How to distinguish material between SUS304 and SUS316 ?
In our daily life, stainless steel is a very common product material, such as what stainless steel pots and pans are often used in daily life. The use of stainless steel is very wide range, to stainless steel small objects, to reach the building materials of stainless steel, but stainless steel is only a general term for a class of materials.
Usually the most common stainless steel varieties are stainless steel 304 and stainless steel 316, but what is the difference between the two?
Is there any way to tell them apart?
So what is an easy way to identify the types of stainless steel?
First, you can tell from the color. Stainless steel that has been washed in acid has a silvery white surface color, and it is very smooth. However, stainless steel that has not been washed in acid has a darker color, for different materials of stainless steel after the display of color is not quite the same.
Second, can use the magnet to identify stainless steel. Because stainless steel contains chromium, it can be attracted to magnets in any state; but stainless steel with high manganese content is non-magnetic; chromium-nickel-nitrogen stainless steel situation is slightly more complicated, a bit can be attracted to iron magnet, some do not, so different types of stainless steel is very easy to distinguish.
What is stainless steel?
Stainless steel is a kind of steel, steel refers to the amount of carbon (C) in 2% or less called steel, more than 2% is iron.
The addition of chromium (CR) , nickel (Ni) , manganese (MN) , silicon (Si) , titanium (Ti) , molybdenum (Mo) and other alloying elements during the smelting process improves the properties of the steel and makes the steel have the corrosion resistance (that is, does not rust) .
Normally we say 304,304 L, 316,316 L and how come, what is the difference of each other?
Why do stainless steels have different steel numbers?
In the smelting process of stainless steel, due to the addition of alloy elements of different varieties, different varieties of addition amount. Its characteristics are also different, in order to make a distinction is crowned with a different steel number, the following is a common decorative stainless steel with different steel”Alloy element” content table for reference only
304 stainless steel
304 stainless steel is the most popular type of steel, as a widely used steel, with good corrosion resistance, heat resistance, low-temperature strength and mechanical properties, stamping, bending and other thermal workability, no heat treatment hardening (no magnetism, then use temperature -196 ° C ~ 800 ° C) .
Scope of application
Household items (tableware, class 1 and 2, cabinets, indoor plumbing, water heaters, boilers, bathtubs)
Auto Parts (windshield wipers, silencers, molded parts)
Medical Appliances, building materials, chemistry, food industry, agriculture, ship parts.
304L stainless steel(L for low carbon)
Introduction to performance
As a low carbon steel, the corrosion resistance of 304 steel is similar to that of 304 steel in general condition, but the corrosion resistance of 304 steel is excellent after welding or stress elimination, it can also maintain good corrosion resistance at temperature of -196 ° C ~ 800 ° C.
Scope of application
Applied to chemical, coal, petroleum industry of high grain boundary corrosion requirements of open-air machines, building materials heat-resistant parts and parts of the difficult heat treatment.
316 stainless steel
316 stainless steel because of the addition of molybdenum, its corrosion resistance, atmospheric corrosion resistance and high temperature strength is particularly good, can be used in harsh conditions;.
Scope of application
Marine equipment, chemical, dye, paper, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal facilities, rope, CD rod, bolts, nuts.
316L stainless steel (L for low carbon)
Introduction to performance
As a low carbon series of 316 steel, it has excellent grain boundary corrosion resistance except that it has the same characteristics as 316 steel.
Scope of application
Products with special requirements for grain boundary corrosion resistance.
Comparison table of chemical composition
316 and 316L stainless steels are mo-containing stainless steels. The molybdenum content in 316L stainless steel is slightly higher than that in 316 stainless steel.
Due to the molybdenum in the steel, the overall properties of the steel are better than those of 310 and 304 stainless steels. At high temperature, 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 and is commonly used in marine environments. The maximum carbon content of 316L stainless steel is 0.03. It can be used in applications where it can not be annealed after welding and where maximum corrosion resistance is required.
Corrosion resistance
316 stainless steel is superior to 304 stainless steel in corrosion resistance, in the pulp and paper production process has good corrosion resistance. The 316 stainless steel is also resistant to marine and corrosive industrial atmospheres.
In general, 304 stainless steel and 316 stainless steel in the chemical corrosion resistance of little difference, but in some specific media are different.
The original stainless steel was 304, which is sensitive to Pitting Corrosion under certain conditions. An additional 2-3% increase in molybdenum reduces this sensitivity, resulting in 316. In addition, these additional molybdenum can also reduce the corrosion of certain thermal organic acids.
316 stainless steel has almost become the standard material of food and beverage industry. The price of 316 stainless steel is higher than that of 304 stainless steel due to a worldwide molybdenum shortage and higher nickel content in 316 stainless steel.
Pitting corrosion is a phenomenon caused mainly by corrosion deposited on the surface of stainless steel. It 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 occurs rarely.
In all types of water media (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 chloride ion content in the medium is very high, then 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 may be very different, the case-by-case analysis. Generally speaking, valve users should be aware of, because they will choose according to the media and pipeline material, do not recommend the material to the user
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 use 316 stainless steel continuously, but outside the temperature range of continuous use of 316 stainless steel, this 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.
Heat Treatment
Annealed in the temperature range of 1850-2050 °C, then annealed rapidly, then cooled rapidly. 316 stainless steel can not be superheated to harden.
Welding
316 stainless steel has good welding performance. All standard welding methods can be used for welding. According to the use of welding, 316CB, 316L or 309CB stainless steel filler bar or electrode for welding.
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
Comparison Table of mechanical properties
Of all steel, Austenitic stainless steel has the lowest yield point. Therefore, Austenitic stainless steel is not the best material for the stem in terms of mechanical properties, as the stem diameter is increased to ensure a certain amount of strength. The yield point can not be increased by heat treatment, but can be increased by cold forming.
Magnetism
The widespread use of Austenitic stainless steel has given the false impression that all stainless steels are non-magnetic.
In the case of Austenitic stainless steel, this basically means that it is non-magnetic, which is true of hardened wrought steel. But the 304 treated by cold forming will be somewhat magnetic. For cast steel, 100% Austenitic stainless steel is not magnetic.
Low carbon type stainless steel
The corrosion resistance of the Austenitic stainless steel comes from the chromium oxide coating that forms on the metal’s surface. If the material is heated to a high temperature of 450 ° C to 900 ° C, the structure of the material changes and chromium carbide is formed along the edge of the crystal.
In this way, chromium oxide protection layer can not be formed at the edge of the crystal, thus resulting in reduced corrosion resistance. This corrosion is called intergranular corrosion.
Thus 304L stainless steel and 316L stainless steel were developed to resist this corrosion. Both 304L and 316L stainless steels have low carbon content, and as the carbon content is reduced there is no chromium carbide and thus no intergranular corrosion.
It should be noted that a 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.
Note that this is due to high temperatures (450 ° C-900 ° C) . Usually welding is the direct cause of reaching this temperature. For soft-seated conventional butterfly valves, it does not make much sense to use low carbon stainless steel since we do not weld on the disc, but most specifications will require 304L or 316L stainless steel.
Why is the stainless steel rusted?
Why is stainless steel rusted? When the stainless steel pipe surface brown rust spots (spots) , people are surprised: that“Stainless steel is not rust, Rust is not stainless steel, may be a problem with steel.”.
In fact, this is a lack of understanding of stainless steel a one-sided view of the wrong. Stainless steel will rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation-that is, stainless, but also has in acid, alkali, salt medium is the ability of corrosion-that is, corrosion resistance.
However, the corrosion resistance of the steel varies with its chemical composition, protective state, service conditions and type of environmental media. Steel 304, for example, has excellent corrosion resistance in a dry, clean atmosphere, but moves it to a coastal area, where it rusts quickly in sea fog with a lot of salt, while steel 316 performs well.
Therefore, not any kind of stainless steel, in any environment can be corrosion-resistant, not ru