The Challenge of Specifying Alloys for Aerospace Applications

After searching through metals references and locating an ideal alloy for a component or product, the realities of the world of supply and demand can lead to delays, problems and disappointment.

Local service centers may not have the specific alloy in the temper and thickness required, or the order may not be big enough to have a large, integrated mill produce a heat lot.

The Challenge of Specifying Alloys for Aerospace Applications

Image Credit: Ulbrich Stainless Steels & Special Metals.

How Specifying Alloy Properties, not Chemistry, Can Address this Issue

The solution to the problems above may be to adopt a more flexible approach to the alloy specification, rather than fixing on a single alloy or chemistry.

This is particularly important when working with stainless steels or nickel-based alloys because there is such a wide range of options available. The extent of the range of options can be a positive though, if the supplier is given some room to explore other suitable options.

Rather than naming a specific chemistry, outline the characteristics and properties that are vital to the success of the application in question. These could include resistance to certain types of corrosion, strength, elongation, operating temperatures of the part, wear resistance or processing characteristics like weldability or formability.

When using this approach, suppliers of alloys can generally meet specific requirements.

The characteristics of nickel-based alloys and stainless steel are dictated by alloy additions to the base metal. Nowadays, there is likely a chemistry available - in either a generic or proprietary alloy - that can accommodate nearly every requirement, regardless of how specialized it is.

How can a Stainless Steel or Special Metals Service Center or Reroll Mill Help?

If an order is substantial enough, an integrated producer may be able to meet the requirements, though it could be several months before a heat of the required metal is produced.

If the required quantity is small or it is not viable to wait until the mill can provide it, a local service center could contact their vendor sources to obtain a similar alloy; that is, one that has been processed to meet, either fully or partially, the specified chemistry.

The service center will be able to make use of their reroll sources to custom manufacture the finished product using hot or cold rolled bands that had been produced previously.

The coils are precision rolled and annealed from thicker gauges to the properties and dimensions specified. This way, a reroll source is actually able to provide their customers with most specialty requirements.

In some circumstances, the reroll mill may even help to write the specification, particularly if the alloy needs precision gauge tolerances or a particular surface finish. With specific techniques like work hardening or thermal processing involved, being able to order these alloys may require a high level of understanding in applied metallurgy.

Factors to Consider When Specifying Aerospace Alloys

Even requesting a common place alloy such as Type 301 series stainless steel can be challenging due to the wide compositional range within the standard AISI chemistry.

Stainless steel Type 301 (high tensile) can attain a tensile strength of 270-300 ksi, but there is a different T-301 composition at the opposite end of the spectrum which work hardens comparatively slowly, making it well suited for deep drawing applications.

The Challenge of Specifying Alloys for Aerospace Applications

Image Credit: Ulbrich Stainless Steels & Special Metals.

Other grades of 300 series stainless steel can be equally flexible, meaning that selecting the right one can be confusing. Stainless grades 309 and 310, for example, are high temperature alloys that are primarily used within furnace fixtures due to their high carbide and oxidation resistance.

Alternatively, should the designer be searching for deep drawing properties, then 309 or 310 will also prove to be good design choices. Both these grades possess a high alloy content which allows extensive forming.

For all but the most specialist applications, there will likely be at least three or four suitable compositional options among the stainless steels or nickel-based alloys, and each of these will likely meet several combinations of wear resistance, corrosion resistance, high temperature cycling requirements or tensile strength.

Considering Yield and Tensile Strength in a Specification

A tensile strength figure is suitable for specifying forgings, castings and other heavier steel applications. However, this is not always suitable for wrought strip products, such as stainless steel and nickel-based alloys.

In some circumstances like the design of springs, yield strength may be a more meaningful attribute than tensile strength. Yield strength estimates the theoretical point at which a material ceases to be elastic and instead, becomes plastic.

Some alloys reach their yield point right before failure while others may reach it comparatively early on the stress/strain curve. With this in mind, specifying solely by tensile strength does not take into account alloys which may have a superior yield strength.

Springback is an area of concern for stampers. This characteristic can be managed by annealing the material at certain key points in the rolling sequence, though springback is a function of the relationship between the yield and tensile strength of metals and as such, it will differ among alloys.

It is also important to consider how ductile the material should be for a particular application.

The Impact of Grain Size on Manufacturing

Grain size can have an effect on the forming or deep drawing of strip products, making it an important consideration for fabricators.

Where grains are too coarse or lacking in uniformity, sidewalls of deep drawn components may become rougher and ‘orange peel’. Should the grains be too fine on the other hand, the part may tear. Should the ideal grain size not be immediately available, it can be custom manufactured using thermal processing.

Smaller grain sizes result in more uniform mechanical properties within alloys. These can also alter yield strength, tensile strength, hardness and the relationship between yield and tensile strength.

Where a maximum yield strength is specified, heat treating to attain a specific grain size should be controlled within the acceptable yield and tensile strength parameters.

The ASTM grain size code has a range of 0 to 13, with the lower end of the range indicating coarser grains and the upper end indicating a finer grain. A standard specification for stamped parts is 7 to 9. While special processing can achieve grain sizes outside of that range, at over 13 the steel would be moving towards non-recrystallization.

Evaluating Annealing and Tempered Conditions

Cold rolled or annealed stainless steels are available in ¼ hard, ½ hard, full hard, and spring temper hardness, the latter being referred to as extra full hard.

These different tempers are accomplished by rolling specific area reductions on fully annealed material. Different alloys will require variations in the amount of area reduction required, and the resulting temper.

Occasionally, soft tempers are required to withstand severe bends and accept deep drawing. Strength increases as the temper does, but this results in reduced formability.

Contemporary technology and advanced processing equipment allows some materials to be varied as much as 200 ksi over a range of tempers, and at any specified tensile or yield strength value.

It is also possible to tailor stainless and nickel-based alloys to meet certain requirements even where these may be beyond the standard specifications of the material.

For example, Armco Steel’s 17-7 PH precipitation-hardening alloy is generally available annealed or in 60% cold rolled, Condition C. It can also be rolled in other forms, such as 1/2C and 3/4C. These new, specially rolled grades are more formable than Condition C, but will precipitation harden at a higher strength than the annealed condition.

Specifying a Metal Surface Finish for Applications

Nickel-based and stainless alloys can be surface finished to make them more aesthetically pleasing, to carry lubricant more efficiently, or to accommodate the extra surface area required for good quality bonding or sealing.

The Challenge of Specifying Alloys for Aerospace Applications

Image Credit: Ulbrich Stainless Steels & Special Metals.

The finish is normally specified in microinches by either an arithmetic average of the surface roughness (measured by a profilometer) or an RMS (root mean square) value. It is good practice to highlight the surface roughness within a series of acceptable finishes.

Overall, it is advisable to be flexible when selecting materials or providing specifications. This allows providers to locate a suitable solution rather than being forced to reject the request outright.

Where alloys are not available at a service center, they are very likely obtainable from a reroll mill.

Cold rolled steel and special metal rerollers are common place throughout the metal industry. In the stainless, nickel and cobalt-alloy market, however, these are essential because these higher priced metals are expected to meet exacting specifications which require high levels of precision.

Even the smallest changes in metallurgy or surface finish can be a key factor in the success or failure of many such applications.

Acknowledgements

Produced from materials originally authored by Ron Brucker from Ulbrich.

This information has been sourced, reviewed and adapted from materials provided by Ulbrich Stainless Steels & Special Metals, Inc.

For more information on this source, please visit Ulbrich Stainless Steels & Special Metals, Inc.

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