Heat Treatment Of Metals: The Ultimate Guide

Heat treatment is an important process in improving physical and mechanical properties of metals. This guide takes you through all the vital stages in heat treatment of metals such as annealing, normalizing, and tempering, amongst others

What Is Metal Heat Treatment?

It is a controlled process where you will heat metal to improve its microstructure. As a result, the metal acquires desirable mechanical and physical properties.

Advantages Of Heat Treatment Of Metals

Improves Strength And Toughness of the Metals

When working with heat treatment as a way of improving the above properties, sometimes it becomes hard to balance between them. When strength is increased, the metal may lose its toughness and become brittle.

Improves Durability And Wear Resistance

Heat treatment of metals can also improve the wear resistance and durability of metals when the metal is hardened.

Improves Machinability And Workability

Through heat treatment of metals, internal stresses caused by initial fabrication processes are removed. Removal of such stresses reduces hardness and makes the metal more machinable and easy to work with.

Advantages Of Heat Treatment Of Metals

Metal Hardening Process

Metal Hardening Process
Metal Hardening Process

Hardening is done on metals with the aim of making them harder, tougher and resistant to wear. Metal hardening process involves the following steps;

Heating Metal

First, the metal has be heated to high temperatures beyond the normalizing temperatures. By temperatures above the critical temperature, we mean, 400C above the higher critical temperature. Heating the metal will change its mechanical properties since heat alters the metal’s microstructure which plays a great role in its properties.

Holding Time

After heating the metal, the temperature is held for a defined period of time, for instance, one hour. During the temperature holding time, the metal is soaked and holding time is determined by the specific requirements. Such requirements are determined by the size and type of metal being hardened, meaning larger parts require more time to soak.

Cooling The Metal

Once the defined period of time has elapsed, the metal is subjected to rapid cooling process. Cooling, also referred to as quenching, can be done through media such as oil, water, air or molten salts. Choosing a given medium is determined by the level of hardness needed in a metal.

Thinner metals like knife blades can be quenched through air while thicker metals can water. Water as a quenching medium presents a challenge by sometimes creating bubbles on the metal surface. In order to prevent the formation of bubbles on the metal surface, manufacturers prefer to use brine solution instead of water.

Oil can also be used as a cooling medium and is usually recommended when metal distortion is a risk. Oil is mostly used with steel alloys.

Thin sections that need to obtain resistance to impact and to avoid cracking, are cooled through salt solutions. The final product, after heating and cooling, depends on factors like heating time, holding time, conditions, quenching time and medium used.

Usually, the choice of parameters is determined by the type of metal, the size of the metal and the heating method used. Once the metal has been cooled, it is now ready for various applications including; hammer, taps, center punches, dies, knife blades.

Metal Case Hardening Process

Metal Case Hardening Process
Metal Case Hardening Process

Case hardening involves the addition of carbon to metals with low carbon content. Metal with low carbon content is combined with another metal that contains more carbon. This combination will definitely produce a product with a harder case or surface compared to the original metals.

Such combination is done in a way that the end product will be easy to mold to for the required shapes. However, most metal case hardening processes take place after almost all the fabrication processes have been done on the metal.

Metal case hardening processes follow the steps described below;

Heating through induction or flame.

First, the metal is subjected to rapid heating to higher temperatures through application of oxy-gas flame or by heating through induction. Other methods under metal case hardening include cyaniding, carburizing, and nitriding.

Annealing Metal Process

Annealing Metal Process
Annealing Metal Process

Annealing is a process that involves heat treating metals to aid in changing their physical and even chemical properties. Changing these properties is meant to reduce metal hardness, increase toughness and ductility making it workable.

Annealing take place when metals are heated above their recrystallization temperature. The temperature is then held at that point for a given period of time before the metal is taken to the next step.

After the holding time is over, the metal is cooled rapidly under different cooling media. Type of metal being anneal will determine the time allocated for cooling. For instance, steel is usually cooled to room temperature in air as the cooling medium.

Silver and copper are cooled in water or can be cooled slowly in air. Heating causes atoms within the metal to move within the crystal lattice reducing the number of dislocations. These processes change the hardness and the ductility of the metal.

Annealing process takes place in a furnace and it is within the furnace that heating, holding and cooling processes take place. After holding time, the metal recrystallizes during the cooling process following elimination of all dislocations.

Annealing happens in three steps including recovery stage, recrystallization stage and grain grown stage. Below is a discussion of each stage as mentioned above;

Recovery Stage

At this stage, the metal is relieved of the internal stresses through heating to temperatures above its crystallization temperatures. The temperature is held for some time at this point to ensure that all the stresses are released before cooling.

Recrystallization Or Cooling Stage

After heating and atoms are moved within the metal lattice, the cooling stage is initiated through a cooling medium of choice. As the metal cools, it forms the crystals that were disintegrated during the first stage. New grans are formed void of the presence of any internal stresses.

Grain Growth Stage

On cooling, the grains now grow to form a product free of stresses with an improved workability. After the annealing process, now other processes can be carried on the metal to further improve properties such as strength.

Normalizing Metal

Normalizing Metal
Normalizing Metal

Normalizing metal is the procedure carried out on metals after the cold working procedures. This process is almost the same as annealing except for one difference. Temperature requirement for normalizing metal is much higher compared to what is used during the annealing process.

Cold working on metals can lead to the development of internal stresses which need to be gotten rid of. During this process, the metal has to undergo heating to temperatures 400C to 500C above the upper normalization temperature.

This is followed by a slow cooling in the air.

Normalizing metal is needed for purposes of improving the electrical, mechanical properties, tensile strength and machinability of the metal. Additionally, normalizing can also be described as the process of restoring the normal structure of a metal through heat treatment.

After normalizing a metal, what is obtained is a uniform structure with a secured grain refinement and better mechanical properties. Steps involved in normalizing metals include;

Heating Metal

The particular metal is heated in a furnace to temperatures high above its highest critical temperature. Approximately 40 – 500C more than the upper critical temperature.

Holding Time

Temperature is held at that point for a given period of time depending on the type of metal and the size. Holding time gives the metal time to soak and do away with all the unnecessary internal stresses.

Cooling Time

Cooling can take place in various media depending on the type of material and the desired end properties. For instance, steel can be cooled slowly in still air as the quenching medium.

After cooling, the product is ready to be taken through other processes as desired. The end product is uniform and machinable. Normalizing metal is mostly used in forgings and castings to relieve stresses and secure grain structure.

Also, normalizing metal can be applied to the removal of dendritic segregation and columnar grains which occurs during casting.

Tempering Process

Tempering Process
Tempering Process

Tempering process involves application of heat through a furnace to a metal to temperatures at its lower critical temperature. Holding time gives the metal time to soak followed by a very slow cooling process in air or any other medium.

Tempering is done in 3 different ways including low, medium or high temperature tempering. Remember, the temperature is determined by the level of hardness or toughness needed for a given application.

Tempering is the procedure aimed at relieving internal stresses that occurred during the hardening processes.

Another reason for carrying out tempering procedure is to add more toughness and ductility to the metal reducing hardness and strength. Also, tempering can enhance dimensional stability by decomposing any retained properties that are not needed. Through tempering, the metal can acquire improved magnetic properties if needed.

Steps in the tempering process include;

Precipitation of e-Carbide

Temperatures in the furnace are raised up to 2000C which leads to reduction of tetragonality of martensite in the metal. This is the temperature equal to the lower critical temperature of steel.

Decomposition of Any Retained Austenite

Temperatures are raised to from 200C to 3000C to ensure that any retained austenite is decomposed.

Formation of Rods

Raising the temperature to from 2000C to 3500C helps to get rid of tetragonality in martensite and e-carbide dissolution.

Recrystallization

Recrystallization takes place during the cooling process to form a product which is free of all internal stresses.

Nitriding

Nitriding
Nitriding

Nitriding is a type of case hardening procedure which involves the use of nitrogen gas to create a hard coating on the metal. During nitriding, the metal is heated in an environment with ammonia in it.

Deposition of an ammonium atom on the surface make it harder than the original metal. Some of the reasons why nitriding is done is to harden the surface of the metal to a given depth.

Another reason for nitriding is to increase wear, fatigue and corrosion resistance.

Nitriding process takes place in the following steps;

Heating The Metal

Heating is carried out in an electric furnace with a temperature range of between 4500C to 5100C. After the metal has been machined and finished, it is placed in a container which is airtight. The container is designed with an inlet and outlet tubes to aid the circulation of the ammonia gas in and out of the container.

Application Of Ammonia Gas

Once the container has been placed into the furnace, ammonia gas is supplied to the container as heating takes place. Heating aids in releasing the nitrogen gas from the ammonia gas which forms an iron nitrate by reacting with the metal surface.

Metal Removal From The Furnace

Once iron nitrate has been formed on the surface of the metal, the metal is removed from the furnace. For this process, no quenching or more heat treatment is required. Nitriding process is mostly applied when hardening medium carbon alloy steel surfaces.

Cyaniding

Cyaniding
Cyaniding

Cyaniding is another type of case hardening process which is mostly used on steel. The steel is heated to high temperatures in the presence of sodium cyanide. For this case, we have nitrogen and carbon atoms being deposited on the surface of the metal making it hard.

Cyaniding is used for increasing surface hardness, wear resistance and fatigue limit and to add a bright and clean look. Cyaniding can take place through the following steps;

Dipping the Metal in a Molten Cyanide

The metal to be cyanide has to be dipped into a liquid cyanide bath which is heated to temperatures of 9500C. The bath is made of sodium carbonate, sodium chloride, soda ash and sodium cyanide.

Soaking

Metal is left to stay in the bath at temperatures maintained at 9500C for around 15 – 20 minutes.

Hardening of the Surface

At high temperatures, the sodium cyanide is decomposed into carbon and nitrogen. Both of them get diffused onto the surface of the metal making it hard.

Cooling.

Once the hardening has been achieved, the metals is removed from the bath and transferred into the cooling medium. Water or oil can be used as the cooling medium for this case. Cyaniding process is suitable for smaller parts such as brushing, gears, screws, hand tools, that need thin and wear resisting surfaces.

Carburization

Carburization
Carburization

Carburizing is the type of case hardening that create hard surfaces through the use of heat in the presence of carbon. From the heating process, the carbon atoms adhere to the surface of the metal making it hard.

Carburization is meant to improve surface hardness, improve fatigue strength, increase wear resistance and produce clean parts. The amount of casing deposited on the metal is determined by the atmosphere, temperature.

Once the metal has been hardened, it is quenched through water or oil to cool it. After cooling, these parts are taken through tempering to meet the required parameters of hardness and ductility.

Surface hardening can be between .002’’ – .250’’ also determined by the intended use and application. Carburizing is suitable for both complex and simple parts.

Applications of carburizing include parts with low carbon content and those that require higher resistance to wear and fatigue.

Applications Of Heat Treatment Process

Automobiles
Automobiles

The commonly used heat treatment methods include tempering, carburizing, hardening and nitriding. Various parts within the automobile industry can be heat treated to increase wear resistance and durability. Such metal parts include hub annulus, friction plates, axle shafts, and gears.

Aerospace
Aerospace

Heat treatment is necessary for making components for the aerospace industry. Through heat treating, parts have strength, parts are machinable, and formable. Some of the heat treatment operations involved include carburizing, cyaniding, hardening etc.

Metal Working
Metal Working

Metal working companies apply the use of heat treatment on metals to improve machinability, formability, wear resistance, ductility, impact strength etc. Such improved properties enhance the quality of parts produced through the metal working processes offering the best customer experience.

Machinery
Machinery

Manufacturers working with different machinery can work around any kind of metal to come up with the best strength and wear resistance. Some of the metals that can undergo heat treatment include bronze, brass, aluminum, copper, steel and cast iron.

Construction
Construction

A lot of thermal procedures are done through forging during manufacturing of construction materials. Most parts in the construction industry undergo normalizing, annealing, tempering and hardening. Some of the parts made through heat treatment include pins, logging jaws and hydraulic cylinders.

People Also Ask:

What Is The Purpose Of Metal Heat Treatment?

Metal heat treatment is carried out on metals for various reasons including increased formability, improved hardness, enhanced strength and high durability. Other reasons for heat treatment include increased abrasion and wear resistance, better corrosion resistance and high resistance to impact.

What Is Annealing Used For?

Annealing is used for doing away with defects arising from hardening processes like during drawing, bending and cold forming. Very hard metals may not be easy to work with, hence annealing makes parts more formable and easier to work with.

How Does Annealing Compare To Tempering?

Annealing requires heating the metal to a specific temperature followed by cooling under controlled and a very slow process. Tempering requires heating the part to a specific temperature just below the critical temperature and does not require cooling.

What Is Quenching In Heat Metal Treatment?

Quenching is described as the process of rapid cooling of a metal using a quenching medium like oil, air or water. In heat treatment, quenching forms part of the most important steps in order to achieve quality heat treated products.

Is There Recommended Tempering Temperature?

The temperature requirement during the tempering process is usually determined by the level of hardness and strength a given application requires. Temperature level could be low, medium or high temperature; however, it varies approximately between 2000C to 7000C.

Will Heat Treatment Make Metal Stronger?

Yes, but this will be determined by the type of heat treatment method you have chosen for your part. For instance, heat hardening will improve the strength, hardness, corrosion resistance, durability and fatigue resistance of your part.

Resources:

Metal Heat Treatment

Metal Surface Heat Treatment

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