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EXPERIMENTAL SECTION
The experimental part of this study consisted in comparing the results
of stress relief performed by heating and by vibration on welds made on
the same material, ASTM A 106 grade B carbon steel, whose chemical
composition is shown in Table 1. Three 4 inch diameter, schedule 40
pieces of pipe were welded with the weld located in the middle. The
first piece was not submitted to any type of treatment, the second was
submitted to heat treatment and the third to VSR. The electrode used
was E-6010 for the first pass and E-7018 for the rest. After the
welding and before treatment the pipes were machined to eliminate the
weld reinforcement, i.e., to have a flush outer pipe surface.
Table 1: Chemical composition of ASTM A-106 grade B steel
Elements |
% |
Carbon |
0,30 max |
Manganese |
0,29 - 1,06 |
Phosphorus |
0,035 max |
Sulfur |
0,035 max |
Silicon |
0,10 max |
7.1. Heat Treatment
The
heat treatment was carried out in the oven of the Metallurgical
Laboratory of Mackenzie School of Engineering, according to Standard
ASME / ANSI B 31.3. The heating took place at a
maximum speed of 315 ºC / h until the soak
temperature of 650 ºC was reached, at which point the pipe was kept for
30 additional minutes. Then, the oven was switched off and allowed to
cool with the pipe in it.
7.2. Vibration Stress Relief
A company in the city of Sao Paulo that owns equipment for VSR kindly
offered its help, performing the treatment on the sample pipe. The
treatment was made according to the equipment’s Instruction Manual. The
pipe was firmly secured and the frequency of vibration was gradually
increased until resonance was reached and maintained for ten minutes.
Then, the vibrator was switched off.
7.3. Tensile test
All tests were carried out according to Standard API 1104 guidelines.
Three specimens were cut, one from each pipe, having the dimensions
indicated in Table 2. The specimens were clamped in the Amsler machine
of the Material Testing Laboratory of Mackenzie School of Engineering,
taking them to rupture. The loading rate was 600 N/s (~60 kgf/s). In
all cases rupture occurred in the base metal and not in the welds or
HAZ. Table 3 shows the information resulting from the tests.
Table 2: Dimensions of specimens before tests.
Dimensions |
No treatment |
VSR |
Heat treatment |
Width, mm |
25,3 |
25,3 |
25,3 |
Length, mm |
225,0 |
225,0 |
225,0 |
Thickness, mm |
6,25 |
6,25 |
6,40 |
Gage length (L 0 ), mm |
125,0 |
125,0 |
125,0 |
Table 3: Tests results
Measurements |
No treatment |
VSR |
Heat treatment |
Yield load kN (kgf) |
52,0 (5300) |
52,0 (5300) |
44,9 (4580) |
Maximum load, kN (kgf) |
74,7 (7620) |
75,9 (7746) |
68,6 (7000) |
Rupture load kN (kgf) |
60,2 (6140) |
61,8 (6300) |
53,9 (5500) |
Gage length (L f ), mm |
148,4 |
148,4 |
153,9 |
The tensile tests calculations are shown on Table 4
Table 4: Tensile tests calculations
Parameter |
No treatment |
VSR |
Heat treatment |
Yield point MPa (kgf/cm 2) |
328,7 (3352) |
328,7 (3352) |
277,4 (2829) |
Maximum stress MPa (kgf/cm 2) |
472,6 (4819) |
480,4 (4899) |
423,9 (4323) |
Ultimate tensile stress MPa (kgf/cm 2) |
380,8 (3883) |
390,7 (3984) |
333,1 (3397) |
Elongation (%) |
18,76 |
18,74 |
23,12 |
7.4 Impact Test
The specimens for the impact test had the same dimensions as those of
the tensile test with the addition of two lateral notches on the
welding bead, as required by Standard API 1104. The three specimens
were submitted to impact test in the Charpy machine, at room
temperature, with a 30 kg hammer. None of the specimens broke. Results
are shown in Table 5.
Table 5: Impact Test
|
No treatment |
VSR |
Heat treatment |
Energy absorbed kJ (kgf.m) |
284,4 (29) |
285,4 (29,1) |
285,2 (29,08) |
7.5 Brinell Hardness
Brinell hardness was measured on the three coupons, with a 10 mm
diameter ball and a load of 3.000 kgf (~30.000 kN). The sizes of the
impressions and their corresponding hardness are shown on Table 6.
Table 6: Brinell Hardness
|
No treatment |
VSR |
Heat treatment |
Indentation diameter, mm |
4,10 |
4,53 |
4,45 |
BHN hardness |
217 |
178 |
183 |
7.6. Metallography
After being polished, the specimens were attacked with nital for nearly
5 seconds and their microstructures observed in the metallographic
microscope. The results are outlined below.
As expected, the micrographs of the original base metal show an
alignment of grains, indicating that the material was manufactured by
hot rolling.
The weld is basically
constituted by ferrite with a dendritic arrangement due to the high
temperature it supported during the welding process. As carbon also
exists, the dark part visible in the micrographs is probably perlite.
On the interface of the weld with the HAZ, dark perlite grains are
visible, with ferrite around the grains, forming a net around the
perlite. The closer the perlite grains are to the weld the bigger is
their size, because they were exposed to higher temperatures than the
ones farther.
It was also observed that
neither the vibration method nor the heat treatment alter the original
metallographic structure of the material.
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