The difference between stainless steel 304, 304L, 316, 316L
What is the meaning of “steel” and “iron”. What are the characteristics? What is the relationship? How is 304, 304L, 316, and 316L produced? What is the difference between stainless steel 304, 304L, 316, 316L?
Steel: A material containing iron as the main element and having a carbon content of generally less than 2% and containing other elements. ——GB/T13304-91 “Steel Classification”
Iron: a metal element, atomic number 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity.
Stainless steel: resistant to weak corrosive media such as air, steam, water. The commonly types of stainless steel grades are 304, 304L, 316, 316L. It is a 300 series steel of austenitic stainless steel.
304 stainless steel
304 stainless steel is the most common steel grade. As a kind of widely used metal, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties. Good hot workability such as stamping and bending. No heat treatment hardening phenomenon (no magnetic , use temperature -196 ° C ~ 800 ° C).
Scope of application
Household items (1, 2 types of 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, etc.
304L stainless steel (L is low carbon)
As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 under normal conditions. However, after welding or after stress relief, it has excellent resistance to intergranular corrosion. Good corrosion resistance can also be maintained without heat treatment. The use temperature is -196 ° C ~ 800 ° C.
Scope of application
It is used in field open-air machines in the chemical, coal, and petroleum industries where high intergranular corrosion resistance is required. Heat-resistant parts for building materials. Parts that are difficult to heat treat.
316 stainless steel
316 stainless steel is particularly good in corrosion resistance, atmospheric corrosion resistance and high temperature strength due to the addition of molybdenum. Can be used under harsh conditions. Excellent work hardenability (non-magnetic).
Scope of application
Equipment for the use of equipment, chemicals, dyes, paper, oxalic acid, fertilizers, etc. in seawater. Photo, food industry, coastal facilities, ropes, CD poles, bolts, nuts.
316L stainless steel (L is low carbon)
As a low carbon series of 316 steel, it has excellent resistance to intergranular corrosion in addition to the same characteristics as 316 steel.
Scope of application
A product that has special requirements for resistance to grain boundary corrosion.
Performance comparison chemical composition
316 and 316L stainless steel are molybdenum-containing stainless steels. The molybdenum content of 316L stainless steel is slightly higher than that of 316 stainless steel. Due to the molybdenum in steel, the total performance of this steel is better than that of 310 and 304 stainless steel. Under high temperature conditions, 316 stainless steel has a wide range of uses when the concentration of sulfuric acid is below 15% and above 85%. 316 stainless steel also has good chloride attack properties. So it is usually used in the marine environment. The maximum carbon content of 316L stainless steel is 0.03. Can be used in applications where annealing is not possible after soldering and where maximum corrosion resistance is required.
316 stainless steel has better corrosion resistance than 304 stainless steel. Good corrosion resistance in the production of pulp and paper. Moreover, 316 stainless steel is also resistant to erosion by marine and aggressive industrial atmospheres.
In general, 304 stainless steel and 316 stainless steel have little difference in chemical resistance. However, there are differences in certain media.
The stainless steel originally developed was 304. In certain cases, this material is sensitive to pitting corrosion (PittingCorrosion). An extra 2-3% increase in molybdenum can reduce this sensitivity. This gave birth to 316. Also, these additional molybdenum can also reduce the corrosion of certain hot organic acids.
316 stainless steel has almost become the standard material for the food and beverage industry. Due to the shortage of molybdenum in the world and the nickel content in 316 stainless steel. The price of 316 stainless steel is more expensive than 304 stainless steel.
Pitting corrosion is a phenomenon mainly caused by deposition corrosion of stainless steel surfaces. This is because oxygen deficiency does not form a chromium oxide protective layer.
Especially in small valves, the possibility of deposits on the valve plate is small. Thus, pitting corrosion is also rare.
Among various types of water media (distilled water, drinking water, river water, boiler water, sea water, etc.), 304 stainless steel and 316 stainless steel have almost the same corrosion resistance. Unless the chloride ion content of the medium is very high. At this time, 316 stainless steel is more suitable.
In most cases, the corrosion resistance of 304 stainless steel and 316 stainless steel is not much different. But in some cases it can also vary a lot. Specific analysis is required. In general, valve users should be aware of it. Because they choose the material of the container and the pipe according to the condition of the medium. It is not recommended to recommend materials to users.
316 stainless steel has good oxidation resistance in intermittent use below 1600 °C and continuous use below 1700 °C. In the range of 800-1575 degrees, it is best not to continuously apply 316 stainless steel. But, when 316 stainless steel is continuously used outside this temperature range, the stainless steel has good heat resistance. 316L stainless steel has better carbide precipitation resistance than 316 stainless steel. It can be used in the above temperature range.
Annealing is carried out at a temperature ranging from 1850 to 2050 degrees, followed by rapid annealing and rapid cooling. 316 stainless steel cannot be hardened by heat treatment.
316 stainless steel has good welding properties. All standard welding methods can be used for welding. When welding, 316Cb, 316L or 309Cb stainless steel filler rods or welding rods can be used for welding according to the application. For best corrosion resistance, the welded section of 316 stainless steel requires post-weld annealing. If 316L stainless steel is used, post-weld annealing is not required.
Among all steels, austenitic stainless steel has the lowest yield point. So, from the viewpoint of mechanical properties, austenitic stainless steel is not the best material for the valve stem. Because of the certain strength, the diameter of the stem will increase. The yield point cannot be increased by heat treatment, but can be improved by cold forming.
Due to the wide application of austenitic stainless steel, it gives people the wrong impression that all stainless steels are not magnetic. For austenitic stainless steels, it can be basically understood as non-magnetic, as is the case with quenched forged steel. However, the 304 processed by cold forming will be somewhat magnetic. For cast steel, if it is 100% austenitic stainless steel, it is not 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 temperature between 450 ° C and 900 ° C, the structure of the material changes and chromium carbide forms along the edges of the crystal. Thus, a chromium oxide protective layer cannot be formed at the edge of the crystal, resulting in a decrease in corrosion resistance. This type of corrosion is called “intergranular corrosion.”
304L stainless steel and 316L stainless steel were developed to combat this corrosion. Both 304L stainless steel and 316L stainless steel have lower carbon content. Since the carbon content is reduced, chromium carbide is not produced and intergranular corrosion is not generated.
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 high chlorine environments.
Please note: This phenomenon is due to high temperatures (450 ° C – 900 ° C). Usually soldering is the direct cause of reaching this temperature. For soft seat conventional butterfly valves, the use of low carbon stainless steel does not make much sense since we do not weld on the valve plate. However, most specifications will require 304L stainless steel or 316L stainless steel.
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