A286 fastener discoloration 3

[DLscrews]

We are a supplier of fasteners and a concern has been ongoing that screws we provide may have an issue because of variations of color from batch to batch, and even within the same batch. Color ranges from light to dark gray. We've been told that heat tinting is somewhat normal due to the base materials and levels of the different elements within. Can someone shed some light on this please?

 

[EdStainless]

What is the final heat treat?
In general small variations in the surface finish, surface cleanliness, and the atmosphere at heat treat will produce variations in tint. The alloy itself is rarely the issue (though different alloys do look different).
In batch heat treat the location within the batch will result in slightly different temp, time, and gases, so you will get color variations.

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P.E. Metallurgy, Plymouth Tube

 

[DLscrews]

Parts are being processed to AMS2759/4 AMS5737M. Trying to find more info

 

[mrfailure]

To build on Ed's comments: color appearance is based on refraction of an oxide film that forms on the surface while the surface is passivating, with the color dependent on the film thickness. It can help understand the maximum temperature seen as the film grows to a maximum thickness where the surface is passivated. After that point there is no further growth. Besides the variables Ed mentioned, you also will see a different color if the exposure time at maximum temperature is too short for the film to fully reach equilibrium, in which case interpretation of the heat tint may interpret a lower temperature than had been seen.

I fully concur with Ed's comments that the variations you are seeing are insignificant, as long as they are in the 50 shades of gray.

 

[israelkk]

What is the procurement specification of the fasteners? AMS2759/4 AMS5737M are the heat treatment and material specifications but more is required according to the procurement specification.

 

[tbuelna]

Parts are being processed to AMS2759/4 AMS5737M...

You are using the wrong spec for heat treating A286 cres (AMS 5737). You should be using AMS 2759/3.

 

[MagBen]

This alloy is very prone to have heat tints due mainly the high Ti content (up to 2%). An XPS surface chemistry analysis indicated the heat tints were mainly Ti nitride and Ti oxide. Surface cleanliness, roughness, residual moisture etc. all affect the formation of tints.

Even under H2 atmosphere with a dew point better than -40 degrees, heat tints can be formed after solution strand anneal (1800F, minutes). The heat tints could affect the die life (if it is to be stamped) due to the abrasiveness of tints. so if the discoloration was on the parts after aging, you got nothing much to worry about.

 

[tbuelna]

Per israelkk's comment, here is a typical mil standard for a bolt made from A286 cres (AMS 5731). If you read note 6 of the mil standard it requires manufacturing in accordance with AMS 7477. AMS 7477 should list any acceptance criteria for surface discolorations, if required.

 

[EdStainless]

It isn't the DP at anneal that matters the most, it is during cooling. The most critical phase is from about 1000F down to 600F, a DP of -50C and O2 less than 1ppm, in pure H (or H + Ar, zero N) will be needed to keep the surface bright.
Though if you form nitrides during anneal then you are stuck with the color and if it is a thin part you will damage the properties (Ti that is tied up in nitrides will not help strengthen the material).

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

OP: where did the discoloration occur? after high temperature short time solution HT, or after low temp long time aging HT?
After solution HT, the tint is normally in brown or gold color, it is very thin (<3 nm), the heaviest I saw was about 2 microns. Acid cleaning or a slight polish grinding is sufficient to remove the heat tints. that's why I don't think this thin layer could impact mechanical properties.

Ed, could you elaborate a bit on this statement? do you mean Ti nitrides and oxides are easily to form at 1000-600F then at higher temperature? unfortunately our furnaces only have a limited capability to control the cooling rate. I understand that it needs more than -70C DP dry H2 at 1800F to reduce TiO.

 

[EdStainless]

In practice you won't be able to reduce TiO.
If you look at charts that show the formation of oxides (Cr mostly, but also Ti) you will see that it is greatest during the cooling, hence you need less oxygen present to suppress it.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

Since it was stated the parts are being heat treated per AMS 2759, note that AMS 2759/3 section 3.3.1 Heating Environment Atmosphere states, "For scale-free or discoloration-free parts, an air atmosphere and air cooling should be avoided."

 

[WKTaylor]

DLscrews...

my 2-cents worth...

After forging, Aerospace A-286 fastener shanks are PHT. This is followed by grinding or machining to final shank and thread-land surface dimensions [immediately before thread and fillet roll-forming/cold-working], to ensure uniform shank and thread finish quality with very tight tolerances. NOTE: dimensional contraction occurs during PHT, which can vary slightly heat-to-heat, so grinding/machining to critical dimensions PRIOR to PHT is unwise.

After these steps, the cleaned part has the finish applied: passivation per AMS2700 is typical when these parts are intended for high temp use; or are in contact with similar metals.

NAS4003 is a classic A-286 [160-KSI-FTU] male-fastener fabrication process [general] spec. 'mild staining' can occur due to the passivation process and/or after exposure to a corrosive environment [deliberate or inadvertent].

Some of the 'stained appearance' could also be from minor thickness variations of the passivation layer... which can arise due to base-metal cleanliness immediately prior to passivation and/or passivation temperature or concentration or bath stagnation during the process. Regardless I would expect some light passivation staining of the forged-head and the thread-tip, with very minor color variations along the ground/machined shank and in the rolled-threads/fillets.

Regards, Wil Taylor

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