Stainless steel is a type of steel. Steel is defined as having a carbon (C) content of 2% or more, while iron is defined as having more than 2% carbon. During the smelting process, chromium (Cr), nickel (Ni), manganese (MN), silicon (Si), titanium (Ti), molybdenum (Mo), and other alloying elements are added to improve the steel’s properties. This enhances the steel’s performance and gives it the corrosion resistance (i.e., it doesn’t rust).
Due to the addition of different types and amounts of alloying elements during the smelting process, stainless steel exhibits different properties, which distinguishes different grades of steel.
Common classifications of stainless steel
1. 304 stainless steel
304 stainless steel is the most commonly used steel. As a widely used steel, it has good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties. It has good ability to be stamped, bent, and undergo other hot processes, and does not exhibit heat treatment hardening (non-magnetic, then usable at temperatures from -196℃ to 800℃).
Applications: Household items (1-2 tableware sets, cabinets, indoor plumbing, water heaters, boilers, bathtubs); automotive parts (windshield wipers, mufflers, molded products); medical equipment, building materials, chemicals, food industry, agriculture, shipbuilding parts.
2. 304L stainless steel (L indicates low carbon)
As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 under normal conditions. However, after welding or stress relief, its resistance to intergranular corrosion is excellent. It also maintains good corrosion resistance without heat treatment, with a temperature range of -196℃ to 800℃.
Applications: Used in the chemical, coal and petroleum industries, where there is a high demand for outdoor machinery with grain boundary corrosion, heat-resistant parts for building materials, and parts that are difficult to heat treat.
3. 316 stainless steel
316 stainless steel, due to the addition of molybdenum, offers excellent corrosion resistance, atmospheric corrosion resistance, and high-temperature strength, making it suitable for use under harsh conditions. It also exhibits excellent work hardening (non-magnetic).
Applications: Seawater equipment, chemical, dye, papermaking, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal facilities, ropes, CD rods, bolts, nuts.
4. 316L stainless steel (L indicates low carbon)
As a low-carbon series of 316 steel, in addition to having the same characteristics as 316 steel, it has excellent resistance to intergranular corrosion.
Application scope: Special requirements for resisting grain boundary corrosion products.
Performance Comparison
1. Chemical Composition
Stainless steel 316 and 316L are molybdenum-containing stainless steels. 316L stainless steel has a slightly higher molybdenum content than 316 steel. Due to the molybdenum in the steel, its overall properties are superior to 310 and 304 stainless steels. 316L stainless steel has a wide range of applications under high-temperature conditions when the concentration of sulfuric acid is less than 15% but exceeds 85%. 316 stainless steel also has good chloride corrosion resistance and is therefore commonly used in marine environments. The maximum carbon content of 316L stainless steel is 0.03%. It is suitable for applications where post-weld annealing is not possible and maximum corrosion resistance is required.
2. CO resistance
316 stainless steel has better corrosion resistance than 304 stainless steel. It exhibits good corrosion resistance in pulp and paper production processes. 316 stainless steel is also resistant to marine and aggressive industrial atmospheres. Generally, the difference in chemical corrosion resistance between 304 and 316 stainless steel is small, but there are differences in certain specific media.
304 stainless steel was originally developed because it is susceptible to corrosion in certain conditions. Adding 2-3% molybdenum can reduce this susceptibility, resulting in 316. Furthermore, this extra molybdenum can reduce corrosion from certain hot organic acids.
316 stainless steel has almost become the standard material in the food and beverage industry. Due to the shortage of molybdenum and the higher nickel content of 316 stainless steel, it is more expensive than 304 stainless steel.
Pitting corrosion is a phenomenon primarily caused by corrosion deposits on the surface of stainless steel due to a lack of oxygen, preventing the formation of a protective chromium oxide layer. It is particularly rare in small valves, where deposits on the disc are minimal.
In various types of aquatic media (distilled water, drinking water, river water, boiler water, seawater, etc.), the chemical resistance of 304 stainless steel and 316 stainless steel is almost the same, unless the chloride ion content in the culture medium is very high, in which case 316 stainless steel is currently more suitable. In most cases, the corrosion resistance of 304 stainless steel and 316 steel is not significantly different, but in some cases it can be very different, requiring case-by-case analysis.
3. Heat resistance
316 stainless steel exhibits good oxidation resistance in discontinuous use below 1600 degrees Celsius and can be used continuously up to 1700 degrees Celsius. Within the temperature range of 800-1575 degrees Celsius, it is best to avoid continuous use of 316 stainless steel, but rather to use it within the specified temperature range, as it possesses good heat resistance. 316L stainless steel has better carbide precipitation resistance than 316 stainless steel and can be used within the aforementioned temperature range.
4. Heat treatment
Annealing is performed in the temperature range of 1850 to 2050 degrees Celsius, followed by rapid annealing and then rapid cooling. 316 stainless steel must not be overheated to harden it.
5. Welding
316 stainless steel has good weldability. All standard welding methods can be used. Depending on the purpose of the welding, 316CB, 316L, or 309CB stainless steel bars or electrodes can be used. For optimal corrosion resistance, the welded portion of 316 stainless steel requires post-weld annealing. If 316L stainless steel is used, post-weld annealing is not required.