The Complete Guide to Heat Treatment Of Metals

Table of Contents

I. Introduction

Heat treatment can be described as the process that involves the heating and cooling of various metals to acquire the needed properties through the use of specified methods. These methods are chosen based on the kind of metal being worked on and the desired properties to be achieved.

In today’s life, various methods have come up and are being used in industries that are involved in manufacturing processes involving metals. Some metals need to be softened while others need to be hardened to fit their preferred roles. The heat treatment process has been one of the commonly used methods to achieve either of the two. This method among others helps make metals with different qualities and clear physical and chemical characteristics particularly.

II. Purpose of Heat Treatment

There are several purposes as to why metals undergo heat treatment. These purposes include the following:

  • Heating treatment is done to enhance the mechanical characteristics of a given metal. These characteristics involve tensile strength, hardness, ductility, the metal’s resistance to shock, and the ability to resist corrosion.
  • It also helps to reduce the metal’s level of fragility and increase its machinability. Machinability refers to the ability of a metal to be easily machined.
  • It helps to ensure that the internal stresses of a metal relax which are brought about during cold or hot working.
  • It helps refine or alter the grain size of metal. This is whereby the grain size refers to the diameter of a grain that is found in the inner composition of a metal.
  • Heating treatment also helps to ensure that the electric and magnetic properties of a metal are increased to the desired level to be similar to other kinds of metal being used.
  • It helps to make a metal more resistant to corrosion and wear to avoid pitting.
  • It helps to boost the hardening of steel to make it possible for it to cut other kinds of metals.
  • Also, heat treatment helps to ensure that a surface looks clean and pleasing due to its bright nature.
  • It also helps that a metallic structure is uniform.

III. Types of Heat Treatment Processes

Heat treatment is a process that can be done in a variety of ways. There are different types of heat treatment processes. They include the following:

A. Annealing heat treatment process

This process of heat treatment involves altering the physical and in other cases the chemical characteristics of a given material to boost its ductility and make it less hard. This is done to make the process of working with the material easier and more efficient. In this process, the material being worked on is exposed to a temperature that is above its recrystallization temperature for a specific period before the cooling process begins. The rate at which the cooling happens is dependent on the type of metal involved in the process. Some are left to cool on their own at room temperature such as steel while others cool down in the air slowly or water might be used for faster results. An example is copper or silver. The metals heated normally recrystallize during the cooling process after the heating had made atoms move and redistribute and get rid of displacements in the workpiece.

This process works in three stages which are:

-The recovery stage where the heating occurs.

-The recrystallization stage where new grains are created without any residual stresses after the metal has been exposed to temperatures above its recrystallization temperature.

-The grain growth stage which is the cooling stage.

Annealing is normally used in cases where there is a need to reverse the outcome that comes as a result of work hardening caused by bending or cold forming.

 B. Normalizing heat treatment process

This is a process used to boost the ductility and toughness of various metals. It can also be referred to as normalizing annealing since it has close similarities to the annealing treatment. In this process, metals are usually heated to a set high temperature and kept at that temperature for a specific amount of time and then later left to cool down slowly in the air till room temperature is reached. This is normally done on metals that have been through metal hardening and need their grains to be normalized. This helps a metal to be more ductile and less hard. Several alloys are fit to be normalized and they include Copper, Nickel, Aluminum, Brass, and other Iron-based alloys.

This process also involves three processes as seen in the annealing process. The recovery stage involves heating the metal, then the recrystallization stage where there is the growth of new grains, and lastly the grain growth stage whereby cooling takes place and the grains formed in the previous stage get to develop completely.

C.Hardening heat treatment process

This process is usually used in instances that involve cutting tools, parts that are made using steel alloys, dies, and other types of materials that are usually used to do heavy work. In this process, there is the heating of steel up to the temperature where hardening happens. The material is kept at the same temperature for a specific amount of time to give it time to reach its austenitic structure. Then, the cooling process follows whereby it is done through the use of cooling means such as water or salt bath. In this process, the metal is not allowed to cool slowly on its own in the air.

D.Case hardening heat treatment method

This is a process that involves boosting the hardness of a metal only on its outer surface. The outer surface is usually the one that is more likely to wear out fast and gets to experience undeviated impacts hence the need to be hard. This process results in the formation of a thin layer on the metal which is usually harder than the metal underneath it. It also involves heating and changes in the structure of the crystal of the metal and new components in the composition of the outer metal surface. It is crucial to note that hardening results in reduced machinability of material therefore, this process is usually done when other manufacturing processes are done.

The hardening of only the surface of a material and not the whole of it helps to save on both time and energy hence being cost-effective when many operations are involved. There are a variety of ways used in case of hardening.

-First, there is heating and quenching which involves heating the exterior surface of a material at high temperatures beyond where there is the altering of the material’s microstructure. This is followed by cooling the heated part rapidly through the use of a quenching means such as water. As a result, a martensite which is usually very hard is formed.

-Nitriding is the other way of case hardening which involves nitrides forming on the surface of a material. The material is usually heated at high temperatures and made to come into contact with ammonia or other substances that may contain nitrogen. This leads to the formation of nitrides which are usually hard and resistant to wearing out. Metals that have elements with the ability to form nitrides are the only ones used in this process such as Chromium. A quenching process is not used in nitriding.

-Carburizing is used when there is a need to boost the mechanical characteristics of a certain steel substrate. It involves heating a steel alloy and letting it come into contact with large portions of carbon on the surface. The presence of carbon results in the formation of carbides on the surface which help to boost its hardness and make it not wear out easily.

E. Aging and precipitation hardening

Aging is the process whereby a metal is heated to a specific heat and then it is allowed to slowly cool down. The resulting material is usually more strong and more resistant to wearing out. This type of hardening can be utilized in only specific metal types which are copper and aluminum alloys. This process does not involve external heat or cold treatment to accelerate it.

Precipitation hardening on the other hand differs from aging hardening because the heat treatment is not involved. As a result, chemical compounds are introduced to the metal and a reaction occurs between the two resulting in a precipitate. The precipitate helps to boost the strength of the metal and how durable it is. This method can be utilized in several metals such as stainless steel. This process is faster compared to aging hardening and there is usually the use of external heat and cold treatment to make it fast.

IV. Heat Treatment Equipment and Furnaces

Heat treatment furnaces are usually used in the heat treatment process whereby utmost heating and/or cooling is involved to come up with the right material to be used. Furnaces can be categorized in various ways as shown below.

A. Batch-type furnaces

These are used in the process whereby the heat treatment method is done on a single load at a time. These furnaces come in different sizes where some are small while others are big enough to need the use of rail cars on rails to transport product into and out of the furnace. These furnaces may in some instances be and work on their while in others there is the attachment of systems like the ones used in the quenching process or to generate atmosphere. There are various types of batch furnaces and they include the following:

-Box furnace which is usually the most regularly used type of furnace. It is made of steel and extremely insulated, containing several gas burners. These furnaces are evaluated about their sizes, whereby bigger boxes are used to process bigger parts, and the rate of temperature, whereby high temperatures are associated with the production of many products and productivity. Fibrous or rigid materials may be used to insulate the boxes.

-Car bottoms which are normally the biggest kinds of batch furnaces. They contain “cars” which help in the transportation of products in and out of the furnace. These furnaces take a very long time to finalize on one load.

-Pit furnaces that deal with products that are usually vertical. The product usually stands 6 feet above the ground and 40 feet below the ground. This is convenient as it helps products that are needed in vertical form avoid bending.

B. Continuous furnaces

They involve a material being passed through the furnace at a given speed and heating occurs at the same time. They are usually used when half-finished materials need reheating for the heat treatment process. Continuous furnaces have a higher performance compared to batch-type furnaces and the cycle of heat treatment is faster. There are various types of continuous furnaces, including Pusher-type furnaces, roller hearth furnaces, rotary hearth furnaces, rotary table furnaces, and walking hearth furnaces.

C. Vacuum furnaces

As the name suggests, the product that goes through the vacuum furnaces usually has a vacuum surrounding it. A vacuum means that there is no air in the space, therefore, making it a good method to avoid oxidation which is usually caused by the presence of oxygen. The vacuum helps to ensure that the product does not lose its heat through convection and also ensures that there is no contamination involved in the process. These types of furnaces are usually employed in the annealing, sintering, and brazing processes.

D. Salt bath furnaces

These are used to heat and cool materials whereby dipping the material in a liquid mixed with salt is involved. The type of salts used is predetermined and their elements are assessed to make sure that the intended motive is achieved on the product. During the heating process, the material is usually immersed into a tub containing salt that is in molten form while cooling is done through the introduction of means to help in the quenching process. These are used in processes such as annealing, brazing, neutral hardening, and austempering among others.

V. Heat Treatment Variables

Various characteristics usually affect the results of the heat treatment process. These variables include the temperature to which the metal is exposed, the duration in which the metal is kept exposed to the heat, and the method that is used in the cooling process. Heating is responsible for the changes in the microstructure of metal therefore the degree to which heating is done determines the final composition of the metal. These variables usually change depending on the kind of material being treated and the outcome that the manufacturer is anticipating. The amount of temperature and time used in heating has effects on the cooling, soaking, and the whole heating process. The right rates have to be used for the right results.

VI. Common Heat Treatment Challenges

Every process has its advantages and the challenges that come with it. In the same way, some challenges are faced during the heat treatment process despite its efficiency. The materials that undergo may experience some flaws which are caused by several occurrences. A number of these defects are discussed below.

  • The metals may have decreased hardness and strength. There are various that causes the metal to not be as hard and strong as expected to be and they include: not soaking the metal for the right amount of time, not heating the metal to the right austenitizing temperature, slow rate of cooling or even not achieving the right hardness during the surface hardening process. This makes it hard for a manufacturer to get the desired end product.
  • The metals might have soft spots which are caused when the hardness of the metal is not evenly distributed in the whole metal. These soft spots are mostly experienced as a result of the treatment processes that use various means in the quenching process. The means used may alter the cooling rate of some parts of the metal hence the soft spots. They may also be caused by heating that is not even, not cleaning the metals properly or the means used for quenching might be of high temperature.
  • Quench cracks may also occur when stresses are caused by the heat treatment process. These stresses may happen to be drastic therefore resulting in cracks in the metal. These cracks are the ones known as quench cracks and make the metals unfit for use.
  • The metals may experience oxidation and decarburization. Oxidation happens when the metal being heated gets into contact with air, water, or carbon dioxide. If no action is taken immediately to reduce the severity of the reaction, small holes might form in the metal making it lose its significant properties. Decarburization, on the other hand, happens when temperatures of 6500C are reached during the heating process leading to the metal being weak.
  • Distortion and warping may also occur on the metal. Distortion is whereby the metal changes in size and shape in a uniform nature while warping happens when the changes are uneven. These two results cannot be undone once they have happened hence being the most costly defects. The changes in the size of the metal happen as a result of expanding and contracting while varying shapes is as a result of bending and twisting.

VII. Heat Treatment of Different Metals

Steel can undergo the heat treatment process to increase the composition of the microstructures. If the method for treatment used is thermo-mechanical, the shape of the steel is altered as well as its structure. If thermos-chemistry is used, the structure and chemistry of the structure are altered. The rate at which heating is done on steel is dependent on its level of conducting heat, the state of the steel, and its size. Processes used for treating heat include annealing, normalizing, tempering, and hardening.

Aluminum is heat treated through precision processes. Processes such as Preheating, annealing, solution heat treatments, and precipitation heat treatment are used in the heat treatment methods for aluminum and its alloys.

On the other hand, copper is heat treated through various processes such as annealing, precipitation hardening, stress-relieving, and homogenizing. The end products that result from the heat treatment of copper include sheets, strips, tubing, powder, bars, wires, and cables.

Titanium and its alloys go through heat treatment to ensure that the stress in the titanium metal is reduced, to ensure machinability and boost its strength. Processes used include annealing, stress relieving, aging process, and solution treatment.

VIII. Conclusion

In conclusion, heat treatment is crucial in ensuring that metals meet the required qualities in its application. To make this process a success, the manufacturer has to know various compositions of materials to know the right type of process to use. It is also necessary to note that not all kinds of metals are suitable for the heat treatment process. The heating process is also crucial in the CNC machining process whereby mold designs are designed and then the mold manufacturing involves the heating and cooling process. Most manufacturers usually use rapid prototyping to assess the efficiency of the various metals being used in the production process. This is done before the mass production of various products to ensure that the customers’ needs are met.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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