Steel | In detail

Ahmed Samy

15 Mar, 2024

Steel, an alloy primarily composed of iron and carbon, it is a fundamental material crucial to modern society. Its importance lies in its exceptional strength, durability, and versatility, making it an essential component in construction, manufacturing, transportation, and infrastructure. From towering skyscrapers to everyday household items, steel plays a vital role in shaping our world and driving technological progress.

Composition of steel

Steels are iron–carbon alloys that may contain appreciable concentrations of other alloying elements; there are thousands of alloys that have different compositions and/or heat treatments. The mechanical properties are sensitive to the content of carbon, which is normally between 0.02 wt% and 2.1 wt%.

Note that:

Alloy steels may contain up to 50% of alloying elements and still be considered alloy steels.

Phases and microstructure of steel

The phases present in steel vary depending on factors such as temperature, carbon content, and the rate of cooling. Common phases include Austenite (γ-phase), Ferrite (α-phase), Pearlite, Martensite, Bainite, and Cementite (Fe₃C). Other factors, such as alloying elements and heat treatment processes, can also influence the formation of specific phases.

Delta ferrite (δ-phase) may be present in certain stainless steel due to the influence of alloying elements like nickel or molybdenum, but it's not commonly encountered in standard carbon steels.

Austenite (γ-phase):

Austenite is a Face centered Cubic (FCC) phase of iron and carbon that forms at high temperatures. It is stable at temperatures above the critical temperature known as the austenite transformation temperature (Ac3). Austenite can dissolve significant amounts of carbon, resulting in high solubility and solid solution strengthening in steel.

Ferrite (α-phase):

Ferrite is a Body Centered Cubic (BCC) phase of iron and carbon. It forms at lower temperatures during the cooling of steel and is stable at temperatures below the eutectoid temperature. Ferrite has limited solubility for carbon, typically less than 0.022% at room temperature, making it a relatively soft and ductile phase.

Pearlite:

Pearlite is a lamellar structure composed of alternating layers of ferrite and cementite (Fe₃C), formed during the eutectoid reaction. It is a common microstructure in steels with a eutectoid composition (0.8 wt% carbon). Pearlite provides a balance of strength and ductility, making it desirable for many engineering applications.

Martensite:

Martensite is a metastable phase formed by rapid quenching of austenite. It has a highly distorted Body Centered Tetragonal (BCT) crystal structure, resulting in high hardness and strength. Martensite is characterized by its needle-like or lath-like morphology and is responsible for the hardening of steel during processes such as quenching and tempering.

Bainite:

Bainite is a microstructure that forms when austenite is cooled at intermediate rates between those for pearlite and martensite formation. It consists of ferrite and cementite phases, providing a combination of strength and toughness. Bainite can offer improved mechanical properties compared to pearlite, especially in terms of fatigue resistance and impact toughness.

Cementite (Fe₃C):

Cementite is a compound of iron and carbon with a fixed composition of 6.67% carbon. It is a hard and brittle phase that can be found in steels as a constituent of pearlite or as isolated particles. Cementite contributes to the overall strength and hardness of steel but can also reduce ductility and toughness when present in excessive amounts.

Properties of steel

Properties of steels vary widely depending on the microstructure and the alloying elements, but most steels are famous for high strength, high toughness, and machinability.

Strength:

Steel is renowned for its high strength-to-weight ratio, making it an ideal material for structural applications. Its strength can be tailored through heat treatment and alloying elements. The tensile strength of steel can range from 300 MPa for mild steel to over 2000 MPa for high-strength alloy steels. Yield strength typically falls within the range of 250 MPa to 1400 MPa.

Hardness:

Steel hardness varies widely depending on the carbon content and heat treatment. The Rockwell hardness (HRC) of steel can range from 20 HRC for soft, low-carbon steels to over 60 HRC for hardened tool steels.

Ductility:

The percentage elongation at fracture, a measure of ductility, can range from 10% to 30% for mild steels and low-alloy steels. High-strength steels may exhibit lower ductility, typically around 5% to 15%.

Toughness:

Steels often exhibit excellent toughness, which is the ability to absorb energy and deform plastically before fracturing. This property makes them resistant to impact and shock loading, making steels ideal for applications such as automotive components and structural members subjected to dynamic loads.

Fatigue Resistance:

Fatigue resistance refers to a material's ability to withstand cyclic loading over time without failure. Steel's fatigue strength typically ranges from 50 MPa to 1000 MPa, depending on factors such as alloying elements, microstructure, and surface conditions. Fatigue properties are often characterized by stress-life (S-N) curves, which show the relationship between stress amplitude and the number of cycles to failure.

Creep Resistance:

Creep is the gradual deformation of a material under constant stress at elevated temperatures. Steels designed for high-temperature applications, such as turbine blades and pressure vessels, are engineered to resist creep deformation. Creep-resistant steels typically exhibit low creep rates, with creep rupture strengths ranging from 50 MPa to 1000 MPa at temperatures above 400°C.

Impact Resistance:

Impact resistance measures a material's ability to absorb energy during sudden loading or impact events without fracturing. Steel's impact resistance depends on its toughness, which is influenced by its chemical composition, microstructure, and processing. Toughness can vary widely, with impact energy absorption capacities ranging from 10 Joules to over 1000 Joules for different steel grades and conditions. Charpy or Izod impact tests are commonly used to assess steel's impact resistance.

Machinability:

Steels with lower carbon content and alloying elements are generally more machinable. Machinability is crucial for manufacturing processes such as drilling, milling, and turning.

Weldability:

The ability of steel to be welded without causing defects or weakening its structure is essential for many fabrication processes. Weldability can be influenced by the steel's composition, impurities, and heat treatment.

Corrosion Resistance:

While plain carbon steels are prone to corrosion, alloying elements such as chromium, nickel, and molybdenum can enhance steel's resistance to corrosion. Stainless steels, for example, are highly corrosion-resistant due to the presence of chromium.

Temperature Resistance:

Steels retain their mechanical properties at elevated temperatures, making them suitable for applications exposed to high temperatures, such as engines, boilers, and industrial furnaces.

Magnetic Properties:

Most steels are magnetic, although the degree of magnetism varies depending on their composition and microstructure. This property is utilized in various applications, including magnetic storage media and electromagnetic devices.

Note that:

Steel changes from ferromagnetic to paramagnetic around 770°C.

Effect of alloying elements in steel

Alloying elements play a vital role in determining the properties and performance of steel. Each alloying element contributes unique characteristics to the steel, influencing its mechanical, physical, and chemical properties.

Carbon is the most essential alloying element in steel. It increases hardness, strength, and wear resistance. Higher carbon content leads to increased hardness and tensile strength but reduced ductility and weldability.

Manganese is a deoxidizer and a desulfurizer. It enhances hardenability, strength, and wear resistance. Manganese also improves machinability and hot working properties. Typical manganese content ranges from 0.3% to 1.65%.

Silicon improves strength, hardness, and elasticity. It enhances the fluidity of molten steel during casting. It is also acts as a deoxidizer and helps control grain size. Silicon content is generally kept below 0.5%.

Nickel improves toughness, strength, and corrosion resistance. It enhances hardenability and impact strength, particularly at low temperatures. Nickel is often added in concentrations ranging from 1% to 5%.

Chromium improves hardenability, wear resistance, and corrosion resistance. It forms a protective oxide layer on the surface, enhancing the steel's resistance to oxidation and corrosion. Chromium content typically ranges from 0.5% to 12%.

Molybdenum enhances hardenability and strength at elevated temperatures. It improves the steel's resistance to wear, corrosion, and hydrogen embrittlement. Molybdenum content varies from 0.2% to 1%.

Vanadium refines grain size, improving strength, toughness, and wear resistance. It increases hardenability, allowing for deeper heat treatment. Vanadium content is usually in the range of 0.1% to 0.5%.

Titanium refines grain structure and enhances strength and toughness. It serves as a deoxidizer and can also improve weldability. Titanium content is typically less than 0.2%.

Boron improves hardenability, enabling deeper and more uniform heat treatment. It enhances strength and wear resistance, particularly in low-alloy steels. Boron content is usually less than 0.005%.

Copper improves corrosion resistance, especially in atmospheric environments. It enhances electrical conductivity and thermal conductivity. Copper content is typically limited to less than 0.4% in most alloy steels.

Classification of steel and their uses

Steels are classified based on their chemical composition, microstructure, and properties. The classification system can vary depending on the region or organization, but the most common methods include Plain Carbon Steels and Alloy Steels.

The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) developed a unified numbering system for classifying steels, known as the AISI/SAE steel designation system. Meanwhile, the American Society for Testing and Materials (ASTM) developed its own system for categorizing materials, including steels. These systems help standardize the identification and classification of steel grades, making it easier for engineers, manufacturers, and consumers to understand and communicate about different types of steel.

AISI/SAE Steel Designation System:

The AISI/SAE system uses a four-digit code to designate the composition of plain carbon and alloy steels. The first digit represents the major alloying element or class of steel. The second digit indicates the approximate percentage of the primary alloying element. The last two digits give the carbon content in hundredths of a percent. For example, AISI/SAE 1045 denotes a plain carbon steel with approximately 0.45% carbon content.

1xxx: Carbon steels.
2xxx: Nickel steels.
3xxx: Nickel-chromium steels.
4xxx: Molybdenum steels.
5xxx: Chromium steels.
6xxx: Chromium-vanadium steels.
7xxx: Tungsten steels.
8xxx: Nickel-chromium-molybdenum steels.
9xxx: Silicon-manganese steels.

ASTM Numbering System:

ASTM uses a letter and number combination to classify steels based on their mechanical properties, chemical composition, and intended use. The letter prefix indicates the steel's basic type or intended application:

A: Ferrous metals (e.g., steel).
B: Nonferrous metals.
C: Welding filler materials.
D: Miscellaneous materials.

The following number indicates the specific alloy or elemental composition, while subsequent numbers differentiate between variations within that classification. For instance, ASTM A36 is a common structural steel grade, while ASTM A516 Grade 70 is a pressure vessel quality steel.

Plain Carbon Steel

Plain carbon steels are primarily composed of iron and carbon, with trace amounts of other elements. They are classified based on their carbon content into low, medium, and high carbon steels.

Low Carbon Steel (0.05% - 0.25% carbon):

Low carbon steel, also known as mild steel, low carbon steel is the most common type of plain carbon steel. It offers good formability, weldability, and machinability. Used in applications requiring moderate strength and where bending, forming, and welding are involved, such as automotive components, construction materials, and pipes.

Medium Carbon Steel (0.25% - 0.60% carbon):

Medium carbon steel offers higher strength and hardness compared to low carbon steel. It is commonly used in machinery parts, shafts, axles, gears, and structural components. Balances strength and ductility, making it suitable for applications requiring moderate toughness.

High Carbon Steel (0.60% - 1.4% carbon):

High carbon steel has the highest hardness and strength among plain carbon steels. It's often used in applications where hardness and wear resistance are critical, such as cutting tools, knives, blades, and springs. However, it tends to be less ductile and more brittle, requiring careful handling and heat treatment to optimize properties.

Note that:

Increasing the carbon content in steels increases strength and hardness.

Properties of plain carbon steels can be further tailored through heat treatment processes such as quenching and tempering, annealing, and normalizing. These processes alter the microstructure of the steel, affecting its hardness, strength, toughness, and other mechanical properties.

Despite their widespread use, plain carbon steels have limitations. Higher carbon content can lead to decreased weldability and increased susceptibility to corrosion. Additionally, while they offer good strength and hardness, they may lack the superior properties of alloy steels in certain specialized applications.

Alloy Steel

Alloy steels are a diverse group of steels that contain additional alloying elements beyond carbon and iron to enhance specific properties. These alloying elements can include elements such as chromium, nickel, manganese, vanadium, silicon, and molybdenum, among others. The addition of these elements can significantly alter the steel's properties, making alloy steels suitable for a wide range of applications across various industries. Alloy steel can be divided into Low alloy steel and High alloy steel.

Low Alloy Steel

Low alloy steels are a category of alloy steels that contain a relatively small percentage of alloying elements compared to high alloy steels. While the exact cutoff between low and high alloy steels can vary, low alloy steels typically contain less than 5-8% total alloying elements by weight. These alloying elements are added to enhance specific properties of the steel while keeping costs relatively low. Examples of low alloy steel include High-Strength Low-Alloy (HSLA) Steel, weathering steel, pressure vessel steel, wear-resistant steel, and rail steel.

High-Strength Low-Alloy (HSLA) Steels:

HSLA steels contain small amounts of alloying elements such as niobium, vanadium, titanium, or a combination thereof. They offer improved mechanical properties, including higher strength, better formability, and enhanced weldability compared to carbon steels, typified by ASTM A572 Grade 50. These steels contain small amounts of alloying elements such as niobium, vanadium, or titanium, contributing to their improved mechanical properties. HSLA steels are commonly used in structural applications, automotive components, pipelines, and offshore structures where high strength-to-weight ratios and good weldability are required.

Weathering Steels:

Weathering steels, also known as atmospheric corrosion-resistant steels or Corten steels, contain small amounts of alloying elements such as copper, chromium, and phosphorus. They develop a protective oxide layer on the surface when exposed to the atmosphere, which inhibits further corrosion and eliminates the need for painting or protective coatings, exemplified by ASTM A588 Grade A/B/C. These steels incorporate small quantities of copper, chromium, and phosphorus, enabling them to develop a protective oxide layer (patina) when exposed to atmospheric conditions. This characteristic provides corrosion resistance. Weathering steels are used in outdoor structures, bridges, buildings, railway cars, and sculptures where corrosion resistance and aesthetic appeal are important.

Pressure Vessel Steels:

Low alloy steels are commonly used in the fabrication of pressure vessels for various industries such as oil and gas, petrochemical, and power generation. These steels are designed to withstand high pressure and temperature conditions and often contain alloying elements such as chromium, molybdenum, and nickel to enhance strength, toughness, and corrosion resistance. ASTM A387 Grade 11 Class 2 is a prime example, containing chromium and molybdenum to enhance corrosion resistance and elevated temperature strength.

Wear-Resistant Steels:

Some low alloy steels are formulated to provide enhanced wear resistance in applications where abrasion and impact are prevalent. These steels typically contain alloying elements such as chromium, molybdenum, and boron to improve hardness, toughness, and wear resistance. Wear-resistant steels like AR400 and AR500 offer exceptional durability. Wear-resistant steels find applications in mining equipment, earthmoving machinery, crushers, and conveyor systems where resistance to abrasive wear is crucial.

Rail Steels:

Low alloy steels are commonly used for the manufacture of railway tracks and components due to their good combination of strength, toughness, and wear resistance. These steels are subjected to stringent quality standards to ensure reliable performance and durability under heavy loads and repeated stresses. A759 Grade R260/R320 is a prime example.

High Alloy Steel

High alloy steels are a subset of alloy steels that contain a significant proportion of alloying elements, typically above 8% by weight. These alloying elements impart exceptional properties to the steel, including superior corrosion resistance, high-temperature strength, wear resistance, and special characteristics tailored to specific applications. High alloy steels are widely used in industries where demanding performance requirements are essential. Examples of High alloy steel include stainless steel, tool steel, high speed steel, heat-resistant alloys.

Stainless Steels:

Stainless steels are the most well-known type of high alloy steel, characterized by their high chromium content (usually 10% or more) along with significant amounts of nickel and other alloying elements. They offer excellent corrosion resistance, both in terms of general corrosion and resistance to localized corrosion such as pitting and crevice corrosion. Stainless steels find applications in a wide range of industries, including food processing, chemical processing, aerospace, automotive, medical devices, and architectural structures.

Tool Steels:

Tool steels are high alloy steels designed to withstand high temperatures and stresses encountered in cutting, shaping, and forming operations. They contain alloying elements such as tungsten, molybdenum, vanadium, and cobalt, which enhance hardness, wear resistance, and heat resistance. Tool steels are utilized in the manufacturing of cutting tools, dies, molds, punches, and other tools used in machining, stamping, and forming processes.

High-Speed Steels:

High-speed steels are specialized tool steels engineered to maintain their hardness and cutting performance at elevated temperatures. They typically contain high levels of tungsten, molybdenum, and vanadium, which form hard carbides that provide wear resistance and retain hardness at high temperatures. High-speed steels are used in cutting tools for machining operations such as drilling, milling, turning, and sawing, where high cutting speeds and temperatures are encountered.

Heat-Resistant Alloys:

High alloy steels with specific compositions are designed to withstand prolonged exposure to high temperatures while maintaining mechanical properties. These alloys are utilized in applications such as heat exchangers, furnaces, boilers, and gas turbines, where resistance to oxidation, creep, and thermal fatigue is critical.

Effects of alloying elements on the eutectoid temperature of steel

The various alloying elements cause the eutectoid temperature of the Fe–Fe₃C phase diagram to be raised or lowered. Manganese and nickel both lower the eutectoid temperature and act as austenite-stabilizing elements enlarging the austenitic region of the Fe–Fe₃C phase diagram. In some steels with sufficient amounts of nickel or manganese, the austenitic structure may be obtained at room temperature. The carbide-forming elements such as tungsten, molybdenum, and titanium raise the eutectoid temperature of the Fe–Fe₃C phase diagram to higher values and reduce the austenitic phase field. These elements are called ferrite-stabilizing elements.

The effect of the percentage of alloying elements

Carbide formation in steel

Carbide formation in steel plays a critical role in determining its microstructure, mechanical properties, and performance characteristics. Carbides are compounds formed by the chemical combination of carbon with other alloying elements present in steel. Understanding carbide formation is essential for designing and engineering steels tailored to specific applications.

Carbon (C):

Carbon readily forms compounds with iron, primarily cementite (Fe3C), a hard and brittle phase that contributes to steel's strength.

Carbon has the highest carbide-forming tendency among the alloying elements in steel. It combines with other elements like chromium, vanadium, and tungsten to form carbides, enhancing hardness and wear resistance.

Manganese (Mn):

Most of the manganese added to carbon steels dissolves in the ferrite phase. However, some manganese forms compounds, such as manganese sulfide (MnS), which improves machinability by acting as a chip breaker. Additionally, manganese can enter the cementite phase as (Fe,Mn)3C.

Manganese has a moderate carbide-forming tendency and can form complex carbides, contributing to hardenability and strength. An example is the formation of manganese carbides (Mn3C) in certain steels, enhancing wear resistance.

Chromium (Cr):

Chromium forms compounds with carbon, such as chromium carbides (e.g., Cr23C6), which improve wear and corrosion resistance. In stainless steels, chromium enhances the formation of a passive oxide layer, preventing further corrosion.

Chromium has a high carbide-forming tendency, forming stable carbides with carbon. Chromium carbides contribute significantly to the hardenability and wear resistance of steels. For example, in high-chromium tool steels, chromium carbides provide excellent wear resistance.

Nickel (Ni):

Nickel stabilizes the austenitic phase in stainless steels, enhancing toughness and corrosion resistance. It dissolves in the α ferrite phase due to its low tendency to form carbides.

Nickel has a low carbide-forming tendency but influences the stability of carbides formed by other alloying elements. In some high-nickel alloys, nickel can form nickel carbides (Ni3C), improving strength and corrosion resistance.

Vanadium (V):

Vanadium forms compounds with carbon, such as vanadium carbides (e.g., V4C3), which improve wear resistance and hardenability. Vanadium carbides contribute to fine grain size and increased toughness.

Vanadium has a high carbide-forming tendency and readily forms stable carbides with carbon. These carbides enhance wear resistance and high-temperature strength. For example, in high-speed tool steels, vanadium carbides improve cutting performance.

Molybdenum (Mo):

Molybdenum forms compounds with carbon, such as molybdenum carbides (e.g., Mo2C), which enhance hardenability and strength at elevated temperatures. Molybdenum also improves resistance to corrosion and pitting.

Molybdenum has a high carbide-forming tendency, forming stable carbides with carbon. These carbides contribute to wear resistance and toughness, particularly in high-speed steels and high-strength low-alloy (HSLA) steels.

Tungsten (W):

Tungsten forms compounds with iron, such as tungsten carbides (e.g., W2C), which are extremely hard and contribute to wear resistance. Tungsten carbides are commonly used in cutting tools and wear-resistant applications.

Tungsten has a very high carbide-forming tendency, forming stable carbides with carbon. These carbides provide exceptional wear resistance and high-temperature strength. In tool steels, tungsten carbides enhance cutting performance and tool life.