For the first time, a raised content of special boundaries (SB) of grains, α-α, (up
to 23 %) and conditional interphase boundaries γ-α (up to 20 %) was found among
ferrite and perlite colonies (former austenite) in the structure of ferrite-pearlite 06Cr1-
U steel with application of the developed procedure [14] (Fig. 1 b, c, d and Fig. 2).
Special boundaries are characterized by close to 180° opposite corners in triple
junctions and zigzag facets (Fig. 1 c, d) and do not exhibit significant surface tension
which is a direct indication of their lower specific surface energy. Calculation of the
specific surface energy of the α-α inter-grain boundaries using the method of
equilibrium of Herring triple junctions [15] and Jung's ratio (
1
/sin
1
=
2
/sin
2
=
3
/sin
3
) has shown that it is equal to 38...33 erg/cm
2
for special boundaries which is
20...24 times lower than for boundaries of the general type (780 erg/cm
2
).
Heating of tubes to a temperature of 950 °C before their calibration contributed to
an increase in specific surface of SB in their metal structure.
а
б
Figure 2. Microstructure of hot-rolled pipes of steel has become
06Cr1-U, 1000, the EBSD method, (SВ in Fig. b is indicated by red lines)
Statistical analysis of distribution of angle values in the study of about 500 triple
joints formed with participation of conditional interphase α-γ boundaries has shown
the highest maximum (≈ 25 %) in the region of 180° angles (Fig. 3). The second
maximum (≈ 15 %) in the region of 90° angles is due to the statistically most probable
combination of such angles with angles close to 180° in the triple joints.
According to the published data [16, 17] as well as the results of authors' own
studies [18-20], the special low-energy boundaries in austenitic chromium-nickel and
chromium-nickel-molybdenum steels have higher corrosion resistance compared to the
boundaries of general type. The increased number of such boundaries obtained using
the principle of grain boundary design during deformation and thermal treatment of
steel has contributed to a significant growth of steel corrosion resistance. It can be
assumed that the high content of special boundaries in the structure of tubes made of
low-alloy ferrite-pearlite steel will also increase their corrosion resistance.
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