1. The document examines the influence of microstructural homogeneity on the uniformity of hardness values in hardness standard blocks. 2. Analysis of a certified 109 HB hardness standard found heterogeneous grain sizes and distributions, with some areas having much larger crystals than others. 3. Hardness measurements within coarse-grained areas showed values of 101 HB, significantly lower than the 108 HB measured in fine-grained areas. Excluding the outlier, the average was 108.5 HB, within specification. 4. The results demonstrate that microstructural heterogeneity can influence both the uniformity and magnitude of hardness values, which is particularly critical for certified reference materials like hardness standard blocks.

1. THE INFLUENCE OF MICROSTRUCTURE IN THE HOMOGENEITY OF
HARDNESS STANDARD BLOCKS
Authors: Iêda Maria Vieira Caminha
Tito Lívio Medeiros Cardoso
Robson Oliveira Centeno
Institution: Instituto Nacional de Tecnologia - INT
Venezuela avenue, n0
82, room 620 - Rio de Janeiro -RJ - Brazil
ZIP CODE 20081-310
Phone: (021)296-6611 Fax: (021)263-6552
e-mail: int@riosoft.softex.br
SUMMARY
The present work demonstrates, through measures of quantitative stereology accomplished by
means of an image digital analyzer, the direct influence among the degree of microstructural homogeneity
and the uniformity obtained in the hardness of the manufactured materials. This relationship is especially
critic in reference materials, such as hardness standards blocks, because of the rigid approaches that they
are submitted with relationship to the specifications of international standards in what concerns to its
mechanical properties and the variations admitted on them.
1. INTRODUCTION
The importance of the hardness for the industry has been a lot emphasized, therefore she is
versatile and useful for the estimate of the mechanical properties, being this the less onerous and faster
method of quantification of this greatness. This can be verified easily when it is observed requirements of
hardness in most of the customers' specifications and suppliers of materials and components of the metal-
mechanic segment, in the country as well as foreing.
Face to the current tendency of internationalization of markets and consequently of the
harmonization of the existent technical norms, the metrologic reliability in this field is constituted in a
survival subject for the institutions and research centers, as well as for the several segments of the
national industry. In this context, the hardness standard blocks are of fundamental importance for the
institutions and research centers as for the productive section, because through them the traceability of the
measures of hardness is established.
INT is involved, together with INMETRO and ITUC/PUC-Rio, in a wide project that has as
purpose to develop a metrologic system with reference scales in Brazil, whose procedures and standards
must have traceability to entities recognized worldwide. To give technical subsidies to the project, were
studied hardness standard blocks of several international origins, everybody with calibration certificates
emitted by institutions recognized internationally in the area of hardness.
1- Metallurgist Eng., M.Sc.
2- Mechanical Eng., M.Sc.
3- Mechanical technician

2. The hardness of the standard is function, among other factors, of the resulting microstructure for
several thermal treatments involved in its production. In the experiences driven in the ambit of the
referred projects, it was observed that, in many cases, the microstructures found in the different standards
didn't present high homogeneity degree, unlike of what would be desirable for hardness standard blocks
, on which the demand in the uniformity of hardness is quite high.
For this reason we decided to develop, in this work, a study for the evaluation of a standard of
hardness Brinell in the range of 100 HB, with calibration certificate emitted by entity recognized
internationally in the area of hardness, with the objective of evaluating the direct influence of the degree
of microstructural homogeneity in the uniformity of the values of hardness obtained in the analyzed
hardness standard.
2. METHODOLOGY
The analyzed hardness standard was manufactured for the indirect calibration of machines for
hardness testing according to the method Brinell and certified by an institute recognized in the area of
hardness, in agreement with the norm DIN EN 10003, in the scale HBW 2,5 mm / 187,5 kgf with a
nominal value of 109 HB and measure uncertainty of  1,0 HB, presenting 0,7% of repeatibility admitted
among successive indentations.
The standard was then obliquely cut-off, being extracted a slice of the same, which was mounted
in baquelite. Later on, the finishing of the sample was proceeded to grindpaper 600 and polish with
diamond pastes of 3 m and 1 m.
The sample was attacked in solution of alcohol and nitric acid at 2% and taken to the microscope
when it was observed that the same exhibited a microstructure predominantly ferritic with pearlite finely
dispersed in the grain boudaries, which were clearly defined.
The microstructural characteristics were studied in detail through a digital image analyze system
LECO model IA-3001 in the amplifications of 100X and 200X. Areas with non homogeneity in the grain
size were marked in its contour through a microhardness testing machine LECO AMH-3000 coupled to
the system images analyzer. Later, the sample was dismounted and some indentations of hardness Brinell
were carried out inside the areas marked by means of an automatic hardness testing machine INSTRON-
WOLPERT model 930 , with sphere of hardmetal (in agreement with the certificate of calibration of the
original block).
In order to obtain larger precision in the determination of the hardness in the selected areas, the
indented sample was taken to the image analizer in which, under an amplification of 50X, it was
proceeded the calculation of the diameter of each indentation through the determination of its area. This
technique besides not being subject to the inherent tendency to the choice of the positions for the
mensurement of the diameters, also presents a better resolution of 0,5 m as function of the micrometric
scale used for the calibration of the objective of the system. Starting from the obtained diameters, the
Brinell hardness corresponding was calculated.
3. RESULTS
The figure 1 presents the micrography of an area in which strong heterogeneity is observed in the
sizes of the grains (in the areas in prominence). The distribution of the grain sizes in this micrography is
represented in the graph of the figure 2.

3. 1) Micrography demonstrating strong heterogeneity in the hardness standard block. The
lines emphasize areas with much larger crystals than its neighborhood. 100X.
2) Distribution of the grain sizes in the micrography of the figure 1.
The graph of the distribution of grain sizes allows to identify a tendency of the curve in presenting
a distribution bimodal. That is resultant of the spreading of the values found for the grain size as well as
of the concentration of the same ones around two values: a minor (in the range of 5 to 10 m) and an
bigger (in the range of 15 to 20 m). In the figure 3, is presented with larger detail, the coarse-grained
area under amplification of 200X, which can be confronted with the fine-grained area presented in the
figure 4 under the same amplification.

4. 3) Details of the coarse-grained area. 200X.
4) Details of the fine-grained area. 200X.
The three micrographies presented in the figures 1, 3 and 4 were submitted to the processing in the
image analizer for the determination of the medium size of grain in each field. The results are presented in
the table 1.
Table 1 - Medium grain sizes in each selected area.
Micrography Description Medium grain size
Figure 1 Heterogeneous granulation. 100 X 13.0 m
Figure 3 Coarse-grained field. 200 X 14.4 m
Figure 4 Fine-grained field. 200 X 11.9 m

5. Clearly, the results manifest the heterogeneity of the microstructure. To identify the consequences
of this heterogeneity in the variation of the obtained hardness, the coarse-grained area on figure 3 was
marked as described in the topic regarding to the methodology, together with other three fine-grained
areas located by means of scanning of the image analizer on sample surface. Then, indentations of Brinell
hardness were carried out on the four areas. The obtained results are presented in the table 2.
Table 2 - Results of Brinell hardness associated to the diameters of the
indentatios on the 109  1,0 HB standard.
Indentation 1 2 3 4
Diameter [mm] 1,4156 1,4185 1,4604 1,4164
Hardness [HB] 108,7 108,2 101,4 108,5
The great difference is observed in the hardness obtained on indentation of number 3 -
accomplished in the coarse-grained area - if compared to the other three indentations - accomplished in
fine-grained areas. The average of these indentatios is 106,7 HB which is out of the measurement
uncertainty admitted in the calibration certificate of the block (1,0 HB). Be noticed that only the point
number 3 produces a difference of 8 HB. If to discard this point, the average of the remaining
indentations is 108,5 HB, which is in agreement with the variation around the reference value of 109 HB,
admitted in the certificate.
In the main parameters for the evaluation of hardness standards, one of the most importan is the
hardness uniformity, that is, the repetibility of values obtained as a result of successive series of
indentations. According to international standards, the repeatibility can be expressed in agreement with
the equation 1.
r = dMÁX - dMÍN (1)
dMÉD
In which dMÁX is the maximum value obtained in the measured diameters on a sequence of
indentations of Brinell hardness, dMÍN is the minimum value obtained in the measured diameters and dMÉD
it is the average of the found diameters. In the case of the sample in analysis, the repeatibility of the four
indentations is 3,14%, out therefore of the maximum value of 0,7% established in the certificate. If to
discard the measure of number 3 identation, the repeatibility it goes down for 0,20%, in agreement with
the value defined in the calibration.
4. CONCLUSION
The obtained results demonstrate as the microstructural heterogeneity can influence in the
uniformity and on magnitude of the values obtained for the hardness.
In the particular case of the indentation number 3 (on the area of the figure 3), it is important to
point out that the same corresponds to a heterogeneous field that mixes coarse-grained and fine-grained
areas. Considering that the produced indentation has a diameter of approximately 1,5 mm, was concluded
that the hardness is an resultant average of the underlying heterogeneous microestructure. Therefore, if
the heterogeneous granulation of the area was more extensive, probably the values of hardness for the
indentation number 3 would be much lower.
It should be attempted for the fact, particularly critical in certified reference materials, such as
hardness standard blocks. In fact, if the situation observed in the extracted sample to repeat symmetrically
for the whole volume of the block, the conclusion wold be that the standard in fact presents a low quality
and that values obtained in successive calibrations of the same will present different and not related
results whose magnitude and dispersion are directly influenced by the underlying microstructure.

6. 5. REFERENCES
1) Iron and Steel Institute, Production and Application of Clean Steels, 1972.
2) ASM Handbook, Vol.1, 10th
edition.
3) Souza, S.A., Propriedades Mecânicas dos Materiais Metálicos, 5th
edition. 1982.
4) Standard European, EN 10.003-3, 1994.
5) Honeycombe, R.W.K., Edward Arnold, London, 1982.