Introduction
FeNi36 is an iron-nickel alloy mainly composed of iron (Fe) and nickel (Ni), containing 36% nickel. This alloy has high strength, high toughness, high thermal stability, and good corrosion resistance, and is suitable for applications in high temperature, high pressure, and corrosive environments.
FeNi36 alloy has an extremely low thermal expansion coefficient and can maintain almost unchanged dimensions at different temperatures. This makes the alloy very suitable for fields that require high stability and precise dimensional control, such as precision instruments, optical equipment, etc. In addition, due to the high proportion of nickel content, FeNi36 alloy has good corrosion resistance and can resist the erosion of most acidic and alkaline media.
FeNi36 alloy shows color stability and a low expansion coefficient at high temperatures, so it is widely used in fields such as thermocouples, high-temperature parts, and electronic components. In addition, it can also be used in medical equipment, aerospace, nuclear energy, and other fields.
Below is the typical chemical composition table for Feni36:
| Element | Content (%) |
|---|---|
| Nickel (Ni) | 35.0 – 37.0 |
| Iron (Fe) | Balance |
| Carbon (C) | ≤ 0.05 |
| Silicon (Si) | ≤ 0.30 |
| Manganese (Mn) | ≤ 0.60 |
| Phosphorus (P) | ≤ 0.025 |
| Sulfur (S) | ≤ 0.025 |
FeNi36 Alloy (Invar 36) Electrical and Mechanical Properties Table
| Property Category | Parameter | Remarks |
|---|---|---|
| Mechanical Properties | ||
| Tensile Strength | 490 – 620 MPa | Typical tensile test values |
| Yield Strength | ≥240 MPa | Stress at 0.2% offset |
| Elongation (%) | ≥35 | Indicates material toughness |
| Brinell Hardness | 130 – 160 HB | |
| Electrical Properties | ||
| Electrical Resistivity (20°C) | 0.78 μΩ·m | Relatively high resistivity |
| Conductivity | 1.3 % IACS | Relative to pure copper |
| Thermal Expansion Coefficient | 1.2 x 10⁻⁶ /°C (20-100°C) | Low thermal expansion |
| Magnetic Properties | ||
| Magnetic Permeability | 1000 – 2000 | Measured at low frequency |
| Density | 8.1 g/cm³ | |
| Thermal Conductivity | 10.2 W/(m·K) (20°C) | Low thermal conductivity |
| Melting Point | 1427 – 1450 °C |
The Invar Effect
Invar – Characteristics of invar
1. Small thermal expansion coefficient, average expansion coefficient ×10-6/℃ at room temperature, and relatively stable at room temperature -80℃~230℃.
2. Low strength and hardness, tensile strength is about 590Mpa, yield strength is about 410Mpa, and Brinell hardness is about 141HBS.
3. Low thermal conductivity, 10W/ , only about 1/4 of the thermal conductivity of 45 steel. The thermal conductivity of 45 steel is 45 W/
4. Plasticity, toughness, elongation, cross-sectional shrinkage, and impact toughness are all very high, elongation δ= 30~45%, shrinkage δ=50~70%. Impact toughness αK=130-310 J/cm2.
In addition to being a crucial method of elucidating the source of magnetism in metals, their alloys, and compounds, the study of the Invar effect is also necessary for low-expansion alloys that need structural stability. Because FeNi36 type Invar can retain a stable austenite state at temperatures near -273°C, it has the greatest practical impact and is used in a wide range of industries, including high-tech items, oil transportation containers, microwave communications, and precise equipment.
In general, the thermal expansion coefficient of most metals and alloys rises linearly, meaning that they expand in volume when heated and contract in volume when cooled. Nevertheless, due to their ferromagnetism, some alloys made of transition metals like iron, nickel, and cobalt in the periodic table do not follow the standard expansion rule within a specific temperature range. Instead, they exhibit aberrant thermal expansion as a result of the Invar effect. From room temperature to roughly 230°C, Invar 36 (FeNi36) has one of the lowest thermal expansion coefficients of any alloy.
Invar Alloy in Metal Injection Molding
MIM, or metal injection molding, is a well-liked substitute for the production of microparts. High dimensional stability and good mechanical properties are appropriate for the low CTE Invar alloy. Nonetheless, throughout the MIM debinding and sintering process, this alloy is susceptible to element contamination. We ought to observe the affection in mechanical characteristics and dimensional stability.
Property Effects of Elements
Invar 36 boasts outstanding mechanical qualities and dimensional stability. However, MIM-processed FeNi36 alloy possesses great ductility and comparatively low strength. As a result, adding elements like Ti, Al, Sn, and carbon can improve age-hardening characteristics. With a manageable low CTE, a small quantity of carbon can enhance the mechanical characteristics of FeNi36.
The environment will affect the mechanical and physical characteristics of Invar parts during the MIM debinding and sintering process. During the bending process, thermoplastic polymers cannot fully break down. The remaining carbon components will change the alloy’s characteristics during the subsequent sintering process. Thus, various sintering atmospheres will affect the sinterability, microstructure, and element contamination of FeNi36 MIM parts.
Effect of the Atmosphere
Various kinds of sintered components with various debinding and sintering atmospheres are used to maximize the final MIM FeNi36 properties:
Vacuum sintering and debinding (V-V)
Sintering in hydrogen (V-H) after debinding in vacuum
Sintering in vacuum versus debinding in air (A-V)
Sintering in hydrogen (A–H) and debinding in air
Then use an X-ray diffractometer and a scanning electron microscope (SEM) to examine the microstructure of the sintered pieces. An elementary analyzer measures the amount of residual carbon and oxygen. Micro-hardness is measured with a micro-hardness tester.
Densification Effect of Elements
Higher volumetric shrinkage and superior densification are observed in hydrogen sintering. On the part surface, there are no original powder particles and the porosity distribution is uniform. These results all support the idea that sintered parts have a low oxygen content, proving that hydrogen can effectively eliminate oxides during the sintering process. Furthermore, the carbon content is higher than that of the original powder particles, and carbon contamination during the sintering process is inevitable.
Analysis of Microstructure
In various sintering atmospheres, a distinct austenitic microstructure with characteristic austenite twin bands is present. with the exception of the A-V process condition (debinding in air and sintering in vacuum).
Different variables affect the austenite’s grain size; larger grains are found during V-H (debinding in vacuum – sintering in hydrogen) process conditions.
The A-V condition has the smallest grain sizes when compared to the V-V and A-H conditions.
CTE & Curie Temperature
When compared to the original FeNi36 powders, sintered parts have a lower Curie temperature, which is related with interstitial elements of C or H, as well as nickel segregation. The nature and size of the austenite grain both influence the Curie temperature.
The Impact of Mechanical Properties
Oxides typically affect densification and negatively impact mechanical characteristics in the final sintered portion of FeNi36. Because it is an interstitial element, the hydrogen atmosphere is useful for part densification and oxide reduction, but it will also deteriorate ductility, Curie temperature, and CTE. In terms of mechanical and physical qualities, vacuum debinding and sintering are the most balanced.
Heat Treatment Method
Invar heat treatment can be divided into: stress relief annealing, intermediate annealing and stabilization treatment.
(1) Stress relief annealing: This process is necessary to remove any remaining stress from the items following machining. Heat for 1 to 2 hours at 530 to 550°C, then let the furnace cool.
(2) Intermediate annealing: To enable subsequent processing by removing the work-hardening phenomena that the alloy produces during cold rolling, cold drawing, and cold stamping. After being heated to 830–880°C for 30 minutes, the workpiece is either furnace or air-cooled.
(3) Stabilization treatment: To achieve stable performance and treatment with a lower expansion coefficient. Three-stage therapy is typically used.
a) Homogenization: Overheating, the alloy’s impurities dissolve completely and the alloying components become more homogeneous. Under protected conditions, the workpiece is heated to 830°C, held there for 20 to 1 hour, and then quenched.
b) Tempering: During this procedure, some of the stress caused by quenching can be removed. After heating the workpiece to 315°C for 1–4 hours, it is furnace cooled.
c) Stabilization aging: The alloy’s dimensions settle. The workpiece is kept warm for 48 hours after being heated to 95°C. In situations when high-temperature treatment is not appropriate for high-precision items following cold processing or machining, the following stress release stabilizing procedure may be utilized: It takes 1–4 hours to heat the workpiece to 315–370°C.
Invar is more challenging to process than austenitic stainless steel, although it shares many of the same properties and cannot be strengthened by heat treatment. High cutting temperature and high cutting force are the primary cutting manifestations. High-performance tools must be employed since it also has soft, sticky qualities and great plasticity during processing, makes chips difficult to break, aggravates tool wear, and lowers the workpiece’s processing accuracy.
Invar surface treatment process
Surface treatment can be sandblasted, polished, or pickled. The alloy can be pickled with 25% hydrochloric acid solution at 70°C to remove the oxide scale.
Application of Invar
Used in environments where extremely low expansion coefficients are required.
Typical applications are as follows:
● Production, storage, and transportation of liquefied gas
● Measuring and control instruments with working temperatures below +200°C, such as temperature regulators
● Screw connector bushings between metals and other materials
● Bimetallic and temperature-controlled bimetallic
● Membrane frames
● Shadow masks
● Tempering molds for CRP components in the aviation industry
● Satellite and missile electronic control unit frames below -200°C
● Auxiliary electron tubes in electromagnetic lenses of laser control devices
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