Compare corrosion resistance of martensitic stainless hard to soft? 2

[is2634]

Recently during a comparison test of various martensitic stainless steels I included a soft sample with my hardened specimens. By soft I mean as delivered from the metal supplier (presumably annealed; for 440C, the hardness measured a 20 on the Rockwell C scaled compared to 60 for the heat treated specimen.)
At 85 °C, 85 % relative humidity for 120 hours, the soft specimen had virtually no corrosion while the hardened specimen exhibited noticable corrosion.
Did the test get compromised? I have always read statements such as "to maximize corrosion resistance in martensitic stainless steel one must meet a minimum hardness value". Does that statement only apply when you perform some type of hardening process?

 

[Maui]

What was the surface condition of the heat treated specimen versus the as-received sample? Was the heat treated specimen machined in any way such that the surface had been subjected to material removal processes prior to heat treatment? Was the entire surface corroded or was it only the areas that had been machined/cut/sectioned. Was it passivated (I suspect that it was not)? A little more detail could go a long way in helping us to zero in on your problem.

Maui

http://www.EngineeringMetallurgy.com

 

[EdStainless]

When you harden these alloys you form Cr carbides, this effectively removes Cr from the base alloy thus lowering the corrosion resistance.
Since they will mostly be M23C6 so each atom of C will tie up ~4 atoms of Cr.
When the material is annealed all of the Cr is in the matrix and helps form the chrome oxide passive film that provides corrosion resistance.
Variation in surface finish will only make this worse as the heat treated samples often have some residual surface oxidation that further lowers corrosion resistance.

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Plymouth Tube

 

[arunmrao]

Ed yet another star for you!

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[is2634]

Thanks EdStainless! That is exactly what I was hoping someone could explain to me.

Maui,
The samples were prepared identically with milling and turning prior to heat treat. The only difference would be that the ones coming through heat treat would have been sandblasted after heat treat. The hardened sample was passivated as well.

Pictures are attached.

 

[Maui]

is2634, do you know what heat treatment recipe was used to harden these specimens? Specifically, can you tell us what tempering temperature was used?

Maui

http://www.EngineeringMetallurgy.com

 

[is2634]

Maui, we used nitrogen blanket furnaces rather than vacuum.
The process applied to the sample was:
Preheat @ 1000F
High Heat @ 1950F
Forced Air Quench
Cold Treatment to -100F
Temper @ 475F for 2 hours
Citric Acid Passivation.