How to distinguish 304 stainless steel from 316 stainless steel

In our daily life, stainless steel is a very common product material. Stainless steel pots and pans are often used in daily life. The scope of use of stainless steel is very wide, ranging from small stainless steel objects to stainless steel building materials, but stainless steel is just a general term for a class of materials.


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? Is there any way to distinguish the two?


The first thing to say is the issue of food grade. In life, we often encounter people who buy stainless steel items. Is 304 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 a stainless steel material for knives. But one thing that needs to be noted is 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 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 devices. So which stainless steel is better in corrosion resistance, 306 or 314? In general, there is no big difference between them, but 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 to be added to medical devices.


So what methods can be used to identify the types of stainless steel?


1. It can be identified by color. The surface color of stainless steel that has been acid-cleaned is silvery white and very smooth, while the color of stainless steel that has not been acid-cleaned is slightly darker. The colors displayed by stainless steel of different materials are also different.


2. Magnets can be used 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. Therefore, 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 and not exactly the same. Moreover, different types of stainless steel are used on different occasions and have very different functions.


For this reason, 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? The naked eye and flexible tools cannot distinguish them. We have more professional methods. Let's talk about it below:


What is the difference between 304, 304L, 316, and 316L?
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 steel containing 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-proof), 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 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. Commonly used steel grades 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 alloying elements are added in different amounts. Their characteristics are also different. In order to distinguish them, different steel grades are given. The following is a table of "alloying element" content of different grades of common decorative stainless steel for reference only:
Chemical composition (mass fraction, %)


- 304 stainless steel -
Performance introduction: 304 stainless steel is the most common steel grade. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength, and mechanical properties; good hot processing properties such as stamping and bending, no heat treatment hardening phenomenon (non-magnetic, convenient use temperature -196℃～800℃).
Scope of application: household goods (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: 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: Applicable to outdoor open-air machines in the chemical, coal, and oil industries with high requirements for intergranular corrosion resistance, heat-resistant parts of building materials, and parts that are difficult to heat treat.


- 316 stainless steel -
Performance introduction: 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: 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)
Performance introduction: 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: products with special requirements for resistance to intergranular corrosion.


Performance comparison
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.


Corrosion resistance
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 resistant to 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 first stainless steel developed was 304, which is sensitive to pitting corrosion under certain circumstances. The additional addition of 2-3% molybdenum can reduce this sensitivity, thus giving birth to 316. 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 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 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 water 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, there may be a big difference, which requires specific analysis. Generally speaking, valve users should have a good idea, because they will choose the material of the container and pipeline according to the medium. It is not recommended to recommend materials to users.


Heat resistance: 316 stainless steel has good oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees. In the range of 800-1575 degrees, it is best not to use 316 stainless steel continuously, 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.
Heat treatment: Annealing in the temperature range of 1850-2050 degrees, then rapid annealing, and then rapid cooling. 316 stainless steel cannot be hardened by overheating.
Welding: 316 stainless steel has good welding properties. 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 for welding according to the purpose. 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.


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.


Magnetism: 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 be basically understood as non-magnetic, and this is true for quenched forged steel. However, 304 that has been cold-formed will be somewhat magnetic. For cast steel, if it is 100% austenitic stainless steel, it is non-magnetic.
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 at the edge of the crystal, 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 low carbon content. Because the carbon content is reduced, chromium carbide will not be produced, and intergranular corrosion will not be generated. It should be noted that a higher intergranular corrosion sensitivity does not mean that non-low-carbon materials are more susceptible to corrosion. This sensitivity is also higher in a high-chlorine environment.


Please note that this phenomenon is caused by high temperature (450℃-900℃). Usually, welding is the direct cause of reaching this temperature. For conventional butterfly valves with soft valve seats, 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.


Why does stainless steel rust?
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, rust is not stainless steel, there may be problems with the steel quality". In fact, this is a one-sided and wrong view of stainless steel due to a lack of understanding. 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. Such as 304 steel pipe, in a dry and clean atmosphere, has absolutely excellent corrosion resistance, but it will be moved to the coastal areas, containing a lot of salt fog, which will soon be rusty; while 316 steel pipe shows well. Therefore, not any kind of stainless steel can be corrosion-resistant and rust-free in any environment.