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Specification of Nickel Base Alloy 718 (UNS N07718) for Oil & Gas Drilling and Production Equipment – API 6A718 for short

One of Broder Metals Group Ltd’s core alloys is Alloy 718.

Please see our product data sheet for details of Alloy 718, and chemistry and metallurgical properties referred to in this blog.

Some years ago we focused on one “flavour” of Alloy 718 – that for the Oil & Gas industry, and particularly to ensure that our products conform to a subset of the American Petroleum Industry’s product standard, API 6A, in this case API6A 718.

The certification and paperwork that accompanies any steel going into the Oil & Gas industry is almost as important as the steel itself. As a company, we therefore have to be aware of, and able to comply with, new editions of the API6A 718 standard as they are released. API intends to review / release new versions every 5 years – and as the First Edition was issued in March 2004, the Second Edition in December 2009, so 2014 could be an important year. In line with this we have seen a draft of the Third edition. We do not know how far this draft is towards a final release, so while we report details of it here, please do not rely on this as being what might actually be in the third edition when released.

The standard covers pressure containing and pressure controlling components, with the purpose of ensuring that such components are not embrittled by presence of an excessive level of deleterious phases (i.e. those secondary phases present in the microstructure of an alloy that have a negative effect on the desired mechanical properties, toughness or corrosion resistance of Alloy 718). Incidentally for budding metallurgists, the standard includes eight pages of photographic examples of acceptable and unacceptable grain structures.

By the way, one noticeable change in the latest draft has been the description – while the first two Editions in 2004 and 2009 were entitled “Specification of Nickel Base Alloy 718 (UNS N07718) for Oil & Gas Drilling and Production Equipment”, the draft third edition is now renamed “Age-Hardened Nickel Based Alloys for Oil & Gas Drilling and Production Equipment” and, while still being headed API6A 718, now includes specifications for Alloys 716, 718, 725, 925, 935 and 945.

The Third Edition draft also removes reference to ISO 10423 (Petroleum & Natural Gas Industries – drilling & production equipment – wellhead & christmas tree equipment) within the text. It also adds a definition for a heat treatment lot – material from the same remelt ingot, processed together using the same procedures and hot/cold work equipment, and of the same nominal dimensions (excluding length) and heat treated in the same furnace loads, including solution annealing, ageing & if applicable re-heat treatment.

Changes between the Editions of standards are obviously of prime importance to ensure that we continue to supply material that is fit for the purpose intended. Some changes can be purely cosmetic but some can be very important in deciding what to stock and what certification changes may be required – for example the draft Third edition introduces new categories of Alloy 718 – 718 120K and 718 140K.

To illustrate this, below please find the main headings of the API6A 718 standard, and changes that have occurred between Editions and what may occur in the next Edition. This is not a definitive or exhaustive list, but hopefully will give you a flavour and sense of the types of changes that have to be kept abreast of:

API 6A 718 2004
API 6A 718 2009
Draft API 6A 718
Chemistry requirements – please see our Alloy 718 data sheet
Measured of a remelt ingot per ASTM E354 / ASTM E1473
No change
Not changed chemistry but added that for 716, 725; 925; 935 and 945
Melt production:
Either:
Option a: Electric Furnace (EF) followed by one of Argon Decarburisation (AOD) or Vacuum Oxygen Decarburisation (VOD); and if required one or more Vacuum Arc Remelting (VAR)
Or option b: Vacuum Induction Melting (VIM) followed by one or more as appropriate of Electroslag Remelting (ESR) or Electroflux Remelting (EFR) or Vacuum Arc Remelting (VAR)
Not changed melt process but added that for 716, 725; 925; 935 and 945
Reduction ratio: 3.75:1 minimum
Reduction ratio: 4:1 minimum
No change
Heat Treatment
Equipment qualified & calibrated to API 6A
Must maintain copies of heat treatment charts for 5 years minimum.
Adds AMS 2750 as a a calibration standard reference. Instrumentation needs to be calibrated at least every 3 months and surveyed no less than once per year
Temperature monitoring – to use either contact surface thermocouple or heat sink as per API 6A or ISO 10423
No change
No change
Soln Anneal – 1021-1052 C for 1-2.5 hrs, cool in air, water, polymer or oil.
No change
No change
Age harden: 774-802C, for 6-8 hours; air cool.
No change
Same for 718 120k, 718 140k changed to 760-802 C for 6-8 hours & cool in air, inert gas, water, polymer or oil.
Complete re-heat treatment is permitted, or re-ageing without re- solution annealing provided cumulative ageing time is less than maximum allowed
No change
No change
Testing:
Microetch
Either a full transverse cross-section slice representative of the top & bottom of each final remelt ingot or (if the product is not tested at the mill & the test piece is not identified as to the relative location within the ingot or product) a full transverse cross-section slice from each end of billet, bar, or product.
No change
No change
Must use Canada’s etchant, Aqua Regia, Kallings etchant, or hydrochloric peroxide
Must use Canada’s etchant, Aqua Regia, Kallings No 2, or hydrochloric peroxide
Must use Canada’s etchant, Aqua Regia, Kallings etchant, hydrochloric peroxide, or dilute heated aqua regia
Product examined and rated to all four classes in ASTM A604 – all no worse than severity A
No change
No change
Microstructure
Tested at Centre, ¼ radius and at surface
No change
No change
Etchant: Kallings no 2 or Super Kallings or Seven acids.
No change
Must use Kallings no 2 or waterless Kallings, or Seven acids or Diluted Glyceregia, (plus others for 925/935/725/716)
Material manufacturer or supplier will retain metallographic specimen mounts for 5 years minimum from date of examination.
Grain size determined in acc with ASTM E112. Should be predominantly no 3 or finer for material less than 254 mm cross section, and no 2 or finer > 254 mm cross section
Average grain size to be no 2 or finer
No change
Duplex grain size defined as per ASTM E1181 – no topological duplex grain size as per ASTM E1181 allowed.
No change
No change
Metallographic
To be examined for deleterious phases at 100x & 500x under light microscopy. Product should be:
No change
No change
  1. free from continuous networks of secondary phases along grain boundary except for isolated individual grains (including of delta or carbides) not representative of bulk of microstructure.
No change
No change
  1. Free from acicular delta phase except for isolated individual grains not representative of bulk of microstructure. No individual grain should be surrounded with acicular delta phase.
No change
No change
  1. No laves phase
No change
No change
Tensile Test
Tensile – one test per remelt ingot per heat treat.
No change
Tensile – one test per remelt ingot per heat treat lot batch.
QTC should be a prolongation cross-section on thickest end or a sacrificial production part.
No change
No change
Measured with ASTM A370
No change
Will change the 0.2% yield strength from a minimum to a range and change the Tensile requirement (elongation & RofA stay the same):
For less than 254 mm dia:
718 120k requirements:
0.2% – 120-145 ski (827-1000 MPa)
TS – 150 ksi (1034 MPa) min
718 140k requirements:
0.2% – 140-150 ksi (965-1034 MPa)
TS – 165 ksi (1138 MPa) min
If results not acceptable, may carry out 2 additional tests (from same QTC with no additional heat treatment) – results of each must satisfy requirements.
Impact Test
Charpy V notch as per ASTM A370 at -60 C
No change
No change
Performed on set of 3 specimens – transverse to primary direction of grain flow unless geometry of QTC prevents use of transverse (less than 76 mm)
No change
No change
718 120 k requirements stays the same as now.
Values for 718 140K become:
<3” (76.2mm) – Longitudinal: Av Joules: 68; Min Joules 61; Lat Exp 0.38 mm
3”(76.2 mm) -10” (254 mm) – Transverse:
Av Joules: 47; Min Joules 41; Lat Exp 0.38 mm
If a test fails, can retest 3 additional specimens from same QTC (with no additional heat treatment) – each test to generate value greater than minimum above.
Hardness
Tested on or near surface – Brinell (10 mm ball at 3000 kgf) per ASTM E10 or Rockwell C as per ASTM E18 after final heat treatment cycle
No change
For Rockwell C, 3 adjacent indentations shall be made & mean hardness reported – no individual hardness number may be higher than 2 HRC units above maximum specified hardness number.
For Brinell testing, at least 1 indentation shall be made.
In case of dispute Rockwell C will be referee method.
Cross section testing will be performed on each QTC using Rockwell C method per ASTM E18. 3 adjacent Indentations will be made near the surface, 3 adjacent indentations at ¼ thickness & 3 adjacent indentations at centre. The mean hardness at each location shall be reported. No individual hardness number may be greater than 2 HRC units above the maximum specified hardness number.
718 120 k stays the same as now (32-40 HRC).
718 140K requires 34-40 HRC:
If any hardness requirements not met, 3 additional indentations can be made in immediate area to determine new hardness numbers – all 3 results must be within specification.
Certification:
Requires:
  1. Chemical analysis
  2. Melt practice
  3. Melt source
  4. Company performing hot working operations (if different from melt source)
  5. Company performing heat treatment operations (if different from melt source)
  6. Total Hot Work reduction ratio
  7. Actual heat treatment times & temps & cooling methods
  8. Name of test lab
  9. Statement that material complies with requirements of macrotech exam
  10. Predominant grain size.
  11. Statement of compliance with topological duplex grain size testing requirement
  12. Statement of compliance with requirements of metallographic examination for deleterious phases
  13. Tensile test results
  14. Impact test temperature, orientation & results
  15. Hardness test results
Adds in requirements for:
  1. Average Grain size
  2. Set of legible photo micrographs
  3. NDE results if performed
Changes 4, 5 and 10 to:
4. Name of facility performing hot working operations
5. Name of facility performing heart treatment operations
10. Average grain size

Hope this little insight into the workings of steel standards and specifications has been of interest, and if you are still awake and interested or have unanswered questions, please contact me or any of our sales team.

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