Stud bolts, threaded bar and bolt blanks
Double-end and full-thread studs, casing bolts, nuts and washers, M10–M100 and 3/8 in – 4 in, rolled or cut thread, to Grade 660 Class A, B, C or D.
Superalloy datasheet & forging capability
AISI 660 Forgings to ASTM A453 / A638 Grade 660, UNS S66286
Alloy 660 · Fe–Ni–Cr precipitation-hardening austenitic stainless steel · Open-die forged, ring rolled and machined to drawing
AISI 660 is an iron-based precipitation-hardening austenitic stainless steel, also designated UNS S66286, Alloy 660, A286, DIN 1.4980 and JIS SUH 660. It is specified for high-temperature bolting, valve internals and pressure-equipment hardware that must hold clamp load at temperature. Its defining property is a coefficient of thermal expansion close to that of austenitic stainless steel, which is why ASTM A453 Grade 660 exists at all. Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory in Jiangyin City, Jiangsu Province, China, forges this grade to customer drawings as bolting bar, seamless rolled rings, discs, shafts, valve stems and extrusion tooling in all four tensile classes, supplied with EN 10204 3.1 mill test certificates.
Key facts at a glance
- Grade
- AISI 660 (Alloy 660)
- Also called
- UNS S66286 · A286 · 1.4980 · SUH 660
- Standards
- ASTM A453 Gr 660 · ASTM A638 Type 660
- Alloy family
- Fe–Ni–Cr precipitation-hardening stainless
- Strengthening
- Gamma-prime Ni₃(Ti, Al) precipitation
- Nominal Ni
- 24.00–27.00 %
- Nominal Cr
- 13.50–16.00 %
- Nominal Ti
- 1.90–2.35 %
- Tensile min
- 895 MPa (130 ksi), all classes
- Yield min
- 585 MPa · Class D 725 MPa
- Service temp
- to ≈ 700 °C (1 300 °F) under load
- Expansion
- 17.6 ×10⁻⁶/°C, matches 300-series
- Melting route
- VIM + ESR (VAR on request)
- Supply condition
- Solution treated + aged, Class A/B/C/D
- Size range
- ⌀ 20–800 mm · up to 3 000 kg/piece
- Certification
- EN 10204 3.1 (3.2 on request)
Send your drawing for a quotation: sales@steelforgepieces.com · +86 189 2135 9659 (WhatsApp / WeChat)
§1
What is AISI 660 (ASTM A453 / A638 Grade 660)?
AISI 660 is an iron-based precipitation-hardening austenitic stainless steel containing roughly 24–27 % nickel, 13.5–16 % chromium and 1.9–2.35 % titanium, hardened by a gamma-prime Ni₃(Ti, Al) precipitate formed during a 720 °C age. The number "660" is the ASTM grade number, taken from ASTM A453 Grade 660 (high-temperature bolting) and ASTM A638 Type 660 (bars and forgings). It is the same alloy the aerospace industry calls A286, and it carries the UNS number S66286.
The design trade-off that matters in service is this: AISI 660 gives room-temperature strength comparable to a quenched-and-tempered alloy steel, keeps useful strength to about 700 °C, and, uniquely among high-strength bolting materials, expands at almost the same rate as the austenitic stainless flange it is clamping. A chrome-moly stud in a hot stainless joint loses preload on every thermal cycle because the flange outgrows it. A Grade 660 stud does not. The full title of ASTM A453 names this property explicitly: High-Temperature Bolting, with Expansion Coefficients Comparable to Austenitic Stainless Steels.
The alloy is fully austenitic and stays essentially non-magnetic in every condition, including after severe cold work, and it keeps good ductility down to cryogenic temperature. That distinguishes it from the martensitic precipitation-hardening stainless grades it is often confused with. 17-4PH and PH13-8Mo are both magnetic and limited to about 315 °C.
Where the alloy is used
- Pressure-equipment bolting. Flange studs, exchanger bonnet studding, nuts and washers on austenitic shells and heads.
- Steam and gas turbine hardware. Casing bolting, forged rings, shafts, spindles and gear blanks at 540–650 °C.
- Valve internals. Stems, seat rings, blocks and bodies for gate, globe, ball, check and plug valves.
- Oil and gas bolting. Wellhead, Christmas tree and subsea fasteners, normally as Class D under NACE MR0175 and API 20F.
- Extrusion press tooling. Container liners, die rings, die holders, mandrels, stems, pressure pads and sleeves.
- Furnace and heat-treatment hardware. Retorts, jigs, fixtures, rolls and manifolds, where scaling resistance runs to about 800 °C.
§2
Designations, standards and equivalents
| System / country | Designation | Notes |
|---|---|---|
| UNS (USA) | UNS S66286 | Root identifier for the chemistry, and the safest single callout on an international order |
| Common names | AISI 660, Alloy 660, A286 | All refer to the same alloy; none of them is a trademark |
| ASTM / ASME (USA) | ASTM A453 Gr 660 ASME SA-453 Gr 660 | High-temperature bolting: bars, bolts, studs, nuts, washers. Classes A, B, C, D |
| ASTM / ASME (USA) | ASTM A638 Type 660 ASME SA-638 Gr 660 | Bars and forgings for high-temperature service |
| SAE AMS (USA) | AMS 5731 / 5732 / 5734 / 5737 / 5853 | Aerospace bar, forgings and rings. See our A286 page |
| DIN / EN (Europe) | 1.4980 X6NiCrTiMoVB25-15-2 | EN 10269 (fasteners), EN 10302 (creep-resisting steels). Tightest sulphur limit |
| JIS (Japan) | SUH 660 | JIS G 4901, heat-resisting steel bars |
| GB/T (China) | GH2132, GH132 06Cr15Ni25Ti2MoAlVB | Chinese high-temperature alloy bar and forging designation |
| AFNOR (France, legacy) | Z6 NCT 25 | Superseded by EN 1.4980; chemistry equivalent |
| BS (UK, legacy) | HR 1810, BS S151 / S152 | Superseded by EN 1.4980; chemistry equivalent |
| Common shorthand | 15Cr–25Ni–1.3Mo–2.1Ti | Nominal composition shorthand, iron balance |
Important: AISI 660 and A286 are the same alloy, not two grades. They are the same chemistry (UNS S66286) approached through two different specification families. ASTM A453 / A638 Grade 660 is the industrial and pressure-equipment route, organised into tensile classes A, B, C and D. AMS 5731 / 5732 / 5734 / 5737 / 5853 is the aerospace route, organised by supply condition and melting practice. A drawing calling for one is not satisfied by a certificate to the other, so certify to the standard your purchase order names. Cross-certification is available on request.
This page covers the Grade 660 industrial route. For AMS aerospace callouts, VIM-VAR melting and turbine disc work, see our A286 / UNS S66286 forgings page.
Do not treat these as free substitutes
Russian and CIS ХН35ВТЮ (ЭИ787) and ХН35ВТ (ЭИ612) appear in many conversion tables as "A286 equivalents". They are not. Both carry 33–37 % nickel and a deliberate 2.5–3.5 % tungsten addition, and ЭИ787 adds 0.7–1.4 % aluminium. Western certification bodies will not accept them against an ASTM A453 Grade 660 callout without a written waiver. If you are converting a GOST drawing, ask for the actual chemistry before assuming interchangeability.
§3
Chemical composition of AISI 660
The composition below is the Grade 660 range common to ASTM A453 and ASTM A638, and is substantially the same under DIN 1.4980 and JIS SUH 660 apart from trace-element control. Every heat we forge is verified by spectrometric analysis, and both ladle and product analysis are reported on the mill test certificate.
| Element | Min % | Max % | Role in the alloy |
|---|---|---|---|
| Carbon (C) | - | 0.08 | Kept low to limit grain-boundary carbide networks |
| Silicon (Si) | - | 1.00 | Residual from deoxidation |
| Manganese (Mn) | - | 2.00 | Austenite stabiliser and deoxidiser |
| Phosphorus (P) | - | 0.040 | Impurity, restricted |
| Sulphur (S) | - | 0.030 | Impurity; tightened to 0.015 max under EN 1.4980 |
| Chromium (Cr) | 13.50 | 16.00 | Oxidation and general corrosion resistance |
| Nickel (Ni) | 24.00 | 27.00 | Stabilises the austenitic matrix; forms gamma prime |
| Molybdenum (Mo) | 1.00 | 1.50 | Solid-solution strengthening at temperature |
| Titanium (Ti) | 1.90 | 2.35 | Principal gamma-prime former, the hardening element |
| Aluminium (Al) | - | 0.35 | Co-former in gamma prime Ni₃(Ti, Al) |
| Vanadium (V) | 0.10 | 0.50 | Strength contribution |
| Boron (B) | 0.0010 | 0.010 | Grain-boundary strengthening; improves creep-rupture life |
| Iron (Fe) | Balance (≈ 52–54 %) | Base element. This is an iron-based alloy, not a nickel-based one | |
Correction to older published tables
A number of Grade 660 composition tables in circulation, including an earlier version of this page, list iron as "0.00 min / 0.00 max" and leave the boron minimum blank. Both entries are transcription errors, and they have been copied across dozens of supplier sites. Iron is the balance, and boron carries a specified minimum of 0.0010 % (EN 1.4980 raises that minimum to 0.0030 %). Boron is not optional in this alloy: below roughly 10 ppm the stress-rupture life falls away sharply.
Where a customer specification is narrower than the table above, we buy or melt to the customer's range and state it on the certificate.
§4
Grade 660 Class A, B, C and D: what actually differs
The four classes of ASTM A453 Grade 660 share one chemistry and one tensile minimum of 895 MPa (130 ksi). They differ only in heat treatment, and therefore in what the material is optimised for. Class A uses the lower solution temperature for best room-temperature tensile properties. Classes B and C use the higher 980 °C solution treatment for best stress-rupture life, differing only in quench medium to suit section thickness. Class D repeats the Class A solution treatment but adds a second age, raising minimum yield strength from 585 to 725 MPa and giving the controlled hardness normally required for sour-service and API 20F bolting.
ASTM A453 Gr 660
Class A
YS ≥ 585 MPa · 85 ksi
900 °C ±14 / 2 h min
→ liquid quench
→ 720 °C ±14 / 16 h
→ air cool
Best room- to moderate-temperature tensile properties and ductility. Default for general pressure-equipment bolting.
ASTM A453 Gr 660
Class B
YS ≥ 585 MPa · 85 ksi
980 °C ±14 / 1 h min
→ liquid quench
→ 720 °C ±14 / 16 h
→ air cool
Highest solution temperature, for optimum stress-rupture life. The usual choice for sustained service near 650 °C.
ASTM A453 Gr 660
Class C
YS ≥ 585 MPa · 85 ksi
980 °C ±14 / 1 h min
→ oil quench
→ 775 °C ±14 / 16 h → AC
→ 650 °C ±14 / 16 h → AC
Same creep intent as Class B, with an oil quench and double age for heavy sections where a liquid quench risks cracking.
ASTM A453 Gr 660
Class D
YS ≥ 725 MPa · 105 ksi
900 °C ±14 / 2 h min
→ liquid quench
→ 720 °C ±14 / 16 h → AC
→ 650 °C ±14 / 16 h → AC
Double-aged high-yield class. Normal pick where NACE MR0175 hardness control and API 20F bolting qualification apply.
| Class | Solution treatment | Quench | Precipitation treatment | Rm min | Rp0.2 min | A min | Z min |
|---|---|---|---|---|---|---|---|
| Class A | 900 °C ±14 (1 650 °F ±25), 2 h min | Liquid | 720 °C / 16 h → AC | 895 MPa 130 ksi | 585 MPa 85 ksi | 15 % | 18 % |
| Class B | 980 °C ±14 (1 800 °F ±25), 1 h min | Liquid | 720 °C / 16 h → AC | 895 MPa 130 ksi | 585 MPa 85 ksi | 15 % | 18 % |
| Class C | 980 °C ±14 (1 800 °F ±25), 1 h min | Oil | 775 °C / 16 h → AC, then 650 °C / 16 h → AC | 895 MPa 130 ksi | 585 MPa 85 ksi | 15 % | 18 % |
| Class D | 900 °C ±14 (1 650 °F ±25), 2 h min | Liquid | 720 °C / 16 h → AC, then 650 °C / 16 h → AC | 895 MPa 130 ksi | 725 MPa 105 ksi | 15 % | 18 % |
AC = air cool. Hardness for Grade 660 product generally falls in the 248–341 HBW band; where NACE MR0175 / ISO 15156 applies, the ceiling is 35 HRC. ASTM A453 also requires a stress-rupture test on finished product: 100 hours minimum life at 650 °C (1 200 °F) at the stress stated in the standard for the class ordered. Read the exact figures off the current revision of A453/A453M before writing them into a purchase specification.
Choosing a class in one paragraph
If the joint is a conventional stainless flange or valve bonnet running below about 550 °C and you want the simplest supply, order Class A. If the bolting sits in continuous elevated-temperature service where creep and stress rupture govern (turbine casings, hot process equipment near 650 °C), order Class B, or Class C if the section is heavy enough that quench cracking is a concern. If the specification chain runs through NACE MR0175 / ISO 15156 or API 20F for wellhead, subsea or valve bolting, order Class D and state the hardness ceiling explicitly on the purchase order.
§5
Mechanical and physical properties
| Property | Class A/B/C min | Class D min | Typical production |
|---|---|---|---|
| Tensile strength, Rm | 895 MPa · 130 ksi | 895 MPa · 130 ksi | ≈ 1 000 MPa · 145 ksi |
| Yield strength, Rp0.2 | 585 MPa · 85 ksi | 725 MPa · 105 ksi | ≈ 660 MPa · 95 ksi (Cl A/B) |
| Elongation, A5 | 15 % | 15 % | ≈ 22–25 % |
| Reduction of area, Z | 18 % | 18 % | ≈ 35–45 % |
| Hardness | 248–341 HB band | 248–341 HB band | ≈ 280–310 HB (29–33 HRC) |
| Charpy V-notch at −60 °C | not required by A453 | not required by A453 | ≈ 57 J · 42 ft·lbf |
| Modulus of elasticity, E | ≈ 201 GPa (29.1 × 10⁶ psi) at room temperature | ||
| Temperature | Rm (MPa) | Rp0.2 (MPa) | Elongation | Comment |
|---|---|---|---|---|
| 20 °C / 68 °F | ≈ 1 000 | ≈ 660 | ≈ 25 % | Reference condition |
| 200 °C / 390 °F | ≈ 960 | ≈ 600 | ≈ 24 % | |
| 400 °C / 750 °F | ≈ 900 | ≈ 560 | ≈ 22 % | |
| 540 °C / 1 000 °F | ≈ 870 | ≈ 530 | ≈ 22 % | Very common flange and valve service point |
| 650 °C / 1 200 °F | ≈ 770 | ≈ 480 | ≈ 20 % | Upper practical limit for loaded bolting |
| 700 °C / 1 290 °F | ≈ 650 | ≈ 410 | ≈ 18 % | Peak service temperature |
| 760 °C / 1 400 °F | ≈ 470 | ≈ 310 | ≈ 22 % | Gamma prime over-ages, transient duty only |
| Property | Value | Condition |
|---|---|---|
| Density | 7.94 g/cm³ (0.287 lb/in³) | 20 °C / 68 °F, aged |
| Modulus of elasticity | ≈ 201 GPa | Room temperature |
| Shear modulus | ≈ 78 GPa | Room temperature |
| Poisson's ratio | ≈ 0.31 | Room temperature |
| Thermal expansion | 16.5 / 17.6 / 18.4 ×10⁻⁶/°C | 20–100 / 20–540 / 20–760 °C |
| Thermal conductivity | ≈ 12.5 W/m·K | Room temperature. Low, so heating rates are controlled |
| Specific heat | ≈ 460 J/kg·K | Room temperature |
| Electrical resistivity | ≈ 0.91 μΩ·m | Room temperature |
| Magnetic response | Non-magnetic, μ ≈ 1.005 | All conditions, including after severe cold work |
| Melting range | ≈ 1 370–1 400 °C | Solidus / liquidus |
| Scaling resistance in air | to ≈ 800 °C | Oxidation limit, not a strength limit |
Data note. Minimum values in Table 4 are the specification requirements a certificate must satisfy. Typical values in Tables 4, 5 and 6 are indicative figures for preliminary design and enquiry only, and they are not design allowables. For ASME pressure-equipment design, use the allowable stresses tabulated in ASME BPVC Section II Part D for SA-453 Grade 660. Certified values for your order are reported per heat and per heat-treatment lot on the EN 10204 3.1 mill test certificate issued with the forgings.
§6
Thermal expansion matching, and why ASTM A453 exists
AISI 660 has a mean coefficient of thermal expansion of about 17.6 ×10⁻⁶/°C over 20–540 °C, within roughly 3 % of Type 304/316 austenitic stainless steel at about 18.2. A chrome-moly ASTM A193 B7 or B16 stud expands at about 13.5, some 25 % less. In a hot austenitic flanged joint the flange grows away from the chrome-moly stud on heat-up, over-strains it, and the joint returns to ambient with reduced clamp load. Grade 660 tracks the flange, so preload survives thermal cycling. This is the engineering problem ASTM A453 was written to solve.
Figure 1. Differential growth in a bolted stainless joint at 540 °C
| Material | CTE ×10⁻⁶/°C | Mismatch vs 316 | Consequence in a hot stainless joint |
|---|---|---|---|
| Type 304 / 316 flange | ≈ 18.2 | reference | The component being clamped |
| AISI 660 / A453 Gr 660 | ≈ 17.6 | −0.6 | Matched, so preload is retained |
| Alloy 800H bolting | ≈ 17.0 | −1.2 | Good match, lower strength |
| Inconel 718 | ≈ 14.4 | −3.8 | Noticeable mismatch, tolerated for its strength |
| Nimonic 80A | ≈ 14.5 | −3.7 | Used where temperature outranks match |
| ASTM A193 B16 (1Cr-Mo-V) | ≈ 13.5 | −4.7 | Significant preload loss per cycle |
| ASTM A193 B7 (4140) | ≈ 13.5 | −4.7 | Significant preload loss per cycle |
Mean coefficients rounded for comparison. Exact values vary with source, product form and the temperature interval used; for joint calculations take the coefficient over your own service interval from the material certificate or the governing design code.
The practical consequence is a maintenance one. On a 316 exchanger bonnet cycling to 500 °C, chrome-moly studding is typically re-torqued after commissioning and again after the first few thermal cycles, and gasket leaks appear as bolt relaxation accumulates. Grade 660 studding in the same joint holds a far more stable clamp load. When the cost case is presented internally, the number that matters is not price per stud. It is unplanned shutdowns per year.
§7
Creep, stress rupture and the real temperature ceiling
Short-time tensile data flatters any high-temperature alloy. For bolting held under load for years, stress rupture and relaxation govern, which is why ASTM A453 mandates a rupture test on the finished product rather than only a tensile test on a bar sample. Grade 660 is required to survive 100 hours minimum at 650 °C (1 200 °F) at the stress stated in the standard for the class ordered, with a minimum elongation on the ruptured specimen.
| Temperature | Stress for rupture in 100 h | Stress for rupture in 1 000 h |
|---|---|---|
| 540 °C / 1 000 °F | ≈ 690 MPa (100 ksi) | ≈ 620 MPa (90 ksi) |
| 595 °C / 1 100 °F | ≈ 570 MPa (83 ksi) | ≈ 480 MPa (70 ksi) |
| 650 °C / 1 200 °F | ≈ 450 MPa (65 ksi) | ≈ 370 MPa (54 ksi) |
| 705 °C / 1 300 °F | ≈ 275 MPa (40 ksi) | ≈ 205 MPa (30 ksi) |
Indicative values for screening only, consistent with published Grade 660 literature and our own rupture testing. They vary with class, section size and melt route and must not be used as design allowables. Lot-specific rupture testing is available as a supplementary requirement.
Where the alloy stops
AISI 660 should not be used for prolonged loaded structural service above about 700 °C. Above that, gamma prime over-ages and coarsens and strength falls away quickly. Table 8 shows rupture strength roughly halving between 650 and 705 °C. Note the distinction that trips people up: the alloy resists scaling in air to around 800 °C, but that is an oxidation limit, not a strength limit. For continuous loaded service above 700 °C, move to Inconel 718, Waspaloy or Nimonic 80A depending on whether strength or oxidation drives the design.
§8
Sour service: NACE MR0175 / ISO 15156 and API 20F
UNS S66286 is recognised in NACE MR0175 / ISO 15156-3 for defined sour-service conditions, with a maximum hardness of 35 HRC. It is normally supplied as Grade 660 Class D, because the double age reaches the higher yield strength while keeping hardness inside that ceiling. Meeting a hardness limit is not the same as being fit for your environment. Published corrosion work shows A286-type material to be severely restricted by chloride stress-corrosion cracking in high-chloride completion brines, failing in calcium-chloride brine at temperatures as low as roughly 95 °C, even while tolerating moderate H₂S. Verify the specific temperature, H₂S partial pressure and chloride limits against the current edition of ISO 15156-3 before specifying.
What the certificate needs to say
- Hardness reported, not just claimed. State "35 HRC maximum per NACE MR0175 / ISO 15156" on the purchase order and require the measured value, test method and locations on the mill certificate.
- The class and the full heat-treatment record. For sour service the aging cycle is what produced the hardness; a certificate naming Class D without temperatures and hold times is incomplete.
- The API 20F bolting specification level where wellhead, subsea or Christmas tree equipment is involved. BSL-2 and BSL-3 add impact testing and traceability requirements above the ASTM baseline. Tell us the BSL level at enquiry stage, because it changes the test plan and not only the paperwork.
Our position on sour service
We will forge and certify Grade 660 Class D to your sour-service specification and report every number. We will not tell you that AISI 660 is a general-purpose sour-service alloy, because it is not. Where the environment combines high chloride with elevated temperature, the alloys that pass the same tests reliably carry appreciably more nickel and molybdenum: Inconel 718, Incoloy 925, Incoloy 945 or Hastelloy C-276. If an enquiry looks like the wrong alloy for the duty, we say so before quoting.
§9
Forging practice, heat treatment, welding and machining
AISI 660 is not a difficult alloy to forge, but it is an unforgiving one to heat treat. The route below describes our standard practice; engineers preparing a supplier-qualification package or a process flow diagram can map it against their own requirements.
| Step | Typical parameters | Purpose |
|---|---|---|
| Melting | VIM + ESR standard; VIM + VAR on request | Titanium and boron are oxygen sensitive, so both are charged late and verified on more than one sample per heat |
| Preheat and soak | ≈ 1 150 °C, soak to through-heat | Avoids steep gradients in a low-conductivity alloy |
| Forging temperature | Start ≈ 1 150 °C, finish above 950 °C | Never above ≈ 1 175 °C (incipient melting) or below ≈ 950 °C (tearing) |
| Forge reduction | ≥ 4:1 on discs and rings, ≥ 6:1 on bar | Breaks down the as-cast dendritic structure |
| Solution treatment | 900 °C (Class A, D) or 980 °C (Class B, C) | Dissolves gamma prime and homogenises the matrix |
| Quench | Liquid or oil per class · transfer under 30 s | The single most critical timing parameter. See the note below |
| First age | 720 °C (775 °C for Class C) / 16 h ± 15 min, air cool | Precipitates the fine gamma prime that carries the strength |
| Second age (Class C, D) | 650 °C / 16 h, air cool | Develops the higher yield strength and controlled hardness |
| Straightening / sizing | Cold or warm, then stress relieve if required | Meets straightness tolerance on long bars and studs |
The 30-second rule
The furnace-to-quench transfer after solution treatment is interlocked under 30 seconds at our works. Linger in the 760–700 °C band and η-phase Ni₃Ti precipitates on the grain boundaries within seconds. It cannot be reversed by any subsequent treatment, it embrittles the boundaries, and it is the origin of most Grade 660 strain-age cracking reported from the field. The 16-hour age is equally non-negotiable: held short, the gamma prime is under-developed and yield falls below minimum; held long, even by one shift to suit furnace scheduling, and Ostwald ripening coarsens the precipitates and room-temperature yield drops measurably.
Exact soak times are set by section thickness and by the customer specification or the applicable A453/A453M revision. Furnace uniformity is surveyed to AMS 2750 pyrometry practice. Where an OEM procedure is supplied, we heat treat to that procedure and record furnace charts with the certificate.
Welding guidance
AISI 660 is weldable by GTAW, GMAW, EBW and laser processes with matching filler per AMS 5805. Weld in the solution-treated condition, not the aged condition. The failure mode to design against is strain-age cracking: in a restrained joint the post-weld thermal cycle causes gamma prime to precipitate at the same time as residual stress relaxes, and boundaries already weakened by η-phase crack intergranularly. Controlled heating and cooling rates through the 600–800 °C window, and minimising restraint, matter more than filler choice. Any welding after final heat treatment requires a full re-solution treatment and the complete age cycle afterwards to restore properties.
Machining and threading
In the aged condition AISI 660 machines at roughly 30–40 % of the rate of Type 304 and work-hardens quickly. Use rigid setups with minimum overhang, sharp positive-rake carbide, heavy positive feeds, cutting speeds around 25–40 m/min for turning, and generous coolant. Never let an insert dwell or rub without cutting, because a work-hardened skin forms immediately and destroys the following pass. Many shops rough in the solution-treated condition and age afterwards, accepting the small dimensional movement that aging brings.
Threads gall against austenitic stainless nuts exactly as 300-series bolting does. Rolled threads outperform cut threads for fatigue life and are the norm for bolting stock. In assembly, use a high-temperature anti-seize compatible with the service. Note that the friction coefficient assumed in a torque figure changes completely with the lubricant, so a torque value quoted without naming the lubricant is not a specification. On critical joints, specify bolt tensioning or turn-of-nut rather than torque alone.
§10
AISI 660 forged forms we produce
All shapes below are forged to customer drawing in AISI 660 / UNS S66286 / ASTM A453 Grade 660 / ASTM A638 Type 660. We are an open-die forging factory, so there are no die costs and no minimum production volume driven by tooling. One-off replacement parts and small maintenance batches are welcome alongside series work.
Round, hex, square and flat bars
Forged bars and rods, turned, peeled or centreless ground, as feedstock for fastener manufacture and machined components.
Seamless rolled rings
Radial-axial ring rolled rectangular, contoured and T-section rings, with grain flow following the circumference. Sealing, spacer and retaining rings.
Discs, blocks and blanks
Upset-forged discs, hubs, blocks, plates and slabs for turbine wheels, tube sheets and near-net machining blanks.
Sleeves, bushes and hollow bar
Bushings, bearing sleeves, cases, shells and hollow sections, trepanned or mandrel forged.
Stems, seats and bodies
Valve stems, seat rings, blocks, bonnets and spindles for gate, globe, ball, check and plug valves in high-temperature or hardness-controlled service.
Liners, die rings and mandrels
Container liners, die rings, die holders, die inserts, liner holders, extrusion stems, mandrels, pressure pads and sleeves for aluminium and copper presses.
Shafts, spindles and custom shapes
Stepped and flanged shafts, eccentric shafts, spindles, gear blanks, forged flanges, tube sheets, nozzles and any non-standard geometry forged to drawing.
§11
Size capability, condition and delivery
| Parameter | Capability |
|---|---|
| Round bar diameter | 20 – 800 mm (0.79 – 31.5 in) |
| Bolting sizes | M10 – M100 · 3/8 in – 4 in · length to 1 200 mm |
| Ring outside diameter | 100 – 2 500 mm (3.9 – 98 in) |
| Disc / block | Up to 1 500 mm diameter, up to 600 mm thick |
| Maximum single-piece weight | Up to approx. 3 000 kg (6 600 lb) |
| Minimum order | One piece. Open-die means no tooling charge |
| Melting route | VIM + ESR standard; VIM + VAR or double ESR on request |
| Delivery condition | As forged · rough turned / black · solution treated only · solution + aged to Class A/B/C/D · finish machined to drawing |
| Surface options | Machined, ground, polished, shot blasted, anti-rust oil, painted |
| Typical lead time | 25 – 45 days from drawing approval, depending on section and testing |
| Packing | Fumigation-free plywood cases, seaworthy, VCI wrapped |
Capability figures describe our standard working envelope. If your part sits outside it, send the drawing anyway and we will tell you plainly whether it is within our press capacity or better placed elsewhere.
§12
Testing, inspection and documentation
Quality control covers the whole route: melting, forging, heat treatment, machining and final inspection before release. The following applies to AISI 660 forgings:
| Item | Method / standard | Scope |
|---|---|---|
| Chemical analysis | Spectrometric, ladle and product, per heat | Standard |
| Tensile & hardness | ASTM A370 / ISO 6892-1, per heat-treatment lot | Standard |
| Ultrasonic testing | ASTM A388 / EN 10228-3 / SEP 1921 or customer class | Standard |
| Liquid penetrant (PT) | ASTM E165 / E1417 | Standard on machined surfaces |
| Magnetic particle (MT) | Not applicable, the alloy is non-magnetic | PT used instead |
| Stress rupture | 100 h at 650 °C per ASTM A453 for the class ordered | Per class / on request |
| Charpy V-notch impact | ASTM E23 at specified temperature. Required by API 20F, not by A453 | On request |
| Elevated-temperature tensile | e.g. 540 °C / 650 °C per specification | On request |
| Grain size / microstructure | ASTM E112, micrograph supplied | On request |
| Macroetch / grain flow | ASTM E381 on disc and ring sections | On request |
| Dimensional report | Against approved drawing | Standard |
| Certificate | EN 10204 3.1 standard; 3.2 with TÜV, SGS, BV or Lloyd's Register | Standard / on request |
| Third-party witness | Customer or agency witnessed inspection at our works | Welcome. Nominate the body at enquiry stage |
Witness points must be planned into the forging and heat-treatment route, not added afterwards. Nominate your inspection body with the enquiry rather than after the purchase order.
§13
AISI 660 compared with alternative bolting grades
AISI 660 is rarely chosen for raw strength, because B7 and B16 are stronger at room temperature and far cheaper. It is chosen when a joint has to hold clamp load hot, in an austenitic structure, without paying nickel-base prices.
| Alloy | Base | Rp0.2 min | Temp ceiling | CTE | Where it is chosen instead |
|---|---|---|---|---|---|
| AISI 660 / A453 Gr 660 | Fe–Ni–Cr | 585 / 725 MPa | ≈ 700 °C | 17.6 | Expansion-matched hot bolting in austenitic joints |
| ASTM A193 B7 (4140) | Cr–Mo steel | 725 MPa | ≈ 400 °C | 13.5 | General purpose, low cost, moderate temperature |
| ASTM A193 B16 (1Cr-Mo-V) | Cr–Mo–V steel | 725 MPa | ≈ 590 °C | 13.5 | Hot ferritic joints where CTE match is irrelevant |
| ASTM A193 B8M Cl 2 | 316 austenitic | ≈ 550 MPa | ≈ 400 °C | 18.2 | Corrosion resistance where temperature is modest |
| Refractaloy 26 / AISI 690 | Fe–Ni–Co–Cr | ≈ 655 MPa | higher | austenitic | Turbine bolting needing rupture strength and rupture ductility |
| Inconel 718 | Ni | ≈ 1 035 MPa | ≈ 650 °C | 14.4 | Highest strength; severe sour service |
| Nimonic 80A | Ni–Cr | ≈ 600 MPa | ≈ 815 °C | 14.5 | Exhaust and turbine bolting above 700 °C |
| 17-4PH H1025 | Martensitic PH | ≈ 1 000 MPa | ≈ 315 °C | 11.6 | High strength at low temperature only; magnetic |
Yield minimums are specification values for common size ranges. Check the size-dependent table in the governing standard. CTE is the mean coefficient over 20–540 °C, ×10⁻⁶/°C.
The three-question decision
- Is the joint above about 450 °C? If not, B7 or B8M will usually do, and Grade 660 is money spent on nothing.
- Is the structure austenitic stainless or nickel alloy, and does it cycle thermally? If yes, expansion matching is worth more than the extra material cost. This is Grade 660's home ground.
- Does the environment combine H₂S with high chloride at temperature? If yes, do not stop at Class D on hardness alone. Read §8 and consider Inconel 718 or Incoloy 945.
We forge all of the alloys above. See Refractaloy 26 / AISI 690, Inconel 718, Nimonic 80A and 17-4PH. If you are not sure which grade a legacy drawing intends, send us the drawing and the operating temperature and we will help you resolve it.
§14
How to request a quotation
Send a drawing in PDF, DWG or STEP format to sales@steelforgepieces.com. To quote accurately in one round rather than three, the following details help:
- Grade and specification revision. For example "AISI 660 / UNS S66286 per ASTM A453 Grade 660 Class B", or the OEM material specification number.
- Tensile class. A, B, C or D. If you are unsure, give the service temperature, the flange or housing material and whether NACE MR0175 applies, and we will recommend a class in writing.
- Finished dimensions and tolerances, and whether you want as-forged, rough machined or finish machined.
- Heat-treatment condition at delivery. Solution treated only for further fabrication, or solution treated and aged as a finished part. State it explicitly rather than implying it from the class.
- Quantity and whether this is a one-off replacement or a repeating order.
- Testing and certification. UT class, PT, elevated-temperature tensile, stress rupture, API 20F BSL level, EN 10204 3.1 or 3.2.
- Delivery port and required date. The date material is needed on site, not the date it should ship.
Quotations are normally returned within 24 hours on working days. Technical questions are answered by our forging and metallurgy staff, not by a call centre.
§15
Frequently asked questions
Is AISI 660 the same as A286 and UNS S66286?
Yes. AISI 660, Alloy 660, A286, UNS S66286, ASTM A453 Grade 660, ASTM A638 Type 660, DIN 1.4980 (X6NiCrTiMoVB25-15-2), JIS SUH 660 and Chinese GH2132 all describe the same iron-based precipitation-hardening austenitic stainless steel. What differs between them is the governing standard's supply condition, tensile class and trace-element control, not the base alloy. None of these names is a trademark. The original development patents expired decades ago.
Is AISI 660 a stainless steel or a nickel alloy?
Strictly, it is an iron-based precipitation-hardening austenitic stainless steel: iron is the balance at roughly 52 to 54 %, with about 25 % nickel. It is commonly catalogued alongside nickel alloys and superalloys because its service temperature and behaviour place it in that family rather than with ordinary 300-series stainless, and because engineers usually arrive at it while shopping for a superalloy. Both classifications appear in supplier catalogues; the chemistry is the same either way.
What is the difference between Grade 660 Class A, B, C and D?
Heat treatment only. The chemistry is identical and the tensile minimum is 895 MPa (130 ksi) for all four. Class A: 900 °C solution, liquid quench, 720 °C for 16 h age; best room-temperature tensile properties. Class B: 980 °C solution, liquid quench, same age; best stress-rupture life. Class C: 980 °C solution, oil quench, 775 °C then 650 °C double age; the heavy-section version of Class B. Class D: 900 °C solution, liquid quench, 720 °C then 650 °C double age; raises minimum yield from 585 to 725 MPa and is the usual pick for NACE MR0175 and API 20F bolting.
What is the chemical composition of AISI 660?
In mass fraction: carbon 0.08 % max, silicon 1.00 % max, manganese 2.00 % max, phosphorus 0.040 % max, sulphur 0.030 % max, chromium 13.50–16.00 %, nickel 24.00–27.00 %, molybdenum 1.00–1.50 %, titanium 1.90–2.35 %, aluminium 0.35 % max, vanadium 0.10–0.50 %, boron 0.0010–0.010 %, with iron as the balance. Note that iron is the balance, not zero, and that boron carries a specified minimum. Both entries are wrong in many copies of this table now in circulation.
Why is AISI 660 specified for high-temperature bolting instead of B7 or B16?
Because its coefficient of thermal expansion, about 17.6 ×10⁻⁶/°C over 20–540 °C, is close to that of austenitic stainless flange material at about 18.2, whereas chrome-moly B7 and B16 run about 13.5. In a hot austenitic joint the flange outgrows a chrome-moly stud, over-strains it, and the joint loses clamp load on each thermal cycle. Grade 660 tracks the flange and holds preload. The full title of ASTM A453 names this property explicitly: high-temperature bolting with expansion coefficients comparable to austenitic stainless steels.
What is the maximum service temperature of AISI 660?
About 700 °C (1 300 °F) for loaded structural service. Above that the gamma-prime precipitates over-age and coarsen and strength falls quickly. Rupture strength roughly halves between 650 and 705 °C. The alloy resists scaling in air to around 800 °C, but that is an oxidation limit, not a strength limit; the two are frequently confused. For continuous loaded service above 700 °C, move to Inconel 718, Waspaloy or Nimonic 80A.
Can AISI 660 be used in sour service under NACE MR0175?
UNS S66286 is recognised in NACE MR0175 / ISO 15156-3 for defined conditions with a 35 HRC maximum hardness, normally supplied as Grade 660 Class D. Hardness compliance alone does not make it fit for service: published testing shows A286-type material to be severely restricted by chloride stress-corrosion cracking in high-chloride completion brines, failing in calcium-chloride brine at temperatures as low as about 95 °C. Check temperature, H₂S partial pressure and chloride content against the current edition of ISO 15156-3, and consider Inconel 718 or Incoloy 945 where the environment is aggressive.
Is AISI 660 magnetic?
No. It is fully austenitic and remains essentially non-magnetic in every condition, with relative permeability around 1.005, and it stays non-magnetic even after severe cold deformation. This distinguishes it from the martensitic precipitation-hardening stainless grades such as 17-4PH and 15-5PH, which are magnetic. Because of this, magnetic particle inspection does not apply, and liquid penetrant is used instead.
How is AISI 660 heat treated?
Solution treat at 900 °C for Classes A and D or 980 °C for Classes B and C, quench, then age 16 hours at 720 °C (775 °C for Class C) and air cool, with a second 16-hour age at 650 °C for Classes C and D. Two parameters do most of the damage when they slip: the aging hold, which must be 16 hours and not "about 16 hours", and the furnace-to-quench transfer, which must stay under about 30 seconds to avoid η-phase Ni₃Ti forming on the grain boundaries.
Can AISI 660 be welded?
Yes, with precautions. Weld in the solution-treated condition using matching filler per AMS 5805. Welding after final heat treatment over-ages the heat-affected zone, so a full re-solution treatment plus the complete age cycle is required afterwards to restore properties. The failure mode to design against is strain-age cracking in restrained joints, so control heating and cooling rates through the 600–800 °C window and minimise restraint. Many OEM specifications restrict or prohibit weld repair on load-bearing hardware; check the drawing note first.
What is the density of AISI 660?
7.94 g/cm³, equivalent to 0.287 lb/in³, in the aged condition at room temperature.
What sizes of AISI 660 forgings can you make?
Round bars from 20 mm to 800 mm diameter, seamless rolled rings from 100 mm to 2 500 mm outside diameter, discs and blocks up to 1 500 mm diameter and 600 mm thick, and single pieces up to approximately 3 000 kg. Bolting is supplied M10 to M100 and 3/8 inch to 4 inch. Because we forge open die, there is no tooling cost and the minimum order is one piece.
What certification is supplied?
An EN 10204 3.1 mill test certificate is standard, reporting chemical composition, tensile and hardness results, the heat-treatment condition and cycle, ultrasonic testing and dimensional inspection, traceable to the heat. EN 10204 3.2 certification witnessed by TÜV, SGS, BV or Lloyd's Register is available on request, as are stress-rupture testing, elevated-temperature tensile testing, Charpy impact testing for API 20F, grain size reports and macroetch grain-flow inspection.
What is the lead time and minimum order quantity?
Typical lead time is 25 to 45 days from drawing approval, depending on section size, class and testing scope. Stress-rupture testing or third-party witnessed inspection adds to that. There is no minimum order quantity for open-die forgings. Single replacement parts for plant overhauls are a routine part of our work.
Who manufactures AISI 660 (ASTM A453 Grade 660) forgings in China?
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, that manufactures AISI 660 / UNS S66286 / ASTM A453 Grade 660 forgings (bolting bar and studs, seamless rolled rings, discs, shafts, valve stems and extrusion tooling) to customer drawings in all four tensile classes, with EN 10204 3.1 certification. Contact: +86 189 2135 9659, sales@steelforgepieces.com.
Do you supply other high-temperature alloys?
Yes. We forge the Inconel, Incoloy, Nimonic, Hastelloy, Monel, Haynes, Udimet, Waspaloy and Refractaloy families, along with precipitation-hardening stainless steels such as 17-4PH, 15-5PH and PH13-8Mo, and carbon, alloy and tool steels. A full grade index is on our nickel alloy forgings page.
§17
About the manufacturer
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory located in Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. The company forges superalloys, stainless steels, alloy steels, tool steels and carbon steels to customer drawings, producing round and flat bars, seamless rolled rings, discs, blocks, sleeves, hollow sections, high-temperature bolting, valve components and extrusion tooling. Products are supplied with EN 10204 3.1 mill test certificates and are exported to power generation, oil and gas, petrochemical, marine and general engineering customers worldwide.
- Company
- Jiangyin Jiangnan Metal Co., Ltd.
- Business
- Open-die forging factory
- Address
- No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
- Telephone
- +86 189 2135 9659
- Website
- www.steelforgepieces.com
Citing this page
This datasheet may be quoted with attribution. Suggested citation:
Jiangyin Jiangnan Metal Co., Ltd. (2026). AISI 660 Forgings to ASTM A453 / A638 Grade 660, UNS S66286: composition, tensile classes, heat treatment and forging capability. Jiangyin, Jiangsu, China.
Available at: https://www.steelforgepieces.com/Nickel-Alloy/AISI-660.html
Contact: sales@steelforgepieces.com · +86 189 2135 9659
Available at: https://www.steelforgepieces.com/Nickel-Alloy/AISI-660.html
Contact: sales@steelforgepieces.com · +86 189 2135 9659
Standards referenced on this page
ASTM A453/A453M · ASTM A638/A638M · ASME BPVC Section II Parts A and D · EN 10269 · EN 10302 · JIS G 4901 · NACE MR0175 / ISO 15156-3 · API 20F · EN 10204 · EN 10228-3 · SEP 1921 · ASTM A388 · ASTM A370 · ASTM E23 · ASTM E112 · ASTM E165 · ASTM E381 · AMS 2750 · AMS 5805. Standards are the property of their issuing bodies and must be purchased from them. Always work from the revision applicable to your purchase order. The values on this page are compiled for reference and can lag a revision.