Abstract
Alloy 725 is a precipitation hardenable nickel-chrome-molybdenum alloy with higher molybdenum content than generally used in common nickel-based alloys used in oil gas applications. This alloy 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 such as springs, fasteners and pump shafts. Alloy X-750, MP35N and Elgiloy are common spring materials for oil and gas application containing sour fluids. With the 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. Alloy K-500 is used in some bolting applications but hydrogen charging can be a concern in certain 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 heat treatment of 45% cold reduced 725 to achieve the desired mechanical properties for these applications. 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.
