According to the use environment and passing media, gate valves can be divided into acid-resistant, corrosion-resistant, high-temperature-resistant and ordinary valves. In this case, stainless steel gate valves are generally used in situations with corrosive media, such as chemical industry, petrochemical, petroleum, papermaking, mining, electric power, liquefied gas, food, pharmaceuticals, municipal water supply, mechanical equipment supporting, electronic industry, urban construction, etc.
What is the difference between stainless steel 304, 304L, 316, and 316L? Introduction to the performance and application scope of stainless steel valves.
What exactly are “steel” and “iron”, what are their characteristics, and what is their relationship? Where do the 304, 304L, 316, and 316L we usually call come from, and what are the differences between them?
Steel: A material with iron as the main element, carbon content generally below 2%, and other elements. ——GB/T 13304 “Steel Classification”
Iron: A metallic element with atomic number 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity.
Stainless steel: Steel that is resistant to weak corrosive media such as air, steam, water, etc. or is stainless. Commonly used steel types are 304, 304L, 316, and 316L, which are 300 series steels of austenitic 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 workability such as stamping and bending, and no heat treatment hardening phenomenon (non-magnetic). , operating temperature -196℃～800℃).
304 stainless steel application scope:
Household supplies (categories 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 is low carbon) performance introduction:
As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel under normal conditions. However, after welding or stress relief, its resistance to intergranular corrosion is excellent; it can also maintain good performance without heat treatment. Excellent corrosion resistance, operating temperature -196℃～800℃.
304L stainless steel application scope:
It is used in outdoor machines in the chemical, coal, and petroleum industries that require high resistance to grain boundary corrosion, as well as heat-resistant building material parts and parts that are difficult to heat treat.
316 stainless steel performance introduction:
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; it has excellent work hardening properties (non-magnetic).
316 stainless steel application scope:
Seawater equipment, chemicals, dyes, papermaking, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal area 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 grain boundary corrosion.
316L stainless steel application scope:
Products with special requirements for resistance to grain boundary corrosion.
316 and 316L stainless steel corrosion resistance:
The corrosion resistance of 316 stainless steel is better than that of 304 stainless steel, and it has good corrosion resistance in the pulp and paper production process. Moreover, 316 stainless steel is also resistant to corrosion from the ocean and corrosive industrial atmosphere.
Generally speaking, there is not much difference in chemical corrosion resistance between 304 stainless steel and 316 stainless steel, but there are differences in some specific media.
The stainless steel originally developed was 304. Under certain circumstances, this material is sensitive to pitting corrosion (Pitting Corrosion). Adding an additional 2-3% of molybdenum can reduce this sensitivity, and thus 316 was born. Additionally, this additional molybdenum can reduce corrosion by 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 greater 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 deposited 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 very small, so pitting corrosion is also rare.
In various types of water media (distilled water, drinking water, river water, boiler water, sea water, etc.), the corrosion resistance of 304 stainless steel and 316 stainless steel is almost the same, unless the content of chloride ions in the medium is very high, in which case 316 stainless steel more suitable.
316L and 316 stainless steel 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 continuously use 316 stainless steel, but when 316 stainless steel is used continuously outside this temperature range, the stainless steel has good heat resistance. The resistance to carbide precipitation of 316L stainless steel is better than that of 316 stainless steel, and it can be used in the above temperature range.
Stainless steel heat treatment:
Annealing is performed in the temperature range of 1850-2050 degrees, followed by rapid annealing and then rapid cooling. 316 stainless steel cannot be hardened by heat treatment.
Stainless steel welding:
316 stainless steel has good welding properties. Welding can be carried out using 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. In order 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, welding annealing is not required.
Stainless Steel Magnetic:
Due to the widespread use of austenitic stainless steel, people are given the wrong impression that all stainless steel is non-magnetic. For austenitic stainless steel, it can basically be understood as non-magnetic, and this is indeed the case for quenched forged steel. However, 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 types of stainless steel:
The corrosion resistance of austenitic stainless steel comes from the protective layer of chromium oxide formed on the metal surface. If the material is heated to a high temperature of 450°C to 900°C, the structure of the material will change and chromium carbide will form along the edge of the crystal. In this way, a protective layer of chromium oxide cannot be formed on the edge of the crystal, resulting in reduced corrosion resistance. This corrosion is called “intergranular corrosion”.
304L stainless steel and 316L stainless steel were developed to combat this corrosion. The carbon content of 304L stainless steel and 316L stainless steel is low. Because the carbon content is reduced, chromium carbide will not be produced, and intergranular corrosion will not occur.
It should be noted that higher susceptibility 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 caused by high temperatures (450℃-900℃). Usually welding is the direct cause of reaching this temperature. For soft-seat conventional butterfly valves, since we are not welding 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 also rust?
Why does stainless steel also rust? When brown rust spots (dots) appear on the surface of stainless steel pipes, people are surprised: “Stainless steel does not rust. If it gets rusty, it is not stainless steel anymore. There may be something wrong with the steel.” In fact, this is a one-sided wrong view due to lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation–that is, stainless steel–it also has the ability to corrode in media containing acid, alkali, and salt–that is, corrosion resistance. However, its corrosion resistance changes with the chemical composition of the steel itself, its protection status, usage conditions and environmental medium types. Like 304 Steel Pipe, in the atmosphere of dry cleansing, absolute good resistant to tarnishing ability is arranged, but if it is moved to riviera, in containing the sea fog of a large amount of salts, will soon get rusty; 316 Steel Pipe performs very well. good. Therefore, not any kind of stainless steel can resist corrosion and rust in any environment.
