Nickel-base superalloy / Open-die forgings

Refractaloy 26 (R26) Forgings R-26 / AISI 690 / Z6NKCDT38 / ASTM A453 Gr.665 / UNS S66545

Published by Jiangyin Jiangnan Metal Co., Ltd., open-die forging factory, Jiangyin, Jiangsu, China. Last reviewed 28 August 2026

Chemical composition of Refractaloy 26

The table gives the specified composition range in weight percent. Heat analysis is reported on the EN 10204 3.1 certificate supplied with each order.

Composition per ASTM A453/A453M Grade 665 and common mill practice. Where a customer specification is narrower, we melt to the customer specification.

Equivalent designations and cross-references

Cross-references are given as a sourcing aid and are not a statement of full interchangeability. Confirm the governing specification on the purchase order.

Mechanical and physical properties

Refractaloy 26 has both high creep rupture strength and high creep rupture ductility, which is why it is used as a high-temperature bolt material. It is an austenitic alloy with a relatively large coefficient of thermal expansion, and is normally matched with austenitic turbine casings rather than ferritic ones.

Note: the values above are indicative, for forged bar in the fully heat treated condition. Guaranteed values are those of the governing specification and of the class ordered. ASTM A453 Grade 665 Class A and Class B differ in strength level. Design allowables should be taken from the specification or from the OEM. The mill test certificate issued with the order is the controlling document.

Behaviour at temperature

Published work on service-exposed R26 turbine bolts reports good creep strength and stress relaxation resistance at 680 °C, and this is the main reason the alloy is used in steam and gas turbines. Suppliers commonly quote useful strength to about 815 °C for shorter exposure. After very long service, tens of thousands of hours, R26 bolts can show hardness drift and microstructural change, and a rejuvenation heat treatment is sometimes applied. We can heat treat to a customer-supplied rejuvenation cycle on request.

Heat treatment of Refractaloy 26

Two cycles are in common use. Which one applies depends on the governing specification and on the section thickness of the part. The cycle is confirmed in writing before production starts.

Soak times are set by section thickness. Furnaces are surveyed and thermocouples calibrated. We can work to AMS 2750 pyrometry requirements when the order calls for it.

Machining and fabrication

Refractaloy 26 falls in ISO machining group S, heat-resistant superalloys, with a machinability rating of about 15-25% of free-cutting steel. Published starting points are 60-100 m/min (200-330 SFM) for turning, 45-75 m/min for milling and 45-75 m/min for drilling. Ground carbide inserts are recommended, with a rake angle of 13-18 degrees, a neutral land of 0.10-0.20 mm and edge honing of 0.02-0.05 mm. Rigid clamping and short tool overhang give more benefit than higher cutting speed. On large parts, rough machining before the final age keeps tool cost down.

The alloy can be welded with matching filler under a controlled procedure that includes preheat and post-weld heat treatment. Where a fabricated assembly can be replaced by a single forged part, the forging is usually the better option for a rotating or pressure-retaining component.

Refractaloy 26 product forms

All items below are forged to drawing at our works in Jiangyin. Send a drawing, a sketch with dimensions, or the finished part dimensions plus your machining allowance.

• Round, square and flat barIncluding turbine blade blanks and rectangular sections
• Hollow bar and sleevesBushes, bushings, barrels, casings, shells, cylinders, hubs, housings
• Seamless rolled ringsRing rolled, not cut from plate
• Gland seal ringsSteam turbine gland sealing rings
• Blocks, discs and platesUpset and drawn forged blanks
• Turbine wheels and discsGas turbine wheels, impeller and blisk blanks
• Valve spindles and stemsGas and steam turbine valve spindles, stems, rods
• Studs and fastenersDouble-headed and double-end studs, bolts, nuts
• Shafts and custom partsOpen-die forged geometry to print

Size range and supply condition

Ranges cover our open-die capability across grades. R26 is a high-cost alloy, so the practical maximum for a single piece depends on ingot availability at the time of enquiry.

Applications

• Steam turbine bolting. High-temperature studs, bolts and nuts where stress relaxation resistance sets the retorque interval.
• Gas turbine hot section. Wheels, discs, spacers and fasteners.
• Turbine blades and blade roots. Applications where creep rupture ductility matters as much as strength.
• Valve internals. Spindles, stems and seats on high-temperature steam valves.
• Gland sealing rings. Steam turbine sealing components.
• Pressure vessel and flange fasteners. Under ASTM A453, where expansion has to match austenitic stainless steel.

Refractaloy 26 compared with other high-temperature bolting alloys

Nominal ranges, given for orientation. Grade selection depends on temperature, stress, expansion match with the mating part and cost. Contact us to check whether a lower-cost grade covers your duty.

Frequently asked questions

Summary

Refractaloy 26 (R26, R-26, AISI 690, AFNOR Z6NKCDT38) is a precipitation-hardening Ni-Cr-Co-Fe superalloy supplied to ASTM A453/A453M Grade 665, UNS S66545, with 35-39% Ni, 18-22% Co, 16-20% Cr, 2.5-3.5% Mo, 2.5-3.0% Ti and balance Fe. It is solution treated at 1025 °C and double aged at 815 °C and 730 °C to precipitate gamma prime. Its combination of creep rupture strength, creep rupture ductility and stress relaxation resistance around 680 °C makes it a standard choice for steam and gas turbine bolting, discs, blades and valve components. Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, forges the alloy to drawing as bar, hollow bar, seamless rolled rings, gland seal rings, discs, turbine wheels, valve spindles and double-end studs, with EN 10204 3.1 certification. Tel +86 189 2135 9659, email sales@steelforgepieces.com.