Nichrome 70/30

About Nichrome 70/30

Nichrome 70/30 is a nickel-chromium alloy, specifically composed of approximately 70% nickel (Ni) and 30% chromium (Cr).¹ This particular composition gives it a unique set of properties that make it highly valuable for demanding high-temperature applications.²

Here are the key properties of Nichrome 70/30:

1. High Electrical Resistivity:

• Nichrome 70/30 exhibits excellent electrical resistivity, meaning it strongly opposes the flow of electric current.³ This characteristic is fundamental to its use as a heating element, as it converts electrical energy efficiently into heat (Joule heating).⁴
• Typical electrical resistivity at 20∘C is around 1.18Ω⋅mm2/m (709Ω/cmf).

2. Exceptional Oxidation Resistance at High Temperatures:

• When heated in air, Nichrome 70/30 forms a stable, self-healing, and highly adherent layer of chromium oxide on its surface.⁵ This protective oxide layer acts as a barrier, preventing further oxidation and corrosion of the underlying alloy, even at elevated temperatures.⁶
• This property is crucial for its longevity in heating applications where exposure to air is continuous.

3. High Maximum Continuous Operating Temperature:

• One of the standout features of Nichrome 70/30 is its ability to withstand very high continuous operating temperatures. It can be used effectively in air up to approximately 1250∘C (2280∘F). This is generally higher than Nichrome 80, making it suitable for more extreme heat applications.

4. Superior Resistance to “Green Rot”:

• This is a critical advantage of Nichrome 70/30, particularly in industrial furnace environments. “Green rot” is a form of intergranular oxidation that can occur in some nickel-chromium alloys (including Nichrome 80) when exposed to reducing atmospheres or atmospheres with fluctuating oxygen levels at high temperatures. The higher chromium content in Nichrome 70/30 significantly improves its resistance to this phenomenon, ensuring better reliability and longer service life in such conditions.

5. Good Ductility and Form Stability:

• Nichrome 70/30 retains good mechanical strength and ductility even after prolonged use at high temperatures.⁷ This allows it to be drawn into fine wires, rolled into strips, and formed into various complex shapes required for heating elements.⁸
• It also exhibits excellent form stability, meaning it maintains its shape under thermal stress.

6. Excellent Weldability:

• This alloy is known for its good weldability, which is important for manufacturing complex heating elements and for joining components.⁹

7. Non-Magnetic:

• Nichrome 70/30 is a non-magnetic alloy, which can be important in certain electrical and electronic applications where magnetic interference needs to be avoided.¹⁰

8. Physical Properties (Approximate Values):

• Melting Point: Around 1380∘C (2516∘F).
• Density: Approximately 8.1 g/cm3.
• Tensile Strength (typical for wire): Around 820 MPa to 900 MPa.
• Yield Strength (typical for wire): Around 430 MPa to 485 MPa.
• Elongation: Typically ¹¹>20% (indicating good ductility).¹²
• Coefficient of Thermal Expansion (20-1000∘C): Approximately 17×10−6/∘C.
• Specific Heat Capacity: Around 0.46 kJ/kg⋅K at 20∘C.
• Thermal Conductivity: Approximately 12.5 W/m⋅K to 16.4 W/m⋅K.

Nichrome 70/30 vs. Nichrome 80:

While both are excellent resistance alloys, their differences make them suitable for different applications:

In summary, the key properties of Nichrome 70/30 are its very high electrical resistivity, outstanding oxidation resistance at elevated temperatures (including resistance to “green rot”), and its ability to maintain mechanical integrity and form at extreme heat.¹³ These characteristics make it a preferred material for high-performance heating elements, particularly in industrial settings with challenging atmospheric conditions.