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Stainless steel valves are usually used in the working medium has a certain corrosive occasions, the use of steel is usually 304,304 L, 316, 316L, this article will introduce you to this small series of four different stainless steel in the hope of the future Work helpful!

304 stainless steel:

304 stainless steel is the most common steel, as a widely used steel, has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; stamping, bending and other hot workability, no heat treatment hardening (non-magnetic , It will use temperature -196 ℃ ~ 800 ℃).

Scope of application:

Household products (1,2 category tableware, cabinets, indoor pipelines, water heaters, boilers, bath)
Auto parts (windshield wipers, muffler, molded products)
Medical equipment, building materials, chemical, food industry, agriculture, ship components

304L stainless steel (L is low carbon):

As a low-carbon 304 steel, in general, its corrosion resistance and 304 just the same, but after welding or stress elimination, its excellent resistance to grain boundary corrosion; in the absence of heat treatment, can also maintain a good Of the corrosion resistance, the use of temperature -196 ℃ ~ 800 ℃.

Scope of application:

Applied to the field of corrosion resistance to grain boundary corrosion chemical, coal, oil field outdoor machinery, building materials, heat-resistant parts and heat treatment of difficult parts.

316 stainless steel:

316 stainless steel by adding molybdenum, so its corrosion resistance, atmospheric corrosion resistance and high temperature strength is particularly good, can be used under harsh conditions; excellent work hardening (non-magnetic).

Scope of application:

Machinery, equipment, chemicals, dyes, paper, oxalic acid, fertilizers and other production equipment; photography, food industry, coastal facilities, rope, CD rods, bolts, nuts.

316L stainless steel (L is low carbon):

As a low carbon steel 316 series, in addition to the same characteristics and 316 steel, the anti-grain boundary corrosion is excellent.

Scope of application:

There are special requirements for grain boundary corrosion resistance.

Extended reading:

316 and 316L stainless steels are molybdenum-containing stainless steels. 316L stainless steel in the molybdenum content slightly higher than the 316 stainless steel due to the molybdenum steel, the total performance of the steel is better than 310 and 304 stainless steel, high temperature conditions, when the concentration of sulfuric acid below 15% and 85% Stainless steel has a wide range of uses. 316 stainless steel also has a good performance of chloride erosion, it is usually used in marine environments. 316L stainless steel has a maximum carbon content of 0.03, can be used for welding can not be annealed and the need for maximum corrosion resistance applications.

Chemical composition

Corrosion resistance:

316 stainless steel corrosion resistance is better than 304 stainless steel, pulp and paper in the production process has a good corrosion resistance. And 316 stainless steel is also resistant to erosion of the marine and erosive industrial atmosphere.

In general, 304 stainless steel and 316 stainless steel in the chemical resistance of the difference is not, but in some specific media under the difference.

Initially developed for the 304 stainless steel, under certain circumstances, this material on the point of corrosion (Pitting Corrosion) is more sensitive. An additional 2-3% molybdenum can reduce this sensitivity, so the birth of the 316. In addition, these additional molybdenum can also reduce the corrosion of certain hot organic acids.

316 stainless steel food and beverage industry has become almost standard material. Because of the worldwide shortage of molybdenum and 316 stainless steel in the nickel content more, 316 stainless steel prices are more expensive than 304 stainless steel.

Pitting corrosion is a phenomenon mainly caused by the deposition of stainless steel surface corrosion, which is due to lack of oxygen and can not form a protective layer of chromium oxide.

Especially in small valves, the possibility of deposition on the valve plate is very small, so pitting corrosion is also rare.
In all types of water medium (distilled water, drinking water, river water, boiler water, sea water, etc.), 304 stainless steel and 316 stainless steel corrosion resistance is almost the same, unless the medium chloride ion content is very high, then 316 stainless steel more suitable.

In most cases, 304 stainless steel and 316 stainless steel corrosion resistance is not much different, but in some cases may also vary widely, the need for specific analysis of specific circumstances. Generally speaking, the valve users should be aware of because they will be based on the media to select the container and pipe material, we do not recommend to the user recommended materials.

Heat resistance:

In 1600 degrees below the intermittent use and in 1700 degrees of continuous use, 316 stainless steel has good oxidation resistance. In the range of 800-1575 degrees, it is best not to continue the role of 316 stainless steel, but the temperature outside the continuous use of 316 stainless steel, the stainless steel has good heat resistance. 316L stainless steel carbide precipitation resistance of 316 stainless steel better than the performance of the above temperature range.

Heat treatment:

Annealing is performed at a temperature in the range of 1850 to 2050 ° C, followed by rapid annealing and then rapid cooling. 316 stainless steel can not be overheated for hardening.

welding:

316 stainless steel has a good welding performance. All standard welding methods can be used for welding. Welding can be based on use, respectively 316Cb, 316L or 309Cb stainless steel filler rod or welding rod. For optimum corrosion resistance, welded sections of 316 stainless steel are subjected to post-weld annealing. If 316L stainless steel is used, no post-weld annealing is required.

Mechanical behavior:

In all steels, austenitic stainless steels have the lowest yield point. Therefore, from the mechanical properties, austenitic stainless steel is not used in the best stem material, because to ensure a certain strength, stem diameter will increase. The yield point can not be improved by heat treatment, but can be improved by cold forming.

magnetic:

Because of the extensive use of austenitic stainless steel, to the people caused by all the stainless steel is not the wrong impression of magnetic. For austenitic stainless steel, the basic can be understood as non-magnetic, quenched forged steel is indeed the case. But the 304 formed by the cold forming process is somewhat magnetic. For cast steels, 100% austenitic stainless steels are not magnetic.

Low carbon type stainless steel:

The corrosion resistance of austenitic stainless steel comes from the chromium oxide coating formed on the metal surface. If the material is heated to 450 ℃ to 900 ℃ high temperature, the structure of the material will change, along the crystal edge of the formation of chromium carbide. As a result, the chromium oxide protective layer can not be formed at the crystal edge, resulting in a reduction in the corrosion resistance. This corrosion is called "intergranular corrosion".

Which developed 304L stainless steel and 316L stainless steel to combat this corrosion. 304L stainless steel and 316L stainless steel carbon content is low, because the carbon content is reduced, so it will not produce chromium carbide, it will not produce intergranular corrosion.

It should be noted that a high susceptibility to intergranular corrosion does not imply that non-low carbon materials are more susceptible to corrosion. In the high-chlorine environment, this sensitivity is also higher.

Please note that this phenomenon is due to high temperature (450 ℃ -900 ℃). Welding is usually the direct cause of this temperature. For the soft seat conventional butterfly valve, the use of low carbon stainless steel does not make much sense, as most of the specifications will require 304L stainless steel or 316L stainless steel, since we do not weld on the valve plate.