Vanadium is a ferrite stabilizer along with Cr, Ti, Nb, Mo.. etc. However, it is beneficial in the sense it is a carbide (VC in microalloyed steel or V4C3 in high speed steel) former and also form cabonitrides ([V(C,N)], both of which are helpful in increasing the strength of steel through precipitation hardening.

ِDear S.S. Nayak

Thanks  a lot for your answer.

Your answer is correct. But, Vanadium is an austenite stabilizer in FeCo alloys. Why???

With Bests

Because it lowers the energy for austenite formation.

Why? Because it does. You could write a PhD thesis on the thermodynamic modelling of the effect of Vanadium on the phase diagram of steel. Probably someone has. Do yourself a favor and don't bother reading it. Just accept that it does and move on to the next step.

By the way, Vanadium turned out to be a key ingredient for recreating the lost art of Damascus steel. Read the work of Paul Verhoeven. I had him as a professor in graduate school and at the time he has been working as a hobby on trying to recreate Damascus steel swords for over 20 years. He finally did it, all thanks to Vanadium. Don't ask why.

Vanadium forms carbides and is a ferrite stabilizer. In modified 9Cr-1Mo steel, V and Nb are added and give better creep strength, and is well known as P91 steel. This P91 steel and P92 steel or Japanese NF 616 steel (i.e., reduced Mo and W added) are preferred for high temeprature applications in new generation power plants. However, boron addition improves type IV cracking resistance and there is enough literature on this subject.

1) you may see this link:  http://www.brighthubengineering.com/power-plants/64886-the-workhorse-of-the-power-industry-p91/

2) You may go through the book: "High Chromium Ferritic and Martensitic Steels for Nuclear Applications" by Ronald L. Klueh and Donal R. Harries, ASTM, 2001 (ISBN 0-8031-2090-7).