Inconel 686 Forgings (UNS N06686 / 2.4606)

What is Inconel 686?

Inconel 686 is a corrosion resistant nickel alloy for service where one component sees both oxidising and reducing attack, usually with chlorides or other halides present. The composition explains the behaviour:

• Chromium 19 to 23 % forms the passive oxide film that gives resistance in oxidising media such as nitric acid, ferric and cupric chloride solutions and hot flue gas condensate.
• Nickel as balance with molybdenum 15 to 17 % gives resistance in reducing media such as hydrochloric and sulphuric acid, where a chromium film alone will fail.
• Molybdenum with tungsten 3.0 to 4.4 % raises resistance to pitting, crevice corrosion and chloride stress corrosion cracking. Localised attack of this kind is the usual failure mode for stainless steel in these services.
• Carbon 0.010 % maximum and controlled iron suppress grain boundary carbide precipitation. Weld heat affected zones keep the corrosion resistance of the parent metal, and post-weld heat treatment is not required.

The alloy is single-phase austenitic and solid solution strengthened. There is no strengthening precipitate, so room temperature strength comes from the alloying elements and from any cold work applied afterwards. It is non-magnetic, welds with ERNiCrMo-14 filler, and is stable in the annealed condition with no ageing step.

Designations and applicable standards

The designations below are the ones used on purchase orders, mill certificates and inspection reports. We certify forgings against whichever of them the project requires.

Chemical composition of Inconel 686

Composition limits for UNS N06686 in weight percent. Nickel is the balance of the analysis. It is not a fixed 59 %, and a mill certificate reports it as the remainder after the other elements are measured.

Note. The sum of Cr, Mo and W in alloy 686 reaches about 40 %, against about 34 % for Hastelloy C-276 and about 36 % for Hastelloy C-22. Resistance to general, pitting and crevice corrosion rises with this total. This is the main reason 686 is specified in place of those grades.

Mechanical and physical properties

Values apply to the solution annealed condition. Two columns are given. The specified minimum is the acceptance value against the product specification. The typical figure is what a sound forging normally achieves.

Effect of section size. Tensile and elongation results fall as section thickness rises, because the quench rate at the core of a heavy forging is slower than at the surface. For sections above about 200 mm, agree the test location (surface, mid-radius or core) and the acceptance values in writing before the order is placed. The contractual values are those printed on the EN 10204 3.1 mill certificate, not the indicative figures on this page.

Corrosion performance

• Critical pitting and critical crevice temperatures above 85 °C in acidified 6 % ferric chloride, ASTM G48 Methods C and D.
• Serviceable in mixed acids at pH below 1 and chloride concentrations above 100,000 ppm.
• Good general and localised corrosion resistance in seawater, both annealed and cold worked.
• High resistance to chloride stress corrosion cracking, which limits austenitic stainless steels in chloride service.
• Weldable without post-weld heat treatment. The ultra low carbon keeps the heat affected zone resistant.

Forging and heat treatment

Alloy 686 cannot be age hardened. This is a common specification error. The alloy is single-phase and solid solution strengthened, so there is no precipitate to develop and an ageing cycle adds no strength. Holding the material in the intermediate temperature range can also precipitate secondary phases at grain boundaries and reduce corrosion resistance, which is the property the alloy is selected for. The correct final condition is a full solution anneal followed by a rapid quench.

Alloy 686 work hardens quickly. Intermediate anneals are used between heavy forging or cold forming operations to restore ductility. Where cold work is used to raise strength, for example in high strength bolting, the trade-off against localised corrosion resistance should be assessed for the specific service.

Inconel 686 forged product forms and size range

All items below are forged from ingot or forged bar in our own shop. They are not cut from purchased plate. Sizes shown are the routine working envelope, and larger pieces are quoted against drawing.

Machining. Forgings are supplied as forged with machining allowance, rough machined, or finish machined to drawing. State which is required on the enquiry, because it changes both the price and the lead time.

Manufacturing route

The sequence below is fixed for every alloy 686 order. Each step is recorded against the heat number, so the finished certificate traces back to the melt.

1. MeltingEAF with AOD or VOD refining, followed by ESR or VAR remelting for pressure retaining and sour service parts. Remelting removes segregation and inclusion clusters that would otherwise appear as ultrasonic indications.
2. Ingot inspection and croppingHot top and bottom discard removed, surface conditioned before the first heat.
3. Open-die forgingUpset and draw cycles on hydraulic press, starting at 1150 to 1200 °C and finishing above 950 °C, to break down the cast structure and reach the specified forging reduction ratio.
4. Ring rollingFor seamless rolled rings: pierce, punch and roll to final section on the radial-axial ring mill. This gives continuous circumferential grain flow.
5. Solution annealing1080 to 1150 °C, held to section, then rapid water quench. Furnace charts are recorded and issued with the certificate.
6. Rough machiningScale and surface layer removed before ultrasonic testing.
7. Testing and NDTChemistry, tensile, hardness, grain size, ultrasonic and penetrant inspection, plus PMI and corrosion testing where specified.
8. Certification and dispatchEN 10204 3.1 or 3.2 certificate issued, parts marked with heat number and hardness, packed in seaworthy cases.

Testing, inspection and certification

Where Inconel 686 forgings are used

Alloy 686 costs more than stainless steel and more than most nickel alloys. It is normally specified where an unplanned shutdown in a chloride bearing or mixed acid process would cost more than the material.

Inconel 686 compared with C-276, C-22 and 625

These four alloys are routinely cross quoted against each other. The choice comes down to the balance of chromium against molybdenum plus tungsten.

* For alloy 625 the figure is Cr plus Mo plus Nb, since it contains no tungsten.

Alloy 686 has the molybdenum and tungsten of C-276 together with the chromium of alloy 625. Where a process is consistently reducing or consistently oxidising, C-276 or 625 is usually the cheaper answer. Where it alternates between the two and carries chlorides, alloy 686 covers both. We forge Hastelloy C-276, Hastelloy C-22 and Inconel 625 on the same equipment, so alternatives can be quoted side by side.

Sourcing Inconel 686 forgings from Jiangyin Jiangnan Metal

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province. The plant sits in the Jiangyin forging cluster on the south bank of the Yangtze, about 90 minutes from the port of Shanghai.

Inconel 686 questions and answers

Related nickel alloy forgings

We forge the full nickel alloy range on the same equipment. These are the grades most often cross quoted against alloy 686.

• Hastelloy C-276 / N10276
• Hastelloy C-22 / N06022
• Hastelloy C-2000
• Inconel 625 / N06625
• Inconel 718 / N07718
• Inconel 600 / N06600
• Inconel 690 / N06690
• Incoloy 825 / N08825
• Monel 400 / N04400
• Alloy 59 / N06059
• All nickel alloy grades