Read This Article To Understand: What Is The Difference between 316 And 316L?

What is stainless steel?

Stainless steel is a type of steel. Steel refers to materials containing less than 2% carbon (C), while anything with more than 2% carbon is iron. During the smelting process, alloying elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), and molybdenum (Mo) are added to improve the properties of steel, making it corrosion-resistant (i.e., not rusting), which is what we commonly refer to as stainless steel. What exactly are "steel" and "iron," what are their characteristics, and how are they related? How do we get terms like 304,304L,316, and 316L, and what are the differences between them? Steel: A material primarily composed of iron, with a carbon content generally below 2%, and containing other elements. ——GB/T 13304-91 "Classification of Steel" Iron: A metallic element with atomic number 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity. Stainless steel: A type of steel that is resistant to weak corrosive media such as air, steam, and water, or has a non-corrosive nature. Commonly used grades include 304,304L,316, and 316L, which belong to the 300 series of austenitic stainless steels.

Why is there a different grade of stainless steel?

Stainless steel is different in its properties due to the different types of alloying elements added and the different amounts of different types added during the smelting process. In order to distinguish them, different steel grades are given. The following is a table of the "alloying element" content of common decorative stainless steel for reference only: Chemical composition (mass fraction%):
① Introduction to the Properties of 304 Stainless Steel: 304 stainless steel is the most common type of steel. As a versatile material, it boasts excellent corrosion resistance, heat resistance, low-temperature strength, and mechanical properties; it has good hot workability for processes such as stamping and bending, and does not undergo hardening during heat treatment (non-magnetic, usable temperature range-196℃ to 800℃). Application Scope: Household items (Tableware, cabinets, indoor piping, water heaters, boilers, bathtubs), automotive parts (windshields, wipers, mufflers, molded products), medical devices, construction materials, chemicals, food industry, agriculture, ship components

②304L Stainless Steel (L stands for low carbon) Performance Introduction: As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel under normal conditions. However, after welding or stress relief, it exhibits excellent resistance to grain boundary corrosion. Even without heat treatment, it maintains good corrosion resistance, with a usable temperature range of-196℃ to 800℃. Application Scope: Suitable for outdoor machinery in the chemical, coal, and petroleum industries where high resistance to grain boundary corrosion is required, as well as for building materials' heat-resistant parts and components that are difficult to heat treat.

③316 Stainless Steel Performance Introduction: 316 stainless steel, due to the addition of molybdenum, has excellent corrosion resistance, atmospheric corrosion resistance, and high-temperature strength, making it suitable for use under harsh conditions; it has excellent work hardening properties (non-magnetic). Application Scope: Equipment used in seawater, chemical, dyeing, papermaking, oxalic acid, fertilizer production; photography, food industry, coastal facilities, ropes, CD poles, bolts, nuts.

④316L stainless steel (L is low carbon) performance introduction: as a low carbon series of 316 steel, in addition to the same characteristics with 316 steel, its anti-grain boundary corrosion is excellent. Application scope: products with special requirements for anti-grain boundary corrosion.

3. Performance Comparison

Chemical composition:

316 and 316L stainless steels are types of molybdenum-containing stainless steel. The molybdenum content in 316L stainless steel is slightly higher than that in 316 stainless steel. Due to the presence of molybdenum in the steel, this grade has superior overall performance compared to 310 and 304 stainless steels. Under high-temperature conditions, when the concentration of sulfuric acid is below 15% and above 85%,316 stainless steel has extensive applications. 316 stainless steel also exhibits excellent resistance to chloride corrosion, making it commonly used in marine environments. The maximum carbon content for 316L stainless steel is 0.03, which allows it to be used in applications where welding cannot be followed by annealing and maximum corrosion resistance is required.

Corrosion Resistance: 316 stainless steel has superior corrosion resistance compared to 304 stainless steel, performing well in pulp and paper production processes. Moreover, 316 stainless steel is also resistant to marine environments and corrosive industrial atmospheres. Generally, there is little difference between 304 and 316 stainless steels in terms of chemical corrosion resistance, although some distinctions exist under certain specific media. The first stainless steel developed was 304, which is relatively sensitive to pitting corrosion (Pitting Corrosion) under certain conditions. Adding an additional 2-3% molybdenum can reduce this sensitivity, leading to the creation of 316. Additionally, this extra molybdenum can also reduce the corrosion caused by certain hot organic acids.

316 stainless steel has almost become the standard material in the food and beverage industry. Due to the global shortage of molybdenum and the higher nickel content in 316 stainless steel, its price is more expensive than that of 304 stainless steel. Pitting corrosion is primarily caused by the deposition of corrosion on the surface of stainless steel, which occurs due to the lack of oxygen that prevents the formation of a chromium oxide protective layer. This is especially true for small valves where the likelihood of deposits on the valve plates is minimal, thus reducing the occurrence of pitting corrosion. In various types of water media (distilled water, drinking water, river water, boiler water, seawater, etc.), the corrosion resistance of 304 stainless steel is almost identical to that of 316 stainless steel, unless the chloride ion content in the medium is very high, in which case 316 stainless steel is more suitable. In most cases, there is little difference in corrosion resistance between 304 stainless steel and 316 stainless steel, but significant differences may occur under certain conditions, requiring specific analysis for specific situations. Generally, valve users should be aware of this, as they will choose the material for containers and pipes based on the medium conditions, and it is not advisable to recommend materials to users.

Heat Resistance: 316 stainless steel exhibits good oxidation resistance when intermittently used below 1600 degrees Celsius and continuously used below 1700 degrees Celsius. Within the range of 800 to 1575 degrees Celsius, it is best not to continuously use 316 stainless steel; however, when continuously used outside this temperature range, the stainless steel maintains excellent heat resistance. The resistance of 316L stainless steel to carbide precipitation is better than that of 316 stainless steel, making it suitable for the aforementioned temperature range.

Heat Treatment: Annealing is performed at temperatures between 1850 and 2050 degrees Celsius, followed by rapid annealing and quick cooling. 316 stainless steel cannot be hardened through overheating treatment.

Welding: 316 stainless steel has excellent weldability and can be welded using all standard welding methods. Depending on the application, 316Cb, 316L, or 309Cb stainless steel filler rods or welding electrodes can be used for welding. To achieve optimal corrosion resistance, post-weld annealing is required for the welded joints of 316 stainless steel. If 316L stainless steel is used, post-weld annealing is not necessary.

Mechanical Properties

Among all steels, austenitic stainless steel has the lowest yield point. Therefore, from a mechanical performance perspective, austenitic stainless steel is not the best material for valve stems, as increasing the diameter of the stem to ensure sufficient strength would be necessary. The yield point cannot be improved through heat treatment but can be enhanced through cold forming.

Magnetism Due to the widespread use of austenitic stainless steel, there is a common misconception that all stainless steels are non-magnetic. For austenitic stainless steel, it can generally be understood as non-magnetic; quenched forged steel is indeed so. However, 304 treated with cold forming will have some magnetic properties. For cast steel, if it is 100% austenitic stainless steel, it is non-magnetic.

⑧ Low-carbon types of stainless steel

The corrosion resistance of austenitic stainless steel comes from the chromium oxide protective layer formed on the metal surface. If the material is heated to temperatures between 450℃ and 900℃, its structure changes, and chromium carbide forms along the crystal edges. This prevents the formation of a chromium oxide protective layer at the crystal edges, leading to a decrease in corrosion resistance. This type of corrosion is called "intergranular corrosion." To combat this, 304L stainless steel and 316L stainless steel were developed. Both 304L and 316L stainless steels have lower carbon content, so less chromium carbide is produced, thus preventing intergranular corrosion. It should be noted that higher sensitivity to intergranular corrosion does not mean that non-low-carbon materials are more prone to corrosion. The sensitivity is even higher in high chloride environments. It is important to note that this phenomenon occurs due to high temperatures (450℃-900℃). Welding is often the direct cause of reaching these temperatures. For soft valve seat conventional butterfly valves, since welding operations are not performed on the valve plate, using low-carbon stainless steel is not particularly significant. However, most specifications require 304L or 316L stainless steel.