304, 304L, and 304H - What are the differences between them

304, 304L, and 304H differ mainly in terms of carbon content, performance, and application scenarios. Here is a detailed analysis:

1. Chemical composition differences

304: Carbon content ≤ 0.08%, Chromium (Cr) 18%-20%, Nickel (Ni) 8%-10.5%.

304L: Carbon content ≤ 0.03%, Chromium (Cr) 18%-20%, Nickel (Ni) 8%-12%.

304H: Carbon content 0.04%-0.10%, Chromium (Cr) 18%-20%, Nickel (Ni) 8%-10.5%. 

II. Performance Characteristics - Corrosion Resistance

Corrosion Resistance:

304L: The ultra-low carbon design gives it the strongest resistance to intergranular corrosion after welding or heat treatment, suitable for chemical industries, acidic environments, etc.

304: The corrosion resistance in the solution state is comparable to 304L, but after welding, chromium carbide may precipitate and cause intergranular corrosion, requiring post-weld heat treatment.

304H: With a higher carbon content, it has the weakest resistance to intergranular corrosion and is not suitable for welding environments prone to corrosion.

Mechanical Properties:

304H: The highest carbon content, with the highest strength and hardness at room temperature, suitable for high-temperature pressure-bearing scenarios.

304: Moderate strength, balanced overall performance.

304L: Low carbon results in the lowest room temperature strength, but better elongation performance.

High Temperature Performance:

304H: Specifically designed for high temperatures, with excellent high-temperature strength and creep resistance above 525°C, suitable for boilers, heat exchangers, etc.

304: The operating temperature is generally limited to below 425°C.

304L: The allowable stress at high temperatures is relatively low, not suitable for pressure-bearing high-temperature environments. 

III. Application Scenarios

304: Highly versatile, widely used in food processing, architectural decoration, household items, general industrial containers, etc.

304L: Commonly used in welding components, chemical equipment, heat exchangers, etc. in scenarios requiring resistance to intergranular corrosion.

304H: Mainly used in high-temperature environments, such as power station boilers, superheaters, steam pipelines (>525℃), etc. 

IV. Processing and Welding Precautions

Weldability:

304L: No annealing is required after welding. It is suitable for thick plate welding.

304H: The welding temperature needs to be controlled (to avoid sensitization). It is recommended to use high-carbon welding wire and combine with annealing.

304: Regular welding is sufficient. However, thick plates may require post-weld heat treatment.

Cost: 304L has a slightly higher cost due to the complex smelting process (such as ultra-low carbon control); 304H has a higher material cost due to the need for high-temperature performance. 

V. Selection Suggestions

High-temperature scenarios (≥550℃): Opt for 304H, considering both strength and oxidation resistance.

Corrosion-sensitive environments (such as Cl⁻, seawater): Choose 304L to avoid intergranular corrosion.

General applications: 304 offers the best cost performance and meets most industrial requirements. 

VI. Data Verification Case

High Temperature Strength: The creep rupture strength of 304H at 650℃ is 30% higher than that of 304, making it suitable for supercritical boilers.

Corrosion Resistance: In an environment containing Cl⁻, the pitting potential of 304H is 50 mV lower than that of 304, and its service life may be shortened to 3 years (while 304 lasts for 5 years).

By considering the carbon content, performance requirements, and cost factors comprehensively, the material selection can be precisely matched. In practical applications, a life cycle cost analysis (LCCA) needs to be conducted in combination with specific operating conditions (temperature, medium, mechanical load).