Comparison Of 304 And 304H: General-Purpose Vs High-Carbon High-Temperature Austenitic Stainless Steel

304 and 304H belong to the 304 series, with the core difference being carbon content (304: C≤0.08%, 304H: C=0.04-0.10%). The controlled high carbon content of 304H significantly improves high-temperature creep strength and structural stability, making them suitable for different temperature and stress service conditions.

Core Parameter Comparison

Parameter	304 Stainless Steel	304H Stainless Steel
Chemical Composition (wt%)	C≤0.08, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=18.00-20.00, Ni=8.00-10.50, Fe=Balance	C=0.04-0.10, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=18.00-20.00, Ni=8.00-10.50, 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) ≥55MPa	Creep rupture strength (1000h) ≥75MPa
Service Temperature	-196℃ to 870℃ (continuous service)	-196℃ to 870℃ (continuous service, preferred for 600-870℃ stress-bearing scenarios)
Equivalent Grades	SUS304 (JIS), EN 1.4301, UNS S30400	SUS304H (JIS), EN 1.4307, UNS S30409

Key Performance Differences: 1. High-temperature performance: 304H's controlled carbon content forms stable carbides at high temperatures, which pin grain boundaries and improve creep resistance; its 1000h creep rupture strength at 700℃ is 36% higher than 304. 2. Intergranular corrosion resistance: 304H is more prone to intergranular corrosion after welding than 304, requiring mandatory post-weld annealing. 3. Weldability: 304 has better welding stability, while 304H requires lower heat input to avoid grain coarsening affecting high-temperature performance. 4. Machinability: Both have similar machinability, with no obvious difference. 5. Cost: 304H is 8-12% more expensive than 304.

Applicable Scenario Distinction: 304 is suitable for general medium-temperature (≤600℃) non-stress-bearing components, such as food processing equipment, decorative pipelines, indoor heat exchangers and low-temperature storage tanks. 304H is suitable for high-temperature stress-bearing components, such as boiler superheater tubes, high-temperature heat exchanger tubes (600-870℃), gas turbine auxiliary components and high-temperature steam pipelines.

Practical Q&A

Q1: Why is 304H suitable for high-temperature stress-bearing components? A1: Its controlled carbon content (0.04-0.10%) ensures the formation of sufficient carbides at high temperatures, which can resist plastic deformation under long-term high-temperature and high-stress conditions, avoiding component failure due to creep.

Q2: What is the mandatory post-weld heat treatment requirement for 304H? A2: Must perform post-weld annealing at 850-900℃, air cooling; this process eliminates residual stress, dissolves precipitated carbides, and restores high-temperature creep performance and corrosion resistance.

Q3: Can 304 replace 304H in high-temperature stress scenarios? A3: No. At temperatures above 600℃, 304's creep resistance is insufficient, and it will undergo obvious plastic deformation after long-term service; 304H is the mandatory material for high-temperature stress-bearing components in this temperature range.

Q4: What is the logic of carbon content control for 304H? A4: The lower limit of carbon content (0.04%) ensures sufficient carbides for high-temperature creep resistance, and the upper limit (0.10%) avoids excessive carbon leading to reduced corrosion resistance, balancing high-temperature performance and corrosion resistance.

Q5: How to select between 304 and 304H? A5: Choose 304 if the service temperature is ≤600℃ and no long-term stress is applied; choose 304H if the service temperature is 600-870℃ and the component bears long-term high-temperature stress.