Low C - 13%Cr martensitic stainless steels have been widely used in the oil and gas industries because of their high strength and excellent corrosion res istance in corrosive conditions. Recently, higher strength(over 110 ksi grade ) corrosion resistant alloys have been requested from users. Nb addition is consider ed to be one of the most appropriate strengthening methods. In this paper, the strengthening of low C - 13%Cr martensitic stainless steel is examined. The e ffect of Nb addition on mechanical properties and corrosion resistance of low C - 13%Cr martensitic stainless steel i s discussed. An optimized chemical composition i s also studied.
Martensitic stainless steels have been widely used for OCTG (oil country tubular goods) because they have high strength and excellent corrosion resistance in CO2 gas wells[1-13]. Recently, much attention has been paid for application of these steels in light sour environments as a partial substitute for 22%Cr duplex stainless steels . Lower C and higher Ni and Mo compared to Type AISI 420 (13%Cr - 0.2%C) steels are the basic modified points , and these grades are called super 13%Cr martensitic stainless steels. Lower C and higher Ni contents lead to high toughness. Therefore, it was possible to achieve high strength steels. Super 13%Cr martensitic stainless steel, whose h ardness and yield strength were HRC27 and 730MPa maximum respectively, has been registered in NACE MR0175[14]. Super 13%Cr martensitic stainless steels showed no significant effect of strength on SSC resistance [12]. These reported studies were limited to its strength up to 110 ksi grade( 758 -861MPa YS ). In this paper, higher strength steel than existing steels was focused. Nb addition combined with high tempering temperature was examined for development of 125 ksi grade( 861-965MPa YS ). Optimization of chemical composition was also studi ed.
Tests were conducted to clarify the following objectives;
to examine the effect of Nb on ability to reach the required strength with a high tempering temperature,
to see how the stee ls with high tempering temperature behave in SSC tests,
to see how combination of Nb and C contents affects the strength achieved with the higher tempering temperature.
: First melt was conducted to confirm effect of Nb addit ion on mechanical properties and corrosion resistance. Six kinds of laboratory melted alloy with changing Nb content from 0 to 0.09% were used. Chemical composition range of laboratory melted alloy s is listed in Table 1. Quenching temperature was 920 degree C. Tempering temperature was 525 - 700 degree C.
: Second melt was conducted to optimize chemical composition of the developed steel. Six kinds of laboratory melted alloy with changing Nb content from 0 .02 to 0.03% and C content from 0.01 to 0.05% were used. Chemical composition range of the second laboratory melted alloy s is listed in Table 2. Quenching temperature was 920 degree C. Tempering temperature was 525 - 700 degree C.
