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

 

[tk90]

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

Hi Ed, there is a temper step in the current ht process (above 200C).

Are you implying that a secondary heating process, below the temper temperature, should not affect hardness?

 

[EdStainless]

As long as you are at least 50F (30C) below the previous temper I wouldn't expect any impact on properties.
Maybe the first temper is too short of time?
I like Mints thought of finding a better coating.

 

[tk90]

I’m certainly not opposed to other coatings, but it seems like Geomet requires a curing step that would likely have the same effect.

I’m thinking my immediate actions are ensuring the h/t process is correct, and explore alternative coatings in parallel.

 

[MintJulep]

I didn't know that Geomet included an elevated temp cure.

Might be worth contacting them to ask if it's entirely temperature dependent, of if it's time and temperature. That is, maybe longer time at lower temperature would get the job done.

 

[TugboatEng]

The coating has thickness and is probably soft. That may be affecting your reading. Can you remove the coating and test again? Perhaps utilize a coating such as electroless nickel that can achieve an equal or higher hardness.

 

[mfgenggear]

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?

See attached file. 4140 data sheet
Look at the recommended tempering temperature. Now look at your heat treat certs.
What was the actual tempering temperature.
Now take the actual tempering temperature -50 degrees F for hydrogen embrittlement bake.
Take a an actual part and send it for a core hardness test. Look at depth on the data sheet.
4140 will not harden all the way through to 55 HRc

 

[tk90]

An update for all- EdStainless was correct, the temper step is at a low temp to maintain surface hardness. Subsequently, any baking/curing process that is above this temp is causing further hardness drop.

@mfgenggear- for reference, we were focused on maintaining case hardness. You are correct that through hardening these components to 55HRC is not possible.

 

[mfgenggear]

OP
4340 will through harden to the center core.
Up to 4 inches per side.
Look at Timkens heat treat data for metalurgist

 

[mfgenggear]

An update for all- EdStainless was correct, the temper step is at a low temp to maintain surface hardness. Subsequently, any baking/curing process that is above this temp is causing further hardness drop.

@mfgenggear- for reference, we were focused on maintaining case hardness. You are correct that through hardening these components to 55HRC is not possible.

Other option is to case harden the surfaces with carburizing or nitriding.
60 HRc but requires grinding for close tolerances