What is Tempered Steel and Why is Tempered Metal Important?

In the world of metallurgy, tempered steel stands as a keystone on the basis of its importance in various industrial usage and applications especially when this material goes through heat treatment process via different temperature variations. The process through which this material is heat treated is known as tempering which comprises of controlled heating and then cooling it rapidly to achieve a microstructure which results in a unique blend of properties of the steel. These properties include good strength, resilience and toughness suiting a wide range of different industrial applications. So, despite your profession, if you are interested in getting basics and knowing wonders of this material, this article surely uncovers all the endless probable of tempered steel- leading material for advance innovation in metallurgical world.

Apart from that, Tuofa is considered as a leading reckon for great precision as well as accuracy especially when dealing with CNC machining. Additionally, having a deep and profound understanding of different types of steel along with alloy steels and carbon steels, Toufa serves in manipulating, designing, and creating different components with excellent dimensional accuracy. Along with good and precise machining capabilities, when merged with different heat treatment processes, they have the ability to raise industrial standards and shape the future with modern solutions, especially in the realm of engineering.

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Fig 1: Tempered steel

What is Tempered Steel?

When a certain type of steel goes through a particular heat treatment process that is tempering, it results in a material with a blend of properties i.e., strength, toughness and ductility and is known as tempered steel. For tempering, the critical temperature is usually around 200°C to 600°C depending on the desired application as well as the composition of that particular type of steel. Low temperature tempering leads to enhanced strength and hardness while high temperature tempering results in increased ductility and toughness.

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Tempered Tools

Such tools are made from tempered steel that undergoes through particular heat treatment process i.e., tempering. These tools possess many properties that are desired by various industrial applications. Some of the reasons that makes them prominent and highly demanding are as follow:

Fig 2: Characteristics of temper tools [6]

There are several tempered tools that are used in different industrial sectors especially in the field of automative, construction along with manufacturing with guaranteed longevity and reliability. Some of them are shown below:

Fig 3: Tempered tools

Composition and Properties

Table 1: Composition of tempered steel

There are many properties that come along when a specific steel is tempered. The major properties that are most prominent are given below:

Hardness: Exhibits a balance between hardness and toughness, making it suitable for a wide range of applications. tempered steel has a Rockwell hardness (HRC) ranging from 20 to 65 HRC.

Tensile Strength: Tensile strength refers to the maximum stress a material can withstand .Tempered steel typically has a high tensile strength, ranging from 400 MPa to over 2000 MPa.

Yield Strength: Tempered steel generally has a high yield strength, ranging from 300 MPa to 1800 MPa or more ensuring to withstand significant load without permanent damage.

1. Hardness: Exhibits a balance between hardness and toughness, making it suitable for a wide range of applications. tempered steel has a Rockwell hardness (HRC) ranging from 20 to 65 HRC.
2. Tensile Strength: Tensile strength refers to the maximum stress a material can withstand .Tempered steel typically has a high tensile strength, ranging from 400 MPa to over 2000 MPa.
3. Yield Strength: Tempered steel generally has a high yield strength, ranging from 300 MPa to 1800 MPa or more ensuring to withstand significant load without permanent damage.

Fig 4: Properties of tempered steel

Tempering Metal Process:

The process is done in order to achieve certain properties required for the desired applications. This is done by heating it to a certain temperature followed by controlled cooling. Below is the phenomena of physical processes that occurs during tempering: 

Diffusion

• Atoms within the steel rearrange themselves due to the heat.
• Allowing for the redistribution of carbon and other alloying elements.
• Relieve internal stresses and refine the microstructure of the steel.

Precipitation

• Depending on the alloying elements present, tempering can induce the formation of fine precipitates within the steel matrix.
• These precipitates contribute to strengthening and stabilizing the material's structure.

These are the basic steps involved in tempering the steel:

Fig 6: Applications of tempered steel

Effects of Higher and Lower Temperatures on Steel's Properties:

Higher Temperatures:

• Reduction of hardness
• Increase in toughness and ductility
• A very high temperature results in low wear resistance and strength

Lower Temperatures: 

• Enhances the hardness of the steel
• Maintain adequate toughness
• But may result in increased brittleness and reduced ductility

Tempering colors of steel

Table 2

Why is Tempering Important?

Tempering is essential for many reasons. Some of them are given below:

Fig 7: Characteristics of tempering

The combination of above-mentioned properties makes tempered steel favorite for large number of applications.

The Process of Tempering

The process comprises controlling both the phases i.e., heating and cooling in order to achieve the desired combination of properties and this can be achieved be adjusting the parameters in such a way that optimum mechanical properties are achieved. Both phases are explained in detail below:

The Heating Phase

During this phase, the steel undergoes particular temperature, generally below the critical point. Factors such as the desired mechanical properties and the composition of the steel determine the exact temperature as well as the duration. Generally, heating temperatures lie between 150°C to 600°C, while duration can vary depending on the dimension of the specimen.

Equipment Used

• Industrial furnaces
• Ovens 
• Heat treatment chambers

The Cooling Phase

Once the steel component has gone through the heated phase, controlled cooling is done that leads to microstructure stability and acquiring optimum mechanical properties. Common cooling methods include:

• Air cooling
• Quenching in oil or water
• Furnace cooling

Cooling rates can be controlled by adjusting parameters such as quenching medium, furnace temperature gradients and agitation. Slower cooling rates lead to increased toughness and ductility. On the other hand, faster cooling rates enhance hardness as well as strength.

The Metallurgical Changes During Tempering

Microstructure Transformation:

Initially, the rapid cooling of the steel specimen leads to martensitic transformation which typically results in a needle like structure when visualized through the optical microscope. Furthermore, this specimen is reheated to a certain temperature but lower this time typically between 150°C to 600°C followed by cooling which results in tempered martensitic transformation. Hence those needles somehow form spherical shapes when observed under microscope leading to enhanced strength, ductility and reduce brittleness.

Another change that is observed is the diffusion of carbon atom in the steel matrix. The tempering process allows these atoms to rearrange and redistribute leading to the formation of carbides promoting strength within the structure. In addition to this, this thing also helps in reliving internal stresses.

Phase Changes and Their Effects

The variation in the tempering temperature as well as the composition of steel can lead to the formation of other phases beside the formation of tempered martensite. These phases changes that occur are mentioned below:

• Bainite: Fine, needle-like ferrite and cementite phases
• Ferrite: Soft phase that forms at higher tempering temperatures

These changes have a great impact on the properties of the steel especially mechanical contributing to varying degrees of hardness, strength, toughness and vice versa.

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Applications of Tempering in Mechanical Engineering

Enhancing Durability of Components

Components comprising of tempered steel play a pivotal role in increasing the durability and performance of various parts in the automotive industry. Below are the components along with their function in different applications:

1. Crankshafts, camshafts, and transmission gears: To withstand the high stresses and loads experienced in internal combustion engines.
2. Springs and control arms: To improve fatigue resistance and longevity under repeated loading conditions.

Secondly, Aerospace engineering has faith in tempered steel parts for crucial aircraft structures as well as systems. Various components like landing gear, structural fasteners and engine mounts are frequently made from tempered steel to ensure safety.

Balancing Hardness and Toughness:

Tempering plays a vital role in mechanical engineering by improving the durability of components in industries like automotive and aerospace. In addition to this, it balances the desired mechanical properties of materials through careful material selection and customized tempering processes. The adequate balance between hardness and toughness highlights the versatility and importance of tempering in optimizing the performance and reliability in various applications.

Advantages of Tempered Steel

Fig 8: Advantages of tempered steel

Comparison with Other Ferrous Alloys

Table 3: Comparison of tempered steel with other ferrous alloys

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Best Practices

Fig 9: Tempered steel best practices

Relation to Carbon Steel and Low Alloy Steels in Manufacturing

Carbon Steel: Enhances the strength, toughness, and wear resistance of carbon steel, making it suitable for a wide range of applications.

Low Alloy Steels: Tempering low alloy steels allows for the customization of mechanical properties to meet the requirements of various applications in aerospace, defense etc.

The Science Behind Tempering

Metallurgical Explanation of Tempered Martensite and Internal Stresses:

Tempered Martensite:  Rapid cooling from a certain austenizing temperature result in a metastable microstructure known as martensite. Thus, prevents austenite from transforming into equilibrium phases which eventually results in supersaturated solution of carbon in iron. Hence, when tempered, carbides are formed because of carbon atoms diffusion at lower temperature. Furthermore, these carbides increase the strength of steel by making hindrance for dislocation movements.

Internal Stresses:  Austenite to martensitic transformation leads to the formation of internal stresses due to non-uniform cooling. When tempered, these internal stresses are released resulting in relaxed atomic structure.

Effects of Tempering Temperatures on Microstructure

Microstructural Changes: The tempering temperature significantly influences the microstructure of the tempered steel.

At lower tempering temperature:

• Carbide precipitation occurs predominantly along the lath boundaries of the martensitic structure.
•  
• Resulting in fine carbide particles dispersed throughout the matrix.
•  
• Increases the hardness and strength of the steel while maintaining adequate toughness.

At high tempering temperature:

• Carbides continue to grow and coalesce
• This leads to coarser precipitates and a reduction in hardness
• Promote the formation of secondary microstructural phases such as bainite and ferrite, which contribute to increased toughness and ductility

Understanding Heat Treatment

Below are the major heat treatment processes along with their impact on steel composition and alloy:

Table 4: Heat treatment processes with their impact

Comparison of Tempering vs. Quenching:

Table 5: Tempering vs Quenching

How do You Temper Steel

Several steps are involved in order to temper the steel component. The major steps are mentioned below starting from initial heat treatment to final testing and quality assurance:

Fig 10: Basic steps for tempering steel components

What is the best method for tempering steel?

It depends on various factors, including the steel type, the mechanical properties desired, as well as the particular application requirements. Usually, the methods for tempering steel include:

Conventional Furnace Tempering
Salt Bath Tempering
Induction Tempering
Flame Tempering
Cryogenic Tempering

Tempering Knife and Swords

In tempering swords or knife, usually they are heated above their critical point followed by quenching. Quenching can be done in oil or water to ensure hardness as well as brittleness. Carefully heating and natural cooling ensures uniform temper. In case of large swords, uniform temper is a bit challenge, therefore differential tempering is done, localized heating with precise flames.

Tuofa Custom Steel Parts Solutions for Diverse Needs

There are huge challenges and problems faced by low temperature applications as well as projects containing steel with high carbon contents. In such conditions, Tuofa CNC machining service proved itself to be the best among others. In terms of low temperature applications, it has a great precision control and has ability to tailor properties in extreme environmental conditions. Secondly, their advanced CNC technologies also delivered great expertise while dealing with high carbon content steel ensuring accuracy in machining and optimized tempering to achieve desired properties.

Partnering with leading aerospace and heavy machinery manufacturers in order to tailor their problems by ensuring reliability and performance. Additionally optimizing the tempering process resulted in exceptional toughness and impact resistance. Hence, clients from both sides showed satisfaction as the objective was achieved successfully i.e., greater efficiency, reliability of the product along with cost effective solutions.

Conclusion

Tempered steel plays an important role in different industrial sectors as far as components comprising of high quality are demanded. Secondly, this steel could provide a blend of properties that helps the components to maintain dimensional accuracy without causing any permanent damage i.e., resistant to permanent deformation. Therefore, this material is beneficial where reliability and durability are prime factors. Apart from that, Tuofa specializes in giving tailored solutions for tempering and CNC machining needs and other customized tempering services. Without any doubt, they can be trusted to tailor applications where precision, accuracy, dimensional stability, all are paramount.