329 Stainless Steel: Properties and Key Applications

329 stainless steel, classified as a duplex stainless steel, is notable for its unique microstructure that combines both austenitic and ferritic phases. This dual-phase structure is achieved through the careful balance of alloying elements, primarily chromium, nickel, and molybdenum, which contribute to its enhanced mechanical properties and corrosion resistance. The typical chemical composition of 329 stainless steel includes approximately 24% chromium, 6% nickel, and 3% molybdenum, which collectively provide excellent strength and toughness, particularly in harsh environments.

Comprehensive Overview

329 stainless steel is primarily utilized in applications requiring high strength and resistance to corrosion, particularly in environments that are both acidic and saline. Its significant characteristics include high tensile strength, good ductility, and excellent resistance to pitting and crevice corrosion. The inherent properties of this steel grade make it suitable for various industrial applications, including chemical processing, oil and gas, and marine environments.

Advantages of 329 Stainless Steel:
- High Strength: The duplex structure provides superior strength compared to standard austenitic stainless steels.
- Corrosion Resistance: Excellent resistance to a wide range of corrosive environments, including chlorides.
- Cost-Effectiveness: Generally lower nickel content compared to other austenitic grades, which can reduce material costs.

Limitations of 329 Stainless Steel:
- Weldability Challenges: While it can be welded, special care must be taken to avoid issues such as hot cracking.
- Ductility at Low Temperatures: May exhibit reduced ductility in extremely low temperatures compared to austenitic grades.

Historically, duplex stainless steels like 329 have gained popularity since the 1980s due to their favorable balance of properties, making them a common choice in various engineering applications.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S32900	USA	Closest equivalent to EN 1.4460
AISI/SAE	329	USA	Minor compositional differences to 2205
ASTM	A240	USA	Standard specification for stainless steel plates
EN	1.4460	Europe	Similar properties to S31803 but with higher nickel content
JIS	SUS329J3L	Japan	Equivalent with slight variations in composition

The differences between these equivalent grades often lie in their nickel and molybdenum content, which can affect corrosion resistance and mechanical properties. For instance, while both 329 and 2205 are duplex grades, 2205 typically has higher nickel content, which enhances its toughness but may also increase costs.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Cr (Chromium)	24.0 - 26.0
Ni (Nickel)	5.0 - 7.0
Mo (Molybdenum)	2.5 - 3.5
N (Nitrogen)	0.08 - 0.20
Fe (Iron)	Balance

Chromium enhances corrosion resistance and contributes to the formation of a passive oxide layer. Nickel improves toughness and ductility, while molybdenum increases resistance to pitting corrosion, particularly in chloride environments. Nitrogen is added to improve strength and corrosion resistance.

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 - 750 MPa	90 - 109 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	450 - 550 MPa	65 - 80 ksi	ASTM E8
Elongation	Annealed	25 - 35%	25 - 35%	ASTM E8
Hardness (Rockwell B)	Annealed	90 - 95	90 - 95	ASTM E18
Impact Strength (Charpy V-notch)	-40°C	40 J	30 ft-lbf	ASTM E23

The combination of high tensile and yield strength makes 329 stainless steel suitable for applications where high mechanical loads are expected. Its elongation values indicate good ductility, allowing for deformation without fracture.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	Room Temperature	7.8 g/cm³	0.28 lb/in³
Melting Point/Range	-	1400 - 1450 °C	2552 - 2642 °F
Thermal Conductivity	Room Temperature	15 W/m·K	87 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	Room Temperature	500 J/kg·K	0.12 BTU/lb·°F
Electrical Resistivity	Room Temperature	0.73 µΩ·m	0.00043 Ω·in

The density of 329 stainless steel is comparable to other stainless steels, while its melting point indicates good high-temperature performance. The thermal conductivity is moderate, making it suitable for applications where heat transfer is necessary but not critical.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3-10	20-60 °C / 68-140 °F	Excellent	Risk of pitting
Sulfuric Acid	10-30	20-40 °C / 68-104 °F	Good	Limited exposure recommended
Hydrochloric Acid	1-5	20-40 °C / 68-104 °F	Fair	Not recommended for high concentrations
Sea Water	-	Ambient	Excellent	Highly resistant to marine environments

329 stainless steel exhibits excellent resistance to pitting and crevice corrosion in chloride environments, making it suitable for marine applications. However, it can be susceptible to stress corrosion cracking (SCC) in certain conditions, particularly in the presence of chlorides and tensile stress.

When compared to other grades, such as 316L and 2205, 329 offers superior resistance to pitting but may not perform as well in reducing acids. The choice of grade often depends on the specific environmental conditions and mechanical requirements of the application.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	300 °C	572 °F	Suitable for high-temperature applications
Max Intermittent Service Temp	400 °C	752 °F	Short-term exposure only
Scaling Temperature	600 °C	1112 °F	Risk of oxidation above this limit

At elevated temperatures, 329 stainless steel maintains its strength and corrosion resistance, although prolonged exposure to temperatures above 300 °C can lead to scaling and oxidation. Careful consideration of service temperatures is essential to ensure long-term performance.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER329	Argon	Preheat recommended
MIG	ER329	Argon + 2% Oxygen	Post-weld heat treatment may be required
Stick	E309L	-	Suitable for thicker sections

329 stainless steel can be welded using standard techniques, but it requires careful control of heat input to avoid issues such as hot cracking. Preheating and post-weld heat treatment are often recommended to relieve residual stresses and improve weld quality.

Machinability

Machining Parameter	329 Stainless Steel	AISI 1212	Notes/Tips
Relative Machinability Index	30%	100%	Requires slower cutting speeds
Typical Cutting Speed (Turning)	30-50 m/min	100-150 m/min	Use carbide tools for best results

Machinability of 329 stainless steel is lower than that of free-machining steels like AISI 1212. Optimal conditions include slower cutting speeds and the use of high-quality tooling to minimize wear.

Formability

329 stainless steel exhibits moderate formability. Cold forming is feasible, but care must be taken to avoid work hardening, which can lead to cracking. Hot forming is also possible, but the material should be heated to avoid excessive strain.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Solution Annealing	1020-1100 °C / 1868-2012 °F	30 minutes	Air or water	Dissolve carbides, improve ductility
Stress Relieving	300-500 °C / 572-932 °F	1 hour	Air	Reduce residual stresses

Heat treatment processes such as solution annealing are critical for achieving the desired microstructure and properties. During this process, the steel is heated to a temperature where the phases can dissolve, followed by rapid cooling to lock in the desired structure.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Oil and Gas	Offshore platforms	High strength, corrosion resistance	Durability in harsh environments
Chemical Processing	Storage tanks	Resistance to pitting and crevice corrosion	Safety and longevity
Marine	Shipbuilding	Excellent resistance to seawater	Longevity and structural integrity

Other applications include:
* Heat exchangers
* Pressure vessels
* Piping systems in corrosive environments

329 stainless steel is often chosen for applications where both strength and corrosion resistance are critical, particularly in environments exposed to chlorides or other aggressive agents.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	329 Stainless Steel	2205 Duplex Steel	316L Stainless Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High Strength	Higher Toughness	Good Ductility	329 offers a balance of strength and corrosion resistance
Key Corrosion Aspect	Excellent Resistance	Good Resistance	Moderate Resistance	329 excels in chloride environments
Weldability	Moderate	Good	Excellent	2205 is easier to weld than 329
Machinability	Moderate	Moderate	High	316L is easier to machine than 329
Approx. Relative Cost	Moderate	Higher	Moderate	Cost can vary based on market conditions
Typical Availability	Moderate	Moderate	High	316L is widely available compared to 329

When selecting 329 stainless steel, considerations include cost-effectiveness, availability, and specific application requirements. Its unique properties make it suitable for niche applications where both strength and corrosion resistance are paramount. Additionally, safety factors and potential environmental impacts should be evaluated during the selection process to ensure optimal performance and longevity in service.