example of a precipitation strengthened component is the tubing used for bicycles with aluminum frames. The alloy is 6061.

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Precipitation strengthening heat treatment

The particles are formed using a series of precipitation heat treatment steps. The first step is solution heat treatment. This involves heating the alloy up to a temperature that results in the atoms of the alloying element being dissolved within the crystal structure of the main element. For Al-Cu alloys, the copper atoms dissolve into the aluminum crystal. This is called a solid solution. The solid solution is then retained at room temperature by cooling the alloy rapidly, such as by water quenching.

After cooling, precipitates are formed either by natural aging or artificial aging.  With natural aging, the precipitates form at room temperature. With artificial aging, the precipitates form when an alloy is heated to a temperature lower than the solution heat treatment temperature. Only certain alloys will undergo natural aging. The other alloys must be artificially aged. Aluminum alloys are examples of alloys that can be naturally and artificially aged.

Regardless of whether an alloy is naturally or artificially aged, as the precipitation process proceeds the precipitates go through a series of stages, with changes in the size, form, and composition of the precipitates. The particular stage influences alloy strength. For artificially aged alloys, this is controlled by the aging temperature and time.

For a particular aging temperature, there is an aging time at which the alloy will reach maximum strength. Maximum strength corresponds to a specific stage of the form and composition of the precipitates. Aging times that are too short or too long will result in less than maximum alloy strength.

For artificially aged alloys, the aging temperature affects the maximum strength that can be obtained, and the time required to reach maximum strength. Time to reach maximum strength decreases as the aging temperature increases. For naturally aged alloys, the strength increases over time. The time required to reach maximum strength depends on the alloy.

Finally, precipitation strengthening can be combined with cold-working to give even greater alloy strength.

More information about the metallurgy of precipitation strengthening and precipitation strengthening heat treatment is in our Precipitation Strengthening course. Also, Heat Treatment: Structure and Properties of Nonferrous Alloys by C. R. Brooks, Precipitation Hardening by J.W. Martin, and ASM Handbook, Volume 4: Heat Treating discusses precipitation strengthening.

Also learn more in the Principles of Metallurgy course.

In this article, I’m going to take a step back to consider the basic science of metallurgy, which includes thinking about the atoms in metals.  On a microscopic level, there are many things going on inside of a metal.  Metals consist of numerous microscopic structures that have a direct and large influence on the properties of metals.  Through composition, mechanical treatment, and thermal treatment these microscopic structures can be modified to impart specific properties.  Whether the desired structures, and resulting properties, are obtained in a completed component or joint between components depends on the knowledge and skill of designers and manufacturers.

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One set of major structures within a metal are the crystal lattice, grains, and phases.  The crystal lattice is the arrangement of the atoms within a metal.  Grains are individual crystals within a metal.  Figure 1 shows grains in a brass alloy.  Phases are different combinations of the elements present in an alloy.  Figure 2 shows pearlite in steel.  The light colored material is the ferrite phase, which is comprised of iron with a little bit of carbon mixed in.  The dark colored phase is cementite, which is comprised of the compound Fe3C.  It is also referred to as iron carbide.  The properties of a metal are affected by the size of the grains and the phases present.

Grains in brass
Pearlite in steel

Defects in the metal crystal lattice make it possible to form alloys and deform metals with the metals cracking.  These defects are not the same as manufacturing defects such as voids, inclusions, seams, and cracks.  Instead, without crystal lattice defects we would only have pure, brittle metals.

Various mechanical (e.g. cold rolling) and thermal (e.g. through hardening and precipitation strengthening) processes take advantage of these crystal defects in order to bring about modification of the grains and phases present in a metal, to obtain the desired properties.

Also, the number of crystal defects in the metal can be modified to obtain desired properties.  For example, cold rolling results in an increase in the number of dislocations in a metal, resulting in increased strength.  Annealing a cold-rolled metal results in a reduction in the number of dislocations and modification of the grains, resulting in a decrease in the metal strength.

Controlling how atoms in metals move

A common representation of the relationship between metal properties, composition, microstructure and processing is shown in the image below.  When the effects of the manufacturing processes on the microscopic structures are properly understood, it is possible to consistently produce metal components and joints that have the desired properties.  Essentially, the people in charge of the manufacturing processes are responsible for making sure that during the processes the atoms in metals move to where they need to be.  And designers are responsible for specifying where the atoms should be.

For more information about the microscopic structures in metals and the atoms in metals, take our online course Principles of Metallurgy or read Metallurgy for the Non-Metallurgist.

Additional Learning Resources
Steel Through Hardening
Steel Case Hardening
Aluminum Heat Treating
Aluminum Metallurgy

Within many common alloys it is possible to alter the phases present with heat treatment.  Forming one or more phases from a different phase is called a phase transformation.   Phase transformations occur when in an alloy is heated or during cooling from an elevated temperature.

Phases are distinct materials that are comprised of the elements in the alloy.  These distinct materials have distinct properties that have an impact on the overall properties of the entire alloy.  Additionally, the size, shape, and location of the phases within the alloy also effect on the overall properties of an alloy.

The metallurgical phases present in an alloy have a huge impact on the properties of a metal component.

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Several types of phase transformations

There are several types of phase transformations that can occur when an alloy is cooled from an elevated temperature.  The type of transformation that can occur depends on the specific alloy.  Not all types of transformations occur in all alloys, and in some alloys no transformations are possible, other than the solid-liquid transformation.

Two of the most common phase transformations encountered with common alloys are eutectoid and precipitation.  For both types of phase transformation, the transformation involves the movement of atoms through the metal to rearrange themselves to form the new phase or phases.

phase transformation
Ferrite (white) and cementite (dark) in steel.

A eutectoid transformation involves a change from a single phase to two other phases when the initial phase is cooled form an elevated temperature.  The most common alloy in which this phase transformation is encountered is steel.  The transformation occurs when steel is cooled from the austentizing temperature.  During slow to moderate cooling, the austenite transforms to ferrite and cementite.  The microstructure consists of cementite plates with ferrite between the plates.  This is commonly referred to as pearlite.  A micrograph of a steel alloy with 0.6% carbon is shown here.

bainite

During faster cooling of some steel alloys, the ferrite forms in the shape of needles or plates and the cementite forms as particles.  This structure is referred to as bainite.  A micrograph of a steel with bainite is shown in Figure 2.

The reverse transformation occurs when steel with ferrite and cementite is heated.  When the temperature is high enough, the ferrite and cementite transform to austenite.  So, the austenite to ferrite + cementite phase transformation is reversible, and repeatable.

Precipitation transformations involve the formation of particles of one phase within an already existing phase.  These particles are called precipitates.  This phase transformation occurs when an alloy is cooled from an elevated temperature.  At the elevated temperature the phase present consists of the main element in the alloy with the alloying elements in solid solution.  When the alloy is cooled the solid solution is not able to hold all the atoms of the alloying elements in solution, so precipitates form that consist of the solute atoms and possibly the atoms of the main element in the alloy.

Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge
Al2Cu precipitates in an aluminum matrix. ©DoITPoMS Micrograph Library, Univ. of Cambridge

For engineered metal components, precipitation  during cooling is undesirable because of the resulting size and location of the precipitates.  So, the process is modified by first quenching the alloy to room temperature to suppress the atom motion.  Then the alloy is either allowed to transformation at room temperature, if room temperature transformation is possible, or the alloy is reheated to an intermediate temperature to speed up the transformation.  An example of a common alloy system in which precipitation is used is the aluminum-copper system.  This figure shows a micrograph of Al2Cu precipitates in an aluminum matrix.

The precipitation transformation occurs in a number of alloys including aluminum alloys (Al-Cu, Al-Mg-Si, Al-Zn-Mg, and Al-Zn-Mg-Cu), precipitation hardened steels (e.g. 17-4 PH, 15-5 PH, and 13-8), some copper alloys (Cu-Be and Cu-Cr), and Zn-Al alloys.

Regardless of the particular transformation, control of the heating temperature, heating time, cooling rate, and, if necessary, reheating temperature and time are all important factors for controlling whether the desired transformation is complete and the shape, size, and location of the phases that form.  These in turn have a big impact on the properties of a metal component.  The relationship between heat treating process conditions, final microstructure, and properties is discussed in our Principles of Metallurgy, Steel Metallurgy, and Precipitation Strengthening courses.

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