Stainless steel is a kind of steel. Steel refers to steel with less than 2% carbon (c), and iron with more than 2%. In the smelting process of steel, chromium (CR), nickel (Ni), manganese (MN), silicon (SI), titanium (TI), molybdenum (MO) and other alloy elements are added to improve the performance of steel and make the steel have corrosion resistance (i.e. no rust), which is what we often call stainless steel.
Stainless steel: resistant to weak corrosive media such as air, steam and water or stainless steel. The commonly used steel grades are 304, 304L, 316 and 316L, which are 300 series steels of austenitic stainless steel.
Today, we will mainly introduce the common performance, application scope and differences of 304, 304L, 316 and 316L.
304 Stainless Steel
Performance introduction: 304 stainless steel is the most common steel. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties. Stamping, bending and other hot workability are good, and there is no heat treatment hardening phenomenon (non-magnetic, the use temperature is -196 ℃ ~ 800 ℃).
Scope of application:
Household products (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)
Auto parts (windshield wiper, muffler, molded products)
Medical appliances, building materials, chemicals, food industry, agriculture, ship parts
304L Stainless Steel (L is low carbon)
Performance introduction: as a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel in general, but its resistance to grain boundary corrosion is excellent after welding or stress relief. It can also maintain good corrosion resistance without heat treatment, and the service temperature is -196 ℃ ~ 800 ℃.
Scope of application:
It is applied to outdoor machines in chemical, coal and petroleum industries with high requirements for grain boundary corrosion resistance, heat-resistant parts of building materials and parts with difficulties in heat treatment.
316 Stainless Steel
Due to the addition of molybdenum, 316 stainless steel has excellent corrosion resistance, atmospheric corrosion resistance and high temperature strength, and can be used under harsh conditions; Excellent work hardening (non-magnetic).
Scope of application:
Seawater equipment, 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 the same characteristics as 316 steel, its grain boundary corrosion resistance is excellent. Scope of application: products with special requirements for grain boundary corrosion resistance.
316 and 316L stainless steels 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 the steel, the overall performance of this steel is better than 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 applications. 316 stainless steel also has good chloride corrosion performance, so it is usually used in marine environment. The maximum carbon content of 316L stainless steel is 0.03, which can be used for applications that cannot be annealed after welding and require maximum corrosion resistance.
316 stainless steel has better corrosion resistance than 304 stainless steel. It has good corrosion resistance in the production of pulp and paper. Moreover, 316 stainless steel is also resistant to the corrosion of ocean and aggressive industrial atmosphere.
Generally speaking, 304 stainless steel and 316 stainless steel have little difference in chemical corrosion resistance, but they are different in some specific media.
The first developed stainless steel is 304, which is sensitive to pitting corrosion under certain circumstances. Adding an additional 2-3% molybdenum can reduce this sensitivity, which gives birth to 316. In addition, these additional molybdenum can also reduce the corrosion of some thermal organic acids.
316 stainless steel has almost become the standard material in the food and beverage industry. Due to the worldwide shortage of molybdenum and the more 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 on the surface of stainless steel, which is due to the lack of oxygen and the inability to form a chromium oxide protective layer.
Especially in small valves, the possibility of deposition on the valve plate is very small, so pitting corrosion rarely occurs.
In various types of water media (distilled water, drinking water, river water, boiler water, sea water, etc.), the corrosion resistance of 304 stainless steel is almost the same as that of 316 stainless steel, unless the content of chloride ion in the media is very high, 316 stainless steel is more suitable at this time.
In most cases, the corrosion resistance of 304 stainless steel and 316 stainless steel is not much different, but in some cases, it may also be very different, which requires specific analysis.
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 continuously act on 316 stainless steel, but when 316 stainless steel is continuously used outside this temperature range, the stainless steel has good heat resistance. The carbide precipitation resistance of 316L stainless steel is better than that of 316 stainless steel, and the above temperature range can be used.
Annealing in the temperature range of 1850-2050 degrees, then rapid annealing, and then rapid cooling. 316 stainless steel cannot be hardened by overheating.
316 stainless steel has good weldability. All standard welding methods can be used for welding. 316cb, 316L or 309cb stainless steel filler rods or electrodes can be used for welding according to the purpose. In order to obtain the best corrosion resistance, the welding section of 316 stainless steel needs to be annealed after welding. If 316L stainless steel is used, post weld annealing is not required.
Among all steels, austenitic stainless steel has the lowest yield point. Therefore, considering the mechanical properties, austenitic stainless steel is not the best material for the valve rod, because to ensure a certain strength, the diameter of the valve rod will increase. The yield point cannot be improved by heat treatment, but can be improved by cold forming. magnetic
Due to the wide application of austenitic stainless steel, it gives people the wrong impression that all stainless steel has no magnetism. For austenitic stainless steel, it can be basically understood as non-magnetic, which is true of quenched forged steel. However, 304 treated by cold forming will be somewhat magnetic. For cast steel, if it is 100% austenitic stainless steel, it has no magnetism. 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 be formed 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 the reduction of corrosion resistance. This corrosion is called “intergranular corrosion”.
304L stainless steel and 316L stainless steel were developed to resist this corrosion. 304L stainless steel and 316L stainless steel have 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 higher susceptibility to intergranular corrosion does not mean that non low carbon materials are more prone to corrosion. In high chlorine environment, this sensitivity is also higher.