Properties of Carbon Steel: Low vs. Medium vs. High Carbon Steel

Carbon steel is ubiquitous in engineering and manufacturing, found in everything from structural beams and pipes to precision components like shafts, gears, springs, and cutting tools. However, as a material primarily composed of iron and carbon, the properties of carbon steel vary significantly depending on its carbon concentration. Key characteristics—such as strength, hardness, ductility, magnetism, and response to heat treatment—are all dictated by this composition. These variations, in turn, directly impact the ease of machining (particularly in modern CNC processes) and the ultimate performance of the finished product.

In this article, we will explore the essential properties of carbon steel: the role that carbon content plays; both the shared and distinct properties within the carbon steel family—covering low, medium, and high-carbon varieties; advice for CNC machining carbon steel selection to help you gain a comprehensive understanding of this versatile material.

What is Carbon Steel?

Carbon steel is defined as a metal alloy consisting primarily of iron and carbon. According to AISI/SAE standards, steel is classified as carbon steel when no minimum content is specified for elements like chromium, cobalt, or nickel; the manganese content does not exceed 1.65%;the silicon and copper contents do not exceed 0.60%.

The properties of carbon steel material are almost entirely dictated by its carbon percentage (typically between 0.04% and 2.1%), and carbon steel family include:

• Low-Carbon (Mild): 0.04%–30% C. Highly ductile and easily welded.
• Medium-Carbon: 0.30%–60% C. Balanced strength and ductility, often heat-treated.
• High-Carbon: 0.60%–50% C. Maximum hardness and wear resistance, primarily for tools and springs

How Does Carbon Affect the Properties of Steel?

Carbon acts as a strengthening agent of carbon steel family. When the carbon content increases, the material undergoes a property trade-off:

• Strength and Hardness increase: The iron lattice becomes more rigid.
• Ductility and Weldability decrease: The material becomes more brittle and harder to join without cracking.
• Toughness decreases: High-carbon variants are more prone to snapping under sudden impact rather than bending.

Shared Properties of Carbon Steel Family

While the carbon percentage dictates whether a steel is “low,” “medium,” or “high,” all carbon steels share similar fundamental physical and chemical properties. Understanding these shared properties of carbon steel material can give you a general overview of carbon steel.

PoorCorrosion Resistance(Oxidation)

Unlike stainless steel, carbon steel lacks a protective chromium oxide layer. When exposed to moisture and oxygen, it reacts to form iron oxide (rust). This makes surface treatments—such as plating, painting, or oiling—a requirement for almost all carbon steel parts.

Ferromagnetism

All carbon steels are magnetic due to its iron content typically accounts for over 98%. This magnetic property of carbon steel is highly beneficial in industrial settings:

• It allows for easy sorting in recycling facilities using electromagnets.
• It enables the use of magnetic work-holding fixtures during CNC machining.
• It makes carbon steel ideal for components in magnetic assemblies and sensors.

High Modulus of Elasticity

Carbon steel is remarkably rigid. It typically possesses a modulus of elasticity of approximately 200 GPa (29,000 ksi) not matter it’s mild, medium, high carbon steel. This means the material can withstand stress and maintain its shape before reaching the point of permanent (plastic) deformation. This rigidity is why carbon steel is the good material for structural frames, heavy-duty machinery, or springs.

Density

Regardless of the carbon content, the density of carbon steel remains stable at approximately 7.85 g/cm³ (0.284 lb/in³). This data provides a reliable basis for calculating the weight and load-bearing requirements of building beams and columns, bridges, and mechanical bases, etc.

Environmental and Temperature Sensitivity

Carbon steel reacts predictably but it is sensitive to temperature changes:

• Low-Temperature Brittleness: In cold environments（-29°C to -45°C, or lower）, carbon steel (especially low-carbon) can become brittle and prone to sudden fracturing.
• High-Temperature Softening: Once temperatures exceed 500°C, carbon steel begins to lose its structural strength and hardness rapidly.

High Melting Point

The melting point for carbon steel ranges between 1425°C and 1540°C. While higher carbon content slightly lowers the melting point, the material remains highly stable under standard industrial heat, making it suitable for forging and welding (provided the specific grade allows for it).

Excellent Surface Treatment Compatibility

Because carbon steel is so prone to rusting, it has been engineered to work perfectly with various finishing processes. From a manufacturing perspective, carbon steel is one of the best substrates for:

• Electroplating (Zinc, Nickel, Chrome)
• Powder Coating and Painting
• Black Oxide Treatment
• Hot-Dip Galvanizing

Relatively LowCost

Carbon steels’ primarily elements iron and carbon are abundantly available in nature, with costs much lower than those of expensive elements required in stainless steel or alloy steel, such as chromium, nickel, or molybdenum. Additionally, its smelting and processing are relatively straightforward, as it does not involve complex alloy formulations. Carbon steel is currently the most widely produced metal material globally. Large-scale standardized production also reduces the cost.

Summary Table: Shared Properties of Carbon Steel

Table 1: Shared Properties of Carbon Steel Family

Property	Typical Value / Behavior	Practical Impact
Corrosion Resistance	Poor	Requires coating, painting, or oiling.
Magnetism	Strong (Ferromagnetic)	Ideal for magnetic sensors and industrial sorting.
Elastic Modulus	~200 GPa	High structural rigidity; resistant to deformation.
Density	~7.85 g/cm³	Predictable weight for engineering calculations.
Temperature Limit	Softens above 500°C; Brittleness around -45°C	Not suitable for extreme high-heat or low temperature environments.
Cost	Relatively Low	Most cost-effective choice for bulk manufacturing.

Properties of Low Carbon Steel (Mild Steel)

Low carbon steel, or mild steel, is the most widely used form of steel due to its affordability and ease of fabrication. Its carbon content is 0.04% – 0.30% and common grades include ASTM A36, AISI 1018, SAE 1008,ect. Except for the shared properties, mild carbon steel has relatively higher ductility and malleability than medium and high carbon steels due to the carbon content, but this is not absolute; Properties of low carbon steel are also different from each other for their specific grades. Here properties of common grades include ASTM A36, AISI 1018, SAE 1008 are listed in below table, and you can find their differences:

Table 2: Properties of Low Carbon Steel ASTM A36, AISI 1018, SAE 1008

Property	ASTM A36	AISI 1018	SAE 1008
Type	Structural Carbon Steel	General Purpose Mechanical Steel	Low Carbon Drawing Steel
Carbon Content (C)	~0.25% – 0.29%	0.15% – 0.20%	≤ 0.10%
Common Form	Hot Rolled (Plates, Beams)	Cold Drawn (Bars, Rods)	Cold Rolled (Sheets, Strips)
Yield Strength	~250 MPa / 36 ksi	~370 MPa due to cold work	~170 MPa / 25 ksi
Machinability	Good (but rough surface)~72%	Excellent (smooth finish)~78%	Poor (Too “gummy” / soft) although moderate data ~55% – 60%
Formability	Fair	Good	Excellent (Deep drawing)
Weldability	Excellent	Excellent	Superior
Typical Applications	Construction, Bridges, Frames	Shafts, Pins, Gears, Bushings	Appliances, Car Panels, Wire

Properties of Medium Carbon Steel

Medium carbon steel(0.30% – 0.60% carbon content) is stronger than low-carbon steel but more ductile and easier to machine than high-carbon steel. Its most significant advantage is its response to heat treatment (quenching and tempering).

Low-carbon steel (e.g., AISI 1018) has a carbon content too low to harden significantly through quenching, unless surface carburizing is applied; it cannot achieve a substantial increase in bulk strength through through-hardening heat treatment. In contrast, high-carbon steel (e.g., AISI 1095) has such high carbon content that it is extremely prone to cracking or distortion during quenching, resulting in excessive brittleness that limits its use primarily to cutting tools or springs.

Medium-carbon steel is uniquely suited for heat treatment. Upon heating and rapid cooling, its internal microstructure transforms into martensite, which dramatically increases both hardness and strength. By subsequently performing tempering (reheating after quenching), the material’s toughness is restored while maintaining its high strength, providing an optimal balance for mechanical components.

Common grades of medium carbon steel include AISI 1045, AISI 1050, AISI 1144, and for their properties you can check in below table:

Table 3: Properties of Medium Carbon Steel AISI 1045, AISI 1050, AISI 1144

Property	AISI 1045	AISI 1050	AISI 1144
Type	Plain Medium Carbon	Plain Medium Carbon	Free-Machining Steel
Carbon Content	0.43% – 0.50%	0.48% – 0.55%	0.40% – 0.48%
Key Elements	Iron + Carbon	Iron + Carbon	S (0.24-0.33%) + Mn
Yield Strength (Q&T)	~450 – 600 MPa	~500 – 650 MPa	~690 MPa
Tensile Strength (Q&T)	~620 – 800 MPa	~700 – 900 MPa	~800 – 950 MPa
Toughness (Impact)	Moderate	Moderate (Lower than 1045)	Low (Due to Sulfur inclusions)
Heat Treatment	Excellent response to Induction Hardening.	Excellent response; achieves higher surface hardness.	Usually used Pre-hardened; prone to quench cracks.
Machinability	Good (Rating: 55%)	Fair (Rating: 45-50%)	Excellent (Rating: 85-90%)
Hardenability	Moderate (Shallow)	Moderate (Better than 1045)	Moderate
Weldability	Limited (Pre-heat required)	Limited (Care required)	Very Poor (Risk of hot cracking)
Typical Use	Gears, Shafts, Axles	Heavy-duty gears, Forgings	Precision Shafts, Hydraulic Parts

Properties of High Carbon Steel

High carbon steel has carbon content of 0.60% – 1.50%, and it often referred to as tool steel; High carbon steel is designed for high hardness and wear resistance.

• Mechanical Properties: Highest tensile strength (600–950+ MPa) but very low ductility (5%–15%).
• CNC Machining: Challenging. It requires slower cutting speeds, robust carbide tooling, and high-pressure coolant.
• Common Grades: AISI 1095, AISI 52100, D2.

For high carbon steel AISI 1095, AISI 52100, D2 properties, you can check in below table:

Table 4: Properties of High Carbon Steel AISI 1095, AISI 52100, D2

Property	AISI 1095	AISI 1075	D2 Tool Steel
Type	High Carbon Steel	High Carbon / Spring Steel	High-Carbon
Carbon Content	0.90% – 1.03%	0.70% – 0.80%	1.40% – 1.60%
Tensile Strength (T)	~620 – 800 MPa	~550 – 750 MPa	~1800 MPa
Yield Strength (T)	~400 – 540 MPa	~380 – 500 MPa	~1500 MPa
Typical Hardness	55 – 62 HRC	45 – 55 HRC	58 – 62 HRC
Machinability	Poor (~45%)	Fair (~55-60%)	Poor (~65% in Annealed state)
Toughness	Moderate	Excellent	Moderate
Heat Treatment	Oil Quench; simple to treat	Oil/Water Quench; very stable	Air Quench; complex/high temp
Wear Resistance	High	Good	Superior
Typical Use	Knives, Scrapers, Springs	Coil Springs, Saws, Lawn Tools	Stamping Dies, Industrial Shredders

Comparison Table for Quick Check: Low vs Medium vs High Carbon Steel

Here list the comparison table of low-carbon (mild) steel vs medium-carbon steel vs high-carbon steel for quick check：

Table 5: Low-Carbon Steel vs Medium-Carbon Steel vs High-Carbon Steel

Characteristics	Low-Carbon (Mild)Steel	Medium-Carbon Steel	High-Carbon Steel
Carbon Content	< 0.30%	0.30% – 0.60%	0.60% – 1.50%
Primary Property	Maximum ductility and weldability	Balanced strength and toughness	Extreme hardness and wear resistance
Strength (Yield)	Lowest (approx. 200–350 MPa)	Moderate (approx. 400–550 MPa)	Highest (600+ MPa)
Machinability	High (easy to cut)	Good (standard machining)	Low (harder to machine)
Weldability	Excellent (no preheat needed)	Fair (may require preheat)	Poor (susceptible to cracking)
Heat Treatment	Case hardening only	Excellent response to quenching	Full hardening and tempering
Common Uses	Beams, car bodies, pipelines, rebar	Axles, gears, railway tracks, shafts	Knives, springs, drill bits, cutting tools
Cost	Most affordable	Moderate	Most expensive

CNC Machining Carbon Steel

CNC Machining is machining, cutting, drilling or milling a whole piece of material into the shape that you’ve designed on computer lathe path. For the main CNC Machining methods are CNC Turning and CNC Milling.

At CNC VMT CNC Machining Factory, we often advise clients on the properties of carbon steel material relative to their budget and part function.

• Low Carbon (e.g., 1018): Best for non-structural, high-volume parts. It is easy to cut, reducing tool wear and cost.
• Medium Carbon (e.g., 1045): The “go-to” for mechanical components. It machines beautifully to a high-quality surface finish.
• High Carbon (e.g., 1095): Reserved for parts that require extreme edge retention or hardness. Specialized carbide tools are suggested to manage the heat generated during the machining of these hard alloys. The complex surface profile of high-carbon steel requires the precision machining capability of 5 axis CNC machining.

Conclusion

In conclusion, understanding the properties of carbon steel is essential for designing and manufacturing considerations between raw material selection and high-performance engineering. By recognizing carbon steel carbon steel shared properties and carbon steel family from low-carbon steel, medium carbon steel to high-carbon steel, engineers can make informed decisions that balance part longevity with production efficiency.

Case Study: High-Precision Drive Shafts for Automotive Transmission Systems

An automotive leader approached VMT CNC Machining Factory to produce 1045 medium carbon steel drive shafts for their new EV platforms. The primary technical hurdles included achieving a stringent surface roughness of Ra 0.8μm and maintaining a tight dimensional tolerance of ±0.01mm. In previous trials, the material’s inherent toughness led to significant heat accumulation and “built-up edge” on cutting tools, resulting in surface tearing and thermal distortion that rendered the slender components out of spec.

To address these issues, VMT’s engineering team transitioned the project to our Advanced 5-axis CNC Machining centers. By optimizing the tool path to maintain a constant chip load and utilizing specialized TiAlN-coated carbide tools, we significantly reduced frictional heat. Furthermore, we implemented a high-pressure through-spindle cooling system that evacuated chips instantly, preventing the re-cutting of hardened particles and ensuring the structural integrity of the 1045 steel throughout the milling process.

The implementation of these advanced techniques yielded exceptional, data-verified results in our first batch production run. We successfully improved surface finish quality by 75%, reaching a consistent Ra 0.4μm, and tightened precision to ±0.008mm. Most notably, the optimized cooling and tooling strategy extended tool life from 4 pieces per edge to 30 pieces, representing a 650% increase in tool efficiency. Consequently, the overall machining cycle time was slashed by 35%, from 52 minutes down to just 34 minutes per unit.

By choosing VMT’s specialized Carbon Steel Machining Services, the client was able to eliminate a costly secondary grinding process, reducing the total cost per part by 25%. Our ability to deliver a 99.8% final yield rate ensured that their delivery schedule remained on track.