Differences Between Stainless Steel 304, 304l, 316, 316l

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-proof), which is what we often call stainless steel.

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: A type of steel that is resistant to weak corrosive media such as air, steam, and water or has stainless properties. Commonly used steel types are 304, 304L, 316, and 316L, which are 300 series steels of austenitic stainless steel.

During the smelting process of stainless steel, different alloying elements are added in different amounts. Their properties 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 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 (no magnetism, convenient use temperature -196℃～800℃).

Scope of application 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 As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel in general 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 It is used in 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.

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; it has excellent work hardening (non-magnetic). Scope of application Equipment for seawater, chemical, dye, papermaking, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal facilities, ropes, CD rods, bolts, nuts.

316L stainless steel - (L is 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

Chemical composition

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. 316L stainless steel has a maximum carbon content of 0.03 and can be used in applications where annealing after welding is not possible 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. 316 stainless steel is also resistant to marine and corrosive industrial atmospheres.

Generally speaking, there is not much 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 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 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 of specific circumstances. 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.

Heat resistance

In intermittent use below 1600 degrees and continuous use below 1700 degrees, 316 stainless steel has good oxidation resistance. It is best not to continuously use 316 stainless steel in the range of 800-1575 degrees, but when 316 stainless steel is continuously used 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

Anneal in the temperature range of 1850-2050 degrees, then rapidly anneal, and then rapidly cool. 316 stainless steel cannot be hardened by overheating.

Welding

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 electrodes 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.

Mechanical properties

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.

Magnetism

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 treated by cold forming will be more or less magnetic. For cast steel, if it is 100% austenitic stainless steel, it is non-magnetic.

Low carbon type 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 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".

Therefore, 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. Because the carbon content is reduced, chromium carbide will not be produced, and intergranular corrosion will not occur. It should be noted that 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.