Case hardening and corrosion resistance

[PeterCharles]

I was watching an advertising video which referring to case hardened as "improving corrosion resistance".
Given that the steel used was typically C 0.18%, Mn 1.15%, Cr 1.05%, Boron treated, will corrosion resistance really be improved by case hardening??

 

[redpicker]

I was watching an advertising video...

That's all you really need to say. Advertising done to sell, not educate.

That said, case hardening can effect the corrosion resistance, depending the hardening process and the type of corrosion you are trying to resist. For example, some case hardening operations will produce a surface scale that will add some protection to atmospheric corrosion. Now, you could remove the surface scale and you would still have the case hardening, but you wouldn't have the corrosion resistance, so you could say that it isn't the case hardening that provided the corrosion resistnace, but the change in the surface. Likewise, you could produce the same surface scale without hardening (or, ineffective hardening) and still get the corrosion resistance.

So, like with most advertising, there is some truth in it, just be careful.

rp

 

[PeterCharles]

It's actually a little more interesting in that!

On my desk is one of our components in a similar steel, case hardened and returned from an application suffering from corrosion. The surface is flaking off and a collegue commented "it looks as if the whole of the case hardened surface has flaked off". Is that likely?

With the passage of time more surface flaking is occuring. Later I intend to check the surface hardness below the flaking to see what the hardness is at this point.

 

[redpicker]

The surface is flaking off and a collegue commented "it looks as if the whole of the case hardened surface has flaked off". Is that likely?

Evidently. The steel chemistry you listed can be carburized. This process puts the case under a compressive load, but under the case, there are residual tensile stresses, which can result in stress corrosion cracking (or enviroonmental cracking) that could cause the case to "flake off" as you are observing. The "usual suapects" would be hydrogen sulfide (H2S), high chloride, or carbonic environments, but there are other ones, too. You could have the corrosion products analyzed to find if that contributed.

On the other hand, it could also be just a excessive example of case crushing and the corrosion is just a red herring.

rp