What are the differences among stainless steel valve 304, 304L, 316 and 316L on the motorized valve（part 2）?

What are the differences among stainless steel valve 304, 304L, 316 and 316L on the motorized valve（part 2）?

Heat resistance of 316L and 316 stainless steel

316 stainless steel has good oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees. It is best not to use 316 stainless steel continuously in the range of 800-1575 degrees, but it has good heat resistance when used continuously outside this temperature range. 316L stainless steel has better resistance to carbide precipitation than 316 stainless steel and can be used in the above temperature range.

Heat treatment of stainless steel

Annealing is carried out in the temperature range of 1850-2050 degrees, followed by rapid annealing and rapid cooling. 316 stainless steel cannot be hardened by overheating.

Stainless steel welding

316 stainless steel has good welding properties. It can be welded by all standard welding methods. When welding, 316Cb, 316L or 309Cb stainless steel filler rods or electrodes can be used according to the application. 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, no welding annealing is required.

Mechanical properties of stainless steel

Among 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 of stainless steel

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 on the edge of the crystal, resulting in reduced corrosion resistance. This corrosion is called "intergranular corrosion".

As a result, 304L stainless steel and 316L stainless steel were developed to combat this corrosion. 304L stainless steel and 316L stainless steel have a low 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 a higher sensitivity to intergranular corrosion does not mean that non-low-carbon materials are more susceptible to corrosion. This sensitivity is also higher in a high-chlorine environment.

Please note that this phenomenon originates from high temperatures (450℃-900℃). Usually welding is the direct cause of reaching this temperature. For soft seat conventional butterfly valves, since we do not perform welding operations 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 rust?

Why does stainless steel also rust? When brown rust spots (dots) appear on the surface of stainless steel pipes, people are surprised: they think "stainless steel does not rust, and if it rusts, it is not stainless steel, and there may be something wrong with the steel." In fact, this is a one-sided and wrong view of stainless steel. Stainless steel will also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation, that is, stainlessness, and also has the ability to resist corrosion in media containing acids, alkalis, and salts, that is, corrosion resistance. However, the size of its corrosion resistance varies with the chemical composition of the steel itself, the protection state, the conditions of use, and the type of environmental media. For example, 304 steel pipes have absolutely excellent corrosion resistance in dry and clean atmosphere, but if they are moved to coastal areas, they will soon rust in sea fog containing a lot of salt; while 316 steel pipes perform well. Therefore, not all kinds of stainless steel can resist corrosion and rust in any environment.

Tianjin Tianfei Haitai Valve Co., Ltd.

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