Invar Steel: Properties and Key Applications Explained

Invar Steel, also known as Fe-Ni alloy, is a specialized steel grade primarily composed of iron and nickel, typically containing around 36% nickel. This unique composition classifies Invar as a low-expansion alloy, making it particularly valuable in applications where dimensional stability is critical. The primary alloying element, nickel, significantly influences the material's thermal expansion properties, resulting in a coefficient of thermal expansion that is nearly zero over a specific temperature range.

Comprehensive Overview

Invar's most significant characteristics include its exceptional dimensional stability, low thermal expansion coefficient, and good mechanical properties at room temperature. These traits make it ideal for precision instruments, aerospace applications, and components requiring high dimensional accuracy.

Advantages of Invar Steel:
- Low Thermal Expansion: Invar's primary advantage is its minimal thermal expansion, making it suitable for applications where temperature fluctuations can lead to significant dimensional changes.
- Good Machinability: Invar can be machined to tight tolerances, which is essential in precision engineering.
- High Strength: It maintains good strength and toughness at room temperature.

Limitations of Invar Steel:
- Cost: The high nickel content makes Invar more expensive than standard steels.
- Limited High-Temperature Performance: While it performs well at room temperature, its mechanical properties can degrade at elevated temperatures.
- Corrosion Resistance: Invar is not as corrosion-resistant as some stainless steels, which may limit its use in certain environments.

Historically, Invar was developed in the late 19th century and has since found applications in various fields, including aerospace, precision measurement tools, and scientific instruments, due to its unique properties.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	K93600	USA	Closest equivalent to Invar 36
ASTM	A 320	USA	Standard specification for Invar
EN	1.3912	Europe	Minor compositional differences to be aware of
JIS	G 4303	Japan	Equivalent to Invar 36 with slight variations
GB	0Cr18Ni9	China	Similar properties but different corrosion resistance

In the 'Notes/Remarks' column, it is important to highlight that while these grades are often considered equivalent, subtle differences in composition can affect performance, particularly in thermal expansion and corrosion resistance.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Fe (Iron)	Balance
Ni (Nickel)	36.0 - 38.0
C (Carbon)	0.03 max
Mn (Manganese)	0.5 max
Si (Silicon)	0.5 max
S (Sulfur)	0.01 max
P (Phosphorus)	0.01 max

The primary role of nickel in Invar is to reduce the thermal expansion coefficient, which is crucial for applications requiring high dimensional stability. Carbon, while present in minimal amounts, helps in enhancing the strength of the alloy, while manganese and silicon contribute to the overall toughness and machinability.

Mechanical Properties

Property	Condition/Temper	Test Temperature	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Annealed	Room Temp	480 - 600 MPa	70 - 87 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	Room Temp	220 - 350 MPa	32 - 51 ksi	ASTM E8
Elongation	Annealed	Room Temp	30 - 40%	30 - 40%	ASTM E8
Hardness (Rockwell B)	Annealed	Room Temp	80 - 90 HRB	80 - 90 HRB	ASTM E18
Impact Strength (Charpy)	Annealed	-20°C	30 J	22 ft-lbf	ASTM E23

The combination of these mechanical properties makes Invar suitable for applications that require high strength and toughness, particularly under mechanical loading conditions. Its low yield strength compared to other high-strength alloys is compensated by its excellent dimensional stability.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	Room Temp	8.0 g/cm³	0.289 lb/in³
Melting Point	-	1450 °C	2642 °F
Thermal Conductivity	Room Temp	13 W/m·K	75 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temp	0.46 kJ/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	Room Temp	0.5 µΩ·m	0.5 µΩ·in
Coefficient of Thermal Expansion	20-100 °C	1.2 x 10⁻⁶ /K	0.67 x 10⁻⁶ /°F

The low coefficient of thermal expansion is particularly significant for applications in precision instruments, where even minor dimensional changes can lead to significant errors.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3%	25°C / 77°F	Fair	Risk of pitting
Sulfuric Acid	10%	20°C / 68°F	Poor	Not recommended
Nitric Acid	20%	25°C / 77°F	Good	Generally resistant
Sea Water	-	25°C / 77°F	Fair	Risk of localized corrosion

Invar exhibits moderate resistance to corrosion, particularly in acidic environments. It is susceptible to pitting in chloride-rich environments, making it less suitable for marine applications compared to stainless steels. When compared to grades like AISI 304 or AISI 316, Invar's corrosion resistance is inferior, particularly in chloride environments, where stainless steels excel.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	300 °C	572 °F	Above this, properties may degrade
Max Intermittent Service Temp	400 °C	752 °F	Short exposure only
Scaling Temperature	600 °C	1112 °F	Risk of oxidation

Invar maintains its mechanical properties up to moderate temperatures, but beyond 300 °C, it can experience significant degradation. Its oxidation resistance is limited, and care must be taken in high-temperature applications to prevent scaling.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ERNi-1	Argon	Preheat recommended
MIG	ERNi-1	Argon	Post-weld heat treatment may be necessary

Invar is generally weldable using TIG and MIG processes, but preheating is often recommended to minimize the risk of cracking. Post-weld heat treatment can help relieve stresses and improve the overall integrity of the weld.

Machinability

Machining Parameter	Invar Steel	AISI 1212	Notes/Tips
Relative Machinability Index	50%	100%	Requires slower speeds
Typical Cutting Speed (Turning)	30 m/min	60 m/min	Use carbide tools

Invar has moderate machinability, requiring slower cutting speeds and specialized tooling to achieve optimal results. The presence of nickel can lead to tool wear, necessitating careful selection of cutting parameters.

Formability

Invar exhibits good formability, both in cold and hot forming processes. However, due to its work-hardening characteristics, careful control of the forming process is necessary to avoid cracking. Bend radii should be larger than those typically used for standard steels to accommodate its unique properties.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	800 - 1000 °C / 1472 - 1832 °F	1 - 2 hours	Air	Softening, improving ductility
Solution Treatment	1000 - 1100 °C / 1832 - 2012 °F	1 hour	Water	Homogenizing microstructure

Heat treatment processes such as annealing can significantly alter the microstructure of Invar, enhancing its ductility and machinability. The metallurgical transformations during these treatments can lead to a more uniform distribution of phases, which is crucial for maintaining the desired properties.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Aerospace	Aircraft components	Low thermal expansion, high strength	Precision and stability
Measurement	Precision instruments	Dimensional stability, machinability	Accuracy in measurements
Electronics	Circuit boards	Low thermal expansion, electrical properties	Stability under temperature changes
Scientific	Laboratory equipment	Corrosion resistance, low expansion	Reliability in experiments

Other applications include:
- Optical devices
- Clocks and watches
- High-precision tooling

Invar is chosen for these applications primarily due to its low thermal expansion, which is critical in environments where temperature variations can lead to significant measurement errors.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	Invar Steel	AISI 304	AISI 316	Brief Pro/Con or Trade-off Note
Key Mechanical Property	Moderate	High	High	Invar is less strong than stainless steels
Key Corrosion Aspect	Fair	Excellent	Excellent	Invar is less resistant to chlorides
Weldability	Moderate	Good	Good	Invar requires special considerations
Machinability	Moderate	High	High	Invar requires slower speeds
Formability	Good	Excellent	Excellent	Invar has specific bending requirements
Approx. Relative Cost	High	Moderate	Moderate	Invar's nickel content drives cost
Typical Availability	Limited	High	High	Invar is less commonly available

When selecting Invar for a specific application, considerations such as cost, availability, and the specific mechanical and thermal properties required must be weighed against alternatives like stainless steels. Invar's unique properties make it invaluable in niche applications, particularly in precision engineering and aerospace, where dimensional stability is paramount. However, its higher cost and limited corrosion resistance compared to stainless steels may limit its use in more general applications.