Stainless steel is the abbreviation of stainless and acid resistant steel, which is called stainless steel for its resistance to weak corrosive media such as air, steam, water or stainless steel, and acid resistant steel for its resistance to chemical corrosive media such as acid, alkali, salt, etc.

Due to the difference in chemical composition, their corrosion resistance is different. Ordinary stainless steel is generally not resistant to chemical medium corrosion, while acid resistant steel is generally stainless. The term "stainless steel" not only refers to one kind of stainless steel, but also represents more than one hundred kinds of industrial stainless steel. Each kind of stainless steel developed has good performance in its specific application field. The key to success is first to understand the purpose, and then to determine the right steel grade. There are usually only six kinds of steel related to the application field of building construction. They all contain 17-22% chromium, and the better steel also contains nickel. The addition of molybdenum can further improve the atmospheric corrosiveness, especially the resistance to chloride atmosphere.

Stainless steel refers to the steel resistant to weak corrosive media such as air, steam, water and chemical corrosive media such as acid, alkali and salt, also known as stainless acid resistant steel. In practical application, the steel resistant to weak corrosion medium is often called stainless steel, while the steel resistant to chemical medium is called acid resistant steel. Due to the difference in chemical composition, the former is not necessarily resistant to chemical medium corrosion, while the latter is generally rust free. The corrosion resistance of stainless steel depends on the alloy elements contained in the steel.

Main types

Stainless steel is usually divided into martensitic steel, ferritic steel, austenitic steel, AUSTENITIC FERRITIC (duplex) stainless steel and precipitation hardening stainless steel according to its microstructure. In addition, it can be divided into: chromium stainless steel, chromium nickel stainless steel and chromium manganese nitrogen stainless steel. There are also special stainless steel plates and strips for pressure vessels (gb24511-2009).

Ferritic stainless steel

It contains 15% - 30% chromium. The corrosion resistance, toughness and weldability of Crl7, cr17mo2ti, Cr25, cr25mo3ti, Cr28 are better than other kinds of stainless steel. Ferritic stainless steel has high chromium content, good corrosion resistance and oxidation resistance, but poor mechanical and technological properties. It is mainly used for acid resistant structures with little stress and as oxidation resistant steel. This kind of steel can resist the corrosion of atmosphere, nitric acid and salt water solution, and has the characteristics of good high temperature oxidation resistance and small thermal expansion coefficient. It can be used in nitric acid and food factory equipment, and can also be made into parts working at high temperature, such as gas turbine parts.

austenitic stainless steel 

It contains more than 18% chromium, about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and can resist corrosion of various media. The common brands of austenitic stainless steel are 1Cr18Ni9, 0Cr19Ni9, etc. WC of 0Cr19Ni9 steel is less than 0.08%, marked as "0" in steel grade. This kind of steel contains a lot of Ni and Cr, which makes the steel austenite at room temperature. This kind of steel has good plasticity, toughness, weldability, corrosion resistance and non-magnetic or weak magnetic properties. It has good corrosion resistance in oxidizing and reducing media. It is used to make acid resistant equipment, such as corrosion-resistant containers and equipment linings, transmission pipelines, nitric acid resistant equipment parts, etc. in addition, it can also be used as the main material of stainless steel clock ornaments. Austenitic stainless steel is generally treated by solution treatment, that is, the steel is heated to 1050-1150 ℃, and then cooled by water or air to obtain single-phase austenite structure.

AUSTENITIC FERRITIC duplex stainless steel

It has both the advantages of austenitic and ferritic stainless steel and superplasticity. Stainless steel with about half of austenite and half of ferrite. In the case of low carbon content, chromium (CR) content is 18% ~ 28%, nickel (Ni) content is 3% ~ 10%. Some steels also contain Mo, Cu, Si, Nb, Ti, N and other alloy elements. This kind of steel has the characteristics of austenite and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, and maintained the 475 ℃ brittleness, high thermal conductivity and Superplasticity of ferritic stainless steel. Compared with austenitic stainless steel, it has higher strength, intergranular corrosion resistance and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel saving stainless steel.

Precipitation hardening stainless steel

The matrix is austenite or martensite structure, and the common grades of precipitation hardening stainless steel are 04cr13ni8mo2al, etc. Stainless steel that can be hardened (strengthened) by precipitation hardening (also known as age hardening).

Martensitic stainless steel

High strength, but poor plasticity and weldability. The commonly used grades of martensitic stainless steel are 1Cr13, 3Cr13, etc. because of its high carbon content, it has high strength, hardness and wear resistance, but its corrosion resistance is slightly poor. It is used for some parts with high mechanical property requirements and general corrosion resistance requirements, such as spring, turbine blade, hydraulic compressor valve, etc. This kind of steel is used after quenching and tempering. Annealing is required after forging and stamping.

Stainless steel plate and strip for pressure equipment

Special stainless steel for pressure vessel, with clear requirements for its classification, code, size, shape and allowable deviation, technical requirements, test methods, inspection rules, packaging, mark and product quality certificate. The commonly used brands are 06cr19ni10, 022cr17ni12mo2, and the numerical codes are s30408, s31603, etc. Mainly used in food machinery, pharmaceutical machinery and other sanitary equipment.

Main characteristics

Weldability

The requirements for welding performance vary with the application of products. A class of tableware generally does not require welding performance, even including some pot enterprises. But most products need good welding performance of raw materials, such as class II tableware, heat preservation cup, steel pipe, water heater, water dispenser, etc.

Corrosion resistance

Most stainless steel products require good corrosion resistance, such as class I and II tableware, kitchen utensils, water heaters, drinking fountains, etc. some foreign businessmen also conduct corrosion resistance tests on the products: use NaCl water solution to warm to boiling, after a period of time, pour out the solution, wash and dry, weigh the weight loss, to determine the degree of corrosion (Note: when the product is polished, because the abrasive cloth or sandpaper contains The composition of Fe will cause rust spots on the surface during the test)

Polishing performance

In today's society, stainless steel products are generally polished in the production process. Only a few products, such as water heaters and drinking water dispensers, do not need polishing. Therefore, the polishing performance of raw materials is very good. The main factors affecting the polishing performance are as follows:

① surface defects of raw materials. Such as scratch, pitting, pickling, etc.

② material of raw materials. If the hardness is too low, it is not easy to polish (BQ is not good), and the hardness is too low, and orange peel is easy to appear on the surface during deep drawing, thus affecting BQ. The hardness of BQ is relatively good.

③ after deep drawing, small black spots and ridging will appear on the surface of the area with large deformation, thus affecting the BQ property.

Heat resistance

Heat resistance refers to the fact that stainless steel can still maintain its excellent physical and mechanical properties at high temperature.

Effect of carbon: carbon is strongly formed and stable in austenitic stainless steel. Elements that define austenite and enlarge austenite zone. The ability of carbon to form austenite is about 30 times that of nickel, and carbon is a kind of interstitial element. The strength of austenitic stainless steel can be significantly improved by solution strengthening. Carbon can also improve the stress and corrosion resistance of austenitic stainless steel in high concentration chloride (such as 42% MgCl2 boiling solution).

However, in austenitic stainless steel, carbon is often regarded as a harmful element, which is mainly due to the fact that under some conditions (such as welding or heating at 450-850 ℃) in stainless steel's corrosion-resistant use, carbon can form Cr23C6 type carbon compound with high chromium with chromium in steel, resulting in local chromium depletion, and the corrosion resistance of steel, especially the intergranular corrosion resistance, decreases. Therefore. Since the 1960s, most of the newly developed Cr Ni austenitic stainless steels are of ultra-low carbon type with carbon content less than 0.03% or 0.02%. It can be known that with the decrease of carbon content, the sensitivity of intergranular corrosion of steel decreases. When the carbon content is less than 0.02%, it has the most obvious effect. Some experiments also pointed out that carbon will increase the pitting corrosion tendency of Cr Ni austenitic stainless steel. Due to the harmful effect of carbon, not only should the carbon content be controlled as low as possible in the smelting process of austenitic stainless steel, but also in the subsequent heat, cold working and heat treatment process to prevent the surface of stainless steel from carburizing and chromium carbide precipitation.

Corrosion resistance

When the amount of chromium atoms in steel is not less than 12.5%, the electrode potential of steel will change suddenly from negative potential to positive potential. Prevent electrochemical corrosion.

Structural components

The corrosion resistance of stainless steel decreases with the increase of carbon content. Therefore, the carbon content of most stainless steels is low, not more than 1.2% at most, and the ω C (carbon content) of some steels is even lower than 0.03% (such as 00Cr12). The main alloy element in stainless steel is Cr (chromium). Only when Cr content reaches a certain value, the steel has corrosion resistance. Therefore, the general Cr (CR) content of stainless steel is at least 10.5%. Stainless steel also contains Ni, Ti, Mn, N, Nb, Mo, Si, Cu and other elements.

Making materials

Due to the different uses of each product, its processing technology and quality requirements of raw materials are also different. Generally speaking, the thickness tolerance of raw materials is different for different stainless steel products, such as class II tableware and heat preservation cup. The thickness tolerance is generally high, which is - 3 ~ 5%. The thickness tolerance of class I tableware is generally - 5%, steel pipe is generally - 10%. The thickness tolerance of materials for hotel freezer is - 8%. The thickness tolerance of dealers is generally - 4% ~ 6%. At the same time, the difference between the internal and external sales of products will also lead to different requirements of customers on the thickness tolerance of raw materials. In general, the thickness tolerance requirements of export customers are relatively high, while that of domestic enterprises is relatively low (mostly due to cost considerations), and some customers even require - 15%.

(1) DDQ (deep drawing quality) material: refers to the material used for deep drawing (punching), that is, the soft material, which is mainly characterized by high elongation (≥ 53%), low hardness (≥ 170%), internal grain grade between 7.0 ~ 8.0, and excellent deep drawing performance. The processing ratio (blanking size / product diameter) of many enterprises producing thermos bottles and pots is generally high, and their processing ratios are respectively 3.0, 1.96, 2.13 and 1.98. Sus304ddq material is mainly used for these products with high processing ratio. Of course, products with processing ratio more than 2.0 generally need to be stretched several times to complete. If the extension of raw materials can not be achieved, the products are easy to crack and pull through when processing deep drawn products, which affects the qualified rate of finished products, and of course increases the cost of manufacturers;

② general material: it is mainly used for materials other than DDQ. This kind of material is characterized by relatively low elongation (≥ 45%), relatively high hardness (≥ 180hb), and internal grain size grade between 8.0 and 9.0. Compared with DDQ material, its deep drawing performance is relatively poor. It is mainly used for products that can be obtained without stretching, such as spoons, spoons, forks, electrical appliances, etc Steel pipe use, etc. But compared with DDQ material, it has an advantage that BQ is relatively good, which is mainly due to its slightly higher hardness.