Introduction
When you’re designing for high-temperature service and need to resist intergranular corrosion after welding, 347H is the stainless steel you want. This niobium-stabilized austenitic grade is specifically formulated for applications where sensitization is a concern. The ‘H’ suffix indicates a higher carbon content (0.04-0.10%) for improved high-temperature strength. In this article, I’ll cover why 347H is ideal for high-temperature environments, its chemistry, mechanical properties, and how to machine it effectively.
Chemical Composition
347H is essentially 321H with niobium instead of titanium as the stabilizing element. Niobium (columbium) forms stable carbides that prevent chromium carbide precipitation at grain boundaries. Here’s the typical composition per ASTM A240:
| Element | Composition Range (%) |
|---|---|
| Carbon (C) | 0.04 – 0.10 |
| Manganese (Mn) | 2.00 max |
| Phosphorus (P) | 0.045 max |
| Sulfur (S) | 0.030 max |
| Silicon (Si) | 1.00 max |
| Chromium (Cr) | 17.0 – 19.0 |
| Nickel (Ni) | 9.0 – 13.0 |
| Niobium (Nb) + Tantalum (Ta) | 10xC min – 1.10 max |
| Molybdenum (Mo) | 0.75 max |
The higher carbon content (0.04-0.10%) compared to standard 347 (0.08% max) provides better creep strength at elevated temperatures. The niobium addition (at least 10 times the carbon content) ensures that all carbon is tied up as stable niobium carbides, preventing sensitization during welding or service in the 800-1500°F range.
Mechanical Properties
347H offers excellent high-temperature strength and creep resistance. Typical mechanical properties are:
| Property | Room Temperature | At 1000°F | At 1400°F |
|---|---|---|---|
| Tensile Strength (ksi) | 85 – 105 | 60 – 75 | 35 – 50 |
| Yield Strength (0.2% offset, ksi) | 35 – 50 | 20 – 30 | 15 – 22 |
| Elongation in 2″ (%) | 40 – 50 | 35 – 45 | 25 – 35 |
| Hardness (Rockwell B) | 85 – 95 | N/A | N/A |
Creep strength is a key advantage of 347H. At 1400°F, the 10,000-hour creep rupture strength is approximately 10-15 ksi, significantly higher than 304H or 321H. This makes it suitable for long-term service in pressure vessels and piping.
Typical Applications
347H is specified for high-temperature environments where welding is involved. Common applications include:
- Chemical and petrochemical processing – Reactors, heat exchangers, and piping for high-temperature hydrocarbons.
- Power generation – Superheater and reheater tubes in boilers.
- Aerospace – Exhaust manifolds, afterburner components, and ducting.
- Heat treatment equipment – Furnace parts, radiant tubes, and muffles.
- Pollution control – Flue gas desulfurization systems and incinerator components.
CNC Machining Guide
Machining 347H is similar to 304H but with slightly higher tool wear due to the niobium carbides. Here are my recommended parameters:
| Operation | Tool Material | Speed (SFM) | Feed (IPR) | Depth of Cut (in) |
|---|---|---|---|---|
| Turning (rough) | Carbide (C2 grade, TiAlN coated) | 200 – 350 | 0.008 – 0.015 | 0.060 – 0.120 |
| Turning (finish) | Carbide (C2 grade, TiAlN coated) | 300 – 450 | 0.003 – 0.006 | 0.010 – 0.030 |
| Milling (rough) | Carbide (TiAlN coated) | 150 – 250 | 0.003 – 0.006 (IPT) | 0.030 – 0.080 |
| Milling (finish) | Carbide (TiAlN coated) | 200 – 350 | 0.002 – 0.004 (IPT) | 0.010 – 0.020 |
| Drilling | Carbide (with coolant-through) | 100 – 180 | 0.002 – 0.005 | N/A |
Key tips: Use rigid setups and minimize tool overhang. High-pressure coolant (800+ psi) is recommended for chip evacuation and heat control. For threading, use single-point threading with multiple passes. Avoid dwell marks—they will work-harden the surface instantly.
Quality Control
At Dongguan Stirling Metal Products Co., Ltd., we ensure your 347H parts meet the highest standards. Our QC includes:
- PMI verification – XRF confirmation of niobium content (10xC min).
- High-temperature tensile testing – Verification of strength at specified service temperatures.
- Intergranular corrosion testing – ASTM A262 Practice E (copper sulfate-sulfuric acid test) to verify stabilization.
- Dimensional inspection – CMM and ultrasonic thickness measurement for pressure-containing components.
Conclusion
347H stainless steel is the go-to material for high-temperature applications where welding is required. Its niobium stabilization prevents sensitization, while the higher carbon content provides excellent creep strength. If you’re designing components for service above 1000°F, 347H is a reliable choice that will stand up to the heat.
Dongguan Stirling Metal Products Co., Ltd. specializes in providing a One-Stop Service for 347H material + CNC machining:
- ✅ Material procurement: Genuine product guarantee, complete material certificates
- ✅ CNC machining: Precision of ±0.01mm, smooth surface finish
- ✅ Sample production: Fast delivery in 3-5 days
- ✅ Batch production: Delivery time of 7-15 days
For a Free Quote, please feel free to contact us!
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