Hi! First time poster.
I'm interested in data, theories and speculation on the roll of niobium as a "microalloying" addition, and how it affects the oxidation of alloy steels. To elaborate, I say microalloying when infact sometime the niobium level is quite high, 0.8wt% for medium carbon steels. And by high alloy I mean 20+ percent Ni and 20+ percent Cr.
I'm sure this is well documented and researched. Any help is sincerely appreciated.
As far as I know, Nb - as well as Ti - is added to steel (as Fe-Nb) because of his high affinity towards carbon. Its role should thus be to fix C in a stable form - a carbide. I seem to recall it is also used to prevent some specific type of corrosion, but I don't know have details - more knowleadgable posters will say more.
Yes I am aware of its role as a carbide former and for strengthing microstructures with niobium carbonitride formation. I am also aware that this stabilization of the carbon increases resistance to certain types of cracking. However, most high alloy steels and stainless steels form a chromium oxide outer layer. I wonder what influence free niobium has on the formation of this oxide.
Niobium at lower concentrations (up to 0.3% wt) helps stop grain growth during heat treatment and also during operation at elevated temperatures (500-600 deg C). Mostly added to stainless steels. Noteable steels exhibiting the above are:
Jethete M160 (S150) FV535 AISI 347 (S130)
15/5PH (AMS 5659) 17/4PH (AMS 5622, AMS 5643) FV520B (S143, S144, S145) Custom 450Look here for details of spec.
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