The Process of Reheating Martensitic Steel to Reduce Brittleness without Significant Loss in Hardness: Tempering

Introduction
Martensitic steel is a type of steel that has been hardened through a process called quenching, which involves rapid cooling from a high temperature. This results in a steel with high hardness but also brittleness. To reduce the brittleness and improve its toughness while maintaining a certain level of hardness, the martensitic steel is reheated in a process called tempering.

Explanation
Tempering is a heat treatment process that involves reheating the hardened martensitic steel to a temperature below its lower critical point. This temperature is typically between 200-600°C, depending on the desired properties and the specific composition of the steel. The steel is held at this temperature for a specific period of time, typically ranging from a few minutes to a few hours, and then cooled down slowly.

Key Points
The process of reheating the martensitic steel to reduce its brittleness without any significant loss in its hardness is called tempering. Here are some key points to understand this process:

1. Purpose: The primary purpose of tempering is to reduce the brittleness of the martensitic steel while maintaining its hardness. This is achieved by relieving the internal stresses generated during quenching and by allowing the microstructure to undergo controlled changes.

2. Reduction in brittleness: During quenching, the martensitic steel undergoes a rapid transformation from austenite to a hard and brittle martensite phase. This high hardness makes the steel prone to cracking and failure under impact or sudden loads. Tempering helps to reduce this brittleness by allowing the transformation of some of the martensite into a more ductile structure, such as tempered martensite or troostite.

3. Controlled microstructural changes: The reheating during tempering causes the diffusion of carbon atoms within the steel, which leads to the precipitation of fine carbides. These carbides act as obstacles to dislocation movement, making the steel tougher and less prone to brittle fracture. The exact microstructural changes depend on the tempering temperature and time.

4. Hardness retention: While tempering reduces brittleness, it is important to note that it also causes a slight reduction in hardness. However, the aim of tempering is to balance the hardness and toughness of the steel, so the loss in hardness is minimal and acceptable. The specific tempering temperature and time are chosen to achieve the desired balance between hardness and toughness.

5. Applications: Tempering is commonly used in the heat treatment of various steels, including tool steels, high-strength steels, and some types of stainless steels. It is an essential step in achieving the desired mechanical properties for these materials.

Conclusion
In conclusion, the process of reheating martensitic steel to reduce its brittleness without significant loss in hardness is called tempering. This heat treatment process helps to improve the toughness and ductility of the steel while maintaining a certain level of hardness. Tempering is a critical step in achieving the desired mechanical properties for martensitic steel and is widely applied in various industries.