Bearing Steel: Properties and Key Applications

Bearing steel is a specialized category of steel designed primarily for the manufacture of rolling element bearings. These steels are characterized by their high hardness, wear resistance, and ability to maintain dimensional stability under load. Typically classified as high-carbon alloy steels, bearing steels often contain alloying elements such as chromium, manganese, and molybdenum, which enhance their mechanical properties and performance in demanding applications.

Comprehensive Overview

Bearing steels are engineered to withstand high levels of stress and friction, making them essential in various mechanical applications, including automotive, aerospace, and industrial machinery. The primary alloying elements in bearing steels include:

• Carbon (C): Increases hardness and strength through heat treatment.
• Chromium (Cr): Enhances hardenability and corrosion resistance, contributing to the steel's overall durability.
• Manganese (Mn): Improves toughness and wear resistance.
• Molybdenum (Mo): Increases strength at elevated temperatures and enhances hardenability.

The most significant characteristics of bearing steels include:

• High Hardness: Achieved through heat treatment, allowing for excellent wear resistance.
• Dimensional Stability: Maintains shape and size under load, crucial for precision applications.
• Fatigue Resistance: Capable of enduring cyclic loading without failure.

Advantages:
- Excellent wear resistance and durability.
- High load-bearing capacity.
- Good machinability and heat treatment response.

Limitations:
- Prone to corrosion if not properly treated or coated.
- Higher cost compared to standard carbon steels.
- Requires precise heat treatment to achieve desired properties.

Historically, bearing steels have played a critical role in the development of machinery and vehicles, evolving from simple carbon steels to advanced alloy compositions that meet modern engineering demands.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	GCr15	USA	Closest equivalent to AISI 52100
AISI/SAE	52100	USA	Commonly used bearing steel
ASTM	A295	USA	Specification for high-carbon chromium bearing steel
EN	100Cr6	Europe	Equivalent to AISI 52100 with slight compositional differences
JIS	SUJ2	Japan	Similar properties, often used in Japanese applications
GB	GCr15	China	Equivalent to AISI 52100

The subtle differences between these grades can affect performance in specific applications. For example, while GCr15 and AISI 52100 are nearly identical, GCr15 may have slightly different impurity levels, which can influence fatigue resistance.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.95 - 1.05
Cr (Chromium)	1.30 - 1.65
Mn (Manganese)	0.30 - 0.50
Mo (Molybdenum)	0.10 - 0.30
Si (Silicon)	0.15 - 0.40
P (Phosphorus)	≤ 0.025
S (Sulfur)	≤ 0.025

The primary role of the key alloying elements in bearing steel is as follows:

• Carbon: Essential for achieving high hardness and strength through heat treatment.
• Chromium: Provides hardenability and enhances resistance to wear and corrosion.
• Manganese: Improves toughness and helps in the deoxidation process during steelmaking.
• Molybdenum: Enhances strength at elevated temperatures and contributes to the overall toughness of the steel.

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	Quenched & Tempered	Room Temp	58 - 65 HRC	58 - 65 HRC	ASTM E18
Impact Strength	Quenched & Tempered	-20°C (-4°F)	20 - 30 J	15 - 22 ft-lbf	ASTM E23

The combination of these mechanical properties makes bearing steel particularly suitable for applications involving high cyclic loads and wear resistance, such as in rolling bearings, where both tensile and yield strength are critical for performance.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	Room Temp	7.85 g/cm³	0.284 lb/in³
Melting Point	-	1420 - 1540 °C	2590 - 2810 °F
Thermal Conductivity	Room Temp	45 W/m·K	31 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	0.00002 Ω·in

Key physical properties such as density and thermal conductivity are significant for applications where weight and heat dissipation are critical. The high melting point indicates good thermal stability, making bearing steel suitable for high-temperature environments.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3-5	25°C (77°F)	Fair	Risk of pitting corrosion
Sulfuric Acid	10-20	25°C (77°F)	Poor	Not recommended
Sea Water	-	25°C (77°F)	Fair	Requires protective coating
Atmospheric	-	-	Good	Susceptible to rust

Bearing steels generally exhibit moderate corrosion resistance. They are susceptible to pitting in chloride environments and should be protected with coatings or surface treatments in corrosive applications. Compared to stainless steels, bearing steels have lower corrosion resistance, making them less suitable for environments with high humidity or corrosive agents.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	150°C	302°F	Beyond this, properties degrade
Max Intermittent Service Temp	200°C	392°F	Short-term exposure only
Scaling Temperature	300°C	572°F	Risk of oxidation beyond this

Bearing steels maintain their mechanical properties up to moderate temperatures but can begin to lose hardness and strength at elevated temperatures. Oxidation can occur at high temperatures, necessitating protective measures 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	ER80S-Ni	Argon	Requires post-weld heat treatment
Stick	E7018	-	Not ideal for thick sections

Bearing steels are generally not recommended for welding due to their high carbon content, which can lead to cracking. Preheating and post-weld heat treatment are essential to mitigate these risks.

Machinability

Machining Parameter	Bearing Steel (AISI 52100)	Benchmark Steel (AISI 1212)	Notes/Tips
Relative Machinability Index	60%	100%	Requires high-speed tooling
Typical Cutting Speed (Turning)	30-50 m/min	60-80 m/min	Use carbide tools for best results

Machinability is moderate; high-speed steel or carbide tools are recommended for effective machining. Proper cooling and lubrication are critical to prevent overheating and tool wear.

Formability

Bearing steels are not typically formed due to their high hardness and strength. Cold forming is limited, and hot forming is generally avoided due to the risk of altering the desired microstructure.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Hardening	800 - 850 °C / 1472 - 1562 °F	1 - 2 hours	Oil or Air	Achieve high hardness
Tempering	150 - 200 °C / 302 - 392 °F	1 - 2 hours	Air	Reduce brittleness, improve toughness

Heat treatment is crucial for achieving the desired hardness and microstructure in bearing steels. The hardening process increases hardness significantly, while tempering reduces brittleness, enhancing toughness for better performance under load.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Automotive	Wheel bearings	High hardness, fatigue resistance	Essential for durability
Aerospace	Engine components	High strength, dimensional stability	Critical for safety
Industrial	Gearboxes	Wear resistance, load-bearing capacity	Ensures long service life
Robotics	Actuator bearings	High precision, low friction	Necessary for performance

Other applications include:
- Railway: Axle bearings for trains.
- Marine: Propeller shaft bearings.
- Construction: Heavy machinery bearings.

Bearing steel is chosen for its ability to withstand high loads and provide long service life, making it indispensable in critical applications.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	Bearing Steel (AISI 52100)	Alternative Grade 1 (AISI 440C)	Alternative Grade 2 (AISI 316)	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High hardness	Good corrosion resistance	Excellent corrosion resistance	Trade-off between hardness and corrosion resistance
Key Corrosion Aspect	Fair	Excellent	Excellent	Bearing steel is less resistant to corrosion
Weldability	Poor	Fair	Good	Bearing steel requires special considerations for welding
Machinability	Moderate	Good	Fair	Bearing steel is more challenging to machine
Formability	Poor	Fair	Good	Alternative grades may offer better forming capabilities
Approx. Relative Cost	Moderate	Higher	Higher	Cost varies significantly based on alloying elements
Typical Availability	Common	Common	Very common	Availability can affect project timelines

When selecting bearing steel, consider factors such as mechanical properties, corrosion resistance, and cost-effectiveness. While bearing steel excels in hardness and wear resistance, alternative grades may be more suitable for applications requiring high corrosion resistance or better weldability.

In conclusion, bearing steel is a vital material in engineering applications where high performance and reliability are paramount. Understanding its properties, advantages, and limitations is essential for making informed material selection decisions in various industries.