×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Compare corrosion resistance of martensitic stainless hard to soft?
2

Compare corrosion resistance of martensitic stainless hard to soft?

Compare corrosion resistance of martensitic stainless hard to soft?

(OP)
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?
 

RE: Compare corrosion resistance of martensitic stainless hard to soft?

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   

www.EngineeringMetallurgy.com

RE: Compare corrosion resistance of martensitic stainless hard to soft?

2
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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Compare corrosion resistance of martensitic stainless hard to soft?

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

RE: Compare corrosion resistance of martensitic stainless hard to soft?

(OP)
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.

RE: Compare corrosion resistance of martensitic stainless hard to soft?

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

www.EngineeringMetallurgy.com

RE: Compare corrosion resistance of martensitic stainless hard to soft?

(OP)
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.
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources