Figure 3.
Statistical analysis of the distribution of angular values in triple joints
It was established that by their level of mechanical properties, the tubes of 06Cr1-
U steel correspond to the X42...Х46 strength groups according to АРI 5L standard
(Table 4) and are characterized by high impact strength at negative temperatures: KCV
-
20... -60
= 365... 410 J/cm
2
.
Table 4 Mechanical properties of specimens taken from 30 batches of tubes
Tube size, mm
в
, МPа
0.2
, МPа
0.2
/
в
5
, %
76…1146…9
420…460
300…345
0.71…0.75
32…37
Requirements of АРІ 5L to tubes
of Х 42/Х 46 strength groups
Not less than
Not more than
Not less than
420/441
294/322
0.93
23/22
As expected, microalloying of steel with niobium in an amount of 0.025 %
increased strength characteristics of tubes made of this steel ((
в
and
0,2
) by an average
of 15... 20 % to correspond to their strength groups X 52... Х 56 according to the АРI
5L standard without a noticeable decrease in relative elongation.
In a case when final deformation of the tubes made of the steel microalloyed with
niobium was completed in the intercritical interval (ICI) of temperatures (810...
830°С), they had a relatively reduced impact strength at negative temperatures when
tested on Charpy specimens with a sharp notch compared to the steel tubes without
niobium: KCV
-60 °С
=90... 170 J/cm
2
. When tested at room temperature, no influence
of deformation end on impact strength was observed in such specimens [22]. The
obtained results are consistent with the published data indicating that the impact
strength at negative temperatures is the most sensitive characteristic depending on the
steel structure [23].
The conducted studies confirmed that the reduced impact strength at negative
temperatures of the tubes made of the steel microalloyed with niobium is explained by
formation of martensitic nuclei in its structure (Fig. 4 a) and the increase in dislocation
density (up to ≈ 10
9
) in ferrite (Fig. 4 b).
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