Short Answer:

Tempering is a heat treatment process done after quenching to reduce the brittleness of hardened steel and improve its toughness. In tempering, the quenched steel is reheated to a lower temperature and then cooled slowly. This helps balance the hardness with flexibility, making the steel safer and more useful in practical applications.

Tempering is necessary after quenching because quenching makes steel very hard but also very brittle. If used in this state, the steel may crack or break under shock or stress. Tempering relieves internal stresses and improves ductility, allowing the steel to perform well under working conditions without failure.

Detailed Explanation:

Tempering process

Tempering is an essential heat treatment technique in mechanical engineering, especially when working with steel that has been hardened by quenching. While quenching increases hardness and strength, it also introduces brittleness and internal stress. Tempering helps to correct these problems without losing too much of the desired hardness.

The process of tempering includes the following steps:

1. Reheating:
   The quenched steel is heated again, but this time to a lower temperature—usually between 150°C and 650°C. The exact temperature depends on the required balance between hardness and toughness.
2. Soaking:
   The steel is held at this temperature for a specific time to allow internal changes in its structure. This time depends on the size and type of steel.
3. Cooling:
   After soaking, the steel is cooled in air or oil at a controlled rate. This step is slower and softer than quenching.

This process allows the transformation of some martensite (hard but brittle structure formed after quenching) into a structure that is slightly softer but much more stable and tough. This change greatly improves the performance of steel parts in real applications.

Why tempering is necessary after quenching

1. Reduces brittleness:
   Quenched steel is extremely hard, but also very brittle. It can break easily under sudden load or impact. Tempering helps to reduce this brittleness and makes the steel more reliable.
2. Relieves internal stresses:
   Rapid cooling during quenching creates uneven contraction and builds up internal stress. These stresses can cause cracks or failure during use. Tempering helps to remove or reduce these internal tensions.
3. Improves toughness:
   Toughness is the ability of steel to absorb energy without breaking. After tempering, steel becomes more resistant to cracks and can withstand shocks better.
4. Maintains controlled hardness:
   Tempering allows engineers to adjust the level of hardness as per need. Lower tempering temperatures maintain more hardness, while higher temperatures reduce hardness but increase ductility and toughness.
5. Increases stability of the structure:
   The steel becomes more stable and less likely to change shape or size over time due to internal reactions. This is important for precision parts and tools.
6. Prepares steel for working conditions:
   Steel parts used in machines, tools, or construction must face real working conditions like load, vibration, and temperature changes. Tempering ensures the steel can handle these without failure.
7. Improves wear resistance and service life:
   Balanced hardness and toughness make the steel long-lasting. This reduces maintenance costs and improves safety in equipment.

Applications of tempered steel

• Cutting tools: After quenching, tools like drills, blades, and chisels are tempered to get sharpness with toughness.
• Machine parts: Gears, shafts, and bearings are tempered to ensure they don’t crack under load.
• Automobile parts: Engine components and suspension parts need to be both strong and flexible.
• Springs and fasteners: These parts require a balance between strength and flexibility, achieved through tempering.
• Structural parts: Steel used in bridges, buildings, and heavy equipment is tempered for safe performance.

Tempering allows steel to be custom-designed for different jobs by carefully selecting the tempering temperature and time.

Conclusion

Tempering is a necessary heat treatment process that follows quenching to improve the toughness and reduce brittleness in steel. It involves reheating the quenched steel to a lower temperature and cooling it slowly, which relieves internal stresses and adjusts hardness. Without tempering, quenched steel would be too brittle and unsafe for most uses. Tempering ensures that steel components are strong, flexible, and durable, making them suitable for real-world mechanical and industrial applications.