440 Stainless Steel: Properties and Key Applications

440 stainless steel is a high-carbon martensitic stainless steel known for its excellent hardness and wear resistance. Classified under the martensitic stainless steel category, it typically contains 16-18% chromium and 1.0-1.2% carbon, which significantly influences its mechanical properties and corrosion resistance. The high carbon content allows for the formation of a hard martensitic structure upon heat treatment, making it suitable for applications requiring high strength and wear resistance.

Comprehensive Overview

440 stainless steel is primarily used in applications where high hardness and moderate corrosion resistance are essential. Its unique combination of properties makes it a popular choice for cutlery, surgical instruments, and various industrial applications. The steel can be categorized into three sub-grades: 440A, 440B, and 440C, each with slight variations in carbon content and hardness.

Advantages:
- High Hardness: 440 stainless steel can achieve a hardness of up to 58 HRC when properly heat-treated, making it ideal for cutting tools and wear-resistant applications.
- Good Corrosion Resistance: While not as corrosion-resistant as austenitic grades, 440 stainless steel offers decent resistance to oxidation and corrosion in mild environments.
- Edge Retention: Its hardness allows for excellent edge retention in cutting applications, making it a preferred choice for knives and blades.

Limitations:
- Brittleness: The high carbon content can lead to brittleness, especially in thinner sections, which may limit its use in certain applications.
- Weldability Issues: 440 stainless steel can be challenging to weld due to its high carbon content, which may lead to cracking in the heat-affected zone.
- Moderate Corrosion Resistance: Compared to austenitic stainless steels, its corrosion resistance is limited, particularly in harsh environments.

Historically, 440 stainless steel has been significant in the development of high-performance cutlery and surgical instruments, establishing itself as a reliable material in various industries.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S44000	USA	Closest equivalent to AISI 440A/B/C
AISI/SAE	440A, 440B, 440C	USA	Variations in carbon content affect hardness
ASTM	A276	USA	Standard specification for stainless steel bars
EN	1.4116	Europe	Equivalent to AISI 440C
JIS	SUS440A, SUS440B, SUS440C	Japan	Similar properties with minor compositional differences

The differences between these equivalent grades can affect selection based on specific application requirements. For instance, while 440C offers higher hardness due to its carbon content, it may also be more susceptible to corrosion in certain environments compared to 440A.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.95 - 1.20
Cr (Chromium)	16.0 - 18.0
Mn (Manganese)	1.0 max
Si (Silicon)	1.0 max
P (Phosphorus)	0.04 max
S (Sulfur)	0.03 max

The primary alloying elements in 440 stainless steel include chromium and carbon. Chromium enhances corrosion resistance and contributes to the formation of a protective oxide layer, while carbon increases hardness and strength through the formation of martensite during heat treatment.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Annealed	620 - 850 MPa	90 - 123 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	450 - 600 MPa	65 - 87 ksi	ASTM E8
Elongation	Annealed	12 - 15%	12 - 15%	ASTM E8
Hardness	Annealed	30 - 40 HRC	30 - 40 HRC	ASTM E18
Impact Strength	-	20 J (at -20°C)	15 ft-lbf (at -4°F)	ASTM E23

The mechanical properties of 440 stainless steel make it suitable for applications that require high strength and wear resistance. Its tensile strength and yield strength indicate its ability to withstand significant loads, while its hardness ensures durability in cutting applications.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	-	7.75 g/cm³	0.28 lb/in³
Melting Point	-	1425 - 1540 °C	2600 - 2800 °F
Thermal Conductivity	20 °C	25.4 W/m·K	17.5 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	20 °C	0.50 J/g·K	0.12 BTU/lb·°F
Electrical Resistivity	20 °C	0.74 µΩ·m	0.0000013 Ω·in

Key physical properties such as density and melting point are crucial for applications involving high-temperature environments. The density indicates the material's weight, while the melting point provides insight into its thermal stability.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3-10	20-60 / 68-140	Fair	Risk of pitting
Sulfuric Acid	10-30	20-40 / 68-104	Poor	Not recommended
Acetic Acid	5-20	20-60 / 68-140	Good	Moderate resistance
Atmospheric	-	-	Good	Performs well in mild environments

440 stainless steel exhibits moderate corrosion resistance, particularly in atmospheric conditions and diluted acids. However, it is susceptible to pitting corrosion in chloride environments, which is a critical consideration for applications in marine or coastal settings. Compared to austenitic grades like 304 or 316, 440 stainless steel is less resistant to corrosion but offers superior hardness.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400	752	Above this, oxidation may occur
Max Intermittent Service Temp	600	1112	Short-term exposure only
Scaling Temperature	800	1472	Risk of scaling beyond this temp

At elevated temperatures, 440 stainless steel maintains its strength but may experience oxidation. Its performance in high-temperature applications is limited compared to other stainless steels, necessitating careful consideration in design.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER440 (AWS A5.9)	Argon	Preheat recommended
MIG	ER440 (AWS A5.9)	Argon + CO2	Post-weld heat treatment may be necessary

Welding 440 stainless steel can be challenging due to its high carbon content, which increases the risk of cracking. Preheating and post-weld heat treatment are often recommended to mitigate these issues.

Machinability

Machining Parameter	[440 Stainless Steel]	[AISI 1212]	Notes/Tips
Relative Machinability Index	60%	100%	Requires high-speed tooling
Typical Cutting Speed (Turning)	30-50 m/min	100-150 m/min	Use carbide tools for best results

440 stainless steel has moderate machinability, requiring specific tooling and cutting speeds to achieve optimal results. The use of carbide tools is recommended to enhance performance.

Formability

440 stainless steel is not highly formable due to its high carbon content, which can lead to cracking during cold working. Hot forming is possible but requires careful temperature control to avoid degradation of properties.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	760 - 800 / 1400 - 1472	1-2 hours	Air	Reduce hardness, improve ductility
Hardening	980 - 1050 / 1800 - 1922	30 minutes	Oil or Air	Increase hardness and strength
Tempering	150 - 400 / 300 - 750	1 hour	Air	Reduce brittleness, improve toughness

Heat treatment processes significantly impact the microstructure and properties of 440 stainless steel. Hardening increases strength and hardness, while tempering helps alleviate brittleness.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Cutlery	Kitchen knives	High hardness, edge retention	Excellent for cutting tools
Medical	Surgical instruments	Corrosion resistance, sterilization capability	Safe for medical use
Automotive	Valve components	Wear resistance, strength	High-performance requirements
Aerospace	Engine components	High strength-to-weight ratio	Critical for safety and performance

440 stainless steel is chosen for applications requiring high hardness and moderate corrosion resistance. Its edge retention makes it particularly suitable for cutlery, while its strength is advantageous in automotive and aerospace applications.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	440 Stainless Steel	AISI 304 Stainless Steel	AISI 316 Stainless Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High hardness	Good ductility	Excellent corrosion resistance	440 is harder but less ductile
Key Corrosion Aspect	Moderate resistance	Good resistance	Excellent resistance	440 is less suitable for harsh environments
Weldability	Poor	Good	Good	440 requires special care in welding
Machinability	Moderate	Good	Moderate	440 requires more effort to machine
Formability	Poor	Good	Good	440 is less formable than austenitic grades
Approx. Relative Cost	Moderate	Low	Moderate to High	Cost varies based on market conditions
Typical Availability	Moderate	High	High	440 may be less readily available

When selecting 440 stainless steel, considerations include its mechanical properties, corrosion resistance, and fabrication characteristics. While it offers high hardness, its limitations in weldability and formability must be accounted for in design and application. Cost-effectiveness and availability also play crucial roles in material selection, particularly in competitive industries.

In summary, 440 stainless steel is a versatile material that excels in applications requiring high hardness and wear resistance, making it a valuable choice in various industries despite its limitations in corrosion resistance and weldability.