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CLARIFICATION OF CHARACTERISTICS OF STEEL WITH TECHNICAL SURVEY DESIGNS

Selection and appointment of the design characteristics of the steel when designing new structures is based on existing standards, which establish the magnitude of the design characteristics based on the statistical processing of the results of sample testing of samples for metallurgical plants. Thus, the normative values and are Delivered Rny values that characterize a probability ensure that the actual resistance of materials will not lower normalized. Because the distribution of the random variables yield limits and temporal tear resistance is subordinate to the normal law, when adopted standard security of actual settlement characteristics of existing designs in most cases exceeds the normative. When setting the impedance calculation of snip II-23-81 * accepted normalized security 0.995 and checked on all kinds of products supplied by GOST 277720-88 * all metallurgical plants.
In terms of operation and reconstruction with the technical examination of structures it is necessary to clarify the characteristics of steel for each design or design of arrays. Such an approach, as shown by the results of screening allows you to dissect a considerable load capacity reserves designs.
Basic properties of steel, important from the point of view of its work in the construction of metal structures are:
-strength;
-plasticity;
-tendency to brittle fracture;
fatigue strength;
-weldability.
The significance of those or other properties depends on the specific conditions of work structures. So, when the exploitation of designs at low temperatures the propensity of particular importance have become brittle fracture. For crane beams are one of the main characteristics of fatigue strength.
Steel properties can be set on the basis of tests carried out on the direct technique through the study of samples taken from constructions, and indirectly by using non-destructive methods of control. Direct control methods give more accurate results, however, have a large labour-intensive research. Non-destructive methods of control less precise, but it does not require the weakening structures notches and holes. They do not require complex laboratory equipment, easy to use and give quick results.
The correct designation of species and the number of tests depends on the analysis of existing documentation. Analysis is subject to the following documentation that identifies are: drawings km which establishes the steel grade, electrodes, metal products and other items in metalware; KMD drawings, which establishes the possible deviations from the project; certificates for which specifies properties and the quality of steel and technical conditions. Of great importance is the establishment period of steelmaking, as provides additional information about intervals expected properties.
In pre-revolutionary building metal designs mostly used welding and cast iron. Normirovalis’ temporary values of resistance and elongation.
Wrought iron low homogeneity. Temporary value of resistance in section rolled-from 267 to 378 Mpa. The strength of the iron in the direction across the rolled on 20-30% cheaper than along. Fibrous structure, with pronounced slag inclusions.
The value of temporary resistance normalized value 320 Mpa varies from 230 to 490 MPA and elongation of 10 to 30%.
Cast iron is more consistency and durability. Tensile strength 350 … 450 MPA and elongation not less than 20%. Structure of homogeneous, fine-grained, chemical composition and mechanical properties is close to modern steel St3kp, but with a few less homogeneity, the coefficient of variation of the yield reaches 13%.
In the 20-ies because of an acute shortage of the metal constructions is often performed from random material mechanical properties which are extremely heterogeneous. For use in designs, sometimes it was enough to conduct tests on cold bending in field conditions. Widely used German steel that had high phosphorus content. Allow use of tomasovskih steels. There were cases when testing samples cut out of the designs, the value of temporary resistance was only 130 Mpa, and in place of gap were voluminous slag inclusion. At the same time on many sites built in those years, the tests showed satisfactory results. In most cases, metal designs these years can be categorized as steel with a value 0 resistance R = 170 Mpa.
In 30-ies of the development and implementation of the frame on steel production contributed to the use of higher quality metal structures. The main building steel became Vs. Significantly improved properties and uniformity coefficient of variation of the yield and the interim resistance amounted to 6-9%. The average value of the yield strength-261 MPA interim resistance-400 Mpa, elongation-26.3%; phosphorus and sulfur content slightly higher than the modern requirements and achieves 0.06-0.08%; toughness value at t =-20° c in average is 56 j/cm2.
With the year 1937 carbon steel mill supplied OST 2897. Quality steel and especially its homogeneity. Chemical composition of modern steel St3kp approached. Application of tomassovskih steel was limited to minor neraschetnymi elements.
Tests of samples taken from the designs of the buildings, built in the 30-ies have shown yield strengths from 250 to 310 MPA and temporary resistance not less than 380 Mpa. Cold brittleness threshold on impact toughness test results amounted to 10-20° c. However, in the construction of 30-ies still found use substandard steel of low quality. A large number of designs run from the German steel with high content of sulphur and phosphorus. All these factors need to be taken into account when assessing quality. Introduced at this time domestic higher-strength steel was used only on unique facilities and wide dissemination.
Feature of steel, released in the period 43-46-x. , is the widespread use of alloyed scrap military years. This has led to an increase in the average values of strength characteristics with increasing spreading. Medium static yield stress values up 300-310 MPA and tensile strength up to 440 Mpa. The coefficient of variation increased to 9-10%.
Starting from the 60-ies, widely used in building constructions were told steel grades of high and extremely high strength. According to the results of statistical studies of the mean values of yield strength of most low-alloyed steels is significantly higher normalized values, normative values of resistances that are installed in a standard have security above 95%.
The main type of connections of steel structures of pre-war manufacture are rivets. Establishment of the design characteristics of the material of the rivets on the test results for each design is very laborious and has no significant variability of the results. Based on the research and section 20 Snip p-23-81 for all rivets can be taken the calculated shear resistance-155 Mpa, estimated tensile strength-120 Mpa.
Settlement characteristics of the weld should be taken on normative documents in force at the time of their execution. Numerous surveys of welded constructions found no damage of welded joints associated with their lack of durability, indicating some strength reserves set out in the regulations.
In designs made before the 50-IES, were often used for welding electrodes with ionizing welding. In this case, given the low quality of the seam, should be working conditions factor γc = 0.8.
As already noted, the pilot studies properties of steel to identify stocks of the bearing capacity of the structures, but in small volumes and the availability of documentation may be omitted. Tests are required in case of insufficient information about the material structures, as well as when damages, related to the quality of metal-rassloj, brittle cracks, etc.
When researching and testing of metal shall be determined by the following indicators:
a) chemical composition with the identification of carbon, Silicon, manganese, sulphur and phosphorus (for all grades), chromium, nickel, and copper (low-alloy steel), as well as nitrogen (for bessemerovskoj and tomasovskoj carbon steel converter and where necessary-for low alloy steel);
b) yield strength, tensile strength and elongation at tensile test;
impact strength) at a temperature of-20° c for carbon steel and-40° c for low;
g) after mechanical toughness of ageing;
d) distribution of sulfur inclusions way prints imprints;
e) microstructure.
For 3 and 4 groups of snip II-23-81 * test), g), d) and (e)) may be dispensed with, as well as not to conduct research and testing in cases where these designs will be operated at voltages not exceeding 170 Mpa and at temperatures above 30° c (Group 3) or-40° c (Group 4), and when the reinforcement is performed without the use of welding.

The number of samples and specimens

Under operating conditions, with technical structures survey number of samples and samples taken from metal structures, should be kept to a minimum while ensuring sufficient reliability results. In addition, the volume of cut metal should also be kept to a minimum. It is obvious that the increase in the number and volume of samples not only increases the effort applied research, but also reduces the reliability and durability of structures. Thus, for testing, you must determine the number and dimensions of samples and places their selection.
The properties and quality of the steel depends on many factors, first and foremost, the type and profile geometry and the metal factory. With the aim of identifying material, metal structures to study, breaks into the party. Metal items are party to the same kinds of rolled steel (thickness, numbers and marks became) included in similar constructions (farms, crane beams, columns, etc.) one queue construction. Lot size metal does not exceed 60 tons and is not more than 25 horizontal otpravochnym marks.
Number of samples (samples) from each batch of metal should not be less than indicated in the table.
Type of testing
The number of items scanned per party
The number of samples (samples)
from item
just from the party
1
Chemical analysis
3
1
3
2
Tensile test
2
1
2
3
Toughness testing (for each temperature and after mechanical aging)
2
3
6
4
Imprint imprints
2
1
2

Sample sizes and sampling

To determine the chemical composition of steel, from the designs shown in shavings drilling locations designs. Chips for chemical analysis shown in handling the entire cross-section rolled or symmetrical half of it. If you cannot take the chips around the section element allowed selection of shavings through the middle third of the drilled widths element or shelves profile. From each element designs chips shown in at least three places on length and thoroughly mixed. Massa finished samples shall be not less than 50 g.
Before taking samples of the metal surface is cleaned of oils, dirt, paint, corrosion products and other mechanical impurities and trimmed to the metallic luster. Select trial without lubrication. The surface of the chips should not have colors discolouration.
For tensile tests of structures cut blanks, of which can be manufactured cylindrical specimens with a diameter of 3 mm or more, or flat, with a thickness of 0.5 mm or more. Both types of samples can be short, with an initial estimated length
and long
where Ao is the cross-sectional area of the test-piece.
Application of short samples in operating conditions is preferable.
During the test of cylindrical specimen samples used as core diameter = 10 mm dî. For testing sheet steel and finished steel with thickness up to 25 mm inclusive apply flat samples with saving them superficial layers of rolled, while non-parallel sides shelves-with preservation of superficial layers of rolled on one side. At a thickness of more than 25 mm flat sample processing is allowed up to 25 mm thickness with saving on one side of the surface pattern rolled or making cylindrical samples.
At a thickness of 25 mm-10 rental, you can perform tests on both flat and cylindrical samples.
When cutting billets for mechanical tests shall be provided with allowances to protect the sample from testing and heating. When cutting samples mechanically allowances must be:
-When thickness up to 4 mm-not less than 5 mm;
-//-from 4 to 10 mm-not less thickness;
-//-from 10 to 20 mm-//-10 mm;
-//-from 20 to 35 mm-//-15 mm;
-//-from 35 to 60 mm-//-20 mm.
When cutting designs fire way allowances from the cut line to the edge of the finished sample shall not be less than 15 mm at a thickness of 60 mm rolled up and 20 mm at a thickness of rolled over 60 mm.
To determine the necessary toughness specimens, form and amount of which regulated GOST 9454-78 *. For conventional steel building structures using the samples of a type of 1-10 with hub type u. for high reliability (pipelines, pressure vessels, etc.) use the samples of a type of 11-14 with a V-shaped hub. For particularly critical structures and in the study of causes of destruction used samples of types of 15-20 hub types t (crack).
Sampling for the detection of distribution of sulfur inclusions method dactylography imprints shall be in accordance with GOST 10243-75 *. Cut template customize: sheet steel and broadband-rolling along the direction of semi-finished and finished steel-across a direction of rolling.
The working surface of the section must lie in a plane perpendicular to the direction of rolling. For sheet steel and broadband grinding must have surface 150 x t mm (t = thickness of rolled products); for semi-finished and finished steel surface of the section must be equal to the cross section profile or half profile from the edge to the axis of symmetry. Tests must be conducted in accordance with all applicable standards and samples and sample are labelled with an indication of the offtake.

Sampling locations and designs with technical survey

When assigning locations for sampling must take into account the degree of loading this element. Defining the properties of steel should guarantee them unprofitable. For these purposes the most interesting properties of metal the most loaded structural elements. Therefore, preliminary it is necessary to define the elements with the greatest level of stress. Specimens are cut from a little tense zones of these elements. For example, in the elements of the farms of the corners of the samples shall be selected speaker shelves corners in nodes; for the belts split beams-priopornyh sections. The cutout must be smooth, without cuts, where necessary, place the tenderloin must be strengthened.
Use tenderloin pieces, usually accomplished by fire way, and strengthen weak spots-tenderloin after over-exposure. These methods impose accessibility requirements metal offtake.
Significant impact on the results of the tests has the texture of rolling elements. To obtain reliable results, you should determine the direction of rolling and produce selection and production of samples in the following directions:
-of semi-finished and finished steel is along the direction of rolling;
-sheet and shirokopolochnogo-across a direction of rolling.
Sheet items that do not have the rolling direction, sampling is performed in the main power flow.

Analysis of the test results and assign the design characteristics

Chemical analysis of samples sets the percentage of steel elements. When considering the results of the analysis should pay attention to the contents of the main contaminants are sulphur and phosphorus, respectively make the steel red-and hladnolomkoj. Increased content of harmful impurities, depending on service conditions may exclude the continued use of the designs.
Particular attention should be paid to the extent of raskislennosti steel, which is tentatively on the content of silicon, the primary steel deoxidizers.
Content of Silicon in percentage is:
-for quiet steel-0.12-0.35;
for semikilled-0.05-0.17;
for boiling-no more than 0.05.
To clarify the extent of oxidation, as well as when you use raskislitelej, do not contain silicon, is determined by the distribution of sulfur inclusions way prints imprints.
Processing thin and fingerprinting imprints are produced in the following order. Prepared by grinding on light normal sheet of photographic paper soaked in a 5% solution of sulphuric acid. After 5-10 min excerpt on photographic paper image appears with the location of the macrostructure of sulfur inclusions (“sulphide stitches). The resulting imprint you must rinse and freeze.
 
Place cuttings samples
Processing thin and fingerprinting imprints are produced in the following order. Prepared by grinding on light normal sheet of photographic paper soaked in a 5% solution of sulphuric acid. After 5-10 min excerpt on photographic paper image appears with the location of the macrostructure of sulfur inclusions (“sulphide stitches). The resulting imprint you must rinse and freeze.
To print makroshlifa boiling steel appear numerous darker bars-sulfide lines balanced-single rare sulfide stitch, calm-sulfide line is missing or there are rare point sulfide inclusion.
Based on the results of the chemical analysis of the samples, you can evaluate the weldability of steel, which is especially important for steels the pre-war smelter. Weldability of steel is an integrated feature that includes strength under different conditions of work; resistance to cold and hot cracks formation; hladnolomkost’, etc.
In existing welded constructions indirect assessment of weldability of the material is a condition of welded joints. If the survey is not detected cracks caused by the welding process itself, additional estimates weldability may be dispensed with. In riveted and other designs with no welds, score weldability is needed if they strengthen assumes the use of welding.
The use of special technological samples requires clippings of constructions and enough labour-intensive tests, so their use can be recommended only in exceptional cases. Weldability of steel structures operated normally estimate the carbon equivalent:
where
ASE-carbon equivalent;
Au = 12-the atomic weight of carbon;
       AI-the atomic weight of the i-th element;
       UI-percentage of the i-th element.
Steel is considered good weldable with Ase ≤ 0.45%. Plain low carbon steel weld well when the number of carbon with ≤ 0.22%, sulfur with ≤ 0.055%, phosphorus S .050% and Silicon Si ≤ 0.22%.
Carbon equivalent for elements that do not have stress concentrators can be determined by empirical formula of fatigue limit of steel
svc = 17 + 25 Se,
Basic strength characteristics of steel are set according to the results of mechanical tests. Because determining the bearing capacity of existing designs currently on the methodology of limit States, according to the results of the tests, you must assign a normative Rm and estimated R resistance steel.
If for each batch of steel structures tested 10 and more samples to determine regulatory resistance can be used in statistical methods. As regulatory resistance that is the value of the yield strength, having the security of not lower than 95%, which corresponds to the basic provisions of the methodology of calculation of building structures by limit States.
As shown by the results of statistical studies of the density distribution of yield strength of the steel sufficiently closely corresponds to the normal distribution.
where
-average value and standard deviation of average yield of a sample;
        k is the number of standards that you must take to obtain the specified security.
When sampling and infinitely large values of sample characteristics approaching population parameters, availability of 95% corresponds to k = 1.65. For limited sampling n statistical characteristics of distribution may differ from the general population parameters and order more than the smaller sample. In view of this, the value of k shall be determined by the formula
where
γ-confidence level sample parameter estimates;
xγis defined by the equation
When γ = 0.9 → xγ = 1.28;
γ = 0.7 → xγ = 0.52;
γ = 0.5 → xγ = 0.
Trust the likelihood of γ be appointed depending on the degree of responsibility of the designs:
-for constructions offensive limit state associated with the collapse of γ = 0.9;
-without the danger of collapse of γ = 0.7;
-for constructions with purely economic responsibility of γ = 0.5.
In all cases the value of regulatory resistance should take no higher than minimum received tests yield stress. For elements that are subjected to the tests as regulatory resistance is taken the result value of the yield stress.
With a limited number of tests, it is not possible to use the results directly to assign normative calculation of resistance. In this case, based on test set steel grade or its equivalent in effect during the construction Gostah and technical conditions and settlement characteristics are assigned based on the experience of the operation of structures made of similar steel.
Design resistance of the steel is defined by the formula
where
γm-coefficient of reliability by material.
Values for existing structures regulated by γm section 20 of snip II-23-81.
Plasticity of steel assessed largest elongation. When received by elongation values below the norms for the corresponding grades, attention should be paid to the possibility of brittle cracks, particularly in the weld and high stress concentrations.
The propensity to become brittle fracture detected according to the results of tests on the impact toughness. When satisfactory results of tests steel may be deemed suitable for further exploitation, even if you did not follow the grade of snip II-23-81 *.
If the results are unsatisfactory, the tests conducted the re-evaluation of toughness on twice the number of samples. Retest results are final. If retesting will give unsatisfactory results, raises the issue of the need to enhance or replace designs, the final decision depends on the State structures, the intensity of impacts and the degree of responsibility.
Based on the results of the chemical analysis and mechanical tests and comparison of these results with the requirements of the existing norms, material matched designs operating conditions and the new conditions during the reconstruction. When you do this, where necessary, restrictions imposed on the operation. So, for buildings with structures made of unkilled steel not allowed negative temperature in winter.

Proximate analysis and indirect methods of estimating the strength of steel

The methods outlined above, as has already been noted, have very high labor costs and may not always be performed. In terms of operation and reconstruction should move nakosvennye non-destructive methods. Currently, such methods are developing intensively, but still at the stage of research and testing.
Replacement of traditional chemical analysis laboratory way possible based on the use of laser technology. Express-analysis based on laser, is the following: on the selected and cleared surface area laser beam structures burned metal, couples which examines a spectrograph. This method will greatly expand the number of samples and thus improve the reliability of the research. Currently, the development of compact apparatus usable in conditions of exploitation.
To obtain the most complete information about the mechanical properties of steel you can use indirect methods of assessing strength without notch specimens. One such method is the evaluation of steel strength on measurements of hardness. Under the hardness refers to the resistance that this material has penetration in other, more solid. Unlike other methods of mechanical testing hardness does not lead to the destruction of samples or structural elements. Existing instruments are simple and compact and allow for tests directly on constructions.
The most common ways to evaluate the hardness are Brinell methods and Poldi.
Based on the results of the determination of the hardness value is determined by the dependency on empirical interim resistance σuts and proof strength σ0, 2.
For construction steels with hardness by Brinell HB usually does not exceed 260 and strength characteristics of value in kN/cm2 can be defined by the equations

The main disadvantage of the method is to define the hardness and durability of surface-only sites that hire due to deformation in rolling process have improved mechanical properties. The precision of the specified method does not exceed 5% and decreases at increasing the thickness of the steel, so it can be used only for preliminary assessment of the strength of steel and the choice of the weakest elements for subsequent testing more precise methods. However, the simplicity and the ability to test virtually all elements make this method very useful in surveys.

Another technique for determining the mechanical properties of steel based on the definition of the efforts needed to cut threads with the chart record deformation feature and allows you to get strength and plastic properties of steel. Based on the results of the statistical material processing massive testing the equations to determine standard specifications, σ0 01, σ0, 2, σuts. Size chart gives an estimate of plastic deformation properties of steel. Analysis of the results of the tensile testing of samples and on the proposed methodology revealed a close correlation. The correlation coefficient is 0.8-0.9. The advantages of the methodology should include determination of mechanical properties with little or no rupture (hole diameter does not exceed 5 mm, depth is a small fraction of the thickness of rolled products), the opportunity to explore the properties of metal in small areas, for example, angular seams, speed of execution and low labor intensity.

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