TO category:
Metalwork and tool work
Strength and hardness of metal
Metals used in mechanical engineering and tool production have a variety of valuable properties, but the most important of them are strength and hardness.
Let's talk briefly about these properties.
Strength, as is known, is the ability of a material to resist destruction. If the metal does not break when stretched or does not collapse when impacted, the metal is said to be strong. But in technology you cannot rely only on a general impression of whether the metal you are dealing with is strong or not strong enough. The strength of a material must be accurately measured, and its ability to resist tearing and its ability to withstand impact loads must be determined separately. To determine the strength of a metal, samples made from it are subjected to tension on special machines until they break. By tracing the force under which the sample ruptured and studying the change in its dimensions at the rupture site, it is possible to obtain a complete and accurate description of the strength of the metal from which the sample is made.
Then, by dividing the magnitude of the force that broke the sample, expressed in kilograms, by the cross-sectional area of the sample, expressed in square millimeters, the stress that the sample withstood was determined, i.e., the tensile strength of the material. The magnitude of this force, per unit sectional area and therefore measured not in kg, but in kg/mm2, is called tensile strength and is designated in all technical literature, drawings and technological documents by the letter sv (sigma be).
Knowing the value of the tensile strength of a particular metal allows not only to calculate the strength of the product, but also to select the necessary cutting conditions when processing it. This is of great importance because the strength of steels varies greatly. So, for example, Art. 1 has = 32 40 kg/mm2, and some high-alloy steels reach 200 kg/mm2.
Studying the torn sample further, one can find that its cross-section at the rupture site has narrowed somewhat, and its overall length has increased. This phenomenon indicates how capable a given material is of resisting destruction and changing its shape without disrupting the molecular connection between its particles, i.e., of being plastic.
If we now calculate how much the cross-sectional area of the sample has decreased, and then divide this value by its original area, we will get a result expressed in. percent and called the relative compression of the cross section. The relative compression of the cross section is denoted by the letter f (psi) and characterizes the viscosity of the material. The value for the softest low-carbon steels reaches 60%, for the least ductile steels - up to 30%.
The measurement of the increased length of the sample is characterized by relative elongation and is designated by the letter 8 (delta). The greater the relative elongation, the greater the ductility of the metal. According to the value of relative elongation 5 and relative compression<|>, indirectly, one can judge the viscosity of the metal. The viscosity of a metal is understood as the property of a material that is the opposite of brittleness.
The second main property of metals is hardness. The higher the hardness, the more durable the part, the slower it wears out. A cutting tool removes chips from a part only because its hardness is much higher than the hardness of the material being processed. Even a small change in hardness significantly affects the performance properties of the part and the tool. All this forces manufacturers to carefully monitor the hardness of the part.
The hardness of a metal is determined by pressing an object into the material being tested. The depth of indentation determines how great this hardness is. Existing hardness measuring instruments operate on this principle: the Brinell press and Rockwell instruments.
Using a Brinell press, the hardness of unhardened steels, as well as cast iron, is measured by pressing a steel ball with a diameter of 10 mm into them with a force of 3000 kg. For other materials, the force of pressing the ball varies: for copper, brass and the like it is 1000 kg, and for soft alloys 250 kg. The Rockwell device determines the hardness of hardened materials by pressing a special diamond cone. The result of the measurement, which characterizes the hardness of the material, is the corresponding hardness numbers: the Brinell hardness number (Hb) and the Rockwell hardness number (HR).
The Brinnell hardness number Ib represents the result of dividing the load (in kg) by the indentation area of the ball, expressed in mm2. To avoid calculations when determining the Hb number, use special tables in which you can find this number based on the diameter of the resulting print. The highest hardness that can be tested on this press is: Yv = 450.
The Brinell press (Fig. 15) operates as follows. The part, cleaned to obtain a flat and even surface, is installed on a ball joint and the flywheel, which rotates the screw, is raised until it comes into contact with the tip ball. Then they close the oil outlet from the cylinder into the reservoir with a screw and create pressure on the piston and ball tip, acting as a pump. The activated pump forces oil into the cylinder from the reservoir, creates pressure on the piston and simultaneously transmits it to the pressure gauge and the lever with weights. The amount of pressure corresponds to the weight of the loads. After some time, the screw opens, some of the oil from the cylinder goes into the reservoir and the pressure drops to zero. After this, lower the screw with the flywheel, release the part and use a special magnifying glass to measure the diameter of the imprint.
Rice. 1. Schematic representation of the Brinell hydraulic press.
Rice. 2. Diagram of the operation of the Rockwell device.
The testing process begins by bringing the object to the diamond tip and applying a preliminary force (10 kg). This is carefully created by a spring located in the spindle bushing of the device. The operating lever 6 acts on the spindle of the device, and its support point is on axis 7, and the place where the force is transmitted to the tip is on the prism. A load acts on this lever.
In the non-working position, the lever rests on the shackle and pressure is not transmitted to the spindle. During the test, the handle is released and then the lever, together with the shackle and the lever, is lowered. The smooth lowering of this entire system is facilitated by an oil damper 8, which allows you to regulate the speed of application of force to the object being tested. Having the opportunity to move, the diamond cone, descending, penetrates the metal. The magnitude of this movement is transmitted by the lever to the indicator.
However, it should be said that not all parts can be tested for hardness using the devices described. It is impossible, for example, with. using them to determine the hardness on the cutting edge of a tool or on the inner surface of some matrix. In such cases, they resort to checking hardness using calibrated files.
This concludes the description of the two most important properties of steel - its strength and hardness. However, these properties are not constant. They can change with changes in the structure of the steel, i.e. its structure. What causes the structure of steel to change?
The main mechanical properties include strength, ductility, hardness, impact strength and elasticity. Most indicators of mechanical properties are determined experimentally by stretching standard samples on testing machines.
Strength- the ability of a metal to resist destruction when exposed to external forces.
Plastic- the ability of a metal to irreversibly change its shape and size under the influence of external and internal forces without destruction.
Hardness- the ability of a metal to resist the penetration of a harder body into it. Hardness is determined using hardness testers by introducing a hardened steel ball into the metal (on a Brinell device) or by introducing a diamond pyramid into a well-prepared sample surface (on a Rockwell device). The smaller the indentation size, the greater the hardness of the metal being tested. For example, carbon steel has a hardness of 100 before hardening. . . 150 HB (Brinell), and after hardening - 500. . . 600 NV.
Impact strength- the ability of a metal to resist impact loads. This quantity, denoted KS(J/cm 2 or kgf m/cm), determined by the ratio of mechanical work A, spent on destruction of the sample during impact bending, to the cross-sectional area of the sample .
Elasticity- the ability of a metal to restore its shape and volume after the cessation of external forces. This quantity is characterized by the elastic modulus E(MPa or kgf/mm 2), which is equal to the voltage ratio a to elastic deformation caused by it. Steels and alloys for the manufacture of springs and leaf springs must have high elasticity.
Mechanical properties of metals
Mechanical properties are understood as characteristics that determine the behavior of a metal (or other material) under the influence of applied external mechanical forces. Mechanical properties usually include the resistance of a metal (alloy) to deformation (strength) and resistance to fracture (ductility, toughness, and the ability of the metal not to collapse in the presence of cracks).
As a result of mechanical tests, numerical values of mechanical properties are obtained, i.e., values of stress or deformation at which changes in the physical and mechanical states of the material occur.
Property evaluation
When assessing the mechanical properties of metallic materials, several groups of criteria are distinguished.
- Criteria determined regardless of the design features and nature of the service of products. These criteria are found by standard tests of smooth samples for tension, compression, bending, hardness (static tests) or impact bending of notched samples (dynamic tests).
- Strength and plastic properties determined during static tests on smooth samples, although they are important (they are included in the calculation formulas), in many cases do not characterize the strength of these materials in real operating conditions of machine parts and structures. They can only be used for a limited number of simple-shaped products operating under static load conditions at temperatures close to normal.
- Criteria for assessing the structural strength of a material, which are in the greatest correlation with the service properties of a given product and characterize the performance of the material under operating conditions.
Design strength of metals
Criteria for the structural strength of metallic materials can be divided into two groups:
- criteria that determine the reliability of metallic materials against sudden destruction (fracture toughness, work absorbed during crack propagation, survivability, etc.). These techniques, which use the basic principles of fracture mechanics, are based on static or dynamic tests of samples with sharp cracks that occur in real machine parts and structures under operating conditions (notches, through holes, non-metallic inclusions, microvoids, etc.). Cracks and micro-discontinuities greatly change the behavior of metal under load, since they are stress concentrators;
- criteria that determine the durability of products (fatigue resistance, wear resistance, corrosion resistance, etc.).
Criteria for evaluation
Criteria for assessing the strength of a structure as a whole (structural strength), determined during bench, full-scale and operational tests. These tests reveal the influence on the strength and durability of the structure of such factors as the distribution and magnitude of residual stresses, defects in the manufacturing technology and design of metal products, etc.
To solve practical problems in metallurgy, it is necessary to determine both standard mechanical properties and criteria for structural strength.
GOST 25.503-97
INTERSTATE STANDARD
CALCULATIONS AND STRENGTH TESTS.
METHODS OF MECHANICAL TESTING OF METALS
COMPRESSION TEST METHOD
INTERSTATE COUNCIL
ON STANDARDIZATION, METROLOGY AND CERTIFICATION
Preface
1 DEVELOPED by the Voronezh State Forestry Academy (VGLTA), the All-Russian Institute of Light Alloys (VILS), the Central Research Institute of Building Structures (TsNIISK named after Kucherenko), the All-Russian Research Institute of Standardization and Certification in Mechanical Engineering (VNIINMASH) of the State Standard of the Russian Federation INTRODUCED by the State Standard of Russia 2 ADOPTED by the Interstate Council for Standardization, Metrology and Certification (Protocol No. 12-97 of November 21, 1997) The following voted for adoption:
|
State name |
Name of the national standardization body |
| The Republic of Azerbaijan | Azgosstandart |
| Republic of Armenia | Armgosstandard |
| Republic of Belarus | State Standard of Belarus |
| The Republic of Kazakhstan | Gosstandart of the Republic of Kazakhstan |
| Kyrgyz Republic | Kyrgyzstandard |
| The Republic of Moldova | Moldovastandard |
| Russian Federation | Gosstandart of Russia |
| The Republic of Tajikistan | Tajikgosstandart |
| Turkmenistan | Main State Inspectorate of Turkmenistan |
| The Republic of Uzbekistan | Uzgosstandart |
| Ukraine | State Standard of Ukraine |
GOST 25.503-97
INTERSTATE STANDARD
Date of introduction 1999-07-01
1 AREA OF USE
This standard specifies methods for static compression tests at °C to determine the mechanical properties of ferrous and non-ferrous metals and alloys. The standard establishes a method for testing samples in compression to construct a hardening curve, determine the mathematical relationship between flow stress s s and the degree of deformation, and evaluate the parameters of the power equation (s s 1 - flow stress at = 1, n - strain hardening index). Mechanical characteristics, hardening curve and its parameters, defined in this standard, can be used in the following cases: - selection of metals, alloys and justification of design solutions; - statistical acceptance control of standardization of mechanical characteristics and assessment of metal quality; - development of technological processes and product design; - calculation of the strength of machine parts. The requirements established in sections 4, 5 and 6 are mandatory, the remaining requirements are recommended.2 REGULATORY REFERENCES
This standard uses references to the following standards: GOST 1497-84 Metals. Tensile test methods GOST 16504-81 System of state testing of products. Testing and quality control of products. Basic terms and definitions GOST 18957-73 Strain gauges for measuring linear deformations of building materials and structures. General technical conditions GOST 28840-90 Machines for testing materials in tension, compression and bending. General technical requirements3 DEFINITIONS
3.1 In this standard, the following terms with corresponding definitions are used: 3.1.1 test diagram (compression): A graph of the load versus the absolute deformation (shortening) of the sample; 3.1.2 hardening curve: A graph of flow stress versus logarithmic strain; 3.1.3 axial compressive load: The load acting on the sample at the current moment of testing; 3.1.4 conditional rated stress s: Stress determined by the ratio of the load to the initial cross-sectional area; 3.1.5 flow stress s s: Stress exceeding the yield strength, determined by the ratio of the load to the cross-sectional area of the sample valid for a given moment of testing under uniform deformation; 3.1.6 limit of proportionality in compression: Stress at which the deviation from the linear relationship between the load and the absolute shortening of the sample reaches such a value at which the tangent of the angle of inclination formed by the tangent to the diagram F - D h at point F pc with the axis of the loads increases by 50% of its value in the linear elastic section; 3.1.7 compressive elastic limit: Stress at which the relative residual deformation (shortening) of the sample (e) reaches 0.05% of the original design height of the sample; 3.1.8 yield strength (physical) under compression: The lowest stress at which the sample is deformed without a noticeable increase in the compressive load; 3.1.9 conditional compressive yield strength: Stress at which the relative residual deformation (shortening) of the sample reaches 0.2% of the original design height of the sample; 3.1.10 compressive strength: Stress corresponding to the highest load preceding failure; 3.1.11 strain hardening index n: Power exponent of the equation approximating the hardening curves, characterizing the ability of a metal to harden under uniform plastic deformation.4 SHAPE AND SIZES OF SAMPLES
4.1 Tests are carried out on samples of four types: cylindrical and prismatic (square and rectangular), with smooth ends of types I - III (Figure 1) and end recesses of type IV (Figure 2).
Figure 1 - Experimental samples of types I - III
Figure 2 - Experimental samples of type IV
4.2 The type and size of the sample are selected according to table 1. Table 1
|
Sample type |
Initial diameter of the cylindrical sample d 0, mm |
Initial thickness of the prismatic sample a 0, mm |
Working (initial design) height of the sample h(h 0)*, mm |
Defined characteristic |
Note |
| Modulus of elasticity, proportionality limit | Picture 1 | ||||
| Proportional limit, elastic limit | |||||
|
6; 10; 15; 20; 25; 30 |
5; 10; 15; 20; 25; 30 |
Determined according to Appendix A |
Physical yield strength, proof strength. Construction of a hardening curve up to logarithmic strain values  |
||
| Construction of the hardening curve | Figure 2. The thickness and height of the bead are determined according to Appendix A | ||||
| * The height of the prismatic sample is determined based on its area b× a, equating it to the nearest area through d 0. ** To construct hardening curves, only cylindrical samples are used. | |||||
| Note - The width of prismatic samples b is determined from the relationship. | |||||
5 REQUIREMENTS FOR EQUIPMENT AND EQUIPMENT
5.1 Tests are carried out on compression machines of all systems and tension machines (compression zone) that meet the requirements of this standard and GOST 28840. 5.2 When conducting compression tests, the testing machine must be equipped with: - a force transducer and a strain gauge or force and displacement transducers with a recording device - when determining the mechanical characteristics E c, . In this case, the strain gauge is installed on the sample in its calculation part, and the recording device is designed to record the diagram F (D h); - force and displacement transducers with a recording device - when determining mechanical characteristics, , and constructing a hardening curve on type III samples. In this case, the displacement transducer is installed on the active grip of the testing machine. It is allowed to measure the absolute deformation (shortening) of the sample D h using measuring instruments and tools; - a force transducer and measuring instruments and tools - when constructing a hardening curve on type IV samples. 5.2.1 Strain gauges must comply with the requirements of GOST 18957. 5.2.2 The total error in measuring and recording movements with an absolute deformation recording device D h should not exceed ± 2% of the measured value. 5.2.3 The recording device must provide recording of the diagram F (D h) with the following parameters: - the ordinate height of the diagram corresponding to the highest limit value of the load measurement range is not less than 250 mm; - recording scales along the absolute deformation axis from 10:1 to 800:1. 5.2.4 The scale division value of measuring instruments and tools when measuring the final height of the sample h k should not exceed, mm: 0.002 - for e £ 0.2% ( ; for samples of types I - III; 0.050 - for e > 0.2% for type IV samples, where A 0 and A k - 0.002 - at £ 0.002 the initial and final areas of the transverse section 0.050 - at > 0.002) 5.2.5 The error in measuring the final diameter of the sample and the cross-sectional dimensions of the prismatic sample should not be more than, mm: 0.01 - for sizes up to 10 mm; 0.05 - for sizes over 10 mm.6 PREPARATION AND CONDUCT OF TESTS
6.1 The number of samples to evaluate the average value of mechanical characteristics E c, , , , and must be at least five*, unless a different number is specified in the regulatory document for the supply of materials. ____________ * If the difference in the determined characteristics does not exceed 5%, you can limit yourself to three samples. 6.2 Number of samples for constructing a hardening curve 6.2.1 To construct a hardening curve on samples of types III, IV with subsequent processing of test results using correlation analysis methods, the number of samples is selected depending on the expected type of the hardening curve and its sections (see Appendix B). For section I of the hardening curve (see Figure B.1a), at least six samples are tested, for section II - at least five samples, for section III - depending on the value of the deformation corresponding to this section (at least one sample per range of degrees of deformation = 0.10). For the hardening curves shown in Figures B.1b - B.1d and B.1e - B.1k, the number of samples must be at least 15, and for the curves presented in Figure B.1e - at least eight samples for each of sections of the curve separated from each other by maxima and minima. 6.2.2 With a limited scope of tests to construct a hardening curve on type III samples with subsequent regression analysis of the test results, the number of samples should be at least five. 6.3 Compression tests of samples are carried out under conditions that ensure minimal eccentricity of load application and safety of experiments. It is recommended to use the device given in Appendix B. 6.4 The hardness of the deforming plates must exceed the hardness of the samples strengthened during testing by at least 5 HRC e. The thickness of the deforming plates is set depending on the forces created in the sample and is taken equal to 20-50 mm. 6.5 It is necessary to monitor compliance with the uniformity of deformation when testing specimens for compression (no barreling or concavity). 6.5.1 When determining the elastic modulus E c, the limit of proportionality and elasticity, control is carried out using instruments installed on opposite sides of the prismatic and cylindrical samples, while the normalized difference in the readings of the two instruments should not exceed 10 (15)%. 6.5.2 When determining the yield strength and tensile strength and when constructing the hardening curve, control is carried out using the equalities for cylindrical and prismatic samples:Where h 0 is the initial calculated height of the cylindrical and prismatic samples, from which the shortening (tensometer base) is determined, mm; h k is the final calculated height of the cylindrical and prismatic samples after testing to a given deformation or upon destruction, mm; A 0 - initial cross-sectional area of the cylindrical sample, mm 2 -; A k is the final cross-sectional area of the cylindrical sample after testing to a given deformation or upon destruction, mm 2; A k.p - the final cross-sectional area of the prismatic sample after testing to a given deformation or upon destruction, mm 2 (A k.p = a k, b k, where a k is the final thickness of the prismatic sample, b k. is the final width of the prismatic sample, mm); A 0p is the initial cross-sectional area of the prismatic sample, mm 2 (A 0p = a b). 6.6 When testing samples of types I and II, the ends of the samples are degreased. Lubricating the ends with lubricant is unacceptable. 6.7 When testing type III samples, the use of a lubricant is allowed, and when testing type IV samples, the use of a lubricant is mandatory. 6.7.1 When testing type III samples, machine oil with graphite, V-32K cutting fluid and Ukrinol 5/5 are used as a lubricant. 6.7.2 When testing type IV samples, stearin, paraffin, a paraffin-stearine mixture or wax are used as a lubricant. The lubricant is applied to the samples in a liquid state. The thickness of the lubricant must correspond to the height of the beads. 6.7.3 It is permitted to use other lubricants that reduce contact friction between the samples and the deforming plate. 6.8 When testing specimens for compression up to the yield point, the relative strain rate is selected from 10 -3 s -1 to 10 -2 s -1 , beyond the yield point - no more than 10 -1 s -1 , and for constructing hardening curves it is set from 10 - 3 s -1 to 10 -1 s -1 . It is recommended to determine the rate of relative deformation taking into account the elastic compliance of the “testing machine - sample” system (see GOST 1497). If the selected relative strain rate in the yield region cannot be achieved directly by adjusting the testing machine, then it is set from 3 to 30 MPa/s [(0.3 to 3 kgf/mm 2 × s)] by adjusting the loading rate before the beginning of the yield region sample. 6.9 Determination of mechanical characteristics 6.9.1 Mechanical characteristics E c, , , are determined: - using strain gauges with manual and automated data collection (analytical and calculation processing methods); - according to the autodiagram recorded by the testing machine in the coordinates “force - absolute deformation (P - D h)”, taking into account the recording scale. The diagrams are recorded under stepwise loading with unloading cycles and continuous application of increasing force in the ranges of specified loading and deformation rates. Recording scale: - along the deformation axis not less than 100:1; - along the load axis, 1 mm of the diagram should correspond to no more than 10 MPa (1.0 kgf/mm 2). The field for recording forces and deformations should, as a rule, be at least 250 ´ 350 mm. 6.9.2 The test results of each sample are recorded in the test report (Appendix D), and the test results of a batch of samples are recorded in the consolidated test report (Appendix E). 6.9.3 The compressive modulus of elasticity is determined on type I samples. The procedure for testing the sample and the methodology for constructing a test diagram based on the readings of the force transducer and strain gauge are given below. The sample is loaded to a voltage s 0 = 0.10 (the voltage corresponds to the expected value of the proportionality limit). At voltage s 0, strain gauges are installed on the sample and loaded with a stepwise increasing voltage up to (0.70-0.80). In this case, the difference between adjacent voltage steps D s is 0.10. Based on the test results, a diagram is constructed (Figure 3). The modulus of elasticity in compression E c, MPa (kgf/mm 2), is calculated using the formula
Where D F - load stage, N (kgf); D h av - average absolute deformation (shortening) of the sample when loaded by D F, mm.
Figure 3 - Test diagram for determining the compressive modulus of elasticity
To determine the modulus of elasticity in compression from the diagram F (D h), recorded on a recording device (see 4.2), the sample is loaded continuously to s = (0.7-0.8). The voltage corresponds to the expected value of the proportional limit. From the diagram, using formula (1), we determine the elastic modulus under compression E c. 6.9.4 The proportionality limit during compression is determined on samples of types I and II. The procedure for testing the sample and the methodology for constructing a diagram based on the readings of the force transducer and strain gauge are given below. The sample is loaded to a voltage s 0 = 0.10 (the voltage corresponds to the expected value of the proportionality limit). At voltage s 0, a strain gauge is installed on the sample and loaded with a stepwise increasing voltage up to (0.70-0.80), while the difference between adjacent voltage steps D s is (0.10-0.15). Next, the sample is loaded in voltage steps equal to 0.02. When the value of the absolute deformation (shortening) of the sample D h at a stress level equal to 0.02 exceeds the average value of the absolute deformation (shortening) of the sample D h (at the same stress level) at the initial linear elastic section by 2, 3 times, the tests are stopped .
Figure 4 - Test diagram for determining the proportional limit in compression
Based on the test results, a diagram is constructed and the proportionality limit during compression is determined (Figure 4). When constructing a diagram, draw a straight line OM, coinciding with the initial straight section. The ordinate axis OF is drawn through point O, and then the straight line AB is drawn at an arbitrary level, parallel to the abscissa axis. On this straight line lay a segment KN equal to half of the segment AK. Through point N and the origin of coordinates, draw a straight line ON and parallel to it a tangent CD to the curve. The tangent point determines the load F pts corresponding to the limit of proportionality during compression, MPa (kgf/mm 2), calculated by the formula
To determine the proportional limit under compression from the diagram F (D h), recorded on a recorder (see 4.2), the sample is loaded continuously to a stress exceeding the expected value of the proportional limit. From the diagram, using formula (2) and carrying out the above constructions, determine the proportionality limit for compression from . 6.9.5 The compressive elastic limit is determined on type II samples. The test procedure based on the readings of the force transducer and strain gauge is given below. The sample is loaded to a stress of 0.10 (the stress corresponds to the expected value of the compressive elastic limit). At voltage s 0, a strain gauge is installed on the sample and loaded with a stepwise increasing voltage up to (0.70-0.80). In this case, the difference between adjacent voltage steps D s is (0.10-0.15). Next, from voltage (0.70-0.80), the sample is loaded in voltage steps equal to 0.05. Tests are stopped when the residual shortening of the sample exceeds the specified tolerance value. Based on the test results, a diagram is constructed and the compressive elastic limit is determined (Figure 5).
Figure 5 - Test diagram for determining the compressive elastic limit
To determine the load F 0.05, calculate the absolute deformation (sample shortening) D h based on the base of the strain gauge. The found value is increased in proportion to the scale of the diagram along the absolute deformation axis and the segment of the obtained length OE is plotted along the abscissa axis to the right of point O. From point E, a straight line EP is drawn parallel to straight line OA. The point of intersection of P with the diagram determines the height of the ordinate, i.e. load F 0.05, corresponding to the compressive elastic limit s 0.05 MPa (kgf/mm 2), calculated by the formula
To determine the elastic limit under compression from the diagram F (D h), recorded on a recording device (see 4.2), the sample is loaded continuously to a stress exceeding the expected value of the elastic limit. From the diagram, using formula (3) and Figure 5, the compressive elastic limit is determined. 6.9.6 The yield strength (physical) under compression is determined on type III samples. The sample is continuously loaded to a voltage greater than the expected value and the diagram is recorded on a recorder (see 4.2). An example of determining the load F t corresponding to the yield strength (physical) is shown in Figure 6.
Figure 6 - Determination of the load F t corresponding to the compressive yield strength
Yield strength (physical), MPa (kgf/mm 2), calculated by the formula
6.9.7 The conditional compressive yield strength is determined on type III samples. The specimen is continuously loaded to a stress greater than the expected proof stress, θ, and the diagram is recorded on a recorder (see 4.2). The scale along the deformation axis is not less than 100:1, and along the load axis - 1 mm of the diagram should correspond to no more than 10 MPa (1.0 kgf/mm 2). Determination from diagrams recorded with a scale along the elongation axis of 50:1 and 10:1 is allowed if the initial height of the sample is greater than or equal to 25 and 50 mm, respectively. The resulting diagram is rebuilt taking into account the rigidity of the testing machine. Using the diagram (Figure 7), the load corresponding to the conditional yield strength (physical) during compression is determined, calculated by the formula
Based on the test results, a diagram F (D h) is constructed (Figure 8) and the load corresponding to the conditional compressive yield strength is determined, which is calculated using formula (5).
1 - characteristic of the rigidity of the testing machine; 2 - diagram F (D h), recorded on a recorder; 3 - diagram F (D h), recorded taking into account the rigidity of the testing machine
Figure 7 - Test diagram for determining the proof compressive yield strength
D h ost t - absolute residual deformation (shortening) of the sample
Figure 8 - Test diagram for determining the proof compressive yield strength
6.9.8 Ultimate compressive strength is determined on type III samples. The sample is continuously loaded until failure. The greatest load preceding the destruction of the sample is taken as the load corresponding to the ultimate compressive strength s in, MPa (kgf/mm 2), calculated by the formula
6.10 Test procedure for constructing a hardening curve 6.10.1 To construct a hardening curve, a series of identical cylindrical samples of types III and IV (see Section 3) are tested at several levels of specified loads. 6.10.2 The hardening curve is plotted in coordinates: ordinate - flow stress s s, abscissa - logarithmic strain (Figure 9) or in double logarithmic coordinates , (Figure 10).
Figure 9 - Experimental hardening curve in coordinates s s -
Figure 10 - Experimental hardening curve in logarithmic coordinates
Flow stress s s, MPa (kgf/mm 2), calculated by the formula
Where F is the axial compressive load, N (kgf). Flow stress s s 1, MPa (kgf/mm 2), is determined graphically from the experimental hardening curve with logarithmic strain (shortening) of the sample equal to 1. Logarithmic strain (shortening) is calculated using the formulas: for type III samples
For type IV samples
The test results of each sample are recorded in the test report (Appendix D), and the test results of a batch of samples are recorded in the summary report (Appendix D). Note - It is possible to construct a hardening curve based on relative deformation (shortening) e 
. 6.10.3 The procedure for testing the sample is given below. The sample is loaded to a specified load. The sample is unloaded to zero load and the final sample diameter dk is measured in two mutually perpendicular directions, and for type III samples, also the final height of the sample hk. The final diameter dk for type IV samples is measured in the middle of the upset sample (at a distance of 0.5 from the ends ). To determine dk of type III samples, measure the diameters of the upset samples at both ends in two mutually perpendicular directions and set the arithmetic mean value of the final diameter of the ends d t, and in the middle of the sample measure the maximum value of the final diameter of the upset workpiece, mm, calculated by the formula
The measurement results d to and h to are averaged. The final cross-sectional area of sample A is rounded as shown in Table 2. For type IV samples, a one-time test is carried out until the shoulders disappear. In order to achieve higher degrees of uniform deformation, a two-stage upsetting is used, and the value of the logarithmic deformation between upsettings must be at least 0.45. In a two-stage test, after the first upsetting, the samples are sharpened to form a cylindrical recess (type IV). The dimensions of the sample beads are selected according to Table 1. The ratio of the height of the ground sample to the diameter is taken according to Appendix A. For type III samples, it is allowed to use intermediate grinding for two-stage upsetting, and the logarithmic degree of deformation between stages must be at least 0.45. 6.10.4 Flow stress s s and the corresponding logarithmic strain values for given load levels are determined according to 6.10.2. 6.10.5 Construct a hardening curve (see Figures 9, 10). The methodology for processing experimental data is set out in Appendix E. 6.10.6 In justified cases (with a limited number of samples or when using the results to calculate processes associated with stepwise loading), type III samples are allowed to be tested with a stepwise increase in load (Figure 11). In this case, the test results for constructing a hardening curve are processed by regression analysis (see Appendix E).
Figure 11 - Carrying out tests with a stepwise increase in load
6.10.7 Testing of samples is considered invalid: - if the beads of type IV samples are torn off during loading; - when the sample is destroyed due to defects in metallurgical production (layers, gas holes, films, etc.). The number of samples to be tested to replace those found invalid must be the same. 6.11 When testing samples of all types, comply with all technical safety rules provided for when working on this equipment. Testing of type IV samples must be performed using a device (see Appendix B).
APPENDIX A
(informative)
DETERMINATION OF THE SIZES OF SAMPLES III, IV TYPES
Type III specimens for constructing the hardening curve are made with a height h0 exceeding the diameter d0. For type IV samples it is allowed. The initial ratio should be as high as possible while ensuring longitudinal stability. The height of the sample h 0 is determined by the formula
, (A.1)
Where n is the strain hardening indicator; n - height reduction coefficient (n = 0.5 - for type III samples; n = 0.76 - for type IV samples). The height of the sample h0 after determination by formula (A.1) is rounded to the nearest whole number. The ratio for ground samples is taken equal to 1.0. The values of n indicators for widely used metals and alloys are given in Table A.1. The thickness of the bead u 0 (section 4) is taken equal to 0.5-0.8 mm for samples made of plastic and medium-strength materials and 1.0-1.2 mm for brittle materials. Large values of u 0 are chosen for samples made from materials with high strength properties, and when preparing samples for repeated upsetting. Table A.1 - Value of the strain hardening index during compression of the rod material
|
Material |
Material condition |
Strain hardening index n |
|
1 TECHNICALLY PURE METALS |
||
| Iron | Normal annealing | |
| Annealing in vacuum | ||
| Aluminum | Annealing | |
| Copper | Annealing | |
| Nickel | Annealing | |
| Silver | Annealing | |
| Zinc | Annealing | |
| Molybdenum | Recrystallization annealing | |
| Magnesium | Pressing | |
| Tin | - | |
| Uranus | - | |
|
2 CARBON STEEL |
||
| With carbon content 0.05-0.10% | Hot rolling | |
| With carbon content 0.10-0.15% | Annealing | |
| Partial annealing | ||
| Normalization | ||
| With carbon content 0.20-0.35% | Annealing | |
| Partial annealing | ||
| Normalization | ||
| Hot rolling | ||
| With carbon content 0.40-0.60% | Annealing | |
| Partial annealing | ||
| Normalization | ||
| Hot rolling | ||
| With carbon content 0.70-1.0% | Annealing | |
| Partial annealing | ||
| Hot rolling | ||
| With carbon content 1.1-1.3% | Partial annealing | |
|
3 ALLOYED STRUCTURAL AND TOOL STEELS |
||
| 15X | Hot rolling | |
| 20X | Annealing | |
| Normalization | ||
| Quenching + tempering at t = 650 °C | ||
| Quenching + tempering at t = 500 °C | ||
| 35X | Hot rolling | |
| 40X | Annealing | |
| Normalization | ||
| Quenching + tempering at t = 400 °C | ||
| 45X | Hot rolling | |
| 20G | Annealing | |
| Normalization | ||
| 10G2 | Annealing | |
| 65G | Hot rolling | |
| 15ХГ | Annealing | |
| Hot rolling | ||
| 40ХН | Annealing | |
| 35ХС | Annealing | |
| Normalization | ||
| 12ХН3А | Annealing | |
| Normalization | ||
| Quenching + tempering at t = 600 °C | ||
| Hot rolling | ||
| 4ХНМА | Annealing | |
| Normalization | ||
| Quenching + tempering at t = 600 °C | ||
| Hot rolling | ||
| 30ХГСА | Annealing | |
| Normalization | ||
| 18ХГТ | Annealing | |
| 17GSND | Normalization + aging at t = 500 °C | |
| 17SSAYU | Normalization | |
| hvg | Annealing | |
| 5ХНВ | ||
| 7Х3 | ||
| X12F | ||
| 3Х3В8Ф | ||
| P18 | ||
|
4 HIGH-ALLOY STEEL |
||
| 20Х13 | Annealing | |
| 12Х18Н9 | Normalization | |
| 12Х18Н9Т | Oil hardening | |
| Quenching in water | ||
| 20Х13Н18 | Oil hardening | |
| 10Х17Н13М2Т | Quenching in water | |
|
Austenitic steels type 09Х17Н7У, 08Н18Н10, 10Х18Н12, 10Х23Н18 |
||
| 17-7 | Hardening | |
| 18-8 | ||
| 18-10 | ||
| 23-20 | ||
|
5 ALUMINUM ALLOYS |
||
| AMg2M | Annealing | |
| A mg6 | Annealing | |
| D1 | Annealing | |
| Hardening + natural aging | ||
| Aging at t = 180 °C | ||
| Aging at t = 200 °C | ||
| 1915 | Hardening | |
| Zone aging | ||
| Aging to maximum strength (steady state) | ||
| Pressing | ||
| AK4-1 | Annealing | |
| Hardening + aging | ||
| AB | Pressing | |
| D20 | Pressing | |
| D16 | Pressing | |
|
6 COPPER ALLOYS |
||
| Brass L63 | Annealing | |
| Brass LS59-1V | Annealing | |
| Brass CuZn15 (15% Zn) | - | |
| Brass CuZn30 (30% Zn) | - | |
| Bronze OF7-0.25 | Annealing | |
| Bronze C u A l 41 (41% A l) | - | |
|
7 TITANIUM ALLOYS |
||
| OT4 | Annealing in vacuum | |
| VT16 | Annealing in vacuum | |
Where m is Poisson’s ratio, the values of which for a number of metals are given in Table A.2. ______________ 1) In the case of repeated upsetting, samples are made with a bead height 0.02-0.03 mm less than the calculated one. Table A.2 - Values of Poisson's ratios m of metals and alloys
|
Name of metals and alloys |
|
| Carbon steels with high manganese content (15G, 20G, 30G, 40G, 50G, 60G, 20G2, 35G2) | |
| Iridium | |
| Steel 20Х13, 30ХНМ | |
| Austenitic steels | |
| Iron, low-carbon steels and high-alloy steels of grades 30Х13, 20Н5, 30ХН3 | |
| Zinc, tungsten, hafnium, steels with high carbon content, steel 40ХН3 | |
| Chromium, molybdenum | |
| Cobalt | |
| Aluminum, duralumin, nickel, zirconium, tin | |
| Titanium, magnesium alloys | |
| Tantalum | |
| Vanadium | |
| Silver | |
| Copper | |
| Niobium, palladium, platinum | |
| Gold | |
| Lead | |
| Indium |
Figure A.1 - Dependence of the optimal value of the bead height on Poisson's ratio
APPENDIX B
(informative)
TYPES OF STRENGTHENING CURVES
There are eight types of hardening curves constructed from the results of compression tests (Figure B.1). The course of the hardening curves s s () is determined mainly by the nature of metals and alloys (Figure B.1a, b, c, d, e), the type and mode of preliminary thermal and plastic treatment (Figure B.1f, g, j). The most common type is the hardening curve shown in Figure B.1a. This type of hardening curve is characteristic of heat-treated and hot-rolled carbon and alloy structural and tool steels, many high-alloy steels, iron, aluminum and its alloys, copper and titanium and most of their alloys, light metals and a number of hard-to-deform metals and their alloys. In these hardening curves, the flow stress increases relatively strongly at the initial stages of deformation; subsequently, the intensity of hardening gradually decreases, and then remains almost unchanged with increasing deformation. For ductile metals and alloys, the intensity of the increase in s s with growth is less than for durable metals and alloys. The second type of hardening curves (Figure B.1b) is characterized by a high intensity of hardening, which may decrease slightly at high degrees of deformation. This type of hardening curve is typical for austenitic steels and some copper and titanium alloys. The third type of hardening (Figure B.1c) describes the dependence s s () of zirconium and the alloy based on it zircolai-2. For such hardening curves, the intensity of hardening at small degrees of deformation is very insignificant, and then increases sharply; an insignificant decrease in the intensity of hardening appears at degrees of deformation close to destruction. The fourth type of hardening curves (Figure B.1d) is different in that after reaching the maximum value of s s, its value either decreases or remains unchanged with a further increase. This type of hardening curves is established for zinc and its alloys with aluminum in the annealed state (curve 2), quenched and aged state (curve 1), as well as for some aluminum alloys at high degrees of deformation. The hardening curves presented in Figure B.1e are typical for superplastic materials. The course of the s s () curve for such materials is complex, with the manifestation of maxima and minima (the fifth type of hardening curves). The hardening curves presented in Figure B.1e (sixth type) are characteristic of various ductile alloys that have received pre-treatment by pressure in a cold state at relatively small deformations (approximately 0.1-0.15), and the directions of loads during preliminary and subsequent deformation are opposite ( for example drawing + upset). In this case, the intensity of the change in s s is less for alloys that have received a higher degree of preliminary deformation (curve 3 compared to curve 1). For such hardening curves, the intensity of the increase in s s growth over the entire range of degrees of deformation is less than for the hardening curves of the first three types (Figures B.1a, b, c). The hardening curves shown in Figure B.1g refer to alloys pre-deformed in a cold state with opposite directions of loads during preliminary and subsequent deformation, ductile steels with large degrees of pre-deformation (more than 0.1-0.15), medium and high steels. strength, brasses and bronzes with high degrees of preliminary deformation. The eighth type (Figure B.1i) of hardening curves corresponds to steels and some alloys based on them that have received pre-treatment in the form of cold plastic deformation, while the direction of load application for both deformations coincides. A flatter slope of the hardening curves (curves 3 and 4) corresponds to higher degrees of preliminary deformation. Such steels are characterized by a low intensity of growth of s s with increasing . The hardening curves of the first type are well approximated by the dependenceWith some approximation, dependence (B.1) describes the hardening curves of the second and third types. It is recommended to use this dependence to approximate the hardening curve of the fourth type in the range of degrees of deformation until a maximum appears on it. The hardening curves of the sixth, seventh and eighth types can be linearized with sufficient accuracy for practice and then, with some approximation, they can be approximated by the equation
Where is the extrapolated yield strength of pre-deformed steels (the segment cut off by the linearized straight line on the ordinate axis); b ¢ is a coefficient characterizing the slope of the linearized hardening curves.
Figure B.1 - Types of hardening curves
DESIGNS OF DEVICES FOR TESTING SPECIMENS IN COMPRESSION
Figure B.1 shows an assembly drawing of a device for conducting compression tests, which makes it possible to eliminate distortions between the sample and the deforming plate and reduce the loading error of the sample. The use of devices of other designs is allowed.
5 - sample; 6 - self-aligning support with replaceable insert
Figure B.1 - Compression test fixture
PROTOCOL Purpose of testing ________________________________________________________________ Testing machine. Type ___________________________________________________ Sample. Type ______________________________________. Hardness according to the Brinell or Rockwell scales _______________________________________________________ |
PROTOCOL Purpose of testing ________________________________________________________________ Testing machine. Type _____________________. Sample. Type ________________ |
|
Sample number |
Brinell or Rockwell hardness |
s s, MPa (kgf/mm 2) |
||||||||
CONSOLIDATED PROTOCOL Name of tests ______________________________________________________________ ___________________________________________________________________________ Characteristics of the tested material: Brand and condition. __________________________________________________________ Fiber direction __________________________________________________________ Type of workpiece _______________________________________________________________ Type and dimensions of the sample _______________________________________________________________ Condition of the surface of the sample ________________________________________________ Hardness on the Brinell or Rockwell scales _________________________________ ________________________________________________________________________ Type and main characteristics of the testing machine and measuring equipment: testing machine _______________________________________________________________ strain gauge _________________________________________________________________ displacement transducer _________________________________________________ measuring instruments and tools ___________________________________________ force transducer ___________________________________________________________ recorder ______________________________________________________________ Test conditions: Materials and hardness of deforming plates (HB or HR C e) _____________________ Relative deformation rate, s -1 _______________________________________ Loading speed, MPa/s (kgf/mm 2 × s) ________________________________________ Speed of movement of the deforming plate, mm/s _____________________________ |
PROCESSING OF EXPERIMENTAL DATA TO CONSTRUCT A STRENGTHENING CURVE. ESTIMATION OF PARAMETERS OF APPROXIMATING EQUATIONS
1 When testing a batch of samples, one sample is tested for each specific value. Strengthening curves described by equations (Figures B.1a, b, c) or (Figures B.1 e, g, j) are constructed based on the results of processing by the least squares method of all experimental points in the entire range of the studied degrees of deformation. Processing should be carried out on a computer. In this case, the parameters of the approximating equations, n, , b¢, are determined for the hardening curves.
Figure E.1 - typical dependences of the strain hardening index n on the degree of deformation
In the case of processing experimental data analytically, it is recommended to use reference literature. 2 With a limited number of tests With a limited number of experiments (five samples), hardening curves are constructed based on processing diagrams of machine records for the settlement of all test samples to the final degree of deformation. s s are calculated for values equal to 0.01; 0.03; 0.05; 0.08; 0.1, and then every 0.05 until the final value of the degree of deformation . For each value s s is determined as the average of the data (five points). The construction of hardening curves and further processing of experimental data are carried out as when testing a batch of samples. 3 Determination of the strain hardening index n at low degrees of deformation and in a narrow range For most metals and alloys, the dependence n () is not a linear function (Figure E.1): with growth n usually decreases, reaching an almost constant value at large values (Figure E.1a), or first increases, reaching a maximum, and then decreases (Figure E.1b). And only in some cases n is linear (Figure E.1 a). The first type of dependence (Figure E.1b) is typical for copper, carbon structural and tool steels, and a number of structural alloy steels. The type of dependence n presented in Figure E.1b is characteristic of materials that undergo structural-phase transformations during deformation - austenitic steels, some brasses. The value of n practically does not change with growth (Figure E.1c) for iron and chromium structural steels. For aluminum alloys, depending on their chemical composition, all three types of dependence n are observed. Due to the change in n with growth for most metals and alloys, there is a need to determine n at small degrees of deformation and in a narrow range. n can be determined by processing experimental data on a computer using the least squares method, however, the number of experimental points must be at least 8-10 in the considered range of degrees of deformation or calculated using the formula
. (E.1)
Mechanical testing of metals. Strength, determination of metal strength.
The choice of metal for the manufacture of machine parts and structures is determined by design, operational, technological and economic requirements.
The metal must have the necessary strength, the ability to deform, meet operating conditions (corrosion resistance, thermal and electrical conductivity, etc.) and have a minimum cost.
Strength is the main requirement for any metal used for the manufacture of machine parts and metal structures.
Strength is the ability of a material to withstand external loads without breaking. The measure of strength is the load that each square millimeter (or centimeter) of the section of the part can withstand.
The strength of a metal is determined by stretching samples of a certain shape and size on a testing machine. When stretched, the cross-sectional area of the sample decreases, the sample becomes thinner, and its length increases. At some point, stretching of the sample along its entire length stops and occurs only in one place; a so-called neck is formed. After some time, the sample ruptures at the site where the “neck” forms.
The stretching process occurs only in viscous materials; in brittle materials (hard steel, cast iron), the sample ruptures with insignificant elongation and without the formation of a “neck.”
By dividing the maximum load that the sample withstood before breaking (the load is measured by a special device - a force meter included in the design of the testing machine), by its cross-sectional area before stretching, the main characteristic of the metal is obtained, called the tensile strength (σ in).
The designer needs to know the tensile strength of each metal to determine the dimensions of the part, and the technologist - to assign processing modes.
At elevated temperatures, short-term tensile tests are performed on conventional testing machines, only a furnace (usually an electric muffle) is built into the machine to heat the sample. The furnace is mounted on the machine frame so that the muffle axis coincides with the machine axis. The test sample is placed inside the oven. For uniform heating, the furnace must be 2-4 times longer than the sample, and therefore it is impossible to secure it directly in the grips of the machine. The sample is secured in special extensions made of heat-resistant steel, which in turn are secured in the grips of the machine.
To obtain consistent results, it is necessary that the sample is kept at the test temperature for 30 minutes. The tensile strength of a heated metal is significantly influenced by the tensile speed: the higher the speed, the greater the tensile strength. Therefore, to correctly assess the heat resistance of steel, the duration of the tensile test should be 15-20 minutes.
Tensile testing of a metal consists of stretching a sample with plotting the dependence of the elongation of the sample (Δl) on the applied load (P), followed by rebuilding this diagram into a diagram of conditional stresses (σ - ε)
Tensile tests are carried out according to the same GOST, and the samples on which the tests are carried out are determined.
As mentioned above, during testing, a metal tensile diagram is constructed. It has several characteristic areas:
- Section OA is a section of proportionality between load P and elongation ∆l. This is the area where Hooke's law is preserved. This proportionality was discovered by Robert Hooke in 1670 and later became known as Hooke's law.
- The OB section is a section of elastic deformation. That is, if a load not exceeding Ru is applied to the sample and then unloaded, then during unloading the deformation of the sample will decrease according to the same law according to which they increased during loading
Above point B, the tension diagram deviates from a straight line - the deformation begins to grow faster than the load, and the diagram takes on a curvilinear appearance. At a load corresponding to Рт (point C), the diagram goes into a horizontal section. At this stage, the sample receives significant permanent elongation with virtually no increase in load. The formation of such a section on the stress-strain diagram is explained by the property of the material to deform under constant load. This property is called the fluidity of the material, and the section of the stress-strain diagram parallel to the abscissa axis is called the yield area. 
Sometimes the yield plateau is wavy in nature. This more often concerns the stretching of plastic materials and is explained by the fact that first a local thinning of the section is formed, then this thinning spreads to the adjacent volume of the material and this process develops until, as a result of the propagation of such a wave, a general uniform elongation occurs, corresponding to the yield area. When there is a yield tooth, when determining the mechanical properties of a material, the concepts of upper and lower yield limits are introduced.
After the yield plateau appears, the material again acquires the ability to resist stretching and the diagram rises. At point D the force reaches its maximum value Pmax. When the force Pmax is reached, a sharp local narrowing appears on the sample - a neck. A decrease in the cross-sectional area of the neck causes a drop in the load and at the moment corresponding to point K of the diagram, the sample ruptures.
The applied load to stretch a specimen depends on the geometry of that specimen. The larger the cross-sectional area, the higher the load required to stretch the sample. For this reason, the resulting machine diagram does not provide a qualitative assessment of the mechanical properties of the material. To eliminate the influence of sample geometry, the machine diagram is reconstructed in coordinates σ − ε by dividing the ordinate P by the original cross-sectional area of the sample A0 and the abscissa ∆l by lo. The diagram rearranged in this way is called a conditional stress diagram. Already from this new diagram, the mechanical characteristics of the material are determined.
The following mechanical characteristics are determined:
Proportionality limit σпз– the greatest stress after which the validity of Hooke’s law is violated σ = Eε, where E is the modulus of longitudinal elasticity, or the modulus of elasticity of the first kind. In this case, E =σ/ε = tanα, i.e. module E is the tangent of the angle of inclination of the rectilinear part of the diagram to the abscissa axis 
Elastic limit σу- conditional stress corresponding to the appearance of residual deformations of a certain specified value (0.05; 0.001; 0.003; 0.005%); the tolerance for residual deformation is indicated in the index at σу 
Yield strength σт– stress at which an increase in deformation occurs without a noticeable increase in tensile load
Also distinguished proof strength- this is the conditional stress at which the residual deformation reaches a certain value (usually 0.2% of the working length of the sample; then the conditional yield strength is denoted as σ0.2). The value of σ0.2 is determined, as a rule, for materials that do not have a plateau or yield tooth on the diagram