304, 304L, 316, and 316L are all types of austenitic stainless steel. The core differences among them lie in aspects such as chemical composition, mechanical properties, corrosion resistance, welding performance, application fields, and price. The details are as follows: 

I. Chemical Composition 

304: Carbon content ≤ 0.08% (typical value is approximately 0.06 - 0.08%), mainly composed of 18% chromium (Cr) and 8% nickel (Ni), it is a basic austenitic stainless steel. 

304L: Contains a carbon content of ≤ 0.03% (the "L" indicates low carbon), and other components are the same as those in 304. By reducing the carbon content, the risk of carbide precipitation during welding or at high temperatures is minimized. 

316: Based on 304, add 2-3% of molybdenum (Mo), with a carbon content of ≤ 0.08%, and the corrosion resistance is significantly enhanced. 

316L: Contains a carbon content of no more than 0.03%, and the molybdenum content is the same as that of 316 (approximately 2-3%), being a low-carbon version of 316. 

II. Mechanical Properties 

Intensity: 

The tensile strength of 304 is ≥ 515 MPa, and the yield strength is ≥ 205 MPa, which is higher than that of 304L (with tensile strength ≥ 485 MPa and yield strength ≥ 170 MPa). 

The tensile strength requirement for 316 is greater than 520 MPa, which is higher than that of 316L (tensile strength ≥ 480 MPa). Since carbon is a strong austenite-forming element, it can significantly enhance the strength. 

Ductility: Due to their low-carbon design, 304L and 316L have superior ductility, making them easier to be processed and shaped. They are suitable for heat processing such as stamping and bending. 

III. Corrosion Resistance 

Normal environment: The properties of 304 and 304L, as well as 316 and 316L, are similar in terms of resistance to acid, alkali and atmospheric corrosion. 

Welding or high-temperature environment: 

Due to their low carbon properties, 304L and 316L have stronger resistance to intergranular corrosion. After welding, no annealing treatment is required and they can be used directly, avoiding the corrosion risk caused by the precipitation of carbides. 

316 and 316L, due to their molybdenum content, exhibit superior resistance to chloride ion corrosion compared to 304 and 304L, and are particularly suitable for marine environments or high chloride ion media. 

High-temperature performance: 316L exhibits superior carbide precipitation resistance within the temperature range of 800 - 1575°C compared to 316, and can be continuously used in high-temperature environments; 316 has better continuous usage performance outside this temperature range. 

IV. Welding Performance 

304 and 316: After welding, they need to be cooled rapidly or undergo annealing treatment to prevent the formation of carbides, which could lead to intergranular corrosion. 

304L and 316L: Due to their low carbon properties, they can be directly welded without any subsequent processing, making them suitable for scenarios where heat treatment is not possible (such as outdoor equipment and pipelines). 

V. Application Areas 

304: Widely used in food equipment, architectural decoration, general chemical pipelines, automotive manufacturing (weather-resistant structural components), and other common scenarios. 

304L: Suitable for welding structures (such as storage tanks, pipelines), petrochemical equipment, low-temperature environments (such as LNG), and high-corrosion-risk scenarios (such as in the chemical, coal, and petroleum industries). 

316: Suitable for use in seawater equipment, chemical industries, dyes, papermaking, food processing, and facilities in coastal areas, etc., in harsh corrosive environments. 

316L: Based on the 316 application, it is more suitable for products with special requirements for resisting intergranular corrosion (such as non-standard equipment material flow channels, surgical instruments). 

VI. Price and Standards 

Price: 304L and 316L have higher requirements for low-carbon smelting processes, so their prices are usually slightly higher than those of 304 and 316. However, the price difference is relatively small, and it depends on market supply and demand. 

Standard and Grade: 

304: The ASTM standard designation is UNS S30400, while the Chinese national standard designation is 06Cr19Ni10. 

304L: The ASTM standard designation is UNS S30403, while the Chinese national standard designation is 022Cr19Ni10. 

316: The ASTM standard designation is UNS S31600, while the Chinese national standard designation is 0Cr17Ni12Mo2. 

316L: The ASTM standard designation is UNS S31603, and the Chinese national standard designation is 00Cr17Ni14Mo2.