660 stainless steel standard and application

660 stainless steel is a high-strength, age-hardenable stainless grade used for bolting, fasteners and hot-section components in chemical, petroleum, aerospace, pharmaceutical and food equipment. Internationally the grade is UNS S66286, and ASTM A453 Grade 660 covers the studs, nuts and other bolting parts that are fitted together with flange gaskets in high-temperature joints.

What 660 Stainless Steel Is

660 stainless steel is an iron-based alloy whose strength comes from chromium, nickel and small additions of titanium, niobium and aluminium that allow age hardening. It keeps useful mechanical properties and corrosion resistance at elevated temperature, which is why the same grade appears in furnace tubes, chemical reactor internals, centrifuge parts and compressor impellers.

In piping assemblies, 660 stainless steel studs and nuts are fitted on pipe flanges that have to stay tight through repeated thermal cycles. Material is normally ordered against the current edition of the applicable standard and released on the mill certificate of the batch.

• Designations: UNS S66286 internationally; ASTM A453 Grade 660 for the product forms; the same grade is also listed under AMS 5737.
• Chinese designation used in domestic practice: 06Cr25Ni20Si2; GB/T 16926-2019 is listed among the domestic standards.
• Main alloying elements: chromium 24.0-27.0%, nickel 19.0-22.0%, with titanium and niobium additions that enable age hardening.

Chemical Composition and Mechanical Properties

The table below lists the specified chemical composition of 660 stainless steel in percent by mass, with iron as the balance.

Note: values in percent by mass; Fe = balance; the symbol - means that no limit is specified.

Mechanical properties measured on our 660 stainless steel material after heat treatment are given below.

Note: MPa = megapascal; HBW = Brinell hardness measured with a tungsten carbide ball.

Corrosion resistance is also specified as a corrosion rate in defined media: in hydrochloric acid solution the rate for 660 stainless steel is limited to 0.1 mm/a. The actual medium, its concentration and the service temperature still decide whether the grade fits a given duty.

Heat Treatment, Machining and Welding

Processing requirements for 660 stainless steel cover heat treatment, cold working, welding and inspection. The purpose of each step is to keep the specified properties through manufacturing.

• Heat treatment - solution annealing at 1050 °C with a holding time of 1 hour, followed by air cooling. The treated material machines well and keeps its corrosion resistance.
• Machining - both cold and hot working are possible. Use hard, sharp tooling when cutting, and control heating temperature and time in hot working so the material stays inside its thermal stability range.
• Welding - conventional arc, TIG and MIG welding all work. Preheat the part, use suitable filler material and shield the weld area to prevent pores and inclusions.
• Surface treatment - mechanical, chemical or electrolytic polishing; keep the surface clean and flat and avoid scratches and dents.
• Inspection - chemical composition analysis, mechanical testing and corrosion testing confirm that the delivered material meets the standard requirements.

Storage and transport: keep the material dry and protected from rust, store it upright and place wooden boards or plastic pads between pieces so surfaces do not touch or rub.

Where 660 Stainless Steel Is Used

The combination of high-temperature strength and corrosion resistance defines where the grade is used: equipment and parts that face heat, corrosive media and wear at the same time.

• Chemical equipment - reactor internals, centrifuge parts and hot process piping.
• Petroleum and gas - high-temperature furnace tubes, compressor impellers and bolting for hot flanged joints.
• Aerospace and power - hot-section fasteners and components where strength retention matters.
• Pharmaceutical and food equipment - parts that need corrosion resistance and clean, smooth surfaces.

The application scope defined in the standard is the reference point for selection: match the grade, the delivery condition and the test requirements to the service temperature and medium, and confirm the heat treatment condition on the certificate before fabrication.

Frequently Asked Questions

How is 660 stainless steel designated in different standards?

Internationally it is UNS S66286, and the product forms are covered by ASTM A453 Grade 660 and AMS 5737. In China the designation is 06Cr25Ni20Si2, and GB/T 16926-2019 is listed among the domestic standards.

What heat treatment does 660 stainless steel need?

Solution annealing at 1050 °C with a holding time of 1 hour and air cooling. After treatment the material machines well and keeps its corrosion resistance.

What mechanical properties does 660 stainless steel deliver?

After heat treatment: tensile strength ≥930 MPa, yield strength ≥635 MPa, elongation ≥15%, reduction of area ≥50% and hardness ≤321 HBW.

Can 660 stainless steel be welded?

Yes, by conventional arc, TIG and MIG welding. Preheat the part, use suitable filler material and shield the weld area to prevent pores and inclusions.

How should 660 stainless steel be stored and transported?

Keep it dry and protected from rust, store it upright and place wooden boards or plastic pads between pieces so surfaces do not touch or rub.