Steel microstructure is the fine structure of its constituents that is made visible by magnification over 25 times. It is a function of the carbon content and the transformations that occur during the process of forming the steel. In addition, it has a significant effect on the mechanical properties of steel, so it is of importance in engineering applications. In this article, we will review the types of steel microstructure, the microstructure of high carbon steel, mild steel, and stainless steel, as well as heat treatment and finishing effects.
Types of Steel Microstructure
The microstructure of pure iron, which is the basic constituent of steel, is like a 3-D lattice of billiard balls with small gaps or interstices between. Depending on the heat treatment process, small elements such as carbon and nitrogen can fit into these interstices. Larger elements including silicon, phosphorus, and magnesium can substitute the iron atoms in the lattice. This is the basis for the formation of steel, where the microstructure is altered to make up different constituents that meet the needs of an application.
When carbon atoms are only in a small fraction of interstices of the iron lattice, the steel has a ferrite microstructure. This type is a body-centered cubic that is soft and ductile, like pure iron. Ferrite microstructural phase limits the amount of carbon in the interstices to 0.02% at 1,340 °F (725 °C), while at room temperature it drops to 0.006%.
This microstructural phase has larger interstitial gaps that can contain up to 2% carbon at 2,100 °F (1,150 °C). Unlike ferrite, this has a face-centered cubic and the increase in carbon content results in higher strength levels.
Cooling austenite at a fast rate (above 86 °F) limits the amount of carbon atoms that diffuse from the crystal structure, resulting in martensite formation. As a result, there are distortions and strains in the iron lattice leading to a body-centered tetragonal structure. This phase is associated with high level of hardness but usually undergoes further heat treatment to reduce distortions and improve toughness.
Microstructure of High Carbon Steel
Generally, carbon steel grades depend on the weight percentage of carbon content present in its structure. For high carbon steel, this value is between 0.6 and 1.25%, while its manganese content is approximately 0.3 to 0.9%. Although this makes it a very hard, wear-resistant steel, they have low levels of ductility. As a result, they are ideal for making edged tools, chains, gear wheels, springs, and high-strength wires.
Microstructure of Mild Steel
Mild steel has a low carbon content ranging from 0.05 to 0.30%, which makes it malleable, ductile, and cheap to produce. Because of these features, it is predominant in several applications in industry. Although it inherently is of low strength, it can form alloys with other elements such as chromium, nickel, and manganese to produce high-tensile steel. This type of steel serves in making cans, pipes, and construction components.
Stainless Steel Microstructure
The key attribute of the stainless-steel microstructure is the presence of a minimum of 10.5% chromium. This results in corrosion-resistant steel (CRES) because the chromium combines with oxygen to form a thin, stable passivation layer. Moreover, if there is a scratch on this layer, a self-healing process quickly forms another protective layer for the material. This feature makes stainless-steel preferable over plated metals in corrosion resistance applications, but it is more expensive.
There are different grades of stainless-steel where the microstructure is altered to suit mechanical and corrosion resistance requirements. These grades include ferritic (10.5-18% chromium and 0.08-0.15% carbon), martensitic (12-18% chromium and 0.10-1.2% carbon), austenitic (16% chromium and 8+% nickel), and duplex (19-32% chromium and 5% molybdenum).
Heat Treatment and Finishing Effects
Heat treatment refers to heating or cooling steel using predetermined methods to achieve desirable changes to its mechanical properties. Moreover, some of the properties that significantly change are ductility, yield strength, and hardness. Others, such as thermal and electrical conductivity alter slightly. There are several heat treatment methods, but this section reviews the major types.
Annealing of steel involves heating it slowly to a set temperature depending on its carbon content. Then it is cooled gradually by burying it in an insulation material or simply leaving it to cool down inside the furnace after switching it off. Moreover, the amount of cooling time relates to the steel type and mass. Due to this treatment, there is stress relief, an increase in ductility, and improvement in the grain structure of the steel microstructure. Also, it enhances the steel’s suitability for cold working and machining. There are various types of annealing such as process, isothermal, and full annealing.
After undergoing casting, forging, machining, or heat treatment, normalizing serves to remove any internal stresses from the steel. During the normalizing process, the steel is first heated until it completely transforms to austenite. Followed by air-cooling at about 100 °F (38 °C) per minute. This gives the steel a fine pearlite microstructure, more uniform than what comes from annealing steel. As a result, the steel has higher strength, hardness, and toughness. Thus, it is ideal for components that support significant loads, and those that require impact strength.
As the name suggests, the aim of this method is to increase the hardness of the steel. This is achieved by heating it to a specified temperature, then cooling it rapidly by submerging it in water, oil, or brine. Although the hardness and the strength of the steel increases, its brittleness increases as well. The hardening process is more effective in alloy steel, rather than carbon steel, and is good for applications where wear resistance is important.
In some situations, the hardening process makes the steel too brittle, so it requires softening. Tempering involves heating quenched steel to a temperature less than its eutectoid temperature, and subsequently cooling it in still air. As a result, this relieves some internal stress in the steel microstructure, making it more ductile.