Incoloy Fasteners: Complete Technical Data – Tensile, Temperature, Corrosion, Torque & More

For engineers specifying bolting in high‑temperature furnaces, chemical processing, or oil & gas wellheads, the Incoloy family offers a versatile solution. Understanding the specific properties of Incoloy 800, 800H, 800HT, 825, and 925 is essential for selecting the right grade. RAYCHIN LIMITED, a specialist global manufacturer of Incoloy and other high‑nickel alloy fasteners with decades of experience, provides this authoritative technical reference. It covers tensile and yield strengths, hardness, maximum temperature limits, creep strength, corrosion resistance, torque values, galling prevention, machining guidelines, and heat treatment protocols. All data is drawn from ASTM specifications and our own production testing, giving you the confidence to design reliable joints for demanding applications.

1. Mechanical Properties of Key Incoloy Grades

The room‑temperature strengths vary significantly among the Incoloy family, from the solid‑solution strengthened 800 series to the age‑hardenable 925. The table below summarizes the minimum properties in the standard heat‑treated condition.

Incoloy 800 tensile strength and Incoloy 825 yield strength reflect their solid‑solution strengthened nature; they cannot be hardened by heat treatment. Incoloy 925 hardness is achieved through precipitation hardening (aging), which significantly increases both strength and hardness while maintaining excellent corrosion resistance.

2. High‑Temperature Capability: 800H, 800HT, and Creep Strength

Incoloy 800H maximum temperature in oxidizing atmospheres is approximately 1100°C (2010°F) for continuous service under light load. However, for loaded fasteners, the practical upper limit is around 800°C (1470°F) to avoid excessive creep deformation. Incoloy 800HT is specifically optimized for high‑temperature strength above 700°C, with improved Incoloy 800HT creep strength due to its controlled carbon and aluminum/titanium content, which stabilizes carbides and gamma‑prime precipitates. In applications such as steam methane reformers, ethylene furnaces, and ultra‑supercritical boilers, 800HT fasteners maintain preload and resist relaxation far better than standard 800 or even 800H.

3. Corrosion Resistance: Sulfuric Acid, Carburization, Pitting, and SCC

• Incoloy 825 sulfuric acid resistance: Incoloy 825 (UNS N08825) is specifically designed to resist sulfuric and phosphoric acids, thanks to its nickel, molybdenum, and copper content. It performs well in concentrations up to 40% at moderate temperatures, making it a workhorse for chemical processing and pickling tanks.
• Incoloy 800 carburization resistance: The high nickel (32%) and chromium (21%) content of the 800 series provides excellent resistance to carburization and oxidation in furnace atmospheres. This prevents carbon diffusion into the metal, avoiding embrittlement in heat‑treating and petrochemical environments.
• Incoloy 825 pitting resistance: With 2.5–3.5% molybdenum, Incoloy 825 has a PREN (Pitting Resistance Equivalent Number) of approximately 31–34. While not as high as 6Mo super austenitic grades, it offers significantly better pitting resistance than 316L and is suitable for many chloride‑containing environments.
• Incoloy 925 stress corrosion cracking resistance: Incoloy 925 retains the chloride SCC resistance of the 825 base while adding high strength through aging. It is NACE MR0175 compliant for sour gas service, making it ideal for wellhead components and downhole tools exposed to H₂S and chlorides.

4. Torque Values and Tightening Recommendations

Achieving the correct preload without galling requires accurate Incoloy torque values. The recommended Incoloy bolt tightening torque depends on the specific grade and its yield strength. The table below provides conservative guidelines based on 50% of yield strength and a lubricated nut factor K = 0.16.

Always use clean, lubricated threads with a nickel‑compatible anti‑seize compound, and a calibrated torque wrench with slow, steady application. RAYCHIN can provide a detailed torque chart specific to your fastener grade, geometry, and coating.

5. Galling Resistance and Assembly Best Practices

Incoloy galling resistance is moderate, typical of high‑nickel austenitic alloys. Under high contact stress, dry assembly or high‑speed tightening can cause thread seizure. RAYCHIN prevents galling through:

• Rolled threads after heat treatment for a smoother, work‑hardened surface.
• Pre‑applied anti‑seize coatings such as MoS₂ or nickel‑based lubricants.
• Dissimilar nut pairing (e.g., Nitronic 60 or Alloy 625 nuts on Incoloy bolts).
• Controlled assembly speed with hydraulic or electric torque wrenches; never use impact tools.

6. Machining Incoloy Fasteners

Machining Incoloy fasteners presents challenges common to high‑nickel alloys: rapid work hardening, high cutting forces, and built‑up edge on tooling. RAYCHIN's production floor employs proven strategies, including rigid CNC setups, positive‑rake carbide tools, moderate cutting speeds with heavy feeds, and high‑pressure coolant. We perform the majority of machining in the annealed condition (for age‑hardenable grades) or solution‑annealed condition (for solid‑solution grades), followed by heat treatment and thread rolling to achieve final properties and surface finish.

7. Heat Treatment Protocols

The heat treatment for Incoloy alloys depends on the grade:

• Incoloy 800 solution annealed: Solution annealing at 1050–1150°C (1920–2100°F), followed by rapid air or water cooling. This dissolves any carbides and ensures a fully austenitic structure with maximum ductility and corrosion resistance. 800H and 800HT are solution‑annealed at slightly higher temperatures (1150–1200°C) to achieve the coarser grain size required for creep strength.
• Incoloy 825: Solution annealing at 930–980°C (1700–1800°F), followed by rapid cooling. The titanium addition stabilizes the alloy against intergranular corrosion.
• Incoloy 925 age hardening: Incoloy 925 is solution annealed at 980–1040°C, quenched, then aged at 732°C (1350°F) for 8 hours, followed by furnace cooling to 621°C (1150°F), holding for 8 hours, and air cooling. This precipitation hardening cycle develops the high strength (140 ksi UTS) and hardness (24–35 HRC) while maintaining corrosion resistance.

RAYCHIN performs all heat treatment in‑house using digitally controlled vacuum furnaces, with complete furnace charts included in every certification package.