Comparison of SUS347 and SUS347H: Niobium-Stabilized vs High-Carbon Niobium-Stabilized Austenitic Stainless Steel

SUS347 and SUS347H are niobium-stabilized austenitic stainless steels of the same series, with the core difference being carbon content (SUS347: C≤0.08%; SUS347H: C=0.04-0.10%). The controlled high carbon content of SUS347H enhances high-temperature creep strength, while maintaining niobium stabilization's high-temperature stability, making them suitable for different ultra-high temperature stress service conditions.

Core Parameter Comparison

Parameter	SUS347 Stainless Steel	SUS347H Stainless Steel
Chemical Composition (wt%)	C≤0.08, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=17.00-19.00, Ni=9.00-13.00, Nb+Ta=8×C-1.00, Fe=Balance	C=0.04-0.10, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=17.00-19.00, Ni=9.00-13.00, Nb+Ta=8×C-1.00, Fe=Balance
Mechanical Properties (Annealed)	Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB	Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB
High-Temperature Creep Strength (700℃)	Creep rupture strength (1000h) ≥65MPa	Creep rupture strength (1000h) ≥90MPa
Service Temperature	-196℃ to 900℃ (continuous service)	-196℃ to 900℃ (continuous service, preferred for 600-900℃ stress-bearing scenarios)
Equivalent Grades	EN 1.4550, UNS S34700, AISI 347	EN 1.4551, UNS S34709, AISI 347H

Key Performance Differences: 1. High-temperature creep strength: SUS347H's controlled carbon content forms more stable NbC carbides, improving creep resistance; its 1000h creep rupture strength at 700℃ is 38% higher than SUS347. 2. High-temperature stability: Both have excellent high-temperature stability; SUS347H's NbC carbides are more stable under high-temperature stress. 3. Intergranular corrosion resistance: Both have excellent intergranular corrosion resistance after welding; SUS347H requires post-weld annealing for thick plates. 4. Weldability: SUS347 has better welding stability; SUS347H requires stricter control of heat input. 5. Cost: SUS347H is 10-15% more expensive than SUS347.

Applicable Scenario Distinction: SUS347 is suitable for general high-temperature non-stress-bearing components, such as nuclear power plant auxiliary equipment, high-temperature heat exchanger tubes (≤900℃), and chemical reaction vessels. SUS347H is suitable for ultra-high temperature stress-bearing components, such as boiler superheater tubes (600-900℃), high-temperature steam pipelines, and aerospace engine components.

Practical Q&A

Q1: Why is SUS347H suitable for aerospace high-temperature components?

A1: Its combination of niobium stabilization and controlled high carbon content ensures excellent high-temperature stability and creep resistance, which can withstand long-term high-temperature stress in aerospace engine environments.

Q2: What is the post-weld heat treatment standard for SUS347H?

A2: Must perform post-weld annealing at 850-900℃, holding time ≥30 minutes, air cooling; this process eliminates residual stress and restores high-temperature performance.

Q3: Can SUS347 replace SUS347H in ultra-high temperature stress scenarios?

A3: No. At temperatures above 600℃, SUS347's creep resistance is insufficient for long-term stress-bearing; SUS347H is the mandatory material for such scenarios.

Q4: What is the significance of niobium-tantalum content in SUS347H?

A4: Nb+Ta=8×C-1.00% ensures sufficient combination with carbon to form NbC, avoiding Cr₂₃C₆ precipitation, and tantalum enhances the stability of NbC, further improving high-temperature performance.

Q5: How to select between SUS347 and SUS347H?

A5: Choose SUS347 if the service temperature is ≤600℃ and no long-term stress is applied; choose SUS347H if the service temperature is 600-900℃ and the component bears long-term high-temperature stress.