Meeting Invar 36 Machinability Challenges

Engineers and product designers prize Invar 36 because of its ability to maintain its size and shape over a wide range of temperatures. This makes it a very useful and common material in various engineered products ranging from watch parts to satellites.

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However, Invar is a tricky material to work with. It can be very difficult to machine and requires special considerations to avoid problems. In this article, we&#;ll discuss Invar 36 machinability challenges and some methods for overcoming them.

What Is Invar 36?

Invar 36 is a nickel-iron alloy containing 64% iron and 36% nickel. This nickel-iron composition is optimized to have an exceptionally low coefficient of thermal expansion&#;one-tenth that of carbon steel at temperatures up to 400°F. As a low-expansion alloy, Invar 36 doesn&#;t expand or shrink at extreme temperatures nearly as much as other high-expansion alloys.

Because it has one of the lowest thermal expansion, Invar 36 is commonly used for applications that demand nearly constant dimensions under temperature variation. For example, radio and electronic devices, aircraft controls, optical systems, surveying tape, and clockwork components are resistant to thermal expansion and maintain superior dimensional stability. In these applications, controlling the rate of thermal expansion is crucial for ensuring functionality across normal atmospheric temperatures. This helps to minimize dimensional changes that can affect performance.

Additionally, Invar 36 is often employed in temperature regulators and environments requiring cryogenic temperatures due to its stability.

Invar 36 Machinability Challenges

Although it looks and feels similar to carbon steel, Invar 36 alloy is much more difficult to machine. This nickel-iron alloy is soft and very ductile, especially in the annealed condition. This can make Invar 36 machinability a challenge.

Cutting tools tend to plow into the material instead of cutting it cleanly. Because of this, Invar 36 produces stringy, gummy chips that tend to &#;bird nest&#; around the cutting tool, interfering with cutting and coolant flow. Despite these challenges, Invar 36 retains good strength, even at cryogenic temperatures, making it suitable for various demanding applications, including maintaining precision in electronic devices.

Like other nickel-iron alloys, an Invar alloy is susceptible to work hardening. Work hardening occurs when the surface of the metal ahead of the cutting tool plastically deforms, forming a hardened layer that is very difficult to penetrate in subsequent passes or operations. This can lead to poor surface finish, part deformation, and broken tools. Managing the rate of thermal expansion during machining is crucial to avoid these issues.

Nickel-iron alloys like Invar 36 demand specific techniques to handle their unique properties effectively.

Tips for Machining Invar 36

Though it&#;s a challenging material to cut compared to high expansion alloys, there are ways to improve Invar 36 machinability, including the following best practices.

Good results when milling an Invar alloy require a careful balance of tool diameter, number of teeth, cutting speed, feed, and chip space. Machine power, condition, and setup are important to reduce chatter and prevent work hardening.

The gummy chips Invar produces can interfere with the cutting tool if too much accumulates. Use a chip breaker and ensure your tooling is sharp and rigidly supported when machining Invar. High-speed steel and carbide tools both work well on Invar 36, though with carbide tools, it is important to use a powerful machine and a rigid setup to minimize vibration. Helical tools resist chatter and will generally provide the best surface finish. Descaling the surface before machining can make machining significantly easier.

Prevent work hardening by moving the tool, minimizing heat, and controlling temperature variation. Maintain a positive feed rate and avoid dwelling so heat doesn&#;t build up in one area. Machining Invar 36 requires higher feeds and lower speeds than other metals. Ensure you have adequate lubrication volume and pressure to keep the cutting surface cool.

When turning Invar 36, use the largest tool possible to provide a large heat sink. Using materials like carbon steel for tooling can help manage dimensional changes and maintain precision. Notably, Invar 36 retains good strength at both normal and cryogenic temperatures, further enhancing its machinability.

Finally, machining Invar 36 introduces stresses that can change the rate of thermal expansion behavior of the finished product. It may need to be annealed after machining to relieve these stresses and ensure nearly constant dimensions. The low coefficient of thermal expansion makes it a reliable choice for high-precision applications.

We Supply Invar 36 to Machinists and Fabricators

For over two decades, Industrial Metal Service has been supplying Invar 36 and other specialty metals to manufacturers, machinists, and fabricators in the San Francisco Bay Area and nationwide.

We sell various forms of new and verified remnant metals, including steel, aluminum, titanium, and high-temperature superalloys. Our metal remnants are verified using x-ray fluorescence analyzers to ensure our customers know the quality of the metal they&#;re receiving.

Additionally, we provide materials like austenitic stainless steels, known for their superior dimensional stability and resistance to dimensional changes across normal atmospheric temperatures.

If you want to learn more, please visit our website INvar Material.

NOTE: Invar® is commonly known as Invar®36

Working and Forming

Invar® may be worked using any conventional working method. Annealed material, that is material with an RB hardness of less than Rockwell B 70, is desirable for material involving deep drawing, hydro-forming or spinning. For blanking, material between 1/4 and 3/4 hard will usually present a cleaner cut. Invar® may be chemically etched. For operation where there is a large quantity of machining. Free Cutting Invar® is available in round rod.

Heat Treatment for Invar®

Invar® can be heat treated using one of the following methods. Heating and cooling rates shall be controlled to prevent damage to the parts (cracking, warpage, etc)

Annealing Method 1

Heat parts to °F +- 25°F and hold at temperature one-half hour per inch of thickness, then furnace cool at a rate not to exceed 200°F per hour to 600°F. No additional machining should be performed on these parts

Annealing Method 2

1. Rough Machine
2. Heat parts to °F +- 25°F and hold at temperature one-half hour per inch of thickness, then furnace cool at a rate not to exceed 200°F per hour to 600°F. Still air cool is acceptable below 600°F
3. Heat Parts for one hour at 600&#;F +- 20°F followed by air cooling
4. Heat parts for 48 hours at 205°F followed by air cooling
5. Finish Machine.

Annealing Method 3

Annealing plus water quench and stabilization method

1. Rough Machine
2. Heat parts to °F +- 25°F and hold at temperature one-half hour per inch of thickness, then water quench
3. Semi finish machine
4. Heat Parts for one hour at 600&#;F +- 20°F followed by air cooling
5. Heat parts for 48 hours at 205°F followed by air cooling
6. Finish Machine

Welding

Conventional welding methods can be used with Invar. Invar filler rod is recommended for those welds requiring filler rod.

Brazing

First anneal the material as above. Avoid over stressing joints during brazing. Use silver and zinc free brazes for brazing Invar®.

Heat Treatment

Because of its affect on the actual structure of the material, there is a distinction made between heat treating the material to facilitate fabrication and heat treating the material to insure optimum conditions for glass sealing, plating, or brazing.

Stress Relief Annealing

To relieve stress and work hardening of parts at intermediate stages o fabrication. It is intended particularly for drawing, forming and spinning operations.

1. Wash and degrease parts
2. Anneal in atmosphere controlled furnace. Atmosphere may be wet or dry hydrogen, dissociated ammonia, cracked gas or similar neutral atmosphere.
3. Annealing temperature is not critical; however, high temperatures (greater than 900°C) or extended time periods (longer than 60 minutes) should be avoided because such treatments promote grain growth.
   Typical cycle &#; 850°C for 30 minutes.
4. Parts should be held at temperature for the indicated time and then furnace cooled to less than 175°C to avoid oxidation and/or thermal shock (which may cause distortion)

Heat Treatment for Oxidation

1. Make sure that proper methods are used to clean, degrease and bright dip parts
2. Oxidation &#; Heat treat in an electric air furnace to 850°C to 900°C until parts are cherry red (dull red heat). The length of heat cycle is approximately 3 minutes, but due to differences n humidity and furnaces, the proper cycle has to be varied. Then reduce heat approximately 10°C per minute. When parts are cooled, oxide will be formed. The oxide may appear from light gray to black in color. Black is normally considered to be over-oxidation and is not necessarily desirable for a good glass to metal seal

Invar® & Super Invar® are registered trademarks of CRS Holding, a subsidiary of Carpenter Technologies &#; Contact Eagle Alloys, your premier Invar® 36 suppliers, today!

Super &#; Invar®

Invar® (36% NI-Balance Iron) Alloy has been the high temp metal of choice for low expansion applications for years. &#;Super-Invar®&#; (31% NI-5% Co-Balance Iron) has found some favor because it has a near zero coefficient of thermal expansion over a limited temperature range. The useful range of &#;Super Invar®&#; is limited to between -32° to + 275°C. because the material begins to transform from Austenite to Martinsite at temperature below-32°F

The C.T.E crosses over the zero frequently, each lot of heat behaves a little differently, but these results are typical for material between 0°F and 200°F

Formability

Super Invar® is easily formed, deep drawn and fabricated.

Weldability

Super Invar is welded using a special Super Invar weld wire, and a variety of other high nickel rods and wires

Machinability

Super Invar® properties make it so the metal is tough and gummy, not hard or abrasive. Tools tend to plow instead of cut, resulting in long stringy &#;chips.&#; Tools must be sharp, feed and speed low to avoid heat and distortion. The use of a coolant is recommended for all machining operations. Machinability similar to Kovar®, Stainless 300 series, and Monel Alloys has been reported. Ni-Fe Alloys generally have a tendency to develop a surface scale during hot working that penetrates the surface. For this reason machining allowances must be increased to eliminate the deep surface oxide. The initial cut is frequently the most difficult.

Invar® & Super Invar® are registered trademarks of CRS Holding, a subsidiary of Carpenter Technologies

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