Incoloy 800H vs 800HT: When Does the Extra Titanium Matter?

Incoloy 800, 800H, and 800HT are three closely related nickel-iron-chromium alloys designed for high-temperature service. They share the same basic composition: approximately 32% nickel, 21% chromium, and 46% iron, with trace amounts of carbon, aluminum, and titanium. Yet the three grades have different ASTM designations, different allowable stresses in ASME code, and different price points. The key difference lies in the control of carbon and titanium contents-and this article explains when those differences matter.

If you specify these alloys, you have likely asked: "Is 800HT worth the premium over 800H? Can I use 800H instead of 800HT? What happens if I substitute 800 for 800H?" These questions arise in petrochemical furnace design, steam superheater tubing, heat-treating baskets, and many other applications operating at 600°C to 1000°C.

This article provides a definitive, data-driven comparison. We will examine the compositional differences, explain the metallurgical role of titanium in creep resistance, compare mechanical properties at elevated temperature, and provide clear selection guidelines with case studies.

Definitive Conclusion: 800HT has higher controlled titanium content (0.85–1.20% vs. 0.15–0.60% in 800H) combined with tighter carbon control. This gives 800HT 20–40% higher creep-rupture strength at temperatures above 700°C. For applications below 700°C, 800H and 800HT are functionally equivalent. Above 700°C in long-term service, 800HT justifies its cost premium through longer creep life and thinner wall designs.

Incoloy 800 Family: Three Grades, One Foundation

Incoloy 800 was developed by Inco (now Special Metals) in the 1940s as a lower-nickel alternative to Inconel 600 for high-temperature applications. The alloy's iron content (about 46%) made it significantly cheaper than Inconel while still providing good oxidation resistance and moderate strength at elevated temperature.

By the 1960s, experience in petrochemical service revealed that the original Alloy 800 had inconsistent creep resistance due to uncontrolled carbon and titanium levels. This led to the development of Alloy 800H (H = "high carbon") with carbon controlled to 0.05–0.10% for improved high-temperature strength.

In the 1970s, further optimization produced Alloy 800HT (HT = "high temperature"), which combined the high carbon of 800H with controlled titanium (0.85–1.20%) and aluminum (0.15–0.60%) to maximize creep-rupture strength through precipitation hardening. Today, all three grades coexist in the market, serving different segments of the high-temperature alloy market.

UNS Numbers and ASTM Specifications

Definitive Conclusion: Note the UNS numbers: N08800 (800), N08810 (800H), and N08811 (800HT). These are NOT interchangeable in ASME code-each has different allowable stress values. Always specify by UNS number, not just trade name.

Chemical Composition

Definitive Conclusion: The critical composition difference is titanium: 800HT requires 0.85–1.20% Ti, while 800 and 800H allow 0.15–0.60%. Combined Ti+Al in 800HT is 1.00–1.80% versus 0.30–1.20% in 800H. This 2-3x increase in precipitation-forming elements is what gives 800HT its superior creep strength.

At high temperatures (above 600°C), metals deform slowly under stress-a phenomenon called "creep." Creep is the life-limiting failure mode for furnace tubes, steam superheaters, and reformer piping. Strengthening against creep requires either:

• Solid solution strengthening: Alloying elements dissolved in the metal lattice impede dislocation movement. In 800-series alloys, chromium and nickel provide this effect.

• Precipitation hardening: Fine particles of a second phase form within the grains, blocking dislocation motion and grain boundary sliding. In 800HT, titanium and aluminum combine with nickel to form gamma-prime (Ni₃(Al,Ti)) precipitates, which are extremely effective at temperatures of 600–900°C.

Alloy 800HT is designed to maximize precipitation hardening. The higher titanium content ensures that during the solution anneal and subsequent service at elevated temperature, a fine dispersion of gamma-prime precipitates forms throughout the matrix. These precipitates are semicoherent with the austenite lattice, meaning they strongly impede dislocation motion without causing brittleness. The result: significantly higher creep-rupture strength than 800H.

Alloy 800H relies primarily on solid solution strengthening (carbon in solution) and carbide precipitation at grain boundaries. It has some gamma-prime from the lower Ti+Al content, but not enough to achieve the full precipitation-hardening effect. For service below 700°C, this is adequate. Above 700°C, the difference becomes critical.

Mechanical Properties at Room and Elevated Temperature

Room Temperature Properties

High-Temperature Tensile Strength

Table 4: High-Temperature Tensile Strength Comparison (Typical Values)

Definitive Conclusion: At room temperature, 800H and 800HT have essentially identical tensile properties. As temperature increases above 700°C, 800HT's strength advantage grows dramatically-up to 30% higher tensile strength at 900–1000°C. This is the precipitation-hardening effect in action.

Creep-Rupture Strength

For high-temperature design, creep-rupture strength is the property that matters most. ASME Boiler and Pressure Vessel Code Section II Part D provides allowable stresses based on creep data. The following table shows the time to rupture at various stress levels and temperatures-the data that determines design life.

Table 5: Creep-Rupture Data Comparison - 800H vs 800HT

ASME Code Allowable Stresses

ASME Boiler and Pressure Vessel Code Section II Part D provides maximum allowable stress values for each alloy at various temperatures. These values are derived from minimum tensile strength, yield strength, and creep-rupture data with appropriate safety factors. The allowable stress determines the minimum required wall thickness for pressure-containing components.

Table 6: ASME Section II Part D Allowable Stresses - 800H vs 800HT

Definitive Conclusion: Above 700°C, ASME allows 19–46% higher stress for 800HT versus 800H. For a tube or pipe under internal pressure, allowable stress is inversely proportional to required wall thickness. A 40% higher allowable stress means 40% thinner walls, 40% less material, and 40% lower material cost-which often exceeds the premium paid for 800HT over 800H.

Oxidation and Carburization Resistance

All three Incoloy 800 grades share the same chromium content (19–23%), which determines their oxidation resistance. The protective chromium oxide (Cr₂O₃) scale that forms on the surface is identical for 800, 800H, and 800HT. Therefore, oxidation resistance is NOT a differentiating factor.

Definitive Conclusion: Do NOT select 800HT over 800H for oxidation or carburization resistance-they are equivalent. The sole reason to specify 800HT is for higher creep-rupture strength at temperatures above 700°C.

Welding and Fabrication

Cost Analysis

800H or 800HT?

Industry Case Studies

Case Study: 1: Steam Methane Reformer - 800HT Enables Thinner Tube Design

A 2024 grassroots hydrogen plant in the Middle East specified steam methane reformer tubes operating at 920°C outlet temperature with 25 bar design pressure. Initial design using Incoloy 800H required 15.2 mm wall thickness to meet the 100,000-hour creep life requirement. Switching to Incoloy 800HT reduced required wall thickness to 11.8 mm (22% reduction), resulting in:

- Material weight reduction: 22%

- Material cost reduction: ~12% (after accounting for HT premium)

- Lower thermal stress due to thinner wall (faster heat transfer, lower temperature gradient)

- Extended creep life margin: estimated 130,000 hours vs. 100,000 hours minimum

Key lesson: At temperatures above 900°C, 800HT's higher allowable stress enables wall thickness reductions that more than offset the material premium.

Case Study: 2: Ethylene Cracking Furnace - 800HT Extends Tube Life in Cyclic Service

An ethylene cracker in Southeast Asia experienced premature radiant coil tube failures after 85,000 hours of service. The original tubes were specified as Incoloy 800H. Failure analysis revealed creep cracking initiated at the outer surface, accelerated by cyclic thermal stress from decoking operations every 30–40 days. During a 2025 turnaround, the plant replaced the radiant coils with Incoloy 800HT.

After 50,000 hours of service with the new 800HT tubes:

- No creep cracking detected

- Remaining life assessment: additional 80,000 hours minimum

- Decoking cycle extended to 45 days (reduced thermal cycling)

Key lesson: For cyclic high-temperature service, 800HT's gamma-prime precipitates provide better resistance to creep-fatigue interaction than 800H.

Case Study: 3: Steam Superheater - 800H Proved Adequate at Moderate Temperature

A 600 MW coal-fired power plant in India specified steam superheater outlet headers operating at 540°C steam temperature, with maximum metal temperature of 580°C. The original design considered both 800H and 800HT. Analysis showed:

- At 580°C, ASME allowable stress for 800H = 92 MPa, 800HT = 97 MPa (only 5% difference)

- Design life requirement: 200,000 hours (easily met by both at this temperature)

- Creep is not the controlling failure mode; oxidation and erosion are concerns

The plant selected Incoloy 800H, achieving a 12% material cost saving over 800HT with no compromise in service life. After 15 years of operation, the headers remain in service with no creep-related issues.

Key lesson: At temperatures below 650°C, 800H and 800HT are functionally equivalent. The 800HT premium provides no benefit.

Case Study: 4: Replacement In-Kind - Matching Original Specification

A 2023 maintenance project at a US refinery required replacement of a section of Incoloy 800H furnace tube that had developed a small leak after 180,000 hours of service. The procurement team considered substituting 800HT to "improve" the replacement, but engineering analysis identified two concerns:

1. The remaining original 800H tubes would continue to creep at a different rate than the new 800HT tube, potentially creating stress concentrations at the transition weld.

2. ASME code-required documentation and inspection regime differs slightly between the two grades.

The project proceeded with an exact in-kind replacement using Incoloy 800H, ensuring metallurgical and mechanical compatibility with the existing system.

Key lesson: For repairs and replacements, always match the original material grade unless a complete engineering reassessment justifies a change.

Product Forms and Specifications

Common Specification Mistakes to Avoid

1. Specifying "Incoloy 800" without H or HT designation

Consequence: May receive Alloy 800 (UNS N08800) which has lower carbon and lower creep strength than 800H/800HT

Correct approach: Always specify by UNS number: N08810 for 800H, N08811 for 800HT

2. Substituting 800H for 800HT without engineering review

Consequence: At temperatures above 750°C, 800H will have 20–40% lower creep life than designed

Correct approach: Never substitute 800H for 800HT without recalculating wall thickness and creep life

3. Using 800HT where 800H is adequate

Consequence: Wastes 10–15% of material budget with no performance benefit at moderate temperatures

Correct approach: Reserve 800HT for applications above 750°C or with extended design life requirements

4. Not verifying grain size

Consequence: Fine grain size (ASTM 6-8) reduces creep strength by 15–30% compared to coarse grain (ASTM 3-5)

Correct approach: Specify grain size 5 or coarser; coarser is better for creep

5. Overlooking solution anneal temperature difference

Consequence: 800HT requires 1175°C minimum; 800H requires only 1150°C. Under-annealed material will not develop full creep strength

Correct approach: Specify correct annealing temperature for each grade

Summary