Mechanical engineering parts and mechanisms, as well as tools intended for their processing, have a set of mechanical characteristics. Hardness plays a significant role among the characteristics. The hardness of metals clearly shows:
- wear resistance of metal;
- possibility of processing by cutting, grinding;
- resistance to local pressure;
- ability to cut other materials and others.
In practice, it has been proven that most of the mechanical properties of metals directly depend on their hardness.
Hardness concept
The hardness of a material is its resistance to destruction when a harder material is introduced into the outer layer. In other words, the ability to resist deforming forces (elastic or plastic deformation).
Determination of the hardness of metals is carried out by introducing into a sample solid, called an indenter. The role of the indenter is performed by: a metal ball of high hardness; diamond cone or pyramid.
After exposure to the indenter, an imprint remains on the surface of the test sample or part, the size of which determines the hardness. In practice, kinematic, dynamic, and static methods of measuring hardness are used.
The kinematic method is based on the compilation of a diagram based on continuously recorded readings that change as the tool is pressed into the sample. Here the kinematics of the entire process is traced, and not just the final result.
The dynamic method is as follows. Measuring tool affects the part. The reverse reaction allows you to calculate the expended kinetic energy. This method allows you to test the hardness of not only the surface, but also a certain volume of metal.
Static methods are non-destructive methods that allow you to determine the properties of metals. The methods are based on smooth indentation and subsequent holding for some time. The parameters are regulated by methods and standards.
- pressing;
- scratching;
- cutting;
- rebound
Machine-building enterprises in this moment To determine the hardness of materials, the Brinell, Rockwell, Vickers methods, as well as the microhardness method, are used.
Based on the tests carried out, a table is compiled indicating the materials, the applied loads and the results obtained.
Hardness Units
Each method of measuring the resistance of a metal to plastic deformation has its own methodology, as well as units of measurement.
The hardness of soft metals is measured using the Brinell method. This method non-ferrous metals (copper, aluminum, magnesium, lead, tin) and alloys based on them, cast iron (except for white) and annealed steel are exposed.
Brinell hardness is determined by indentation of a hardened, polished ball made of ShKh15 ball bearing steel. The circumference of the ball depends on the material being tested. For hard materials - all types of steel and cast iron - 10 mm, for softer materials - 1 - 2 - 2.5 - 5 mm. Required load applied to the ball:
- iron alloys – 30 kgf/mm2;
- copper and nickel – 10 kgf/mm2;
- aluminum and magnesium – 5 kgf/mm2.
The unit of hardness measurement is a numerical value followed by a numerical index HB. For example, 200 NV.
Rockwell hardness is determined by the difference in applied loads to the part. First, a preliminary load is applied, and then a general load, at which the indenter is introduced into the sample and held.
A pyramid (cone) of diamond or a ball of tungsten carbide (hardened steel) is introduced into the test sample. After removing the load, the depth of the indentation is measured.
The unit of measurement for hardness is conventional units. It is generally accepted that one is the amount of axial displacement of the cone, equal to 2 μm. The hardness designation is marked with three letters HR (A, B, C) and numerical value. The third letter in the marking indicates the scale.
The technique reflects the type of indenter and the load applied to it.
Basically, measurement scales A and C are used. For example, the hardness of steel is HRC 26...32, HRB 25...29, HRA 70...75.
Products of small thickness or parts with a thin, hard surface layer are measured by Vickers hardness. The blade used is a regular tetrahedral pyramid with an apex angle of 136°. The display of hardness values is as follows: 220 HV.
Hardness measurement using the Shore method is carried out by measuring the rebound height of a fallen striker. Indicated by numbers and letters, for example, 90 HSD.
Microhardness is determined when it is necessary to obtain the values of small parts, thin coatings, or individual alloy structures. The measurement is made by measuring the imprint of the tip a certain shape. The value notation looks like this:
N □ 0.195 = 2800, where
□ — tip shape;
2800 – numerical value of hardness, N/mm 2.
Hardness of base metals and alloys
The hardness value is measured on finished parts sent for assembly. Control is carried out for compliance with the drawing and technological process. Tables of hardness values have already been compiled for all basic materials, both in the initial state and after heat treatment.
Non-ferrous metals
The Brinell hardness of copper is 35 HB, the values of brass are 42-60 HB units, depending on its brand. For aluminum, the hardness is in the range of 15-20 HB, and for duralumin it is already 70 HB.
Black metals
The Rockwell hardness of cast iron SCH20 HRC 22, which corresponds to 220 HB. Steel: tool – 640-700 HB, stainless steel – 250 HB.
To convert from one measurement system to another, tables are used. The values in them are not true, because they are derived imperially. Not the full volume is presented in the table.
| HB | H.V. | H.R.C. | HRA | HSD |
| 228 | 240 | 20 | 60.7 | 36 |
| 260 | 275 | 24 | 62.5 | 40 |
| 280 | 295 | 29 | 65 | 44 |
| 320 | 340 | 34.5 | 67.5 | 49 |
| 360 | 380 | 39 | 70 | 54 |
| 415 | 440 | 44.5 | 73 | 61 |
| 450 | 480 | 47 | 74.5 | 64 |
| 480 | 520 | 50 | 76 | 68 |
| 500 | 540 | 52 | 77 | 73 |
| 535 | 580 | 54 | 78 | 78 |
Hardness values, even if produced by the same method, depend on the applied load. The lower the load, the higher the readings.
Hardness measurement methods
All methods for determining the hardness of metals use mechanical action on the test sample - indentation of an indenter. But this does not destroy the sample.
The Brinell hardness method was the first to be standardized in materials science. The principle of testing samples is described above. It is subject to GOST 9012. But you can calculate the value using the formula if you accurately measure the imprint on the sample:
HB=2P/(πD*√(D 2 -d 2),
- D – ball circumference, mm;
- d – imprint circumference, mm.
The ball is selected relative to the thickness of the sample. The load is pre-calculated from accepted standards for the relevant materials:
iron alloys - 30D 2;
copper and its alloys - 10D 2;
babbitts, lead bronzes - 2.5D 2.
Download GOST 9012-59
Measuring the hardness of metals.
Methods for measuring the hardness of metals. One of the widespread types of metal testing is hardness determination. The hardness of a metal can be determined by direct and indirect methods.
Direct hardness testing methods involve pressing a special hard tip (hardened steel, diamond or carbide) into the sample. various shapes(ball, cone, pyramid). After removing the load, an imprint remains, the size of which characterizes the hardness of the sample.
With indirect methods, the properties of the metal are assessed, proportional to its hardness.
Hardness tests can be static or dynamic. The first type includes tests by the indentation method, the second - the impact indentation method.
Depending on the nature and method of application of the load, hardness indirectly characterizes various mechanical properties metals If the tip is pressed into the sample, the hardness characterizes the resistance to plastic deformation. If the tip scratches the
For example, hardness characterizes resistance to fracture. Hardness, determined by tip rebound, characterizes the elastic properties of the sample metal.
Based on the hardness of a metal, you can get an idea of the level of its properties. For example, the higher the hardness, determined by indentation of the tip, the lower the ductility of the metal, and vice versa.
The hardness measurement method has a number of advantages over other methods of mechanical testing of metal: simplicity of technique and speed of testing, simplicity of form and small sizes samples, the ability to test directly on the product without destroying it.
Hardness is determined using special devices - hardness testers.
Hardness testers can be stationary or portable. Basic device hardness testers for all hardness testing methods are the same.
The main components of hardness testers are a frame, a work table, a tip (a unit consisting of a mandrel and an indenter), a loading device, and a device for measuring the amount of deformation.
General scheme The test is as follows: a part or sample is placed on a work table, an indenter is pressed into the sample using a loading device, and after removing the load, the hardness is determined.
Depending on the purpose of the test, the properties of the metal being tested, the size of the sample, the shape, size and material of the indenter, the magnitude and duration of the load are selected.
The most common methods used to determine hardness are: Brinell hardness measurement - according to GOST 9012 - 59; Rockwell hardness measurement - according to GOST 9013 - 54; Vickers hardness measurement - according to GOST 2999 - 75; change in hardness using the impact imprint method - according to GOST 18661 - 73; measurement of microhardness by indentation of diamond tips - according to GOST 9450 - 76.
Exist General requirements for sample preparation and testing:
1. The production of samples and surface preparation must be carried out in ways that exclude changes in the properties of the metal due to heating or work hardening.
2. The surface of the sample must be clean, without oxide films, traces of rust or scale, cracks or other defects.
3. Samples must be of a certain thickness. After imprinting back side the sample should not show any signs of deformation.
4. The sample should lie rigidly and stable on the table. During testing, the sample must not move or bend.
5. The applied load must act perpendicular to the surface of the sample.
6. The load must be applied and increased smoothly to a given value, and then maintained constant for a certain time.
Brinell hardness measurement. When determining hardness by the Brinell method, a metal ball is pressed into the test sample or product for a certain time (Fig. 5). After removing the load, a spherical imprint remains on the surface of the sample. The size of the imprint depends on the hardness of the metal: the harder the metal, the smaller the size of the imprint will be. The Brinell hardness number is designated HB.
Rice. 5. Scheme of the location of the print when determining hardness using the Brinell method
To determine the hardness number HB (MPa or kgf/mm 2), you need the magnitude of the applied load R divided by the area of the print F:
,
Where D- ball diameter, m (or mm);
d- imprint diameter, m (or mm);
In order not to make calculations every time, when determining the hardness number, a specially compiled table is used (appendix to GOST 9012-59). Knowing the load, the diameters of the ball and the imprint, using this table you can determine the hardness number HB.
Balls made of hardened steel or hard alloy with a diameter of 2.5 are used for testing; 5.0 and 10 mm. The diameter of the ball is selected depending on the thickness of the test sample and its hardness: the thinner and harder the sample, the smaller the diameter of the ball should be. Typically, the test is carried out on a specially prepared horizontal sample area.
The thickness of the test sample must be at least ten times the depth of the indentation. The depth of the indentation is determined by a test test or, if the hardness level is known, by the formula
Where h- print depth;
D- ball diameter;
NV- hardness number.
There is the following relationship between tensile strength and hardness number HB:
For steel σ V= 0.34 HB;
For copper alloys σ V= 0.45 HB;
For aluminum alloys σ V= 0.35 HB.
The distance from the center of the print to the edge of the sample must be at least 2.5d, and between the centers of two adjacent prints - no less 4d.Print diameter d measured using a magnifying glass or a reading microscope (Fig. 6) in two mutually perpendicular directions and determine the arithmetic mean of the two determinations.
Depending on the hardness of the metal, the load on the ball can vary from 15.6 to 3000 kgf. In order for the test results to be comparable for any diameter of the ball taken, the following ratio must be maintained between the load and the diameter of the ball: P = 2.5D 2, P = 10D 2, P = = 30D 2.
The duration of application of the load must be sufficient to undergo deformation and increase with decreasing hardness of the test metal from 10 to 30 and 60 s.
When choosing the ball diameter D,loads R, duration of exposure under load t and the minimum thickness of the sample are guided by the table. 1.
The test results are recorded as follows. If the test is carried out with a ball with a diameter D= 10 mm under load R= 3000 kgf with holding time D= 10 s, then the hardness number is written with the symbol HB. For example, the hardness of steel is 350 HB. If the test conditions are different, this is indicated by the corresponding indices. For example, the hardness number is 230 and the test was carried out with a ball with a diameter of D= 5.0 mm at a load of 750 kgf with a load hold of 10 s. In this case, the results are written as follows: HB 5/750/10/230.
Rice. 6. Measuring the diameter of the print using a magnifying glass scale
Table 1
Selection of test parameters when determining hardness
Brinell method
| Material | Hardness range in Brinell numbers | Minimum thickness test sample, mm | Relationship between load P and ball diameter | Ball diameter D, mm | Duration under load, s | |
| Black metals | 140-150 | From 6 to 3 From 4 to 2<2 | P = 30D 2 | 10,0 5,0 2,5 | 187,5 | |
| <140 | >6 From 6 to 3<3 | P = 10D 2 | 10,0 5,0 2,5 | 62,5 | ||
| Non-ferrous metals | >130 | From 6 to 3 From 4 to 2 >2 | P = 30D 2 | 10,0 5,0 2,5 | 187,5 | |
| 35-130 | From 6 to 3 From 6 to 3<2 | P = 10D 2 | 10,0 5,0 2,5 | 62,5 | ||
| 8-35 | >6 From 6 to 3<3 | P = 2.5D 2 | 10,0 5,0 2,5 | 62,5 15,6 |
Rockwell hardness measurement. When measuring hardness by this method, a diamond cone or steel ball is pressed into the test sample under the action of a total load R. Moreover, a preload is applied first P 0, and then main P 1, i.e. P = P 0 + P 1. Hardness is determined by the depth of the indentation (Fig. 7). The Rockwell hardness unit is a conventional value corresponding to the axial movement of the tip by 0.002 mm. Depending on the hardness of the test sample, the test is carried out by indenting a diamond cone or ball at different values of the main and total load. When testing, hardness can be measured on three scales: A, B and C (Table 2).
The test surface can be flat or curved. The radius of curvature of the surface must be at least 15 mm. The minimum sample thickness must be no less than eight times the indenter penetration depth after removing the main load P1.
When measuring hardness, the distance between the centers of two adjacent indentations or the distance from the center of the indentation to the edge of the sample must be at least 3.0 mm. At least three measurements are carried out on each sample.
Rice. 7. Hardness test scheme using the Rockwell method
table 2
Selection of parameters when determining hardness by the Rockwell method
Vickers hardness measurement. When measuring hardness using this method, a diamond tip shaped like a regular tetrahedral pyramid is pressed into the sample. Load R valid for a certain time.
The load value can be as follows: 1.0; 2.0; 5.0; 10.0; 20.0; 30.0; 50.0; 100.0 kgf. The greater the load, the more accurate the result.
The duration of exposure of the sample under load is usually 10-15 s.
The surface of the test sample must be well prepared - its roughness should not exceed 0.16 microns. The minimum thickness of a steel sample should be 1.2 times greater than the diagonal of the imprint, and for samples made of non-ferrous metals 1.5 times. The radius of curvature of the surface must be at least 5 mm.
The prints are placed so that the distance between the center of the print and the edge of the sample or the edge of an adjacent print is at least 2.5 times the length of the print diagonal (Fig. 8).
Rice. 8. Scheme of the location of the print when determining hardness using the method
Vickers
The error when measuring diagonals should be no more than ±0.001 mm for diagonal lengths up to 0.2 mm, and for longer lengths no more than 0.5%.
Vickers hardness (HV) is calculated using the formula:
,
α - the angle between the opposite faces of the pyramid at the apex, equal to 136°;
d- arithmetic mean value of the lengths of both diagonals of the print after removing the load, mm.
If tests are carried out under standard conditions, then in order not to carry out calculations, use the table (appendix to GOST 2999-75), which shows the hardness depending on the length of the diagonal of the indent under different loads.
When recording test results under normal conditions, Vickers hardness is designated by the symbol HV. The usual test conditions are a load of 300 N (30 kgf) and a holding time of 10-15 s. In this case, the hardness is recorded, for example, HV 300. If the test conditions are different, then this is indicated by indices, and the load value is indicated first, then the holding time. For example, writing HV 20/40 - 250 means that with a load of 200 N (20 kgf) and a holding time of 40 s, the Vickers hardness is 250.
2.1. Determination of Brinell hardness
Hardness is determined by pressing a steel ball of a certain diameter (D) with a certain load (P) into the product.
The Brinell hardness number HB (N/m2) is equal to the ratio of the load to the indentation area (F).
НВ = Р/ F = Р/ π D h = 2P/ π (D – D 2 – d 2)
where d is the indentation diameter measured after removing the load, h is the indentation depth calculated from D and d.
To determine the hardness of a metal, balls of the following diameters are used: 2.5 mm; 5 mm and 10 mm, for metal with a thickness of up to 3 mm, respectively; 3-6 mm; more than 6 mm.
There is a certain relationship between the diameter of the ball and the load:
For ferrous metals P = 30 D 2;
For copper, brass, bronze P = 10 D 2;
For aluminum and its alloys P = 2.5 D 2.
2.2. Rockwell hardness determination.
The tip is pressed into the surface of the test material under the action of preliminary (P 1 = 100 N) and final (P 2) loads. A diamond cone with an apex angle of 120º or a hardened steel ball with a diameter of 1.59 mm for soft metals are used as tips for hard metals. Depending on the type of material being tested, the type of tip is selected and the final load is assigned. (see table 1.1). Readings are taken on one of the instrument scales (A, B or C). Depending on the scale by which the hardness number is determined, the following designations are accepted: HRA, HRB and HRC.
Table 1.1.
|
Type of metal being tested |
Tip type |
Brand designation |
||
Rockwell hardness is determined by the formula:
HR = K – (h 2 – h 1) / b
where h 1 and h 2 are the penetration depths of the tip under the action of preliminary (P 1) and final (P 2) loads, respectively, mm; K is a constant number with dimensions in mm; b – indicator scale division value corresponding to a tip recess of 0.002 mm.
2.3. Vickers hardness determination.
When determining hardness, a tetrahedral diamond pyramid with an apex angle of 136º is pressed into the test material. In this case, loads from 50 to 1200 N are used. After the load is applied, an imprint in the form of a square remains on the sample.
The hardness number is defined as the load per unit surface of the print.
НV = 2 P sin 0.5α / d 2
where P is the load on the pyramid; α – angle at the top of the pyramid; d – print diagonal length.
3. MEASUREMENT OF STRENGTH AND PLASTICITY OF REINFORCEMENT
BY STATIC STRETCH METHOD
Tensile testing is carried out on tensile testing machines with automatic recording of the tensile curve.
Test specimens are classified according to cross-sectional area normal And proportional . Normal samples have a cross-sectional area of 314 mm 2 (d 0 = 20 mm). They come in two types:
long (length of the design part ℓ 0 = 200 mm, and ratio ℓ 0 / d 0 = 10);
short (ℓ 0 = 100 mm, ℓ 0 / d 0 = 5);
The cross-sectional area of proportional samples can be arbitrary, and the design length is determined by the formula:
ℓ 0 = 11.3 F 0 or ℓ 0 = 6.65 F 0
where F 0 is the initial cross-sectional area of the samples, mm 2 .
Cast samples and samples from brittle materials are made with a design length ℓ 0 = 2.82 F 0 .
The load P is plotted on the vertical axis of the diagram, and the absolute elongation of the sample Δℓ is plotted along the horizontal axis.
In the section OR p, the elongation Δℓ of the sample increases in direct proportion to the load P p, called the proportional limit load. In this area, elastic (reversible) deformations of the sample occur and Hooke’s law is preserved (ε = σ / E). Limit of proportionality σ p is the maximum stress up to which the relative elongation of the sample remains directly proportional to the load P p.
σ р = Р р / F 0
The load P e at which the sample receives a residual elongation equal to 0.005% of the calculated length is called the elastic limit load. Elastic limit σ e is the stress at which the residual elongation is equal to 0.005% of the original length of the sample.
σ e = P e / F 0
The load P t at which the metal begins to flow is called the yield stress load, and the horizontal section of the curve is called the yield plateau.
σ t = Р t / F 0
Yield strength σ t is the lowest stress at which the sample is deformed without a noticeable increase in load.
Beyond the yield point, the load again increases to a certain maximum value P in, after which the formation of a local narrowing (neck) begins on the sample. A decrease in the cross-section in the neck area causes a decrease in the load, and at point K under load P z the sample ruptures. The highest load P at which necking begins is called the tensile strength load.
Tensile strength is the ratio of the highest load at which necking begins to the cross-sectional area of the sample.
σ in = P in / F 0
True tear resistance σ z is determined by the formula
σ z = Р z / F 1
where F 1 is the cross-sectional area of the sample at the fracture site.
The total deformation of the sample Δℓ p consists of the residual Δℓ rest and elastic deformation Δℓ ex. To determine these deformations, it is necessary to draw a straight line from point K on the tensile diagram, parallel to the straight section of the curve (Fig. 1) until it intersects with the abscissa axis.
R 
Rice. 1. Tension diagram
Relative elongation δ is the ratio of the increment in sample length after rupture to its calculated length, expressed as a percentage
δ = 100 (ℓ 1 – ℓ 0) / ℓ 0 (%)
where ℓ 1 is the length of the sample after rupture, mm; ℓ 0 – estimated sample length, mm.
Relative contraction ψ is the ratio of the reduction in cross-sectional area after rupture to the initial cross-sectional area, expressed as a percentage.
Ψ = 100 (F 0 – F 1) / F 0 (%)
where F 0 is the initial cross-sectional area of the sample, mm 2 ; F 1 – final cross-sectional area of the sample, mm 2.
EXPERIMENTAL PROCEDURE
Familiarize yourself with the theoretical part of the work. Define metallographic macroanalysis. Write down what determines the fibrous structure of steel. Write down the main defects of the weld. Define carburization, for what purpose and how carburization of steels is carried out. Define segregation and the effect of segregation of sulfur and phosphorus on the properties of steels.
Describe the methodology for preparing sections intended for studying steel fibrousness, defects in welded joints, depth of carburization and segregation of sulfur and phosphorus. Draw thin sections of the products studied in class.
Familiarize yourself with the operating principle of Rockwell and Vickers hardness testers and use them to determine the hardness of three reference metal samples. Enter the test results into the table.
CONTROL QUESTIONS
What is metallographic macroanalysis called?
What causes the fibrous structure of metals? Method for determining steel fiber content?
How is the depth of steel carburization determined?
The influence of sulfur and phosphorus segregation on the properties of steel.
Method for determining segregation of sulfur and phosphorus in steels.
The structure of the weld and methods for its examination.
Methods for determining the hardness of metals.
What is the difference between hardness HRC, HRB, HRA?
How is the strength, ductility and fluidity of a metal determined?
In order for parts and mechanisms to serve for a long time and reliably, the materials from which they are made must meet the necessary operating conditions. That is why it is important to control the permissible values of their main mechanical parameters. Mechanical properties include hardness, strength, and ductility. The hardness of metals is a primary structural characteristic.
Concept
The hardness of metals and alloys is the property of a material to create resistance when another body penetrates its surface layers, which does not deform or collapse under accompanying loads (indenter). Determined for the purpose of:
- obtaining information about acceptable design features and operational capabilities;
- state analysis under the influence of time;
- monitoring the results of temperature treatment.
The strength and aging resistance of the surface partly depend on this indicator. Both the source material and finished parts are examined.
Research options
The indicator is a value called the hardness number. There are various methods for measuring the hardness of metals. The most accurate studies involve the use of various types of calculations, indenters and corresponding hardness testers:
- Brinell: the essence of the apparatus is pressing a ball into the metal or alloy being tested, calculating the diameter of the indentation and subsequent mathematical calculation of the mechanical parameter.
- Rockwell: Uses a ball or diamond cone tip. The value is displayed on a scale or determined by calculation.
- Vickers: The most accurate measurement of metal hardness using a diamond pyramid tip.
To determine parametric correspondences between indicators of different measurement methods for the same material, there are special formulas and tables.
Factors that determine the measurement option
In laboratory conditions, if the necessary range of equipment is available, the choice of research method is carried out depending on certain characteristics of the workpiece.
- Approximate value of the mechanical parameter. For structural steels and materials with low hardness up to 450-650 HB, the Brinell method is used; for tool, alloy steels and other alloys - Rockwell; for hard alloys - Vickers.
- Dimensions of the test sample. Particularly small and thin parts are examined using a Vickers hardness tester.
- The thickness of the metal at the measurement location, in particular the cemented or nitrided layer.
All requirements and compliance are documented by GOST.
Features of the Brinell technique
Hardness testing of metals and alloys using a Brinell hardness tester is carried out with the following features:
- Indenter - a ball made of alloy steel or tungsten carbide alloy with a diameter of 1, 2, 2.5, 5 or 10 mm (GOST 3722-81).
- Duration of static indentation: for cast iron and steel - 10-15 s., for non-ferrous alloys - 30, a duration of 120 and 180 s is also possible in some cases.
- Limit value of the mechanical parameter: 450 HB when measured with a steel ball; 650 HB when using carbide.
- Possible loads. Using the weights included in the kit, the actual deformation force on the test sample is adjusted. Their minimum permissible values: 153.2, 187.5, 250 N; maximum - 9807, 14710, 29420 N (GOST 23677-79).
Using formulas, depending on the diameter of the selected ball and the material being tested, the corresponding permissible indentation force can be calculated.
Example notation:
400HB10/1500/20, where 400HB is the Brinell hardness of the metal; 10 - ball diameter, 10 mm; 1500 - static load, 1500 kgf; 20 - period of indentation, 20 s.
To establish accurate figures, it is rational to examine the same sample in several places, and determine the overall result by finding the average value of the obtained ones.
Determination of hardness using the Brinell method
The research process proceeds in the following sequence:
- Checking the part for compliance with the requirements (GOST 9012-59, GOST 2789).
- Selecting the required ball, determining the possible force, installing weights for its formation, and the period of indentation.
- Starting the hardness tester and deforming the sample.
- Measuring the diameter of the recess.
- Empirical calculation.
where F is load, kgf or N; A - print area, mm 2.
НВ=(0.102*F)/(π*D*h),
where D is the diameter of the ball, mm; h - indentation depth, mm.
The hardness of metals measured by this method has an empirical connection with the calculation of strength parameters. The method is accurate, especially for soft alloys. It is fundamental in systems for determining the values of this mechanical property.
Features of the Rockwell technique
This measurement method was invented in the 20s of the 20th century, and is more automated than the previous one. Used for harder materials. Its main characteristics (GOST 9013-59; GOST 23677-79):
- Availability of a primary load of 10 kgf.
- Holding period: 10-60 s.
- Limit values of possible indicators: HRA: 20-88; HRB: 20-100; HRC: 20-70.
- The number is visualized on the dial of the hardness tester and can also be calculated arithmetically.
- Scales and indenters. There are 11 different scales depending on the type of indenter and the maximum permissible static load. The most common in use: A, B and C.
A: diamond cone tip, apex angle 120˚, total permissible static influence force - 60 kgf, HRA; Thin products, mainly rolled products, are being studied.
C: also a diamond cone, designed for a maximum force of 150 kgf, HRC, suitable for hard and hardened materials.
B: 1.588 mm ball made of hardened steel or hard tungsten carbide, 100 kgf load, HRB, used to evaluate the hardness of annealed products.
A ball-shaped tip (1.588 mm) is applicable for Rockwell scales B, F, G. There are also scales E, H, K, for which a ball with a diameter of 3.175 mm is used (GOST 9013-59).
The number of tests performed with a Rockwell hardness tester on one area is limited by the size of the part. A repeat test is allowed at a distance of 3-4 diameters from the previous place of deformation. The thickness of the tested product is also regulated. It should be no less than the tip penetration depth increased by 10 times.
Example notation:
50HRC is the Rockwell hardness of a metal, measured using a diamond tip, its number is 50.
Rockwell Study Design
Measuring metal hardness is more simplified than for
- Assessment of the dimensions and surface characteristics of the part.
- Checking the health of the device.
- Determination of tip type and permissible load.
- Sample installation.
- Implementation of a primary force on the material of 10 kgf.
- Exercising full appropriate effort.
- Reading the resulting number on the dial scale.
A mathematical calculation is also possible to accurately determine the mechanical parameter.
Provided that a diamond cone is used with a load of 60 or 150 kgf:
HR=100-((H-h)/0.002;
when performing a test with a ball under a force of 100 kgf:
HR=130-((H-h)/0.002,
where h is the penetration depth of the indenter at a primary force of 10 kgf; H - indenter penetration depth at full load; 0.002 is a coefficient that regulates the amount of movement of the tip when the hardness number changes by 1 unit.
It is simple, but not accurate enough. At the same time, it allows the measurement of mechanical properties for hard metals and alloys.
Characteristics of the Vickers technique
Determining the hardness of metals using this method is the simplest and most accurate. The hardness tester works by pressing a diamond pyramidal tip into the sample.
Key Features:
- Indenter: diamond pyramid with apex angle 136°.
- Maximum permissible load: for steel - 5-100 kgf; for copper alloys - 2.5-50 kgf; for aluminum and alloys based on it - 1-100 kgf.
- Static load holding period: from 10 to 15 s.
- Test materials: steel and with a hardness of more than 450-500 HB, including products after chemical-thermal treatment.
Example notation:
where 700HV is the Vickers hardness number; 20 - load, 20 kgf; 15 - period of static force, 15 s.
Vickers study sequence
The procedure is extremely simplified.
- Checking the sample and equipment. Particular attention is paid to the surface of the part.
- Selection of permissible force.
- Installation of the test material.
- Putting the hardness tester into operation.
- Reading the result on the dial.
The mathematical calculation for this method is as follows:
HV=1.8544*(F/d 2),
where F is load, kgf; d - average value of the lengths of the print diagonals, mm.
It allows you to measure high hardness of metals, thin and small parts, while providing highly accurate results.
Ways to switch between scales
Having determined the diameter of the print using special equipment, you can use tables to determine the hardness. The metal hardness table is a proven assistant in calculating this mechanical parameter. Thus, if the Brinell value is known, the corresponding Vickers or Rockwell number can be easily determined.
Example of some match values:
Imprint diameter, | Research method |
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Brinell | Rockwell | Vickers |
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The metal hardness table is compiled on the basis of experimental data and is highly accurate. There are also graphical dependences of Brinell hardness on the carbon content in an iron-carbon alloy. So, in accordance with such dependencies, for steel with an amount of carbon in the composition equal to 0.2%, it is 130 HB.
Sample requirements
In accordance with the requirements of GOSTs, the tested parts must meet the following characteristics:
- The workpiece must be flat, lie firmly on the hardness tester table, its edges must be smooth or carefully processed.
- The surface should have minimal roughness. Must be sanded and cleaned, including using chemical compounds. At the same time, during machining processes, it is important to prevent the formation of work hardening and an increase in the temperature of the treated layer.
- The part must comply with the selected method for determining hardness using parametric properties.
Fulfillment of primary requirements is a prerequisite for measurement accuracy.
The hardness of metals is an important fundamental mechanical property that determines some of their other mechanical and technological features, the results of previous processing processes, the influence of temporary factors, and possible operating conditions. The choice of research methodology depends on the approximate characteristics of the sample, its parameters and chemical composition.
Hardness is the property of a material to resist penetration into it by another, harder body, such as a tool. The area of application of materials, their behavior during operation and preservation of appearance depend on hardness. This characteristic is used to evaluate the quality of metals, plastics, ceramics, wood, stone and other materials.
It significantly affects the nature and complexity of processing the material.
There are several ways to determine the hardness of materials: scratching, indentation, puncture with a standard needle, tests using a striker and oscillations of a pendulum. All of them are based on the introduction of a mineral, ball, pyramid, or punch into the test sample under a certain pressure: the lower the force and the greater the depth of penetration, the lower the hardness of the material, and vice versa.
The simplest and most common way in practice to determine the hardness of natural stone materials is to scratch them with other minerals on the hardness scale. Proposed in the last century by the German scientist F. Mohs, this scale contains 10 minerals from the softest (talc) to the hardest (diamond), and the serial number of the mineral in the scale corresponds to its hardness and each subsequent mineral leaves a line (scratch) on the previous one, but he himself is not drawn by it (see Table 3).
The hardness of other materials is determined in various ways, usually using special instruments. The hardness of metals, concrete, wood and plastics (except porous) is assessed by pressing a steel ball or diamond cone into the samples. The value of hardness is judged either by the depth of indentation of the ball or cone, or by the diameter of the resulting imprint.
Numerical characteristics of the hardness of materials are hardness numbers, which are summarized in various scales corresponding to different methods of measuring it. Hardness numbers are indicated in units of HB (Brinell method), HV (Vickers method), HR (Rockwell method), where H is the first letter of the English word hardness - hardness.
When determining hardness by the Rockwell method, additional designations are introduced: B (ball), C and A (cone, with different loads). Let us explain this using the example of determining the hardness of metals: for non-hardened parts, a hardened steel ball and a weight weighing 100 kg are used, the hardness is measured on the red scale B and designated HRB; for hardened parts of high hardness, a diamond cone and a load weighing 150 kg are used, the hardness is measured on the black scale C and denote HRC; For particularly hard or thin parts, a diamond cone is also used, but the load is 60 kg, the hardness is measured on the A scale of a special device and is designated HRA.
It should be noted that the hardness of a material does not always correspond to its strength. For example, wood, although significantly inferior to concrete in hardness, has the same strength as it.