Properties of Invar alloy Ni36
Invar alloy 4J36, the jewel of iron-based high-nickel alloy, contains 32%-36% nickel, just like an oasis in the desert. Trace elements such as sulfur, phosphorus, and carbon shine like stars dotted within. The remaining 60% or so is made up of iron, which forms the main body of the alloy like towering mountain peaks.
Nickel, the ambassador of austenite, allows the phase transition from austenite to martensite to take place at room temperature or even lower. It was a deep cold -100 -120, like a cold winter wax moon. However, after annealing, the Invar alloy exhibits a wonderful austenite structure. It exists in the form of a face-centered lattice structure, as if a dancer is gliding freely on the ice. The fusion of nickel and -Fe is like the fusion of spring water and peach blossoms, forming a beautiful solid solution.
Invar therefore has a unique property that is the perfect balance between cold and warmth, between toughness and flexibility, between strength and beauty.
1 Very small coefficient of expansion.
Invar alloys, also known as non-expanding steels, have stable physical properties. Its average coefficient of expansion is as fine as velvet and is 15×10-6℃。And when its nickel content reaches 36%, this value is reduced to a staggering 18 ×10-8℃。Whether at room temperature -80 or +100, the alloy retains its original properties and is not affected by changes in ambient temperature.
2. High plasticity and toughness.
Invar alloys have excellent ductility with an elongation of up to 25-35% δ, exhibiting unrivalled flexibility. What's even more impressive is that its impact toughness is equally excellent, ranging from 18-33 kg m cm2, allowing it to remain intact in the face of impact.
3. The strength and hardness are not high.
The carbon content of the Invar alloy is extremely low, only 005% or less, so its hardness and strength are not outstanding. However, its tensile strength can still reach about 517 MPa, the yield strength is about 276 MPa, and the Vickers hardness is about 160. The uniqueness of this alloy is that it retains good plasticity even with increased strength. With proper cold deformation, its strength can be significantly improved.
4 Low thermal conductivity.
The thermal conductivity of Invar alloys is surprisingly low at just 0026～0.032cal cm sec, which is lower than the thermal conductivity of 45 steel, only 1 3-1 4 of the latter. It's like setting up an insurmountable obstacle on the road to heat dissipation, allowing the heat to falter under its guidance.
5 Other features.
Invar alloys are rich in nickel, which gives them excellent hardenability and quenchability. This not only enhances the toughness of the steel, but also gives it better resistance to gas, corrosion and wear.
Through an in-depth analysis of the chemical composition, metallographic structure, and mechanical and physical properties of Invar alloys, we found that their machinability is quite similar to that of austenitic stainless steels, and even more problematic. In the machining process, Invar alloys exhibit three characteristics: high cutting forces, high cutting temperatures and fast tool wear. The soft, sticky properties behind this, as well as the great plasticity, make it difficult for the chips to break. This undoubtedly increases the friction between the chips and the rake face, further accelerating tool wear.
This not only significantly reduces the durability of the tool, but also affects the machining accuracy of the workpiece. To break this dilemma, we had to adopt high-performance carbide-coated tools and explore completely new machining methods. Only in this way can we ensure that the machining process runs smoothly. With the right method, even difficult-to-machine Invar alloys can be easily machined.
Transforming Invar from "difficult-to-machine" to "easy-to-machine" is not only an important goal for us to study its material properties, but also the ultimate vision we pursue.