Laboratory work -4
Topic: Determination of hardness of steels by carbon content. Brinnel and
Rockwell methods.
Necessary tools and materials
:
1. TSH-2M type hardness measuring device. 2. Magnifier for measuring the
mark on the metal surface. 3. Sample set. 4. Shtangentcircul' SHTS-1. 5. yellow
paper.
Work material:
steel
Theoretical information
The property of being able to resist the immersion of a body harder than this
material on the surface of any material is called its
hardness .
There are several ways to determine the hardness of metals. Among these
methods, Brinell and Rockwell methods are common.
The Brinell method is used to determine the hardness of base metals, non-
ferrous metals and alloys based on them. 2.5 diameter depending on the thickness of
the metals whose hardness should be determined; 5 and 10 mm rolled steel ball 1.875
per test sample; 2.5; 5.0; 7.5; With a force of 10 and 30 kN, it is slowly immersed
for a certain time (10, 30 and 60 sec), as a result, a trace of a steel ball remains on
the surface of the tested metal, the hardness of the metal is determined depending on
the diameter of this trace.
Figure 1 shows the scheme for determining metal hardness according to the
Brinell method.
Brinell hardness of the metal "NV"* force "R" pressing the ball on the tested
metal (N) on the surface of the ball trace formed on the surface of the tested metal
due to this force F( m ∙ m
2
) is determined by the ratio:
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If we express the surface of the trace left by the ball on the metal by the
diameter of the ball "D" and the depth of the trace "h", then the surface of the trace
will be as follows:
Figure 1.
Brinell method
Since it is difficult to measure the depth of the trace, F is filled from the following
formula:
The sign of hardness
N
is the initial letter of the English word Nagdness. The
letters after
N
indicate the corresponding hardness test methods, such as
V
- Brinell,
R
- Rockwell and
V - Winkers.
It usually takes the form of the Brinell hardness of the metal:
where: D is the diameter of the sphere, (mm).
d—diameter of the trace left by the ball on the metal, (mm).
The diameter of the track is measured with a special caliper.
In order to quickly determine the hardness of the sample, special tables are
used in practice. Hardness (NV) in these tables
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the values corresponding to the force (R) and the diameter of the trace (d) are
given (Appendix 1).
Balls are made of solid steel of SHX15 grade. They have a hardness of at least
8500 Vickers units after hardening and then releasing at low temperatures.
should not exceed 450 N/mm
2
according to Brinell, that is, the hardness of
the tested metals and the hardness of the sheet materials with a thickness of less than
1 mm are determined by this method. determination will not be appropriate. This is
considered a disadvantage of the Brinell method. The Brinell method has a number
of advantages as well as disadvantages.
They are the simplicity of the press and the approximation of the hardness
value (NV) determined by this method to the tensile strength limit ( d
v
); ie
In this formula, K is a dimensionless coefficient, which is determined by
experiment. For example, the value of K for steel is from 0.34 to 0.36.
Usually, before testing the sample, its surface to be tested is smoothed and
brought to a flat state. In the standard test, the load for a ball with a diameter of 10
mm is always 30 kN (3000 kg). The standard conditions for determining the Brinell
hardness of materials are listed in Table 1.
Brinell test conditions include load, ball diameter, and load exposure time.
For example, in NV 10 (3000)10-2500, the first number (10) is the diameter of the
ball, the second number (3000) is the load, the third number (10) is the time the load
is applied, and the fourth number (2500) is Brinell. represents hardness.
The scheme of the Brinell press is shown in Fig. 2. The sample or part to be
tested is placed on the base (1), and when the spindle (2) is rotated clockwise, the
ball rises to (3). After that, the electric motor (4) is activated, and the motor, in turn,
activates the system of levers in the press. When the lever system is activated, the
ball begins to sink into the sample under the influence of the pressure (5).
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