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.
The impetus for this alloy development work was a need to create a high strength, high performance, lower cost material to fill a gap that exists between X-750 and the cobalt-based superalloys shown in Table 1. The need became apparent for an alloy with yield strength over 200 ksi [1379 Mpa] with sour service resistance approaching or equivalent to MP35N. High tensile strength is required for spring applications. The end use for such a product is currently restricted to small diameter applications generally less than 1 inch (25.4mm), for example spring, pump shafts and fasteners1. To this end, prototype spring wire was produced by cold working alloy 725 to a diameter of 0.330 inches (8.4mm), and processed at various aging temperatures to enhance mechanical properties to strength levels required for spring, fastener and pump shaft components.
The product was drawn to final size using 45% cold work. The material was subjected to a series of cold draw/annealing cycles to refine and optimize the microstructure. Standard wire drawing techniques and lubricants were used. These are the same processes employed for other common spring alloys such as X-750 and MP35N. The final product will require peeling and shaving prior to spring manufacture similar to other nickel and cobalt based alloys to insure a surface free of any imperfections from the drawing process. The microstructure of the straightened alloy 725 rod (0.330 inch OD) with 45% CW and subsequent aging at 1350F is shown in Figures 3 and 4. The structure shows uniform grains with twinning and a uniform hardness from surface to center.
