Alloy 800HT

Titanium

Nickel

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Nickel–Chromium Alloys

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Nickel–Iron Alloys

Alloy 800

Alloy 800H

Alloy 800HT

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Alloy 800HT

Alloy 800HT is a nickel–iron–chromium (Ni–Fe–Cr) alloy engineered for maximum creep and rupture strength at elevated temperatures within the 800 series.
It is an advanced version of Alloy 800 and 800H, distinguished by tighter control of carbon content, aluminum, titanium, and grain size, making it suitable for long-term service under high temperature and pressure.

Within the Nickel–Iron alloy family, Alloy 800HT represents the highest-temperature, highest-strength grade.

Chemical & Metallurgical Characteristics

Alloy 800HT builds on the Ni–Fe–Cr base chemistry with additional metallurgical controls to enhance high-temperature performance.

Key Metallurgical Features

  • Controlled carbon range for improved creep strength

  • Controlled aluminum and titanium additions to stabilize the microstructure

  • Coarse, stable austenitic grain structure

  • Nickel stabilizes the FCC structure at elevated temperatures

  • Chromium improves oxidation and carburization resistance

These features allow Alloy 800HT to maintain structural integrity during prolonged high-temperature exposure.

High-Temperature & Creep Performance

Alloy 800HT is specifically designed for severe high-temperature pressure service.

Excellent Performance In:

  • Long-term exposure to high temperatures

  • Applications where creep and stress rupture govern design

  • Oxidizing and carburizing atmospheres

  • Continuous thermal operation with minimal deformation

Compared with 800 / 800H:

  • Highest allowable stress at elevated temperatures

  • Superior creep rupture resistance

  • Preferred grade for the most demanding thermal conditions

Corrosion & Oxidation Behavior

Typical Performance

  • Good resistance to oxidation at elevated temperatures

  • Suitable for carburizing environments

  • Stable performance in steam and hydrocarbon service

Limitations

  • Not designed for strong acidic environments

  • Limited resistance to chloride-induced corrosion

As with other Ni–Fe alloys, Alloy 800HT is selected primarily for thermal-mechanical performance, not chemical corrosion resistance.

Mechanical Properties (Typical)

Alloy 800HT provides:

  • Highest creep and rupture strength in the 800 series

  • Good tensile and yield strength at elevated temperatures

  • Stable ductility under long-term thermal exposure

These properties make it suitable for ASME-coded pressure equipment operating near design temperature limits.

Typical Applications

Alloy 800HT is widely used in extreme high-temperature, pressure-containing equipment.

Common Applications Include:

  • High-temperature heat exchanger tubing

  • Pressure vessels and headers

  • Industrial furnaces and reformers

  • Petrochemical and refinery heaters

  • Power generation and superheater components

It is often specified when Alloy 800H no longer meets creep design requirements.

Product Forms Available

Alloy 800HT is commonly supplied in:

  • Seamless tubes

  • Pipes

  • Plates and sheets

  • Bars and rods

  • Forgings and tube sheets

For thermal and pressure equipment, seamless Alloy 800HT tubes are widely specified due to:

  • Uniform wall thickness

  • Reliable pressure performance

  • Stable long-term high-temperature behavior

Manufacturing & Fabrication

Alloy 800HT can be fabricated using standard nickel-alloy practices, with additional attention to metallurgical control:

  • Hot and cold working

  • Solution annealing

  • Conventional welding procedures

Proper heat treatment and cleanliness are essential to preserve creep strength and grain stability.

Standards & Specifications

Alloy 800HT products are commonly supplied in accordance with:

  • ASTM material standards

  • ASME Boiler & Pressure Vessel Code

  • Project-specific high-temperature service requirements

Typical Inspection & Testing:

  • Chemical composition analysis

  • Mechanical and creep-related testing

  • Dimensional inspection

  • Eddy current or ultrasonic testing (for tubes)

  • PMI verification

Full Mill Test Certificates (MTC) and traceability are standard.

Alloy 800HT vs Alloy 800 / 800H (Concept)

  • Alloy 800

    • General high-temperature stability

    • Lowest creep strength

  • Alloy 800H

    • Improved creep resistance

    • Suitable for most pressure applications

  • Alloy 800HT

    • Maximum creep and rupture strength

    • Preferred for extreme high-temperature service

Selection depends on temperature level, pressure, and code-required allowable stress.

Summary

Alloy 800HT is positioned within Nickel → Grades & Materials → Nickel–Iron Alloys and is selected for the most demanding high-temperature pressure applications.

Explore next:

  • Grades: Alloy 800 · Alloy 800H

  • Applications: Heat Exchangers · Power Generation

  • Guide: Nickel Selection Guide