1、 Physical properties of stainless steel

Compared with the physical properties of stainless steel and carbon steel, the density of carbon steel is slightly higher than that of ferritic and martensitic stainless steel, but slightly lower than that of austenitic stainless steel; The resistivity increases in the order of carbon steel, ferritic stainless steel, martensitic stainless steel and austenitic stainless steel; The order of linear expansion coefficient is similar, austenitic stainless steel is the highest and carbon steel is the lowest; Carbon steel, ferritic stainless steel and martensitic stainless steel are magnetic, while austenitic stainless steel is non-magnetic, but it will be magnetic when cold work hardening generates martensitic transformation. Heat treatment can be used to eliminate this martensitic structure and restore its non-magnetic. The

Austenitic stainless steel has the following characteristics compared with carbon steel:

1) high conductivity, about 5 times that of carbon steel. The

2) the large linear expansion coefficient is 40% larger than that of carbon steel, and the value of linear expansion coefficient increases with the increase of temperature. The

3) low thermal conductivity, about 1/3 of carbon steel. The

The

2、 Mechanical properties of stainless steel

Regardless of stainless steel plate or heat-resistant steel plate, austenitic steel plate has the best comprehensive performance, with sufficient strength, excellent plasticity and low hardness, which is one of the reasons why they are widely used. Austenitic stainless steel is similar to most other metal materials. Its tensile strength, yield strength and hardness increase with the decrease of temperature; The plasticity decreases with the decrease of temperature. Its tensile strength increases uniformly in the temperature range of 15~80 ° C. More importantly, with the decrease of temperature, the impact toughness decreases slowly, and there is no brittle transition temperature. Therefore, stainless steel can maintain enough plasticity and toughness at low temperature.



3、 Heat resistance of stainless steel

Heat resistance refers to the thermal stability that has both anti-oxidation or gas medium corrosion resistance under high temperature and sufficient strength at high temperature. The

1. 304l/022cr19ni10 stainless steel

As a Low-C 304 steel, in general, its corrosion resistance is similar to that of 304 steel, but its resistance to grain boundary corrosion is excellent after welding or stress relief; It can also maintain good corrosion resistance without heat treatment, and the service temperature is -196 ℃ ~800 ℃. It is applied to outdoor machines in the chemical, coal and petroleum industries with high requirements for grain boundary corrosion resistance, heat-resistant parts of building materials and parts with difficulties in heat treatment, household goods (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs), automobile accessories (windshield wipers, silencers, moulds), medical appliances, building materials, food industry, agriculture, and ship parts.

As a Low-C 304 steel, in general, its corrosion resistance is similar to that of 304 steel, but its resistance to grain boundary corrosion is excellent after welding or stress relief; It can also maintain good corrosion resistance without heat treatment, and the service temperature is -196 ℃ ~800 ℃. It is a kind of universal stainless steel, which is widely used to make equipment and parts requiring good comprehensive performance (corrosion resistance and formability).

2. 316l/ss316l/022cr17ni12mo2

316 and 317 stainless steels (see the following for the properties of 317 stainless steel) are molybdenum containing stainless steels. The molybdenum content of 317 stainless steel is slightly higher than that of 316 stainless steel Due to the molybdenum in the steel, the overall performance of this steel is better than that of 310 and 304 stainless steel. Under high temperature conditions, when the concentration of sulfuric acid is lower than 15% and higher than 85%, 316 stainless steel has a wide range of applications. 316 stainless steel also has good chloride corrosion performance, so it is usually used in marine environment. The

316l/ss316l/022cr17ni12mo2 low carbon

As a Low-C series of 316 steel, in addition to having the same characteristics as 316 steel, its grain boundary corrosion resistance is excellent.

316 steel has special requirements for grain boundary corrosion resistance. It contains molybdenum, so its resistance to pitting corrosion in marine and chemical industrial environments is much better than that of 304 stainless steel.

316L stainless steel has a maximum carbon content of 0.03, which can be used in applications where annealing cannot be performed after welding and where maximum corrosion resistance is required. The

Corrosion resistance: the corrosion resistance is better than 304 stainless steel. It has good corrosion resistance in the production of pulp and paper. Moreover, 316 stainless steel is also resistant to the corrosion of marine and corrosive industrial atmosphere. The

Heat resistance: 316 stainless steel has good oxidation resistance in interrupted use below 1600 ° C and continuous use below 1700 ° C: in the range of 800-1575 ° C, it is better not to continuously act on 316 stainless steel. However, when 316 stainless steel is continuously used outside this temperature range, the 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. The

Heat treatment: annealing within the temperature range of 1850-2050 ℃, then rapid annealing, and then rapid cooling. 316 stainless steel cannot be hardened by heat treatment. The

Welding: 316 stainless steel has good welding performance. All standard welding methods may be used for 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 post weld annealing treatment. If 316L stainless steel is used, post weld annealing is not required. The

Typical uses: equipment for pulp and paper making, heat exchanger, dyeing equipment, film processing equipment, pipeline, materials for external use of buildings in coastal areas.



4、 Stainless steel processing and construction

Drawing deep processing: it is easy to generate friction heat, so the stainless steel with high pressure resistance and heat resistance shall be used for simultaneous forming. After the completion of processing, the oil attached to the surface shall be removed. The

Welding: before welding, the rust, oil, moisture, paint, etc. harmful to welding shall be completely removed, and the welding rod suitable for the steel grade shall be selected. The time interval of spot welding is shorter than that of carbon steel. Stainless steel brush shall be used to remove welding slag. After welding, the surface shall be ground or cleaned to prevent local corrosion or strength reduction. The

Cutting and stamping: as stainless steel has higher strength than ordinary materials, higher pressure is required for stamping and shearing, and poor shear and work hardening can not occur only when the gap between the knives is correct. Plasma or laser cutting is best used. When gas cutting or arc cutting has to be used, grinding and necessary heat treatment shall be carried out for the heat affected zone. The

Bending processing: the thin plate can be bent to 180, but in order to reduce the crack size of the bending surface with the same radius, it is best to give a radius of 2 times the thickness of the plate when the thick plate is bending along the rolling direction, and a radius of 4 times the thickness of the plate when bending perpendicular to the rolling direction. Especially during welding, in order to prevent processing cracking, it is necessary to carry out surface grinding on the welding area. The

Construction and construction precautions

In order to prevent scratches and pollutants from adhering during construction, stainless steel construction shall be carried out under the film state. However, with the extension of time, the residue of the adhesive solution shall be cleaned according to the service life of the film. After the construction, the surface shall be washed when the film is removed, and special stainless steel tools shall be used. When cleaning public tools with general steel, it shall be cleaned to prevent iron filings from sticking. The

It should be noted that the highly corrosive magnetic and stone waste cleaning drugs should not be allowed to contact the stainless steel surface. If they are contacted, they should be washed immediately. After the construction, neutral detergent and water shall be used to wash the cement, powder ash and other substances attached to the surface.



5、 Storage and transportation of stainless steel

Storage: pay attention to moisture, dust, oil, lubricating oil, etc. during storage, as well as rust on the surface, or poor welding and reduced corrosion resistance. The

The corrosion rate is faster when water is soaked between the film and the steel substrate than when there is no film. The warehouse shall be kept in a clean, dry and ventilated place to maintain the original packaging state. The stainless steel coated shall be protected from direct light. The coating shall be inspected periodically. If the coating deteriorates (the service life of the coating is 6 months), it shall be replaced immediately. If the packaging material is wet when adding a pad, the pad shall be removed immediately to prevent surface corrosion. The

Transportation: in order to avoid surface scratches, rubber or sleepers shall be used during transportation, and stainless steel protection special materials shall be used as far as possible. In order to avoid surface pollution caused by fingerprints, gloves shall be worn during operation.

The

6、 Why is stainless steel rusty? The

Why is stainless steel rusty? When brown rust spots (spots) appeared on the surface of stainless steel pipes, people were surprised: they thought that "stainless steel is not rusty, rusty is not stainless steel, and there may be a problem with the steel". In fact, this is a one-sided wrong view of the lack of understanding of stainless steel. Stainless steel will rust under certain conditions. The

Stainless steel has the ability to resist atmospheric oxidation, that is, rust resistance. At the same time, it also has the ability to corrode in the medium containing acid, alkali and salt, that is, corrosion resistance. However, the corrosion resistance of the steel varies with its chemical composition, interaction state, service condition and environmental medium type. For example, 304 steel pipe has excellent corrosion resistance in a dry and clean atmosphere, but if it is moved to the coastal area, it will soon rust in the sea fog containing a lot of salt; While 316 steel pipe performs well. Therefore, not any kind of stainless steel can resist corrosion and rust in any environment. The

Stainless steel is a thin, firm, fine and stable chromium rich oxide film (protective film) formed on its surface to prevent the continuous penetration and oxidation of oxygen atoms and obtain the ability to resist corrosion. Once the film is continuously damaged for some reason, the oxygen atoms in the air or liquid will continuously penetrate or the iron atoms in the metal will continuously separate out, forming loose iron oxide, and the metal surface will be continuously corroded. This surface facial mask is damaged in many ways, and the following are common in daily life:

1. There are dust containing other metal elements or attachments of dissimilar metal particles on the surface of stainless steel. In the humid air, the condensate between the attachments and stainless steel will connect them into a micro cell, causing electrochemical reaction and damaging the protective film, which is called electrochemical corrosion. The

2. Organic juice (such as melons and vegetables, noodle soup, phlegm, etc.) adhered to the surface of stainless steel constitutes organic acid in the presence of water and oxygen. For a long time, organic acid will corrode the metal surface. The

3. The stainless steel surface contains acid, alkali and salt substances (such as alkali water and lime water splashing on the decoration wall), causing local corrosion. The

4. In polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide and nitrogen oxide), sulfuric acid, nitric acid and acetic acid liquid points are formed in the presence of condensate, causing chemical corrosion. The

The above conditions can cause the damage of the protective film on the surface of stainless steel and lead to corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, we suggest:

1. The decorative stainless steel surface must be cleaned and scrubbed frequently to remove attachments and external factors causing decoration. The

2. 316 stainless steel shall be used in coastal areas, which can resist seawater corrosion. 3. the chemical composition of some stainless steel pipes in the market can not meet the corresponding national standards, which can not meet the 304 material requirements. Therefore, it will also cause rust, which requires users to carefully select products from reputable manufacturers.



7、 Why is stainless steel magnetic? The

People often think that the magnet adsorbs stainless steel to verify its advantages and disadvantages and authenticity. If it does not adsorb non-magnetic, it is good and genuine; If the user is magnetic, it is considered to be a fake. In fact, this is an extremely one-sided, impractical and wrong method of discrimination. The

There are many kinds of stainless steel, which can be divided into several types according to the organizational structure at room temperature:

1. Austenite body type: such as 304, 321, 316, 310, etc; The

2. Martensitic or ferritic type: such as 430, 420, 410, etc; The

Austenite type is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Most stainless steels used as decorative tube sheets are austenitic Type 304 materials. Generally speaking, they are non-magnetic or weakly magnetic. However, magnetism may also occur due to chemical composition fluctuations or different processing states caused by smelting. However, this cannot be considered as counterfeit or different. What is the reason? The

As mentioned above, austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to component segregation during smelting or improper heat treatment, a small amount of martensite or ferrite in Austenitic 304 stainless steel will be caused. In this way, 304 stainless steel will have weak magnetism. The

In addition, after cold working, the microstructure of 304 stainless steel will also transform to martensite. The greater the cold working deformation, the more martensite transformation, and the greater the magnetic properties of the steel. Production of steel strips of the same batch number Φ 76 tube, no obvious magnetic induction, production Φ 9.5 pipe. The magnetic induction is more obvious due to the large bending deformation. The deformation of square rectangular tubes is larger than that of round tubes, especially in the angle part. The deformation is more intense and the magnetism is more obvious. The

In order to completely eliminate the magnetism of 304 steel caused by the above reasons, the stable austenite structure can be recovered through high-temperature solid solution treatment, so as to eliminate the magnetism. The

In particular, the magnetism of 304 stainless steel caused by the above reasons is not at the same level as that of other stainless steels, such as 430 and carbon steel, that is to say, the magnetism of 304 steel always shows weak magnetism. The

This tells us that if stainless steel has weak magnetism or no magnetism at all, it should be judged as 304 or 316 material; If it is the same as the magnetism of carbon steel, it shows strong magnetism, because it is judged that it is not made of 304 material.