HIGH-STRENGTH CORROSION RESISTANT NICKEL-BASE ALLOY 725HS FOR SEVERE SOUR OIL AND GAS FIELD APPLICATIONS

摘要

Today's global exploration of oil and gas leads to a range of highly corrosive environmentsrnthat, in turn, require corrosion resistant high nickel content alloys for subsurface and wellheadrnequipment. Materials have to meet criteria for corrosion resistance and mechanical properties in severernenvironments for the required service life. This paper presents the latest development of a higherrnstrength grade of alloy 725 (UNS N07725), alloy 725HS. This new age-hardenable nickel base alloyrn725HS offers many advantages such as higher-strength, toughness and excellent corrosion resistance.rnThe standard alloy 725 grade is typically specified with 120 ksi (827 Mpa ) minimum yield strengthrncompared to alloy 725HS with 140 ksi (965 Mpa ) minimum yield strength. The grades are approvedrnto NACE MR0175 up to hardness levels of 40 HRC and 43 HRC maximum for the alloys 725 andrnalloy 725HS, respectively. The higher strength level of alloy 725HS offers higher strength for sourrnwell service compared with the traditionally used alloy 718 (UNS N07718). Corrosion data is alsornpresented showing the performance of the age-hardenable nickel alloys in seawater, resisting chloridernstress corrosion cracking, and in sour well environments using the slow strain rate and C-ring testrnmethods. Alloys 725 and 725HS are also shown to be suitable candidate materials in severelyrncorrosive high temperature oil patch environments. Sour environments in which alloys 725 and 725HSrnhave met the criteria for corrosion resistance are defined with and without free sulfur.