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The metal manufacturing world uses several processes to convert raw materials into usable products. One such crucial process is tempering. Not sure what it is? Do not worry, as Sree Metaliks is here to explain it to you in detail. This comprehensive post by our team will introduce you to what is tempering, why it is done, what its advantages are and how it is done.
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What is Tempering?
Tempering is a heat treatment for steel. It is also known as drawing. Tempering mainly involves heating the components and holding them to a predefined temperature for a certain duration. The temperature set in this process is below the critical point. Following this heating process, the components are then cooled in still air to bring down the temperature to room temperature.
The tempering process aims to alter the mechanical properties of the metal just like other processes of heat treatment for steel do. This heating treatment changes the mechanical properties of a metal from its surface to the core. However, manufacturers also perform partial tempering if required. Tempering is usually used for products that need to have a certain level of flexibility for some specific applications.Manufacturers also use tempering to reduce the hardness of a metal that has recently been welded. The welding process uses high localised temperature that results in making a product hard. It is where tempering is used to reduce the hardness. Although theoretically, tempering can be used for a wide range of metals, it is commonly used for carbon steel.
When is Tempering Used?
After understanding the definition of tempering, you must be wondering- What is the purpose of tempering a steel? Usually, tempering is performed after hardening processes to reduce the hardness of a metal. Hardening processes involve heating the metal components above the critical temperature. The heating process is followed by a quenching operation that involves rapidly cooling the heated metal by immersing it in hot water, oil or forced air. The hardening process makes the material brittle. Although metal products must be hard, the brittle nature that accompanies this hardness makes them unsuitable for certain applications.
The purpose of heat-treating steel or other metals is to reduce this increased brittleness. Tempering helps restore their ductility by reheating them to a lower temperature than the critical point. The resulting product has a perfect balance between ductility and hardness. The cooling rate during tempering is also lower than quenching. The top manufacturers like Sree Metaliks carry out tempering right after quench-hardening for the best results. We also avoid any errors during this process to prevent warping or distorting of the material. Tempering is also done after other manufacturing processes that harden the material, like the welding process.
Understanding the Tempering Process
After understanding the purpose of heat-treating steel, let's delve into the process of tempering. The tempering procedure is divided into three stages as follows -
1. Heating
2. Dwelling
3. Cooling
1. Heating
The first stage of the tempering procedure involves heating the metal. This metal component is heated to a set temperature. The predefined temperature is known as the tempering temperature, and it is between the room temperature and the lower critical temperature. Now, this stage does not involve heating the metal at any rate. If it is heated too quickly, the metal can get damaged due to cracking. Hence, a controlled rate is opted for heating the metal.
The temperature and heating rate depend significantly on the type of steel and the properties desired. For example, spring steel is heated at 300- 400°C, tool steels at 200- 300°C and structural steel at 450- 650°C. The metal components at this stage are heated in a gas, induction or electric furnace. Also, it is done in the presence of a vacuum or inert gas to avoid oxidation. However, certain steels are heated in the presence of air or salt baths.
2. Dwelling
The next stage in the tempering process is dwelling. Dwelling refers to holding the metal for a predefined duration at the set temperature below its critical point. The duration for which the metal is held depends on the steel type, charge size, component cross sections and the desired mechanical properties. This is because the tempering temperature and dwelling time play a crucial role in altering the mechanical properties of the resulting metal. The tensile strength of a metal decreases with the increasing tempering temperature. On the other hand with increasing temperature and dwell time, the ductility, toughness and impact strength of the metal increases.
3. Cooling
The final stage in the tempering procedure is the cooling stage. It might sound like an easy and unimportant process. But you must understand that this stage is as crucial as the remaining two. The heated and dwelled component is cooled in the presence of still air. The rate of cooling is crucial to maintain here, and it is predetermined based on varying factors.
Advantages of Tempering
Tempering offers several advantages, as follows:
- Reduces brittleness
- Increases flexibility and ductility
- Enhances wear resistance properties of metal's surface and core
- Make the metal long-lasting and durable
- Helps adjust excess hardness to acceptable levels
- Increases toughness
- Enhances formability and machinability for upcoming manufacturing processes
- Quicker than other heating treatments like the annealing process
- Tempered steels are harder and stronger than annealed steels.
- Relieves internal stresses that might cause damage by hydrogen cracking
- Enhances mental strength by improving microstructure
Types of Tempering
Mainly tempering is categorised into the following three types -
Low-Temperature Tempering
The tempering temperature in low-temperature tempering is between 160? and 300?. The hardness requirement for this tempering type is around 60 HRC. The duration for low-temperature tempering is longer than that for high-temperature one as it aims to achieve a homogenous temperature throughout the part. It reduces the internal stresses in the steel. Usually, low-temperature tempering is used for cutting tools that need to maintain hardness and wear resistance.
High-Temperature Tempering
Another type of tempering is high-temperature tempering. In this type, the tempering temperature is more than 500?. It is mainly used for carbon construction steels, high-speed steels and hot work tool steels. The high-temperature tempering is used to increase the toughness of the metal. The hardness of the resulting product in this tempering type will be between 300 HB and 65 HRC.
Spring Steels Tempering
Spring, die or coil steels use tempering temperatures between 300? and 500?. The hardness of the resultant product is around 45 HRC. It is used for steels with a high yield strength and firmness. The resulting products are suitable to withstand torsional stresses and strong bending.
Read more: Stainless Steel vs Carbon Steel: Understanding the Fundamental Differences!
Final Thoughts
Tempering is an indispensable procedure in the metal manufacturing industry. From altering the mechanical properties of the metal to ensuring it is ready for use, this process is quite significant in the manufacturing industry. Sree Metaliks understands the importance of this process. As a result, we ensure the accuracy of each stage in the tempering process. We also comprehend the purpose of heat-treating steel and choose the most suitable type of tempering to ensure the end product has the desired properties.