：
MARTENSITIC STRUCTURE OF CARBON STEEL
Introduction
Carbon steel is an alloy of iron and differing levels of carbon. It is used in a wide variety of products and applications due to its nearly unparalleled strength, good ductility, hardness, wear resistance and machinability. Thousands of years have passed since carbon steel has been utilized in manufacturing processes, with its properties being heavily researched and refined throughout that time. One of the most important properties of carbon steel is the structure it forms upon cooling, called the Martensitic structure. This paper seeks to explain the composition, features and processes involved with the formation of this structure in carbon steel.
Composition and Structure
The component which provides the strength in carbon steel is called ferrite, a form of iron with a body-centered cubic lattice structure. This is then combined with trace amounts of carbon and other elements such as manganese, for added strength or hardness. The carbon combines with the ferrite within the steel in order to form a compound called iron carbide, or cementite. This compound dominates the properties of the steel such as itshardness, malleability and wear resistance. Carbon steel possesses two distinct structures due to its composition: ferritic and martensitic.
Martensitic Structure
When heated, iron undergoes a process known as austenitization. In this process, iron is heated to its austenite transformation temperature, causing it to convert from ferrite to austenite. The amount of heat needed to reach this point will differ depending on the amount of carbon present in the steel. Austenite is a form of iron composed of a body-centered cubic lattice structure and is much more malleable than ferrite. Furthermore, the incorporation of carbon molecules into the lattice causes it to be significantly more brittle than ferrite. This intern makes austenite more prone to cracking and deformations.
Nevertheless, it is only through the austenitization process that the martensitic structure is able to be obtained. Martensite is achieved when the austenitic steel is rapidly cooled from the austenite transformation temperature to room temperature, often by quenching, which is a process in which the steel is submerged in a bath of oil or water. This rapid cooling prevents the austenite from converting back to ferrite and instead causes it to transform into the martensitic structure. This structure has a highly organized distorted lattice and is composed mainly of iron and carbon in unequal amounts, along with other elements such as manganese and silicon. This structure is much harder and stronger than both ferrite and austenite and is essentially the highest level of strength obtainable via cold working.
Processes
Since the structure of martensite can allow for extreme levels of strength and toughness, many processes use it as a form of hardening.Hot forging, quenching and tempering are just a few examples of these processes and take advantage of the abilities of martensite.
Hot Forging
Hot forging is the process of quickly combining metals through extreme temperatures and pressure. Carbon steel blanks are heated to the austenite transformation temperature in order to soften them, at which point a die is used to shape them into the desired form. Once the desired shape is achieved, the piece of steel is quickly cooled, causing it to form the martensitic structure. While this process helps to shape the steel, it also increases its strength and Hardness as a result of the formation of martensite.
Quenching
Quenching is a process in which materials are rapidly cooled in order to obtain specific properties such as greater hardness. This is achieved by quickly cooling the austenite steel from its austenite transformation temperature to room temperature, allowing it to form the martensitic structure. In order to rapidly cool the steel, it is submerged in a bath of oil or water.
Tempering
Tempering is the process in which a previously hardened material is heated to a specific temperature in order to reduce its hardness, increase its ductility and improve its overall toughness. After the steel has been heated in a tempering process, it is once again cooled, allowing it to re-form the martensitic structure.
Conclusion
In conclusion, the Martensitic Structure of carbon steel is a highly organized, distorted lattice consisting mainly of iron and carbon in unequal amounts. It is achieved by rapidly cooling austenite steel from the austenite transformation temperature to room temperature and is able to provide extremely strong and tough properties. This is especially advantageous in processes such as hot forging, quenching and tempering which put the steel to extreme temperatures and pressures for long periods of time. Carbon steel is a very unique and versatile material due to its ability to form the martensitic structure and as such, it is used in a wide variety of products and applications.
