Invar Alloy Machining – A Guide

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About Invar Alloys

Metalworking shops frequently receive requests to machine parts and components from Invar alloy (UNS K), a 36% nickel-iron alloy known for its unique low expansion properties. The thermal expansion rate of Invar alloy is approximately one-tenth that of carbon steel at temperatures up to 400 °F (204 °C). This characteristic makes the alloy suitable for a growing number of applications, including:

• (a) Applications where dimensional changes due to temperature variations must be minimized, such as radio and electronic devices, aircraft controls, optical and laser systems.
• (b) Uses involving high expansion alloys in applications where motion is desired with temperature changes, such as bimetallic thermostats and rod and tube assemblies for temperature regulators.

This alloy is available in two variations. The conventional Invar alloy is generally used for its optimum low expansion properties. The second variation is known as Free-Cut Invar 36 alloy (UNS K and ASTM F), which demonstrates improved machinability for high productivity applications. This variation contains a small addition of selenium to enhance machinability.

Free-Machining Variation

Free-Cut Invar 36 alloy, the world’s first free-machining Invar alloy, is preferred by machine shops producing high volumes of components such as controls for hot water heaters, filters for microwave instruments, and precision parts for optical mounting in lenses. High-production shops report that the free-machining alloy is advantageous for executing various machining operations, particularly for intricate shapes and/or close tolerances.

Compared to conventional Invar grade, the downside of Free-Cut Invar 36 alloy is negligible. Its coefficient of thermal expansion is only slightly higher than that of the basic alloy; this difference is generally insufficient to impact part performance.

While there is a minimal loss in transverse toughness and corrosion resistance, it may be necessary to clean and passivate the free-cut alloy to remove selenides from the surface. Nevertheless, the ease of machining with the free-machining alloy allows for productivity gains, frequently reaching 250%. From a machinist's perspective, it is challenging to justify not using the free-machining grade.

Fabricating Characteristics

Both types of Invar 36 alloys are soft like Type 304 and Type 316 austenitic stainless steels; the free-cut variation, in particular, machines similarly to these stainless grades. All these materials exhibit a high work-hardening rate, necessitating care during machining.

The standard Invar alloy tends to produce stringy, gummy chips that can "bird nest" around the tools, obstructing coolant flow. Chip breaking is essential using chip breakers, which are also required for the free-cut alloy, though they do not need to be as deep since free-cut chips are more brittle.

It is recommended to use large, sharp, and rigidly supported tooling for both grades. A positive feed rate should be maintained throughout machining operations to prevent glazed, work-hardened surfaces. In certain cases, increasing the feed and reducing the speed might be necessary, while interrupted cuts or successive thin cuts should be avoided.

In general, free-cut Invar alloy produces not only good surface finishes but also higher productivity. Care must be taken to ensure effective lubrication and cooling during all cutting operations.

Both grades are very ductile, making them readily cold-headed and formed. Stamping from cold-rolled strips is also easier, and deep drawing from properly annealed strips is feasible.

Fabrication can introduce stresses that, if unrelieved, may alter thermal expansion behavior. Parts placed in service without stress relief may not meet design requirements. Therefore, annealing and stress-relieving thermal treatments are recommended to enhance structural uniformity and dimensional stability.

After significant forming, bending, and machining, stress relief from these operations can be achieved by annealing at temperatures ranging from 760 °C to 980 °C long enough to heat the section thoroughly. However, these alloys have a tendency to oxidize at elevated temperatures.

When annealing cannot occur in a non-oxidizing atmosphere, sufficient material must be available for cleaning via light grinding or pickling following the annealing process. For sections subjected to light finishing cuts or grinding post-annealing, stress relief can be attained by heating between 315 °C and 425 °C long enough to uniformly heat the workpieces.

Machining Parameters

No universal rules or formulas apply to all machining scenarios. Besides the materials, job specifications and equipment must be considered to determine the relevant machining parameters.

Moreover, operations like turning on automatic screw machines, turret lathes, and CNC lathes involve numerous variables, making it impossible to provide specific recommendations applicable to all conditions. Thus, the parameters below should be viewed as starting points for initial machine setups.

Turning

Properly ground tools are crucial for turning Invar alloy. Suggested starting geometries for high-speed steel single-point turning tools include angles aiming for efficiency. Tools with a 5 to 10° positive top rake angle yield reduced heat generation and smoother cuts.

Utilizing a larger tool allows for greater heat dissipation as well as a more stable setup. Minimal front clearance angles (7 to 10°) are recommended to support the cutting edge adequately. Invar alloys may require tools ground with top rake angles towards the higher side of the 5 to 10° range for effective chip control and may also necessitate increased side clearance angles to minimize rubbing and localized work hardening.

Carbide tools in single-point turning operations permit higher speeds than high-speed tool steels; however, they require greater attention to rigidity of the tooling and workpiece. Avoid interrupted cuts whenever possible.

For Invar alloy applications, either blade-type or circular cutoff tools can be employed. Blade-type cutoff tools typically maintain sufficient bevel for side clearance (with a minimum of 3°) but may need more clearance for deeper cuts. Proper grinding to achieve top rake and front clearance is essential:

• Front clearance angle: 7 to 10°
• Top rake angle: 7 to 10°

Angles for circular cutoff tools mirror those for blade-type tools, including a top rake angle of 7 to 10°. Due to their rigidity, circular cutoff tools can endure increased shock and may be preferred in automatic screw machine operations.

Drilling

When drilling Invar alloys, several rules should be followed:

• (a) Keep the work clean and remove chips frequently to prevent drill dulling.
• (b) Select drills carefully and ensure proper grinding.
• (c) Align drills and firmly support the work.
• (d) Direct cutting fluid at the hole correctly.
• (e) Use the shortest drilling length to avoid whip or flexing, which could break drills or cause inaccurate work.

For Invar 36 alloys, using a sharp three-cornered punch is advisable to avoid work-hardening the material. Drilling templates or guides can also assist. To alleviate chip packing, drills may occasionally need to be backed out. General rules suggest drilling a depth of three to four times the diameter of the drill for the initial bite, and subsequently one or two diameters for the next bites.

It is imperative to avoid allowing the drill to dwell during cutting to prevent glazing at the bottom of the hole, complicating subsequent drilling. Proper feed settings are critical in enhancing production rates, with feeds and speeds for various drill sizes indicated in tables as references.

Tapping

Two hole types are prepared for tapping: open (through) and blind holes. For open holes, either spiral-fluted or straight-flute spiral-point taps can be utilized, proving beneficial for working with soft Invar alloys.

The spiral-point tap cuts using a shearing motion, offering minimal resistance and enhancing chip removal, thus reducing the likelihood of breaking the tap during the backing out process. For blind holes, special taps are available, and taps should ideally be matched to ensure effective chip removal.

Milling

Various high-speed steel cutters can be employed, and tooling with carbide inserts may also function effectively with Invar grades. Helical or spiral cutters are generally preferred for obtaining exceptional finishes, particularly for broader cuts. Coarse-tooth cutters allow for less stress and higher speeds than fine-tooth cutters.

Broaching

For Invar materials, high-speed steel broaches are the best choice, as they simplify operations by incorporating roughing, semi-finished, and precision cuts into one tool. Proper lubrication and cooling, using sulfochlorinated oils diluted with paraffin, is crucial.

Reaming

Use high-speed reamers for Invar alloys, preferably spiral-fluted reamers with a helix angle of approximately 7°. Left-hand spiral reamers with right-hand cutting are recommended for better cutting efficiency. Feeds and speeds for different operations should be carefully regulated to prevent overheating.

Cutting Fluids

Two types of cutting fluids are suitable for machining Invar alloys: sulfochlorinated oils to prevent seizing and emulsifiable fluids with superior cooling capacity. Sulfochlorinated oil is commonly required for most machining activities.

Summary

If machine shops experience challenges while working with Invar 36 alloys, reevaluating their procedures and addressing common issues can be helpful:

• A. If parts productivity is inadequate and machining shapes is insufficient, consider using the free-cut variation of Invar alloy.
• B. If machined surfaces are glazed, ensure you maintain a positive feed rate.
• C. If tools are chattering and not cutting cleanly, review tool geometry against provided guidelines.
• D. If tools heat excessively, verify that tooling is appropriate for heat dissipation and check the cutting fluid composition.

The information provided is available in the public domain. While we strive to keep it accurate, no representations or warranties of any kind are made regarding its correctness. Any loss or damage arising from its use is not the liability of the provider.

Using the information below is strictly at your own risk.

Please contact City Special Metals for further questions or if you wish to place an order for Invar.