Effect of alloying elements on the properties of steel. Types, marks and purposes of steels
Steel is one of the most sought-after materials inworld today. Without it, it is difficult to imagine any existing construction site, machine-building enterprises, and many other places and things that surround us in everyday life. At the same time, this alloy of iron with carbon is quite different, therefore this article will consider the effect of alloying elements on the properties of steel, as well as its types, brands and uses.
Today, many types of steel are widely usedpractically in any sphere of human activity. This is largely due to the fact that in this alloy the whole complex of mechanical, physical-chemical and technological properties that do not have any other materials is perfectly combined. The process of smelting steel is continuously improved and, therefore, its properties and quality make it possible to obtain the required performance indicators of the resulting mechanisms, parts and machines.
Classification according to purpose
Each steel, depending on what it is designed for, can be compulsorily listed in one of the following categories:
- Special purpose with special properties.
The most numerous class is structuralsteel, designed to create a variety of building structures, appliances, machines. Structural grades are divided into improved, cemented, spring-spring, high-strength.
Instrumental steel is differentiated independing on what tool they are made for: cutting, measuring, etc. It goes without saying that the effect of alloying elements on the properties of steel in this group is also great.
Special steels have their own division, which provides the following groups:
- Stainless (they are also corrosion-resistant).
- Heat resistant.
- Heat resistant.
Groups of steels in chemical composition
The classification of the steel is made according to the chemical elements that form them:
- Carbon steel grades.
At the same time, both these groups are further divided also by the amount of carbon they contain in:
- Low-carbon (carbon less than 0.3%).
- Medium carbon (the concentration of carbon is 0.3-0.7%).
- High-carbon (carbonate more than 0,7%).
What is alloy steel?
This definition should be understood as steel, inwhich are kept, in parallel with the constant impurities, also additives introduced into the structure of the alloy, in order to increase the mechanical properties of the material eventually obtained.
A few words about the quality of steel
This parameter of this alloy impliesa set of properties, which, in turn, are determined directly by the process of its production. To similar characteristics, which obey and alloyed tool steel, include:
- Chemical composition.
- Homogeneity of the structure.
- Mechanical properties.
The quality of any steel is directly dependent on,how much it contains oxygen, hydrogen, nitrogen, sulfur and phosphorus. Also, the method of steel production plays an important role. The most accurate from the point of view of falling into the required range of impurities is the comparison of steel smelting in electric furnaces.
Alloyed steel and its properties change
Alloyed steel, the brands of which containto its marking, the letter designation of elements forcibly introduced, changes its properties not only from these external substances, but also from their mutual action with each other.
If we consider carbon specifically, then, by interacting with it, the alloying elements can be conditionally divided into two large groups:
- Elements that form a chemical compound (carbon carbide) - molybdenum, chromium, vanadium, tungsten, manganese with carbon.
- Elements that do not create carbides - silicon, aluminum, nickel.
It should be noted that steels that are alloyed with carbide-forming substances have very high hardness and increased resistance to wear.
Low alloy steel (grades:20ХГС2, 09Г2, 12Г2СМФ, 12ХГН2МФБАЮ and others). A special place is taken by alloy 13X, which is hard enough to make surgical, engraving, jewelery equipment and razors from it.
The content of alloying elements in steel can be determined by its labeling. Each of these components introduced into the alloy has its own letter designation. For example:
- Chrome - Cr.
- Vanadium -V.
- Manganese -Mn.
- Niobium - Nb.
- Tungsten -W.
- Titanium is Ti.
Sometimes at the beginning of the steel index there are letters.Each of them has a special meaning. In particular, the letter "P" means that the steel is high-speed, "Ш" signals that the steel is ball-bearing, "A" - automatic, "E" - electrical, etc. High-quality steels have in their alphanumeric designation at the end letter "A", and especially qualitative ones contain the letter "Ш" at the very end of the marking.
Effects of alloying elements
First of all it should be said thatCarbon has a fundamental effect on the properties of steel. It is this element that, with increasing concentration, increases the strength and hardness with decreasing viscosity and plasticity. In addition, an increased concentration of carbon guarantees a deterioration in machinability by cutting.
The chromium content in steel directly affects itscorrosion resistance. This chemical element forms a thin protective oxide film on the surface of the alloy in an aggressive oxidizing environment. However, to achieve such an effect, chromium steel should be at least 11.7%.
Special attention should be paid to aluminum.It is used in the process of alloying steel to remove oxygen and nitrogen after its purging, in order to contribute to reducing the aging of the alloy. In addition, aluminum significantly increases the toughness and fluidity, neutralizes the extremely harmful effect of phosphorus.
Vanadium is a special alloying element, due to which alloyed tool steels receive high hardness and strength. In this case, the grain decreases in the alloy and the density increases.
Alloyed steel, the brands of which containtungsten, is endowed with high hardness and redness. Tungsten is also good because it completely eliminates brittleness during the planned release of the alloy.
To increase the heat resistance, magnetic properties andresistance to significant impact loads, steel is alloyed with cobalt. But one of those elements that does not have any significant effect on steel is silicon. However, in those steel grades that are intended for welded metal structures, the silicon concentration must necessarily be within the range of 0.12-0.25%.
Significantly increases the mechanical properties of magnesium steel. It is also used as a desulphurizer in the case of using out-of-service desulfurization of cast iron.
Low alloy steel (its grades containalloying elements less than 2.5%) very often contains manganese, which provides it with an indispensable increase in hardness, durability, while maintaining optimum plasticity. But the concentration of this element should be more than 1%, otherwise it will not be possible to achieve the indicated properties.
Carbon steel grades, melted for various large-scale building structures, contain copper, which provides the maximum anti-corrosive properties.
To increase the redness, elasticity,tensile strength and corrosion resistance, molybdenum is necessarily introduced into the steel, which also increases the resistance to oxidation of the metal when heated to high temperature values. In turn, cerium and neodymium are used to reduce the porosity of the alloy.
Considering the influence of alloying elements onproperties of steel, you can not ignore the nickel. This metal allows the steel to obtain excellent hardenability and strength, increase plasticity and impact resistance and lower the cold-brittle limit.
Very widely used as an alloying agentadditives and niobium. Its concentration, which is 6-10 times higher than the amount of necessarily present carbon in the alloy, allows to eliminate intercrystalline corrosion of the stainless steel and protects weld seams from extremely undesirable destruction.
Titanium allows you to get the most optimalstrength and ductility, and improve corrosion resistance. Those steels that contain this additive are very well treated with various special tools on modern metal cutting machines.
The introduction of zirconium into the steel alloy makes it possible to obtain the required grain size and, if necessary, to influence precisely the growth of the grain.
Extremely undesirable elements that are verynegatively affect the quality of steel, are arsenic, tin, antimony. Their appearance in the alloy always leads to the fact that the steel becomes very brittle over the boundaries of its grains, which is especially noticeable when looking at steel bands and during the annealing of low-carbon steel grades.
In our time, the effect of alloying elements onthe properties of steel have been fairly well studied. Specialists carefully analyzed the effect of each additive in the alloy. The obtained theoretical knowledge allows metallurgists to form a schematic diagram of steel smelting at the stage of order registration, to determine the technology and quantity of required consumables (ore, concentrate, pellets, additives and others). Most often steelmakers use chromium, vanadium, cobalt and other alloying elements, which are quite expensive.