4140, Surface Hardness, and Coatings

[tk90]

Hello, our company currently specs a 4140 component, induction hardened to 55+ HRC. However, we then apply a corrosion resistant (ZnNi) coating which involves a baking step at 200°C to mitigate hydrogen embrittlement. In doing so, there is a mild annealling effect which decreases the surface hardness below our requirements. For full transparency, the coating/corrosion resistance is for aesthetic purposes but is a customer requirement; they would be open to alternatives that pass 300Hrs Salt spray testing.

As a workaround, I'd like to test modifying the process flow such that the coating is applied after quenching, but before tempering. I would then combine the tempering and baking step into one; baking at a much higher temperature.

Is this idea entirely off-base?

 

[EdStainless]

Isn't there a temper after your induction hardening?
IS 200C above that temp?
This sounds like something is incorrect in the process.

 

[mfgenggear]

OP
Look at the AMS 2759/9 spec, it specifies to bake at 50 degrees below the tempering .
Or 375 dgrees

 

[MintJulep]

Look into zinc-aluminum flake coatings such as Dacromet or Geomet.

They would:

• Avoid the Nickel toxicity problems
• Avoid potential for hydrogen embrittlement
• Easily meet 300 hours salt-spray
• Probably be less expensive