Excited to know about the tempering? So, in this article, we are going to discuss all the insights of a tempering process. Keep reading this article to know more about this heat treatment technique.
The History Behind Tempering
You may be amazed to know that tempering was in practice from 1200 to 1100 BC. The best example of tempering is a pick axe. This heat-treating method was in used all over the world. People from Asia, Africa and Europe are using the tempering from many years and this process is done mainly in the continents like Asia, etc.
So, let’s have a close look at
What is Tempering?
Tempering is one of the heat treatment processes in which the toughness of the material increases. In this heat-treating process, the material that are used are mostly related with iron or we can say that this heat treatment process is especially performed for all the alloys of iron. Along with the increasing toughness, the hardness of the metal decreases in the tempering. In this process, the workpieces are heated below the lower arrest temperature. The lower arrest temperature is often known as lower critical temperature or lower transformation temperature. If you want to achieve the exact physical properties that you wanted then, you must have a tight control on temperature that you are providing to the workpieces as well as the time for which the temperature is being maintained. If this process is used for the carbon steels then, it changes the size as well as results in forming the tempered martensite.
There are some terms in tempering like:
1. Through Tempering:
When hardness is produces in steel uniformly then, the process is termed as a through tempering
2. Differential Tempering:
When tempering results in a non-uniform hardness it is called as a differential tempering.
What is Main Purpose of Tempering?
The basic purpose of tempering is to make changes in the properties of the steel in such a way that it can be used for larger number of applications. The properties that are being changed by the tempering are hardness, ductility, toughness, tensile strength, etc.
This process is applied on normalized steels to make them more softer and on welded steel to eliminate the hardness of the steel.
Do you ever hear the term Tempering colours? Well, tempering colours are developed on the unoxidized steel as a result of tempering. When the steel is heated its temperature increases which in turn causes an increase in the thickness of the iron oxide. As these oxide layers are not transparent, they do not allow light rays to pass through them. Hence, the phenomenon of the thin film interference occurs resulting in the formation of tempering colours. These tempering colours vary from faint-yellow to grey-blue.
Following are the tempering colours along with the temperature range in which they produced:
- Light-straw: It is formed about 205º C
- Faint yellow: It is formed about 176º C
- Purple: This color is formed in the tempering process when you heat it about 282º C
- Dark straw: It is formed at 226º C
- Light blue: This color is formed at 337º C
- Grey blue: This color is formed at about 371º C during the tempering
Types of Tempering Techniques
1. Differential Tempering:
Differential tempering is also called as a graded tempering or selective tempering. This process was in practice since ancient time in the Europe. The people used this differential tempering in making various tools as well as equipment which are used in battle like swords, knives, etc. Differential tempering results in the sharp as well as hard edges of knives.
Austempering is one of the tempering processes which is particularly used for ferrous metals. It is applied to steel gave rise to bainite whereas when it is applied to cast irons it forms ausferrite. The main intention behind using austempering is to eliminate deformation inside the metal parts.
There are the four processes that are involved in the Austempering:
In Austemperising, the ferrous metals are first heated between the temperatures 790 to 915º C. When material is heated in this temperature range it gets converted into Austenite.
The heated metal is now quench into the solution of nitrite-nitrate. The heated metal can also be quenched into different oils.
After quenching, the heated parts are cooled slowly until they reach the room temperature.
If the given metal part is completely converted into bainite or Ausferrite then, there is absolutely no need of tempering. Tempering is required only if there are some issues with Austempering.
Following are the advantages of the Austempering:
- One of the most important advantage of the Austempering is that it almost eliminates the distortion inside the metal.
- Austempering causes higher ductility and enhances the fatigue strength of the material.
- It also improves the wear resistance of metal.
Martempering is quite familiar to the Austempering. The only difference is that in Martempering, the quenching is done at much lower temperature than Austempering. The main focus in Martempering is to form a martensite rather than bainite. It is also known as stepped quenching or interrupted quenching. In this process, the steel is heated in such a way that it surpasses its upper critical temperature. First of all, all the steel is converted into Austenite and then, by quenching process it is converted into martensite process. The quenching temperature lies between 150- 300º C and the steel is quenched in the solution of salt and oil. There is only one disadvantage of Martempering is that the larger steel parts cannot be heated with the help of this process.
4. Black Tempering`
Black tempering results in a black-iron. Black tempering is done in the presence of the inert gases. For black tempering, temperatures must be above 950ºC and the metal is heated for consecutive 20 hours at constant temperature.
5. White Tempering
During white tempering, the temperature must be as high as 1,000º C. The metal is heated at such high temperatures for almost 60 hours. After such long heating, the material is allowed to cool at slower rate. The cooling rate during white tempering is 10º C per hour. This process results in the formation of the malleable cast iron or the porous cast iron.
In this article you have learnt about what is tempering, its types and process. If you get some valuable knowledge from this article then, don’t hesitate to share this knowledge with your friends.