The quantitative regularities of structure phase states formation in different Hbeam
cross sections under accelerated cooling in different regimes are established. The
gradient structure-phase states formation being characterized by the regular change of
dislocation substructure parameters, nanosized range -phase fragments and cementite
particles on cross section are revealed by methods of transmission electron diffraction
microscopy.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2068

Gromov, V.E.

Ivanov, Yu.F.

Konovalov, S.V.

Kosterev, V.B.

Electronic Sumy State University Institutional Repository

Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  293  NANOSIZE PHASES FORMATION UNDER LOW CAR-BON STEEL THERMOMECHANICAL STRENGTHENING Victor E. Gromov1*, Yurii F. Ivanov2, Vadim B. Kosterev3,  Sergey V.  Konovalov1  1 Siberian State Industrial University, Kirov str., 42, Novokuznetsk, 654007, Russia  2 Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences, av.Akademicheskii, 2/3, Tomsk, 634055, Russia  3 West Siberian Iron and Steel Plant, Kosmicheskoe av., 16, 654043, Novokuznetsk, Russia   ABSTRACT The quantitative regularities of structure phase states formation in different H-beam cross sections under accelerated cooling in different regimes are established. The gradient structure-phase states formation being characterized by the regular change of dislocation substructure parameters, nanosized range Į-phase fragments and cementite particles on cross section are revealed by methods of transmission electron diffraction microscopy.   Key words: structure-phase states, dislocation substructure, strengthening, H-beam   INTRODUCTION Currently, the production of rolled steel uses thermo-mechanical hardening technology, providing enhanced mechanical properties without the use of expensive alloying additives [1-3]. Targeted management of the rolled steel operational properties, and development of optimal modes of its hardening should be based on knowledge of the structure-forming process under various technological operations [1-3]. The aim of this study was to investigate the formation of nanosize struc-tural-phase states of 09G2S steel H-beam subjected to thermomechanical strengthening (accelerated cooling). MATERIALS AND METHODS  Temperature-speed parameters of rolling and accelerated cooling of two technological options are presented in Table.  1. As the research material we used H-beam DP155 made from 09G2S steel (0.095 wt. % C, 1.560 wt. % Mn, 0.660 wt. % Si). Analysis of structure phase states and dislocation structures were performed by transmission electron microscopy of thin foils on the different distances from cooling surface.                                                * e-mail: gromov@physics.sibsiu.ru, tel: +7(3843)462277 294                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II            Table 1. The modes of accelerated cooling Mode Number  Rolling speed,  V, m/s Ɍ, ºɋ Water pressure at water supply, atm I II I after 3rd roll stand after 9th roll stand when entering the cooler 1b 2t 3b 4t 1m Ɋ1 4.5 1050 -1150 1040 - 1080 690-730 1.5 1.5 2.5 2.5 3.5 Ɋ2 6.0 1050 -1160 1060 - 1100 800-850 1.5 1.5 2.5 2.5 3.0  The plates with thickness of 0.3 mm were cut out from the segment parallel to the inner surface of the profile of the H-beam at a distances of 4, 7 and 10 mm from the surface cooling, and in addition, the layer structure, directly adjacent to the cooling surface was analyzed.  The diffraction analysis with the dark-field technique and subsequent indexing of microelectron diffraction pictures was used for identification of the phases. The images of material's fine structure (bright-field image) were used to classify the morphological features of the structure; determine the size, the volume fraction and the localization of the particles of the second phases; measure the scalar U dislocation density [4, 5]. RESULTS AND DISCUSSION Thermostrengthening of the 09G2S steel of the H-beam performed on the device of accelerated cooling according to regimes P1 and P2, leads to the formation of a multilayer (surface, transitional and central layers) of the microstructure of the H-beam profile. The structure of the stele in the transition layer and the central zone of the shelve under these cooling conditions resulted from the transformation according to diffusion mechanism, and consists of ferrite, perlite, a "degenerate" pearlite and carbide precipitates along the ferrite grain boundaries. The structure of the surface layer is formed as a result of the intermediate (regime P2) and shear (regime F1) mechanisms of J  D transformation, followed by "self-tempering" process. Quantitative characteristics of the surface (hardened) layer, reflecting the most significant effect of hardening regime on the substructure of steel and identified according to the results of electron microscopic studies are presented in Table. 2. Accelerated cooling of the surface is accompanied by formation of the gradient of the structure phase states. Quantitative relationships that characterize the gradient nature of the organization of the defect substructure of H-beam subjected to forced water cooling are shown in Fig. 1. Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  295  Table 2. Quantitative characteristics of the structure of the 09G2S steel hardened layer Process mode The parameters of the structure 'V1 d1, ȝm U1, 1010  cm-2 'V2 d2, ȝm U2, 1010 cm-2 <U>, 1010  cm-2 <d>, ȝm Ɋ1 0.9 0.2 5.0 0.1 0.5 2.8 4.78 0.23 Ɋ2 0.6 0.3 4.8 0.4 0.5 3.6 3.84 0.38 Note: 'V1, 'V2 - the volume fraction of lamellar (martensite or bainite) and subgrain type, respectively; d1,  d2 - the average transverse sizes of plates and subgrains, respectively; U1, U2 -  the scalar density of dislocations arranged in plates and subgrains, respectively; <U> - the average scalar density of dislocations in the layer (taking into account the types of structures); <d> - the average size of the substructure in the layer (taking into account the types of structures).  Analysis of the results presented in this figure confirms the conclusion about the gradient nature of the defected substructure. Namely, the closer to the cooling surface the scalar density of dislocations in the ferrite grains (Fig.  1, curve 2) and ferritic interlayers of pearlite grains (Fig.  1, curve 1), are increasing and the average size of fragments of the ferrite (Fig. 2, curve 1) and particle size of cementite (Fig. 2, curve 2) are reducing.  It can be seen that in the surface layer of H-beam the state is formed, which based on the average size of fragments of Į-phase and cementite particles can be regarded as a nanostructure.    Fig. 1 – Dependence of the scalar density of dislocations located in the ferrite component of the pearlite grains (curve 1) and ferrite grains (curve 2) on the distance to the surface of treatment Fig. 2 – Dependence of the cementite fragments average size (curve 2) and ferrite fragments D average size (curve 1) on the distance to the surface of treatment  Formation of nanostructured phases during thermomechanical hardening Analysis of the results presented above gives us grounds to conclude that the formation of nanoscale phase in the studied steel under thermomechanical processing and subsequent accelerated cooling of rolled products is possible with  the  implementation  of  such  a  processes.  First,  by  dispersing  of  pearlite  cementite plates colonies by cutting them by moving dislocations. Secondly, in 296                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II            the process of dissolution of pearlite colonies cementite plates and repeated precipitation of cementite particles on the dislocations, the boundaries of blocks, subgrains and grains. Third, during the decay of solid solution of carbon in the D-iron, formed under the conditions of accelerated cooling of the steel ("self-tempering" martensite). Fourth, when during the final transformation of the retained austenite in the structure of carbideless beinite with the formation of D-iron and cementite particles. Fifth, in the implementation of the diffusion mechanism of J  D transformation under the conditions of high degree of deformation and high temperature treatment.  Dispersion of the pearlite colonies cementite plates by cutting them by moving dislocations Fig.  3 shows electron microscopy images of the pearlite colony, the cementite plates which are divided into separate fragments (the blocks). Sizes of fragments vary from 5 to 30 nm. Simultaneously, cementite particles are found in ferritic interlayers of the pearlite colony, the sizes of whose particles vary from 5 to 10 nm (Fig. 3, the particles are indicated by arrows).    Fig. 3 – The fragmentation of the cementite plates of perlite grains, a, c – light-field image, b - dark field image obtained in the reflection [121] Fe3C; d - electron diffraction pattern image, the arrow indicates the reflection, in which a dark field image was obtained. On c) the arrows indicate cementite particles located in the plates of ferrite Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  297  Nanoscale range of the cementite structure of given pearlite colony is confirmed by quasicircular construction of the electron diffraction image derived from this plot foils (Fig. 3, d). Presented micrographs of the steel structure suggest that the thermomechanical processing is accompanied not only by mechanical destruction of the plates of cementite, but also their dissolution of with the departure of the carbon atoms on the dislocation and the subsequent precipitation in the body of ferrite plates. Dispersion of the cementite plates may be accompanied by the formation of the block (subgrain) structure (Fig.  4). The newly evolved cementite particles in such a structure are located at block boundaries, stabilizing their sizes.  Dissolution of cementite plates of the pearlite colonies and repeated precipitation of the cementite particles on the dislocations, the boundaries of the blocks, subgrains and grains  Removal of carbon atoms from destructed particles of cementite is possible and even at much greater distances. Studies of the block (subgrain) structure of D-iron grains by methods of dark-field analysis revealed the cementite particles in the body of the blocks on the dislocations and block boundaries (Fig. 5). Particles have a rounded shape, particle sizes vary from 5 to 15 nm.   Fig.  4 – The formation of a subgrain structure of the steel and fragmentation of the pearlitic grain cementite plates a - light-field image, b - dark field image obtained in the reflection [121] Fe3C; c - electron diffraction pattern image, the arrow indicates the reflection, in which a dark field image was obtained 298                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II             Fig. 5 – Precipitations of second phase particles (cementite) within the body and at the boundaries of D-phase subgrains: a - dark field image, obtained in the reflections [012] Fe3C + [110] D-Fe (particles indicated by arrows), b - electron diffraction pattern image, the arrow indicates the reflection, in which a dark field image was obtained.  The decay of solid solution of carbon in the D-iron, formed under the conditions of accelerated cooling of steel ("self-tempering" of martensite)  Accelerated cooling of the H-beam leads to the formation of the martensitic structure in the surface layer. Subsequent "self-tempering" of the steel under the influence of the residual heat is accompanied by relaxation of the dislocation substructure, which manifests itself in reducing the scalar density of dislocations, the destruction of low-angle boundaries of martensite crystals, precipitation on dislocations within the body of martensite crystals (Fig. 6a) and along the boundaries of cementite particles (Fig. 6b). The sizes of particles located on dislocations, vary between 5 ... 10 nm (Fig. 6a), located on the borders - in the 10 ... 30 nm range.   Fig. 6 – The microstructure of the hardened layer of the H-beam, cooled by P1 regime, a - bright-field image, b - dark field image obtained in the reflection [120] Fe3C; ɫ - electron diffraction pattern image, the arrow indicates the reflection, in which a dark field image was obtained. On (b) the arrows indicate cementite particles. Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  299  Formation of cementite particles during the process of complete transformation of retained austenite presented in the structure of carbideless bainite In the surface layer of the steel sample cooled according to P2 regime, along with grain-subgrain structure the structure of plate type, the so-called carbideless bainite was observed. As shown above, the plates are arranged parallel to each other and there was an alternation of plates of light and dark contrast. Microdiffraction analysis of these structures revealed the presence of only the reflections of D-phase. Reflections of the retained austenite and carbide phase particles are not detected. At the same time, mottled contrast reminiscent of the contrast from the pre-precipitates of second phase particles (Fig.  7) is revealed within the structure of the darker plates (formed, presumably as a result of the process of complete transformation of residual austenite).   Formation of nanosize phases as a  result  of  polymorphic J  D transformation The high level of steel plastic deformation, which is realized by thermomechanical processing of rolled products, leads to dispersion of structures formed in the process of diffusion of the J  D transfor-mation. Fig.  8 shows electron microscopy images of the structure of lamellar pearlite. The measurements show that the thickness of plates of D-phase, separated by the plates of carbide, is about 70 nm; the thickness of the plates of the carbide phase of ~ 25 nm. Formation of nano-sized particles of the carbide phase, is also observed in the formation of the so-called pseudoperlite, namely ferrite grains, containing particles of cementite of globular morphology (Fig. 9). The sizes of particles of cementite in these grains vary in the range 40 ... 60 nm.   Fig. 7 – Electron microscopic image of the structure formed at the transformation of the residual austenite; a - light field image, b - electron diffraction pattern image of (a). Arrows indicate pre-precipitates of second phase particles 300                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II             Fig. 8 – Electron microscopic image of the structure of the lamellar (plate) perlite: a – light field image, b - dark field image obtained in [021] Fe3C the reflection; c – electron diffraction pattern image, the arrow indicates the reflection, in which a dark field image was obtained  . Fig. 9 – Electron microscopic image of the pseudoperlite structure: a – light field image, b - dark field image obtained in the reflection [012] Fe3C; c - electron diffraction pattern image, the arrow indicates the reflection, in which a dark field image was obtained.  CONCLUSIONS A study of the H-beam's structure phase states after thermomechanical strengthening was carried out using the methods of transmission diffraction electron microscopy.  The formation of gradient structure, characterized by regular changes of parameters of the structure-phase states and dislocation substructure as it approaches to the surface of the accelerated cooling was revealed. It was established that nanosize structure-phase states are formed in the surface layer. The analysis of the processes leading to the formation of the structure within the beam profile of 09G2S steel nanoscale phases was carried out.  It  is  shown  that  the  formation  of  nanoscale  phases  was  carried  out  possibly in the implementation of the following processes. First, during dispersing of the cementite plates of pearlite colonies by cutting them by Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  301  moving dislocations. Secondly, during the dissolution of pearlite colonies cementite plates and repeated precipitation of the cementite particles on the dislocations, the boundaries of blocks, subgrains and grains. Third, during the decay of solid solution of carbon in the D-iron, formed under the conditions of accelerated cooling of steel ("self-tempering" of martensite). Fourth, during final transformation of the retained austenite in the structure of carbideless beinite with the formation of D-particles of iron and cementite. Fifth, during the implementation of the diffusion mechanism of J  D transformation under the conditions of high degree of deformation and high temperature processing.   Acknowledgements This work was supported by the Federal Program "Research and scientific-pedagogical personnel of innovation in Russia 2009-2013" (contract No P332) and Analytical service programme “The development of scientific potential of high school (2009 – 2011)” (project No 2.1.2/13482).  REFERENCES [1] V.E. Gromov, O.Yu. Efimov, V.B. Kosterev et al. Structure phase states and proper-ties of the strengthened rolled steel and cast iron rolls. Novokuznetsk. Inter-Kuzbass Publ., 2011. [2] E.G. Belov, L.M. Poltoratskii, O.Yu. Efimov, S.V. Konovalov, V.E. Gromov, Steel in Translation, 2010, 40(2), P. 114-118. [3] O.Yu. Efimov, A.B. Yur’ev, Yu.F. Ivanov, V.E. Gromov, M.M. Morozov. Steel in Translation, 2008, 38(12), P. 982-986. [4] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley, M.J. Whelan Electron mi-croscopy of thin crystals, Krieger Publishing Co., Melbourne, 1977. [5] N.A. Koneva, E.V. Kozlov, Yu.F. Ivanov, N.A. Popova, A.N. Zhdanov // Material Science and Engineering, 2005, A 410-411, P. 341-344.    

Nanosize phases formation under low carbon steel thermomechanical strengthening

SocioEconomic Challenges Journal (Sumy State University)

Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  293 
 
NANOSIZE PHASES FORMATION UNDER LOW CAR-
BON STEEL THERMOMECHANICAL STRENGTHENING 
Victor E. Gromov1*, Yurii F. Ivanov2, Vadim B. Kosterev3,  
Sergey V.  Konovalov1 
 
1 Siberian State Industrial University, Kirov str., 42, Novokuznetsk, 654007, Russia  
2 Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences, 
av.Akademicheskii, 2/3, Tomsk, 634055, Russia  
3 West Siberian Iron and Steel Plant, Kosmicheskoe av., 16, 654043, Novokuznetsk, Russia  
 
ABSTRACT 
The quantitative regularities of structure phase states formation in different H-
beam cross sections under accelerated cooling in different regimes are established. The 
gradient structure-phase states formation being characterized by the regular change of 
dislocation substructure parameters, nanosized range Į-phase fragments and cementite 
particles on cross section are revealed by methods of transmission electron diffraction 
microscopy.  
 
Key words: structure-phase states, dislocation substructure, strengthening, H-
beam  
 
INTRODUCTION 
Currently, the production of rolled steel uses thermo-mechanical 
hardening technology, providing enhanced mechanical properties without the 
use of expensive alloying additives [1-3]. Targeted management of the rolled 
steel operational properties, and development of optimal modes of its hardening 
should be based on knowledge of the structure-forming process under various 
technological operations [1-3]. 
The aim of this study was to investigate the formation of nanosize struc-
tural-phase states of 09G2S steel H-beam subjected to thermomechanical 
strengthening (accelerated cooling). 
MATERIALS AND METHODS  
Temperature-speed parameters of rolling and accelerated cooling of two 
technological options are presented in Table.  1. As the research material we 
used H-beam DP155 made from 09G2S steel (0.095 wt. % C, 1.560 wt. % Mn, 
0.660 wt. % Si). 
Analysis of structure phase states and dislocation structures were 
performed by transmission electron microscopy of thin foils on the different 
distances from cooling surface. 
                                               
* e-mail: gromov@physics.sibsiu.ru, tel: +7(3843)462277 
294                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
Table 1. The modes of accelerated cooling 
M
od
e 
N
um
be
r  
R
ol
lin
g 
sp
ee
d,
  
V
, m
/s
 
Ɍ, ºɋ 
Water pressure at water supply, 
atm 
I II I 
after 
3rd roll 
stand 
after 
9th roll 
stand 
when 
entering 
the cooler 
1b 2t 3b 4t 1m 
Ɋ1 4.5 1050 -1150 
1040 - 
1080 690-730 1.5 1.5 2.5 2.5 3.5 
Ɋ2 6.0 1050 -1160 
1060 - 
1100 800-850 1.5 1.5 2.5 2.5 3.0 
 
The plates with thickness of 0.3 mm were cut out from the segment 
parallel to the inner surface of the profile of the H-beam at a distances of 4, 7 
and 10 mm from the surface cooling, and in addition, the layer structure, 
directly adjacent to the cooling surface was analyzed.  
The diffraction analysis with the dark-field technique and subsequent 
indexing of microelectron diffraction pictures was used for identification of the 
phases. The images of material's fine structure (bright-field image) were used to 
classify the morphological features of the structure; determine the size, the 
volume fraction and the localization of the particles of the second phases; 
measure the scalar U dislocation density [4, 5]. 
RESULTS AND DISCUSSION 
Thermostrengthening of the 09G2S steel of the H-beam performed on the 
device of accelerated cooling according to regimes P1 and P2, leads to the 
formation of a multilayer (surface, transitional and central layers) of the 
microstructure of the H-beam profile. The structure of the stele in the transition 
layer and the central zone of the shelve under these cooling conditions resulted 
from the transformation according to diffusion mechanism, and consists of 
ferrite, perlite, a "degenerate" pearlite and carbide precipitates along the ferrite 
grain boundaries. The structure of the surface layer is formed as a result of the 
intermediate (regime P2) and shear (regime F1) mechanisms of J  D 
transformation, followed by "self-tempering" process. Quantitative 
characteristics of the surface (hardened) layer, reflecting the most significant 
effect of hardening regime on the substructure of steel and identified according 
to the results of electron microscopic studies are presented in Table. 2. 
Accelerated cooling of the surface is accompanied by formation of the 
gradient of the structure phase states. Quantitative relationships that 
characterize the gradient nature of the organization of the defect substructure of 
H-beam subjected to forced water cooling are shown in Fig. 1. 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  295 
 
Table 2. Quantitative characteristics of the structure of the 09G2S steel hardened 
layer 
Process 
mode 
The parameters of the structure 
'V1 d1, ȝm 
U1, 1010  
cm-2 'V2 
d2, 
ȝm 
U2, 1010 
cm-2 
<U>, 1010 
 cm-2 
<d>, 
ȝm 
Ɋ1 0.9 0.2 5.0 0.1 0.5 2.8 4.78 0.23 
Ɋ2 0.6 0.3 4.8 0.4 0.5 3.6 3.84 0.38 
Note: 'V1, 'V2 - the volume fraction of lamellar (martensite or bainite) and subgrain type, 
respectively; d1,  d2 - the average transverse sizes of plates and subgrains, respectively; U1, U2 -  the 
scalar density of dislocations arranged in plates and subgrains, respectively; <U> - the average 
scalar density of dislocations in the layer (taking into account the types of structures); <d> - the 
average size of the substructure in the layer (taking into account the types of structures). 
 
Analysis of the results presented in this figure confirms the conclusion 
about the gradient nature of the defected substructure. Namely, the closer to the 
cooling surface the scalar density of dislocations in the ferrite grains (Fig.  1, 
curve 2) and ferritic interlayers of pearlite grains (Fig.  1, curve 1), are 
increasing and the average size of fragments of the ferrite (Fig. 2, curve 1) and 
particle size of cementite (Fig. 2, curve 2) are reducing.  
It can be seen that in the surface layer of H-beam the state is formed, 
which based on the average size of fragments of Į-phase and cementite 
particles can be regarded as a nanostructure.  
  
Fig. 1 – Dependence of the scalar 
density of dislocations located in the 
ferrite component of the pearlite grains 
(curve 1) and ferrite grains (curve 2) on 
the distance to the surface of treatment 
Fig. 2 – Dependence of the cementite 
fragments average size (curve 2) and ferrite 
fragments D average size (curve 1) on the 
distance to the surface of treatment 
 
Formation of nanostructured phases during thermomechanical hardening 
Analysis of the results presented above gives us grounds to conclude that 
the formation of nanoscale phase in the studied steel under thermomechanical 
processing and subsequent accelerated cooling of rolled products is possible 
with  the  implementation  of  such  a  processes.  First,  by  dispersing  of  pearlite  
cementite plates colonies by cutting them by moving dislocations. Secondly, in 
296                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
the process of dissolution of pearlite colonies cementite plates and repeated 
precipitation of cementite particles on the dislocations, the boundaries of 
blocks, subgrains and grains. Third, during the decay of solid solution of carbon 
in the D-iron, formed under the conditions of accelerated cooling of the steel 
("self-tempering" martensite). Fourth, when during the final transformation of 
the retained austenite in the structure of carbideless beinite with the formation 
of D-iron and cementite particles. Fifth, in the implementation of the diffusion 
mechanism of J  D transformation under the conditions of high degree of 
deformation and high temperature treatment. 
 
Dispersion of the pearlite colonies cementite plates by cutting them by 
moving dislocations 
Fig.  3 shows electron microscopy images of the pearlite colony, the 
cementite plates which are divided into separate fragments (the blocks). Sizes 
of fragments vary from 5 to 30 nm. Simultaneously, cementite particles are 
found in ferritic interlayers of the pearlite colony, the sizes of whose particles 
vary from 5 to 10 nm (Fig. 3, the particles are indicated by arrows).  
 
 
Fig. 3 – The fragmentation of the cementite plates of perlite grains, a, c – light-field 
image, b - dark field image obtained in the reflection [121] Fe3C; d - electron diffraction 
pattern image, the arrow indicates the reflection, in which a dark field image was 
obtained. On c) the arrows indicate cementite particles located in the plates of ferrite 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  297 
 
Nanoscale range of the cementite structure of given pearlite colony is 
confirmed by quasicircular construction of the electron diffraction image 
derived from this plot foils (Fig. 3, d). Presented micrographs of the steel 
structure suggest that the thermomechanical processing is accompanied not 
only by mechanical destruction of the plates of cementite, but also their 
dissolution of with the departure of the carbon atoms on the dislocation and the 
subsequent precipitation in the body of ferrite plates. 
Dispersion of the cementite plates may be accompanied by the formation 
of the block (subgrain) structure (Fig.  4). The newly evolved cementite 
particles in such a structure are located at block boundaries, stabilizing their 
sizes. 
 
Dissolution of cementite plates of the pearlite colonies and repeated 
precipitation of the cementite particles on the dislocations, the boundaries of 
the blocks, subgrains and grains  
Removal of carbon atoms from destructed particles of cementite is 
possible and even at much greater distances. Studies of the block (subgrain) 
structure of D-iron grains by methods of dark-field analysis revealed the 
cementite particles in the body of the blocks on the dislocations and block 
boundaries (Fig. 5). Particles have a rounded shape, particle sizes vary from 5 
to 15 nm. 
 
 
Fig.  4 – The formation of a 
subgrain structure of the 
steel and fragmentation of 
the pearlitic grain cementite 
plates a - light-field image, b 
- dark field image obtained 
in the reflection [121] Fe3C; 
c - electron diffraction 
pattern image, the arrow 
indicates the reflection, in 
which a dark field image 
was obtained 
298                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
 
Fig. 5 – Precipitations of second phase particles (cementite) within the body and at the 
boundaries of D-phase subgrains: a - dark field image, obtained in the reflections [012] 
Fe3C + [110] D-Fe (particles indicated by arrows), b - electron diffraction pattern image, 
the arrow indicates the reflection, in which a dark field image was obtained. 
 
The decay of solid solution of carbon in the D-iron, formed under the 
conditions of accelerated cooling of steel ("self-tempering" of martensite)  
Accelerated cooling of the H-beam leads to the formation of the 
martensitic structure in the surface layer. Subsequent "self-tempering" of the 
steel under the influence of the residual heat is accompanied by relaxation of 
the dislocation substructure, which manifests itself in reducing the scalar 
density of dislocations, the destruction of low-angle boundaries of martensite 
crystals, precipitation on dislocations within the body of martensite crystals 
(Fig. 6a) and along the boundaries of cementite particles (Fig. 6b). The sizes of 
particles located on dislocations, vary between 5 ... 10 nm (Fig. 6a), located on 
the borders - in the 10 ... 30 nm range.  
 
Fig. 6 – The microstructure of the hardened layer of the H-beam, cooled by P1 regime, a 
- bright-field image, b - dark field image obtained in the reflection [120] Fe3C; ɫ - 
electron diffraction pattern image, the arrow indicates the reflection, in which a dark 
field image was obtained. On (b) the arrows indicate cementite particles. 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  299 
 
Formation of cementite particles during the process of complete 
transformation of retained austenite presented in the structure of carbideless 
bainite 
In the surface layer of the steel sample cooled according to P2 regime, 
along with grain-subgrain structure the structure of plate type, the so-called 
carbideless bainite was observed. As shown above, the plates are arranged 
parallel to each other and there was an alternation of plates of light and dark 
contrast. Microdiffraction analysis of these structures revealed the presence of 
only the reflections of D-phase. Reflections of the retained austenite and 
carbide phase particles are not detected. At the same time, mottled contrast 
reminiscent of the contrast from the pre-precipitates of second phase particles 
(Fig.  7) is revealed within the structure of the darker plates (formed, 
presumably as a result of the process of complete transformation of residual 
austenite).   
Formation of 
nanosize phases as 
a  result  of  
polymorphic J  D 
transformation 
The high level 
of steel plastic 
deformation, which 
is realized by 
thermomechanical 
processing of rolled 
products, leads to 
dispersion of 
structures formed 
in the process of 
diffusion of the J 
 D transfor-
mation. Fig.  8 
shows electron 
microscopy images 
of the structure of lamellar pearlite. The measurements show that the thickness 
of plates of D-phase, separated by the plates of carbide, is about 70 nm; the 
thickness of the plates of the carbide phase of ~ 25 nm. 
Formation of nano-sized particles of the carbide phase, is also observed in 
the formation of the so-called pseudoperlite, namely ferrite grains, containing 
particles of cementite of globular morphology (Fig. 9). The sizes of particles of 
cementite in these grains vary in the range 40 ... 60 nm. 
 
 
Fig. 7 – Electron microscopic image of the structure formed 
at the transformation of the residual austenite; a - light field 
image, b - electron diffraction pattern image of (a). Arrows 
indicate pre-precipitates of second phase particles 
300                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
 
Fig. 8 – Electron microscopic image of the structure of the lamellar (plate) perlite: a – 
light field image, b - dark field image obtained in [021] Fe3C the reflection; c – electron 
diffraction pattern image, the arrow indicates the reflection, in which a dark field image 
was obtained 
 
. 
Fig. 9 – Electron microscopic image of the pseudoperlite structure: a – light field image, 
b - dark field image obtained in the reflection [012] Fe3C; c - electron diffraction pattern 
image, the arrow indicates the reflection, in which a dark field image was obtained. 
 
CONCLUSIONS 
A study of the H-beam's structure phase states after thermomechanical 
strengthening was carried out using the methods of transmission diffraction 
electron microscopy.  
The formation of gradient structure, characterized by regular changes of 
parameters of the structure-phase states and dislocation substructure as it 
approaches to the surface of the accelerated cooling was revealed. It was 
established that nanosize structure-phase states are formed in the surface layer. 
The analysis of the processes leading to the formation of the structure 
within the beam profile of 09G2S steel nanoscale phases was carried out.  
It  is  shown  that  the  formation  of  nanoscale  phases  was  carried  out  
possibly in the implementation of the following processes. First, during 
dispersing of the cementite plates of pearlite colonies by cutting them by 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  301 
 
moving dislocations. Secondly, during the dissolution of pearlite colonies 
cementite plates and repeated precipitation of the cementite particles on the 
dislocations, the boundaries of blocks, subgrains and grains. Third, during the 
decay of solid solution of carbon in the D-iron, formed under the conditions of 
accelerated cooling of steel ("self-tempering" of martensite). Fourth, during 
final transformation of the retained austenite in the structure of carbideless 
beinite with the formation of D-particles of iron and cementite. Fifth, during the 
implementation of the diffusion mechanism of J  D transformation under the 
conditions of high degree of deformation and high temperature processing.  
 
Acknowledgements 
This work was supported by the Federal Program "Research and 
scientific-pedagogical personnel of innovation in Russia 2009-2013" (contract 
No P332) and Analytical service programme “The development of scientific 
potential of high school (2009 – 2011)” (project No 2.1.2/13482). 
 
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Nanosize phases formation under low carbon steel thermomechanical strengthening

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