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The history of alloying is the history of human development. From bronze to iron, the process of refining and improving metals is the foundation of our progress.
And the history of alloys of steel is the history of the industrial revolutions – Industry 1.0 to Industry 4.0 and beyond.
Steel is the most universal material in the world, thanks to its versatility, strength, and durability. And much of that springs directly from the astonishing diversity of the family.
Two of the most common classifications of steel are alloy steel and stainless steel, each with unique and often overlapping properties, applications, and benefits.
Understanding the differences and commonalities between these material categories is pivotal in selecting the most appropriate type of steel for every application.
In this blog, we’ll discuss the composition, performance, and relative benefits of alloy steel and stainless steel to equip you to make informed decisions about their selection and implementation in your components.
Sean B.
Mechanical Engineer
"I've recommended it to all my colleagues in manufacturing."
Jiga is the best way to get the parts you need, when you need them.
What is Alloy Steel?
Alloy steel is the broadest possible category of steel that contains a mix of metals (alloying additions) and not just carbon and Iron. By adding elements like chromium, manganese, nickel, molybdenum, vanadium, and others, alloy steels are engineered with greatly enhanced properties – strength, hardness, wear resistance, temperature resilience, corrosion resistance, and toughness. A subset of alloy steels are referred to as stainless steels, as their composition is particularly focussed on specific corrosion resilience – making them ideal applications in biological environments, chemical processing, marine applications, etc.
Classes of Alloy Steel
Alloy steel can be broadly categorized into two main types:
- Low-alloy steels, containing a lesser percentage of alloying elements (usually sub 5%), aiming for moderate improvements in strength and toughness, enhanced by close control of grain size and crystalline structure.
- High-alloy steels which contain above 5% alloying agents, delivering improved heat and corrosion properties, higher tensile strengths, improved toughness, maintained ductility, and more.
Composition of Alloy Steel
The primary elements in alloy steel, in addition to iron and carbon, include:
- Chromium: Increases hardness, corrosion resistance, and strength.
- Nickel: Improves toughness and ductility.
- Molybdenum: Enhances strength and heat resistance.
- Vanadium: Increases strength, toughness, and resistance to wear.
- Manganese: Enhances toughness and wear resistance.
Superalloys may also include both commonplace and exotic alloying elements:
- Titanium: Improves yield and ultimate tensile strength, reduces elongation at fracture.
- Copper: Improves corrosion resistance up to 0.2%, but deteriorates many properties when higher.
- Aluminum: Improves toughness but reduces creep resistance.
- Tantalum: Improves strength without reducing toughness.
- Niobium: At 0.02 to 0,05% this refines austenitic grain size and uniformity.
Properties of Alloy Steel
- Strength: Alloy steels are known for their excellent mechanical strength, making them ideal for heavy-duty applications.
- Hardness: The addition of elements like chromium and vanadium makes alloy steel harder and more resistant to wear.
- Toughness: Alloy steels maintain good toughness, even at low temperatures, which is why they are often used in challenging environments.
- Corrosion Resistance: While not as corrosion-resistant as stainless steel, certain grades of alloy steel offer improved corrosion resistance compared to carbon steel.
- Heat Resistance: High-alloy steels, especially those containing molybdenum and chromium, perform well at elevated temperatures, making them suitable for high-heat environments.
What is Stainless Steel?
Stainless steel is a family of steel grades with varied degrees of remarkable corrosion resistance, achieved through the addition of Chromium (at least 10.5%) and, in some applications, nickel. These elements form a self healing, passive layer of metal oxide on the steel’s surface, preventing further moisture based oxidation (and other chemical attacks such as chloride ion oxidation) to deliver various levels of corrosion durability, according to alloy type.
Types of Stainless Steel
Stainless steel is commonly divided into five broad classes based on its microstructure and properties – each class including a range of variants within the type:
- Austenitic: Most stainless steel is formed primarily from austenite – a face centered cubic (FCC) crystalline structure, known for its excellent blend of corrosion resistance and formability. This class contains high levels of Chromium (16-26%) and Nickel (up to 35%), typified by stainless steel 304 alloy, also called 18/8 alloy.
- Ferritic: These contain no Nickel and 10.5-27% Chromium and are known for good corrosion resistance and magnetic properties. These are typified by stainless steel 403.
- Martensitic: These are appreciated for hardness and strength but have lower resistance to corrosion. They contain 11.5-18% Chromium, typically 1.2% Carbon and may contain Nickel in addition. This type is often used in cutlery and tools and lie in the 410 to 440 stainless steel naming range.
- Duplex: A combination of austenitic and ferritic microstructures, offering higher strength and improved resistance to pitting and stress corrosion.
- Precipitation hardening: These are appreciated for high strength and excellent corrosion resistance, optimized heat treatment processes.
Composition of Stainless Steel
The key alloying elements in stainless steel include (but are by no means limited to):
- Chromium: Provides corrosion resistance by forming a protective oxide layer.
- Nickel: Improves formability and increases resistance to extreme temperatures and some corrosive environments.
- Molybdenum: Enhances resistance to pitting and crevice corrosion in chloride environments.
- Carbon: Added in controlled amounts to improve strength and hardness (at the expense of ductility and workability) in some grades of stainless steel.
General properties of Stainless Steels
- Corrosion resistance: Stainless steels are renowned for their ability to resist corrosion, particularly in wet or humid environments and, in the case of some alloys, under very aggressive chemical conditions.
- Hygiene: Stainless steel is non-porous and easy to clean, making it ideal for food, medical, and pharmaceutical industries. It tolerates the most aggressive sterilization processes.
- Strength: Some grades of stainless steel offer high tensile strength, though typically less strong than many alloy steels.
- Heat resistance: Stainless steel performs well in high-temperature environments, especially austenitic and ferritic types.
Comparing Alloy Steel and Stainless Steel
Composition and Alloying elements
The primary difference between alloy steel and stainless steel lies in their constituent alloying elements and their proportions, which greatly alter crystalline composition and most physical and thermal properties as a consequence. Stainless steel by definition contains a minimum of 10.5% chromium, which is responsible for its corrosion resistance. In contrast, alloy steels may contain a wide variety of metals and even some non-metals in varied proportions, to deliver a wide spectrum of properties according to application needs.
Corrosion resistance
Stainless steel is outstanding in terms of corrosion resistance. The chromium content forms a self healing and impervious protective chromium dioxide layer that prevents progressive oxidation, even in harsh environments like marine or industrial settings. Alloy steel, while more resistant to corrosion than carbon steel, cannot typically equal the corrosion resistance of stainless steel, especially in wet or acidic environments.
Strength and durability
Both alloy steel and stainless steel can be high strength compared with more basic steels, but alloy steels are developed to be stronger in high-stress, high-impact applications. Alloy steels, particularly low-alloy steels, are designed to provide an excellent balance of strength, toughness, and wear resistance, rendering them suited to heavy-duty construction, heavy machinery, and industrial equipment applications.
Stainless steel, while strong compared with most basic steels, is often chosen for applications where corrosion resistance is more important than pure mechanical strength, such as in chemical processing or marine environments. Martensitic stainless steels, however, offer high hardness and strength at the cost of reduced corrosion resistance.
Heat resistance
Both alloy steel and stainless steel can be used in high-temperature environments, but certain grades of each material are more suitable for aggressive thermal conditions and rapid temperature fluctuations. High-alloy steels containing molybdenum and vanadium are particularly resistant to heat, while austenitic stainless steels are also known for their high-temperature stability and resistance to oxidation at elevated temperatures.
Weight and ductility
Stainless steel is generally more dense than alloy steel due to the high chromium and nickel content. However, some stainless steels offer better ductility than typical alloy steels. This property makes stainless steel more appropriate for applications that require complex shapes, such as kitchen utensils, medical instruments, and architectural detailing.
Alloy steel, depending on the specific composition, can be lighter and more rigid, making it ideal for applications where weight (rather than extreme corrosion resistance) is the critical factor, such as in aerospace or automotive industries.
Machinability and formability
Some stainless steels are considerably more challenging to machine and form than alloy steels due to their hardness and the strong tendency to be work-hardened. Alloy steel, on the other hand, is generally easier to machine and can be formed into more extreme shapes with less wear on cutting/forging tools.
Cost
Stainless steel is typically more expensive than alloy steel due to the considerably higher cost of chromium and nickel. The cost of stainless steel is application-appropriate because of its superior corrosion resistance and, therefore, service life.
Alloy steel, while lower cost initially, may require more maintenance or protective coatings to prevent corrosion, especially in harsh environments. Its lower cost makes it a better cost-choice where corrosion resistance is of lower concern.
Javier L
Principal Systems R&D Mechanical Engineer
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Jiga is the best way to get the parts you need, when you need them.
Applications of Alloy Steel vs. Stainless Steel
Alloy Steel applications
Alloy steels are widely used where strength, toughness, and wear resistance are the most essential characteristics. Example applications include:
- Construction: Alloy steel is used in the construction of bridges, buildings, and other infrastructure projects due to its high strength and durability. S355 grade is typical, though S275 is common in railway bridges where fatigue resistance is given a higher priority.
- Machinery: Many heavy-duty machines and equipment are made from alloy steel to withstand high stresses and impacts.
- Aerospace: Low-weight, high-strength alloy steels are used in aircraft structures and components.
- Automotive: Alloy steel is often used in the manufacturing of higher stress or wear/fatigue exposed automotive components such as gears, timing chains, and crankshafts.
Stainless Steel Applications
Stainless steel is heavily favored in industries that demand corrosion resistance, cleanliness/hygiene, and higher quality aesthetics.
- Medical devices: Stainless steel is typical in surgical instruments, in-tissue implants, and medical apparatus, based on its intrinsic biocompatibility and ease of sterilization.
- Food processing: Stainless steel is essential in food and beverage processing equipment for hygiene and food-acid corrosion resistance.
- Marine applications: High Nickel stainless steel is necessary in wetted-marine environments, such as in boat fittings and underwater pipelines, for the added resistance to chloride ion attack.
- Architecture: Stainless steel serves in building facades/cladding, hand-rails, and other architectural detailing for its appearance and environmental durability.
Conclusion
In the decision process demanded in selecting between alloy steel and stainless steel for your component needs, the choice is derived from the specifics of the application and an understanding of the material properties.
Alloy steel delivers superior strength, toughness, and generally heat resistance – the right choice for heavy-duty, high-stress industrial and construction applications. Stainless steel is the preferred material when corrosion resistance, hygiene, and aesthetics are more critical than strength/mechanical durability – medical devices, food processing, and marine applications, people-touched surfaces.
Both materials offer great benefits and limitations, and a deep understanding of these differences is key to making the right material choice for your project. Whether you’re designing machinery, constructing a building, or manufacturing medical equipment, the differentiating factors in choosing between alloy steel and stainless steel will depend on the need for strength, corrosion resistance, weight saving, cost saving, thermal resilience, and environmental impact.
