“I AD AOSS 129 CONSTRUCTION ENGINEERING RESEARCH LAB (ARMY) CHAMPAI—ETC P/S 13/5

NONDESTRUCTIVE TESTING FOR FIELD WELDS: REAL TIME WELD QUALITY ——ETC (U).

AUG 78 F KEARNEY

UNCLASSIFIED CERL—I R M 251 NL

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construction

engineering INTERIM REPORT

Augud 1978

research L

laboratory ____________________

NONDESTRUCTIVE TESTING FOR FIELD WELDS:

REAL TIME WELD QUALITY MONITOR

~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~ 

F. Kearney

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~~~~ Approved fur public reIea~ ; distribution unlimited.

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The contents of this report are not to be used for advert ising, publication, or

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The findings of this report are not to be construed as an official Department

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• 12. GOVT ACCESSION NO. 3. RECIPIENT S CATALOG NUMBER

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) NONDESTRUCTIVE TESTING FOR FIELD ~ EL DS :7 C9 ~

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EAL JIME MELD ZUALITY MONI1’OR 4 p . ~ _________________________

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10. PROGRAM ELEME~~~~~~~~~ 9. PERFORMING ORGANIZATION NAM E AND ADDRESS CT , TASK

CONSTRUCTION ENGINEERING RESEARCH LABORATORY AREA S WORK UN~~~~~~~ BERS

P.O. Box 4005

Champaign , IL 61820

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IS. KEY WORDS (Continu. on .v.r.. aid. SI n. c..ony ond Sd.ntify by block numb .,)

welding

weld quality

nondestructive testing

20. )I

~~TRAcr (cS ~~ . on ,s~~~~~~~ sf* N nsc s~~y ~~d S*ntifr by block numb.t)

This report describes part of a study to develop a field portable weld

quality monitor . A real time weld quality monitor was configured and

subjected to laboratory and field tests . Based on the tests, it was

determined that the WQM design is adequate and has field applicability .

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FOREWORD

Thi s Investi gation was conducted by the Engineering and Material s

Division (EM), Construction Engi neeri ng Research Laboratory (CERL ) for

the Directorate of Military Construction, Office of the Chief of

Engineers (XE). The research was funded under Project 4A762719AT41, Desi gn , Construction and Operations and Maintenance Technology for Mi litary Facilities TM

; Task 04, TMConstructlon Systems Technology ’; Work Unit

U09 , ‘Plondestructive Testing for Field Welds.” The QCR is 1.06.004. Messrs. C. Damico and E. Hunt were the OCE Technical Monitors.

Dr. G. R. William son Is Chief of EM. COL U. E. Hays is Conmiander

and Di rector of CERL, and Dr. 1. R. Shaffer is Technical Director.

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CONTENTS

DO FORM 1473 1

FOR EWORD 3

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . 5

Background

Objective

Approach

Mode of Technol ogy Transfer

2 LABORATORY TEST. . . . . . . . . . . . . . . . . . . . . . . 7

Procedure

Resul ts

3 FIELD TEST . . I.. . 8

Site Selection

Test Operations

Results

4 CONCLUSION. . . . . . . . . . . . . . 10

APPENDIX A: Factors Affecting Wel d Metal Mechanical Properties 15

APPENDIX B: Circui t Description 18

APPENDIX C : Weld Repai r Procedure - Ozark Hydroel ectri c Shaft 19

REFERENCES 21

DISTRIBUTION

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NONDESTR UCTIVE TESTING FOR FIELD WELDS:

REAL TIME W ELD QUALITY MONITOR

1 INTRODUCTION

Back ground

S During the wel ding process, changes in arc vol tage, travel speed , and heat input can occur wi thout the operator’ s knowledge . These

chan ges can cause defects such as porosity , slag inclusions , incompl ete

fusion , and undercut in the deposited weld metal (Appendi x A ). The cost

of locating and repairing these defects can be a major portion of constructi on costs; wel ding inspection can consti tute 25 to 40 percent of

the wel d fabrication cost. In addition , wel d defects decrease the service life of wel ded joints.

Consequently, it is necessary to monitor the welding parameters to

detect, i denti fy , and locate possible defects. A wel d monitor wi th real

time output woul d aid the inspector in designating suspect areas for

nondestructive testing. To address this need , the U.S. Army Construction En gineering Research Laboratory (CERL) is conducting research to

develop a fiel d portable wel d quality monitor ( WQM).

In the initial phase of study ,

’ the fol lowing requirements were established for the device. It should:

1. Monito r the three primary signals from the weld system: arc

vol tage, current , and travel speed ; compare them to preset limits ; and

al ert the operator if the limi ts are exceeded .

2. Calcul ate the heat i nput , nugget area , and cooling rate from

the three primary signals; compare these val ues wi th preset limits ; and

alert the operator if these limits are exceeded .

3. Be fiel d portable.

4. Interface easily with in situ welding equipment.

1 R. Weber , F. Kearney , and S. Joshi , D,

~v~1opnent of Weld QuaZ.ity

?Ion itoi’, Interim Report M- 183/A0A027644 (U.S. Army Construction

Engineering Research Laboratory ECERLI, July 1976).

5

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,

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Essentially, the W QM is Intended to provide a mechanism to merge the

welding engineer’s desi gn intent wi th the actual field wel ding process.

Following devel opment of these requirements , a prototype WQM was

desi~~ied, fabricated , and tested using input from a fully automati c gas metal-arc (GMA) welding machine. 2 The automated GMA process was chosen

to obtain close control and reproducibility of the wel ding variables for

initial testing.

Appendix B describes the circui try of the WQM.

Objective

The objectives of thi s phase of study were (1) to confi gure a portable , real time WQM system , and (2) conduct laboratory and field tests

to determine the adequacy and field appl icability of the design .

Future phases of the WQM study will aim at devel oping (1) suitable

speed sensing systems for manual wel ding situations , (2) specifi c radiometric measurement techniques involving acoustic and thermal spectral analysis , and (3) digital processing features using microprocessors to

facilitate progranmiing the W QM.

Approach

The desi gn of the prototype WQM developed in the earlier phase of

study was modified to incorporate improvements indicated by actual wel ding si tuations In the l aboratory. Hardware was assembl ed and packaged for fiel d use.

In the transitional period from l aboratory prototype to fiel d prototype, personnel in Government and the private sector were consul ted

and their suggestions were used to further improve the unit.

The unit was then instal led in a welding situation that woul d thoroughl y test all modes of operation.

Mode of Technology Transfer

Thi s study will impact on TM 5-805-7 , Weldi ~

zg; Design Procedure s

.i.nd Inspection (Department of the Army , 15 Marc h 1968).

2 R. Weber et al , Interim Report 11-183.

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2 LABORATORY TEST

Procedure

Each channel of the l aboratory prototype monito r was Individually

tested wi th a vari able signal similar in current and vol ta ge level to

the signal from a welding machine . The limits for each channel were

set , and the test vol tages were varied to simulate chan ges In the pri- S

mary si gnals. When the status lights indicated that the limit had been

exceeded , the test vol ta ge was compared to the preset limit val t

~

e to

check the accuracy of the comparator circuit.

After each channel had been tested successfully, the three simulated primary signal s were fed into the monito r simul taneously. The

limits were again set and the Input vol tages varied . All circui ts , incl uding the analog computer section , were checked for accuracy and reproducibility .

The monitor was then connected to the CERL wel ding machine to test

the circui try wi th ac tual signals. After the limits were set , a welding

arc was establ ished on a test plate .

Resul ts

Results of the laboratory testing showed that all channel s performed satisfactorily , both independently and in conjunction wi th each

other. The warning lights were tri ggered when the input signal exceeded

the limits set by the reference signal , and no difficul ties were encountered when the limit span was changed.

While investi gating the si gnal s of the three parameters ( vol tage,

current , and speed) , it was found that the vol tage and amperage si gnal s

contained spurious noise signals. These si gnal s were removed by (1) incorporatIng filters in the data channel to eliminate the peaks and

smooth out the si gnal s , thus reducing the chance of damage to components , and (2) repl acing the shunt as the amperage si gnal source wi th

a Hall effect solid state transductor. (The advantage of using the

transductor Is that it is not directly connected to the welding cable as

the shunt is; Instead , it fi ts around the cable and measures the magnetic fi el d generated by the current passing through the cable.) The

transductor minimi zed amperage transient si gnal problems ; filters were

installed in all channel s for fiel d contingencies.

The modifications indicated by the l aboratory testing program were

incorporated into the monitor before fiel d testing. The system assembled for fiel d testing is shown In Fi gure 1.

7

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3 FIELD TEST

Si te Sel ection

The two general types of welding operations considered for field

testing were shop fabrication , which uses automated welding equipment, and field fabrication/repair , which Invol ves manual or semi-automatic

welding and Is more dependent on the operator’s subjective judgiient.

In addition , it was decided that fiel d tests woul d be more conclusi ve if the wel d quality monitor were used in conjunction wi th some

other form of NDT. Two sites were available that offered these combinati ons: Flint Steel Corporation , Tul sa, OK , and a hydroelectric turbine shaft repair job at Ozark Hydroel ectric Plant , Ozark, AK . The

fiel d repair job at Ozark Power Pl ant was chosen since it woul d entail

si tuations that coul d not be simulated during the CERL laboratory eval uations. It was fel t that the time and space constraints of the field

repai r* situation woul d assess the unit ’s adaptability most ri gorously.

In addition to the hardware eval uation , the field test woul d provide an

opportunity for wel ding personnel from industry to apprai se the WQM .

Test Operations

The WQM and auxiliary equipment were transported from CERL to the

Ozark plant in a conventional automobile with no special handling. The

equipment was set up by maintenance personnel and was ready for operation in less than 1 day.

The WQM was set up remote from the repair location , approximately

50 ft (15 m) from the access door (Figure 2). Fi gure 3 shows the unit

connected for testing.

The Hal l effect current transductor is shown in Fi gure 4. Installati on of this device involves simpl e di sconnection and reconnection of

one of the leads from the wel der power unit; no hard wiring is required .

The welding operation in Fi gure 5 shows the WQM vol tage sensing

lead attached to the welding cable leading to the wel ding gun. The lead

does not encumber the operator in any way .

*Repair welding procedure is given in Appendix C.

8
Since no speed measurement system was avai l abl e for this test, a

precision vol tage source was used to provide an equival ent speed si gnal

to compute heat input and nugget area . For thi s mode of operation , a

vol ta ge corresponding to a particular wel ding speed is input to the

analog computer modul e to compute heat input and nugget area (Equations

Al and A3, Appendix A). For exampl e , if the analog module were scaled

for 1 vol t = 1 in./min , then a 6-vol t si gnal from the precision vol tage

source would be input for a welding speed of 6 in./min.

A typical vol tage and current time trace obtained from the WQM is

given in Fi gure 6. These si gnal s were taken at the output of the si gnal

conditioners before fi l tering for inputting the comparators. As can be

seen , the si gnal is not distorted and shows the response of the sensors

to the vol tage and current variations occurring in the arc . Because the

parameter data content is preserved in the tranduction and conditi oning

process, several data utilization options are possible -- from simpl e

al arms to adaptive control systems.

Resul ts

m etal 1 ation

The central unit and associated sensors were interconnected and energized wi thout disrupting the welding contractor or requiring welding

equi pment modification. This verified the adaptability and flexibility

of the desi gn objective. Installation of the system was accompl i shed by

a ceramic engi neer; an el ectrical engineer was not required.

Operation and Thta Interpre tation

The WQM w~s operated by nonelectronic personnel (a welding

engineer) wi th minimal instruction. The data displ ay and printout were

understandable to both Corps and contractor personnel .

Tro ub ’

, eahooting

During the start up of the WQM , erratic operation was indicated by

the visual display ; the modular packaging method enabled the probl em to

be diagnosed and repaired rapidl y by interchanging modules. Agai n , this

was accomplished by nonel ectronic personnel using predefined troubl eshooti n g procedures.

9
S ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~ - S- S.S --_~~- ~~~~~~~ ~~~~~~~~~ 5 - -

4 CONCLUSION

~ 

real time WQM was confi gured and subjected to laboratory and

field tests . Based on the tests , it was determined that the WQM desi gn S 

Is adequate and has fiel d applicability .

A

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Figure 2. Ozark shaft weld joint.

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Figure 3. WqM connected for test with auxiliary

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FIgure 5. WQM field test; WQM arc vol tage sense

lead attached to welding stick cable.

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APPENDIX A:

FACTORS AFFECTING WELD METAL MECHANICAL PROPERTIES

Defects

Changes in the welding parameters of arc vol tage , travel speed, and

heat input can cause several types of defects In the deposited wel d

metal .

Porosity is a void or gas pocket trapped In solidifying wel d metal . The reduced solubility of the gas in the metal caused by the decreasing

temperature forces the gases out of solution. The gases are ori ginally

introduced either by poor shielding, which entrains air , or by chemical

reactions in the mol ten weld metal . With stick electrodes, too long an

arc resul ting from excessive arc vol tage can reduce the shielding effectiveness , thus introducing gas.

Slag inclusion is the entrapment of an oxide or other nonmetallic

material under the wel d bead. The major source of slag is the coatings

on stick electrodes. This defect is related to heat input.

Incomplete fusion Is the failure of adjacent l ayers of the weld

metal or wel d base plate to fuse . Incomplete fusion may result when the

adj acent metal is not heated to the mel ting point because of insufficient heat Input .

An undercut is a groove mel ted into the basepl ate at the toe of the

wel d and is caused primarily by excessive travel speed in relation to

the welding current.

In addition to the defects caused by improper control , the heat

generated by the wel ding process can cause the following changes in the

base metal :

1. GraIn coarsening

2. Softening ( annealIng effects)

3. Hardening ( phase precipi tation or transformation)

4. Segregation of consti tuents

5. Grai n boundary mel ting

6. Loss of ductilit y

15
7. Loss of toughness

8. Residual stresses causing di stortion or cracking.

The type of change which occurs depends on the chemical composition of

the base metal and electrode and the heat history of the base plate . 5

In the two co.iinonly used field wel ding processes--shiel ded metal - arc ( stick electrodes) and gas metal-arc (bare-wi re)--the source of heat

for mel ting the materi al s is an electric arc . Control of the arc param- eters will control the amount of heat generated , the length of time at

an elevated temperature , and the cooling rate of the wel d zone .

Base Metal Microstruc ture

The cooling cycle after a wel d pass determines the microstructure

of the wel d metal and the heat-affected zone . With fast cooling rates , some steel s become very hard because of a martensitic transformation.

If the cooling rate is sufficiently slow , the metal may be more ductile

and the structure ferritic and pearl itic. The type of steel generally

determines which of these structures is desired . For l ow-carbon and

low-al loy steel s , the pearlitic structure is desirable , while for high- strength quenched and tempered steel , the martensitic structure is desirable.

Martensite is undesirable in l ow-carbon and low -~lloy steel s desi gned for yiel d strengths less than 80 ksi (552 MN/rn’) because of its

hardness and low solubility for hydrogen at ambient temperatures. The

combination of characteri stics increases the likel ihood of hydrogen

cracking in the joint. Use of l ow-hydrogen stick el ectrodes and the gas

metal-arc wel ding system reduces this tendency toward hydrogen-induced cracking.

Cooling Rate Control

Control of the cooling rate is essential in preventing undesirable

microstructure in the wel d and heat-affected base pl ate. A mathematical

combination of arc vol tage , current , and travel speed known as heat

input (HI) has been used as a means of controlli ng cool i ng rate for many

years. The equation for calcul ating heat input Is

HI (Ju n ) = Vol tage x Pinperage x 60 (E Al) Travel Speed (tn./mln.) q

16
_____________________ 5~~~~ 5 .

The normal maximum has been 55 ,000 to 60 ,000 JI m . (21 654 to 23

622 J/cm) for the field processes mentioned above. Another means of

controlling cooling rate has been preheat treatment. Dorschu’ has shown

that the relationship between heat input , preheat temperature , and cooling rate is:

m (T-T )2

CR 

HI +c (Eq A2 )

where CR = cooling rate

S T = test temperature, 1000°F (538°C)

T = preheat temperature = constants

HI = heat input (kJ/in.)

Eq A2 indicates that the hi gher the preheat temperature and heat input , 5

the slower the cooling rate.

Shultz and Jackson ’5 have shown that the cross sectional area of the

wel d bead is a useful indicator of wel d metal mechanical properties and

that a rel ationship exists between the area and cooling rate. They al so

found that arc vol tage has littl e or no effect on the nugget area and

cooling rate. The relationshi p that Shul tz and Jackson have developed for nugget area , arc current , and speed is

na = i _____ 122 X 10 

s 0903 (Eq A3)

where na = nugget area (sq in.)

i = arc amperage

S = arc travel speed (in./min.).

K. E. Dorschu , “Control of Cooling Rates in Steel Weld Metal ,” Welding

Research Supplement (February 1968).

B. L. Shul tz and C. E. Jacks on , “Influence of Wel d Bead Area on Wel d

Metal Mechanical Properties,” Welding Research Supplement (January 1973).

L 

. 511 

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APPENDIX B:

CIRCUIT DESCRIPTION

Fi gure B1 is a block diagram of the weld quality monitor showing the input si gnal s from the welding arc . These si gnal s are conditioned

to standard val ues and sent to the comparator module , which compares the Input si gnal s wi th a set of limit si gnals. ’ If the Input signal s are too high or low , the appropriate alarm is triggered. The input signal s are

al so transmitted to the analog computer modul e for cal cul ation of the heat Input, cooling rate , and nugget area . The cal culated val ues are

then compared to reference signals and the appropriate alarm Is trig- gered if needed.

I REFERENCE1

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WELD NEASURANDS

ANALOG

COMPUTER

MODULES

Figure Bi. Block diagram of weld quality monitor.

R. A. Weber and C. E. Jackson , Review of WeidabiUty of Conetr ’ uction

~~teriale, Interim Report M-168/ADA027383 (CERL , 1976).

18

_____________ 5 

-- —...

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APPENDIX C:

WELD REPAIR PROCEDURE -- OZARK HYDROELECTRIC SHAFT

Parameters

Process : SMAW

Filler Material : E7018

Electrode Diameter: 1/8 in. (.3tmi) 5/32 In. (4imn)

Current Range : 115-165 amps 150-220 amps Vol tage Range : 21-24 vol ts 22-25 volts S

Polarity : Direct current-8lectod8 positive

Preheat Temperature: 3000

F (1490C) m m . Interpass Temp~rature~ 350 F (178 C) max.

Postheat: 300 F (149 C) 6 hours

Heat input Vol tage x Amperage x 60 - 40 to 55 JIm Travel Speed

Technique (Figure Cl)

Check root pass temperature before welding beams.

1. The root pass shal l be deposited using a back step wel ding

technique ; root should be nondestructively eval uated by some

method before wel ding over.

2. Block welding shal l al so be employed to minimize transverse

wel d shrinkage .

3. Each wel d bead shal l be peened to control distortion. Peening

of the first and final layers is prohibited.

4. The peening tool shal l be ground to el iminate any sharp edges.

5. The air pressure used to drive the peening tool s shal l fall

wi thin a 95 to 100 psi pressure range.

6. An insIde micrometer shal l be used to measure between reference

blocks for monitoring distortion while welding. These measurements may be taken only when all four heat zones are wi thin

the preheat and interpass temperature range , and the variance

in temper

~ture0from any one zone to any other zone does not

exceed 20 F (7 C).

7. The joint shall be wel ded at least one layer above flush and

magnetic particle inspected.

19
—S

8. During the 6 hour postheat at 300°F ( 149°C), the reinforcement

S may be removed by grinding. The grinding marks must be

parallel to the shaft axis.

9. A final Inspection shall be performed by magnetic particle testing and ul trasonic testing, 24 hours after the weld has

cooled to an ambient temperature.

I

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AREA !- WELDED FIRST

AREA fl—WELDED SECOND

AREA IlL — WELD ED THIRD

CRACKED AREA AIR-ARC GOUGED S GROUND SMOOTH

20 In

SHAFT CROSS-SECTION

WALLS SUTT~~ED SEQUENTIAL ~ SROUND SMOOTH

CROSS SECTION OF WELD JOINT

Figure Cl. Cross sections of shaft and weld joint,

1 

20

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REFERENCES

Dorschu , K. E., “Control of Cooling Rates in Steel Weld Metal ,” Welding

Research Suppl ement (February 1968).

Schul tz, B. 1. and C. E. Jackson , “Infl uence of Wel d Bead Area on Wel d

Metal Mechanical Properties,” Welding Research Supp l ement (January

1973).

Weber, R. A. and C. E. Jackson , Review of Weldability of Constr uc tion

ttiterials, Interim Report M-168/ADA027383 (CERL, 1976).

Weber , R., F. Kearney, and S. Joshi , Deveiopn~~t of a Weld Quality

Monitor, interim Report M-183/A0A027644 (CERL , July 1976).

21
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Kearney, Frank

Nondestructive testing for field welds : real time

weld quality monitor. -- Champaign, Ii. : Construction

Engineering Research Laboratory ; Springfield , Va .

available from National Technical Information Service,

1978.

21 p. : ii). ; 27 cm. (Interim report -- Construction Eng ineering Research Laboratory ; M-251)

1. Welded joints-testing . 2. Nondestructi ve testing . I. Title. II. Series: U.S. Construction

Engineering Research Laboratory. Interim report

M-25l.

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