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

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.

Table 3: Tests results

The tensile tests calculations are shown on Table 4

Table 4: Tensile tests calculations

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

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

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.