The strength class of fasteners with metric threads made of carbon alloy and non-alloy steels in accordance with ISO 898/1-78 and at a temperature of 16-24 degrees Celsius is indicated in the marking: 4.6; 4.8; 5.6; 5.8; 8.8 (up to 16 diameters); 8.8 (over 16 diameters); 10.9; 12.9. Since bolts, screws, and studs “work” to pull out, these figures show the ratio of the first number multiplied by 100, which determines the nominal temporary resistance in N/mm², and the second number (separated by a dot from the first) multiplied by 10, which is the ratio of yield strength to temporary resistance in percent.
The product of the numbers multiplied by 10 determines the nominal yield strength in N/mm².
For example: strength class 5.8: 5 * 100 = 500 N/mm2 (minimum tensile strength); 8 * 10 = 80% (ratio of yield strength to temporary resistance);
Based on this, 5 * 8 * 10 = 400 N / mm2 is the yield strength of a bolt (screw, stud) with a strength class of 5.8.
The classification of steels by chemical composition and physical properties is called a steel grade. In different countries of the world, the following are used
different ways of marking steels. Sometimes you can find different markings for similar steel. But there are cases when
it is impossible to find an analog of steel for the current region, due to technological or other features.
Steel designations correlate with the amount of carbon in iron.
Carbon structural steel of State Standard GOST 380-88.
We use the St2 and St3 markings, where “St” stands for steel, and 2 and 3 for carbon content – 0.2% and 0.3%, respectively.
There is also a breakdown into groups where the steel undergoes the necessary controls:
A – controlled by chemical composition (carbon);
B – controlled only by mechanical properties;
B – controlled by mechanical properties and chemical composition.
In addition, steel grade A is not indicated in the name of the steel grade, while B and C are indicated by Bst3 and Bst2, respectively.
After deoxidation (with ferromanganese), the following stages are applied to the steel:
Kp is a boiling steel (e.g., BSt3Kp) that has a porous structure after the smelting cools down;
Ps – semi-quiescent (VSt2Ps), has fewer pores than boiling;
C – calm (not marked), has no time.
Only boiling water is used for high-drawing, stamped products, but they cannot be welded.
Structural carbon steel of quality GOST 1050-88.
These steels include Steel 20. (carbon content percentage of 0.2%). This group of steels is not divided into groups A, B, C; but is divided by the level of deoxidation. The carbon content is allowed from 0.08 to 0.60%. Harmful additives for this steel are sulfur (creates the effect of “red brittleness” – brittleness when heated), the permissible content is up to 0.035%; and phosphorus (causes cold resistance (at temperatures below -30 degrees Celsius, the steel loses its load-bearing capacity), up to 0.035% is allowed).
An example of the designation of this type of steel: Steel 35Kp GOST 1050-88.
Alloy steels of GOST 4543-88.
This type of steel is steel that, in addition to the usual impurities, contains elements that are specially introduced in certain quantities to ensure
required physical or mechanical properties. These elements are called alloying elements.
The formula includes markings that indicate the percentage of a particular impurity (all but boron):
Cr-X (chromium); Si-S (silicon); Mn-G (manganese); Ni-N (nickel); B-P (boron); Al-Y (aluminum); Ti-T (titanium); W-B (tungsten); V-F (vanadium).
Example of designation: 40X13 steel – 0.40% C, 0.13% Cr.