Stainless steel. How to choose stainless steel

A4 stainless steel in terms of low carbon content and the percentage composition of manganese, silicon, chromium and nickel is almost identical to stainless steel A2. The improved technological parameters of A4 stainless fasteners are due to the addition of 2-3 percent molybdenum to the chemical composition of steel. A4 stainless steel is resistant to the aggressive effects of salts, acids and sea water. It retains its strength characteristics at higher temperatures compared to A2 stainless steel. GOST 7805 bolts (analogues of DIN 931 standard bolts), GOST 5927 (DIN 934) nuts and GOST 11371 (DIN 125) flat washers made of A4 steel do not change their mechanical properties in the temperature range from minus 60 to plus 450 degrees Celsius. High corrosion resistance in stainless steel occurs due to a thin protective film of chromium oxide, the formation of which occurs upon contact with air. A4 bolts are completed with nuts and washers only from a similar grade of A4 steel. They are slightly more expensive than A2 stainless fasteners with similar strength and size characteristics. However, for finished stainless steel products, which include fasteners from this section of the catalog, and which are operated in contact with aggressive media, it is strongly not recommended to use A2 steel fasteners instead of bolts, washers and nuts. The analogs of A4 stainless steel are steel 10X17H13M2, AISI 316 and AISI 316L steel, which contains a lower percentage of carbon than previous grades.

A4 stainless steel in terms of low carbon content and the percentage composition of manganese, silicon, chromium and nickel is almost identical to stainless steel A2. The improved technological parameters of A4 stainless fasteners are due to the addition of 2-3 percent molybdenum to the chemical composition of steel. A4 stainless steel is resistant to the aggressive effects of salts, acids and sea water. It retains its strength characteristics at higher temperatures compared to A2 stainless steel. GOST 7805 bolts (analogues of standard boltsDIN   931), nuts GOST 5927 (DIN   934) and flat washers GOST 11371 (DIN 125) made of A4 steel do not change their mechanical properties in the temperature range from minus 60 to plus 450 degrees Celsius. High corrosion resistance in stainless steel occurs due to a thin protective film of chromium oxide, the formation of which occurs upon contact with air. A4 bolts are completed with nuts and washers only from a similar grade of A4 steel. They are slightly more expensive than A2 stainless fasteners with similar strength and size characteristics. However, for finished stainless steel products, which include fasteners from this section of the catalog, and which are operated in contact with aggressive media, it is strongly not recommended to use A2 steel fasteners instead of bolts, washers and nuts. The analogs of A4 stainless steel are steel 10X17H13M2, AISI 316 and AISI 316L steel, which contains a lower percentage of carbon than previous grades.

Fasteners of high accuracy class A from A4 stainless steel, in addition to resistance to acidic environments, have such remarkable properties as hygiene and lack of toxicity. In addition, austenitic steel of this brand can be easily machined and polished. On polished products, the protective layer of chromium oxide is restored much faster than on parts with mechanical defects. A4 stainless steel is non-magnetic and easy to weld due to its low carbon content and the absence of additional coatings. Welds are easy to clean and polish. Fasteners and hardware made of steel A4, as well as steel A2, can not be painted with organic dyes.

The strength characteristics of fasteners made of A4 stainless steel have three values \u200b\u200bequal to 50, 70 and 80 kG / mm 2. The marking of bolts and nuts made of austenitic stainless steel of the specified brand is carried out by applying the manufacturer’s trademark (brand) and the symbol of the tensile strength, which looks, for example, like this: A4-70 or A4-80.

The Mashpropezh company, one of the leading suppliers of fasteners, sells bolts, washers and nuts from A4 stainless steel to its customers in large and small wholesale lots. We offer to buy stainless steel fasteners in factory packaging weighing 5 kilograms (bolts and nuts) and weighing 1 kilogram (washers).

Stainless steel at the dawn of its history, corrosion-resistant properties gave rise to many myths among people, and its products were considered miraculous, as they were timeless. What this alloy represents today, we will discuss in the article.

1

Today, stainless steels are a rather large set of alloys with various properties that are described by numerous GOSTs and TUs. But they share one common property - resistance to moisture and oxygen, the main enemies of iron-containing materials. To achieve this "survivability" allows a special chemical composition. All types of this alloy contain more than 10% chromium in their composition, which easily starts the passivation process on the steel surface.

Stainless steel pipes

The inactivity of the stainless steel surface is explained by the thinnest layer of the oxide film that chromium forms under the influence of oxygen. This protects the product from any other interaction, including water - the main activator. Moreover, the charm of such a composition is that even if the integrity of the surface is violated, such a layer very quickly reappears. For example, if a chip or deep scratch is formed, then chromium, evenly present in the entire volume of steel, will again react with oxygen and create a protective film. This kind of healing effect.

But stainless steels have a weak spot due to their unusual properties. In oxygen-free environments or environments with a low content of this oxidizing agent, a layer of chromium oxide will form slowly and unevenly, which will necessarily be reflected by the appearance of foci of corrosion. Also, a simple violation of production technology can become the cause of material damage. Then corrosion is called crevice. It also happens to be of an electrochemical nature, therefore, it is also not necessary to discount the danger of interaction with other metals and salt medium (for example, sea water).

Despite the enviable properties, stainless steels can change their quality depending on the alloying elements. For example, alloy can be strengthened with sulfur, but to the detriment of anticorrosion abilities, and nickel will increase immunity to acidic media. The same properties are given to steel by additives from Mn (manganese), Mo (molybdenum), Cu (copper) and other metals of this family. More exotic metals like Ti (titanium), Nb (niobium) or Ta (tantalum) will make the alloy more heat resistant.

2

By structure, chromium stainless steels are divided into 5 types, 3 - ferritic (F), martensitic (C) and austenitic (A) are of interest to the general user. The first variety contains little carbon, so it is softer and may have magnetic properties. The second hardest, less resistant to corrosion, can also act as a magnetic material. Application is suitable for tableware, cutting tools and in some areas of mechanical engineering. Austenintic is considered the most popular. It is a non-magnetic alloy with a high content of chromium (almost 20%) and nickel (up to 15%), resistant to corrosion. Such stainless steel can be processed and is used in a large number of industrial tasks and for the manufacture of fasteners.

Stainless steel alloys

According to domestic GOST, ferritic stainless steel is often designated 12X17, it is a heat-resistant variety, but poorly welded. Therefore, it is mainly rolled products, pipes or rods that are made from it, and a sheet form of release is also found. For each product, you can find the corresponding GOST with quality requirements. To access all the characteristics of stainless steels, you can use GOST 5632–72. All types of alloy with a brief description of the application, chemical composition and physical properties can be found in this document alone. It is better to look at more detailed data and special instructions in separate GOST, which almost every brand of stainless steel has. Martensitic types of anticorrosive steel are recognized leaders in strength due to the most stable structure. They also have a peculiar metal memory from a technological point of view. Quite often, such steel is labeled as heat-resistant.

As can be seen from GOST 5632–72, it is presented most widely, this is a very diverse list of alloys both in composition and properties, but they are all heat-resistant and very resistant to corrosion. These are the so-called stainless steel 300 series. Such steel is universal, so it is so popular in the market. We will discuss its types in a separate paragraph.

3

Steel A1 is characterized by a high sulfur content, this leaves a certain imprint on its corrosion resistance, although it is very heat-resistant, sometimes the threshold reaches 1000–1100 ° C. True, it is important to monitor the acidity of the medium, the atmosphere should be reducing, and the sulfur there should not be more than 2 g / 2 m. Elements of such steel are used in the manufacture of alkalis or hydrogenation, of course, all installations for thermal operations are made of them (furnaces, branch pipes of motors and turbines, cracking units, reforming machines). Doors, pins and brackets for furnaces are also made of such an alloy.

A4 steel items

A2 steel is easily welded, without losing strength. Like all types discussed, it resists corrosion well, does not contain toxins, and does not exhibit magnetic properties. Although the last statement can be corrected if properly processed product. This is how magnetized washers and screws get. This is a fairly common steel grade, but it is not acid resistant, so it will not work to use fasteners made of this alloy in a pool where there is a lot of chlorine or salt water. According to GOST 5632–72, A2 steel products do not lose strength at low temperatures up to -200 ° C.

Within this type, there are several analogues with different but substantially low carbon contents. These steels are resistant to intercrystalline corrosion (hidden from the human eye and detected already in the later stages), which is why they are leading in those industries where this property is important. Mostly products from A2 you will find in installations for light, chemical, and in pharmaceutical and plastics manufacturing. Also, GOST 5632–72 allows the equipment of catering units with steel materials, for example, kitchens, restaurants, bars.

Steel A3 is very similar in characteristics to A2, but has useful alloying additives (Ti, Nb, Ta), because of this it is more heat-resistant than the previous variety. Even at high temperatures, the product is able to not lose quality and not be covered by corroding corrosion. Such alloy withstands decent quality up to 800 ° C. Therefore, it is often used for chemical equipment, in boiler bodies, as compensation compounds.

A4 steel is the most acid resistant. Its composition is slightly different from A2, mainly due to the presence of molybdenum in a small amount (about 2-3%). But even this small amount makes it less susceptible to intergranular corrosion, even in aggressive environments. Products from A4 can maintain their properties at a decent level - up to -60 ° C in the negative range and up to 450 ° C in the positive. Under this marking there are also various combinations of steels in chemical composition, more detailed proportions that such an acid-resistant grade may have can be found in GOST 5632–72. Alloy A4 is the main contender for use in the food and chemical industries. A tool is made from it, which will have contact with sea water. You can also find quite often all kinds of hardware from A4 steel. The reason for their popularity is versatility, they are not afraid of either water or acids and are quite durable.

Under the A5 brand, a certain averaged version is assembled between A4 and A3, so the properties obtained are the same in demand. This steel is heat resistant and can withstand aggressive chemical environments, that is, it can also act as acid resistant.

Special chemical processes inside the crystal lattice create a strong immunity to intergranular corrosion. The scope of products from such steel is similar to the description of A4. The marking is expressed according to the DIN standard, but almost every country with a large steel industry has its own standard, summary tables can be found on the open spaces of the network. Also, in each type of stainless alloy there is a more fractional separation - the steels are marked based on the composition and proportions of the elements included in it. This is clearly seen in GOST 5632–72, which lists a huge number of brands and their analogues.

4

Since the set of stainless alloys is huge, and it’s rather difficult to navigate them, you need to know some special markings. For example, high-strength 16X16H3MAD is most often used in the aviation industry. It is capable of not suffering from corrosion and can do this under harsh environmental conditions and element loads. Such a high-strength brand is also used for cables for bridges and building structures. In view of the responsibility assigned to products made of this material, all rolled metal has a lot of requirements set forth in numerous OST, GOST and TU.

Steel 16X16H3MAD

Acid-resistant steel is also a very important component of industrial and not only plants, and mistakes in its choice can also be very expensive. And the danger here is that chemicals are able to destroy the alloy secretly, at the level of the crystal lattice, and will open to the human eye even when an accident occurs. The most typical representative of such steels is 10X17H13M2T. This acid-resistant brand has several close analogues - 15X25T, 08X22N6M2T.

Stainless steel  - This steel is resistant to aggressive and acidic environments. Steel grades A1, A2, A4 belong to the group of austenitic steels.
Austenitic steel  It is a chromium-nickel alloy that also contains carbon, silicon, manganese, phosphorus, sulfur and molybdenum. It has good plastic properties, which is a big plus in its processing and use for the production of fasteners. Good anti-corrosion properties of stainless steel are ensured by the chromium content in the alloy. Its weight fraction is more than 12%. Due to this, a passive film is formed on the surface of the product, consisting of oxides and other insoluble materials. Depending on the requirements for the formation of a protective film, the content of chromium and other components may be increased. In addition to chromium, nickel is an equally important constituent element of stainless steel. The increase in its content provides higher stability of the alloy.

A2 grade steel  subjected to heat, as the alloy contains a minimum amount of sulfur. Domestic analogue of steel is steel grade 10X17H13M2T according to GOST 5632-72.

A4 steel  possesses the highest acid resistance, due to which it is possible to use fasteners made of A4 steel in aggressive oxidizing environments, in particular in nitric acid with a strength of up to 50%. Domestic analogue of this steel grade is steel grade 12X18H9T, in accordance with GOST 5632-72.

Chemical composition

Mechanical properties

Designation example

steel A2-70
A2  - steel grade
70   - strength class

The advantage of stainless steels

• non-toxicity, which allows them to be used in the food industry, as well as in medicine;
• stainless steel is non-magnetic;
• acid resistance;
• resistance to corrosion.

  Stainless steel application

It is traditionally used in pump engineering as a standard grade of stainless steel, characterized by high strength properties and heat resistance. In addition, this material is very resistant to organic solutions.

See also

Austenitic steels contain 15-26% chromium and 5-25% nickel, which increase corrosion resistance and are practically non-magnetic.

It is austenitic chromium-nickel steels that exhibit a particularly good combination of machinability, mechanical properties and corrosion resistance. This group of steels is most widely used in industry and in the manufacture of fasteners.

The austenitic steels are indicated by the initial letter "A"  with an additional number that indicates the chemical composition and applicability within this group:

From the warehouse we deliver products from the following steels:

Steel A2 (AISI 304 \u003d 1.4301 \u003d 08X18H10)  - non-toxic, non-magnetic, non-hardening, corrosion-resistant steel. Easy to weld and does not become brittle. May exhibit magnetic properties as a result of machining (washers and some types of screws). This is the most common group of stainless steels. The closest analogues are 08X18H10 GOST 5632, AISI 304 and AISI 304L (with reduced carbon content).

Fasteners and A2 steel products are suitable for use in general construction works (for example, when installing ventilated facades, stained glass structures from aluminum), in the manufacture of fencing, pumping equipment, instrument making from stainless steel. steel for oil and gas, food, chemical industry, shipbuilding. It retains strength properties when heated to 425 ° C, and at low temperatures to -200 ° C.

A4 steel (AISI 316 \u003d 1.4401 \u003d 10X17H13M2) - differs from A2 steel by the addition of 2-3% molybdenum. This greatly increases its ability to resist corrosion and acid. A4 steel has higher anti-magnetic characteristics and is absolutely non-magnetic. The closest analogues are 10X17H13M12 GOST 5632, AISI 316 and AISI 316L (low carbon content).

Fasteners and rigging made of A4 steel are recommended for use in shipbuilding. Fasteners and A4 steel products are suitable for use in acids and environments containing chlorine (for example, in pools and salt water). It can be used at temperatures from -60 to 450 ° C.

Strength classes

All austenitic steels (from “A1” to “A5”) are divided into three strength classes, regardless of grade. The lowest strength are steel in the annealed condition (strength class 50).

Since austenitic steels are not hardened, they have the greatest strength in the cold-deformed state (strength classes 70 and 80). The most widely used fasteners are made of A2-70 and A4-80 steels.

The main mechanical properties of austenitic steels:

The main mechanical properties of bolts made of A2 and A4 steels of various strength classes:

Estimated values \u200b\u200bof tightening torques and pre-tightening forces for screws made of stainless and acid-resistant steel - A2 / A4:

The chemical composition of stainless steel:

1) Maximum values, unless otherwise specified.
  2) Sulfur can be replaced by selenium.
  3) If the mass fraction of nickel is below 8%, then the mass fraction of manganese should be at least 5%.
  4) There is no minimum limit for the mass fraction of copper if the mass fraction of nickel is more than 8%.
  5) Molybdenum is allowed at the discretion of the manufacturer. If a limitation of the molybdenum content is necessary for certain applications, this should be indicated by the client.
  6) Molybdenum is also allowed at the discretion of the manufacturer.
  7) If the mass fraction of chromium is below 17%, then the mass fraction of nickel should be at least 12%.
  8) In austenitic steel with a mass fraction of carbon of a maximum of 0.03%, nitrogen should be a maximum of 0.22%
  9) For stabilization, titanium ≤ 5xC must be contained up to a maximum of 0.8% and be designated in accordance with this table or niobium and / or tantalum ≤ 10xC up to a maximum of 1% and shall be designated in accordance with this table.

Austenitic chromium-nickel steels exhibit a particularly good combination of machinability, mechanical properties and corrosion resistance. Therefore, they are recommended for many applications and are the largest group of stainless steels. The most important property of this group of steels is its high corrosion resistance, which increases with increasing content of alloying, especially chromium and molybdenum.

A2 and A4 stainless steels. Practical application and structural features. A2 steel and A4 steel is the shortened name for Austenitic stainless steel grades. Austenitic steel has a number of remarkable properties that have provided it with very widespread use in the national economy. Steel A2 and A4 are non-toxic and corrosion resistant. They are well subjected to mechanical and heat treatment, as well as welding. Fasteners made of A2 steel and A4 steel are practically non-magnetic, strong and durable. They perfectly retain their properties at high and low temperatures.

A2 steel has a domestic analogue - stainless steel grade 08X18H10 and a foreign analogue - stainless steel grade AISI 304 (in the USA). Assembly units, parts and fasteners from A2 steel are used in the oil, food, chemical and gas industries; in instrument making and shipbuilding; in construction during the installation of ventilated facades and stained glass structures, as well as in the manufacture of pumping equipment. Products made from A2 steel retain their strength properties in a wide temperature range: from low (-200 degrees Celsius) to high (+425 degrees Celsius).

Steel A4 is similar in its characteristics to steel A2, but its scope has expanded significantly due to the addition of 2-3% molybdenum, which contributes to its higher resistance to corrosion in environments containing acids, salts and chlorine. A4 stainless steel products retain their strength properties at low (up to -60 degrees Celsius) and at high (up to +450 degrees Celsius) temperatures. These products are used: in the chemical industry, where they are exposed to aggressive environments; in shipbuilding (fasteners and rigging products) to protect against the damaging effects of sea water; in pools containing chlorinated water. A4 steel, like A2 steel, also has a domestic analogue - steel of the type 10X17H13M2 and a foreign analogue - AISI 316 steel (in the USA).

Steel A2 and steel A4 are excellent for the manufacture of fasteners with a high accuracy class A, which are used to create strong and durable critical joints. Bolts and nuts of this class are made, for example, on lathes with numerical control (CNC). The difference between the diameters of the thread, external for the bolt and internal for the nut, after finishing on the machine does not exceed 0.25 ... 0.3 mm. However, the price of parts made of stainless steel will be significantly higher than that of parts made of ordinary carbon steel. Strength class for bolts made of austenitic stainless steel grade A2 and steel grade A4 is 50, 70 or 80.