Generally speaking, the most common types of stainless steel are 304 stainless steel and 316 stainless steel, but what is the difference between the two?
The first thing to talk about is the issue of food grade. In life, we often encounter people who buy stainless steel items, and ask whether 304 is food grade. According to the material, 304 stainless steel has indeed reached the food grade. There are three common food-grade stainless steels on the market, namely 304, 316, and 430 stainless steel. 304 is usually used for small household appliances or water cups, 316 is used for medical equipment, and 430 is used as stainless steel for knives. However, it is worth noting that the so-called food-grade does not only refer to the material reaching the food grade but also includes the standards of its production process.
The second thing to talk about is the issue of corrosion resistance. For different stainless steel materials, there must be some differences in corrosion resistance, otherwise, 304 can only be used as daily household appliances, while 314 can be used in medical equipment. So which stainless steel, 306 or 314, has better corrosion resistance? In general, there is no big difference between them. However, if the chloride ion content in the medium is very high, 316 stainless steel will be slightly better than 304 stainless steel and more suitable. This is why 316 stainless steel is selected for medical devices.
So how can we distinguish the types of stainless steel?
First, we can distinguish them by color. The surface color of stainless steel washed with acid is silvery white and very smooth, while the color of stainless steel that has not been washed with acid is slightly darker. The colors displayed by stainless steel of different materials are also different.
Second, we can use magnets to identify stainless steel. Because stainless steel contains chromium, it can be attracted by magnets in any state; but stainless steel with a high manganese content is non-magnetic; the situation of chromium-nickel-nitrogen stainless steel is slightly more complicated. Some can be attracted by magnets, but some cannot, so different types of stainless steel are still very easy to distinguish.
The above is a brief introduction to stainless steel 304 and stainless steel 316. From this, we can know that the components contained in different types of stainless steel are different, not exactly the same, and the occasions used for different types of stainless steel are also different, and the functions they have are also very different.
Therefore, choosing the right stainless steel is something we need to pay attention to in daily life. It is not just a matter of choosing it casually. We need to determine whether it reaches the food grade and whether it can be used reasonably in life. What do you think of different types of stainless steel?
1. What kind of steel is 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. In 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 (ie, rust-proof), which is what we often call stainless steel. What exactly are “steel” and “iron”, what are their characteristics, and what is their relationship, 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, atomic number 26. Iron materials have strong ferromagnetism, 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 stainless properties. The commonly used steel types are 304, 304L, 316, and 316L, which are 300-series steels of austenitic stainless steel.
2. Why do stainless steel have different steel grades?
During the smelting process of stainless steel, different types of alloy elements are added, and the amount of different types added is different. Their characteristics are also different. In order to distinguish them, they are given different steel grades. The following is a table of “alloy element” content of different grades of common decorative stainless steel for reference only:
Chemical composition (mass fraction%)
- 304 stainless steel -
304 stainless steel performance introduction: 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, easy-to-use temperature -196℃～800℃).
Scope of application of 304 stainless steel:
Household items (category 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)
Auto parts (windshield wipers, mufflers, molded products)
Medical equipment, building materials, chemicals, food industry, agriculture, ship parts
- 304L stainless steel – (L stands for low carbon)
Performance introduction of 304L stainless steel: As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel under normal conditions, 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 operating temperature is -196℃～800℃.
Scope of application of 304L stainless steel: It is used in outdoor open-air machines in the chemical, coal, and petroleum industries with high requirements for resistance to intergranular corrosion, heat-resistant parts of building materials, and parts that are difficult to heat treat.
- 316 stainless steel -
Introduction to the performance of 316 stainless steel: 316 stainless steel has excellent corrosion resistance, atmospheric corrosion resistance, and high-temperature strength due to the addition of molybdenum, and can be used under harsh conditions; excellent work hardening (non-magnetic).
Scope of application of 316 stainless steel: equipment for use in seawater, chemical, dye, papermaking, oxalic acid, fertilizer, and other production equipment; photography, food industry, coastal facilities, ropes, CD rods, bolts, and nuts.
- 316L stainless steel – (L stands for low carbon)
Introduction to the performance of 316L stainless steel: 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.
Scope of application of 316L stainless steel: products with special requirements for resistance to intergranular corrosion.
Performance comparison of stainless steel:
① 316 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 chloride corrosion resistance, 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.
② Corrosion resistance of stainless steel: 316 stainless steel has better corrosion resistance than 304 stainless steel, and has good corrosion resistance in the production process of pulp and paper. Moreover, 316 stainless steel is also resistant to corrosion in 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 stainless steel originally developed was 304, which is sensitive to pitting corrosion under certain circumstances. Adding an additional 2-3% of molybdenum can reduce 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 almost become a standard material in the food and beverage industry. Due to the worldwide shortage of molybdenum 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 is due to the lack of oxygen and the inability to form a protective layer of chromium oxide. 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.), the corrosion resistance of 304 stainless steel and 316 stainless steel is almost the same, 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, it may be very different, which requires specific analysis. Generally speaking, valve users should have a good idea in mind, because they will choose the material of the container and pipeline according to the medium. It is not recommended to recommend materials to users.
③ Heat resistance of stainless steel: 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.
④ Heat treatment of stainless steel: Annealing is carried out in the temperature range of 1850-2050 degrees, followed by rapid annealing and rapid cooling. 316 stainless steel cannot be hardened by overheating.
⑤ Welding of stainless steel: 316 stainless steel has good welding performance. It can be welded by all standard welding methods. When welding, 316Cb, 316L, or 309Cb stainless steel filler rods or welding rods can be used according to the purpose. To obtain 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.
Among 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.
⑥ Magnetism of stainless steel: Due to the wide application of austenitic stainless steel, people have the wrong impression that all stainless steel is non-magnetic. Austenitic stainless steel can basically be understood as non-magnetic, and this is true for quenched forged steel. But 304 treated by cold forming will be more or less magnetic. For cast steel, if it is 100% austenitic stainless steel, it is non-magnetic.
⑦ Low-carbon type 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 a high temperature of 450℃ to 900℃, the structure of the material will change and chromium carbide will form along the edge of the crystal. In this way, the chromium oxide protective layer cannot be formed on the edge of the crystal, resulting in reduced corrosion resistance. This corrosion is called “intergranular corrosion”.
Therefore, 304L stainless steel and 316L stainless steel were developed to combat this corrosion. 304L stainless steel and 316L stainless steel have a lower carbon content. Because the carbon content is reduced, chromium carbide will not be produced, and intergranular corrosion will not occur. It should be noted that 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-chlorine environments. Please note that this phenomenon is due to high temperatures (450℃-900℃). Usually, welding is the direct cause of reaching this temperature. For soft seat conventional butterfly valves, since we do not perform welding operations on the valve plate, it does not make much sense to use low-carbon stainless steel, but most specifications will require 304L stainless steel or 316L stainless steel.
Post time: Feb-05-2025