DETERMINATION OF HARDNESS OF METALS AND ALLOYSPurpose of the work: to develop practical skills at Brinell, Vickers and Rockwell hardness determination; to analyze the relationship between normalized steel hardness and its tensile strength; to investigate the carbon content influence on steel hardness. Theory. Hardness is the property of external layers of a material to resist plastic deformation (less frequently, brittle fracture) upon penetration from side of harder body called an "indentor". Indentor, however, has to remain without residual strain. In many cases a definite relationship exists between the hardness of a metal and its mechanical, service and technological properties (yield point or tensile strength, wear resistance, machinability, workability, etc). Therefore hardness determination tests have found exceptionally wide application in the quality control of metals and metal components. The main standard hardness determination methods are following: Brinell, Vickers (diamond pyramid) and Rockwell hardness tests. The testing conditions for these methods are regulated by GOST 9012-59, GOST 2999-75, GOST 9013-59 correspondently. The principle of the Brinell hardness test is shown in Fig. 1.7 a. This method consists in forcing the hardened steel ball 2.5, 5 or 10 mm in diameter D at a constant load P into a flat surface of a metal. When the load is removed, an impression (indentation) remains on the surface of the metal. Its diameter d is measured by optical magnifying device with a scale marked every 0.05 mm. The Brinell hardness number (HB) is the ratio of the load, applied to the ball in the test P (N), to the surface area of the impression obtained, F (m2):
Special standard table is used in practice to avoid the prolonged calculations. The diameter of the impression is measured and is used to find the HB number directly from the table. The ball diameter (2.5, 5 or 10 mm), the load P (from 156 to 30000 N) and the pressure holding (10, 30 or 60 sec) are selected to suit the expected hardness and the thickness of the test specimen (Table 1.3). The simplicity of the test and the accuracy of measurements are among the advantages of the method. Besides, the definite relationships in the form of su = kHB are known for several ductile alloys. The k is the coefficient equal to 0.35 for steel and duralumin; 0.55 – for copper and its alloys after annealing; 0.4 – for copper and its alloys after strain-hardening. Nevertheless, it is not recommended to employ the Brinell tests for metals of a hardness exceeding HB 4500 MPa as the ball may be deformed, thereby introducing errors into the test results. It is also impossible to testify thin components (with the thickness less than 2…3 mm) and thin surface layers (less than 1 mm). Sometimes the impression of 2…6 mm in diameter may be regarded as a demerit of the method.
Fig. 1.7. Principle of hardness tests: a – Brinell test; b – diamond pyramid (Vickers) test; c – Rockwell test
Vickers (diamond pyramid) method enables to avoid the drawbacks mentioned above. It consists in forcing a square-based diamond pyramid (with an angle of 136 degrees between opposite faces) into ground, or even polished, surface being tested (Fig. 1.7b). The Vickers hardness number (HV) is the ratio of the load P (N) applied to the diamond pyramid to the area of the permanent indentation F:
where d – arithmetic average of the two diagonals of the indentation after removing of the load. The average diagonal d is usually converted to the Vickers hardness number by the use of tables supplied with a testing machine. The applied load changes to suit the thickness of a specimen or surface layers and their hardness. The load may be equal to 10, 20, 50, 100, 200, 300, 500, 1000 N. Such variety of loads as well as the extreme hardness of the diamond pyramid enables to determine a hardness of almost any material. It is possible to measure the Vickers hardness on specimens with the minimal total thickness of 0.03…0.05 mm.
1.3. Table for selection of the Brinell hardness test parameters
According to the GOST 9012-59 and GOST 2999-75 hardness obtained by Brinell and Vickers methods have the same dimensions and coincide (approximately) for materials with hardness up to about 4500 MPa. They are denoted as follows: HB 230, HV 200 if the measurements are conducted in kgf/mm2, and HB 2300 MPa, HV 2000 MPa if the measurements are conducted in MPa. In Rockwell test the hardness is determined by the depth of an indentation remained after the removal of a load. The indentor or penetrator is a diamond cone (called a brale) with an apex angle of 120 degrees and radius of 0.2 mm, or a hardened steel ball 1.588 mm in diameter. The brale or ball are indented by two consecutive loads: the minor (preliminary load P0 equal to 100 N and the main (additional) load P1 (Fig. 1.7c). The sum of the mentioned loads represents the total load P, which is chosen in accordance with the type of indentor and expected hardness (Table 1.4).
1.4. The Rockwel1 test conditions
The hardness number is read directly on the dial of an instrument. Rockwell hardness is determined in conventional units. If the brale is employed, with the total loads of 1500,or 600 N, the hardness number is read on the black scale of the instrument and is denoted by HRC and HRA correspondently. The Rockwell hardness number is conventionally calculated by the formula:
where e is found by the formula:
h0 is depth of penetration of the indentor when preliminary P0 is applied, mm; h1 is depth of penetration of the indentor due to the total load P, mm. When the hardened steel ball is employed, the hardness number is depicted on the red scale shifted (displaced) fromthe initial point of the black one by 30 units. The formula, therefore modifies:
Thus, in a Rockwell test the hardness is determined by the depth of the indentation (after application of the two consecutive loads P0and P1), but not by its diameter or diagonals. The indicator's pointer marks not the depth, but the value of (100-e) on the black scale and (130-e) on the red one. It is necessary to take into account that Rockwell hardness numbers determined by different scales do not coincide. Scales A and C are used for testing rather hard materials and scale B for softer ones. The specimen thickness for Rockwell tests has to exceed 0.5…1 mm (8 depths of an indentation). The correspondence between hardness numbers determined by different methods is shown in Fig. 1.8. The Vickers hardness scale is recognized to be the basic standard for comparison in this case.
Fig.1.8. Relationship between hardness numbers determined by different methods.
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