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Author: Site Editor Publish Time: 2025-02-10 Origin: Site
Material Matters: A Full Comparison of 304, 304L, 316, and 316L Stainless Steel
Stainless steel is a type of steel. Steel refers to steel containing less than 2% carbon (C), and more than 2% is iron. During the smelting process, alloy elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), and molybdenum (Mo) are added to improve the performance of steel and make it corrosion-resistant (i.e., rust-resistant), which is what we often call stainless steel.
What exactly are "steel" and "iron", what are their characteristics, and what is the relationship between them? How did the 304, 304L, 316, and 316L we usually talk about come from, and what is the difference between them?
Steel: A material with iron as the main element, a carbon content generally below 2%, and other elements. ——GB/T 13304-91 "Steel Classification"
Iron: A metal element with an atomic number of 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity.
Stainless steel: steel that is resistant to weak corrosive media such as air, steam, water, or stainless steel. Commonly used steels are 304, 304L, 316, and 316L, which are 300 series steels of austenitic stainless steel.
2. Why do stainless steels have different steel grades?
During the smelting process of stainless steel, different alloy elements are added in different amounts. Their properties are also different, so different steel grades are given to distinguish them.
304 stainless steel is the most common type of steel. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; it has good hot processing properties such as stamping and bending, and no heat treatment hardening phenomenon (non-magnetic, convenient use temperature -196℃～800℃).
Household items (1, 2 types of tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)
Automotive accessories (windshield wipers, mufflers, molded products)
Medical equipment, building materials, chemicals, food industry, agriculture, ship parts
(L stands for low carbon)
As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel in general state, but after welding or stress relief, its resistance to intergranular corrosion is excellent; it can also maintain good corrosion resistance without heat treatment, and the use temperature is -196℃～800℃.
Applied to outdoor open-air machines in the chemical, coal, and petroleum industries with high requirements for intergranular corrosion resistance, heat-resistant parts of building materials, and parts that are difficult to heat treat.
Due to the addition of molybdenum, 316 stainless steel has particularly good corrosion resistance, atmospheric corrosion resistance, and high-temperature strength, and can be used under harsh conditions; excellent work hardening (non-magnetic).
Equipment for seawater, chemical, dye, papermaking, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal facilities, ropes, CD rods, bolts, nuts.
(L stands for 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.
Products with special requirements for resistance to intergranular corrosion.
316 and 316L stainless steel are molybdenum-containing stainless steels. The molybdenum content in 316L stainless steel is slightly higher than that in 316 stainless steel. Due to the molybdenum in the steel, the overall performance of this steel is better than that of 310 and 304 stainless steel. Under high temperature conditions, when the concentration of sulfuric acid is lower than 15% and higher than 85%, 316 stainless steel has a wide range of uses. 316 stainless steel also has good resistance to chloride corrosion, so it is usually used in marine environments. The maximum carbon content of 316L stainless steel is 0.03, which can be used in applications where annealing cannot be performed after welding and maximum corrosion resistance is required.
316 stainless steel has better corrosion resistance than 304 stainless steel and has good corrosion resistance in the production process of pulp and papermaking. Moreover, 316 stainless steel is also resistant to corrosion by marine and corrosive industrial atmospheres.
Generally speaking, there is little difference between 304 stainless steel and 316 stainless steel in terms of chemical corrosion resistance, but there are differences in certain specific media.
The original stainless steel developed was 304, which was sensitive to pitting corrosion under certain conditions. The addition of 2-3% molybdenum reduced this sensitivity, and thus 316 was born. In addition, this additional molybdenum can also reduce corrosion from certain hot organic acids.
316 stainless steel has become almost a standard material in the food and beverage industry. Due to the shortage of molybdenum worldwide and the higher nickel content in 316 stainless steel, 316 stainless steel is more expensive than 304 stainless steel.
Pitting corrosion is a phenomenon mainly caused by deposit corrosion on the surface of stainless steel, which cannot form a protective layer of chromium oxide due to lack of oxygen.
Especially in small valves, the possibility of deposits on the valve plate is small, so pitting corrosion rarely occurs.
In various types of aqueous media (distilled water, drinking water, river water, boiler water, seawater, etc.), 304 stainless steel and 316 stainless steel have almost the same corrosion resistance, unless the chloride ion content in the medium is very high, then 316 stainless steel is more suitable.
In most cases, there is not much difference in the corrosion resistance of 304 stainless steel and 316 stainless steel, but in some cases there may be a big difference, which requires specific analysis. Generally speaking, valve users should be aware of this, because they will choose the material of the container and pipeline according to the medium. It is not recommended to recommend materials to users.
316 stainless steel has good oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees. It is best not to use 316 stainless steel continuously in the range of 800-1575 degrees, but when 316 stainless steel is used continuously outside this temperature range, the stainless steel has good heat resistance. 316L stainless steel has better resistance to carbide precipitation than 316 stainless steel and can be used in the above temperature range.
Anneal in the temperature range of 1850-2050 degrees, then anneal rapidly, and then cool rapidly. 316 stainless steel cannot be hardened by overheating.
316 stainless steel has good welding performance. All standard welding methods can be used for welding. Depending on the application, 316Cb, 316L or 309Cb stainless steel filler rods or welding rods can be used for welding. For the best corrosion resistance, the welded section of 316 stainless steel needs to be annealed after welding. If 316L stainless steel is used, no post-weld annealing is required.
Of all steels, austenitic stainless steel has the lowest yield point. Therefore, from the perspective of mechanical properties, austenitic stainless steel is not the best material for valve stems, because to ensure a certain strength, the diameter of the valve stem will increase. The yield point cannot be increased by heat treatment, but it can be increased by cold forming.
Due to the wide application of austenitic stainless steel, people have the wrong impression that all stainless steels are non-magnetic. For austenitic stainless steel, it can basically be understood as non-magnetic, and this is true for quenched forged steel. But 304 that has been cold formed will be somewhat magnetic. For cast steel, if it is 100% austenitic stainless steel, it is non-magnetic.
The corrosion resistance of austenitic stainless steel comes from the protective layer of chromium oxide formed on the metal surface. If the material is heated to high temperatures of 450℃ to 900℃, the structure of the material changes and chromium carbides form along the edges of the crystals. This prevents the formation of a protective layer of chromium oxide at the edges of the crystals, resulting in reduced corrosion resistance. This type of corrosion is called "intergranular corrosion".
As a result, 304L stainless steel and 316L stainless steel were developed to combat this corrosion. Both 304L stainless steel and 316L stainless steel have a lower carbon content, and because the carbon content is reduced, chromium carbides will not be produced, and intergranular corrosion will not occur.
It should be noted that a higher sensitivity to intergranular corrosion does not mean that non-low carbon materials are more susceptible to corrosion. This sensitivity is also higher in high chloride environments.
Please note that this phenomenon is due to high temperatures (450℃-900℃). Usually welding is the direct cause of reaching these temperatures.
Why does stainless steel rust? When brown rust spots (spots) appear on the surface of stainless steel pipes, people are surprised: they think "stainless steel does not rust, and if it rusts, it is not stainless steel, and there may be something wrong with the steel." In fact, this is a one-sided and wrong view of stainless steel. Stainless steel will also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is, stainlessness, and also has the ability to corrode in media containing acids, alkalis, and salts, that is, corrosion resistance. However, the size of its corrosion resistance varies with the chemical composition of the steel itself, the protection state, the use conditions and the type of environmental media. For example, 304 steel pipes have absolutely excellent corrosion resistance in dry and clean atmosphere, but if they are moved to coastal areas, they will soon rust in sea fog containing a lot of salt; while 316 steel pipes perform well. Therefore, not all kinds of stainless steel can resist corrosion and rust in any environment.
As a high-performance alloy material, stainless steel plays an important role in modern industry and life. The four stainless steel materials, 304, 304L, 316 and 316L, have their own characteristics and significant differences in composition, performance and application scenarios. By deeply understanding these differences, users can choose the most suitable stainless steel material according to actual needs to ensure the performance and service life of the product. Whether in home, chemical plant or marine environment, stainless steel can meet various complex requirements with its excellent performance.
