304H Stainless Steel High-Carbon

304H is the high-carbon variant of 304 stainless steel, with controlled carbon content to enhance high-temperature creep strength. It is specifically designed for high-temperature stress-bearing components, balancing high-temperature performance and corrosion resistance, widely used in the power generation industry.

Chemical Composition (wt%): C=0.04-0.10, Cr=18.00-20.00, Ni=8.00-10.50, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Fe=Balance

Mechanical Properties (Annealed): Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB

Performance Advantages: Excellent high-temperature creep strength, especially stable at 600-870℃; good high-temperature oxidation resistance; similar room-temperature corrosion resistance to 304; suitable for high-temperature stress-bearing scenarios.

Applications: Boiler superheater tubes, high-temperature steam pipelines, gas turbine auxiliary components, industrial furnace heating elements, high-temperature reaction vessel flanges in power generation industry.

Equivalent Grades: UNS S30409, JIS SUS304H, EN 1.4307, GB 07Cr19Ni10

Q&A

Q1: Why is 304H suitable for high-temperature stress-bearing components? A1: 304H is suitable for high-temperature stress-bearing components mainly due to its controlled high carbon content (0.04-0.10wt%). At high temperatures, carbon in 304H combines with chromium to form stable chromium carbides, which can pin grain boundaries and prevent grain sliding, thereby significantly improving high-temperature creep strength. At 700℃, the 1000h creep rupture strength of 304H (≥75MPa) is 36% higher than that of 304 (≥55MPa), enabling it to maintain structural stability under long-term high-temperature and high-stress conditions. In contrast, 304's low carbon content results in insufficient carbides at high temperatures, leading to poor creep resistance and easy plastic deformation. Additionally, 304H retains good high-temperature oxidation resistance, forming a dense oxide film to resist high-temperature gas corrosion.

Q2: What is the mandatory post-weld heat treatment requirement for 304H? A2: 304H stainless steel must undergo post-weld annealing at 850-900℃, followed by air cooling. This heat treatment process is mandatory because welding can cause residual stress in the component, which may lead to stress corrosion cracking in high-temperature environments. Annealing at 850-900℃ can effectively eliminate residual stress, reduce the risk of cracking. Meanwhile, this temperature range can dissolve excessive chromium carbides precipitated during welding, avoiding the formation of chromium-depleted zones and restoring the corrosion resistance of the weld area. The holding time of the annealing process should be at least 30 minutes per 25mm of thickness to ensure sufficient heat penetration. Air cooling after annealing helps maintain the austenitic structure and avoid the formation of brittle phases, ensuring the mechanical properties of the component.

Q3: Can 304 replace 304H in high-temperature stress scenarios? A3: No, 304 cannot replace 304H in high-temperature stress scenarios. The key reason is the significant difference in high-temperature creep strength between the two. At temperatures above 600℃, 304's creep resistance is insufficient; under long-term high-temperature and high-stress conditions, it will undergo obvious plastic deformation, leading to component failure. For example, in boiler superheater tubes operating at 700℃, 304 will experience excessive deformation within a short period, while 304H can maintain stable performance for a long time. Additionally, 304H's controlled carbon content balances high-temperature performance and corrosion resistance, while 304's low carbon content results in poor high-temperature structural stability. Using 304 in high-temperature stress scenarios will not only reduce the service life of the component but also pose potential safety hazards such as pipeline leakage.

Q4: What is the difference in carbon content control logic between 304H and 304? A4: The carbon content control logic of 304H and 304 is fundamentally different due to their different application scenarios. 304 is designed for general low-corrosion environments, so its carbon content is controlled at a low level (≤0.08wt%) to reduce the risk of intergranular corrosion, prioritizing room-temperature corrosion resistance and formability. In contrast, 304H is designed for high-temperature stress-bearing environments, so its carbon content is controlled within a specific range (0.04-0.10wt%). The lower limit of 0.04wt% ensures sufficient carbon to form carbides at high temperatures, providing necessary creep strength. The upper limit of 0.10wt% avoids excessive carbon, which would lead to excessive carbide precipitation, reducing room-temperature corrosion resistance and toughness. This precise carbon content control enables 304H to balance high-temperature performance and corrosion resistance, which is not achievable with 304's carbon content range.

Q5: What are the key factors affecting the service life of 304H in high-temperature environments? A5: Several key factors affect the service life of 304H in high-temperature environments. First, the operating temperature: exceeding the maximum continuous service temperature (870℃) will accelerate oxidation and carbide coarsening, significantly reducing service life. Second, the level of applied stress: higher stress will increase creep deformation rate, shortening the creep rupture life. Third, the quality of post-weld heat treatment: insufficient annealing temperature or holding time will leave residual stress, increasing the risk of stress corrosion cracking. Fourth, the composition of the high-temperature medium: corrosive gases such as sulfur dioxide or chloride ions in the medium will accelerate the corrosion of 304H, reducing its service life. Fifth, the material's purity: impurities such as phosphorus and sulfur will reduce the high-temperature toughness and creep strength of 304H, affecting service life. To extend service life, it is necessary to strictly control operating temperature and stress, ensure proper post-weld heat treatment, and avoid corrosive medium environments.