EN31 Steel (52100): Properties and Key Applications

EN31 Steel, also known as Bearing Steel or 52100, is a high-carbon alloy steel primarily classified as a medium-carbon alloy steel. This steel grade is notable for its excellent hardness, wear resistance, and ability to withstand high loads, making it particularly suitable for manufacturing rolling elements in bearings, such as balls and rollers. The primary alloying elements in EN31 include chromium, which enhances hardenability and corrosion resistance, and carbon, which contributes to its hardness and strength.

Comprehensive Overview

EN31 steel is characterized by its high carbon content (approximately 0.95% to 1.05%) and chromium (around 1.30% to 1.60%), which together impart significant mechanical properties. The steel's microstructure typically consists of a fine pearlite matrix with dispersed cementite, resulting from appropriate heat treatment processes.

The most significant characteristics of EN31 include:

• High Hardness: Achievable hardness levels can reach up to 60 HRC after proper heat treatment.
• Excellent Wear Resistance: Its hardness and microstructure provide superior wear resistance, making it ideal for high-load applications.
• Good Toughness: Despite its hardness, EN31 maintains reasonable toughness, which is essential for dynamic applications.

Advantages (Pros):
- Exceptional fatigue strength and wear resistance.
- Suitable for high-stress applications, particularly in rolling contact.
- Good machinability in the annealed condition.

Limitations (Cons):
- Limited corrosion resistance compared to stainless steels.
- Requires careful heat treatment to achieve desired properties.
- Can be brittle if improperly processed.

Historically, EN31 has been widely used in the automotive and aerospace industries for components such as gears, shafts, and bearings, establishing its position as a critical material in engineering applications.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G52100	USA	Closest equivalent to EN31
AISI/SAE	52100	USA	Commonly used in bearing applications
ASTM	A295	USA	Specification for high-carbon chromium steel
EN	EN31	Europe	Standard designation in Europe
DIN	100Cr6	Germany	Similar properties, minor compositional differences
JIS	SUJ2	Japan	Comparable grade with slight variations in properties

The differences between these equivalent grades can affect selection based on specific application requirements. For instance, while G52100 and EN31 are often interchangeable, G52100 may offer slightly better hardenability due to its specific carbon content.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.95 - 1.05
Cr (Chromium)	1.30 - 1.60
Mn (Manganese)	0.30 - 0.50
Si (Silicon)	0.15 - 0.40
P (Phosphorus)	≤ 0.025
S (Sulfur)	≤ 0.025

The primary role of carbon in EN31 is to enhance hardness and strength, while chromium improves hardenability and wear resistance. Manganese contributes to toughness and aids in deoxidation during steelmaking, whereas silicon enhances strength and elasticity.

Mechanical Properties

Property	Condition/Temper	Test Temperature	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Tensile Strength	Quenched & Tempered	Room Temp	1000 - 1200 MPa	145 - 174 ksi	ASTM E8
Yield Strength (0.2% offset)	Quenched & Tempered	Room Temp	850 - 1000 MPa	123 - 145 ksi	ASTM E8
Elongation	Quenched & Tempered	Room Temp	10 - 15%	10 - 15%	ASTM E8
Hardness (HRC)	Quenched & Tempered	Room Temp	58 - 62 HRC	58 - 62 HRC	ASTM E18
Impact Strength (Charpy)	Quenched & Tempered	-20°C (-4°F)	20 - 30 J	15 - 22 ft-lbf	ASTM E23

The combination of high tensile and yield strength makes EN31 suitable for applications involving dynamic loads, such as bearings and gears, where resistance to deformation is critical.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	Room Temp	7.85 g/cm³	0.284 lb/in³
Melting Point	-	1425 - 1540 °C	2600 - 2800 °F
Thermal Conductivity	Room Temp	45 W/m·K	31.2 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temp	460 J/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	Room Temp	0.0006 Ω·m	0.000006 Ω·in

The density of EN31 contributes to its weight and strength, while the thermal conductivity is significant for applications involving heat dissipation. The specific heat capacity indicates how much energy is required to raise the temperature, which is crucial in processes involving thermal cycling.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Water	0 - 100	20 - 100 / 68 - 212	Fair	Risk of rusting
Acids	0 - 10	20 - 100 / 68 - 212	Poor	Susceptible to pitting
Chlorides	0 - 5	20 - 100 / 68 - 212	Poor	Risk of stress corrosion cracking
Alkalis	0 - 10	20 - 100 / 68 - 212	Fair	Moderate resistance

EN31 steel exhibits limited corrosion resistance, particularly in acidic and chloride environments, making it less suitable for applications exposed to harsh conditions without protective coatings. Compared to stainless steels, such as AISI 304 or AISI 316, EN31's susceptibility to corrosion is significantly higher, necessitating protective measures in corrosive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	200	392	Beyond this, properties may degrade
Max Intermittent Service Temp	300	572	Short-term exposure only
Scaling Temperature	600	1112	Risk of oxidation at higher temperatures

At elevated temperatures, EN31 maintains its hardness and strength up to a certain limit, but prolonged exposure can lead to oxidation and loss of mechanical properties. Proper heat treatment can enhance its performance in high-temperature applications.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG	ER70S-6	Argon + CO2	Preheat recommended
TIG	ER70S-2	Argon	Requires careful control
Stick	E7018	N/A	Post-weld heat treatment recommended

EN31 is not typically recommended for welding due to its high carbon content, which can lead to cracking. Preheating and post-weld heat treatment are essential to mitigate these risks.

Machinability

Machining Parameter	EN31	AISI 1212	Notes/Tips
Relative Machinability Index	60%	100%	EN31 is more challenging to machine due to hardness.
Typical Cutting Speed (Turning)	30-50 m/min	60-80 m/min	Use carbide tools for best results.

Machining EN31 requires careful consideration of tooling and cutting speeds due to its hardness. Carbide tools are recommended to achieve optimal results.

Formability

EN31 exhibits limited formability due to its high carbon content, making it less suitable for processes requiring significant deformation. Cold forming is possible but should be approached with caution to avoid cracking. Hot forming can be performed at elevated temperatures to improve ductility.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	600 - 700 / 1112 - 1292	1 - 2 hours	Air	Reduce hardness, improve machinability
Quenching	800 - 850 / 1472 - 1562	30 minutes	Oil or Water	Increase hardness and strength
Tempering	150 - 200 / 302 - 392	1 hour	Air	Reduce brittleness, improve toughness

The heat treatment processes significantly affect the microstructure of EN31, transforming it from a softer, more ductile state to a hard, wear-resistant condition. Proper tempering is crucial to balance hardness and toughness.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Automotive	Ball bearings	High hardness, wear resistance	Essential for durability
Aerospace	Gear shafts	High tensile strength, fatigue resistance	Critical for safety
Manufacturing	Tooling components	Toughness, machinability	Required for precision
Oil & Gas	Valve components	Corrosion resistance, strength	Essential in harsh environments

Other applications include:

• Industrial machinery: Used in components requiring high wear resistance.
• Robotics: For parts that endure high stress and require precision.
• Medical devices: In applications where strength and reliability are paramount.

The selection of EN31 for these applications is primarily due to its excellent mechanical properties, which ensure reliability and performance under demanding conditions.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	EN31	AISI 4140	AISI 440C	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High hardness	Moderate hardness	High hardness	EN31 offers superior wear resistance but lower toughness than 4140.
Key Corrosion Aspect	Fair	Good	Excellent	EN31 is less corrosion-resistant than 440C, which is critical in certain environments.
Weldability	Poor	Fair	Good	EN31 requires careful welding practices compared to 4140.
Machinability	Moderate	Good	Poor	EN31 is harder to machine than 4140, requiring specialized tools.
Formability	Limited	Good	Limited	EN31 is less formable than 4140, which can be advantageous in certain applications.
Approx. Relative Cost	Moderate	Moderate	Higher	Cost considerations may influence selection based on project budgets.
Typical Availability	Common	Common	Less common	EN31 is widely available, while 440C may be harder to source.

When selecting EN31, considerations include the specific application requirements, cost-effectiveness, and availability. Its high hardness and wear resistance make it ideal for applications where durability is paramount, while its limitations in corrosion resistance and weldability must be addressed through appropriate design and processing choices.

In conclusion, EN31 steel is a versatile and robust material that plays a critical role in various engineering applications, particularly where high performance and reliability are essential. Understanding its properties and limitations allows engineers to make informed decisions for optimal application outcomes.