400 Series Stainless Steel: Properties and Key Applications

The 400 Series Stainless Steel is a category of stainless steel that primarily consists of ferritic and martensitic stainless steels. These grades are characterized by their high chromium content, typically ranging from 11% to 30%, which provides excellent corrosion resistance and high-temperature strength. The primary alloying elements in the 400 Series include chromium, carbon, and, in some cases, nickel. The presence of chromium is crucial as it forms a passive layer on the steel surface, enhancing its resistance to oxidation and corrosion.

Comprehensive Overview

The 400 Series is classified into two main types: ferritic and martensitic stainless steels. Ferritic grades, such as 430, are known for their good corrosion resistance and formability, while martensitic grades, like 410 and 420, offer higher strength and hardness but are less resistant to corrosion. The balance of chromium and carbon in these steels influences their mechanical properties, making them suitable for various applications.

Significant Characteristics:
- Corrosion Resistance: Generally good, but varies by specific grade.
- Strength and Hardness: Martensitic grades exhibit higher strength and hardness due to their carbon content.
- Weldability: Varies significantly; ferritic grades are more weldable than martensitic grades.
- Magnetic Properties: Ferritic grades are magnetic, while martensitic grades can be magnetic depending on their heat treatment.

Advantages:
- High strength and hardness (especially martensitic grades).
- Good resistance to oxidation and scaling at elevated temperatures.
- Cost-effective compared to austenitic stainless steels.

Limitations:
- Limited corrosion resistance compared to austenitic grades.
- Susceptibility to stress corrosion cracking in certain environments.
- Lower ductility and toughness, particularly in martensitic grades.

Historically, the 400 Series has been significant in applications requiring moderate corrosion resistance and high strength, such as automotive components, kitchen utensils, and industrial equipment.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S41000	USA	Martensitic, good hardness
AISI/SAE	410	USA	Commonly used for cutlery
ASTM	A240	USA	Standard specification for stainless steel plates
EN	1.4006	Europe	Ferritic grade, good formability
DIN	X20Cr13	Germany	Similar to AISI 410, with minor compositional differences
JIS	SUS410	Japan	Equivalent to AISI 410
GB	0Cr13	China	Equivalent to AISI 410

The differences between these grades can affect selection based on specific performance requirements. For instance, while UNS S41000 and AISI 410 are equivalent in terms of mechanical properties, the specific processing and heat treatment can lead to variations in performance.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Cr (Chromium)	11.5 - 13.5
C (Carbon)	0.08 max
Ni (Nickel)	0.75 max
Mn (Manganese)	1.0 max
Si (Silicon)	1.0 max
P (Phosphorus)	0.04 max
S (Sulfur)	0.03 max

Chromium is the primary alloying element that enhances corrosion resistance and oxidation resistance. Carbon increases hardness and strength, particularly in martensitic grades. Nickel, while present in low amounts, can improve toughness and ductility.

Mechanical Properties

Property	Condition/Temper	Test Temperature	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Tensile Strength	Annealed	Room Temp	480 - 620 MPa	70 - 90 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	Room Temp	275 - 410 MPa	40 - 60 ksi	ASTM E8
Elongation	Annealed	Room Temp	20 - 30%	20 - 30%	ASTM E8
Hardness (Rockwell C)	Annealed	Room Temp	20 - 30 HRC	20 - 30 HRC	ASTM E18
Impact Strength	Annealed	-20°C (-4°F)	30 - 50 J	22 - 37 ft-lbf	ASTM E23

The combination of these mechanical properties makes the 400 Series suitable for applications requiring high strength and moderate corrosion resistance, such as in the automotive and aerospace industries.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	Room Temp	7.75 g/cm³	0.28 lb/in³
Melting Point/Range	-	1400 - 1450 °C	2550 - 2642 °F
Thermal Conductivity	Room Temp	25 W/m·K	14.5 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temp	500 J/kg·K	0.12 BTU/lb·°F
Electrical Resistivity	Room Temp	0.73 µΩ·m	0.0000013 Ω·in
Coefficient of Thermal Expansion	20 - 100 °C	10.5 x 10⁻⁶/K	5.8 x 10⁻⁶/°F

Key physical properties such as density and melting point are significant for applications involving high-temperature environments. The thermal conductivity indicates how well the material can dissipate heat, which is crucial in applications like exhaust systems.

Corrosion Resistance

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

The 400 Series exhibits varying degrees of corrosion resistance depending on the environment. While it performs well in atmospheric conditions, it is susceptible to pitting corrosion in chloride environments and should be avoided in acidic conditions. Compared to austenitic grades like 304, the 400 Series has lower resistance to corrosive agents, making it less suitable for harsh environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	815	1500	Suitable for high-temperature applications
Max Intermittent Service Temp	870	1600	Short-term exposure only
Scaling Temperature	600	1112	Risk of scaling above this temp
Creep Strength	600	1112	Begins to degrade at this temp

At elevated temperatures, the 400 Series maintains its strength but can suffer from oxidation and scaling. The maximum continuous service temperature indicates the upper limit for prolonged exposure, while the scaling temperature highlights the risk of surface degradation.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER410	Argon	Preheat recommended
MIG	ER308L	Argon + CO2	Good for thin sections
Stick	E410	-	Suitable for outdoor work

Weldability varies significantly within the 400 Series. Ferritic grades are generally more weldable than martensitic grades, which may require preheating to avoid cracking. Post-weld heat treatment can improve the properties of the weld.

Machinability

Machining Parameter	[400 Series]	AISI 1212	Notes/Tips
Relative Machinability Index	60	100	Lower machinability than 1212
Typical Cutting Speed (turning)	30 m/min	50 m/min	Adjust tooling for better performance

Machinability is moderate in the 400 Series, with martensitic grades being more challenging to machine due to their hardness. Proper tooling and cutting speeds are essential for optimal performance.

Formability

The 400 Series exhibits limited formability, particularly in martensitic grades, which are prone to cracking during cold working. Ferritic grades offer better formability and can be cold-formed with appropriate techniques.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	800 - 900 / 1472 - 1652	1 - 2 hours	Air	Relieve stress, improve ductility
Hardening	1000 - 1100 / 1832 - 2012	30 minutes	Oil	Increase hardness and strength
Tempering	400 - 600 / 752 - 1112	1 hour	Air	Reduce brittleness, improve toughness

Heat treatment processes significantly affect the microstructure and properties of the 400 Series. Annealing can enhance ductility, while hardening increases strength, making it essential to select the appropriate treatment based on the desired application.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Automotive	Exhaust systems	High-temperature strength, corrosion resistance	Durability and performance
Kitchenware	Cutlery	Hardness, edge retention	Sharpness and longevity
Oil and Gas	Valve components	Strength, resistance to high temperatures	Reliability in harsh environments
Construction	Fasteners	High strength, moderate corrosion resistance	Structural integrity

Other applications include:
- Industrial equipment
- Marine hardware
- Architectural applications

The selection of the 400 Series for these applications is often due to its balance of strength, hardness, and moderate corrosion resistance, making it suitable for environments where these properties are critical.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	[400 Series]	[AISI 304]	[AISI 316]	Brief Pro/Con or Trade-off Note
Key Mechanical Property	Moderate	High	High	304 and 316 offer better corrosion resistance
Key Corrosion Aspect	Fair	Excellent	Excellent	400 Series is less resistant to chlorides
Weldability	Moderate	Good	Good	400 Series may require preheating
Machinability	Moderate	Good	Fair	400 Series is harder to machine
Formability	Limited	Good	Good	Ferritic grades are more formable
Approx. Relative Cost	Lower	Higher	Higher	Cost-effective for moderate applications
Typical Availability	Common	Very Common	Common	400 Series is widely available

When selecting the 400 Series, considerations include cost-effectiveness, availability, and specific mechanical and corrosion resistance requirements. While it may not match the performance of austenitic grades in corrosive environments, its strength and hardness make it suitable for many applications where these properties are prioritized. Additionally, the magnetic properties of ferritic grades can be advantageous in certain applications, such as in electrical components.

In conclusion, the 400 Series Stainless Steel offers a unique combination of properties that make it suitable for a variety of engineering applications. Its balance of strength, hardness, and moderate corrosion resistance, along with its cost-effectiveness, positions it as a valuable material in the stainless steel family.