Age-hardened nickel alloys
What is API 6ACRA?4th July 2019
API 6A CRA is a supplement to API 6A, covering a group of high-performance alloys – specifically age-hardened nickel alloys, which includes Alloy 718, Alloy 725 and Alloy 925 as well as Alloys 716, 935 and 945. From a material supplier’s perspective, it covers the most relevant aspects of the metals properties: production route, chemical composition, mechanical properties, microstructure and test conditions.
API is the American Petroleum Institute. Formed in 1919, the trade association aims to promote safety across the natural gas and oil industry globally. One of its main activities is to bring together subject matter experts to develop and maintain standards that cover materials and operational practices.
API 6A is a large document, at over 400 pages, that covers all aspects of materials specification for wellhead and Christmas tree equipment. It goes into great detail to cover the requirements for specific components such as tubing and casing hangers, valve bodies and bonnets, stems, lock screws, flanges, hubs, clamps and connectors. It covers identification and marking of components, general dimensional control and operational parameters.
Langley Alloys stocks a number of these very high-strength nickel alloys, certified to both the relevant API (API 6A and API 6ACRA) and NACE (MR0175 – ISO15156-3) standards.
In all cases these alloys develop their high strength through a secondary ageing heat treatment. After solution annealing and quenching, they are aged by holding them at an intermediate temperature (c. 700degC) for several hours. This additional heat treatment results in the precipitation of very fine particles throughout the microstructure of the alloy – this help to ‘pin’ grain boundaries in place and thereby strengthen the alloy. These precipitates are created by alloying each of these grades with small quantities of Nb, Ti and Al which combine with Ni to form small particles.
Alloy 718 (UNS N07718, ASTM B637, W No. 2.4668) is perhaps the most widely specified of this family of alloys. Langley Alloys stocks in more than 30 different sizes, from ½” to 9” diameter, for the manufacture of components.
Alloy 725 (UNS N07725, ASTM B637) is less widely specified. It is a high-strength variant of Alloy 625, so offers comparable strength to Alloy 718. It has an increased alloy content compared with Alloy 718, containing significantly more Mo, which helps with resistance to pitting corrosion. Therefore, it is somewhat more expensive. Langley Alloys stocks a focused range of sizes, from 1” to 4” diameter.
Alloy 925 (UNS N09925, ASTM B805) is a relatively younger alloy. It is the high-strength variant of Alloy 825, and therefore is less highly alloyed than both Alloy 725 and Alloy 718. It is of interest to a number of large users as an alternative to Alloy 718 given the cost-out opportunities, where the absolute level of corrosion resistance required is not as high. Langley Alloys has a significant stock of solid bars from 4 ½” to 9” diameter, used in the manufacture of various oil tool components.What is NACE MR0175?What is precipitation hardening?