The selection of 9% Ni steels for high pressure CO2 injection lines adopted by a Brazilian user in offshore oil production plant was based on the high strength and low temperature toughness properties that this material may achieve.The adopted material specification for piping included seamless ASTM A333 grade 8 pipes; ASTM A522 type 1 flanges and ASTM A420 grade WPL8 fittings. The piping material specification was created by the user to meet ASME B31.4 and ASME B31.8 codes in ASME pressure class 10.000.Minimum specified YS of 75 Ksi was important to reduce the thickness of the pipes and weight of the lines. Even adopting in design a high strength material the calculated pipe wall thickness for 8” diameter is 1.25” and for 14” diameter pipes is 2” which is a heavy wall pipe to be welded.The requirements for low temperature toughness for these high pressure gas lines come from minimum design temperature of down to -100°C and this is not a problem since the 9%Ni alloy steel presents a ductile behavior for temperatures down to -195°C tested in Charpy V.Also the weldability of this material is considered good since no preheating and no post welding heat treatment (for stress relief) are required. The only potential problem during welding is related to hot cracking and the adoption of AWS ER NiCrMo-3 filler metal and heat input control were considered the main solutions to avoid welding problems.The question about the suitability of the material for this specific condition appeared when it was stated by basic design that the produced CO2 lines should be also H2S resistant since this material is not prequalified by ISO 15156. More this steel is not classified as a low alloy (and cannot be treated according to ISO 15156-2) as well as is not a corrosion resistant alloy (and cannot be treated according to ISO 15156-3).To understand materials behavior exposed to the expected environmental conditions (dry CO2 eventually wet) the user tested several materials in a corrosion testing program and qualify the 9%Ni steel based on their own sampling and considered it approved based on an ideal chemistry (maximum 0.06%C and nominal 0.59%Mn 0.28%Si 9.08%Ni P< 0.005% and S< 0.001%) and maximum hardness values of 22HRc. Now all forgings fittings pipes and valves suppliers shall test their materials according to NACE TM 0177 and meet chemistry and hardness values to be considered qualified.But the main discussion which is presented in this paper is related to the welding effects on a heavy wall thickness pipe. Are the hardness values limits adopted for base material valid for multipass welded joints mainly heat affected zones? And the hardness limits can be adopted in WPS qualification tests even if the microstructures are different than the base metal? Small chemistry variations change SSC resistance of welded joints? The lack of stress relief for 2” thickness welded joint can be adopted without SSC risk? What are the essential variables to be adopted to qualify a WPS?Microstructures and hardness values of joints welded using different heat inputs are presented and discussed in this paper. Stress relief treatment cycles were also simulated and the hardness values and microstructures are presented.