A lot of metal disappears annually due to corrosion. However, even more damage is caused by the failure of metal products as a result of corrosion. The costs required to replace parts or to repair equipment, automobiles, sea and river vessels, equipment used in chemical production are many times higher than the cost of the material that went into their manufacture.
Significant losses are indirect. These include, for example, gas or oil leakage from pipelines damaged by corrosion, food spoilage, destruction of building structures, and much more. Therefore, the fight against metal corrosion is of paramount importance.
Why is the destruction of metallic materials?
Before turning to the question of what corrosion-resistant steel is, let's look at the concept of corrosion and the essence of this process.
Translated from the Latin corroder - erosion. Slow spontaneous destruction of metals and alloys based on them, occurring under the chemical influence of the environment, is called corrosion. The cause of this destruction is the chemical interaction (redox reactions) of metallic materials with the gaseous or liquid medium in which they are located.
What are corrosion resistant steels and alloys?
Products made of stainless and heat-resistant steel or their alloys are designed to work in aggressive environments at high or ordinary temperatures. Therefore, the main requirement for the materials of this group is heat resistance (resistance to the action of a gaseous medium or high temperature steam) or corrosion resistance (the ability to effectively resist the effects of aggressive factors at ordinary temperature).
Corrosion resistance is characteristic of metal products, on the surface of which in a hostile environment a strong passivating film is formed, which prevents the penetration of deeper layers of the metal and the interaction of aggressive substances with them.
In other words, corrosion-resistant steel is steel that is resistant to intergranular, chemical, electrochemical and other corrosion.
Chemical composition
The properties of a metal are determined by its chemical composition. When the chromium content is 12-13%, the steel becomes stainless, that is, stable in the atmosphere and chemical environments. An increase in chromium content up to 28-30% makes it stable in aggressive environments.
Other elements used for alloying include manganese, aluminum, titanium, nickel. The most widely used alloys, in which the average nickel content is 10%, chromium - 18%, carbon - from 0.08 or 0.12%, titanium - 1% (12X18H10T - corrosion-resistant steel, GOST 5632).
Classification by microstructure type: austenitic grade stainless steel
The corrosion resistance of this class increases due to the elements of alloying nickel (from 5 to 15%) and chromium (from 15 to 20%). Austenitic alloys are insensitive to intergranular corrosion, provided that the carbon content in them is less than the solubility limit in austenite (0.02-0.03% or less). Non-magnetic, well welded, cold and hot deformation. They have excellent adaptability. This is the best steel for the manufacture of fasteners, welded structures and applications in various industries.
Martensitic class
Stainless steels included in the martensitic class can be magnetic and have higher - in comparison with austenitic - indicators of maximum hardness. Hardening is achieved by hardening and tempering. Well suited for the manufacture of products intended for use in medium and low intensity environments (for example, a number of food products or the manufacture of razor blades).
Ferrite class
With high corrosion resistance, the properties of these grades are similar to low carbon steel. The average chromium content is 11-17%. They are used in the production of household appliances, elements of the architectural decor of the interior, kitchen utensils.
Austenitic-ferritic class
Corrosion-resistant stainless steels of this class are characterized by a reduced nickel content and a high chromium content (from 21 to 28%). As additional alloying elements are niobium, titanium, copper. After the heat treatment, the ratio of ferrite to austenite is approximately one to one.
The strength of austenitic-ferritic steels exceeds austenitic twice. At the same time, they are ductile, withstand shock loads well, have a low level of corrosion cracking and high resistance to intergranular corrosion. Recommended for use in construction, manufacturing, for the manufacture of products that will be in contact with sea water.
Austenitic-martensitic class
The chromium content is from 12 to 18%, nickel is from 3.7 to 7.5%. Additional elements - chrome and aluminum. Hardened by hardening (t> 975 ° C) and subsequent tempering (t = 450-500 ° C). Austenitic-martensitic stainless steels weld well and have high mechanical properties.
Stainless steel: price (factors influencing the formation)
Corrosion resistant metals include expensive alloying elements such as chromium, nickel, titanium, and molybdenum. Their value is crucial in pricing. Since other grades (carbon, structural, ball-bearing, tool, etc.) contain the listed elements in much smaller quantities, the cost of corrosion-resistant steels is always higher in comparison with them. However, the price may vary depending on market conditions and the costs required for the production of stainless steel.
Mechanical properties
Grades of stainless steels must have mechanical properties that meet the requirements of established manufacturing standards. These include:
- Brinell maximum hardness (HB);
- relative extension (%);
- yield strength (N / mm 2 );
- tensile strength (H / mm 2 ).
After production, each batch (smelting) of marketable products is checked for compliance with the mechanical properties and microstructure of the steel grade GOST. The results of laboratory testing of samples are indicated in the certificate of manufacture.
Steel grading system
An extensive range of alloys and steels is produced in various countries of the world. At the same time, there is still no unified international system for labeling them.
In the United States of America there are several signage systems. This situation, due to the large number of standardization organizations (AJS, ANSI, ACJ, SAE, AWS, ASTM, ASME), creates certain difficulties for partners, contractors and customers of metal products from American manufacturers from other countries.
In Japan, steel is marked with letters and numbers indicating their group (low-alloy, high-alloy, special-purpose alloys, medium-alloyed, high-quality, high-quality, etc.), its serial number and metal properties.
In the countries of the European Union, the designations are governed by the standard EN 100 27, which defines the procedure according to which the name and serial number are assigned.
In the Russian Federation, an alphanumeric system developed during the Soviet Union operates, in accordance with which steel grades are indicated. GOST prescribes to indicate each alloying chemical element that is part of the metal, in capital Russian letters.
For manganese, it is G, silicon - C, chromium - X, nickel - H, molybdenum - M, tungsten - B, vanadium - F, titanium - T, aluminum - Yu, niobium - B, cobalt - K, zirconium - C, boron - R.
The numbers following the letter indicate the percentage of alloying elements. If the composition of the steel contains less than 1% of the alloying element, then the figure is not put down, with a content of 1 to 2% after the letter they put 1. The two-digit number indicated at the beginning of the grade is necessary to indicate the average carbon content in hundredths of a percent within the brand composition.
Range of products manufactured from stainless steel
Corrosion-resistant steel is used to produce the following products:
- heat-treated etched and polished sheets;
- heat-treated non-etched sheets;
- thermally untreated and non-etched sheets;
- heat, cold and hot deformed seamless pipes;
- hot-rolled steel strips for general purposes;
- calibrated hexagons;
- stainless circles;
- stainless wire (heat treated and cold drawn);
- castings with special properties;
- forgings;
- other types for which GOSTs and technical instructions (TU) are developed.
Application area
Being one of the best examples of strength, aesthetics, resistance to the destructive force of corrosion and high temperatures, recyclability and durability, having excellent surface finish that meets all sanitary and hygienic requirements, corrosion-resistant steel is widely used in almost all areas of economic and economic activities.
The stainless steel is in high demand in the petrochemical, chemical, pulp and paper, food industries, in the construction industry, electric power, shipbuilding and transport engineering, in the areas of instrument making and environmental protection.
The efficiency and durability of products made of stainless steel is determined by the correct choice of its class and brand, understanding of the physicochemical properties and structure of the microstructure. Using metals that are resistant to the damaging effects of corrosion, in strict accordance with their properties, we get the opportunity to take advantage of all the undeniable advantages of modern technology.