characteristics of stainless steel
Stainless steel has the characteristics of being stainless and corrosion-resistant, with a chromium content of at least 10.5% and a carbon content of no more than 1.2%.
Commonly used steel grades are 304, 304L, 316, and 316L.
304 stainless steel is a common steel grade with excellent corrosion resistance and heat resistance, as well as good low-temperature strength and mechanical properties.
(Mechanical properties refer to the ability of a material to resist deformation and damage when subjected to external forces. It includes strength, hardness, toughness, wear resistance, fatigue characteristics, creep characteristics, etc.)
In addition, it has good thermal processing properties, such as stamping and bending, and does not harden during processing.
(Thermal workability refers to the ability of metal materials to deform under thermoplastic conditions. It includes casting, forging, welding and other processes.)
Another feature is that after heat treatment, it remains non-magnetic and can be used in temperatures ranging from -196°C to 800°C.
304L steel is a low carbon steel, and its corrosion resistance is similar to 304 steel under normal conditions. After welding or stress relief treatment, its resistance to grain boundary corrosion is excellent.
It maintains good corrosion resistance even without heat treatment. The service temperature range of this steel grade is from -196°C to 800°C.
304L steel contains a lower carbon content, which helps reduce the precipitation of carbides at grain boundaries, thereby reducing the possibility of intergranular corrosion.
Because it contains high levels of chromium, molybdenum and other elements, it can form a dense oxide film to isolate the material from the environment, thereby slowing down or avoiding the occurrence of intergranular corrosion.
304L steel has good grain boundary stability.
It contains high nitrogen elements, which can effectively suppress the precipitation of grain boundaries and prevent the occurrence of intergranular corrosion.
304 steel also has good heat treatment capabilities, and the heat treatment process can be controlled to make it have good grain boundary stability and resistance to intergranular corrosion.
316 stainless steel is a special type of steel. Due to the addition of molybdenum, its corrosion resistance, atmospheric corrosion resistance and high temperature strength have been significantly improved.
It not only has excellent corrosion resistance, but also has good work hardening properties, which means that it can be processed to obtain higher hardness and better mechanical properties. And 316 stainless steel is not magnetic.
In addition to having the same characteristics as 316 steel, the 316 low carbon series has excellent resistance to grain boundary corrosion.
316 and 316L are both molybdenum-containing stainless steels, and the molybdenum content in 316L is slightly higher.
Due to the molybdenum in the steel, the performance of this steel is better than that of 310 and 304. It is widely used in environments with sulfuric acid concentration of 15% to 85% at high temperatures.
316 is also resistant to chloride corrosion and is often used in marine environments.
The maximum carbon content of 316L is 0.03, which is suitable for applications that cannot be annealed after welding and are corrosion-resistant.
The corrosion resistance of austenitic stainless steel comes from the protective layer of chromium oxide formed on the surface. However, at high temperatures of 450-900°C, the material structure changes and chromium carbide is formed. As a result, the chromium oxide protective layer cannot be formed and the corrosion resistance is reduced, which is called “intergranular corrosion”. (Austenitic stainless steel includes 304, 316, 321 and other stainless steels)
To combat this corrosion, 304L and 316L stainless steels were developed. They have lower carbon content and are less likely to form chromium carbide to avoid intergranular corrosion. However, high chlorine environments can also increase susceptibility to intergranular corrosion.
316 reduces pitting corrosion susceptibility by increasing the molybdenum content and reduces corrosion by certain hot organic acids. (Pitting corrosion is mainly caused by deposition on the surface of stainless steel. In various water media, unless the chloride ion content is high, the corrosion resistance of the two stainless steels is similar.)
316 has become a standard material in the food and beverage industry, but it is more expensive. Except for specific usage conditions, the performance of 304 and 316 is not much different.
The corrosion resistance of 304 stainless steel can be attributed to the presence of chromium (18-20%) and nickel (8-10.5%). These elements react with oxygen in the air or water to form a thin layer of oxide on the surface, which acts as a barrier against further oxidation or rust.
321 stainless steel is added with titanium element on the basis of 304 stainless steel, which can maintain good corrosion resistance and strength at high temperatures.
316 stainless steel is a stainless steel material with excellent heat resistance.
At high temperatures, 316 stainless steel can withstand oxidation, corrosion and high temperature strength. Its heat resistance is mainly due to the Mo element in the composition, which can improve the corrosion resistance and high temperature strength of the material.
316 stainless steel has good heat resistance in intermittent use below 871°C and continuous use above 927°C.
Several steel types have excellent heat resistance, but different steel types have different durability at different temperatures.
Oxidation resistance refers to the ability of the material to resist oxidative erosion under high temperature, high humidity and high oxygen environment.
In metal materials, when the metal is exposed to high temperature, high humidity, high oxygen environment, the metal surface oxidation reaction will occur, forming an oxide film, which will gradually thicken over time, resulting in a decline in the performance of metal materials.
Some metal materials have high oxidation resistance, such as 316L and 321 in stainless steel, because these metal materials contain high chromium, nickel and other elements.
Heat treatment methods for 304 stainless steel include annealing and quenching
Annealing will relieve internal stresses within the metal structure while increasing its ductility.
Quenching will form a hardened surface layer while maintaining a degree of flexibility within the material.
Heat treatment of 304L stainless steel includes annealing and solution treatment
Annealing will eliminate stress and structural defects in the material and improve its plasticity and toughness.
Solution treatment can improve the material’s processing properties and increase its strength and corrosion resistance.
Heat treatment of 316 stainless steel includes solid solution treatment and precipitation treatment
Solid solution treatment makes the grains more uniform and finer, thereby improving its strength and plasticity. Next is the precipitation treatment. During this process, some metal elements will precipitate from the solid solution to form new phases, further improving the performance of 316 stainless steel.
Heat treatment of 316L stainless steel includes annealing and solution treatment
Annealing will relieve stress in the 316L stainless steel material and improve its plasticity and workability. In addition, annealing can also improve the grain structure of 316L stainless steel, making it smaller and more uniform, thus improving the toughness and strength of the material.
During heat treatment, rapid cooling from 1500 to 800°F (816 to 427°C) avoids reprecipitation of chromium carbide and provides optimal corrosion resistance.
Solid solution treatment will increase the strength and hardness of the material and also improve the corrosion resistance of 316L stainless steel, making it more stable in corrosive media such as acids and alkalis.
304 and 304L stainless steel have better welding properties. 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 304 stainless steel needs to be annealed after welding.
316 stainless steel also has good welding properties and can be welded 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, post-weld annealing is not required.
Stainless steel is basically non-magnetic, and even if it is, it is not very strong.
Pickling is a metal surface treatment method, the main use of acid solution to remove metal surface pollutants and oxide layer, make the stainless steel surface more beautiful, improve its appearance quality, improve processing performance, improve corrosion resistance (reduce the adhesion of corrosive media on the surface) and other purposes.
A physical method, using wire drawing technology to form a variety of patterns, such as straight lines, threads, random lines, ripples, spiral lines, etc., to make the stainless steel appearance more smooth and delicate, while improving the wear resistance.
Mirror treatment includes physical and chemical methods.
Mechanical polishing (polishing with polishing discs)
Mechanical polishing relies on the grinding and rolling action of very fine polishing powder to remove a very thin layer of metal on the grinding surface of the sample.
Electrolytic polishing is a process that uses the principle of electrolysis to polish metal surfaces. Electropolishing can greatly improve the surface quality and aesthetics of metal by removing defects such as oxide layers, burrs, and scratches on the metal surface.
Using high-speed spraying technology, spray material is sprayed onto the outer layer of stainless steel to change its shape and appearance.
Change the color of stainless steel through coloring technology, increase color diversity, and improve wear resistance and corrosion resistance.
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