Development of Spring and Fastener Characteristics for Alloy 725 for Oil and Gas Applications

Alloy 725 is a precipitation hardenable nickel-chrome-molybdenum alloy with high molybdenum content for oil and gas equipment applications and is used where high strength and superior corrosion resistance is required.  It is listed in NACE MR0175/ISO15156 at a maximum hardness of 43 HRC in the annealed and aged condition. There are applications where higher strength along with corrosion and cracking resistance are required. Alloy X-750, MP35N and Elgiloy are common spring materials for oil and gas application containing sour fluids. With high cost of raw materials MP35N is very expensive for many applications whereas X-750 may be marginal or not accepted by oil companies in sour environments. The ability for alloy 725 to be cold worked and aged to higher strength levels offers opportunities to use the alloy at the relatively high strength levels required for springs, pump shafts and fasteners. This paper presents the work done to optimize the amount of cold work required for this application as well as the heat treatment to achieve desired mechanical properties. Stress corrosion testing for this alloy was carried out in the spring temper (cold worked and aged condition). The results of initial studies are presented in this paper.