What are the main characteristics of stainless steel ?
Stainless steel refers to steel that is resistant to corrosion by weakly corrosive media such as air, steam, and water, and chemically corrosive media such as acid, alkali, and salt. It is also called stainless acid-resistant steel. In practical applications, steel that is resistant to weakly corrosive media is often referred to as stainless steel, and steel that is resistant to chemical media is referred to as acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to corrosion by chemical media, while the latter are generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.
The main characteristics of stainless steel:
Different products have different requirements for welding performance. A type of tableware generally does not require welding performance, and even includes some pot enterprises. But most products need good welding performance of raw materials, such as second-class tableware, insulation cups, steel pipes, water heaters, water dispensers, etc.
Most stainless steel products require good corrosion resistance, such as Class I or II tableware, kitchenware, water heaters, water dispensers, etc. Some foreign merchants also perform corrosion resistance tests on the products: use NACL solution to heat to boiling, and then pour Remove the solution, wash and dry, and weigh the weight loss to determine the degree of corrosion (Note: When the product is polished, the surface of the test will cause rust spots due to the Fe component in the abrasive cloth or sandpaper).
In today's society, stainless steel products are generally polished during production. Only a few products, such as water heaters and water dispenser liners, do not need polishing. Therefore, it is required that the polishing performance of the raw material is good. The factors that affect the polishing performance are the following:
① Surface defects of raw materials. Such as scratches, pitting, over pickling and so on.
② the problem of raw material. If the hardness is too low, it will not be easy to be polished when polishing (BQ property is not good), and the hardness is too low, and the orange peel phenomenon will appear on the surface during deep drawing, which will affect the BQ property. BQ with high hardness is relatively good.
③ After deep drawing, small black spots and RIDGING will appear on the surface of the area with a large amount of deformation, which will affect the BQ performance.
Heat resistance means that stainless steel can still maintain its excellent physical and mechanical properties at high temperatures.
Effect of carbon: Carbon is strongly formed and stable in austenitic stainless steel. Element that sets austenite and enlarges the austenite region. The ability of carbon to form austenite is about 30 times that of nickel. Carbon is a gap element. The strength of austenitic stainless steel can be significantly improved by solid solution strengthening. Carbon can also improve the stress and corrosion resistance of austenitic stainless steel in highly concentrated chlorides (such as 42% MgCl2 boiling solution).
However, in austenitic stainless steel, carbon is often regarded as a harmful element. This is mainly due to the conditions under which the stainless steel is used for corrosion resistance (such as welding or heating at 450 ~ 850 ° C). Chromium forms a high chromium Cr23C6 type carbon compound, which results in the depletion of local chromium, which reduces the corrosion resistance of the steel, especially the resistance to intergranular corrosion. therefore. Most of the newly developed chromium-nickel austenitic stainless steels since the 1960s have ultra-low carbon types with a carbon content of less than 0.03% or 0.02%. It can be known that as the carbon content decreases, the sensitivity of intergranular corrosion of steel decreases. When the carbon content is lower than 0.02% has the most obvious effect, and some experiments have also pointed out that carbon will also increase the pitting corrosion tendency of chromium austenitic stainless steel. Due to the harmful effects of carbon, not only the carbon content should be controlled as low as possible during the austenitic stainless steel smelting process, but also the surface of the stainless steel should be prevented from increasing carbon during the subsequent hot, cold working and heat treatment, and chromium carbides should be avoided. Precipitation.
When the amount of chromium atoms in the steel is not less than 12.5%, the electrode potential of the steel can be abruptly changed from a negative potential to a positive electrode potential. Prevents electrochemical corrosion.