Stainless Steel
Stainless steel: grades, types and composition
Stainless steel is one of the most sought-after materials in construction, mechanical engineering, medicine, food, and chemical industries. Its popularity stems from a unique combination of key properties: strength, corrosion resistance, high performance, and durability. Due to the ability to vary its composition and structure, stainless steels come in different types, each with its own characteristics and intended uses.
Modern stainless steel grades are developed to meet specific requirements for chemical composition, physical and mechanical properties, and resistance to aggressive environments. The use of these alloys is regulated by standards, including GOSTs and international equivalents, where each grade has a designation and detailed description.
What is stainless steel: definition of the term
Stainless steel is a highly alloyed iron-based material. Its key feature is its high corrosion resistance. This metal contains additives that impart resistance to moisture, acids, alkalis, and other aggressive factors. The presence of additional elements in the composition allows for properties that prevent oxidation and degradation of the material.
The name "stainless steel" refers to the alloy's primary property—its resistance to rust. However, in reality, the degree of corrosion resistance depends on the specific grade of stainless steel and the operating conditions.
Stainless steels are classified as alloy steels because they contain additional components: nickel, molybdenum, titanium, copper, nitrogen, manganese, and others. These additives allow the alloy's properties to be significantly modified depending on its intended use.
Chemical composition of stainless steel
The alloy's composition plays a key role in determining its properties. The main components include:
- Iron.
It is the basis of any alloy. - Chromium.
This element provides corrosion resistance. The chromium content in stainless steel is at least 10.5%. - Nickel.
It improves steel's ductility and resistance to acids. - Molybdenum.
This additive enhances chloride resistance and increases the alloy's strength. - Manganese.
This element improves the structure of steel. - Nitrogen.
It increases strength and resistance to pitting corrosion. - Silicon and titanium.
These additives stabilize the structure and prevent the formation of carbides.
The combination of the above elements determines what type of stainless steel the alloy will be and what tasks it can perform.
Properties
Stainless steel is in demand due to its balanced performance characteristics. Below, we examine the key properties that determine the alloy's application in various industries.
Corrosion resistance
The main property of stainless steel is its resistance to corrosion. The presence of chromium forms a passive oxide film on the metal's surface, preventing the material from deteriorating under the influence of moisture, acids, and alkalis. Some grades of stainless steel with added molybdenum and titanium exhibit particularly high corrosion resistance in seawater, concentrated solutions, and other aggressive environments.
Strength
Stainless steels are high-strength materials that can withstand significant static and dynamic loads. Their strength level depends on the material grade, its heat treatment, and its structure. For example, martensitic and duplex steels are characterized by high hardness, making them suitable for critical mechanical structures.
Melting point
This value varies between 1400–1530°C depending on the alloy's chemical composition. Austenitic steels, which contain a large amount of nickel, have a higher melting point than ferritic or martensitic steels.
Conductivity
Stainless steel has low thermal and electrical conductivity compared to pure metals such as copper or aluminum. This property makes the alloy useful in applications requiring minimal heat transfer, including the manufacture of containers for storing liquid reagents or food products.
Magnetic properties
The response to a magnetic field depends on the steel's structure. Martensitic and ferritic steels are magnetic. Austenitic steels are non-magnetic or weakly magnetic, but can become magnetic after cold deformation. The difference in magnetic properties allows the appropriate steel type to be selected depending on the application, for example, in electronics, construction, or medicine.
Wear resistance
Alloyed stainless steels typically exhibit excellent wear resistance, especially when used under friction and abrasive loads. This property is widely used in the production of cutting tools, valves, and machine components.
Density
This figure fluctuates between 7.7 and 8.0 g/cm³. Austenitic stainless steel grades have the highest density due to their nickel content, which affects the weight of finished products and is taken into account in structural design.
Types of stainless steel
Stainless steel comes in various types, differing in composition, structure, and application. Classification depends on the type of crystal lattice and alloying components. Thus, the following are distinguished:
- martensitic steels;
- martensitic-ferritic steels;
- ferritic steels;
- austenitic steels;
- duplex steels;
- stainless steels with the addition of titanium;
- high-alloy stainless steels
Each category has unique properties that determine its scope of application. Below, we'll examine the steel grades and their characteristics.
Stainless steel grades
A grade is a standard designation for an alloy, including an alphanumeric code that reflects the chemical composition, production method, and structure of the material. In Russia, GOST designations are used, while international practice uses AISI standards.
The choice of brand depends on a number of factors. These include:
- Operating conditions. These include humidity, temperature, and chemical environment.
- Required properties include strength, wear resistance, and ductility.
- Processing method. This may include welding, stamping, or rolling.
Examples of popular brands:
- AISI 304 is a general-purpose austenitic steel with excellent corrosion resistance;
- AISI 316 - steel with molybdenum, which improves resistance to chlorides;
- 12Х18Н10Т is a domestic analogue of AISI 321, stabilized with titanium.
Classification of stainless steel
Classification is based on the microstructure of materials and determines the type of specific alloys. Let's look at the characteristics of different steel categories.
Martensitic steels
These are high-strength alloys that acquire hardness and strength through heat treatment (quenching and tempering). Their main difference from other steels is their martensitic structure, which forms during cooling. These steels possess magnetic properties and high hardness, but comparatively low corrosion resistance compared to austenitic steels.
Examples of brands:
- AISI 410;
- AISI 420;
- 20Х13, 30Х13 (according to GOST).
Application area : manufacturing of cutting tools, knives, pump parts, valves, turbine blades.
Martensitic-ferritic steels
This type combines the properties of martensitic and ferritic steels, achieving a balance between strength, ductility, and corrosion resistance. The microstructure of this steel is composed of approximately equal parts ferrite and martensite. Advantages include high strength and excellent corrosion resistance, particularly to pitting and intergranular corrosion. Weldability is moderate.
Examples of brands:
- AISI 329;
- SAF 2205;
- 08Х22Н6Т (according to GOST).
Application areas: petrochemical industry, pulp and paper production, marine engineering.
Ferritic stainless steels
These steels are composed primarily of chromium and contain little or no nickel. They have a ferritic (body-centered cubic) lattice, are magnetic, and have moderate corrosion resistance. Ferritic steels are resistant to atmospheric and alkaline corrosion, are less expensive than austenitic steels, and are easily workable under pressure. Their weldability and ductility are low.
Examples of brands:
- AISI 430;
- AISI 409;
- 08X17 (according to GOST).
Application areas: manufacturing of automotive exhaust systems, household appliances and architectural finishing elements.
Austenitic stainless steels
This is the most common and versatile type of stainless steel. These alloys contain significant amounts of nickel (8–12%) and chromium (18–20%), providing high corrosion resistance, non-magnetic properties, and good weldability. Austenitic steels exhibit excellent resistance to acids, alkalis, and seawater, and good weldability. They are less hard than martensitic steels. Disadvantages include high cost.
Examples of brands:
- AISI 304 (08Х18Н10 according to GOST);
- AISI 316 (10Х17Н13М2Т according to GOST);
- AISI 321 (12Х18Н10Т according to GOST).
These grades are considered the main ones for the production of utensils, pipes, tanks, and surgical instruments.
Application areas: food industry, medicine, pharmaceuticals, architecture, construction, chemical industry.
Duplex stainless steels
These steels consist of ferrite and austenite phases. They offer twice the strength of standard austenitic steels, better resistance to chloride corrosion, and a reduced susceptibility to cracking during welding.
Examples of brands:
- AISI 2205;
- AISI 2507;
- AISI 318;
- AISI 329.
Application areas: oil and marine industry, bridge construction, chemical tanks.
Stainless steels with added titanium
The addition of titanium provides resistance to intergranular corrosion. The element binds carbon, preventing the formation of carbides, especially during welding. This makes titanium-containing stainless steels an ideal choice for high-temperature applications and where good weldability is required.
Examples of brands:
- AISI 321 (12Х18Н10Т according to GOST).
Application areas: aviation, energy, heat exchanger production.
High-alloy stainless steels
Alloys in this category contain more than 25% alloying elements. In addition to chromium and nickel, they may include molybdenum, copper, tungsten, niobium, etc. These steels are used in extremely aggressive conditions, including contact with acids, seawater, and fluorine-containing substances.
Examples of brands:
AISI 904L (03ХН28МДТ according to GOST).
Application areas: mechanical engineering, chemical, petrochemical, pulp and paper, food industries.
GOSTs for stainless steel
In Russia and the CIS, the properties, composition, and classification of stainless steels are defined by state standards—GOSTs. These documents describe the components required in alloys, their properties, and the conditions under which they are permitted to be used. GOSTs also set requirements for the mechanical properties, heat treatment, and surface quality of steel products.
Let's look at the main GOST standards for stainless steel:
- GOST 5632-2014
— "Corrosion-Resistant, Heat-Resistant, and Heat-Resistant Alloys." This document contains the classification of steels, their chemical composition, tensile strength, and service temperature. It is the basic GOST standard for stainless steels and alloys. - GOST 7350-77
— "Corrosion-Resistant Steel." Regulates intergranular corrosion resistance testing and methods for verifying steel resistance. It is especially important for quality control of welded structures. - GOST 9940-81/GOST 9941-81
— "Corrosion-Resistant Steel Pipes." This document describes the range, tolerances, wall thickness, and quality of steel pipes used in the chemical, oil and gas, and food industries. - GOST 5582-75
— "Corrosion-Resistant Steel Sheets." This standard establishes specifications for the production of sheets of varying thickness, width, and mechanical strength. - GOST R ISO 15510-2007
is an international standard harmonized with ISO. It establishes designations for stainless steels, specifying Russian and international grades, including AISI equivalents.
Compliance with GOST standards guarantees high-quality products that meet precise requirements. When designing and manufacturing stainless steel structures, it's important to ensure that the material meets the required standard. Otherwise, there's a high risk of defects, including poor corrosion resistance, poor composition, and incorrect thickness tolerances.
Conclusion
Stainless steel is not just a single material, but an entire category of alloys with diverse properties and applications. Their unique combination of strength, corrosion resistance, and processability have made these alloys indispensable in modern industry.
The choice of the appropriate grade depends on operating conditions, strength requirements, temperature conditions, and the presence of aggressive environments. It's important to consider not only the steel's name, but also its chemical composition, GOST or AISI designation, and its specific type.
Proper classification and understanding of the differences in the composition and properties of alloys allows for the creation of durable and safe structures that meet modern standards of quality and reliability.
Steel pipe 610x36 mm
S.I.R Group supplies rolled metal tubular products with a round cross-section of 610 millimetres and a wall thickness of 36 mm for the needs of oil and gas refining, construction, utilities, and energy.
High-strength metal products with a solid surface are manufactured using hot or cold deformation and are suitable for ensuring the tightness of general-purpose and special-purpose pipelines.
Welded profiles are produced by welding sheet steel blanks using a less expensive technology, so they are cheaper than their seamless counterparts.
Technical specifications:
Our company's catalog features various metal products:
- types of rolled pipes: thick-walled, seamless, hot-deformed, electric-welded, cold-deformed, stainless steel, boiler;
- steel grades: st 20, 35, 40X, 30KhGSA, 45, 09G2S, etc.;
- regulatory documents: GOST 8732, 8734, 10705, 10706, 23270, 32528, etc.