Submerged arc welding (SAW) haslong been used in the fabricationof high-energy and heavy wall

piping. This process is utilized for sev-eral reasons including high depositionrates, relatively low operator skill re-quirements, no visible arc flash, andthe ability to deposit welds that gener-ally possess very good mechanical andmetallurgical properties. Alloying ofthe fluxes also allows further refine-ment of weld properties depending onthe demands of the application.

Of these advantages, the most ap-pealing one is arguably the high depo-sition rate. Schedules, available manpower, end user demand, and globalcompetition have forced manufactur-ers and fabricators to rely heavily onSAW to deposit a relatively largeamount of weld metal when comparedto other common arc processes. Theability to deposit 10 lb an hour ormore can speed the fabrication processconsiderably. This is normally accom-plished by depositing weld beads thatare considerable in size and penetra-tion, in addition to high heat input.

Creep Strength-Enhanced Applications

A high deposition, high heat inputpractice is more common than not formost carbon- and low-alloy steels de-pending on the application. Lower al-loyed materials generally do not en-counter complex metallurgical trans-formations during the heating andcooling cycles of welding. However, inthe never ending quest to develop al-loys with higher performance, newmaterials have been developed that areunlike anything that has come before.This is especially true in high-temper-ature, high-pressure applications (such

as in power generation) where creepresistance is a primary design consid-eration. The development of creepstrength-enhanced ferritic steels(CSEF) such as Grade 91 have becomea mainstay in designs and applicationsdue to their high creep strength andsignificant weight savings offered.

Because SAW is such a simple andeasy to use process, many times engi-neers and operators will overlook theconsiderations that need to be madewhen welding CSEF steel with SAW. Itis easy to put the correct spool of wireon the feeder, load the correct flux inthe hopper, push the start button andbe on your way. However, using tradi-tional “hot and heavy” SAW practices,the engineer may find the weld pos-sesses poor mechanical propertieswhen tested. Materials with low duc-tility in the weld metal and heat-af-fected zone (HAZ) will likely fail tomeet many equipment owners’ specifi-cations. Scattered hardness readingsmay also fall out of specification.

Considerations WhenUsing SAW

For the reasons discussed above,careful consideration must be takenwhen developing welding proceduresfor SAW CSEF applications. These ad-vanced materials require special weld-

ing and heat treating practices in orderto remain safe and reliable duringtheir operating life. One of the mosteffective and practical ways to achievethe desired metallurgical structures isthrough regulated deposition rates,heat input and, most importantly,control of bead size, shape, and place-ment.

Depositing a high amount of fillermetal takes a considerable amount ofheat input. In most cases this will alsolead to a very large and deep penetrat-ing bead that also contributes to ahigh amount of dilution — Fig. 1.When welding similar metals with amatching filler metal, a high dilutionisn’t necessarily a problem; however,when welding dissimilar metals, highdilution rates can be a potential haz-ard. A region of uncertainty will bepresent along the weld interface adja-cent to the HAZ. This hybrid alloy mayor may not have the properties one de-sired. Regardless of dilution rates, avery large bead with high heat inputhas the potential to degrade the me-chanical properties of any metal (Refs.1, 2). While a slow cooling rate is gen-erally desirable with many metals, thecooling rate of a very heavy bead on amaterial that already has a 400°F pre-heat will be far too slow. It can be ex-pected the yield strength, hardness ,and impact properties will all be re-duced significantly.

Follow a DifferentFormula

In many cases, a welding engineerwould turn to the traditional heat in-put formula to monitor and predictheat input as well as bead shape. Thismethod should be used with caution,as the heat input formula simply

WELDING JOURNAL / MAY 20151

When Using SAW to Weld Creep-Resistant Steels: Less Is MoreControlling bead size, shape, and placement isimportant when using submerged arc welding to join creep strength-enhanced ferritic steels such as P91

BY MATTHEW A. COX AND WILLIAM F. NEWELL JR.

THE AMERICAN WELDERTHE AMERICAN WELDER

Fig. — 1 High deposition SAW weld.

measures the electrical energy spent,and does not take into account the ef-fectiveness of that energy or how effi-ciently it was used. There are severalstudies that show SAW cross-sectionalbead size and penetration can vary agreat deal with the same or similarheat input using the traditional for-mula as illustrated in Figs. 2 and 3.

Research performed by Linde andmany others in the past 60 years hasshown this to be true in many cases

for a variety of processes, not justSAW (Refs. 1–4). The reason for this isthere are too many other variablesthat affect the weld, including the useof constant voltage vs. constant cur-rent, preheat, material thickness, elec-trode extension (this significantly af-fects I2R heating), and current densitywhen using different-diameter elec-trodes and characteristics of differentbatches of flux.

The Way to GoAn effective way to obtain desirable

mechanical properties from welds inCSEF steels is deposit beads of a moreconservative size that can be placed ac-curately and evenly as the joint is filled— Fig. 4. The smaller cross section ofthese beads also lend themselves tofurther tempering and promote grainrefinement by the heat from subse-quent beads. This tempering tech-nique helps the weld metal obtain thedesired mechanical properties, espe-cially ductility and toughness. Usingsuch a technique, it is not uncommonto see impact properties at room tem-perature ranging 40–50 ft-lb or morethroughout the weld metal and HAZ.These values are only a fraction ofwhat can be obtained with othersteels, but they are considered verygood for CSEF materials.

ConclusionSubmerged arc welding is an excel-

lent process for joining CSEF materi-als, but careful consideration needs tobe taken into account when develop-ing procedures and executing produc-tion. Submerged arc welding has al-ways lured fabricators with promisesof high deposition rates and relativelymoderate operator skill. While this istrue in many cases, a CSEF applicationrequires diligence by the engineers andproduction personnel to develop pro-cedures and techniques that obtainwelds with controlled bead size, pro-files, and the desired properties.

Acknowledgment

The authors would like to acknowl-edge James Hales, president of SpecialWelding and Machining, LLC, for hiscontributions.

References

1. Jackson, C. E. 1959. The science ofarc welding. Adams Lecture. Welding Jour-nal, Part I, 129-s to 140-s, April 1960; PartII, 177-s to 190-s, May 1960; and Part III,225-s to 230-s, June 1960.

2. Schultz, B. L., and Jackson, C. E.1973. Influence of weld bead area on weldmetal mechanical properties. Welding Jour-nal 52(1): 26-s to 37-s.

3. Wilson, J. L., Claussen, G. E., andJackson, C. E. 1956. The effect of I2R heat-ing on electrode melting rate. Welding Jour-nal 35(1): 1-s to 8-s.

4. Jackson, C. E., and Shrubsall, A. E.1953. Control of penetration and meltingratio with welding technique. Welding Jour-nal 32(4): 172-s to 178-s.

MAY 2015 / WELDING JOURNAL 2

WJ

THE AMERICAN WELDER

WILLIAM F. NEWELL, Jr. is vice presi-dent of Euroweld, Ltd., Mooresville,N.C.MATTHEW A. COX is a technicalwelding consultant and sales repre-

sentative with Euroweld, Ltd.,www.euroweld.com.

Figs. 2 and 3 — Each weld bead de-posited with an average heat input of1.78 kJ/mm (45 kJ/in.) (Ref. 1).

Fig. 4 — 2.0-in.- (50-mm)-thick Grade 91 SAW weldment using consistent and evenly placedbeads.