Short Answer:
Tempering is a heat treatment process applied to martensitic steel to reduce its brittleness and improve toughness after it has been hardened. Martensitic steel becomes very hard but also very brittle after quenching. Tempering helps to make the steel strong yet more ductile and safe to use in practical applications.
The main purpose of tempering is to adjust the hardness, relieve internal stresses, and improve mechanical properties like toughness, strength, and impact resistance. It allows martensitic steel to retain most of its hardness while becoming more useful and reliable in tools, springs, and structural parts.
Detailed Explanation:
Purpose of Tempering Martensitic Steel
Martensitic steel is known for its high hardness and strength, which is achieved through a process called quenching—rapid cooling after heating. However, this rapid cooling also creates high internal stresses and makes the steel extremely brittle, meaning it can crack or break easily under shock or impact. To overcome this problem, the steel is tempered.
Tempering is a controlled reheating process that adjusts the steel’s microstructure to improve its toughness, ductility, and overall performance without losing much hardness. It is a critical step in the heat treatment of martensitic steel.
Why Tempering Is Needed After Hardening
After quenching, martensitic steel contains:
- Very high hardness due to trapped carbon in the iron structure.
- High internal stresses due to rapid cooling.
- Low toughness and high brittleness, which can lead to sudden failure.
Without tempering, such steel is unsuitable for real-world applications like tools or machine parts that face repeated loading, vibration, or shock.
Main Purposes of Tempering Martensitic Steel
- Reduce Brittleness
- Tempering allows some carbon atoms to move and form more stable structures.
- This reduces the brittleness and makes the steel less likely to crack or break.
- Improve Toughness and Ductility
- The steel becomes more capable of bending or absorbing shock without failing.
- This is essential for tools, springs, and blades that must resist sudden impact.
- Relieve Internal Stresses
- Quenching introduces stress within the metal, which can cause warping or cracking over time.
- Tempering releases these stresses and stabilizes the steel structure.
- Adjust Hardness
- Tempering slightly reduces the extreme hardness gained from quenching.
- However, the final steel still remains hard enough for cutting or bearing loads, while being much safer and stronger.
- Improve Dimensional Stability
- Tempering prevents the part from changing shape or size during use, which is important in precision components.
How Tempering Is Done
- After Quenching, the hardened martensitic steel is reheated to a specific temperature, usually between 150°C to 650°C, depending on the desired properties.
- The steel is held at that temperature for a fixed time (often 1–2 hours).
- Then it is allowed to cool slowly, usually in air.
- Low tempering temperature: Keeps high hardness but improves toughness slightly (used in tools like drills, knives).
- High tempering temperature: More toughness and ductility, less hardness (used in springs, gears).
Applications Requiring Tempered Martensitic Steel
- Cutting tools: chisels, drills, blades
- Machine parts: gears, axles, shafts
- Springs and shock-absorbing parts
- Molds and dies used in shaping metal
- Automobile components needing both strength and durability
All these applications need steel that is strong and wear-resistant, but also safe and reliable under load or impact—qualities that tempering provides.
Conclusion
The purpose of tempering martensitic steel is to make it tough, durable, and safer to use while maintaining its high strength. Tempering reduces brittleness, removes internal stress, and adjusts the hardness to a useful level. This process is essential after hardening because it transforms the steel from a very hard but fragile state into a balanced material that can perform under stress without failure. Without tempering, martensitic steel would not be suitable for most engineering applications.