AWS Welding Journal - February 2012 p25-27

5/26/2018 AWS Welding Journal - February 2012 p25-27

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In places where manual welding is ex-tensively used, such as boilermaking andplateworking shops, and for piping andstructural steel prefabrication and jobsites, it is common to cut a finished weld,then reweld it. Two reasons for doing thisare to correct the position of a piece andto repair flaws detected during nonde-structive tests. It is also not uncommonfor cutting and rewelding to be done twoor more times. The cutting operation canbe done in several ways, such as with anoxyacetylene or plasma arc torch, or aircarbon arc gouging. For low-carbon steel,oxyacetylene cutting is the process mostcommonly used.

During welding, the metallographicstructure of the heat-affected zone (HAZ)undergoes changes, due mainly to thetemperature increase in this region andto the carbon content of the base metal.Therefore, the common opinion is thatrepeated cutting and rewelding of thesame weld increases the metallographicchanges in the HAZ up to a point wherefurther cutting and rewelding is no longerpossible.

This situation has been, and still is, rea-son for endless discussions between sup-pliers (plateworking shops and contrac-tors) and clients, with the former attempt-

ing to justify successive cutting andrewelding and the latter trying to forbidthem from doing so.

As far as we know, this problem hasnot yet been studied in depth, and thereis no unanimous opinion among weldingengineers as to how many times it is pos-sible to cut and remake a weld without ru-

ining the structure and properties of thebase metal. Welding standards from the

AWS, ASME, API, AISC, and others aresilent on this matter, and the solution tothe problem is left up to the techniciansinvolved in the work. Of course, there arerules of thumb used by welding profes-sionals or that have been stated by largecorporations and engineering and con-struction companies, but they are basedon empirical experience rather than ac-tual research This is how we did itonce, we had no problems, and so we willkeep on doing it in the same way.

Before writing this article, the authorsdid some research on the AWS Forum

(Ref. 1) . The result of the research wasthat none of the Forum participants knewof any experimental basis for the rules ofthumb used in these cases.

Hence, the authors decided to conducttests to establish what would be the maxi-mum number of times a low-carbon steel

weld could be cut and rewelded. Themethodology, the procedure followed, theresults obtained, and the conclusions that

were reached are described in this article.

Methodology

Two low-carbon steel flat plates, with

known, laboratory-checked chemical andmechanical properties, were welded to-gether with the gas metal arc welding(GMAW) process, with a wire compatible

with their chemical composition. Thebevels were hand made with an oxyacety-lene torch and then cleaned with a grind-ing disk. The welder was qualified in the

flat (1G) position, in accordance with threquirements of Section IX of the ASME

Boiler and Pressure Vessel Code.Oxyacetylene cutting was chosen be

cause it is the method most frequentlused at workshops and job sites to cut cabon steel. The intention was to reproducas closely as possible the real conditionexisting in practice.

Specimens were taken after welding iorder to carry out the following tests, according to widely accepted standardbending, ultimate tensile, impact, elongation, average grain size, and metallographic structure of the HAZ.

The following conditions would hav

deserved special attention if one of themhad occurred: The specimen did not pass the bend tes The ultimate tensile strength of th

specimen was lower than what the applicable standard required for the basmetal.

The impact strength and elongatiowere significantly lower than that of specimen with a single cut and weld.

The average grain size was significantbigger than that of a specimen with single cut and weld.

The metallographic structure of thHAZ was not compatible with that o

the base metal in good conditions.

Test Conditions

Tested Metal. The tested metal was 3in.-thick, low-carbon steel plate. Laboratory analyses performed before the tesshowed the following properties:

25WELDING JOURNAL

ANTONIO GONALVES de MELLO, JR., GIOVANNI S. CRISI (gscrisi@ mackenzie.br), and EVERALDO VITOR are professors aMackenzie Engineering School, So Paulo, Brazil. ROGERIO A. LOPES DA SILVA is chief technician of the Metallurgical Laboratory oMackenzie Engineering School.

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

How Often Can Joints Be

Cut and Rewelded in

Low-Carbon Steel?

BY ANTONIO GONALVES de MELLO, JR

GIOVANNI S. CRISI, EVERALDO VITOR

AND ROGERIO A. LOPES DA SILVA


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1. Chemical composition: 0.122% car-bon, 0.35% manganese, 0.013% silicon,0.04% phosphorus, and 0.014% sulfur.

2. Mechanical properties: 285.9 MPayield point, 398 MPa ultimate tensilestrength, 40.2% total elongation on a 200-mm-long specimen, 52% elongation atboth sides of the rupture.

These results classify the metal asbeing ASTM A 283 GrB. This standarddoes not require a given impact strength,grain size, and metallographic structure;however, these parameters were alsomeasured to compare them to those of themetal resulting from repeated cutting andrewelding. Therefore, the following meas-urements were obtained:

3. Impact test: performed on two spec-imens with a 30-kg hammer: 205 kJ/cm2.The specimen did not break in either case.

4. Average grain size: 75. Metallographic structure: ferrite,

with small pearlite grains.The micrograph of the base metal isshown in Fig. 1.

Wire and Gas Used for Welding. Thewire used was ER 70S-6 (from AWSA5.18, Specification for Carbon Steel Elec-trodes and Rods for Gas Shielded Arc Weld-ing) for direct current, which is recom-mended for the welding of low-carbonsteel. The diameter was 1.2 mm. The

chemical composition was 0.060.15%carbon, 1.41.85% manganese, 0.81.15%silicon, maximum 0.025% phosphorus,and maximum 0.035% sulfur.

The brand was a high quality one, widelyknown in Brazil. The gas composition was75% argon and 25% carbon dioxide.

Bevel Preparation. The bevel anglewas 60 deg, which we considered accept-able for a 38-in.-thick groove weld. As ex-

plained previously, the bevel was cut withan oxyacetylene torch and cleaned with a

grinding disk. As the cut was hand made,even though done carefully, the 60-degangle was approximate.

Position of the weld. The weld was per-formed in the flat (1G) position.

Preheating and postweld heat treat-

ment. No preheating nor postweld heattreatment was conducted because they arenot required by Section VIII of the ASMECode for low-carbon steel 38 in. thick. Nospecial precautions were taken for slowcooling of the metal after welding. Brazilis a tropical country and welds were never

made at a room temperature of less than25C.

Standards followed for the tests.

Bend testing: ASME Section IX, para-graph QW160 and subsequent. Accord-ing to this standard, the test is approved

when the overall length of all the cracksthat may have appeared after bending isnot higher than 3.2 mm (18 in.).

Ultimate tensile: ASME IX, paragraphQW462 and subsequent.

Impact, with triangular notch: ABNTNBR 281-1.2003. (ABNT is the Brazil-ian Association of Technical Stan-

dards.) Average grain size: ASTM E 112/04,Comparison Procedure, Plate I.

Procedure

Six sections 200 mm wide 440 mmlong were used for the tests. To identifythem, a number from one to six wasstamped on a corner of each one. A first

bevel was made on all sections, as described in a previous paragraph.

Next, Section No. 1 was welded. Oncthe weld was concluded, the root wagouged by means of a triangular file anthe resulting groove was filled1. The result was a metal section with one torch cuand one weld, from which we took off thspecimens to be used for root and facbending, ultimate tensile, elongation, an

root and face impact tests. The remaininsection was used to verify the averaggrain size and the metallographic struture of the HAZ. The results are showin Table 1 and Fig. 2.

Again, on Section No. 2 the first welwas applied and the root was gouged anfilled. The weld was cut and the bevel waredone, always as described above. A ne

weld was appl ied for the second timeOnce again, the root was removed and refilled. The resulting section had two torccuts and two welds, from which we extracted the specimens for the tests anchecking described above.

Then, the first weld was applied to section No. 3 and the root was removed anfilled. The weld was cut and the bevel waredone. A second weld was applied and throot removed and filled. After a new cuand rebeveling, the third weld was applied

with the root once again removed anfilled. This resulted in a section with thretorch cuts and three welds, from which thspecimens for the tests were extracted.

This procedure was followed up to thsixth specimen, which resulted in six cuand six welds.

Test Results

For a quick comparison, the tests results, including those of the base metaare shown Table 1. The micrographs arshown in Fig. 2.

The metallographic structure is thsame in all the cases. Observed are the existence of clear ferrite grains and darkegrains where, on a ferrite matrix, the ex

FEBRUARY 201226

Fig. 1 Micrograph of the base metal.

Table 1 Results of Tests on Welded Metal

Section No. UTS (MPa) Elongation (%) Bend Face Bend Root Impact Face (kJ) Impact Root (kJ) Average grain size

Base Metal 398 40.2 Not executed Not executed 205(a) Not executed 7

1 419 17.6(b) OK OK 112 106 7

2 417 15.3 OK OK 150 120 7

3 414 16.6 OK OK 107 170 6

4 415 16.5 OK OK 187 137 7

5 417 17.0 OK OK 114 115 9

6 422 17.0 OK OK 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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27WELDING JOURNAL

istence of cementite is seen. The shape ofthe cementite is sometimes spots, some-times small flakes, and in a few cases theshape of small stains. These are neitherferrite nor martensite, because the equiv-alent carbon content is too small to pro-duce martensite. The structure is the typ-ical one of a heat-affected zone.

Interpretation of Results

The ultimate tensile strength shows anincrease of approximately 5% in compar-ison to the base metal, beginning in Sec-tion No. 1, and remains approximatelyconstant up to the last section.

The elongation shows a decrease toless than half in comparison to the basemetal, beginning in section No. 1, and re-mains approximately constant up to thelast specimen.

The impact strength shows a decreasein comparison to the base metal. Not con-

sidering the face test of Section No. 2, theroot test of Section No. 3 and face androot tests of Section No. 4, the average de-crease of the other tests in comparison tothe base metal is approximately 40%.

The changes in these three parametersare due to the fact the welds were not sub-

mitted to any postweld heat treatment.Also, no precautions were taken for slowcooling after the conclusion of welding.Consequently, there was a decrease inductility in both the weld bead and theHAZ. As stated previously, our intention

was to reproduce as closely as possible theprocedures followed in workshops and jobsites, where those precautions are not usu-ally taken when a 38-in.-thick, low-carbonsteel weld has to be cut and redone, espe-cially when the ambient temperature isnever below 25C, which happens not onlyin tropical countries but also in the sum-mertime in cold ones.

The face and root bend tests were sat-isfactory in all cases, i.e., in some speci-mens there were no cracks, and in the oth-ers the overall crack length was less than18 in., as specified in the ASME Code, Sec-tion IX, paragraph QW163. No bendtests were carried out on the original basemetal because that was not considered

necessary.The average grain size of the heat-affected zones were not significantly dif-ferent from that of the base metal. This isdue to the fact the sizes were not meas-ured in the region immediately next to the

weld bead, but in the fine-grain region ofthe HAZ, and in any case, always withinthe HAZ.

Conclusions

Our conclusion is that the main char-acteristics that ensure the mechanical

strength and ductility of the weld bead anthe heat-affected zone, reported by thultimate tensile strength and bend testremained unchanged after six cuts anrewelds in the same region of the originabase metal. The elongation also remaineconstant after the first cut and reweld.

The research demonstrated that thcutting and subsequent welding operatioin the same region can be performesafely at least six times on low-carbosteel.

Further research may confirm the conclusions of this one, and may also shothe possibility that cutting and welding cabe executed more times, or also on othematerials, such as alloy and stainlessteels.

Acknowledgments

The authors wish to thank PROAQ

Empreendimentos Tecnolgicos Ltd. anVOITH Hydro Ltd., both of So Paulofor performing the metallographic analyses mentioned in this article.

References

1. Multiple welding repairs in thsame area. Discussion on the AWForum available at www.aws.org/cg

bin/mwf/topic_show.pl?tid=7304. Last access was January 2, 2012.

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

1. In workshops and at job sites, gouging oflow-carbon steel is usually done by meansof air carbon arc. However, because the uni-versitys weld lab does not have this equip-ment, we used the file.

Specimen 1

Specimen 4 Specimen 5 Specimen 6

Specimen 2 Specimen 3

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AWS Welding Journal - February 2012 p25-27