Seek the basic knowledge of stainless steel

The corrosion resistance of stainless steel decreases with the increase of carbon content, so the carbon content of most stainless steels is low, and the wC (carbon content) of some steels is even lower than 0.03% (for example, 00Cr 12). The main alloying element in stainless steel is Cr, and only when the Cr content reaches a certain value can the steel have corrosion resistance. Therefore, the wCr of general stainless steel is above 13%. Stainless steel also contains elements such as nickel, titanium, manganese, nitrogen and niobium.

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kind

Stainless steel is often divided into martensitic steel, ferritic steel and austenitic steel according to its microstructure. In addition, it can also be divided into chromium stainless steel, chromium nickel stainless steel and chromium manganese nitrogen stainless steel.

1, ferritic stainless steel: containing 12% ~ 30% chromium. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its chloride stress corrosion resistance is better than other kinds of stainless steel.

There are Crl7, Cr 17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Ferritic stainless steel has good corrosion resistance and oxidation resistance because of its high chromium content, but its mechanical properties and technological properties are poor, so it is mostly used in acid-resistant structures with less stress and as oxidation-resistant steel. This kind of steel can resist the corrosion of atmosphere, nitric acid and brine solution, and has the characteristics of good oxidation resistance at high temperature and small thermal expansion coefficient. It can be used in nitric acid and food factory equipment, and can also be used to manufacture parts that work at high temperature, such as gas turbine parts.

2. Austenitic stainless steel: containing more than 18% of chromium, about 8% of nickel and a small amount of elements such as molybdenum, titanium and nitrogen. Good comprehensive performance, corrosion resistance to various media. Commonly used brands of austenitic stainless steel are 1Cr 18Ni9, 0Cr 19Ni9, etc. lavatory

3. Austenite-ferrite duplex stainless steel: It has the advantages of both austenitic and ferritic stainless steel and is superplastic.

About half austenite and half ferrite stainless steel. In the case of low C content, Cr content is 18%~28%, and Ni content is 3%~ 10%. Some steels also contain alloying elements, such as molybdenum, copper, silicon, niobium, titanium and nitrogen. This steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferritic steel, it has higher plasticity and toughness, no brittleness at room temperature, significantly improved intergranular corrosion resistance and weldability, and maintained the brittleness, high thermal conductivity and superplasticity of ferritic stainless steel at 475℃. Compared with austenitic stainless steel, it has high strength and obviously improved intergranular corrosion resistance and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

4. Martensite stainless steel: high strength, but poor plasticity and weldability.

Commonly used brands of martensitic stainless steel are 1Cr 13, 3Cr 13, etc. Martensite stainless steel has high strength, hardness and wear resistance due to its high carbon content, but its corrosion resistance is slightly poor. Used for some parts with high mechanical performance and general corrosion resistance requirements, such as springs, steam turbine blades, hydraulic machine valves, etc. This kind of steel is used after quenching and tempering.

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Stainless steel history

Harry buhrer, a famous metallurgist who graduated from the University of Sheffield in England, invented stainless steel in the early 20th century.

The invention and use of stainless steel can be traced back to the First World War. British scientist BuHenry buhrer was commissioned by the British military factory to study the improvement of weapons. At that time, the rifle bore used by soldiers was very easy to wear, and buhrer wanted to invent an alloy steel that was not easy to wear. The stainless steel invented by buhrer Leigh was patented in Britain in 19 16, and mass production began. Since then, stainless steel accidentally found in garbage has swept the world, and Henry buhrer Leigh is also known as the "father of stainless steel".

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Stainless steel action

Stainless steel will not cause corrosion, pitting, rust or wear. Stainless steel is also one of the strongest building metal materials. Because stainless steel has good corrosion resistance, it can permanently maintain the integrity of engineering design. Chromium-containing stainless steel also has both mechanical strength and high ductility, which is easy to manufacture and meets the needs of architects and structural designers.

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Stainless steel brand grouping

Precipitation hardening stainless steel. It has good formability and weldability, and can be used as ultra-high strength material in nuclear industry and aerospace industry.

It can be divided into Cr series (400 series), Cr-Ni series (300 series), Cr-Mn-Ni series (200 series) and precipitation hardening series (600 series).

200. Cr-Mn-N series austenitic stainless steel

300. Cr-Ni austenitic stainless steel series

30 1- has good ductility and is used for molding products. It can also be rapidly hardened by machining. Good weldability. Wear resistance and fatigue strength are better than 304 stainless steel.

302—— Corrosion resistance is the same as that of 304, and its strength is better because of its relatively high carbon content.

303— It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.

304— namely 18/8 stainless steel. GB brand is 0Cr 18Ni9.

309- has better temperature resistance than 304.

316 —— The second most widely used steel after 304, mainly used in food industry, pharmaceutical industry and surgical instruments. Adding molybdenum element makes it obtain a special corrosion-resistant structure. Because it has better chloride corrosion resistance than 304, it is also used as "marine steel". SS3 16 is usually used in nuclear fuel recovery devices. 18/ 10 stainless steel usually meets this application level. [1] Model 32 1— The performance is similar to that of 304, but the risk of weld corrosion is reduced by adding titanium.

400. Series-ferritic and martensitic stainless steels.

408- Good heat resistance and weak corrosion resistance, 1 1% Cr, 8% Ni.

409- the cheapest model (Britain and America), usually used as an automobile exhaust pipe, belongs to ferritic stainless steel (chrome steel).

4 10— Martensite (high strength chromium steel) has good wear resistance and poor corrosion resistance.

416 —— Adding sulfur can improve the processability of materials.

420—— "Blade Grade" martensitic steel, similar to Brinell high chromium steel, the earliest stainless steel. It can also be used for scalpels, which can be made very bright.

430 ferritic stainless steel, used for decoration, such as automobile parts. Good formability, but poor temperature resistance and corrosion resistance.

440— High-strength cutting tool steel, with a slightly higher carbon content, can obtain a higher yield strength after proper heat treatment, and its hardness can reach 58HRC, making it one of the hardest stainless steels. The most common application example is "shaving blade". There are three commonly used models: 440A, 440B, 440C and 440F (easy to process).

500. Heat-resistant chromium alloy steel series.

600. Series martensite precipitation hardening stainless steel.

Stainless steel 630- the most commonly used precipitation hardening stainless steel model, usually also called17-4; 17% Cr, 4% Ni.

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Why is stainless steel resistant to corrosion?

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

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Types of stainless steel

The word "stainless steel" refers not only to one kind of stainless steel, but to 100 kinds of industrial stainless steel, and each developed stainless steel has good performance in its specific application field. The key to success is to find out the purpose first, and then determine the correct steel grade. For more details about stainless steel, please refer to the "Stainless Steel Guide" floppy disk compiled by NiDI.

Fortunately, there are usually only six kinds of steel related to the application field of building structures. They all contain 17 ~ 22% chromium, and the better steel also contains nickel. Adding molybdenum can further improve the atmospheric corrosivity, especially the corrosion resistance of stainless steel in chloride-containing atmosphere.

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Advantages of stainless steel

Experience shows that the degree of atmospheric corrosion is different in different regions. For the sake of explanation, it is suggested that the regions should be divided into four categories, namely rural areas, cities, industrial areas and coastal areas.

Rural areas are basically pollution-free areas. The population density in this area is low and there are only pollution-free industries.

This city is a typical residential, commercial and light industrial area, where there is light pollution, such as traffic pollution.

Industrial areas are areas where heavy industry causes air pollution. Pollution may be caused by gases formed by fuel oil, such as oxides of sulfur and nitrogen, or other gases released by chemical plants or processing plants. Suspended particles in the air, such as dust or iron oxide precipitation produced during steel production, will also aggravate corrosion.

Coastal areas usually refer to areas within a mile from the sea. However, the ocean atmosphere can spread inland, especially on the island. The prevailing wind comes from the ocean and the climate is bad. For example, the weather in Britain is like this, so the whole country belongs to the coastal areas. If the wind is mixed with sea fog, especially because of salt deposition and accumulation caused by evaporation, coupled with less rain, it is not often washed away by rain, and the conditions in coastal areas are even more unfavorable. If there is industrial pollution, it is more corrosive.

Studies in the United States, Britain, France, Italy, Sweden and Australia have determined the influence of these areas on the atmospheric corrosion resistance of various stainless steels. The related contents are briefly introduced in the Brief Introduction of Architects published by NiIDI. The tables in this book can help designers choose the most cost-effective stainless steel for different fields.

When making a choice, it is important to determine whether there are still local factors that affect the use of the site environment. For example, when stainless steel is used under the factory chimney, near the exhaust baffle of air conditioner or near the waste yard, abnormal conditions will occur.

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Maintenance and cleaning

Like other materials exposed to the atmosphere, stainless steel will get dirty. Future lectures will analyze the design factors that affect maintenance and cleaning costs. But rain washing, hand washing and dirty surface are still related.

The effect of rain erosion is determined by placing the same slats directly in the atmosphere and in the shed. The effect of manual cleaning is determined by manually scrubbing the right side of each lath with a sponge dipped in soapy water every six months. The results show that, compared with being placed in a shed or an unwashed place, washing with rainwater and manual scrubbing to remove dust and deposits on the surface have a good effect on the surface condition. In addition, it is also found that the surface processing conditions also have an influence, and the lath with smooth surface is better than the lath with rough surface.

Therefore, the interval of washing is influenced by many factors, and the main influencing factor is the required aesthetic standard. Although many stainless steel curtain walls are only cleaned when cleaning the glass, generally speaking, the stainless steel used for external walls is cleaned twice a year.

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Typical use

Most of the use requirements are to keep the original appearance of the building for a long time. When determining the type of stainless steel to be selected, the main considerations are the required aesthetic standards, the corrosiveness of the local atmosphere and the cleaning system to be adopted.

However, more and more other applications only seek structural integrity or impermeability. Such as roofs and side walls of industrial buildings. In these applications, the owner's construction cost may be more important than beauty, and the surface is not very clean.

The effect of using 304 stainless steel in dry indoor environment is quite good. However, in rural and urban areas, if you want to keep its appearance outdoors, you need to clean it frequently. In heavily polluted industrial areas and coastal areas, the surface will be very dirty and even rusty. However, in order to obtain the beautiful effect in outdoor environment, nickel-containing stainless steel must be used. Therefore, 304 stainless steel is widely used in curtain walls, side walls, roofs and other building applications, but in the industrial or marine atmosphere with severe corrosiveness, it is best to use 3 16 stainless steel. Stainless steel sliding door Now, people have fully realized the advantages of using stainless steel in structural applications. Some design standards include 304 and 3 16 stainless steel. Because "duplex" stainless steel 2205 combines good atmospheric corrosion resistance with high tensile strength and elastic ultimate strength, it is also included in European standards.

Product shape

In fact, stainless steel is made according to all standard metal shapes and sizes, and there are many special shapes. The most commonly used products are thin plate and strip steel, and special products are also medium plate, such as hot rolled structural steel and extruded structural steel. There are also round, oval, square, rectangular and hexagonal welded or seamless steel pipes and other forms of products, including profiles, bars, wires and castings. X. Surface Conditions As will be mentioned later, in order to meet the aesthetic requirements of architects, a variety of commercial surface treatments have been developed. For example, the surface can be highly reflective or dull; Can be smooth, polished or embossed; It can be colored, colored, electroplated or etched on the surface of stainless steel, and can also be drawn to meet various requirements of designers for appearance.

It is easy to keep the surface state. Dust can only be removed by occasional cleaning. Because of its good corrosion resistance, graffiti pollution or similar other surface pollution can also be easily removed.

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Future prospects

Because stainless steel has many ideal properties required by building materials, it can be said to be unique among metals, and its development continues. In order to improve the performance of stainless steel in traditional applications, the existing types have been improved, and new stainless steel is being developed to meet the strict requirements of advanced building applications. Due to the continuous improvement of production efficiency and quality, stainless steel has become one of the most cost-effective materials chosen by architects. Stainless steel combines performance, appearance and use characteristics, so it will still be one of the best building materials in the world. China Stainless Steel Marketing Network integrates the service platform of stainless steel and stainless steel supply chain information, and provides information and consulting services for members and users around the world by setting up several columns such as stainless steel industry information, industry observation, enterprise management, stainless steel design, stainless steel forum, equipment and materials, exhibition information, stainless steel knowledge and talent recruitment. , through the latest information, database, database, analysis and prediction, communication platform, etc. Provide trade information and seek business opportunities for stainless steel industry and related industries; Spread stainless steel culture and home life art, and provide stainless steel consumption knowledge.

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Identification method of stainless steel

Number and representation of steel

(1) Use international chemical element symbols and national symbols to represent chemical components, and use Arabic letters to represent component content;

Such as: China-Russia 12CrNi3A.

(2) The series or number of steel is represented by a fixed number of digits; Such as: USA, Japan, 300 series, 400 series, 200 series;

(3) The serial number is composed of Latin letters and sequences, and the serial number only indicates the purpose.

Numbering rules in China.

(1) Use element symbols

② uses: hanyu pinyin, open hearth steel: p, boiling steel: f, killed steel: b, grade a steel: a, T8: Te8,

GCr 15: ball

◆ Alloy steel and spring steel, such as 20CrMnTi 60SiMn (C content is expressed in parts per ten thousand).

◆ Stainless steel and alloy tool steel (C content is expressed in thousandths), such as: 1Cr 18Ni9 thousandths (i.e.

0. 1%C), stainless steel C≤0.08%, such as 0Cr 18Ni9, ultra-low carbon C≤0.03%, such as 00Cr 17Ni 13Mo.

International stainless steel marking method

The American Iron and Steel Association uses three numbers to represent various standard grades of malleable stainless steel. These include:

① Austenitic stainless steel is marked with 200 and 300 series numbers, such as some common austenitic stainless steels.

Marked with 20 1, 304, 3 16 and 3 10.

Stainless steel ② ferritic and martensitic stainless steels are represented by 400 series numbers.

③ ferritic stainless steel is marked with 430 and 446, and martensitic stainless steel is marked with 4 10, 420 and 440C.

Remember, biphasic (austenite-ferrite),

④ Stainless steel, precipitation hardening stainless steel and high alloy with iron content less than 50% are usually named by patent names or trademarks.

4). Classification and grading of standards

4- 1 classification:

① National standard GB ② Industry standard YB ③ Local standard ④ Enterprise standard Q/CB

4-2 Classification:

① Product standard ② Packaging standard ③ Method standard ④ Basic standard

4-3 standard grades (divided into three grades):

Y: international advanced level I: international general level H: domestic advanced level.

4-4 National Standards

GB 1220-84 stainless steel bar (grade I) GB424 1-84 stainless steel welding plate (grade h)

GB4356-84 stainless steel welding plate (grade I) GB 1270-80 stainless steel pipe (grade I)

Gb12771-91stainless steel welded pipe (grade y) GB3280-84 stainless steel cold plate (grade I)

GB4237-84 Stainless Steel Hot Plate (Grade I) GB4239-9 1 Stainless Steel Cold Strip (Grade I)

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Cleaning stainless steel surface damage and entrained foreign matter.

dust

Production is often carried out in dusty places, and there is often a lot of dust in the air, which keeps falling on the surface of equipment. They can be removed with water or alkaline solution. However, adhesive dirt requires high-pressure water or steam to clean it.

Floating iron powder or embedded iron

Free iron will rust and corrode stainless steel on any surface. Therefore, it must be deleted. Floating powder can usually be removed with dust. Some of them have strong adhesion and must be embedded with iron. In addition to dust, there are many sources of surface iron, including cleaning with ordinary carbon steel wire brush, shot peening with sand, glass beads or other abrasives previously used on ordinary carbon steel, low alloy steel or iron castings, or grinding the above-mentioned non-stainless steel products near stainless steel parts and equipment. If the stainless steel is not protected during unloading or hoisting, the steel wire rope, spreader and iron pieces on the workbench are easy to embed or dirty the surface.

Order requirements and post-production inspection can prevent and find the existence of free iron. ASTM standard A380[3] specifies the rust test method for checking iron or steel particles on the surface of stainless steel. This test method should be used when no iron is required. If the result is satisfactory, clean the surface with clean pure water or nitric acid until the dark blue disappears completely.

As the standard A380[3] points out, if the corrosion test solution cannot be completely removed, it is not recommended to adopt this test method on the technical surface of the equipment, that is, the direct contact surface used to produce human consumer goods. A relatively simple test method is to expose to water for 12~24 hours to check whether there is rust spot. This test is insensitive and time-consuming. These are tests, not cleaning methods. If iron is found, it must be cleaned by chemical and electrochemical methods described later.

gap

In order to prevent the accumulation of process lubricants or products and/or dirt, scratches and other rough surfaces must be cleaned mechanically, usually with a special polishing machine for stainless steel.

Thermal tempering color and other oxide layers

If stainless steel is heated to a certain high temperature in the air during welding or grinding, chromium oxide hot tempering color will appear on both sides, lower surface and bottom of the weld. The thermal tempering color is thinner than the oxide protective film, which is obviously visible. Depending on the thickness, the color can be rainbow, blue, purple to light yellow and brown. Thicker oxides are generally black. It is caused by staying in high temperature or high altitude for a long time. When any kind of oxide layer emerges, the chromium content on the metal surface will decrease, thus reducing the corrosion resistance of these areas. In this case, not only the thermal tempering color and other oxide layers should be removed, but also the chromium-poor metal layer below them should be removed.

rust patch

Stainless steel products or equipment sometimes appear rust before or during production, indicating that the surface is seriously polluted. The equipment must be derusted before being put into use, and the thoroughly cleaned surface should be inspected by iron test and/or water test.

Rough grinding and machining

Both grinding and machining will cause surface roughness, leaving defects such as grooves, overlaps and burrs. Each defect may also cause a certain depth of damage to the metal surface, so that the damaged metal surface cannot be cleaned by pickling, electropolishing or shot blasting (such as dry sand blasting and glass beads for abrasives). Rough surface may be the origin of corrosion and deposition products, and it is not allowed to use rough grinding to clean weld defects or remove redundant weld reinforcement before re-welding. In the latter case, fine abrasives should be used for grinding.

Welding arc starting point

When the welding machine arcs on the metal surface, it will cause the surface roughness defect. The protective film was damaged, leaving a potential corrosion source. The welder should strike the arc on the weld bead or the side of the welded joint. Then the arc trace is melted into the weld.

welding slag

Welding spatter has a great relationship with welding technology. For example, GTAM (gas shielded tungsten arc welding) or TIG (inert gas shielded tungsten arc welding) has no splash. However, when GMAW (gas shielded metal arc welding) and FCAW (flux-cored wire arc welding) are used, if the welding parameters are not used properly, a lot of spatter will be caused. When this happens, the parameters must be adjusted. If we want to solve the problem of welding spatter, we should apply splash inhibitor on each side of the joint before welding, so as to eliminate the adhesion of spatter. After welding, anti-splash agent and various splashes can be easily removed without damaging the surface or causing slight damage.

traffic

Welding processes using flux include manual welding, arc welding with flux core and submerged arc welding. These welding processes will leave fine flux particles on the surface, which cannot be removed by ordinary cleaning methods. This kind of particles will become the corrosion source of crevice corrosion, and these residual fluxes must be removed by mechanical cleaning.

weld defect

Welding defects such as undercut, incomplete penetration, dense pores and cracks not only reduce the firmness of the joint, but also become the corrosion source of crevice corrosion. In order to improve this result, they also carry solid particles during cleaning operations. These defects can be repaired by re-welding or re-welding after grinding.

oil

Organic substances such as oil, grease and even fingerprints will become the corrosion source of local corrosion. Because these substances can play a barrier role and affect the chemical and electrochemical cleaning effect, they must be thoroughly cleaned. There is a simple waterproof test for
TM A380 to detect organic pollutants. During the test, water is injected from the top of the vertical surface, and in the process of downward flow, water will separate along the periphery of the organic matter. Flux and/or acidic chemical cleaning agent can remove oil and grease.

Residual adhesive

When the adhesive tape and protective paper are torn off, a part of the adhesive always remains on the stainless steel surface. If the glue is not hard, it can be removed with organic solvent. However, when exposed to light and/or air, the adhesive hardens, forming a corrosion source of crevice corrosion. Then mechanical cleaning with fine abrasive is needed.

Paint, chalk and marker marks

The effects of these pollutants are similar to those of oil and grease. It is recommended to use a clean brush and clean water or alkaline cleaning agent, or use high-pressure water or steam for cleaning. Hard 30 1 stainless steel is a kind of ferritic stainless steel in use. The chromium content is 1 1%~30%, and it has a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of elements such as molybdenum, titanium and niobium. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance and excellent stress corrosion resistance, and is mostly used to manufacture parts resistant to atmospheric, steam, water and oxidative acid corrosion. This kind of steel has some disadvantages, such as poor plasticity, obviously reduced plasticity after welding and corrosion resistance, which limits its application. The application of refining technology outside the furnace (AOD or VOD) can greatly reduce the interstitial elements such as carbon and nitrogen, so this kind of steel is widely used.

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Comparison of physical properties data between 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 is carbon steel, ferrite, martensite and austenitic stainless steel in turn; The order of linear expansion coefficient is similar, with the highest in austenitic stainless steel and the lowest in carbon steel. Carbon steel, ferritic stainless steel and martensitic stainless steel are magnetic, while austenitic stainless steel is non-magnetic, but they will be magnetic when martensitic transformation occurs during cold working and hardening. Heat treatment can be used to eliminate this martensite structure and restore its non-magnetism.

Compared with carbon steel, austenitic stainless steel has the following characteristics:

1) has a high resistivity, which is about 5 times that of carbon steel.

2) The linear expansion coefficient is large, which is 40% larger than that of carbon steel. With the increase of temperature, the value of linear expansion coefficient also increases accordingly.

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