AWS Welding Journal - February 2012 p25-27

 

How Often Can Joints Be Cut and Rewelded in Low-Carbon Steel? Tests were conducted to determine how often a weld could be cut and rewelded  without making deleterious changes to the metallurgical structure of the HAZ  BY ANTONIO GONÇALVES GONÇALVES de MELLO, JR., GIOVANNI GIOV ANNI S. CRISI, EVERALDO VITOR, AND ROGERIO A. LOPES DA SILVA

In places where manual welding is extensively used, such as boilermaking and plateworking shops, and for piping and structural steel prefabrication and job sites, it is common to cut a finished weld, then reweld it. Two reasons for doing this

ining the structure and properties of the base metal. Welding standards from the  AWS, ASME, ASME, API, AISC, and others are silent on this matter, and the solution to the problem is left up to the technicians involved in the work. Of course, there are

flat (1G) position, in accordance with the requirements of Section IX of the ASME  Boiler and Pressure Pressure Vessel Vessel Code. Oxyacetylene cutting was chosen because it is the method most frequently used at workshops and job sites to cut car-

are to correct the position of a piece and to repair flaws detected during nondestructive tests. It is also not uncommon for cutting and rewelding to be done two or more times. The cutting operation can be done in several ways, such as with an oxyacetylene or plasma arc torch, or air carbon arc gouging. For low-carbon steel, oxyacetylene cutting is the process most commonly used. During welding, the metallographic structure of the heat-affected zone (HAZ) undergoes changes, due mainly to the temperature increase in this region and to the carbon content of the base metal. Therefore, the common opinion is that repeated cutting and rewelding of the

rules of thumb used by welding professionals or that have been stated by large corporations corporatio ns and engineering and construction companies, but they are based on empirical experience rather than actual research — “This is how we did it once, we had no problems, and so we will keep on doing it in the same way.” Before writing this article, the authors did some research on the AWS Forum (Ref. 1) . The result of the research was that none of the Forum participants knew of any experimental basis for the rules of  thumb used in these cases. Hence, the authors decided to conduct tests to establish what would be the maximum number of times a low-carbon steel

bon steel. The intention was to reproduce as closely as possible the real conditions existing in practice. Specimens were taken after welding in order to carry out the following tests, according to widely accepted standards: bending, ultimate tensile, impact, elongation, average grain size, and metallographic structure of the HAZ. The following conditions would have deserved special attention if one of them had occurred: • The specimen did not pass the bend test. • The ultimate tensile strength of the specimen was lower than what the applicable standard required for the base metal.

same weld increases the metallographic changes in the HAZ up to a point where w here further cutting and rewelding is no longer possible. This situation has been, and still is, reason for endless discussions between suppliers (plateworking shops and contractors) and clients, with the former attempting to justify successive cutting and rewelding and the latter trying to forbid them from doing so.  As far as we know know,, t his probl em has not yet been studied in depth, and there is no unanimous opinion among welding engineers as to how many times it is possible to cut and remake a weld without ru-

 wel d coul  weld could d be cut and rewe lded . The methodology, the procedure followed, the results obtained, and the conclusions that  were reached are described in this article.

• The impact strength and elongation  were signi s ignifican ficantly tly lower l ower than t han that th at of a specimen with a single cut and weld. • The average grain size was significantly bigger than that of a specimen with a single cut and weld. • The metallographic structure of the HAZ was not compatible with that of  the base metal in good conditions.

Methodology Two low-carbon steel fl at plates, with known, laboratory-checked chemical and mechanical properties, were welded together with the gas metal arc welding (GMAW) (GMA W) process, with a wire compatible  with  wit h thei theirr chem chemical ical comp composit osition. ion. The bevels were hand made with an oxyacetylene torch and then cleaned with a grinding disk. The welder was qualified in the

Test Conditions 3 The tested metal was  ⁄  8in.-thick, low-carbon steel plate. Laboratory analyses performed before the tests showed the following properties:

Tested Metal.

 ANTONIO GONÇAL GONÇALVES VES de MELLO, JR., GIOVANNI GIOVANNI S. CRISI (gscrisi@ mackenzie.br) , and EVERALDO VITOR are professors at  Mackenzie Engineering Engineering School, São Paulo, Paulo, Brazil. ROGERIO ROGERIO A. LOPES DA SILV SILVA A is chief technician of the Metallurgical Metallurgical Laboratory of   Mackenzie Engineering Engineering School.

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chemical composition was 0.06–0.15% carbon, 1.4–1.85% manganese, 0.8–1.15% silicon, maximum 0.025% phosphorus, and maximum 0.035% sulfur. The brand was a high quality one, widely known in Brazil. The gas composition was 75% argon and 25% carbon dioxide. Bevel P reparatio reparation. n. The bevel angle  was 60 deg, deg , which we w e considered conside red acceptaccep t3 able for a  ⁄  8-in.-thick groove weld. As explained previously, the bevel was cut with an oxyacetylene torch and cleaned with a

bevel was made on all sections, as described in a previous paragraph. Next, Section No. 1 was welded. Once the weld was concluded, the root was gouged by means of a triangular file and the resulting groove was filled 1. The result was a metal section with one torch cut and one weld, from which we took off the specimens to be used for root and face bending, ultimate tensile, elongation, and root and face impact tests. The remaining section was used to verify the average

grinding disk. As the cut was hand made, even though done carefully, the 60-deg angle was approximate. Position of the weld. The weld was performed in the flat (1G) position.

grain size and the metallographic structure of the HAZ. The results are shown in Table 1 and Fig. 2.  Again, on Section Se ction No. 2 the th e first weld w eld  was applied and the root was w as gouged and filled. The weld was cut and the bevel was redone, always as described above. A new  weld  wel d was appl ied for the secon d time . Once again, the root was removed and refilled. The resulting section had two torch cuts and two welds, from which we extracted the specimens for the tests and checking described above. Then, the first weld was applied to section No. 3 and the root was w as removed and filled. The weld was cut and the bevel was redone. A second weld was applied applie d and the

 Fig. 1 — Micrograph Micrograph of the base base metal.

1. Chemical composition: 0.122% carbon, 0.35% manganese, 0.013% silicon, 0.04% phosphorus, and 0.014% sulfur. 2. Mechanical properties: 285.9 MPa  yiel d poin point, t, 398 MPa ulti ultimate mate tens tensile ile strength, 40.2% total elongation on a 200mm-long specimen, 52% elongation at both sides of the rupture. These results classify the metal as being ASTM A 283 GrB. This standard does not require a given impact strength, grain size, and metallographic structure;

Preheating and postweld heat treatment. No preheating nor postweld heat

however, these parameters were also measured to compare them to those of the metal resulting from repeated cutting and rewelding. Therefore, the following measurements were obtained: 3. Impact test: performed on two specimens with a 30-kg hammer: 205 kJ/cm2. The specimen did not break in either case. 4. Average grain size: 7 5. Metallographic structure: ferrite,  with small pearlite grains. The micrograph of the base metal is shown in Fig. 1.  Wire and an d Gas Us ed for Welding . The  wire used was ER 70S70S-6 6 (fro (from m AWS  A5.18, Specification for Carbon Steel Electrodes and Rods for Gas Shielded Arc Weld-

graph QW–160 and subsequent. According to this standard, the test is approved  when the overall overall length of all the cracks that may have appeared after bending is 1 not higher than 3.2 mm ( ⁄  8 in.). • Ultimate tensile: ASME IX, paragraph QW–462 and subsequent. • Impact, with triangular notch: ABNT NBR 281-1.2003. (ABNT is the Brazilian Association of Technical Standards.) • Average grain size: ASTM E 112/04, Comparison Procedure, Plate I.

ing ) for direct current, which is recommended for the welding of low-carbon steel. The diameter was 1.2 mm. The

treatment was conducted because they are not required by Section VIII of the ASME Code for low-carbon steel  ⁄   3⁄ 8 in. thick. No special precautions were taken for slow cooling of the metal after welding. Brazil is a tropical country and welds were never made at a room temperature of less than 25°C. Standards followed for the tests.

• Bend testing: ASME Section IX, para-

root removed and filled. After a new cut and rebeveling, the third weld was applied,  with the root once again remove removed d and filled. This resulted in a section with three torch cuts and three welds, from which the specimens for the tests were extracted. This procedure was followed up to the sixth specimen, which resulted in six cuts and six welds. Test Results

440 mm

For a quick comparison, the tests results, including those of the base metal, are shown Table 1. The micrographs are shown in Fig. 2. The metallographic structure is the

long were used for the tests. To identify them, a number from one to six was stamped on a corner of each one. A first fi rst

same in all the cases. Observed are the existence of clear ferrite grains and darker grains where, on a ferrite matrix, the ex-

Procedure

Six sections 200 mm wide

×

Table 1 — Results of Tests on Welded Metal

Section No. Base Metal

UTS (MPa)

Elongation (%)

Bend Face

Bend Root

Not executed

Not executed

398

40.2

1 2 3 4 5

419 417 414 415 417

17.6(b) 15.3 16.6 16.5 17.0

OK OK OK OK OK

OK OK OK OK OK

6

422

17.0

OK

OK

Impact Face (kJ) 205(a)

Impact Root (kJ)

Average grain size

Not executed

7

112 150 107 187 114

106 120 170 137 115

7 7 6 7 9

114

111

8

(a) This was the only impact test that was executed, because the base metal has neither face nor root. (b) The elongation was measured between the farthest points of the specimen narrowing, before and after the tensile test, equal to 68 mm in all cases before the test.

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Specimen 1

Specimen 2

Specimen 3

Specimen 4

Specimen 5

Specimen 6

 Fig. 2 — Micrographs of the HAZs of the successive welds made on the base metal.

istence of cementite is seen. The shape of  the cementite is sometimes spots, sometimes small flakes, and in a few cases the shape of small stains. These are neither ferrite nor martensite, because the equivalent carbon content is too small to produce martensite. The structure is the typical one of a heat-affected zone.

Interpretation of Results The ultimate tensile strength shows an increase of approximately 5% in comparison to the base metal, beginning in Section No. 1, and remains approximately constant up to the last section. The elongation shows a decrease to less than half in comparison to the base metal, beginning in section No. 1, and remains approximately constant up to the last specimen. The impact strength shows a decrease in comparison to the base metal. Not considering the face test of Section No. 2, the root test of Section No. 3 and face and root tests of Section No. 4, the average decrease of the other tests in comparison to the base metal is approximately 40%. The changes in these three parameters are due to the fact the welds were not sub-

1. In workshops at job sites, of   low-carbon steeland is usually donegouging by means  of air carbon arc. However However,, because because the uni versity’s weld lab does not have this equip ment, we used the file.

mitted to any postweld heat treatment.  Also, no precautions were taken for slow cooling after the conclusion of welding. Consequently, there was a decrease in ductility in both the weld bead and the HAZ. As stated previously, our intention  was to reproduce as closely as possible the procedures followed in workshops and job sites, where those precautions are not usu3 ally taken when a  ⁄  8-in.-thick, low-carbon steel weld has to be cut and redone, especially when the ambient temperature is never below 25°C, which happens not only in tropical countries but also in the summertime in cold ones.

strength and ductility of the weld bead and the heat-affected zone, reported by the ultimate tensile strength and bend tests, remained unchanged after six cuts and rewelds in the same region of the original base metal. The elongation also remained constant after the first cut and reweld. The research demonstrated that the cutting and subsequent welding operation in the same region can be performed safely at least six times on low-carbon steel. Further research may confirm the conclusions of this one, and may also show the possibility that cutting and welding can

The face andcases, root bend tests were satisfactory in all i.e., in some specimens there were no cracks, and in the others the overall crack length was less than 1  ⁄  8 in., as specified in the ASME Code, Section IX, paragraph QW–163. No bend tests were carried out on the original base metal because that was not considered necessary. The average grain size of the heataffected zones were not significantly different from that of the base metal. This is due to the fact the sizes were not measured in the region immediately next to the  weld bead, but in the fine-grain f ine-grain region regi on of  the HAZ, and in any case, always within the HAZ.

be executedsuch moreas times, also stainless on other materials, alloyorand steels.

Conclusions

1. “Multiple welding repairs in the same area.” Discussion on the AWS Forum available at  www.aws  www.aws.org .org/cgi/cgi bin/mwf/topic_show.pl?tid=7304.. Last ac bin/mwf/topic_show.pl?tid=7304 cess was January 2, 2012.

Our conclusion is that the main characteristics that ensure the mechanical

 Acknowledgments The authors wish to thank PROAQT Empreendi mentos Tecnológicos Empreendimentos Tecnológicos Ltd. and VOITH Hydro Ltd., both of São Paulo, for performing the metallographic analyses mentioned in this article.

 References

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