Mechanical properties of steel

Without doubt the most important properties of steels, thanks to which they find such widespread use, are their mechanical properties. These properties include a combination of very high strength with the ability to significantly change shape., eg, plastic deflection, before final destruction. To characterize strength and ductility (measure of plastic deflection) steels and other metals, various test methods have been developed.

Strength of steels

Mechanical properties of steels, like other metallic materials, are most often defined using tensile tests. Tensile testing consists of applying a tensile force to a specimen - most often, in the form of a rod - and measuring the change in the length of the sample with increasing applied force (picture 1). The sample is cut from the material or product of interest.. The test result is stretch diagram – schedule, on which vertically is laid voltage (force per unit area of ​​the sample), force per unit area of ​​the sample deformation (force per unit area of ​​the sample).

diagramma-rastyazheniyaPicture 1 force per unit area of ​​the sample
force per unit area of ​​the sample

At small deformations, the rod behaves elastically - it "springs" back to its original length, if the applied voltage is removed. When voltage higher than, which is called limit fluidity, the bar begins to deform plastically. It means, that after removing the applied stresses, the bar no longer returns to its original length, a gets irreversible lengthening. By stretching the bar to failure, the maximum stress is found in the tensile diagram. This maximum stress is called tensile strength or temporary tensile strength material, from which the sample was made.

Ductility of steels

If, in a simple bending test, the metal fails only after a large plastic deflection, then it is considered plastic. If there is no such deflection at all or it is insignificant, the material is called brittle.. Good ductility of the metal is manifested in a tensile test with a high elongation of the sample and / or its contraction.. Elongation expresses in percentage the increase in the length of the specimen after failure to its original length. (cm. picture 1). Similarly, the constriction expresses as a percentage the decrease in the area of ​​the sample compared to its original area. (picture 2).

diagramma-xrupkogo-plasticheskogoPicture 2 - Tensile diagram for brittle and ductile metals

Most often, the mechanical properties of steels as a whole are assessed by three indicators: ultimate strength, yield strength and elongation. Ultimate strength and ductility are usually expressed in megapascals. (MPa), elongation - percentage (%). Almost always, with an increase in the strength of a metal or alloy, its ductility decreases..

Hardness of steels

When hardness tests the mechanical properties of steels are assessed by introducing a solid material into it at a given force, so called indentora (picture 3). Often such an indenter is made of diamond.. As a result of the test, an imprint is formed in the material - its dimensions are judged from the hardness of the steel: in Rockwell test – by print depth, in tests by Brinell and Vickers - by its width.

ispytanie-tverdostiPicture 3 - Scheme of hardness test and its main characteristics

The ratio of strength and hardness of steels

In hardened and tempered steels, there is a good correlation between tensile strength and hardness - hardness can be used to assess strength and vice versa. For heat-hardened steels, hardness 45 HRC and above is common. Hardness 45 HRC corresponds to tensile strength 1480 MPa. Compared to the toughest industrial aluminum alloys, copper and titanium, which roughly have strength respectively 570, 1220 and 1350 MPa, it will become clear, that steel is stronger than all these materials.

Toughness of steels

An important mechanical property of steel is its toughness.. Usually the term viscosity apply, as a measure of the ability of a metal to break down non-brittle.

The nature of fracture - brittle or ductile - is good to consider on the example of ferritic steels. All metals with a body-centered cubic atomic lattice - like ferritic steels - have one common disadvantage. They break down brittle at low temperatures, whereas at sufficiently high temperatures they are destroyed normally - plastically. whereas at sufficiently high temperatures they are destroyed normally - plastically brittle-ductile transition temperature. It is defined as the temperature, below which brittle fracture occurs. The brittle transition temperature can in principle be determined by a tensile test, but under uniaxial tension, its value is much lower, than that, observed in complex steel parts. Experience has shown, that Charpy impact tests are in much better agreement with experimental data on brittle fracture of complex parts. A schematic diagram of the Charpy impact test method is shown in the figure. 4.

ispytanie-sharpiPicture 4 - Charpy Impact Test Scheme

Fatigue of steels

Fatigue destruction is a type of destruction, which happens in metal parts, which are subjected to cyclic loads.

Consider an axle on wheels, to which “presses” pretty heavy load. This weight causes a bend in the center at a point midway between the wheels., as shown schematically in the figure 5.

Picture 5 - Change of compressive and tensile stresses
on the surface of the rotating axis

This bend causes the metal to stretch at point T and contract at point C.. In other words, it means, that at point T the metal is subjected to tensile stresses, and at point C - contracting. therefore, since the axis rotates, each point in the middle of the axis is subjected to cyclic stresses - tensile, when she is down and squeezing, when at the top.

In a well-designed axis, the maximum tensile stresses will be well below the yield point and all deformations, which occur on the surface of the metal during rotation, will be in the elastic region, as shown at the bottom of the picture 5.

but, if there is a small scratch on the metal surface, then in this place of the surface the so-called stress concentration arises. If the value of stresses at this point exceeds the yield point, then a crack might start here. Every time, when the axis makes a revolution, this crack will grow, until it's big enough, to lead to the destruction of the axis. This process is called fatigue failure or steel fatigue.. The ability of steel to resist cyclic stresses is called fatigue strength or cyclic strength.. Fatigue failures occur in metal parts, which are subjected to cyclic stresses, such as rotating parts, valves, springs, as well as vibrating parts, such as airplane wings.

Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007