Technical knowledge
Inconel 718 heat treatment explained: solution anneal and double ageing
Why Inconel 718 is solution annealed then double aged at 720 °C and 620 °C, what the gamma double prime phase actually does, and how condition affects machining, welding and API 6A compliance.
The three strengthening phases and why only two are wanted
Inconel 718 contains three phases that matter: gamma prime Ni3(Al,Ti) which contributes moderate hardening, gamma double prime Ni3Nb which provides the main strength increase, and the delta phase Ni3Nb which is essentially the same chemistry as gamma double prime but forms as coarse platelets at grain boundaries and contributes little strength. Delta phase is useful in controlled amounts because it pins grain boundaries and limits grain growth during forging, but too much reduces strength and toughness.
The standard heat treatment cycle step by step
- Solution anneal at 955-1010 °C. Dissolves niobium, aluminium and titanium into the matrix and establishes grain size. Higher solution temperature produces coarser grains and better creep strength; lower temperature gives finer grains and higher tensile strength.
- Quench, usually oil or water depending on section. Freezes the solute in place so nothing precipitates prematurely.
- First age at 720 °C for 8 hours. Nucleates gamma double prime and some gamma prime.
- Furnace cool at around 55 °C per hour to 620 °C. Slow cooling through this range is essential - it is what grows the precipitates to their strengthening size.
- Second age at 620 °C for 8 hours then air cool. Completes precipitation. Total ageing time is roughly 18 hours.
The result is a minimum of 1275 MPa tensile strength, 1035 MPa yield strength and 12 % elongation for bar to ASTM B637, with typical hardness around 36-42 HRC.
Oil and gas variants: direct ageing and API 6A limits
For oil patch components the requirement is strength plus sour service compatibility. Instead of a full solution anneal, material may be hot worked and then direct aged, which gives higher strength with slightly different toughness. Whatever route is used, NACE MR0175 compliance means keeping hardness typically at or below 40 HRC for API 6A 718, so heat treatment parameters must be tightly controlled and verified on prolongation coupons.
Practical consequences for machining and welding
- Machine before ageing where possible. Solution treated material cuts far more freely and gives much better tool life; age afterwards to develop strength.
- Weld in solution treated or over-aged condition. The slow precipitation kinetics of gamma double prime are precisely why 718 resists strain-age cracking, unlike faster hardening alloys.
- Avoid stress relieving between 595 °C and 815 °C. This range promotes strain-age cracking during subsequent ageing.
- Age the finished assembly. Welding already aged material cannot restore full strength in the heat affected zone.
How to verify you received correctly heat treated 718
Require calibrated furnace charts, test coupons from the same heat treatment batch rather than the same heat alone, tensile and yield results, elongation, hardness values and, for critical parts, microstructural examination showing grain size and absence of excessive delta phase. Hangbo supplies all of this with the EN 10204 3.1 certificate and can add an SGS third party test report on request.
Questions about this topic
Can Inconel 718 be used without heat treatment?
It can be used in the annealed condition where strength is not critical, but then you are paying for an age-hardenable alloy and getting roughly half the available yield strength. If strength matters, always specify the aged condition.
Is GH4169 heat treated the same way as Inconel 718?
Yes. GH4169 is the Chinese GB equivalent of UNS N07718 with matching chemistry and the same solution plus double ageing window, and it is normally heat treated to the identical cycle.
Why does Inconel 718 resist weld cracking better than other superalloys?
Because gamma double prime precipitates slowly, the heat affected zone can relax welding stresses before hardening occurs. Faster ageing alloys such as Waspaloy or Alloy 706 harden while stress is still high, which is when strain-age cracking starts.
What hardness will I typically measure on aged 718?
Typically 331 to 401 HB, roughly 36 to 42 HRC. Sour service specifications commonly cap this at 40 HRC, so always state the limit on your order.
How long does the full heat treatment take?
The ageing cycle alone takes about 18 hours including the controlled furnace cool, so plan furnace capacity accordingly for large batches or heavy sections.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email demi@nickel-alloy.com or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
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