Inconel 686 (UNS N06686): Composition, Properties and Application Guide

Inconel 686 is a single-phase, austenitic nickel-chromium-molybdenum-tungsten alloy — UNS N06686, W.-Nr. 2.4606, EN NiCr21Mo16W — developed for the duty that standard Ni-Cr-Mo grades handle poorly: service in which oxidising and reducing media alternate, and in which welded joints must keep their corrosion resistance without post-weld heat treatment.

The grade carries 19.0–23.0% Cr, 15.0–17.0% Mo and 3.0–4.4% W, giving a combined Cr+Mo+W content of 40.7% and a PREN of 51 — the highest of the commercial Ni-Cr-Mo family — while carbon, silicon and iron are capped at ≤0.010%, ≤0.08% and ≤2.0%. The result is a corrosion-resistant nickel alloy that behaves like C-276 in reducing acids, like C-22 in oxidising media, and unlike either of them after welding.

This guide covers the metallurgical logic behind the grade, verified physical and mechanical data, selection rules against C-276, C-22, 625 and 690, and the parameters to specify on a purchase order. Figures are taken from the Special Metals technical bulletin SMC-024 and cross-checked against ASTM B575 / ASME SB 575 and ASME Code Case 2198.

Inconel 686 at a Glance

Key designations and properties of Inconel 686

These six figures answer most first-contact questions. The designations allow a UNS drawing, a European 2.4606 material number and a Chinese purchase specification to be cross-referenced in one step. Density converts a square-metre plate weight into freight cost. The melting range sets the welding window and the annealing temperature. PREN 51 is the fastest way to rank 686 against C-276 or C-22 on paper, and the standards list is what an inspector will ask for on the mill certificate.

Alloy 686 plate and coil in mill-finish condition. The grade is stocked as plate, sheet, bar, wire, seamless tube and welded pipe.

1. Where Inconel 686 Comes From

Inconel 686 was developed by Inco Alloys International, now Special Metals Corporation, and is protected by US patent 5,019,184. Its design brief was explicit: combine the reducing-acid performance of C-276 with the oxidising-media performance of C-22, then remove the weakest link in both — heat-affected-zone sensitisation after welding.

The starting point was Hastelloy C-276 (UNS N10276). C-276 is outstanding in hydrochloric and phosphoric acids and in reducing chloride service, but its 15.5% nominal chromium leaves it exposed in strongly oxidising conditions; after exposure to 760 °C its corrosion rate in the ASTM G-28 B test rises above 1000 mpy. C-22 (UNS N06022) solves the oxidising side with 20.5% Cr, but carries less molybdenum and therefore less resistance to reducing acids and to chloride pitting.

Alloy 686 takes the C-22 chromium level and pushes molybdenum and tungsten beyond both: 16.3% Mo and 3.9% W nominal, against 14.2% and 3.2% for C-22. Iron, the element that accelerates detrimental intermetallic precipitation in high-molybdenum austenitic alloys, is capped at 2.0% instead of the 4–7% typical of C-276.

The commercial consequence is a grade positioned at the top of the corrosion-resistant nickel alloy ladder. General chemical service is normally served more economically by C-276 or C-22; 686 is specified where chlorine, hydrogen chloride or hydrogen fluoride are present, where oxidising and reducing stages alternate in the same vessel, or where a welded fabrication must pass an intergranular corrosion test without a post-weld heat treatment.

2. Inconel 686 Chemical Composition

The specification limits below follow ASTM B575 / ASME SB 575. Tighter limits may be agreed for specific product forms or for DIN 17750 supply.

Chemical composition of UNS N06686

Optical emission spectrometry on a machined coupon. Carbon, silicon and iron are the three values that separate a compliant 686 heat from a substituted C-276 or C-22.

The engineering logic is a set of three high additions and three low ones. Chromium governs the oxidising side: it supplies the Cr₂O₃ film that reforms within seconds in nitric acid, wet chlorine or oxygen-bearing chloride streams. Molybdenum governs the reducing side and, together with tungsten, raises the pitting resistance equivalent to 51 — four points above C-22 and six above C-276 on the same calculation basis. Tungsten is not decorative: at 3.9% nominal it contributes roughly 1.7 PREN points and improves repassivation at pit initiation sites.

The low limits carry equal weight. Carbon at ≤0.010% removes the chromium-carbide grain-boundary network that causes sensitisation in welded 625 or 316L. Silicon at ≤0.08% suppresses the formation of intermetallic phases in the heat-affected zone, where segregation is most severe. Iron at ≤2.0% slows the kinetics of σ and μ phase precipitation, which is the mechanism by which high-molybdenum alloys lose toughness and intergranular corrosion resistance after elevated-temperature exposure.

For buyers, these three numbers are the practical audit point. A mill certificate showing C at 0.02%, Si at 0.4% or Fe at 4% does not describe alloy 686, whatever the heat number says; it describes a different grade. Positive material identification, spectral analysis and a G-28 corrosion test on a welded coupon are the three checks that close the gap between certificate and component.

3. Microstructure: Why 686 Does Not Sensitise After Welding

Alloy 686 is a solid-solution alloy: it derives strength from molybdenum and tungsten dissolved in a face-centred-cubic nickel matrix, not from precipitation hardening. That single structural fact explains most of its behaviour.

In the annealed condition the microstructure is equiaxed austenite with straight annealing twins and clean grain boundaries. During welding, the heat-affected zone passes through the 600–1000 °C window in which carbon, silicon and molybdenum diffuse. In C-276 and C-22 this window produces grain-boundary carbides and, on longer exposure, μ and σ phases; the boundary regions become chromium- and molybdenum-depleted and corrode preferentially.

Annealed 686 microstructure: equiaxed austenite, straight annealing twins, clean grain boundaries. Precipitation at these boundaries is what welding and elevated-temperature exposure are designed to avoid.

In 686 the same thermal cycle produces far less of this damage, because there is very little carbon and silicon available to form grain-boundary compounds and the low iron content restricts intermetallic nucleation. The measured effect is decisive. In the ASTM G-28 B ferric chloride test, annealed 686 corrodes at 12 mpy; after one hour at 760 °C the rate is 13 mpy, at 870 °C it is 17 mpy, and at 980 °C it is 27 mpy. Under the same treatment C-22 rises from 7 mpy to 2022 mpy and C-276 from 45 mpy to more than 1000 mpy.

This is why alloy 686 fabrications are normally placed in service without post-weld heat treatment: welding does not need to be "repaired" metallurgically, because the heat-affected zone never loses the passive film stability that the base metal relies on. The practical benefit is a shop-floor one — no furnace cycle, no distortion correction, no re-pickling of a heat-treated surface.

The trade-off is toughness at temperature. 686 is a corrosion alloy, not a creep-resistant superalloy. Impact energy of annealed material is around 405 J at room temperature and remains near 400 J after exposure at 540–650 °C, but falls to about 25 J after one hour at 760 °C and to 8 J at 870 °C. Continuous service above roughly 650 °C therefore requires a design justification, not just a datasheet temperature rating.

4. Inconel 686 Properties

Physical Properties

Physical properties of Inconel 686

The density of 8.73 g/cm³ is high for a nickel alloy and follows directly from the molybdenum and tungsten loading; it matters in weight-sensitive skid design and in freight costing, not in corrosion performance.

Mechanical Properties

Typical room-temperature values for annealed product, from mill test data. Minimum values for a specific order are those stated in ASTM B575 / ASME SB 575 for the product form and thickness concerned, and must be confirmed on the mill certificate.

Typical tensile properties of annealed Inconel 686 by product form

Tensile testing of an annealed specimen. Elongation of 56–71% across product forms is the practical marker of a properly solution-annealed, non-embrittled 686.

Strength falls slowly with temperature, which is adequate for pressure-retaining process equipment but not for hot-section structural duty:

Elevated-temperature tensile properties of Inconel 686

Ductility is the number worth noting. A 56–71% elongation in a 16% molybdenum austenitic alloy is unusual, and it is the property that makes cold forming, flanging and field repair practical. It also explains why the grade tolerates the thermal cycle of welding without cracking in the base metal.

Corrosion Performance

Corrosion performance comparison for Inconel 686

The crevice-corrosion figure is the commercial separator. In the standard oxidising chloride test solution, C-276 begins to suffer crevice attack at 50 °C and C-22 at 75 °C; 686 remains resistant above 85 °C, the upper limit of the test. Where a flanged joint, gasket face or tube-to-tubesheet crevice sits at 60–80 °C in a chloride stream, that difference decides the material selection.

5. Inconel 686 vs C-276, C-22, 625 and 690

Comparison of Inconel 686 with C-276, C-22, 625 and 690

Coupons of the five grades most often quoted against each other. Surface colour and finish differ with mill condition, not with corrosion performance — selection has to be made on chemistry and test data.

Three selection rules cover most enquiries:

1. General chemical service, reducing acids, no halogens at temperature — C-276 or C-22 will normally do the job at lower cost per kilogram and shorter lead time.

2. Wet chlorine, HCl or HF present, or oxidising and reducing stages in the same item — 686 is the defensible choice, because neither C-276 nor C-22 covers both ends of the redox range.

3. Fabrication cannot be post-weld heat treated, or the client's specification requires a G-28 test on the welded joint — 686 removes the sensitisation risk that C-276 and C-22 carry.

Where the decision hinges on 60–80 °C chloride crevice service, the critical crevice temperature data in the previous section is more informative than any PREN comparison, because PREN ranks pitting initiation while crevice corrosion is usually the life-limiting mechanism in gasketed and flanged equipment.

6. Typical Applications

Flue gas desulfurisation absorber and ducting — one of the largest single applications for alloy 686, where wet, aerated, chloride-bearing sulfuric acid attacks both the reducing and the oxidising side of the duty.

Chemical processing. Chloride-bearing organic process streams, acid-catalysed chlorination stages, and reactor internals where the vessel cycles between oxidising and reducing conditions. Alloy 686 is also used for overmatched weld overlays on carbon steel and low-alloy components, where a thin 686 deposit outperforms a thicker C-276 overlay.

Flue gas desulfurisation. Absorber internals, reheaters, outlet ducting, bypass dampers, fans and stack liners in wet FGD systems. The service combines sulfuric acid at pH 1 or below with chlorides above 100,000 ppm, aerated by the flue gas itself — a textbook alternating redox environment.

Pulp and paper. Continuous digesters, bleach plant washers, and chlorine dioxide stage equipment, where chloride and oxidising bleach chemistries meet.

Halogen service. Chlorine and hydrogen chloride handling, hydrogen fluoride alkylation, vinyl chloride monomer production, and uranium conversion to uranium hexafluoride, where HF and fluorinating agents rule out lower-alloyed grades.

Other duties. Chloride-route titanium dioxide production, chlor-alkali and corrosive alkali-metal handling, nuclear fuel reprocessing equipment, marine fasteners and seawater systems, and furnace furniture where oxidation resistance rather than load-bearing creep strength is the requirement.

Across all of these, the same selection logic applies: the material is chosen not because it is the most corrosion-resistant grade available, but because the duty crosses a redox boundary that single-character alloys cannot cross.

7. Inconel 686 FAQ

What is the difference between Inconel 686, C-276 and C-22?

686 is the high-chromium, low-iron evolution of the C-family: 19–23% Cr for oxidising media, 15–17% Mo plus 3–4.4% W for reducing media and pitting resistance, and Fe capped at 2.0% to suppress intermetallic precipitation. C-276 is the economical choice for reducing chloride service, C-22 for oxidising mixed acids, and 686 for duties that combine both or that must survive welding without heat treatment.

Why do the carbon, silicon and iron limits matter so much?

They determine whether the alloy still resists intergranular corrosion after welding or elevated-temperature exposure. A heat with C above 0.010%, Si above 0.08% or Fe above 2.0% will precipitate grain-boundary carbides and μ/σ phases in the heat-affected zone, and the welded joint will fail a G-28 test even though the base metal passes. Always reconcile these three values against the mill certificate before release.

Does Inconel 686 need post-weld heat treatment?

No, not for restoration of corrosion resistance. The grade is designed so that the heat-affected zone retains its passive film stability after welding, which is why it is often selected for shop fabrication without a furnace. Post-weld heat treatment may still be specified for dimensional or stress reasons, but it is not a corrosion requirement.

What are the welding precautions?

Use overmatching filler metal — ERNiCrMo-14 (INCO-WELD 686CPT) for GTAW/GMAW or ENiCrMo-14 for SMAW. Keep heat input low, use stringer beads, control interpass temperature and allow rapid cooling; high-molybdenum austenitic alloys are susceptible to solidification cracking and to elemental segregation in the weld pool if heat input is excessive. Qualify the procedure with a G-28 test on the welded coupon, and clean thoroughly before welding — chlorine, sulfur and lead contamination are the usual cause of field cracking.

Is Inconel 686 a high-temperature alloy?

It is a corrosion alloy with useful oxidation resistance, not a creep-resistant superalloy. ASME Code Case 2198 provides design allowables for Section VIII Division 1 service to 427 °C, and VdTÜV approves it to 400 °C. Above roughly 650 °C, impact toughness falls sharply after prolonged exposure, so higher-temperature use is normally limited to non-load-critical components where oxidation resistance is the governing requirement. For hot-section structural duty, 617 or 602CA are the appropriate grades.

How is Inconel 686 machined?

It work-hardens readily and has low thermal conductivity, so the practical rules are low surface speed, heavy feed, sharp positive-rake carbide tooling and generous coolant. Avoid dwelling in the cut, which work-hardens the surface and accelerates tool wear. Hot working is carried out between 870 and 1230 °C with heavy reduction above 1090 °C, followed by annealing at 1180–1200 °C and rapid cooling.

Which standards and product forms are available?

Plate, sheet and strip to ASTM B575 / ASME SB 575; bar and rod to ASTM B574; seamless tube to ASTM B622; welded pipe to ASTM B619; welded tube to ASTM B626; fasteners to ASTM F467 / F468; DIN 17750 for European supply. The grade is listed in NACE MR0175 / ISO 15156-3 for oil and gas service and covered by ASME Code Case 2198 (P-No. 43).

8. Specifying Inconel 686 on a Purchase Order

Final dimensional and chemistry verification before dispatch. On alloy 686, the mill certificate and the positive material identification result matter as much as the dimensional report.

A compliant order states more than the trade name. The following items remove almost all of the ambiguity that leads to substituted or out-of-spec material:

• Designation and condition — UNS N06686, solution annealed, with the product standard and edition (for example ASTM B575-23).

• Chemistry verification — mill certificate showing C, Si, Fe, Ti, Cr, Mo and W, plus positive material identification on receipt.

• Corrosion acceptance — ASTM G-28 Method B result on the base metal, and on a welded coupon where the item will be fabricated.

• Mechanical properties — tensile, yield and elongation against the specification minimum for the actual thickness, not a generic grade value.

• Fabrication requirements — welding procedure specification, filler metal classification (ERNiCrMo-14 / ENiCrMo-14), interpass temperature limit, and any restriction on post-weld heat treatment.

• Traceability — heat number, EN 10204 3.1 certificate, and dimensional and surface inspection records.

Alloy 686 is specified where the cost of a corrosion failure exceeds the cost of the alloy by an order of magnitude — typically in unplanned shutdowns on FGD absorbers, bleach plants and halogen handling systems. Buying on chemistry verification and welded-coupon test data, rather than on the grade name alone, is what converts that premium into service life.

Need alloy 686 plate, tube or welding consumables? Send the specification, product form and quantity — our metallurgy team will confirm availability, mill origin and certificate package within one working day.