What is tempering of steel?

Tempering can be done to both hot rolled and cold rolled steel. Hot rolled steel is roll-pressed at very high temperatures, while cold rolled steel is processed at or near room temperature and then annealed or temper rolled. Tool steels are tempered at around 200-300 degrees celsius, spring steels at 300-400 degrees celsius and structural steels at 450-650 degrees celsius. In this blog, we delve into the tempering of steel, uncovering its meaning, process, and the transformative effects it has on this metal.

Understanding tempering of steel

Tempering steel is a heat treatment process that plays a crucial role in adjusting the properties of hardened steel. This process follows the initial step of hardening, during which the steel is heated to a high temperature and then rapidly cooled to achieve maximum hardness. However, steel in its fully hardened state can be brittle and prone to cracking, making it unsuitable for many applications where toughness, resilience, and flexibility are desired.

This is where tempering comes into play. Tempering involves reheating the hardened steel to a specific temperature range, typically below its critical point, and then allowing it to cool gradually.

This controlled cooling process results in a change in the steel's microstructure. The microstructure of martensite and the morphology of carbides were found to evolve over time during tempering. The type, size, and amount of carbides formed during tempering are important for the properties of the steel

The science behind tempering

The science behind tempering lies in the transformation of the steel's crystal structure. Initially, during hardening, the steel undergoes a phase transformation known as martensitic transformation, where the carbon atoms are trapped in a supersaturated solution. Martensitic transformation is a diffusionless transformation that results in a change in crystal structure from the parent austenite phase to a distorted and supersaturated martensite phase. The resulting martensite phase is known for its high strength and hardness, but also known for its brittleness.

Tempering allows some of these carbon atoms to diffuse and rearrange within the crystal lattice, reducing the internal stresses that lead to brittleness. This diffusion of carbon atoms leads to the formation of small, evenly distributed carbide particles within the steel matrix. These particles act as obstacles to the movement of dislocations, effectively strengthening the material while also increasing its toughness and resilience.

How is tempering different from annealing, hardening and quenching?

Annealing, hardening, tempering, and quenching are distinct heat treatment processes used to modify the properties of steel.

Tempering: Tempering involves controlled heating after the steel has been quenched and hardened. The purpose of tempering is to reduce the brittleness that can occur in steel after rapid quenching. It is done at a range between 150 degrees celsius and 600 degrees celsius.

Annealing: Annealing involves heating steel to a specific temperature and then allowing it to cool slowly in a controlled manner. This process is used to soften the metal and involves heating the steel to a temperature typically between 700 degrees celsius and 900 degrees celsius.

Hardening: Hardening is the process of heating steel above its critical temperature (the highest temperature at which the substance can exist as a liquid) and then rapidly cooling it, usually through quenching in water, oil, or other cooling mediums. This rapid cooling locks the steel's crystal structure in a hardened state and is done between 750 degrees celsius to 950 degrees celsius.

Quenching: Quenching is a process that involves rapidly cooling metal after it has been heated to a temperature greater than normal conditions. This rapid cooling changes the internal structure of the metal, leading to altered mechanical properties.

In summary, annealing softens the steel, hardening makes it harder and stronger, tempering balances hardness and toughness, and quenching adjusts the steel's mechanical properties by rapid cooling.

Effects of tempering

Increased toughness: By relieving internal stresses and promoting the formation of carbide particles, tempering enhances the steel's ability to absorb energy without fracturing. Carbides can help make the steel even stronger through a process called "precipitation hardening”. This process happens when tiny pieces of the carbides spread out inside the steel. These little carbide pieces act like reinforcements, making the steel tougher. This makes tempered steel ideal for applications that require resistance to impact and vibrations.

Reduced brittleness: The martensite structure, which is formed during the quenching process of steel, is hard and brittle. Tempering mitigates the extreme brittleness that can result from the hardening process. This property is particularly important in applications involving sudden load changes or variable stresses.

Retention of hardness: While tempering reduces the overall hardness of the steel, it ensures a balance between hardness and toughness. The steel remains sufficiently hard for most applications while gaining improved fracture resistance.

Improved machinability: Fully hardened steel can be challenging to machine due to its extreme hardness. Tempering slightly softens the material, making it more manageable for machining processes.

Enhanced structural stability: tempered steel is less prone to warping or cracking due to temperature changes or stress, making it an ideal choice for components subjected to varying conditions.

Conclusion
By carefully controlling the heating and cooling cycles, engineers and metallurgists achieve a delicate balance between hardness, toughness, and resilience, making tempered steel an indispensable material in a wide range of applications, from construction to manufacturing and beyond. Understanding the science and significance of tempering steel provides a profound appreciation for the complex interplay between heat, microstructure, and mechanical properties that underlie the durability and versatility of this remarkable alloy.