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tonino59 (Aeronautics)
27 Jul 08 14:09
Chromium plated bolts are not easy to buy and often my company procurement ask for substitute parts. What would you use as subsitute part for bolts like NAS660()C()?
Would you recommend use of NAS670()U() as replacement, which is cres passivated?
RPstress (Aerospace)
28 Jul 08 8:10
Not sure what CRES alloy NAS670 is.

We usually use A-286 CRES as a general stainless bolt for airframe, including external use where salt spray and hydraulic fluid, etc., can be an issue. We would usually consider it as resistant as chromium plate, and even somewhat superior (for whatever reason we don't use chromium plate). A-286 bolting is generally very resistant to stress corrosion cracking. SCC might be an issue with A-286 at strengths above UTS 160 ksi.
 
wktaylor (Aeronautics)
28 Jul 08 14:44
tonino59...

Why were chrome plated bolts origionally specified? Chrome plating usally implies a high wear requirement.

Regards, Wil Taylor

RPstress (Aerospace)
29 Jul 08 8:19
Wil T. makes a very good point. Addressing corrosion/SCC by itself is not enough. If these fasteners are in stainless bushes or similar, then using bare stainless could result in bad galling, potentially giving service failures.
 
tonino59 (Aeronautics)
29 Jul 08 13:45
NAS6703-20 are A286 cres bolts, passivated. Since we are unable to find NAS6603-20 alloy steel chromium plated bolts, I asked what do you think of replacing them with cres passivated bolts, in areas where wear is a concern.  
wktaylor (Aeronautics)
29 Jul 08 14:11
tonino59...

I repeat...

Why were chrome plated bolts ORIGIONALLY specified?

Chrome plating usally implies a high wear requirement... and/or as RPStress elaborated... for anti-galling protection.
 
Regards, Wil Taylor
 

 
 

Regards, Wil Taylor

RPstress (Aerospace)
30 Jul 08 8:15
Ah - you say "in areas where wear is a concern."

A re-post in the tribology forum might get more recommendations, but I suspect that A-286 will cause significantly more wear/get significantly more worn than chrome plated.

You don't mention the nature of the part or the sort of work you company does...if it's a drawing for a flying part or GSE then a substitute of this nature would need approval from a whole bunch of people.
 
tonino59 (Aeronautics)
4 Aug 08 7:50
Chromium plated bolted were specified on drawing by other engineers, so I can only guess for what reason. It seems to me that the main reason is the temperature in one case and the wear in another one. Both are drawings for flying parts. Now I feel confident to replace them with cres passivated bolts in the case of critical temperature (600°F), but I am seeking  advise for use in the other case, where the chromium bolt is basically a hing pin.
wktaylor (Aeronautics)
4 Aug 08 10:44
tonino59...

1. I've discovered that when a part becomes non-available, then the following factors may be in-play...

1.1 Parts are being replaced at a higher rate than anticipated during routine maintenance due to in-service failures. This implies a poor engineering definition of the service environment, to begin-with. NOTE: in some cases, the service environment may have changed substantially.

1.2 Parts Procurement failed to anticipate production needs and/or the need for spare parts for fleet maintenance.

1.3 Your procurement folks failed to anticipate rising costs and lead-times for special parts procurement.

1.4 Your vendors failed to meet their commitments.

2. CAUTION.

2.1 Chrome plated steel is a fairly high wear capable combination... even though the steel is corrosion prone.

2.2 Passivated A285 is a tough alloy that is highly corrosion resistant... but has galling issues... especially if used at high temperatures with similar alloys.

2.3 KNOWING... not guessing... the circumstances surrounding the use of hardware is essential to finding an adequate substitute. An expensive and specialized coating (such as chrome plate) implies a need for specific service environment protection that cannot be randomly and "lightly" replaced. Substantial justification is essential.

2.4 CAUTION: if Your acft is certificated (FAA) or is built/maintained for/by the DoD, then configuration controls are in-place and material/parts substitutions must be carefully evaluated and approved by the regulating agency. Once an "easy to procure substitute" is in-place for a notoriously hard-to-get part, it is impossible to reverse course when an "aw-shit" is discovered, without an Airworthieness Directive [AD] or a TCTO or STCTO (USAF).

2.5 NOTE: passivated A286 bolt shanks are nominal diameter (no allowance for coating). The fasteners MIGHT be altered by an engineering drawing (creating a new part number) with flash-chrome plating (0.0002 max) or solid film lubricant. This option usually presents it's own challenges and headaches...
 

Regards, Wil Taylor

tonino59 (Aeronautics)
4 Aug 08 16:54
Thanks Wil for your detailed reply. Parts shortage was clearly a procurement failure, and I am obliged to choose an alternative among available ones. You know production line cannot stop. When you say A286 passivated bolts have galling issues, you mean "that sort of superficial welding"?
The term is not familiar for us italians. Can you advise for a different replacement in this situation? I believe that lot of designers wouldn't have used chromium plated bolts as hinge pin. I have designed hundreds of single pin joints, but never used such a bolt finish. I remember that kind of finish only on doors shoot bolts, but they are really "under wear" let me pass the term!. I just want to share my concern with you all before taking a decision, which is the reason why us engineers are paid for!
wktaylor (Aeronautics)
5 Aug 08 10:47
Tonino59..

A286 is a high temp alloy with nickel that is prone to the phenonema of galling... much as You described.

Per AGARD Multilingual Aeronautical Dictionary [1980]...

Galling = [It] escoriazione (or) consumo

Fretting = [It] sfregamento (or) usura

NOTE: Galling is the severe form of Fretting... where mechanical adhesion occurs between similar parts.

Low alloy steel is a superior material for friction/wear when plated with cadmium, zinc, nickel-flash, zinc-nickel, etc. The Problem: several of these finishes are becoming "banned materials" in the EU... and will eventually wear-off exposing steel to corrosive elements [eventual rusting]. NOTE: if the hinge is heavily lubricated with a grease filled with corrosion inhibitors, then conventionally plated steel might work "well-enough"... but caution is still advised.  

Regards, Wil Taylor

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