It is eminent that treating metal by heat will change its physical properties in some preferred way. The range of terms and techniques of heat treating metals and the precise methods of treating them is often not clear.
The main aim of this article is not to give professional advice on heat treatment, but to give you a thorough and detailed explanation of various treatments and their differences.
In this article, the processes that we are comparing are annealing, normalizing, and tempering. All of these techniques are used in various manufacturing projects for various reasons.
- 1 Why Different Heat Treatments Produce Different Results
- 2 How We Should Compare Those Techniques
- 3 Basics of Annealing
- 4 Basics of Normalizing
- 5 Basics of Tempering
- 6 Differences Between Annealing, Normalizing, Quenching, and Tempering
- 7 What You Have to Remember From This Article
Why Different Heat Treatments Produce Different Results
Metals that are also chemical elements like iron, bronze, and copper can be treated with heat without altering the metal’s microstructure. Since there are no other elements inside the piece of metal, once it cools down its normal shape, its properties are brought back.
However, steel is very different and can undergo structural modifications after heat treatment; this is made possible because it is an iron-carbon alloy. Keep in mind that there are substantial amounts of other metals in many types of steel.
Therefore, in alloys, the composition of it, the rate that the heat is applied to it, the rate that the fabricator cools it, the maximum temperature that the metal is heated are all very important parameters. Those parameters affect the microstructure of the metal hence its properties. This is one of the many reasons why steel is considered the most used industrial metal for many decades not. Steel can be tempered, hardened, and depending on its composition and method you can give it almost any properties you want.
How We Should Compare Those Techniques
One thing that applies to all techniques of heat treatment is that the metal should be heated to more than its critical range, normally until it glows red. The cooling process is the second part of those techniques. Usually, the accuracy and speed in which the metal is cooled down will determine the flexibility and strength of it.
Annealing, normalizing and tempering are the basic processes that manufacturers use for altering properties of metals like steel.
Keep on reading to know more about annealing, normalizing, and tempering treatments.
Basics of Annealing
Annealing treatment can be advantageous as the course of heating and slow cooling will generate a uniform and finer grain and get rid of internal strains and the bend resulting from cold working. This improves a lot of the properties of perlite steel. Once a piece of steel is not in the pearlite state, annealing would offer even more advantages to the metal’s characteristics. Annealing can be performed in 3 different stages which are heating the metal, keeping it at a constant temperature, and slow cooling it down to room temperature. The maximum temperature that steel is heated exceeds 100 degrees Fahrenheit, more than its critical range for almost an hour. So, let’s see the advantages and problems of annealing.
Key Takeaways of Annealing
The main characteristic that distincts annealing among other heat treatments is the fact that cooling takes place at a controlled rate. The metal is heated to a fixed temperature for a specific time and then cooled down slowly with the furnace, also known as complete annealing. The objective is to lessen the hardness of the metal to get rid of a rough structure and internal stress.
The metal is heated to 750 degrees Fahrenheit for a specific time and then cooled down gradually to 500 degrees Fahrenheit. Last but not least, it is cooled in the air, also known as ball annealing. The objective is to lessen the metal’s hardness to boost the performance in cutting, largely for steels that contain a lot of carbon.
Stress relief annealing which many people call it low temp-annealing refers to heating steel in temperature between 500 to 600 degrees Fahrenheit. The next step of the process is letting steel stay at that temperature for a while. Then, the metal has to be gradually cooled down below 300 degrees Fahrenheit in furnace controlled temperature. The final step of the process is letting the metal cool down to room temperature. An interesting fact about this structure is that the structure of steel doesn’t change. However, all the internal stresses and tension inside the metal has been greatly reduced.
Basics of Normalizing
Normalizing is another process similar to annealing. The way heat is added to steel is the same on both methods. The difference between those two processes is related to the cooling step. In normalizing, the metal cools down with the help of air. The microstructure of steel changes with this method. A huge factor in the whole process is the carbon concentration inside the metal.
When it comes to grain types when the process completes, most of the time a mixture of sorbitite and pearlite is created and in some cases only pearlite. Furthermore, in tinier pieces such as rings, faster cooling takes place because of their size. It will certainly result in less formation of pearlite but this fact makes the steel less squashy and harder. So, faster cooling which takes place in smaller objects allows the development of sorbitite, the type of grain that increases the hardness of the material.
Key Takeaways of Normalizing
The main objective of this process is to make the metal less hard
Opposed to annealing, the cooling rate in normalizing is a bit faster. Also, this rate is not controlled. Therefore the heat treatment cycle is short. On both of these methods, parts are able to meet similar performance requirements.
Basics of Tempering
Tempering is another technique that is vastly used in the industry. Any metal that is hardened is certainly tougher but this fact makes it brittle to work on it. Therefore, manufacturers are always trying to find for the composition of the metal, the right balance between ductility and hardness.
The process is similar to annealing. However, the finished product has completely different properties because in tempering the metal is heated in lower temperatures. Like in annealing the rate that the temperature drops during the cooling step is controlled.
Before any tempering takes place, the metal should be first and foremost hardened. After that, the metal must be preheated and immediately cooled down.
Now it’s time for the tempering process. There are lots of temperature ranges wherein a piece of metal might be tempered. Each of those ranges gives different combinations of ductility and hardness. So, two pieces of the same alloy tempered at different temperatures will have different amounts of those characteristics. As I have mentioned those qualities of the metal are opposite. Therefore, one of the metals will become harder after the completion of the process.
Variations of Tempering
Also, the process where the metallurgist keeps a large amount of steel’s hardness but causes the steel to be pretty brittle is known as drawing. Even if this process is named tempering, people refer to it as drawing. Therefore, they don’t consider it as tempering while technically it is one. The drawing process is also called troostitizing.
Another variation of tempering is sorbitizing. In sorbitizing, the metal is tempered but the final product properties are completely opposite with pieces of metal that are treated with drawing. The final product is ductile and not hard.
Characteristics of Tempering
Steel tempering generates a remarkable effect on the color of steel because it darkens it. During this process thin layers of oxide are created and once reheated, they will blaze in different shades. There is a strong correlation between temperature and color. Each temperature corresponds to a specific shade.
In troostitizing process, the metal needs to be heated at 600 degrees Fahrenheit until you notice a shiny blue color. However, the metal needs a milder heating temperature of fewer than 600 degrees Fahrenheit to be tempered. The metal in those temperatures gets a dark purple or deep blue color. Keep in mind that there are lots of stages at which metal might be tempered or drawn. Once a piece of steel is heated under extremely high temperature, it will begin to re-acquire a pearlite formation. In this scenario, the process of hardening stops.
A distinctive feature of this process the maximum temperature that the metal is heated. However, the duration that the metal stays in that temperature affects its properties. As I have previously mentioned the temperature that tempering takes place affects the color of the metal. However, letting it for a while at that temperature will also make the color of the metal shadier. So, it is possible to produce two pieces of the same metal with the same color but with completely different properties. They have different properties because one of the pieces was tempered at a specific temperature for a certain amount of time while the other was tempered at a lower temperature for a longer time.
Key Takeaways of Tempering
Low heat tempering from 50 to 250 degrees Celsius removes the mechanical stresses that exist inside the metal. Also, the metal becomes very elastic and that’s why it becomes wear-resistant in quenching.
Medium heat tempering is from 350 to 500 degrees Celsius. In this case, the metal is boosted in both strength and elasticity.
High heat tempering is from 500 to 650 degrees Celsius. The metal becomes tough when it is tempered in over 500 degrees Celsius.
Parts of the metal quenched after high heat tempering have complete mechanical properties, not just has a specific hardness, strength, but also plasticity and toughness. Therefore, it is not uncommon for carbonated steels to be treated with high-temperature tempering subsequent to quenching.
Differences Between Annealing, Normalizing, Quenching, and Tempering
As I have written above there are many differences among these treatments. You will find variations in the materials’ structure as well as in the materials’ properties.
When it comes to normalizing, the workpiece is heated to Ac3. Ac is the end temperature at which ferrite is changed to austenite during the process of heating. In general, it is located between 727 degrees Celsius and 912 degrees Celsius or also known as Acm. The critical line of temperature for comprehensive steel austenitization is 30 to 50 degrees Celsius higher than that temperature.
Tempering means that a normalized or quenched metal is cooled at a particular level after being heated for a period under the critical temperature.
Annealing heats the steel to a specific temperature for an adequate time and then cooled down at an ideal rate, normally, slow cooling or also known as controlled cooling.
Other Differences in Grain Level
Quenching is also the process of steel heating to a heat that is more than the desired temperature of AC1 or Ac3. If the metal stays for a specific period in those temperatures then the whole metal gets austenitized. Then, the metal which is in this state at grains level is quickly cooled down to room temperature.
Low heat tempering leads to martensite formation. Extreme tempering, as well as quenching treatment, leads to a tempered sorbitite formation. Subsequent to annealing, the grain is purified; the structure is changed as well as eliminating the defects in the metal.
Quenching will cause the cooled austenite to go through bainite or martensite change. This also allows you to obtain a bainite or martensite structure. Lastly, a disturbed formation conquered by martensite is acquired. Also, it is having bainite, or keeping one-stage austenite is required.
Differences in Material Performance
In the event of normalizing, crystal grains of metal can be purified in a bit faster cooling. Faster cooling makes the metals more hard and tough. The problem is that these metals might break easily. A lot of low alloy hot rolled steel sheets, low alloy steel castings and forgings, after undergoing normalizing treatment, their mechanical properties are greatly improved. Also, it boosts their performance in cutting.
It’s not uncommon for a metal that has been tempered to be normalized and quenched. A way to get rid of some residual tension is to rapidly cool the metal. So, the flexibility and toughness of steel can be enhanced and improved to a greater extent.
Annealing, on the other hand, can lessen the hardness of steel, and at the same time, enhance its machinability. Also, this method is used for eliminating residual stresses, tensions. Moreover, it is a great method for stabilizing the size of the metal while at the same time reducing the probability of cracking and fracturing.
Quenching can boost the hardness, and rigidity to a higher level. This method also enhances the toughness, resistance to fatigue, and strength of the metal. Exclusive chemical and physical properties like corrosion and ferromagnetism resistance of specific special metals can be met using this process.
So, it is obvious that a combination of those methods can be used for creating mechanical tools and parts with unique properties.
What You Have to Remember From This Article
Annealing: It’s the process of heating steel to a specific temperature. Then, the metal is left warm for a period of time. The final step is cooling the metal at a controlled rate.
Normalizing: This refers to heating steel from 30 to 50 degrees Celsius above the critical temperature. The process wherein air is cooled after a specific time is also called annealing. Just the normalizing cooling level is a bit faster, while the production cycle is a bit short. However, you can use both methods to meet the needed performance of parts. You can make use either of those methods when it is possible.
Quenching: It refers to heating the steel above the critical temperature. At the quenching temperature, the state and structure of the steel with alter. The type of grains is called austenitic. The metal has to stay warm for a specific amount of time at that high temperature. Then, the metal is rapidly cooled. This is done to get a metastable martensite formation or a lower bainite formation.
Subsequent to quenching, a martensite formation is acquired. However, the internal formation of this condition is not balanced. Even if the toughness is high, the hardness, as well as the plasticity, is reduced. Also, the metal is brittle. So, the metal that undergoes quenching will not be delivered as an end product.
When it comes to tempering, it is certain that this method has a lot to offer in some projects. Once the steel is toughened or hardened, it is heated to a temp under the critical temperature. In this method, the metal stays warm for a few minutes. What we want is to have all parts of the metal at the same temperature. Then, we can cool the metal to room temperature. The end product will have remarkable strength, toughness, as well as specific plasticity and hardness.
In these four processes related to heating treatments, tempering and quenching are applied in many different projects and in some cases together. I question that you might currently is which method should I use. Another question is what is the right sequence of a combination of those methods. I have given some glimpses of what is the answer to those questions throughout the article. However, the right answer is that it depends.
Each project and its requirements are unique. Also, the materials that you are using are not the same every time. Different alloys contain different metals in different quantities. That is why there are material engineers that have extensively analyzed these subjects.
Keep in mind that there’s no fixed sequence. The processes have to be customized to a particular case or situation.