Inconel 800HT
Introduction to Inconel 800HT
Inconel 800HT is a high-strength, solid-solution nickel-iron-chromium alloy designed for optimal performance in high-temperature, high-stress environments. As the most advanced variant of the Inconel 800 series, Inconel 800HT combines the oxidation and corrosion resistance of Inconel 800 with superior creep rupture strength and long-term dimensional stability above 600°C.
The alloy is produced with tighter control of carbon (0.06–0.10%), aluminum (0.25–0.60%), and titanium (0.25–0.60%) contents than Inconel 800H, enabling improved structural reliability in cyclic or base-load heat service. It is extensively used in high-efficiency heat exchangers, power boiler tubing, reformer systems, and structural furnace components. CNC machining of Inconel 800HT parts ensures precision tolerance and high mechanical integrity for mission-critical assemblies.
Chemical, Physical, and Mechanical Properties of Inconel 800HT
Inconel 800HT (UNS N08811 / ASTM B409 / ASME SB409 / DIN 1.4959) is delivered in solution-annealed condition and used in applications requiring enhanced mechanical performance at sustained high temperatures.
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Nickel (Ni) | 30.0–35.0 | Base alloy; provides oxidation and carburization resistance |
Chromium (Cr) | 19.0–23.0 | Promotes oxide scale formation and high-temperature corrosion resistance |
Iron (Fe) | Balance (≥39.5%) | Structural matrix and thermal stability |
Carbon (C) | 0.06–0.10 | Enhances creep rupture strength |
Aluminum (Al) | 0.25–0.60 | Strengthens γ′ phase and improves oxidation resistance |
Titanium (Ti) | 0.25–0.60 | Grain boundary stabilization and γ′ formation |
Manganese (Mn) | ≤1.5 | Supports hot workability |
Silicon (Si) | ≤1.0 | Oxidation resistance at high temperatures |
Sulfur (S) | ≤0.015 | Minimized for weldability and surface integrity |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 7.94 g/cm³ | ASTM B311 |
Melting Range | 1357–1385°C | ASTM E1268 |
Thermal Conductivity | 11.0 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 1.18 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 14.5 µm/m·°C (20–1000°C) | ASTM E228 |
Specific Heat Capacity | 460 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 190 GPa at 20°C | ASTM E111 |
Mechanical Properties (Solution Annealed Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Tensile Strength | 520–650 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 230–320 MPa | ASTM E8/E8M |
Elongation | ≥30% (25mm gauge) | ASTM E8/E8M |
Hardness | 160–190 HB | ASTM E10 |
Creep Rupture Strength | ≥110 MPa @ 815°C, 1000h | ASTM E139 |
Key Characteristics of Inconel 800HT
Superior Creep Rupture Strength: Enhanced by tighter carbon, aluminum, and titanium control to improve long-term thermal load resistance at 750–950°C.
High-Temperature Oxidation and Carburization Resistance: Maintains surface integrity and mechanical strength in furnace, reformer, and power boiler environments.
Thermal Fatigue Stability: Resistant to embrittlement and grain boundary weakening under cyclic heat exposure.
CNC Machinability: Annealed condition allows for tight tolerance machining (±0.01 mm) with finish values of Ra ≤ 0.8 µm.
CNC Machining Challenges and Solutions for Inconel 800HT
Machining Challenges
Thermal Hardening and Alloy Toughness
High strength and γ′ phase content increase cutting forces and rapid edge wear if tooling and feeds are not optimized.
Work Hardening
Sensitive to low feed or multi-pass finishing, leading to surface hardening and reduced dimensional accuracy.
Low Thermal Conductivity
Generates localized heat buildup at the tool tip, increasing tool wear without high-pressure coolant systems.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | PVD-coated carbide or ceramic (SiAlON) | Maintains cutting edge integrity at high heat |
Coating | AlTiN or AlCrN (2–5 µm) | Reduces adhesion and oxidation at tool-work interface |
Geometry | 10–12° positive rake, reinforced edge preparation | Promotes smoother cutting and chip control |
Cutting Parameters (ISO 3685)
Operation | Speed (m/min) | Feed (mm/rev) | DOC (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 25–40 | 0.20–0.30 | 2.0–3.0 | 80–100 |
Finishing | 45–70 | 0.05–0.10 | 0.3–0.8 | 100–150 |
Surface Treatment for Machined Inconel 800HT Parts
Hot Isostatic Pressing (HIP)
HIP eliminates microporosity and enhances creep resistance in cast or heavy-section parts for power and process equipment.
Heat Treatment
Heat Treatment involves solution annealing at 1120–1150°C followed by rapid air cooling to optimize mechanical properties and grain structure.
Superalloy Welding
Superalloy Welding uses GTAW and matching filler wires (ERNiCr-3) to ensure metallurgical compatibility and resistance to intergranular attack.
Thermal Barrier Coating (TBC)
TBC Coating applies 125–250 µm of YSZ ceramics via APS or EB-PVD to shield surfaces from extreme radiant heat in reformers and radiant boilers.
Electrical Discharge Machining (EDM)
EDM provides tight-tolerance contouring and slotting with precision up to ±0.01 mm, especially in aged or hardened Inconel 800HT sections.
Deep Hole Drilling
Deep Hole Drilling enables L/D ≥ 40:1 internal passages for heat exchanger tubing and manifold distribution systems.
Material Testing and Analysis
Material Testing includes long-term creep simulation, microstructure examination (ASTM E112), and stress rupture validation.
Industry Applications of Inconel 800HT Components
Petrochemical Reactors
Radiant tubes, outlet manifolds, transfer piping.
Operates under 800–1000°C conditions with hydrogen-rich or carburizing gases.
Power Generation
Boiler components, reheater coils, and superheater tubes.
Delivers long service life under creep stress and thermal fatigue.
Chemical Processing
High-temperature pressure vessels and ethylene cracking tubes.
Maintains corrosion resistance and structural integrity in dual-phase environments.
Nuclear & Heat Treating
Core internals, trays, baskets, and thermowells.
Provides stress corrosion resistance in halide-rich and thermal cycling applications.
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