Tips to simplify designs
Engineers love steel. We love steel. Its implicit value across all aspects of the built environment and products, large to small is felt everywhere. In so many ways, steel is the essential material in virtually every aspect of human effort. It houses us, transports us, equips us to work and play, it feeds us, waters us, protects us and heals us.
It is a miraculous material that combines strength, toughness, processability, versatile utility, and affordability. It is used to build everything from skyscrapers and bridges to automobiles and appliances, from screws to a train (and their tracks).
However, not all steel is created equal. Steel comes in many conditions and grades, with two of the most widely used broad classifications being alloy steel and carbon steel.
The terminology is somewhat unclear. Both carbon steel and alloy steel are alloys of Iron and carbon, differentiated by the inclusion of a spectrum of additional alloying elements that making the family of alloy steels. These significantly alter its mechanical and chemical properties in a variety of ways, depending on constituents and proportions.
The differences in composition result directly in well-understood differences in strength, durability, corrosion resistance, and heat resistance, making each type better suited for different applications. In this blog, we will explore the key differences between alloy steel and carbon steel, compare their properties, advantages, and limitations, and discuss their most common applications.
Jerry S.
Mechanical Engineer
"The holy grail of good speed, quality, and price for custom parts"
Jiga is the best way to get the parts you need, when you need them.
Understanding Carbon Steel
Steel, the basic alloy of Iron and Carbon, can also include trace amounts of silicon, manganese, sulfur, and phosphorus. However, the Carbon content in Carbon steel is the defining characteristic, and it is typically discussed in three characteristics ranges- low, medium, and high Carbon.
With increasing Carbon content, the strength, hardness and heat treatability increase. This comes at the expense of ductility, which is the ability of the material to stretch or deform without fracture.
Note that the strength benefits of Carbon are reduced as percentages rise further—so commercial (cast) Iron often contains 4% Carbon and it is brittle and relatively weak in tension.
Carbon steel is generally divided into three categories based on the percentage of Carbon:
- Low-carbon steel (mild steel): This contains less than 0.3% Carbon and is appreciated for its toughness, malleability, and ductility. It is the most widely used type of Carbon steel due to its low cost and ease of fabrication.
- Medium-carbon steel: This contains between 0.3% and 0.6% Carbon and offers a balance between strength, and ductility, making it suitable for applications requiring higher strength than low-Carbon steel, but retaining good ductility, such as automotive body components.
- High-carbon steel: This contains more than 0.6% Carbon and up to 2%. It is characterized by its very high strength and abrasion resistance. However, it is more brittle and difficult to work, making it only suitable for specialized applications like cutting tools and springs.
Properties of Carbon Steel
Carbon steel exhibits a range of properties depending on the carbon content. Some key characteristics include:
- Strength and hardness: Carbon steel’s strength and hardness increase with the carbon content (up to a point), making higher-carbon grades suitable for applications that require durability and wear resistance.
- Ductility: Lower carbon steels are more ductile and can be bent, formed, or shaped easily. As the Carbon content rises, the material tends more towards brittle rather than ductile – the two ends of a property continuum.
- Machinability and weldability: Low and medium-Carbon steels are relatively easy to machine and weld. High-carbon steels, while stronger, are more challenging to work with due to their hardness.
- Corrosion resistance: Carbon steel has limited corrosion resistance, especially when compared to some alloy steels or stainless steel (itself an extreme example of alloy steel). Carbon steel requires protective coatings, such as galvanizing or painting, to prevent rust in even mildly damp conditions.
Applications of Carbon Steel
Due to its low cost and versatility, Carbon steel is a primary material in most industries:
- Construction: Low-carbon steel is commonly used for structural components, beams, and rebar in ferro-concrete.
- Automotive: Medium-carbon steel is typically used in the production of gears, shafts, structural components and other parts in vehicles.
- Tool manufacturing: High-carbon steel is suited to making cutting tools, knives, and blades due to its hardenability and wear resistance.
- Pipes and tubes: Low-carbon steel pipes are commonly used for transporting oil, gas, and water due to their strength and cost-effectiveness.
Understanding Alloy Steel
Alloy steel is (Carbon) steel that has been enhanced by being alloyed with one or more typically metallic elements, such as Chromium, Nickel, Molybdenum, Vanadium, or Manganese, to adjust its mechanical properties. The presence of these alloying elements can significantly alter the resultant alloy’s strength, toughness, corrosion resistance, heat tolerance, and abrasion resistance.
Alloy steel is typically categorized into two types:
- Low-alloy steel: These contain a relatively low percentage of alloying elements (typically less than 5%) and are generally used for structural applications. These steels strike a cost-favoring balance between strength, toughness, and cost-effectiveness.
- High-alloy steel: Such materials contain a higher percentage of alloying elements (generally greater than 5% by mass) and are used in applications requiring higher levels of corrosion resistance, heat resistance, or abrasion durability. Stainless steels, which typically contain at least 10.5% Chromium, are clear examples of high-alloy steels.
Properties of Alloy Steel
The properties of alloy steels vary very widely, depending on the specific alloying constituents added and their proportions. Some of the key properties sought/enhanced by alloying include:
- Strength: Alloy steels generally offer higher strength than Carbon steel, particularly when they contain Vanadium and Molybdenum.
- Toughness: Alloy steels tend to be tougher and more resistant to impact than Carbon steels, making them suitable for applications where shock resilience is essential.
- Corrosion resistance: While Carbon steel is prone to oxidation under relatively benign conditions, alloy steels, and particularly stainless steel, offers significantly better corrosion resistance, often due to the formation of an impervious and self-repairing Chromium dioxide surface patina.
- Heat resistance: Alloy steels, particularly those containing Nickel, Molybdenum, and Chromium, offer maximum service temperatures than can far exceed those of Carbon steels.
- Cost: The addition of alloying elements renders alloy steels more expensive than Carbon steel, but the enhanced performance characteristics typically compensate for the higher cost.
Applications of Alloy Steel
Alloy steels of highly selected compositions are used in industries where specific performance characteristics are demanded, including:
- Aerospace: Alloy steel is used in the production of aircraft components that must withstand high stress and extreme temperatures, such as turbine blades and landing gear.
- Automotive: Alloy steel is commonly used for engine components, drive shafts, sub-assembly housings, and gears due to the combination of high strength and toughness.
- Oil and gas: Various alloy steels are used in pipelines, valves, and fittings for oil and gas applications due to their high corrosion resistance and ability to withstand high-stress environments.
- Power generation: Alloy steel is used in gas turbine components, boilers, and heat exchangers in power plants, exploiting its heat resistance and strength.
Key differences between Alloy Steel and Carbon Steel
Composition
The primary difference between alloy steel and Carbon steel is the range of alloying elements included. Carbon steel is essentially composed of Iron and Carbon, while alloy steel contains various additional elements—vanadium, Nickel, Molybdenum, Chromium and many others – that modify its properties in selected and highly controlled ways.
Strength and hardness
Alloy steels are generally stronger and harder than Carbon steels due to the presence of alloying elements which alter the crystalline matrix of the base material in diverse ways. While Carbon steel can achieve high strength by increasing the carbon content, this also leads to embrittlement and reduced ductility/malleability. Alloy steels, on the other hand, can achieve similarly improved strength without sacrificing toughness or ductility.
Corrosion resistance
Carbon steel is typically susceptible to corrosion, especially in humid or aggressive environments. It requires protective coatings to prevent degradation. Alloy steels, particularly stainless steels, offer significantly enhanced corrosion resilience due to the critical level of chromium and, to a lesser extent nickel, which lend their resilient oxide-forming capabilities to the resulting alloy.
Heat resistance
Alloy steels can achieve dramatically superior heat resistance compared to Carbon steels. For example, stainless steel can maintain its strength at greatly elevated temperatures, while Carbon steel loses strength and becomes more prone to structural disruption.
Cost
Carbon steels are considerably lower cost than typical high-alloy steels, because many of the potential alloying agents are scarce and of higher value. Alloy steel is also more expensive due to the more complex manufacturing process entailed. Low alloy steels such as the HSLA (high strength low alloy steels) used in automotive bodywork/structures somewhat bridge this gap by using precision alloying and grain size control to deliver strength and ductility with low addition levels.
Machinability and weldability
Low-Carbon steel is much easier to machine and weld than high-carbon steel or alloy steel. However, alloy steels with lower carbon content can be quite weldable, while high-alloy steels, such as stainless steel, often require more specialized welding techniques. The machinability of alloy steel varies greatly depending on the alloying elements and the state of hardening/heat treatment.
Alan B.
Mechanical Engineer
"Hands-Down the Best Platform and Partner for Fast, Quality Parts"
Jiga is the best way to get the parts you need, when you need them.
Choosing between Alloy Steel and Carbon Steel
The choice between alloy steel and carbon steel depends on the demands of the application, but generic considerations include:
- Strength to weight: If strength is a critical factor, alloy steel may be the better choice due to its higher strength-to-weight ratio and resistance to stress and fatigue. Carbon steels will require more material to deliver the same strength in a component
- Corrosion resistance: In environments where corrosion is a concern, alloy steel, particularly stainless steel, is the only logical choice.
- Heat resistance: For applications that involve high temperatures, such as turbines or boilers, highly specialized alloy steels are the practical option.
- Cost: Carbon steel is considerably lower cost than typical alloy steels, making it the logical choice in applications where cost is a primary consideration.
- Ductility and ease of fabrication: Low-carbon steel is easier to work/process and is highly suitable for applications where the material needs to be heavily deformed and welded. Where strength beefs override processing costs, alloy steel selection is often necessary.
Conclusion
Both alloy steel and carbon steel have distinct advantages and are selected to suit the individual demands of various divergent applications.
Carbon steel, offering affordability and ease of fabrication, is well-suited for bulk tasks of moderate stress, such as construction/rebar, heavy machinery, and non-extreme applications. Alloy steel, on the other hand, delivers enhanced strength, toughness, corrosion resistance, and heat resistance – at a price. This makes it the material of choice for more demanding applications in aerospace, power generation, and the oil and gas industries.
Understanding the subtle but critical performance differences between these two broad classes of steel is essential for making informed decisions about selection, balancing often conflicting criteria. Selecting the appropriate grade of steel ensures that products meet the extremes of required performance while optimizing cost.
