Heat Treatment of Metals: Everything You Need To Know

Users of metals in the manufacturing industry have learned how to improve vast varieties of metals. This is mostly done to tailor their properties to fit into the task at hand such as reaction to precision machining.

There are vast methods to improve metals, one of which includes heat treatment of metals. This process can alter a number of different properties including strength, formability, elasticity, hardness, ductility, and machinability.

As the topic of this article implies, this article focuses on everything you need to know about the heat treatment of metals.

What is Heat Treatment of Metals

Heat treatment is a general process of the usage of heating and cooling operations at various staged levels to alter the physical properties of metals (microstructure) such as steel, aluminum, and many more. The major purpose of such treatment is to improve the physical and structural properties for some specific use or future work of the metal.

There are vast varieties of heat treatment processes out of which include case hardening, annealing, tempering, decarburizing, normalizing, case hardening, aging, quenching, and more. While each of these heat treatment brings about different results in metal, they all involve three basic steps. These steps include heating, soaking, and cooling.

The Advantages of Heat Treatment of Metals

In the world of manufacturing, heat treatment of metals is generally used and it is a precisely controlled process of heating and cooling. Heat treatment does not only make the metal harder, but it also makes it softer too. The softening allows metals for working operations such as cold forging, machining, deep drawing, and many more. The heat treatment of metal is beneficial and they include the following:

• Improvement in machinability or workability:

Heat treatment helps to improves a metal’s manufacturability. This is done by the removal of internal stress from previous fabrication processes such as hot work, cold work, machining, welding, and stamping. For example, if a metal is highly hard to bend or machine, it can be subjected to annealing or stress relieving. This will help to reduce the hardness of such material. If a material deforms when machined, to keep it from deformation, the material can be annealed or stress relieved. Heat treatment using induction or flame can also be used the soften a specific area of the metal, leaving the remaining part of the metal untouched.

• Improvement in Durability and Wear Resistance

There are several heat treatment processes out there. Some of these processes can be used to improve wear resistance by hardening the metals involved. Metals such as titanium, steel, Inconel, and some alloys of copper can be hardened either on the surface (case hardening) or through (through hardening). This is done to make the material stronger, more durable, tougher, and more resistant to wear and tear. This method is the best method commonly used to increase the durability of inexpensive steel including 1018 or A-36.

Localized hardening can be done either by induction or by flame. This can also help to harden a specific part leaving the rest part of the material untouched or unchanged. Lastly, nitriding is used to harden the part surface at low temperatures to reduce distortion.

• Improvement in Toughness and Strength

Toughness and strength are a trade-off, as increasing strength as measured by hardness can help to reduce toughness and introduce brittleness. Consequently, heat treatment can affect the tensile strength, yield strength, and fracture toughness. Through hardening or case hardening will help to increase the material’s strength. However, the material will be required to be drawn back or tempered to reduce brittleness. The extent of tempering is determined by the ultimate strength required in the part. Besides, if the received material is too brittle, it can be heat treated either re-tempered or annealed to make it more usable (ductile).

• Improvement in Magnetic Properties

Many metals including 316 or 1008 tend to gain magnetism which is measure as magnetic permeability. This is mostly obtained when the materials in question are work-hardened using methods including machining, stamping, forming, and bending. Aside from gaining magnetism, there is a specific type of annealing process that helps to reduce magnetic permeability. This is important to be carried out if the part has an application in an electronic environment.

What Metals Can Be Heat Treated?

In the world of heat treatment, ferrous metals account for the majority of heat-treated materials. About 80% of heat-treated ferrous metals are the different grades of steel. Other examples of ferrous metals that are heat treatable include stainless steel and cast iron.   However, other metals including, magnesium. Aluminum, nickel, titanium, brass, copper alloys, and many more are heat treatable.

Heat Treatment Aluminum

Heat treatment of aluminum helps to strengthen and harden a specific subset of alloys of aluminum. This includes wrought and cast alloys that are precipitation hardenable. These precipitation-hardenable alloys of aluminum include 2XXX, 6XXX, 7XXX, and 8XXX grades. Annealing may also be required for parts that have undergone strain hardening in their forming process.

The typical heat treatment of aluminum includes annealing, natural & artificial aging, homogenizing,  and solution heat treatment. While the heat treatment of aluminum differs from other metals such as steel, its furnace temperature can range between 240 and 1000oF depending on the exact process being used.

Heat Treatment of Steel

As mentioned earlier, the most heat-treated ferrous metal is steel. The adjustment of the carbon content of steel is the simplest heat treatment of steel. This helps to change the mechanical properties of steel. Additional changes are done by heat treating – for example by accelerating the rate of the cooling through the austenite-to-ferrite transformation point. Also, increasing the rate of cooling of pearlitic steel (0.77% carbon) to about 200oC per minute generates a DPH of about 300, and cooling at 400oC per minute rases the DPH to about 400. The increasing hardness is attributed to the formation of a finer pearlite and ferrite microstructure that can be obtained during slow cooling under ambient air.

In general, the commonly used heat treatment process for the steel include annealing, quenching, tempering, boronizing, carburizing, case hardening, nitriding, decarburizing, cyanide hardening, and many more.  However, not every steel grade is required to go through all of the mentioned heat treatment but all steel needs to be treated.

Heat Treatment Stainless Steel

Another heat-treatable metal is stainless steel. For stainless steel, they are generally treated based on the grade or alloy type. Heat treatment methods including hardening, stress-relieving, and annealing help to strengthen the corrosion resistance and ductility properties of stainless-steel during fabrication. It also helps to generate a hard structure that can resist abrasion and high mechanical stresses.

The heat treatment of stainless steel is mostly done under controlled conditions to prevent decarburization, carburization, and scaling on the surface of stainless steel. The commonly used methods of heat treatment of stainless steel include annealing (quench annealing, process annealing, and stabilizing annealing), hardening, stress-relieving, and many more.

Heat Treatment Titanium

Titanium and its alloys undergo heat treatment to reduce residual stresses developed during fabrication (Stress relieving). Besides, it leads to the production of an optimum combo of dimensional stability and machinability (Annealing). For increased strength of titanium and its alloy Solution Treating & Aging are used. When it comes to heat treatment, titanium alloys are classified as Alpha, near Alpha, Alpha-Beta, or Beta alloys.

• While Alpha & near Alpha alloys of titanium can be stress relieved and annealed, the high strength cannot be developed by any type of heat treatment
• The commercial Beta alloys are regarded as metastable Beta titanium alloys. When exposed to selected high temperatures, the retained Beta phase decomposes and strengthening of the material occurs. To obtain Beta alloys, aging and stress relieving treatment can be combined while annealing and solution treatment may be identical operations.
• As the name implies, Alpha-Beta alloys are two-phase alloys that comprise both Alpha and Beta Phases. They seem to be the most versatile and most common of the three varieties of alloys of titanium.

Heat Treatment Copper

Copper as a metal exhibit a pleasing color, but the most important features of copper are its high thermal and electrical conductivity, strength, machinability, good corrosion resistance, non-magnetic, and ease of fabrication. The end products of the fabrication of copper are generally described as foundry and mill products. These may include cable and wire, strip, rod, tubing, casting, powder metallurgy shapes, sheet, plate, bars, forgings, and more. These aforementioned products are manufactured using copper and its alloy and may be heat treated for vast varieties of purposes.

The most commonly used heat treatment methods for copper include homogenizing, stress relieving, annealing, precipitation hardening, and many more.

Common Heat Treatment Methods

Annealing

Annealing is a heat treatment method that consists of heating a metal to a particular temperature and then cooling the same metal at a slow rate that will produce a refined microstructure. This process can be done either fully or partially by separating the constituents. This method is usually used to soften a metal for cold working to enhance its features or properties such as machinability, electrical conductivity, ductility, and toughness.

It is beneficial in relieving stresses in the metal that arises as a result of prior cold working processes. During the recrystallization, the plastic deformation that occurred is removed when the temperature of the metal crosses the upper critical temperature.

Using this method of heat treatment, parts to be heat treated may go through a vast variety of techniques. These techniques include and are not limited to partial annealing, full annealing, recrystallization, and final annealing.

Ferrous alloys can undergo either process annealed or full annealed. In this case, the process annealed involves a faster cooling rate up to and including normalizing to produce a uniform microstructure. Full annealing on other hand involves slow cooling to produce a form of coarse pearlite.

For non-ferrous metals, they are mostly subjected to a vast variety of annealing methods. This includes partial annealing, full annealing, recrystallization annealing, and final annealing.

Normalizing

The normalizing heat treatment technique is used when the relieving of internal stresses is required. This stress may be caused by processes such as casting, welding, or quenching. This process requires heating the metal parts to a temperature that is 40oC greater than its upper critical temperature.

Another usefulness of normalizing is to provide uniformity in size and composition when an alloy is to be created. Normalizing can also be used for austenitized ferrous alloys that have been cooled in the open air.

This technique is beneficial because it produces martensite, pearlite, and even bainite. This produces harder and stronger steel than annealed steel. It is factual that normalized steel is tougher than any heat-treated steel. Due to this, parts that are needed to support massive external loads or have impact strength applications are always normalized. This will help the part to meet up with the requirement of the part needed for the project.

Stress relieving

When some parts undergo processes including forming, rolling, straightening, or machining it leaves some specific internal stress on the part. In a bid to relieve this internal stress, the stress-relieving heat treatment technique is used.

Stress relieving heat treatment technique is used to reduce or remove stresses that have built up in a part due to prior technical activities performed on such parts. It is mostly done by heating the parts to a temperature that is lower than the critical temperature and then cooled uniformly.

The stress-relieving heat treatment technique is used on items including, boilers, air tanks, pressure vessels, and many more.

Aging

Aging is otherwise known as precipitation hardening. This heat treatment technique is majorly known for its application in the increase of yield strength of malleable metals. The mechanism of action of this technique produces uniformly dispersed particles within a metal’s grain structure that result in changes in properties.

After the heat treatment technique that reaches high temperatures comes precipitation hardening. Aging on the other hand only elevates the temperature to an optimum level and lowers it down quickly again.

While some metals age naturally (at room temperature), others age artificially – in essence, elevated temperatures. It is very easy to store naturally aging metals at lower temperatures. Naturally aging alloys in some applications are kept in the freezer to avoid hardening until its time for its use. Alloys that can undergo precipitation hardening include an alloy of aluminum (2000 series, 6000 series & 7000 series), steel (maraging steel), and many more.

Quenching

Quenching or quench hardening involves the heating of parts above their upper critical temperature rapidly return such part’s temperature to room temperature. The returning to room temperature is done by placing the hot metal in the oil, brine, a polymer dissolved in water, or another suitable liquid to harden the structure fully. This process is carried out in a rapid state. Quenching is done for both ferrous alloys and non-ferrous alloys. While non-ferrous metal produces softer than normal parts, ferrous alloys produce a harder part.

The quenched hardness of the desired part is dependent on the method of quenching used and the chemical composition of the metal. Quenching is done for ferrous metals including iron and steel and non-ferrous metals including alloys of nickel, copper, aluminum, and many more. However, most non-ferrous metals produce an opposite effect when they are quenched. Such materials include aluminum, copper, or nickel, austenitic stainless steel like 316 and 304.

Hardening

In the heat treatment space, hardening is the most common technique used to increase the hardness of parts. In some situations, only the surface of these parts is hardened.

To do this, the desired part for heat treatment is hardened by heat treatment to a specified temperature then it is rapidly cooled by insertion into a cooling medium. The cooling medium to be used include and not limited to brine, water, or oil. The end product of the heat treatment by hardening will increase strength and hardness, however, the brittleness of the material will increase simultaneously.

A type of hardening process is case hardening in which only the metal parts exhibit outer layer hardness. This means that the resultant piece will have a softer core but a harder outer layer. This outer layer hardness is common for shafts because it protects its outer layer from material wear.

Tempering

Tempering is a heat treatment technique used to increase the resilience of iron-based alloys such as steel. While iron base alloys display a high level of hardness, they are often too brittle to be used for most applications. As a result, tempering is used to alter the ductility, hardness, strength, and brittleness to make it easier to machine. To do this, the part undergoes heat treatment below the critical point as lower temp reduces brittleness while maintaining the part’s hardness. On the other hand, if increased plasticity is required with less hardness and strength, a higher temperature is required.

Another approach to this is to purchase parts that have been hardened or to hardened the part before machining. Unlike a post-machining treatment process, it may be difficult to machine, but it eliminates the risk of changes in part’s size. This process also helps to eliminate the need for a grinding shop to obtain tight tolerances or finishes.

Decarburization

Decarburization involves the removal of carbon from the surface of the desired parts either through the normal aging process of oxidation or by applying heat. It is a surface degradation phenomenon in the heat treating and forging of steel. It can also be described as a metallurgical process in which the surface of the steel is depleted of its carbon content. This is done mostly by chemical action or heating the steel part above the lower critical temperature.

The carbon content of metal influences the metal’s hardness. During the decarburization process, the carbon diffuses from the surface of the metal, thus leading to the weakening of the metal. While the process lowers the strength of the metal, it also increases the shear strain below the metal surface. It also decreases the fatigue resistance while the wear rate and crack growth are increased.

Common Heat Treatment Process

There are vast varieties of heat treatment techniques used in the manufacturing space. Each of the heat treatment technique has different results, but they have common steps including:

• Heating
• Soaking
• Cooling

Heating

In most of the heat-treating processes, heating is the first step. Many of the heat treatable alloys change structure whenever they are heated to a specific temperature. At room temperature, the structure of an alloy can either be a solid solution mechanical mixture, or a combination of mechanical and solid solution mixture.

For example, a mechanical mixture can be likened to concrete just as the sand and gravel are held together in one piece by cement. Likewise, in a mechanical mixture, the elements and compounds are visible and are held together by a matrix of base metals.

A solid solution is referred to as a solution in which two or more metals are absorbed into another to form one piece. Therefore, when an alloy is in form of a solid solution, the element and compounds that make up the piece are absorbed into each other.

At room temperature, metal in its mechanical mixture goes into a partial solution, or solid solution when it is heated. During this process, the chemical properties and composition of the piece can be altered in grain size and structure. There is a possibility that the alloy ends up in one of the three states explained earlier depending on the technique used.

Soaking

This stage is also regarded as the holding stage, the metal undergoing heat treatment is kept at the required temperature.  The metal must remain at this temperature until the heat is evenly distributed which is referred to as soaking. The duration of the time it will spend at this temperature depends on the requirements. For example, the higher the mass of the part the longer it takes to soak the part. Another factor that affects the duration is the type of material.

Cooling

After the part has been soaked properly, the next step is to cool it. Here, the part structure may undergo changes from one chemical composition to another, it may go back to its original form, or remain the same. Depending on the rate of cooling and the type of metal, a solid solution metal may stay the same during cooling, change to a combination of the two, or change to a mechanical mixture. It is an interesting fact that the result is predictable, therefore the part would be predicted to end up as expected.

Consequently, a vast variety of metals can be made to conform to specific structures to increase their toughness, hardness, tensile strength, ductility, and many more.

Necessary Equipment For Heat Treating

To obtain a successful heat treatment process, close control over all factors affecting the heating and cooling of the part is required. This control is only possible when the proper equipment is available and they properly fit into the requirement of the project. Hence, the furnace to be used must be of proper type and size in which the temperature must be controlled and kept within the limits prescribed for each operation.

Also, the atmospheric condition within the furnace can affect the condition of parts to be heat treated. Furthermore, the quenching medium and quenching equipment must be elected to fit the material being used and the heat-treating process. Lastly, there must be provision for equipment for parts and materials handling, straightening parts, and cleaning metals. The following are the different types of furnaces required for the heat treatment process:

• Batch Furnace: made up of insulated chambers featuring a heating system, a steel shell, and access to the chamber.
• Bell-type furnace: made up of removable covers named bells which can be lowered over the load and hearth by crane
• Box-type furnace: most has been upgraded to a semi-continuous batch furnace with feature including slow-cool chambers and integrated quench tanks.
• Car-type furnace: also regarded as bogie hearth which is an extremely large batch furnace with a movable insulated car that can be moved in and out of the furnace.
• Elevator -type furnace: the hearth and the car are rolled into position under the furnace unlike the car furnace
• Fluidized bed furnace: made up of cylindrical retort produce from high-temperature alloy.
• Pit Furnace: furnace is constructed in a pit and extended to floor level or slightly above
• Salt bath furnace: uses a variety of salt for heat treatment

Summary

In conclusion, heat treatment is a way of using controlled heating and cooling techniques to alter the physical properties of metals to improve the metal. As a result, such metals are given the ability to be used in large varieties of industries.  Furthermore, heat treatment is an essential part of the precision machining process to transform parts and to ensure that your parts perform as expected for your projects.