If we characterize the concept of yield strength briefly, then in the strength of materials the yield strength
is the stress at which plastic deformation begins to develop. Yield strength refers to strength characteristics.
According to [1], fluidity
is macroplastic deformation with very low
hardening
dτ/dγ.
Physical yield strength
is a mechanical characteristic of materials: stress corresponding to the lower position
of the yield point
in
the tensile diagram
for materials having this site (figure),
σ
T =
P
T/
F
0. Here
P
T is the yield stress load, and
F
0 is the initial cross-sectional area of the sample.
Yield strength
establishes the boundary between the elastic and elastic-plastic deformation zones.
Even a small increase in stress (load) above the yield point
causes significant deformation. [2]
What is the yield plateau called?
turnover area
— The stress in the material is usually less than the maximum stress at which an increase in strain occurs without an increase in stress.
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The value of the metal's yield strength
At the time when atomic physics began to develop to a significant extent, it was possible to calculate the value of the parameter through theory. As for this work, it was first performed by the famous Yakov Frenkel, back in 1924.
As a matter of fact, his work was quite complex and really required attention, it was for this reason that a special formula was invented that would help everyone cope with the task.
The fluidity value of the metal is equal to Tm=G/2mt, where G is the shear modulus.
For the onset of plastic deformation, it was believed that it would be sufficient for the first half of the body to undergo a shift relative to the other, to the effect that returning to the initial position is impossible.
Composition of steel alloys
The properties of the metal depend on the crystal lattice formed , which, in turn, is determined by the carbon content. The dependence of lattice types on the amount of carbon is clearly visible in the structure diagram. If, for example, there is up to 0.06% carbon in the steel lattice, then this is classic ferrite, which has a granular structure. This material is fragile, but fluid and has a high impact strength limit.
According to the structure of steel, they are divided into:
- ferritic;
- pearlite-ferrite;
- cementite-ferritic;
- cementite-perlite;
- pearlite
Carbon Additives and Strength
The law of additivity is confirmed by the percentage changes in cementite and ferrite in steel. If the amount of carbon additive is about 1.2%, the yield strength of the steel material is increased and the hardness, strength and temperature resistance are improved. With a subsequent increase in carbon content, the technical parameters deteriorate. Steel is difficult to weld and is difficult to stamp. Alloys with a low carbon content behave best when welding.
Manganese and silicon
Manganese is additionally added as an additive to increase the degree of deoxidation. In addition, this element reduces the harmful effects of sulfur. The manganese content is usually no more than 0.8% and it does not affect the technological properties of the alloy. Present as a solid component.
Silicon also does not particularly affect the characteristics of the metal. It is necessary to increase the quality of welding parts. The content of this element does not exceed 0.38% and it is added during the deoxidation process.
Sulfur and phosphorus
Sulfur is contained in the form of brittle sulfites. An increased amount of this element affects the mechanical properties of the alloy. The more sulfur, the worse the ductility, fluidity and viscosity of the alloy. If the 0.06% limit is exceeded, the product is more susceptible to corrosion and becomes susceptible to severe abrasion.
The presence of phosphorus increases the fluidity index, but at the same time the plasticity and viscosity decreases. In general, an excessive phosphorus content significantly deteriorates the quality of the metal. The combined high content of phosphorus and carbon has a particularly harmful effect on the characteristics. Acceptable limits for phosphorus content are considered to be values from 0.025 to 0.044%.
Nitrogen and oxygen
These are non-metallic impurities that reduce the mechanical properties of the alloy. If the oxygen content is more than 0.03%, then the metal ages faster, and the values of ductility and viscosity decrease. Nitrogen additives increase strength, but in this case the yield strength decreases. The increased nitrogen content makes steel brittle and contributes to the rapid aging of the metal structure.
Behavior of alloying additives
To improve all the physical characteristics of steel, special alloying elements are added to the alloy. Such additives can be tungsten, molybdenum, nickel, chromium, titanium and vanadium. Joint addition in the required proportions gives the most acceptable results.
Alloying significantly increases fluidity, impact strength and prevents deformation and cracking.
Alloy check
Before launching into production, tests are carried out to study the properties of the metal alloy. Metal samples are subjected to various loads until all properties are completely lost.
The loads are:
- Statistical load.
- Testing the endurance and fatigue of steel.
- Stretching an element.
- Bend and torsion testing.
- Combined bending and tensile endurance.
For these purposes, special machines are used and conditions are created that are as close as possible to the operating mode of the future structure.
Testing
To carry out tests on a cylindrical sample with a cross-section of twenty millimeters and a calculated length of ten millimeters, a tensile load is applied. The sample itself has a length of more than ten millimeters so that it can be securely grasped, and a length of ten millimeters is marked on it and this is what is called the calculated length. The tensile force is increased and the increasing elongation of the sample is measured. For clarity, the data is plotted on a graph. It is called a conditional stretch diagram.
With a small load, the sample elongates proportionally . When the tensile force increases sufficiently, the limit of proportionality will be reached. After passing this limit, disproportionate elongation of the material begins with a uniform change in the tensile force. Then a limit is reached, after which the sample cannot return to its original length. When passing this value, the test part changes without increasing the tensile force. For example, for steel rod Art. 3 this value is equal to 2450 kg per square centimeter.
Unexpressed flow point
If, under constant force, a material is capable of deforming on its own for a long time, then it is called ideally plastic.
During testing, it often happens that the yield point is not clearly defined, then the definition of the conditional yield strength is introduced. This means that the force acting on the metal caused a deformation or permanent change of about 0.2%. The value of the residual change depends on the ductility of the metal.
The more ductile the metal, the higher the residual deformation value. Typical alloys in which such deformation is not clearly expressed are copper, brass, aluminum, and steels with low carbon content. Samples of these alloys are called densified.
When a metal begins to “flow,” as experiments and research demonstrate, strong changes occur in its crystal lattice. Shear lines appear on its surface and the layers of crystals shift significantly.
After the metal spontaneously stretches, it passes into the next state and again acquires the ability to resist. Then the alloy reaches its strength limit and the weakest area clearly appears on the part, where a sharp narrowing of the sample occurs.
The cross-sectional area becomes smaller and this is where rupture and destruction occur. The magnitude of the tensile force at this moment drops along with the value of stress and the part breaks.
High-strength alloys can withstand loads of up to 17,500 kilograms per square centimeter. The tensile strength of ST.3 steel is in the range of 4–5 thousand kilograms per square centimeter.
Plasticity characteristic
The ductility of a material is an important parameter that must be taken into account when designing structures. Plasticity is determined by two indicators:
- residual elongation;
- narrowing upon rupture.
Permanent elongation is calculated by measuring the total length of the part after it has broken. It consists of the sum of the lengths of each half of the sample. Then the ratio to the original conditional length is determined as a percentage. The stronger the metal alloy, the lower the elongation value.
Residual narrowing is the percentage ratio of the narrowest point of the rupture to the original cross-sectional area of the rod under study.
Fragility index
The most brittle metal alloys are considered to be tool steel and cast iron. Fragility is the inverse property of plasticity, and it is somewhat conditional, since it strongly depends on external conditions.
Such conditions may be:
- Ambient temperature. The lower the temperature, the more fragile the product becomes.
- The rate of change of applied force.
- Ambient humidity and other parameters.
When external conditions change, the same material behaves differently. If a cast iron bar is clamped on all sides, it will not break even under significant loads. And, for example, when there are grooves on a steel rod, the part becomes very fragile.
Therefore, in practice, it is not the concept of the brittleness limit that is used, but the state of the sample is determined as brittle or rather ductile.
Material strength
This is a mechanical property of the workpiece and is characterized by its ability to withstand loads without completely collapsing. For the test sample, conditions are created that best reflect future operating conditions and a variety of influences are applied, gradually increasing the load. An increase in impact forces causes plastic deformations in the sample. In plastic materials, deformation occurs in one, pronounced area called the neck. Brittle materials can fail in several areas at the same time.
Steel is tested to accurately determine various properties in order to obtain an answer about the possibility of its use in certain conditions during construction and the creation of complex structures.
The fluidity values of various steel grades are included in special Standards and Specifications. There are four main classes. The fluidity value of products of the first class can reach up to 500 kg/cm2, the second class meets the load requirements of up to 3 thousand kg/cm2, the third - up to 4 thousand kg/cm2. and the fourth class can withstand up to 6 thousand kg/cm2.
Physical characteristics
Yield strengths refer to strength indicators. They represent macroplastic deformation with rather small strengthening. Physically, this parameter can be represented as a characteristic of the material, namely: stress, which corresponds to the lower value of the yield area in the graph (diagram) of tension of materials. This can also be represented in the form of a formula: σТ=PT/F, where PT means the yield stress load, and F corresponds to the original cross-sectional area of the sample in question. PT establishes the so-called boundary between the elastic-plastic and elastic deformation zones of the material. Even a slight increase in stress (above PT) will cause significant deformation. The yield strength of metals is usually measured in kg/mm 2 or N/m 2. The value of this parameter is influenced by various factors, for example, heat treatment mode, sample thickness, the presence of alloying elements and impurities, type, microstructure and crystal lattice defects, etc. The yield strength changes significantly with temperature. Let's consider an example of the practical meaning of this parameter.
Manganese and silicon additives
An alloying additive in the form of manganese is used to deoxidize the alloy and compensate for the negative effects of harmful sulfur-containing impurities. Due to its similar properties to iron, manganese does not have a significant independent effect on the properties of the alloy. Typical manganese content is about 0.8%.
Silicon has a similar effect; it is added during the deoxidation process in a volume fraction not exceeding 0.4%. Since silicon significantly worsens such a technical indicator as the weldability of steel. For structural steels intended for welding, its proportion should not exceed 0.25%. Silicon does not affect the properties of steel alloys.
What is PT for valves?
These products are an integral part of reinforced concrete, intended, as a rule, to resist tensile forces. Usually steel reinforcement is used, but there are exceptions. These products must work together with the mass of concrete at all stages of loading a given structure without exception, and have plastic and durable properties. And also meet all the conditions for the industrialization of these types of work. The mechanical properties of steel used in the manufacture of reinforcement are established by the relevant GOST and technical specifications. GOST 5781-61 provides for four classes of these products. The first three are intended for conventional structures, as well as non-prestressed rods in prestressed systems. The yield strength of reinforcement, depending on the class of the product, can reach 6000 kg/cm2. So, for the first class this parameter is approximately 500 kg/cm2, for the second – 3000 kg/cm2, for the third – 4000 kg/cm2, and for the fourth – 6000 kg/cm2.
Nitrogen and oxygen in the alloy
These non-metals from the beginning of the periodic table are harmful impurities and reduce the mechanical and physical characteristics of steel, such as the viscosity threshold, ductility and brittleness. If oxygen is contained in amounts greater than 0.03%, this leads to accelerated aging of the alloy, and nitrogen increases the fragility of the material. On the other hand, the nitrogen content increases strength by reducing the yield strength.
Microstructure of an alloy containing nitrogen and oxygen
Tensile strength
Let's look at how this characteristic of mechanical properties is determined. Strength is the ability of a material, under certain limits and conditions, to withstand various impacts without collapsing. Mechanical properties are usually determined using conventional stress-strain diagrams. Standard samples should be used for testing. Test instruments are equipped with a device that records the diagram. Increasing loads above normal causes significant plastic deformation in the product. The yield strength and tensile strength correspond to the highest load preceding the complete destruction of the sample. In plastic materials, deformation is concentrated in one area where a local narrowing of the cross section appears. It is also called the neck. As a result of the development of multiple slips, a high dislocation density is formed in the material, and so-called nucleation discontinuities also arise. Due to their enlargement, pores appear in the sample. Merging with each other, they form cracks that propagate transversely to the tensile axis. And at a critical moment the sample is completely destroyed.
Effect of carbon content on the properties of steels
Changes in the physical properties of materials are determined by the presence of carbon. In the event that up to 1.2 percent carbon is present, the most durable material can be achieved.
However, you need to understand that a higher carbon content does not lead to the most positive consequences. For example, weldability and ultimate deformation are reduced. And this is a very important indicator that is definitely worth paying attention to.
I really hope that you really found it interesting and useful!
