The effect of Nb microalloying on microstructure,mechanical and corrosion properties of quenched and tempered 13% Cr-5% Ni-0.03% C-O.01% N martensitic stainless steels was investigated in comparison with a high nitrogen microalloyed steel.The microstructure,density and dispersion of high angle boundaries,nano-scale precipitates,amount of reverse austenite and microchemistry of precipitates were characterized by using optical,SEM,EBSD,TEM,XRD and atom probe analysis.The resistance to pitting corrosion was evaluated by using anodic polarization curves to determine pitting potential,Eb in 3.5% NaCl solution.The effect of increasing Mo content from 1% to 2% was investigated in low nitrogen Nb microalloyed steel.The combined effects of lowering nitrogen content in 13% Cr- 5% Ni-1% Mo-O.03% C steel to 0.01%,and adding Nb and Ti are to decrease the amount of Cr rich precipitates,as Ti and Nb preferentially combine with nitrogen to form Ti-Nb carbo-nitrides,which suppress the formation of Cr2N and Cr23C6.Nb rich precipitates of less than 20 nm size are found,which are very effective in imparting additional strength.By tempering at 600℃,an optimum amount of reverse austenite is obtained,which is used to advantage to impart adequate toughness.If nitrogen content is kept high in order to stabilize austenite,Cr rich precipitates occur irrespective of microalloying additions of Nb and V.The most significant finding is the improvement of pitting corrosion resistance with the suppression of Cr rich precipitates by Nb addition to steels with lower nitrogen content both in 1% Mo and 2% Mo bearing steels.The pitting corrosion resistance is found to decrease with the occurrence of Cr rich precipitates.Atom probe studies are being carried out to gain insight into the underlying mechanisms.The distinct advantages of enhanced strength and pitting corrosion resistance due to Nb addition are novel findings,which are yet to be exploited commercially for technological applications in oil and gas industries.