Stainless steel

Stainless steel is a modern material with a 60-year history of development

Since the invention of stainless steel at the beginning of this century, stainless steel has changed the image of modern materials and architectural applications. Its reputation for excellence is the envy of its competitors.

As long as the steel type is selected correctly, processed properly, and maintained properly, stainless steel will not cause corrosion, pitting, rust or wear. Stainless steel is also one of the strongest materials among metal materials used in construction. Because stainless steel has good corrosion resistance, it allows structural components to permanently maintain the integrity of their engineering design. Chromium-containing stainless steel also combines mechanical strength and high elongation, making it easy to process and manufacture components and meeting the needs of architects and structural designers.

In the construction, building and structural industries, the key to the success of stainless steel is its good corrosion resistance.

Stainless steel grade grouping

200 series—Chromium-nickel-manganese austenitic stainless steel

300 series—Chromium-nickel austenitic stainless steel

Model 301—good ductility, used for molded products. It can also be hardened quickly through mechanical processing. Good weldability. Wear resistance and fatigue strength are better than 304 stainless steel.

Model 302—The corrosion resistance is the same as 304, but the strength is better due to the relatively higher carbon content.

Model 303 - By adding a small amount of sulfur and phosphorus, it is easier to cut than 304.

Model 304—universal model; i.e. 18/8 stainless steel. The GB grade is 0Cr18Ni9.

Model 309—has better temperature resistance than 304.

Model 316—the second most widely used steel type after 304, mainly used in the food industry and surgical equipment. The addition of molybdenum gives it a special structure that resists 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. Grade 18/10 stainless steel is also generally qualified for this application grade. [1]

Model 321—Similar to 304 except that the addition of titanium reduces the risk of material weld corrosion.

400 Series—ferritic and martensitic stainless steels

Model 408—good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.

Model 409—the cheapest model (British and American), commonly used as automobile exhaust pipes, is a ferritic stainless steel (chromium steel).

Model 410—martensitic (high-strength chromium steel), good wear resistance, poor corrosion resistance.

Model 416—Added sulfur improves the material's processing properties.

Model 420—"cutting grade" martensitic steel, similar to Brinell high chromium steel, the earliest stainless steel. Also used in surgical knives, which can be made very shiny.

Model 430—ferritic stainless steel for decorative purposes, such as automotive accessories. Good formability, but poor temperature resistance and corrosion resistance.

Model 440—high-strength cutting tool steel, with slightly higher carbon content. After appropriate heat treatment, it can obtain higher yield strength, and its hardness can reach 58HRC, which is among the hardest stainless steels. The most common application example is "razor blades". There are three commonly used models: 440A, 440B, 440C, and 440F (easy-to-process type).

500 Series—Heat-resistant chromium alloy steel.

600 Series—Martensitic precipitation hardening stainless steel.

Model 630—the most commonly used precipitation-hardened stainless steel model, usually also called 17-4; 17Cr, 4Ni.

Why is stainless steel resistant to corrosion?

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, allowing the rust to expand and eventually form holes.

Carbon steel surfaces can be protected by electroplating with paint or oxidation-resistant metals (such as zinc, nickel, and chromium), but, as we all know, this protection is only a thin film. If the protective layer is damaged, the steel underneath begins to rust.

The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different.

When the added amount of chromium reaches 10.5%, the atmospheric corrosion resistance of steel increases significantly. However, when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that alloying steel with chromium changes the type of surface oxide to one similar to that formed on pure chromium metal. This tightly adherent chromium-rich oxide protects the surface from further oxidation. This kind of extremely thin oxide layer, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself and re-form the "passivation film" to continue to protect.

Therefore, all stainless steels have the same characteristics, that is, the chromium content is above 10.5%.

Types of stainless steel

The word "stainless steel" does not simply refer to a kind of stainless steel, but refers to more than one hundred industrial stainless steels. Each stainless steel developed has its own characteristics. Good performance in specific application areas. The key to success is to first understand the application and then determine the correct steel grade. Further details on stainless steel can be found in the "Stainless Steel Guide" floppy disk produced by NiDI.

Fortunately, there are usually only six types of steel relevant to building construction applications. They all contain 17 to 22% chromium, and the better steel types also contain nickel. Adding molybdenum can further improve the atmospheric corrosion resistance, especially the corrosion resistance of chloride-containing atmosphere.

Atmospheric corrosion resistance

Experience shows that the degree of atmospheric corrosion varies from region to region. For ease of explanation, it is recommended to divide regions into four categories, namely: rural, urban, industrial areas and coastal areas.

Rural areas are basically pollution-free areas. The area has low population density and only non-polluting industries.

Cities are typical residential, commercial and light industrial areas with mild pollution, such as traffic pollution.

Industrial areas are areas where heavy industry causes air pollution. Contamination may be due to gases formed from fuel oil, such as sulfur and nitrogen oxides, or other gases released from chemical plants or processing plants. Particles suspended in the air, such as dust from the steel production process or deposits of iron oxide, can also increase corrosion.

Coastal areas generally refer to areas within one mile of the sea. However, the oceanic atmosphere can spread deep inland, especially on islands, where prevailing winds come from the ocean and the climate is harsh. For example, climate conditions in the UK are such that the entire country is coastal. Conditions in coastal areas are even more unfavorable if the wind is mixed with marine fog, especially if salt deposits accumulate due to evaporation, and if there is little rain and they are not regularly washed by rainwater. If there is industrial pollution, it will be even more corrosive.

Research work conducted in the United States, the United Kingdom, France, Italy, Sweden and Australia has determined the impact of these areas on the atmospheric corrosion resistance of various stainless steels. This content is briefly introduced in the "Architect's Note" published by NiIDI, and the tables in the book can help designers choose the most cost-effective stainless steel for various areas.

When making a selection, it is important to determine whether there are any local factors that affect the use of the site environment. For example, if stainless steel is used under factory chimneys, near air-conditioning exhaust baffles or near scrap steel yards, unusual conditions will exist.

Maintenance and Cleaning

Like other materials exposed to the atmosphere, stainless steel can get dirty. Future lectures will analyze design factors that affect repair and cleanup costs. However, there is an interrelationship between rain wash, manual wash and soiled surfaces.

The effect of rain washout was determined by placing the same slats directly in the atmosphere and in a covered area.

The effectiveness of manual flushing is determined by manually scrubbing the right side of each slat every six months with a sponge and soapy water. It was found that the removal of dust and siltation on the surface through rainwater washing and manual scrubbing had a good effect on the surface condition compared with slats placed in covered places and in places where they were not washed. It was also found that the surface processing conditions also have an impact. Slats with a smooth surface are better than those with a rough surface.

Therefore, the washing interval is affected by many factors, the main influencing factor being the required aesthetic standards. Although many stainless steel curtain walls are only washed when the glass is cleaned, generally speaking, stainless steel used for exterior use is washed twice a year.

Typical uses

Most uses require maintaining the original appearance of the building for a long time. When determining the type of stainless steel to choose, the main considerations are the required aesthetic standards, the corrosive nature of the local atmosphere, and the cleaning regime to be adopted.

However, other applications increasingly seek structural integrity or impermeability. For example, roofs and side walls of industrial buildings. In these applications, the owner's cost of construction may be more important than aesthetics, and the surface may not be very clean.

The effect of using 430 stainless steel in dry indoor environments is quite good. However, maintaining its appearance outdoors in the countryside and cities requires frequent cleaning. In heavily polluted industrial areas and coastal areas, the surface will be very dirty and even rusty. But to achieve aesthetic results in outdoor environments, nickel-containing stainless steel is required. Therefore, 304 stainless steel is widely used in curtain walls, side walls, roofs and other construction purposes. However, in severely corrosive industries or marine atmospheres, it is best to use 316 stainless steel.

Now, people have fully realized the advantages of using stainless steel in structural applications. There are several design guidelines that include 304 and 316 stainless steel. Because "duplex" stainless steel 2205 has integrated good atmospheric corrosion resistance with high tensile strength and elastic limit strength, this steel is also included in the European standards.

Product Shape

In fact, stainless steel is manufactured in fully standard metal shapes and sizes, and there are also many special shapes. The most commonly used products are made of thin plates and strips, but medium and thick plates are also used to produce special products, such as the production of hot-rolled structural steel and extruded structural steel. There are also round, oval, square, rectangular and hexagonal welded pipes or seamless steel pipes and other forms of products, including profiles, bars, wires and castings.

Surface Conditions

As will be discussed later, a variety of commercial surface finishes have been developed to meet the aesthetic requirements of architects. For example, the surface can be highly reflective or matte; it can be glossy, polished or embossed; it can be tinted, colored, electroplated or patterned on the stainless steel surface to satisfy the designer's desire for appearance. various requirements.

Maintaining appearances is easy. Dust can be removed with just an occasional rinse. Due to its good corrosion resistance, surface graffiti contamination or other similar surface contamination can also be easily removed.

Design

For more than sixty years, architects have chosen stainless steel to build cost-effective permanent buildings. Many existing buildings illustrate the correctness of this choice. Some are very ornamental, such as the Chrysler Building in New York City. But in many other applications, stainless steel plays a less dramatic role, but plays an important role in the aesthetics and performance of the building. For example, because stainless steel is more resistant to wear and indentation than other metal materials of the same thickness, it is the material of choice for designers when building sidewalks in high-traffic areas.

Stainless steel has been used as a structural material in the construction of new buildings and in the restoration of historic monuments for more than 70 years. Early designs were calculated on first principles.

Today, design specifications such as the American Society of Civil Engineers' standard ANSI/ASCE-8-90 "Design Specification for Cold-Formed Structural Stainless Steel Members" and the "Design Manual for Structural Stainless Steel" published jointly by NiDI and Euro Inox have simplified long service life, Design of structural components for buildings with good integrity.

Future Outlook

Because stainless steel already possesses many of the ideal properties required for building materials, it can be said to be unique among metals, and its development continues. Existing types are constantly being improved to make stainless steel perform better in traditional applications, and new stainless steels are being developed to meet the stringent requirements of advanced construction applications. Due to increasing productivity and quality improvements, stainless steel has become one of the most cost-effective materials chosen by architects.

Stainless steel combines performance, appearance and usage characteristics, so stainless steel will remain one of the best building materials in the world.

Identification method of stainless steel

Numbering and representation method of steel

①Use international chemical element symbols and national symbols to indicate chemical composition, use Arabic letters to Indicates ingredient content:

Such as: China, Russia 12CrNi3A

②Use fixed-digit numbers to express steel series or numbers; such as: United States, Japan, 300 series, 400 series, 200 series;

③Use Latin letters and sequence to form the serial number, which only indicates the purpose.

my country’s numbering rules

①Use element symbols

②Use, Chinese pinyin, open-hearth steel: P, boiling steel: F, killed steel: B, Class A steel: A, T8: Special 8,

GCr15: Balls

◆Combined steel, spring steel, such as: 20CrMnTi 60SiMn, (C content expressed in parts per ten thousand)

◆Stainless steel and alloy tool steel (C content expressed in thousandths), such as: 1Cr18Ni9 thousandth (i.e.

0.1C), stainless steel C≤0.08 such as 0Cr18Ni9, ultra-low carbon C≤0.03 such as 0Cr17Ni13Mo

International stainless steel labeling method

The American Iron and Steel Institute uses three digits to label various standard grades of forgeable stainless steel. Among them:

①Austenitic stainless steel is represented by 200 and 300 series numbers,

②Ferrite and martensitic stainless steel 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 steels are marked with 430 and 446, Martensitic stainless steel is marked with 410, 420 and 440C

and is dual-phase (austenite-ferrite).

④ Stainless steel, precipitation hardening stainless steel and iron-containing stainless steel High alloys with a weight below 50 are usually named by patented names or trademarks.

4). Classification and classification of standards

4-1 classification:

①National standard GB

②Industry standard YB

③Local standards

④Enterprise Standard Q/CB

4-2 Category:

①Product Standard

②Packaging Standard

③Method standards

④Basic standards

4-3 standard level (divided into three levels):

Level Y: International advanced level

Level I: International General Level

Level H: Domestic Advanced Level

4-4 National Standard

GB1220-84 Stainless Bar (Level I)

GB4241-84 Stainless welding disk (Grade H)

GB4356-84 Stainless welding disk (Grade I)

GB1270-80 Stainless Pipes (Grade I)

GB12771-91 Stainless welded pipe (Grade Y)

GB3280-84 Stainless cold plate (Grade I)

GB4237-84 Stainless hot plate (Grade I)

GB4239-91 Stainless cold strip (Grade I)

Stainless steel

Stainless acid-resistant steel is referred to as stainless steel. It is composed of two parts: stainless steel and acid-resistant steel. In short, steel that can resist atmospheric corrosion is called stainless steel, while steel that can resist corrosion by chemical media is called acid-resistant steel. Generally speaking, steel with a Wcr content greater than 12 has the characteristics of stainless steel. Stainless steel can be divided into five categories according to its microstructure after heat treatment: ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic stainless steel, and austenitic stainless steel. Text on ferritic stainless steel and precipitation carbonized stainless steel

After acidification, whether to polish or frost

Only good materials can be polished to a mirror surface.

One refers to titanium mirror

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