UNS N07718 (Alloy 718) is often seen as the workhorse of the Oil & Gas nickel-chromium alloy range, but also has numerous applications in Aerospace and General Engineering. The grade is one of a type that is precipitation-hardenable, and which, with its addition of niobium, molybdenum, aluminium and titanium, provides a metal that is relatively high strength, and has excellent creep-stress rupture strength and good corrosion resistance. It is easy to weld and durable at high temperatures, although has a spread of operating temperatures of between -200°C and +550°C.
However, as with most grades, there are different variations rather than just one “Alloy 718”.
This article is one of a series that will explain each variant and pin-point differences:
This first article will discuss the “Oil & Gas” API 6A CRA properties, the second the AMS5662, a third AMS5663 properties, finally the fourth will discuss the general engineering ASTM B637. Comparisons between the classes will be drawn.
Alloy 718 to API 6A CRA
Broder Metals Group stocks Alloy 718 in sizes from 1/2” (12.7 mm) to 124” (304.8 mm). in diameter.
Our stock is bought to meet API 6A CRA to the material designation 120k* and NACE MR0175/ISO 15156 and as such meets most application uses for the oil industry. However be aware that many end users of Alloy 718 for Oil & Gas applications may write their own specifications, usually extending certain requirements of the API 6A CRA standard, and such specifications must always be checked to ensure material conforms with the requirements.
*The designation refers to the latest API 6A CRA split of Alloy 718 into 2 classes according to the minimum requirement of the 0.2% Yield Strength, being either 120 ksi or 140 ksi.
Chemistry:
| Ni | Cr | Fe | Nb | Nb+Ta | Mo | Ti | Al | C | |
| Min | 50.0 | 17.0 | - | 4.87 | 2.80 | 0.80 | 0.40 | - | |
| Max | 55.0 | 21.0 | Bal | - | 5.20 | 3.30 | 1.15 | 0.60 | 0.045 |
| Co | Mn | Si | P | S | B | CU | Pb | Se | |
| Min | - | - | - | - | - | - | - | - | - |
| Max | 1.00 | 0.35 | 0.35 | 0.010 | 0.010 | 0.0060 | 0.23 | 0.0010 | 0.0005 |
| Bi | Ca | Mg | |||||||
| Min | - | - | - | ||||||
| Max | 0.00005 | 0.0030 | 0.0060 |
Melt Practice:
Dual melted – usually VIM (Vacuum Induction Melted) & VAR (Vacuum Arc Remelted) or ESR (ElectroSlag Remelted).
Hot Work Reduction Ratio:
Greater than 4.0:1
Heat Treatment:
Equipment and controls used are to be qualified as per API 6A or to a similar standard such as AMS 2750. Temperatures are only monitored by either a contact surface thermocouple or heat sink. Heat treatment monitoring charts are provided.
Furnaces are loaded such that the presence of one part does not adversely affect the heat treatment response of any other part in the same heat treatment load. Either a furnace load map or a statement of compliance is reported.
Phase 1 – Solution Anneal – 1021o C to 1052o C for 1 hour to 2.5 hours and usually cool in liquid
Phase 2 – Age Hardening – 774o C to 802o C for 6 hours to 8 hours and air cool.
Strict controls and reporting are maintained if the material has to be re-heat treated.
Material Testing Requirements:
Test Laboratories used must be ISO 17025 approved / accredited.
1.1 Macroetch examination is carried out on full transverse cross-sectional slices representative of the top and bottom of the final remelt ingot or product thereof, and the acceptance criteria is no worse than severity A on all 4 classes of ASTM A604.
1.2 Microstructural Analysis is also carried out in the final heat-treated condition. This examination reports the:
- Grain size (ASTM no 2 or finer)
- Duplex Grain Size (no topological duplex grain size allowed)
- Deleterious phases (at 100x and 500x magnification) (free from acicular phases except for individual and isolated grains, free from continuous networks of secondary phases along grain boundaries, and no laves phases).
1.3 Tensile Properties: This examination reports the:
1.3.1 Tensile
Carried out at room temperature, with the following acceptance criteria:
| Section thickness | 0.2% Yield Strength (min) ksi | 0.2% Yield Strength (max) ksi | Tensile Strength (min) Ksi | Elongation in 4D (min) % | Reduction in Area (min) % |
| ≤ 10” (254 mm) Ø | 120 | 145 | 150 | 20 | 35 |
| > 10” (254 mm) Ø | 120 | 145 | 150 | 20 | 25 |
1.3.2 Impact Toughness
Carried out at -60o C, with the following acceptance criteria:
| QTC cross section thickness | Test Orientation | Min Average Joules | Min single reading Joules | Min Lateral Expansion mm |
| <3” (76 mm) Ø | Longitudinal | 68 | 61 | 0.38 |
| ≥3” (76 mm) Ø – 10” (254 mm) | Transverse | 47 | 41 | 0.38 |
| >10” (254 mm) Ø | Transverse | 41 | 37 | 0.38 |
1.3.3 Hardness Test Requirements
Measured after the final heat treatment cycle, with the following acceptance criteria:
| Min HRC | Max HRC |
| 32 | 40 |
1.4 Non-Destructive Examination
100% ultrasonic tests shall be carried out in accordance with API 6A PSL 3 or PSL 4 (with Operator qualified to minimum SNT level 2.)
The next paper in the series will consider AMS5662 properties.
Please contact Broder Metals Group Limited, either through the webform section of the website or by any of the other contact methods list on the website if you have any questions.
