Archival Expert Content: the article preserves historical designations, tables, and claims about the composition and mechanical properties of steel, but is not a valid standard, material specification, certificate, structural calculation, or heat treatment and testing instruction. Design values depend on the accurately identified type and batch of steel, manufacturing condition, thickness, temperature, weldability, toughness, fatigue and applicable standard. The bearing material must have traceable documentation and be selected and checked by a responsible expert.
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Composition and historical markings of construction steel
Crude iron is obtained by melting iron ores mixed with fuels, and with the addition of admixtures - usually limestone - as well as by introducing heated air under pressure into the blast furnace. Pig iron contains over 1,7% carbon; it cannot be forged, rolled, chiseled or pressed - therefore, it cannot be shaped in various ways. Wrought iron is produced from pig iron by blowing air in a basic converter, or from scrap iron and pig iron in a furnace (Siemens-Martin furnace). Various forms can be created from it by rolling, pressing and forging. All wrought iron is called steel.
Historical Tables: The old steel designations, standards, allowances and bending procedures in the tables shown must not be used to source, identify or size today’s material. Only the label and documentation of the delivered batch are valid, with a check of equivalence according to the agreed and valid standard.
Chemical composition of construction steel
Technical iron is not pure iron, but an alloy of the basic elements of iron and carbon, usually with the addition of manganese, silicon, phosphorus, sumbor and copper, and less often with aluminum, chromium, nickel, molybdenum, etc. The most important admixture is carbon. The yield strength and tensile strength of molten steel increase with increasing carbon content until about 0,9% carbon is reached; elongation at break decreases as the carbon content increases. Steel with a high carbon content must be handled more carefully when straightening, bending, buckling, etc.
Construction steels St 37.12 as a rule contain 0,08-0,15% carbon, then about 0,5% manganese, up to 0,3% silicon. The content of phosphorus and sulfur must be low; it is respectively 0,09% and 0,05%. As a rule, copper has more than 0,2%.
When evaluating the results of chemical analyses, care must be taken as to whether they are analyzes from melts or analyzes of valid goods; for the latter, it is important whether the sample data refer to samples originating from the entire section, or to samples taken from a specific place (from the core, from the edge, from one end).
Steel structure, hardening, tempering and annealing
Observation of the structure is carried out on flat, carefully processed, ground and polished surfaces which, as a rule, are corroded by acids. In certain cases (eg on superheated steel), steel grains can be recognized with the naked eye on prepared samples.
Figure 1 shows the structure of slowly cooled steel containing about 0,1% carbon, and Figure 2 shows the structure of slowly cooled steel containing about 0,3% carbon. Light grains are carbon-free and consist mainly of pure iron (hence the name ferrite); under high magnification, the dark areas appear striped (perlite). Under the microscope, it can be recognized that the dark spots consist of harder and softer layers; the harder layers are iron carbide (Fe3C), called cementite, embedded in the softer ferrite. Locals - crystals formed at high temperature disintegrate upon gradual cooling. Sudden cooling of the hot steel prevents the crystals from disintegrating. If, on the other hand, sudden cooling is carried out before the transformation of the solid solution begins, then the solution - which otherwise can only exist at high temperatures - will continue to be maintained at room temperature. In this case, we do not obtain a material whose carbon is distributed in the form of spots, but a material with evenly distributed carbon. Steel in that condition shows a far higher yield strength and higher tensile strength than slow-cooled steel of the same chemical composition, but it also shows less machinability; such steel is called hardened steel.
It is natural that the cooling effect is most pronounced on the surfaces, while it weakens towards the interior, which is of great importance for the use of steel. Hardened steel contains high internal stresses.
Laboratory and thermal processes: metallographic acid etching, heating, tempering and annealing require a controlled laboratory or workshop, identified material, appropriate equipment and protection. Improper heat treatment can cause cracks, deformations, loss of toughness and high residual stresses.
Tensile behavior of steel
1. Ordinary structural steels in rolled condition
From the tensile test, in the first place the modulus of elasticity E can be determined for both total and elastic elongations. The modulus of elasticity of steel is 200 GPa.
Permanent elongations occur at low voltages, but they are of no particular importance in construction. Ordinary steel begins to “flow” under loads that for commercial construction steel St 37 and concrete steel I mostly exceed 0,24 GPa, and in large sections this value can go down to 0,18 GPa. As a result of the flow, large permanent deformations and “yield lines” occur. The line of the steel elongation diagram (picture below) shows the exceeding of the yield strength (in tension loading, this limit is also called the yield strength).
When, in the case of reinforced concrete beams, the stretching limit of the reinforcement is exceeded in the tension zone, significant elongations occur in the steel, whereby cracks and bends are created in the tension zone of the beam, which impairs the load-bearing capacity of the beam. On steels with a high carbon content, on hardened steels and in general on non-ferrous metals, the characteristic yield point shown in the picture above does not appear; permanent elongations generally increase, and there is no distinct yield point. In such cases, as a rule, the yield strength is assumed to be equal to the load at which a permanent elongation of 0,2% occurs.
Values are not design data: modulus of elasticity, yield strength, strength and elongation in the text are transferred from a historical source. For the calculation, the characteristic and design values of the exact modern class, thickness and delivery condition are taken, along with the relevant partial factors, tolerances and requirements for toughness, weldability and fatigue.
2. Cold drawn steels
In every steel bar that is straightened or bent, locally cold drawn steel occurs. In doing so, the steel is locally stressed beyond the yield point; on the drawn part, the drawing limit is increased. If a plain steel rod is loaded in tension until the yield strength is exceeded, on reloading it is observed that the value of the yield strength has increased. The longer the “pause” time interval between two loads, the more the yield strength will increase.
In addition, since during cold stretching, the diameter also decreases in proportion to the elongation of the bar, the steel that has been exposed to significant stretching before the test will show a significantly higher yield strength and tensile strength than steel that has not been stretched; at the same time, it will show less impact resistance.
Cold drawing is used in the production of concrete steels IIb, IIIb, and IVb (tor-steel, steel construction mesh), as well as in the production of wires for prestressed reinforcement and ropes of suspension bridges.
Cold Working and Reinforcement: Historical grades and descriptions of cold drawing are not indicative of modern product. Bending, straightening, re-bending, welding or heating the reinforcement can change the properties and is allowed only when permitted by the product documentation, the design and the applicable standard.
The behavior of steel in pressure testing
What has been said about the elasticity and the yield strength of the molten steel in tension applies in a corresponding sense to the pressure load. The values of the yield strength of structural steel (in the case of pressure it is called the yield strength) obtained during the compression test and the tension test do not differ much. However, under pressure, an increase in load beyond the crushing limit is only possible for very short elements.
For slender rods, the laws of buckling apply. The relevant assumptions must be considered first, whether they concern the support and restraint of the rod ends or deviations of the rod axis from its established position.
Compressed elements: the strength of the material alone does not determine the bearing capacity of the rod. Slenderness, initial curvature, residual stresses, local and global buckling, supports, joints and imperfections must be included in the calculation of the specific element.
The behavior of steel in the bending test
As for the yield point (observed from the outside by the large deflections of the supports or by the yield lines), it is important that it will occur during the bending test at slightly lower but higher stresses than those obtained when testing the tensile strength of the test rod. After exceeding the yield strength of the steel in the tension zones of the girders, the loads can be increased if the girders are sufficiently secured against buckling or twisting.
Bending test: photo shows laboratory procedure, not product acceptance criteria. Specimen geometry, supports, speed, temperature, rolling direction and grading criteria must match the applied standard and the exact type of steel.
Fatigue strength of steel
Strength properties determined during ordinary tensile, compressive or bending tests provide detailed information on the behavior of steel under static loading. At the same time, it should be taken into account that the yield strength during long-term static loading will be slightly lower than the limit obtained by ordinary testing until failure. If static loads are followed by frequent loads (e.g. bridges, crane tracks, vehicles), or if only frequent loads act at all, then data can be obtained about the material only based on its behavior under conditions that most closely match practical conditions. Therefore, steels for building elements, which will be mainly exposed to moving loads, are tested by frequent loading.
Most often, stationary and moving loads occur at the same time. The following examples outline the essence of the problem. The rods shown in the picture 1 are made of the same material, and taken from the same rod. The lower rod is slowly bent by gradual loading; therefore, it could be bent. The upper rod is repeatedly subjected to repeated loading, with stresses far lower than those prevailing in the first rod. After some time, the upper rod broke without showing any permanent deformation around the fracture. According to this, the behavior of the material at failure depends significantly on the way in which the load is applied. The given example, which refers to bending loads, is also characteristic of other types of loads. Figures 2 and 3 show examples of tensile tests; Fig. 2 refers to a simple example of a tear test, and Fig. 3 to unidirectional variable tension tests.
Regarding the application of these facts, it is important that - apart from the behavior shown during the ordinary tearing test - the value of the construction element, which has been drilled or whose cross-section has undergone other changes, must be evaluated in particular. The resistance of a drilled rod to frequent loads is much lower than the resistance of an undrilled rod, since voltage thresholds appear at the edges of the hole. The stress at the edge of the hole, if caused by frequent loads, will always be significantly higher than the average stress. The consequence is that drilled building elements under frequent loads show far less strength than non-drilled elements.
A final note on fatigue: fatigue cracking can occur at stresses lower than those that cause visible plastic deformation. Holes, welded details, sudden changes in section, corrosion and damage change the category of details and life. Structures under repeated loading require a special fatigue calculation, an inspection plan and an expert assessment of each crack detected.