Steel Microstructure: Definition, Determination, and Its Properties

Steel microstructure refers to the arrangement and characteristics of the tiny atomic and crystalline features within steel. It is crucial because it determines steel's mechanical properties, such as: strength, hardness, and ductility. Understanding steel microstructure is necessary for designing and producing materials with specific performance attributes, ensuring the reliability and safety of countless applications, from buildings and bridges to machinery and vehicles.

This article will discuss the definition, properties, and factors involved in determining the microstructure and phases of steel. 

What Is Steel Microstructure?

Steel microstructure refers to the arrangement and distribution of the different phases or constituents within a sample of steel at the microscopic level. These phases are typically composed of iron and other alloying elements. They can vary in structure and properties based on factors such as composition, heat treatment, and processing methods. The microstructure of steel stores the material’s genesis and it determines its chemical and physical properties. 

Are Steel Microstructure and Cast Steel the Same?

No, steel microstructure and cast steel are not the same. Steel microstructure refers to how the tiny parts of steel are arranged at a microscopic level. It's like looking at the small building blocks of steel. This arrangement affects steel’s strength, hardness, and flexibility. By changing the microstructure through processes like heating and cooling, we can make steel suitable for different applications.

Cast steel, on the other hand, is a type of steel made by casting. The process involves melting steel and pouring it into molds to create shapes. These shapes can have different microstructures depending on how they are made. Cast steel is used to create various things, from car parts to machinery.

What Is the Importance of Steel Microstructure?

The importance of the microstructure of steel lies in that it is the main determining factor of the mechanical properties of steel, including properties like: strength, hardness, ductility, and toughness. This means that the material’s performance is highly reliant on the particle distribution, as well as the shape and sizes of the phases within the microstructure. The microstructure of steel can have different appearances, which are influenced by parameters like rolling setup, alloying elements, cooling rates, heat treatments, and post-treatments of the steel. 

Engineers can tailor steel for specific applications by understanding and controlling its microstructure. Heat treatment processes are employed to modify the microstructure, enhancing properties like hardness and toughness. Additionally, microstructure affects weldability, corrosion resistance, formability, and durability of steel components. It plays an important role in quality control, ensuring that steel products meet performance standards. 

What Determines Steel Microstructure?

The steel microstructure is influenced by factors like: the alloying elements, the cooling rate, mechanical processes like rolling, forging, and extrusion, heat treatments applied, and the different post-processing treatments of the steel. Alloying elements, such as: carbon, manganese, and chromium, significantly influence microstructure by dictating the types and proportions of phases that form within the steel. The cooling rate during fabrication and heat treatment also plays an important role; rapid cooling can transform austenite into the hard martensite phase, while slower cooling permits the development of phases like pearlite and bainite. Mechanical processes like: rolling, forging, and extrusion exert mechanical deformation on the steel, affecting grain size and uniformity, which, in turn, impact strength and ductility. Heat treatments, like annealing and tempering, offer precise control over the microstructure to enhance desired properties. Subsequent post-processing treatments, including cold working and precipitation hardening, further refine the microstructure.

What Are the Properties of Steel Microstructure?

Steel microstructure is the arrangement of atoms within steel, and it's a critical factor that determines the material's properties and behavior. This microstructure can be visualized as a lattice of iron atoms, much like stacked billiard balls. When carbon is added to iron, it forms various phases within the microstructure, each with distinct properties. For example, ferrite, which has a body-centered cubic (BCC) crystal structure, is soft and ductile. Austenite, with a face-centered cubic (FCC) structure, can hold more carbon and is stable at high temperatures. However, when steel cools, austenite can transform into harder phases like martensite, which has a body-centered tetragonal (BCT) structure. Heat treatments can further modify the microstructure, impacting steel's hardness and toughness. 

What Is the ASTM Standard for Steel Microstructure?

There isn't a specific ASTM (American Society for Testing and Materials) standard that defines steel microstructure in detail. Instead, ASTM provides various standards related to the testing and characterization of steel microstructures. These standards cover techniques and procedures for examining and analyzing steel microstructures. Some relevant ASTM standards include:

1. ASTM E3: Covers the preparation of metallographic specimens, including steel, for microscopic examination. It outlines techniques for sectioning, mounting, grinding, polishing, and etching of specimens to reveal their microstructures.
2. ASTM E112-13: Defines methods for determining the average grain size in metals like steel using various microscopic techniques.
3. ASTM A923: Provides procedures for examining the microstructure of steel and alloy samples to detect the presence of harmful intermetallic phases, such as the sigma phase, which can affect material properties.
4. ASTM A1084-15a: Defines a systematic method for detecting detrimental phases in lean duplex stainless steel. It involves a metallographic procedure to visualize these phases, assess their quantity, and correlate the microstructure results with toughness and corrosion tests. This standard is essential for ensuring the reliability and safety of structures and components made from lean duplex stainless steel.
5. ASTM A892-09: Outlines the process for defining and rating the microstructure of carbide structures found in annealed high-carbon bearing steels. It includes specifications for the optical metallography equipment and specimen preparation, which encompasses procedures like polishing and etching. This standard is crucial for ensuring the quality and performance of high-carbon-bearing steels used in various applications. 

What Does the Steel Phase Diagram Look Like?

The iron-carbon phase diagram depicts the relationship between carbon concentration (X-axis) and temperature (Y-axis) and reveals critical information about the microstructure of these alloys. Each region or phase field within the phase diagram signifies the phases present for a specific alloy composition and temperature as seen in Figure 1 below:

In this diagram, carbon is considered an interstitial impurity within the iron lattice, which can form either a face-centered cubic (FCC) or body-centered cubic (BCC) lattice, leading to different phases. The phases include α-ferrite, γ-austenite, δ-ferrite, and Fe3C (cementite).

What Are the Different Phase Structures of Steel Microstructure?

The behavior of steel, from its soft and ductile form to its hardest phases, is influenced by the interplay between carbon content, temperature, and cooling rates. Listed below are the different phase structures of steel microstructure:

1. Ferrite

When a minute fraction of interstices houses carbon atoms, the resulting microstructure is ferrite. Ferrite boasts a body-centered cubic (BCC) crystal structure, making it soft, ductile, and akin to pure iron. The maximum carbon content that can fit in ferrite is about 0.02% at high temperatures but drops to 0.006% at room temperature.

2. Martensite

Rapid cooling, typically faster than 30 °C per second, locks excess carbon within the crystal structure, distorting it into a body-centered tetragonal (BCT) structure, forming martensite. Higher carbon levels result in harder martensite. When heated, martensite allows carbon to diffuse out, reducing hardness and increasing toughness.

Heating martensite leads to tempered martensite, which combines ferrite and iron carbide (Fe3C). This microstructure disperses Fe3C more evenly than pearlite, reducing compression stresses.

3. Austenite

Austenite has larger gaps than ferrite, with a face-centered cubic (FCC) crystal structure. At temperatures around 1,150 °C, it can accommodate up to 2% carbon. However, it's unstable at temperatures below the eutectic point (727 °C) unless rapidly cooled. Austenite is non-magnetic.

Some austenite doesn't transform into martensite during quenching, creating retained austenite. Its presence depends on factors like carbon content and alloying. For example, austenitic stainless steels like 304 and 316 maintain their austenitic structure at room temperature.

4. Cementite

Cementite, also known as iron carbide (Fe3C), is a hard and brittle compound that forms in steel when excess carbon is present. It has a fixed composition and is often used to strengthen steel. Cementite typically appears as small, hard particles in the microstructure.

5. Bainite

Formed at moderate cooling rates, bainite contains ferrite, cementite, and retained austenite. The rounded hard components in bainite make it more resistant to crack initiation and propagation than flat, elongated structures like pearlite. This balance of strength and ductility makes bainitic steels suitable for applications like automotive wheels and suspension arms.

6. Pearlite

As steel cools, austenite transforms into pearlite, a combination of ferrite and cementite (Fe3C). The amount of cementite depends on the carbon content. Unlike ferrite, cementite is hard, brittle, and less resistant to cracks. It forms a lamellar structure, resembling a mother of pearl (nacre) under a microscope.

What Is the Strongest Steel Microstructure?

Martensite is the strongest and hardest steel microstructure, but it is also the most brittle. It derives its strength from its unique crystal structure and the way it forms. Martensite has a body-centered tetragonal (BCT) crystal structure, which results from the rapid cooling or quenching of austenite, another steel microstructure. During this rapid cooling, carbon atoms are trapped in the crystal structure, creating internal stresses that make martensite exceptionally hard and brittle.

What Is the Weakest Steel Microstructure?

Ferrite is considered the weakest steel microstructure because it is relatively soft and ductile compared to other steel microstructures like martensite. Ferrite has a body-centered cubic (BCC) crystal structure, which gives it lower hardness and strength compared to microstructures like martensite, which have a more complex crystal structure and higher hardness. This lower hardness and strength make ferrite less suitable for applications in which high strength and hardness are required. However, ferrite's softness and ductility can be advantageous in applications in which toughness and formability are more important than hardness and strength.

How Do Atomic Mechanisms of Transformation Affect Steel Microstructure?

The atomic mechanisms of solid-state phase transformations in steel are critical in determining the resulting microstructure, morphology, and chemical composition. These mechanisms define how atoms move and rearrange during phase transformations, ultimately shaping the material's properties. Two fundamental atomic mechanisms of transformation are displacive and reconstructive mechanisms. In the displacive mechanism, the crystal structure changes without disrupting the relative order of atoms. This deformation leads to a change in the overall shape of the material, resulting in significant strain energy. 

Conversely, the reconstructive mechanism involves breaking the bonds in the parent phase (austenite) and rearranging atoms into the structure of the new phase (e.g., ferrite) while maintaining the overall shape. This mechanism requires atoms to diffuse over distances comparable to the size of the transformation product. Figure 2 illustrates how major transformation products in steel can be classified based on these atomic mechanisms: 

Displacive transformations typically result in plate-shaped products, while reconstructive transformations are characterized by changes in chemical composition and a range of product shapes.

What Are the Heat Treatments for Steel Microstructure?

The heat treatments for steel microstructure are:

1. Annealing

With annealing, the steel is heated to a specific temperature and held there for a certain period, followed by slow cooling. This method alleviates internal stresses, enhances the grain structure, and imparts increased softness and ductility to the steel. It is frequently used to improve machinability or prepare the material for further processing.

2. Tempering

Tempering is performed after quenching (hardening). It involves reheating the hardened steel to a temperature below its critical point and then cooling it. Tempering reduces the brittleness and hardness of the steel, while simultaneously improving toughness and reducing internal stresses. The precise temperature and duration for tempering can be customized to attain the preferred balance between toughness and hardness.

3. Hardening

Hardening, also known as quenching, is the process of heating the steel to a high temperature (above its critical point) and then rapidly cooling it, often in a quenching medium such as oil or water. This rapid cooling "freezes" the steel's microstructure in a hardened state, resulting in increased hardness and strength. However, it also makes the steel more brittle, so tempering is often performed afterward to restore some toughness.

4. Normalizing

Normalizing is a heat treatment used to refine the grain structure of the steel and improve its mechanical properties. It involves heating the steel to a temperature slightly above its critical point and then allowing it to cool in still air. Normalizing eliminates internal stresses, improves machinability, and enhances the steel's uniformity and strength.

What Is the Advantage of Steel Microstructure?

Steel microstructures offer a wide range of advantages including: exceptional strength, durability, and versatility. Steel can be tailored to meet specific requirements through precise control of its microstructure. This results in steel grades that can be made suitable for everything from structural applications to cutting tools. Steel’s excellent heat resistance and fire resistance make it ideal for high-temperature environments. Additionally, steel is readily available and cost-effective, making it a preferred choice for many engineering and construction projects.

What Is the Disadvantage of Steel Microstructure?

Some steel microstructures have certain disadvantages that need to be considered. One significant drawback is susceptibility to corrosion in certain environments, which can compromise steel’s structural integrity over time. Some steel microstructures, especially those with high carbon content like martensite, can be brittle, limiting their use in applications requiring toughness. In general, the weight of steel can be a disadvantage in weight-sensitive industries like aerospace. Additionally, the production of steel can have a significant environmental impact due to energy consumption and greenhouse gas emissions. Complex heat treatment processes may be required to achieve desired properties, adding to production costs. Despite these disadvantages, the adaptability and extensive range of steel microstructures continue to make it a material of choice across numerous applications.

What Is the Difference Between Cast Iron and Steel Microstructure?

The main difference between the microstructures of cast iron and cast steel lies in their carbon content. Cast iron typically contains more than 2% carbon, while cast steel often contains between 0.1–0.5% carbon. This carbon content significantly affects their properties and characteristics.

Cast iron is known for its relatively high carbon content, which makes it easier to cast due to its better flowability and lower shrinkage during cooling. It has excellent vibration-damping properties, making it ideal for applications like engine blocks, in which reducing vibration is crucial. It also exhibits good compressive strength, making it suitable for parts that need to withstand forces that reduce size. However, cast iron lacks impact resistance and can be brittle, limiting its range of applications.

On the other hand, cast steel contains lower carbon content, which makes it more challenging to cast compared to cast iron. Steel has excellent impact resistance, allowing it to withstand sudden impacts without deforming or breaking. This makes steel ideal for applications in which toughness is essential. While steel may be more challenging to machine, it offers a good balance of strength and ductility, making it widely used in various mechanical and structural applications.

Summary

This article presented steel microstructure, explained it, and discussed its various properties. To learn more about steel microstructure, contact a Xometry representative.

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