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METALLURGY AND FOUNDRY ENGINEERING Vol. 34, 2008, No. 2
Janusz Krawczyk, Bogdan Paw³owski
THE EFFECT OF NON-METALLIC INCLUSIONS
ON THE CRACK PROPAGATION IMPACT ENERGY
OF TOUGHENED 35B2+Cr STEEL
1. INTRODUCTION
The investigations concerning the methods of restricting the content of non-metallic
inclusions in structural steels as well as modification of the morphology of these inclusions
for the improvement of the properties of such steels are still carried out by many researchers
[1–4]. Such research is being performed in spite of the fact, that today’s metallurgical tech-
nologies assure the content of non-metallic inclusions on the level required by correspond-
ing technical standards (for example PN-64/H-04510, DIN 50 602, and ASTM E45-
standards) [5]. Despite the fact, that many research works [6–8] describe the role of non-
metallic inclusions in initiation of cracking or in fatigue wear [8–10], there is still a need to
investigate the relation between non-metallic inclusions content and parameters describing
the impact energy of steels, independent of notch geometry (for example in the case of the
samples for impact energy testing). Such research is also important for the sake of verifica-
tion of the proposed theories of the influence of non-metallic inclusions on the properties of
steels [11, 12] and numerical models based on these theories [13]. One of such parameters
(material constant) can be for example a critical coefficient of stress intensity KIC [12].
However, this coefficient describes the crack resistance under static conditions and at plane
state of strain. To obtain such a parameter describing crack resistance under dynamic condi-
tions, it is necessary to apply Gulaev’s interpretation of it [14], allowing for division of the
energy of nucleation and development of the crack during impact testing.
The aim of this research is to describe the influence of non-metallic inclusions content
in 35B2+Cr toughened steel on the impact energy of crack development, described assum-
ing linear dependence between notch-root radius and the impact energy of crack nucleation,
as described by Gulaev [14].
2. MATERIAL FOR TESTING
The research was performed on 35B2+Cr steel used for screws, delivered by three
various suppliers. The chemical compositions of ingots are given in Table 1. After sphe-
roidizing annealing, samples taken from the ingots were toughened. Austenitizing was per-
formed at the temperature of 860o^ C for 60 minutes. Endothermic protective atmosphere of
+5 o^ C dew-point and 0.4% carbon potential was applied. Hardening was performed in
Hartenol 70S oil, and tempering was done at 450o^ C for 100 minutes. The microstructures of
the investigated toughened steels are show on Figure 1. It can be noticed, that the micro-
structures of steels obtained from different suppliers are very similar. The hardness of these
steels is also similar (see Tab. 2).
Table 1. The chemical composition (weight %) of investigated steels
Fig. 1. Microstructure of investigated steels: a) steel no. 1; b) steel no. 2; c) steel no. 3
Table 2. The hardness of investigated steels after toughening
- EXPERIMENTAL PROCEDURE
Impact tests were carried out on 10×10×55 mm samples. Two notch geometries were
applied: both were 2 mm deep, but with different notch-root radius (0.25 mm and 1 mm).
Samples were deformed on Charpy hammer (maximum energy of 150 J). All the tests were
performed at room temperature. Five impact tests were performed for each steel and for
each notch geometry.
Steel C Si Mn P S Cr Mo Ti Cu Al B N
no. 1 0.37 0.07 0.75 0.009 0.010 0.27 0.008 0.030 0.030 0.036 0.0040 0.
no. 2 0.38 0.08 0.73 0.010 0.005 0.28 0.006 0.035 0.046 0.042 0.0033 0.
no. 3 0.37 0.08 0.68 0.008 0.012 0.24 – – 0.050 0.042 0.0027 0.
a) b) c)
Steel no. 1 Steel no. 2 Steel no. 3
11
− HV 30^
15
− HV 30^
11
− HV 30
fine dispersion inclusions (Fig. 2b, c). The classic oxide strings were not observed. Non-
metallic inclusions of large dimension, characterized by “fuzzy” shape, were considered as
exogenous inclusions (Fig. 2c). The inclusions, described above as exogenous inclusions,
may partly be endogenous inclusions of silicates, but identifying them basing on the obser-
vations of polished cross-section, with a use of the light microscope, is difficult. For sim-
plicity, these inclusions are considered as exogenous in this work.
Summarized fraction of non-metallic inclusions as well as after dividing them into par-
ticular kind of inclusions in steels comming from the particular supplier is shown in Table 3.
Table 3. Non-metallic inclusions contribution (volume %) in investigated steels
The impact energy of crack development was evaluated assuming, according to Gulaev
theory, a linear dependence between notch-root radius and crack nucleation energy. For this
purpose, an average values of the impact energy determined in impact tests for the samples of
different notch-root radius were used. The macroscopic picture of fracture surfaces of sam-
ples is shown on Figure 3. The pictures, that were used for the approximation of the impact
energy of the crack development for steels delivered by particular supplier is shown on Fig-
ure 4. Steel No 2 was characterized by the greatest impact energy of crack development.
Having the measurements of the content of non-metallic inclusions as well as the val-
ues of impact energy of crack development, the relationship between them was searched
(Fig. 5–9).
There was no clear relationship between a total fraction of non-metallic inclusions and
impact energy of crack development (Fig. 5). Similar situation was in the case of relation-
ship between the oxide content and and the energy of the crack development (Fig. 6). The
advantageous influence of the content of sulfides (Fig. 7) and nitrides (Fig. 8) on the impact
energy of the crack development was proved. This phenomenon may be explained by the
ductility of sulfides and their elongation in the direction perpendicular to the plane of the
fracture during impact test. Such a morphology of the sulfides can restrict the propagation
of the crack. It is, however, difficult to explain such an influence of nitrides due to their
properties and morphology. Maybe it should not be considered in respect to the aluminum
bounded in a form of inclusions, but in respect to the aluminum dissolved in the alloy ma-
trix. In the case of a large number of unbounded in nitrides aluminum atoms present in the
alloy matrix, they may segregate to the grain boundaries, causing weakening if these areas.
Such an interpretation of the results of this research seems to be in agreement with the
analysis of the chemical composition of the investigated steels, especially in respect to the
aluminum content and nitrogen content (see Table 1). As opposed from mentioned above
sorts of non-metallic inclusions, exogenous inclusions strongly decrease the impact energy
of the crack development (Fig. 9).
Steel
non-metallic
inclusions
oxides sulfides nitrides
exogenous
inclusions
no. 1 0.299±0.032 0.157±0.012 0.091±0.041 0.024±0.011 0.027±0.
no. 2 0.270±0.015 0.136±0.006 0.095±0.023 0.027±0.003 0.012±0.
no. 3 0.210±0.012 0.115±0.017 0.062±0.023 0.016±0.001 0.016±0.
Fig. 3. The fracture surfaces after impact toughness tests of investigated steels according to samples
notch: a) steel no. 1; b) steel no. 2; c) steel no. 3; A) notch radius 1 mm; B) notch radius 0.25 mm
a)
b)
c)
Fig. 5. The effect of fraction of non-metallic inclusions on the crack propagation energy of investi-
gated steels
Fig. 6. The effect of fraction of oxide inclusions on the crack propagation energy of investigated steels
Fig. 7. The effect of fraction of sulfide inclusions on the crack propagation energy of investigated steels
sulfides, %
Fig. 8. The effect of fraction of nitride inclusions on the crack propagation energy of investigated
steels
Fig. 9. The effect of fraction of exogenous inclusions on the crack propagation energy of investigated
steels
- CONCLUSIONS
Obtaining similar microstructure and hardness in steels delivered by three different
suppliers, characterized by different content of non-metallic inclusions, with application of
linear approximation of the relation between notch-root angle and the impact energy of the
crack development, allowed to derive the following conclusions:
- The differences in non-metallic inclusions content, in the range given by techni- cal standard, have an influence on the impact energy of the crack development in toughened 35B 2 +Cr steel.
- A clear evidence of the influence of total content of non-metallic inclusions on the impact energy of crack development in the investigated toughened steel was not ob- served.
[13] Niezgodziñski T., Kubiak T., M³odkowski A.: Phenomenon of lamellar tearing in numerical calculation. Zeszyty Naukowe Politechniki wiêtokrzyskiej. Mechanika 7 3 ( 200 1) 2 33 239 (in Polish).
[14] Gulaev A.P.: Ðàçëîæåíèå óäàðíîé âÿçêîñòè íà åå ñîñòàâëÿþùèå ïî äàííûì èñïûòàíèÿ îáðàçöîâ ñ ðàçíûì íàäðåçîì (Rozloenie udarnoj vjazkosti na ee sostavljajuèie po dannym ispytanija abrazcov s raznym nadrezom - The decomposition of impact energy on the components on the ground of tests of impact strength samples with different notches). Zavodskaja Laboratorija 33 (1 967 ) 4 7 34 7 5 (in Russian).
Received December 2008
