9840 Steel: Properties and Key Applications

9840 steel is classified as a medium-carbon alloy steel, primarily known for its excellent hardenability and strength. It is part of the AISI/SAE 9000 series of steels, which are designed for a variety of engineering applications. The primary alloying elements in 9840 steel include carbon (C), manganese (Mn), chromium (Cr), and molybdenum (Mo). These elements significantly influence the steel's mechanical properties, such as tensile strength, toughness, and wear resistance.

Comprehensive Overview

The unique combination of alloying elements in 9840 steel contributes to its notable characteristics. The carbon content typically ranges from 0.36% to 0.44%, which provides a good balance between strength and ductility. Manganese enhances hardenability and tensile strength, while chromium and molybdenum improve corrosion resistance and toughness, particularly at elevated temperatures.

Advantages of 9840 Steel:
- High Strength and Toughness: Suitable for applications requiring high load-bearing capacity.
- Good Hardening Response: Can achieve high hardness levels through heat treatment.
- Wear Resistance: Ideal for components subjected to abrasive wear.

Limitations of 9840 Steel:
- Weldability Issues: Requires careful consideration during welding to avoid cracking.
- Cost: Generally more expensive than low-carbon steels due to alloying elements.
- Machinability: Can be more challenging to machine compared to simpler steel grades.

Historically, 9840 steel has been used in various applications, including automotive and aerospace components, due to its favorable mechanical properties. Its market position is solid, particularly in industries that demand high-performance materials.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G98400	USA	Closest equivalent to AISI 4140
AISI/SAE	9840	USA	Medium-carbon alloy steel
ASTM	A829	USA	Standard specification for alloy steel
EN	1.6511	Europe	Minor compositional differences
DIN	34CrMo4	Germany	Similar properties, but different alloying elements
JIS	SCM440	Japan	Comparable performance, but with different heat treatment requirements

The table above highlights various designations and standards associated with 9840 steel. Notably, while G98400 and 4140 are often considered equivalents, the differences in alloying elements and heat treatment processes can lead to variations in performance, particularly in high-stress applications.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.36 - 0.44
Mn (Manganese)	0.60 - 0.90
Cr (Chromium)	0.80 - 1.10
Mo (Molybdenum)	0.15 - 0.25
Si (Silicon)	0.15 - 0.40
P (Phosphorus)	≤ 0.035
S (Sulfur)	≤ 0.040

The primary alloying elements in 9840 steel play crucial roles:
- Carbon (C): Enhances hardness and strength through heat treatment.
- Manganese (Mn): Improves hardenability and tensile strength.
- Chromium (Cr): Increases corrosion resistance and toughness.
- Molybdenum (Mo): Enhances high-temperature strength and hardenability.

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	850 - 1000 MPa	123 - 145 ksi	ASTM E8
Yield Strength (0.2% offset)	Quenched & Tempered	Room Temp	600 - 800 MPa	87 - 116 ksi	ASTM E8
Elongation	Quenched & Tempered	Room Temp	15 - 20%	15 - 20%	ASTM E8
Hardness (Rockwell C)	Quenched & Tempered	Room Temp	28 - 34 HRC	28 - 34 HRC	ASTM E18
Impact Strength (Charpy)	Quenched & Tempered	-20 °C	30 - 50 J	22 - 37 ft-lbf	ASTM E23

The mechanical properties of 9840 steel make it suitable for applications that require high strength and toughness. Its ability to withstand significant loads and resist deformation under stress is critical for components in machinery and structural applications.

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 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temp	460 J/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	Room Temp	0.0000017 Ω·m	0.0000017 Ω·in

Key physical properties such as density and thermal conductivity are essential for applications involving heat treatment and thermal management. The density indicates the material's weight, while thermal conductivity affects how heat is dissipated in high-performance environments.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C)	Resistance Rating	Notes
Chlorides	3-10	20-60	Fair	Risk of pitting corrosion
Sulfuric Acid	10-30	25-50	Poor	Not recommended
Sodium Hydroxide	5-20	20-80	Good	Moderate resistance
Atmospheric	-	-	Good	Generally resistant

9840 steel exhibits moderate corrosion resistance, particularly in atmospheric environments. However, it is susceptible to pitting corrosion in chloride-rich environments and should be protected or coated when used in such conditions. Compared to other grades like AISI 4140, 9840 may show slightly lower resistance to certain corrosive agents due to its composition.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400 °C	752 °F	Suitable for prolonged exposure
Max Intermittent Service Temp	500 °C	932 °F	Short-term exposure
Scaling Temperature	600 °C	1112 °F	Risk of oxidation at high temps

At elevated temperatures, 9840 steel maintains its strength and hardness but may experience oxidation if not properly protected. Its performance in high-temperature applications makes it suitable for components in engines and turbines.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG	ER80S-D2	Argon + CO2	Preheat recommended
TIG	ER80S-D2	Argon	Requires post-weld heat treatment
Stick	E8018-B2	-	Careful control of heat input

Weldability of 9840 steel can be challenging due to its alloying elements. Preheating is often necessary to prevent cracking, and post-weld heat treatment is recommended to relieve stresses.

Machinability

Machining Parameter	[9840 Steel]	[AISI 1212]	Notes/Tips
Relative Machinability Index	60	100	More difficult to machine
Typical Cutting Speed (Turning)	30 m/min	50 m/min	Adjust tooling accordingly

Machinability of 9840 steel is moderate, requiring appropriate tooling and cutting speeds to achieve optimal results. The presence of alloying elements can lead to increased tool wear.

Formability

9840 steel exhibits moderate formability, allowing for both cold and hot forming processes. However, it is essential to consider work hardening effects during cold forming, as this can lead to increased strength but reduced ductility.

Heat Treatment

Treatment Process	Temperature Range (°C)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	700 - 800	1 - 2 hours	Air	Softening, improving ductility
Quenching	800 - 900	30 minutes	Oil or Water	Hardening, increasing strength
Tempering	400 - 600	1 hour	Air	Reducing brittleness, improving toughness

Heat treatment processes significantly affect the microstructure and properties of 9840 steel. Quenching increases hardness, while tempering balances strength and ductility, making it suitable for various applications.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection
Automotive	Gears and Shafts	High strength, toughness	Required for load-bearing components
Aerospace	Aircraft Components	High strength-to-weight ratio	Essential for performance and safety
Oil & Gas	Drill Bits	Wear resistance, toughness	Critical for harsh environments
Machinery	Crankshafts	High fatigue resistance	Necessary for durability

Other applications include:
- Structural components in heavy machinery
- Tooling and dies
- Fasteners and fittings

The selection of 9840 steel for these applications is driven by its mechanical properties, which provide the necessary strength and durability for demanding environments.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	[9840 Steel]	[AISI 4140]	[AISI 4340]	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	High strength	Higher toughness	9840 is more cost-effective
Key Corrosion Aspect	Moderate	Good	Fair	4140 offers better corrosion resistance
Weldability	Moderate	Good	Fair	9840 requires preheating
Machinability	Moderate	Good	Fair	4140 is easier to machine
Formability	Moderate	Good	Fair	9840 has better work hardening
Approx. Relative Cost	Moderate	Higher	Higher	9840 is often more affordable
Typical Availability	Common	Common	Less common	9840 is widely available

When selecting 9840 steel, considerations include cost-effectiveness, availability, and specific application requirements. Its balance of properties makes it a versatile choice for various engineering applications, though careful attention must be paid to welding and machining processes to ensure optimal performance.