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Stainless steel is a modern material with a history of 60 years Since the invention of stainless steel at the beginning of this century, stainless steel combines the image of modern materials with the reputation of excellence in building applications to envy its competitors.
As long as the steel type is selected correctly, the processing is appropriate, and the maintenance is appropriate, stainless steel will not produce corrosion, pitting, corrosion or wear. Stainless steel is also one of the strongest materials in construction metal materials. Because stainless steel has good corrosion resistance, it can make structural components permanently maintain the integrity of the engineering design. Chromium-containing stainless steel also combines mechanical strength and high extensibility, making it easy to process and manufacture components that meet the needs of architects and structural designers.
The key to the success of stainless steel in the construction, building and structural industries is its excellent corrosion resistance.
Stainless Steel Branding 200 Series - Chromium-Nickel-Manganese Austenitic Stainless Steel 300 Series - Chromium-Nickel Austenitic Stainless Steel Model 301 - Good ductility for forming products. It can also be hardened by mechanical processing. Good weldability. The wear resistance and fatigue strength are better than 304 stainless steel.
Model 302 - Corrosion resistance with 304, due to the relatively high carbon content and therefore better strength.
Model 303 - It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.
Model 304 - Universal Model; 18/8 stainless steel. The GB grade is 0Cr18Ni9.
Model 309 - better temperature resistance than 304.
Model 316 - After 304, the second most widely used steel grade, mainly for the food industry and surgical equipment, adding molybdenum elements to obtain a special structure against corrosion. Because it has better resistance to chloride corrosion than 304, it is also used as "marine steel". SS316 is usually used in nuclear fuel recovery devices. 18/10 grade stainless steel also generally meets this application level. [1]
Model 321 - Other properties are similar to 304 except that the addition of titanium reduces the risk of material weld rust.
Series 400 - ferritic and martensitic stainless steel model 408 - good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
Model 409 - the cheapest model (British and American), commonly used as automotive exhaust pipe, is a ferritic stainless steel (chrome steel).
Model 410 - Martensitic (high strength chrome steel), good wear resistance, poor corrosion resistance.
Model 416 - Added sulfur improves the material's processability.
Model 420 - "Blade-grade" martensitic steel, similar to Brinell high chromium steel, the first stainless steel. Also used for surgical tools, can do very bright.
Model 430—ferritic stainless steel, for decoration, for example for automotive accessories. Good formability, but poor temperature resistance and corrosion resistance.
Model 440—High-strength cutting tool steel, with slightly higher carbon content. After proper heat treatment, it can obtain higher yield strength, and the hardness can reach 58HRC, which is among the hardest stainless steels. The most common application example is the "razor blade." There are three commonly used models: 440A, 440B, 440C, in addition to 440F (easy processing type).
500 series - heat-resistant chromium alloy steel.
600 Series - Martensitic precipitation hardened stainless steel.
Model 630 - the most commonly used precipitation hardened stainless steel model, also commonly called 17-4; 17% Cr, 4% Ni.
Why is stainless steel corrosion resistant?
All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, causing the rust to continuously expand, eventually forming holes. The carbon steel surface can be ensured by electroplating with paint or oxidation-resistant metals (for example, zinc, nickel and chromium), but, as is known, this protection is only a thin film. If the protective layer is destroyed, the underlying steel begins to rust.
The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection method is not the same.
When the addition amount of chromium reaches 10.5%, the atmospheric corrosion resistance of the steel significantly increases, but when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that the alloying treatment of steel with chromium changes the type of surface oxide to a surface oxide similar to that formed on pure chromium metal. This tightly adhered chromium-rich oxide protects the surface from further oxidation. This oxide layer is extremely thin, through which the natural luster of the steel surface can be seen, giving stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface reacts with the atmosphere to repair itself, and this “passivation film†is re-formed to continue protection.
Therefore, all stainless steels have a common characteristic that the chromium content is above 10.5%.
The term "stainless steel" in the stainless steel category does not simply refer to stainless steel but to more than one hundred industrial stainless steels. Each type of stainless steel developed has a good performance in its specific application area. The key to success is first to understand the use, and then determine the correct steel. Further details on stainless steel can be found in the "Stainless Steel Guide" floppy disk compiled by NiDI.
Fortunately, there are usually only six types of steel related to the application of building structures. All of them contain 17-22% of chromium, and the preferred steel also contains nickel. The addition of molybdenum can further improve the atmospheric corrosion, especially to the chloride containing atmosphere.
The experience of resistance to atmospheric corrosion shows that the degree of atmospheric corrosion differs from region to region. For ease of explanation, it is recommended that the area be divided into four categories: rural areas, cities, industrial areas, and coastal areas.
The countryside is basically a pollution-free area. The area has a low population density and there are only pollution-free industries.
The city is a typical residential, commercial and light industrial area with mild pollution, such as traffic pollution.
Industrial areas are areas where heavy industry causes atmospheric pollution. Pollution may be due to gases formed by fuel, such as sulfur and nitrogen oxides, or other gases released by chemical plants or processing plants. Particles suspended in the air, such as dust or iron oxides produced during steel production, also increase corrosion.
Coastal areas usually refer to areas within a mile of the sea. However, the oceanic atmosphere can spread deeper inland, especially on the islands. The prevailing winds come from the sea and the climate is harsh. For example, the British climate conditions are such that the entire country belongs to the coastal region. If the wind is mixed with sea fog, especially due to evaporation and salt concentration, together with less rain, it is not often washed by rain and the conditions in the coastal areas are even more unfavorable. Corrosion is even greater if there is industrial pollution.
Research conducted by the United States, the United Kingdom, France, Italy, Sweden, and Australia has determined the effect of these regions on the atmospheric corrosion resistance of various stainless steels. The content is briefly described in the "Architectural Notes" published by NiIDI, which helps designers select the most cost-effective stainless steel for various regions.
When making a choice, it is important to determine whether there are local factors that affect the use of the site environment. For example, stainless steel is used under the factory chimney and is used near air-conditioner exhaust baffles or near scrap yards, which can cause unusual conditions.
Maintenance and cleaning As with other materials exposed to the atmosphere, stainless steel can also be dirty. Future lectures will analyze the design factors that affect the maintenance and cleaning costs. However, there is an interrelationship between rain flushing, manual flushing and dirty surfaces.
The effect of rain erosion is determined by placing the same slats directly in the atmosphere and in a shed place. The effect of manual rinsing is determined by manually scrubbing the sponge with soapy water every six months to scrub the right side of each slat. As a result, it was found that the removal of dust and sludge from the surface by rainwater and artificial scrubbing had a good effect on the surface condition compared to the slats in the shed areas and areas where they were not washed. It has also been found that the condition of the surface finish is also influential, and the smooth surface of the slats is better than the rough surface slats.
Therefore, the interval between washings is affected by many factors. The main influencing factors are the required aesthetic standards. Although many stainless steel curtain walls are only flushed when the glass is rubbed, stainless steel used for external purposes is generally washed twice a year.
Typical uses Most of the requirements for use are to maintain the original appearance of the building for a long period of time. In determining the type of stainless steel to be selected, the main considerations are the required aesthetic standards, the corrosivity of the local atmosphere, and the cleanup system to be used.
However, more and more other applications only seek structural integrity or impermeability. For example, roofs and side walls of industrial buildings. In these applications, the construction cost of the owner may be more important than the aesthetic, and the surface may not be clean.
Using 430 stainless steel in a dry indoor environment is quite good. However, in rural areas and cities, if they want to maintain their appearance outdoors, they need to be cleaned frequently. In heavily polluted industrial areas and coastal areas, the surface is very dirty and may even cause corrosion. However, in order to obtain the aesthetic effect in the outdoor environment, it is necessary to use nickel-containing stainless steel. Therefore, 304 stainless steel is widely used for curtain walls, side walls, roofs, and other construction purposes, but 316 stainless steel is best used in aggressive industrial or marine atmospheres.
Nowadays, people have fully realized the superiority of using stainless steel in structural applications. There are several design guidelines that include 304 and 316 stainless steels. Because the "duplex" stainless steel 2205 has combined good atmospheric corrosion resistance with high tensile strength and elastic limit strength, the steel was also included in the European Code.
Product Shape In fact, stainless steel is produced in full standard metal shapes and sizes, and there are many special shapes. The most commonly used products are made from thin sheets and strips, and special products are also produced from medium and thick plates, for example, hot-rolled structural steels and extruded structural steels. There are also circular, elliptical, square, rectangular and hexagonal welded tubes or seamless steel tubes and other forms of products, including profiles, rods, wires and castings.
The surface state As will be discussed later, in order to meet the aesthetic requirements of architects, a variety of different commercial surface finishes have been developed. For example, the surface may be highly reflective or matte; may be glossy, polished, or embossed; may be colored, colored, electroplated, or etched with a pattern on the stainless steel surface to meet the designer's appearance The various requirements.
It is easy to keep the surface state. Just an occasional flush can remove dust. Due to good corrosion resistance, surface smearing or other surface contamination can also be easily removed.
For more than 60 years, architects have been using stainless steel to build cost-effective permanent structures. Many existing buildings fully illustrate the correctness of this choice. Some are very ornamental, such as the Chrysler Building in New York City. However, in many other applications, the role played by stainless steel is not so eye-catching, but it plays an important role in the aesthetics and performance of buildings. For example, because stainless steel is more wear-resistant and indent-proof than other metal materials of the same thickness, it is the designer's preferred material when constructing a walkway in a place with a large population flow.
Stainless steel has been used for more than 70 years to build new buildings and structural materials used to repair historic sites and monuments. Early designs were calculated according to basic principles. Today, design specifications, such as the American Society of Civil Engineers' standard ANSI/ASCE-8-90 "Design Specification for Cold-Molded Stainless Steel Structural Parts" and the "Structural Stainless Steel Design Manual" jointly published by NiDI and Euro Inox have simplified the long life, Design of structural parts for structural integrity.
Future Prospects Since stainless steel already possesses many desirable properties required for building materials, it can be said to be unique in metals, and its development continues. In order to make stainless steel perform better in traditional applications, the existing types have been improved, and new stainless steels are being developed in order to meet the stringent requirements of advanced building applications. Due to continuous improvement of production efficiency and continuous improvement of quality, stainless steel has become one of the most cost-effective materials selected by architects.
Stainless steel combines performance, appearance and use characteristics, so stainless steel will still be one of the best building materials in the world.
Stainless Steel Marking Method Steel Numbering and Representation 1 Chemical composition is expressed using international chemical element symbols and national symbols, and ingredients are expressed in Arabic letters: eg: China, Russia 12CrNi3A
2 Use a fixed number of digits to represent steel series or numbers; for example: the United States, Japan, 300 Series, 400 Series, 200 Series;
3 Use Latin letters and sequence numbers to indicate only the purpose.
China's numbering rule 1 uses the element symbol 2 purpose, Pinyin, open hearth steel: P, boiling steel: F, killed steel: B, A steel: A, T8: special 8, GCr15: ball â—† joint steel, spring steel , such as: 20CrMnTi 60SiMn, (in thousands of minutes to indicate C content)
◆ stainless steel, alloy tool steel (with a thousandth part of the C content), such as: 1Cr18Ni9 one thousandth (that is, 0.1% C), stainless steel C ≤ 0.08% such as 0Cr18Ni9, ultra-low carbon C ≤ 0.03% such as 0Cr17Ni13Mo
International Stainless Steel Marking Method The American Iron and Steel Institute uses three digits to mark various standard grades of malleable stainless steel. among them:
1 Austenitic stainless steel is marked with 200 and 300 series numbers.
2 Ferritic and martensitic stainless steels are represented by 400 series numbers. For example, some of the more common austenitic stainless steels are marked with 201, 304, 316, and 310,
3 Ferritic stainless steel is marked with 430 and 446. Martensitic stainless steel is marked with 410, 420, and 440C. It is a duplex (austenitic-ferritic).
4 Stainless steels, precipitation hardened stainless steels, and high alloys containing less than 50% iron are usually named after the patent name or trade mark.
4). Classification and Classification of Standards 4-1 Classification:
1 National Standard GB
2 industry standard YB
3 Local Standard 4 Corporate Standard Q/CB
4-2 classification:
1 Product Standard 2 Packing Standard 4 Method Standard 4 Basic Standard 4-3 Standard Level (3 levels):
Grade Y: International Advanced Level I: International General Level H: Domestic Advanced Level 4-4 GB GB1220-84 Stainless Steel (Grade I)
GB4241-84 Stainless Welding Disk Park (Grade H)
GB4356-84 Stainless Welding Disk Park (Class I)
GB1270-80 stainless steel pipe (Class I)
GB12771-91 stainless steel welded pipe (Y grade)
GB3280-84 stainless cold plate (I level)
GB4237-84 stainless hot plate (Class I)
GB4239-91 stainless cold band (I level)
Stainless steel stainless steel is referred to as stainless steel. It is composed of stainless steel and acid-resistant steel. In short, steel that resists atmospheric corrosion is called stainless steel, and steel that resists chemical corrosion is called acid-resistant steel. In general, steels with a WCr content of more than 12% have the characteristics of stainless steel. The microstructures of stainless steels after heat treatment can be further divided into five categories: ferritic stainless steels, martensitic stainless steels, austenitic stainless steels, Austenite-ferritic stainless steel and precipitation-stained stainless steel text stainless steel equipment will show damage, defects and some materials that affect the surface, such as: dust, float powder or embedded iron, thermal tempering color and other oxidation Layers, rust, abrasive burrs, welding arc marks, welding spatters, fluxes, welding defects, oils and greases, residual adhesives and paints, chalks, markers, etc. The vast majority are ignoring or not doing well because they ignore their harmful effects. However, they are potentially harmful to oxidation protection films. Once the protective film is damaged, thinned or otherwise altered, the underlying stainless steel begins to corrode. Corrosion is not generally throughout the entire surface but at or near the defect. This total corrosion will usually be pitting or crevice corrosion, both of which will progress to depth and breadth, while most of the surface will not be eroded. Let us talk about the various causes of these problems.
Stainless steel surface damage and the cleaning of entrained foreign materials 1. Dust production is often carried out in a dusty place, often with a lot of dust in the air, and they continuously fall on the surface of the equipment. They can be removed with water or an alkaline solution. However, adhesion dust requires high pressure water or steam for cleaning.
2. Floating iron powder or embedded iron on any surface, free iron will rust and stainless steel corrosion. Therefore, it must be cleared. Floats are generally removed with the dust. Some adhesion is very strong and must be treated with embedded iron. In addition to dust, there are many sources of surface iron, including shot blasting using plain carbon steel wire brushes and shot peening using sand, glass beads, or other abrasives previously used on plain carbon steel, low alloy steel, or cast iron, or The aforementioned non-stainless steel products were ground near stainless steel parts and equipment. If the stainless steel is not protected during blanking or hanging, the steel wire rope, spreader, and iron on the work surface can easily embed or stain the surface. Ordering requirements and post-manufacturing inspections can prevent and detect the presence of free iron. ASTM Standard A380 [3] specifies the rust test method for inspecting iron or steel particles on stainless steel surfaces. This test method should be used when absolutely no iron is required. If the result is satisfactory, clean the surface with clean water or nitric acid until the dark blue disappears completely.
As the standard A380 [3] points out, if the rust test solution cannot be completely removed, this test method is not recommended on the process surface of the equipment, ie the direct contact surface for the production of human consumer products. A relatively simple test method is to expose the water for 12 to 24 hours and check for rust spots. This test is not sensitive and time consuming. These are test tests, not cleaning methods. If iron is found, it must be cleaned up using the chemical and electrochemical methods described later.
3. Scratches Scratches and other rough surfaces must be mechanically cleaned to prevent the accumulation of process lubricants or products and/or contaminants.
4. Hot tempering color and other oxide layers If the stainless steel is heated to a certain high temperature in the air during the welding or grinding process, chromium oxide thermal tempering color will appear on both sides of the weld seam, the lower surface and the bottom of the weld seam. . The hot tempering color is thinner than the protective oxide film and is clearly visible. The color depends on the thickness and can be seen in rainbow, blue, purple to light yellow and brown. Thicker oxides are generally black. It is due to staying at higher temperatures at high temperatures or for long periods of time. When any of these oxide layers are present, the chromium content of the metal surface is reduced, resulting in reduced corrosion resistance in these areas. In this case, not only the hot tempering color and other oxide layers should be eliminated, but also the chromium-depleted metal layer below them should be cleaned.
5, stainless steel products or equipment will sometimes see rust before or during the production of rust, which indicates that the surface is seriously polluted. The rust must be removed before the equipment is put into use. Thoroughly cleaned surfaces should be inspected by iron and/or water tests.
6. Rough Grinding and Machining Grinding and machining can cause surface roughness, leaving defects such as grooves, overlaps, and burrs. Each type of defect can also damage the metal surface to a certain depth, so that the damaged metal surface cannot be removed by pickling, electropolishing or shot blasting. The rough surface can be the source of corrosion and deposition products. Weld defects can be cleaned by rough grinding before rewelding or by removing excess weld reinforcement. In the latter case, fine abrasives should be used for grinding.
7. Arc welding spot welders will cause rough surface defects when they arc at the metal surface. The protective film is damaged, leaving a potential source of corrosion. The welder should start the arc either on the welded bead or on the side of the weld joint. Then the arc strikes are melted into the weld.
8, welding splash welding splash and welding process have a great relationship. For example: GTAM (gas protection tungsten arc welding) or TIG (inert gas tungsten arc welding) does not splash. However, the use of GMAW (gas-shielded metal arc welding) and FCAW (arc welding with a flux core) welding process results in a large amount of spatter when the welding parameters are used improperly. When this happens, parameters must be adjusted. If you want to solve the problem of welding spatter, apply anti-spatter agent on each side of the joint before welding. This will eliminate spatter adhesion. After the welding, the anti-spatter agent and various splashes can be easily removed without damaging the surface or causing minor damage.
9. Soldering flux The process of welding with flux consists of manual welding, flux cored arc welding and submerged arc welding. These welding processes leave fine flux particles on the surface, and ordinary cleaning methods cannot remove them. This particle will be the source of crevice corrosion corrosion and must be removed by mechanical cleaning.
10, welding defects Welding defects such as: undercut, incomplete penetration, dense pores and cracks will not only reduce the joint's firmness, but also will become a crevice corrosion corrosion source. When this result is improved for the cleaning operation, they also carry solid particles. These defects can be repaired by re-welding or regrind after re-welding.
11, oil and grease organic substances such as: oil, grease and even fingerprints will become a source of local corrosion corrosion. Since these substances can act as barriers, they can affect chemical and electrochemical cleaning effects and must be thoroughly cleaned. ASTM A380 has a simple WATERBREAK test to detect organic pollutants. During the test, water was poured from the top of the vertical surface and the water separated along the surrounding of the organic matter during the downflow. Flux and/or acidic chemical cleaners remove grease and grease.
12. Residual Adhesive When tearing off the tape and protecting the paper, a part of the adhesive remains on the surface of the stainless steel. If the adhesive is not hard enough, it can be removed with organic flux. However, when exposed to light and/or air, the sticky agent hardens, forming a source of crevice corrosion corrosion. Then you need to use a fine abrasive for mechanical cleaning.
13. Paints, chalk, and markings The effects of these pollutants are similar to those of oils and greases. It is recommended that you use a clean brush and clean water or alkaline cleaners, or use high-pressure water or steam.
Stainless steel knowledge
Resistance to air, steam, water and other weak corrosive media and acid, alkali, salt and other chemical etching medium corrosive steel. Also known as stainless steel. In practical applications, steels that are resistant to weak corrosive media are often referred to as stainless steels, and steels that are resistant to chemical corrosion are known as acid-resistant steels. Because of the difference in chemical composition between the two, the former is not necessarily resistant to chemical media corrosion, while the latter is generally non-rust. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element that gives corrosion resistance to stainless steel. When the chromium content in the steel reaches about 12%, chromium reacts with oxygen in the corrosive medium to form a thin oxide film on the surface of the steel (self-passivation film). , can prevent further corrosion of the steel substrate. In addition to chromium, the commonly used alloying elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the various requirements of the stainless steel structure and performance requirements. Stainless steel is usually divided into matrix structure: 1 ferritic stainless steel. Chromium 12% to 30%. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is superior to other types of stainless steel. 2 Austenitic stainless steel. Chromium is more than 18%, also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good overall performance, resistant to many media corrosion. 3 Austenitic-ferritic duplex stainless steel. It combines the advantages of austenitic and ferritic stainless steels and has superplasticity. 4 martensitic stainless steel. High strength but poor plasticity and weldability.