446 Stainless Steel: Properties and Key Applications

446 stainless steel is classified as an austenitic stainless steel, notable for its high chromium content and the addition of molybdenum, which enhances its resistance to oxidation and corrosion. This steel grade is primarily composed of chromium (24-27%) and nickel (19-22%), with molybdenum (up to 1.5%) as a significant alloying element. The high chromium content provides excellent resistance to oxidation and high-temperature environments, while nickel contributes to its toughness and ductility.

Comprehensive Overview

446 stainless steel is particularly recognized for its exceptional high-temperature strength and oxidation resistance, making it suitable for applications in environments where other stainless steels may fail. Its ability to maintain mechanical properties at elevated temperatures, combined with good weldability and formability, makes it a preferred choice in various industrial applications, especially in the petrochemical and power generation sectors.

Advantages:
- High-Temperature Resistance: Capable of withstanding temperatures up to 1200°C (2192°F) without significant degradation.
- Corrosion Resistance: Excellent resistance to oxidation and scaling in high-temperature environments.
- Durability: Good mechanical properties, including strength and ductility, contribute to its longevity in service.

Limitations:
- Cost: Higher alloy content can lead to increased material costs compared to lower-grade stainless steels.
- Weldability Challenges: While it can be welded, care must be taken to avoid issues such as hot cracking.
- Limited Availability: Not as commonly stocked as other stainless steel grades, which may affect lead times.

446 stainless steel has a strong market position in specialized applications, particularly where high-temperature performance is critical. Its historical significance lies in its development for use in furnace components, heat exchangers, and other high-temperature applications.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S44600	USA	Closest equivalent to EN 1.4762
AISI/SAE	446	USA	Commonly used designation
ASTM	A240	USA	Standard specification for stainless steel plates
EN	1.4762	Europe	Minor compositional differences to be aware of
JIS	SUS446	Japan	Equivalent grade with similar properties
GB	00Cr25Ni20	China	Comparable grade with slight variations

The differences between these equivalent grades often lie in minor compositional variations that can affect performance in specific environments. For instance, while both UNS S44600 and EN 1.4762 offer similar high-temperature resistance, the specific alloying elements and their percentages can influence corrosion resistance and mechanical properties.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Cr (Chromium)	24.0 - 27.0
Ni (Nickel)	19.0 - 22.0
Mo (Molybdenum)	0.5 - 1.5
C (Carbon)	≤ 0.03
Mn (Manganese)	≤ 1.0
Si (Silicon)	≤ 1.0
P (Phosphorus)	≤ 0.045
S (Sulfur)	≤ 0.03

The primary role of chromium in 446 stainless steel is to enhance corrosion resistance and oxidation resistance at elevated temperatures. Nickel contributes to the steel's toughness and ductility, while molybdenum improves resistance to pitting and crevice corrosion, particularly in chloride environments.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Annealed	515 - 690 MPa	75 - 100 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	205 - 310 MPa	30 - 45 ksi	ASTM E8
Elongation	Annealed	40 - 50%	40 - 50%	ASTM E8
Hardness (Rockwell B)	Annealed	85 - 95 HRB	85 - 95 HRB	ASTM E18
Impact Strength (Charpy V-notch)	-20°C	40 J	30 ft-lbf	ASTM E23

The combination of high tensile and yield strength, along with good elongation, makes 446 stainless steel suitable for applications that require structural integrity under mechanical loading. Its impact strength at low temperatures also ensures reliability in colder environments.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	Room Temperature	7.8 g/cm³	0.283 lb/in³
Melting Point/Range	-	1400 - 1450 °C	2552 - 2642 °F
Thermal Conductivity	Room Temperature	25 W/m·K	17.3 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	Room Temperature	500 J/kg·K	0.119 BTU/lb·°F
Electrical Resistivity	Room Temperature	0.73 µΩ·m	0.0000013 Ω·in
Coefficient of Thermal Expansion	20 - 100 °C	16.5 x 10⁻⁶ /K	9.2 x 10⁻⁶ /°F

The density of 446 stainless steel indicates its robustness, while its melting point signifies its suitability for high-temperature applications. The thermal conductivity and specific heat capacity are critical for applications involving heat exchangers, where efficient heat transfer is essential.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Sulfuric Acid	10-20	25/77	Fair	Risk of pitting
Chlorides	3-5	60/140	Good	Susceptible to pitting
Sea Water	-	25/77	Excellent	Good resistance
Hydrochloric Acid	5-10	25/77	Poor	Not recommended

446 stainless steel exhibits excellent resistance to various corrosive environments, particularly in high-temperature applications. Its performance in chloride-rich environments is noteworthy, although it is susceptible to pitting corrosion. Compared to grades like 304 and 316 stainless steel, 446 offers superior oxidation resistance but may not perform as well in reducing environments or in the presence of strong acids.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	1200	2192	Optimal for high-temperature applications
Max Intermittent Service Temp	1300	2372	Short-term exposure only
Scaling Temperature	1150	2102	Begins to oxidize significantly
Creep Strength Considerations Begin	800	1472	Creep may become a concern

446 stainless steel maintains its mechanical properties at elevated temperatures, making it ideal for applications such as furnace components and heat exchangers. Its oxidation resistance allows it to perform well in environments where other steels may fail due to scaling or oxidation.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER446	Argon	Preheat recommended
MIG	ER446	Argon + 2% O₂	Good for thin sections
Stick	E446	-	Requires careful control

Weldability of 446 stainless steel is generally good, but it requires careful attention to avoid hot cracking. Preheating before welding can help mitigate this risk, and post-weld heat treatment may be necessary to relieve stresses.

Machinability

Machining Parameter	[446 Stainless Steel]	Benchmark Steel (AISI 1212)	Notes/Tips
Relative Machinability Index	30%	100%	Requires slower speeds
Typical Cutting Speed (Turning)	20 m/min	60 m/min	Use carbide tools

Machinability of 446 stainless steel is lower than that of more common grades like AISI 1212. It is advisable to use carbide tooling and slower cutting speeds to achieve optimal results.

Formability

446 stainless steel exhibits moderate formability. Cold forming is possible, but care must be taken to avoid work hardening. Hot forming is preferred for complex shapes, and recommended bend radii should be adhered to in fabrication.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	1050 - 1150 / 1922 - 2102	1 - 2 hours	Air	Relieve stresses, improve ductility
Solution Treatment	1000 - 1100 / 1832 - 2012	1 hour	Water	Enhance corrosion resistance

During heat treatment, 446 stainless steel undergoes metallurgical transformations that enhance its microstructure, improving its mechanical properties and corrosion resistance.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Power Generation	Boiler Tubes	High-temperature strength, oxidation resistance	Essential for high-efficiency systems
Petrochemical	Furnace Components	Corrosion resistance, durability	Critical for long-term operation
Aerospace	Exhaust Systems	High-temperature performance	Required for extreme conditions

Other applications include:
* Heat exchangers
* Industrial furnaces
* Chemical processing equipment

The selection of 446 stainless steel for these applications is primarily due to its ability to withstand high temperatures and corrosive environments, ensuring reliability and longevity.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	446 Stainless Steel	304 Stainless Steel	316 Stainless Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High tensile strength	Moderate tensile strength	Moderate tensile strength	446 offers superior high-temp performance
Key Corrosion Aspect	Excellent oxidation resistance	Good general corrosion resistance	Better resistance to chlorides	446 is less effective in reducing environments
Weldability	Moderate	Good	Good	446 requires more care during welding
Machinability	Moderate	Good	Good	446 is harder to machine than 304/316
Formability	Moderate	Good	Good	446 is less formable than 304/316
Approx. Relative Cost	Higher	Lower	Moderate	Cost can be a deciding factor in selection
Typical Availability	Limited	Widely available	Widely available	Availability may affect project timelines

When selecting 446 stainless steel, considerations such as cost-effectiveness, availability, and specific application requirements must be taken into account. Its unique properties make it suitable for niche applications where high-temperature performance is critical, but its higher cost and limited availability may necessitate careful planning and sourcing strategies.

In summary, 446 stainless steel is a high-performance material that excels in demanding environments, making it a valuable choice for industries requiring durability and reliability under extreme conditions.