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Nickel Plating on 9Ni-4Co-0.3C Steel HT 220-240-KSI [Acft engine mount parts].

Nickel Plating on 9Ni-4Co-0.3C Steel HT 220-240-KSI [Acft engine mount parts].

Nickel Plating on 9Ni-4Co-0.3C Steel HT 220-240-KSI [Acft engine mount parts].

(OP)
Help! I have a bizarre problem.

Our 9Ni-4Co-0.30C steel alloy parts [HT220-KSI] [turbine engine aft mount] have a 0.0035--0.0055" thick nickel plating per BAC5746 applied to ~80% of the surface area.

BAC5746 requires post plating hydrogen embrittlement [PPHE] relief bake at 375F+/-15F for 23-hours min, which is in-line with other PPHE relief bakes for nickel plated HS steel parts [AMS2759/9, etc]. However, drawing notes for each of these 9Ni-4Co-0.30C steel parts EXPLICITLY requires PPHE relief bake at 475F+/-15F for 24-hours min.

We are now in the overhaul process of stripping-off the nickel plate, inspecting, repairing [corrosion, etc] and then re-plating.

Serious problem: our plating shop, like MOST plating shops, has capability to accurately PPHE bake at 275F+/-15F or 375F+/-15F... but these same ovens cannot reliably bake the parts at 475F+/-15F.

Unfortunately there is NO tech data available to explain why 475F bake was chosen. Soooo... I have beaten-up our local M&P guys for a real answer as to "why the 475F bake?", but have gotten a general [unsatisfying non-scientific] response: "because the drawings say-so".

A higher level [corporate] M&P plating expert thought that MAYBE there was a hydrogen diffusion-path issue, due to the [high density] deep plating thickness and large surface area coverage; so, perhaps, the extra temperature bump [100F] was needed to drive the hydrogen to bare steel areas within a reasonable time frame [24-hrs]. Another thought proposed was that the extra temperature would harden/densify the nickel plating, along with PPHE relief... but 450F seems a little low for that purpose.

NOTE.
The Corporate M&P engineer was quite disturbed that we would rely on nickel plating for corrosion protection of steel parts that could see extreme service temperatures up-to 850F: depending on the reference, max service temp for Nickel plate is either 700F or 800F.

Any experience along these lines out there??? PLEASE ADVISE!!!

Regards, Wil Taylor

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RE: Nickel Plating on 9Ni-4Co-0.3C Steel HT 220-240-KSI [Acft engine mount parts].

Nickel coatings that contain phosphorus will form Ni3P precipitates in the range of 420 to 500 F. However, electrodeposited nickel from sulfamate baths typically do not contain P. So, I can only surmise that the reason for the elevated temperature is for enhanced embrittlement relief of a large component with a thick coating.

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