Key Concepts in Welding Engineeringby R. Scott Funderburk

Selecting Filler Metals:Low Hydrogen

This is part two in a series on select-ing filler metals. When selecting fillermetals, the specifier may elect torequire “low hydrogen electrodes.”Such electrodes may be required tominimize the possibility of hydrogenrelated cracking. In some cases theengineer may specify low hydrogenelectrodes because he believes theseelectrodes will also provide welddeposits exhibiting a high minimumlevel of notch toughness. While thismay be true, it can not be assumed.This article will address specifyingfiller metals that resist hydrogen relat-ed cracking while also providing goodmechanical properties.

The term “low hydrogen” has beenaround for about 60 years. It was firstintroduced to differentiate this classifi-cation of shielded metal arc welding(SMAW) electrode (e.g., E7018) fromother non-low hydrogen SMAW elec-trodes (e.g., E6010). They were creat-ed to avoid hydrogen cracking on highstrength steels, such as armor plate.1

Confusion About the TermAlthough so-called “low hydrogen elec-trodes” have been around for manyyears, there is some confusion aboutwhat is meant by the term. Manycodes and specifications use the des-

ignation, however, neither “low hydro-gen” nor “low hydrogen electrodes” arelisted in the American WeldingSociety’s (AWS) Standard WeldingTerms & Definitions (AWS A3.0-94)2.This may come as a surprise to some,especially to engineers that have beenspecifying that “only low hydrogenelectrodes shall be permitted,” or “allwelds shall be low hydrogen”, or that“all welding processes shall be lowhydrogen.” Without a formal definition,the term “low hydrogen” can be under-stood differently by engineers, contrac-tors, or inspectors, which can lead toconfusion and conflicts.

“Low HydrogenElectrode” MeansSMAW ElectrodeThe closest thing to a formal definitionfor low hydrogen SMAW electrodes isfound in the AWS A5.1 filler metalspecification3. This specification listsseveral electrode classifications with“low hydrogen” coatings. These classi-fications must have a coating moisturelevel of less than 0.6% when tested at1800 °F (980 °C), according to AWSA5.1. This moisture level correspondsto a relatively low diffusible hydrogenlevel in the deposited weld metal, typi-cally less than 16 mL/100g. For exam-ple, AWS A4.3, Standard Methods forDetermination of Diffusible Hydrogen4,shows that when E7018 is welded at70 °F and 60% relative humidity a0.6% coating moisture equates toapproximately 12 mL/100g of diffusiblehydrogen. Many of today’s E7018products have actual coating moisture

content levels much lower than themaximum of 0.6% in the as-receivedcondition. Table 1 lists the SMAWelectrodes with low hydrogen coatingcontained in A5.1.

Can Hydrogen Affect MechanicalProperties?The influence of hydrogen can beobserved in mechanical testing; how-ever, its effects on the test results arelimited. A high hydrogen content in atensile specimen can produce “fish-eyes” on the fracture surface as seenin Figure 1.

Figure 1. “Fish-eyes” on an all-weld-metal tensile specimen fracture surface.

“Low hydrogen” can be understood

differently by engineers, contractors,

or inspectors….

Table 1. AWS SMAW Electrodes withLow Hydrogen Coverings

EXX15-x

EXX16-x

EXX18-x

EXX18M-x

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Welding Innovation Vol. XVII, No. 1, 2000

Additionally, the presence of hydrogencan reduce ductility (as expressed byelongation and reduction in area).Hydrogen, however, does not typicallyinfluence the impact toughness, ultimatetensile strength or yield strength results.It is only in severe cases that it caninfluence the ultimate tensile strength.

Since low hydrogen SMAW electrodeslike E7018 are also required to have aminimum specified level of Charpy V-notch (CVN) impact energy, low hydro-gen is sometimes equated with aminimum CVN level. This has ledsome people to specify low hydrogenwhen the real desire is for notchtoughness. The better approach is tospecify notch toughness requirementssince there is no automatic linkbetween low diffusible hydrogen con-tent in the weld and CVN values.Actually, some deposits with highhydrogen levels can deliver relativelyhigh levels of notch toughness. Forexample, the E6010 classification(non-low hydrogen, 30-50 mL/100g)has a minimum CVN requirement of20 ft-lbs at minus 20°F.

Use of the Term in Codes andSpecifications Some codes and specifications referto hydrogen control in terms of either(1) requiring low hydrogen SMAWelectrodes or (2) placing specific limitson diffusible hydrogen. The StructuralWelding Code – Steel (AWS D1.1-2000)5 has provisions related to hydro-gen in the preheat table (Table 3.2). Inthe table, Category “A” is applicable to“shielded metal arc welding with otherthan low hydrogen electrodes.” Theminimum preheat temperatures listed

in Category “A” are higher thanCategory “B” because Category “B” isfor “shielded metal arc welding withlow hydrogen electrodes, submergedarc welding, gas metal arc welding,flux cored arc welding.”

In the Interim Guidelines: Evaluation,Repair, Modification and Design ofWelded Steel Moment FrameStructures6 published by the FederalEmergency Management Agency(FEMA), a comparison between lowhydrogen SMAW electrodes andFCAW and SAW is made. This docu-ment states, “All of the electrodes thatare employed for flux cored arc weld-ing (both gas shielded and self shield-ed), as well as submerged arcwelding, are considered low hydro-gen.” Implied is the assumption thatFCAW and SAW will provide welddeposits with diffusible hydrogen levelssimilar to SMAW electrodes with lowhydrogen coverings.

Weld DepositHydrogen LevelsAs mentioned above, no definitionexists for a “low hydrogen welddeposit.” The word “low” is an impre-cise description. The preferredmethod of controlling the level ofhydrogen in a weld deposit is to usethe optional hydrogen designators asdefined by the American WeldingSociety. These designators are in theform of a suffix on the electrode classi-fication (e.g., H8, H4, and H2). Thefiller metal manufacturer may chooseto add the hydrogen designator to theelectrode classification if the fillermetal meets the diffusible hydrogenrequirements in the applicable AWSA5.x filler metal specification.Following are examples of the desig-nator requirements:

To avoid hydrogen induced cracking,the hydrogen level in the material mustbe held to a certain maximum level.This level is a function of themicrostructure susceptibility, constraint(or restraint), and residual stresses.Microstructure susceptibility to hydro-gen induced cracking often increaseswith increasing steel strength.Therefore, for higher strength steelslower levels of hydrogen are required.To simply state “use low hydrogen” isnot enough. For example, “low” for a50 ksi steel may not be “low” for a 100ksi steel. Rather than require that “onlylow hydrogen electrodes can be used,”engineers and fabricators are shoulduse statements such as, “only elec-trodes or electrode-flux combinationscapable of depositing weld metal with amaximum diffusible hydrogen content of8 mL/100g (H8) are permitted.”

Codes That UseHydrogen DesignatorsThe AWS D1.1 Structural WeldingCode also has several provisions thatutilize hydrogen designators (e.g., H8).For example, Category “D” in the mini-mum preheat and interpass tempera-ture table (Table 3.2) allows only“…electrodes or electrode-flux combi-nations capable of depositing weldmetal with a maximum diffusiblehydrogen content of 8 mL/100 g (H8).”This is a good example of properlyusing the H-designators.

The AWS D1.1 Code also has analternate method to determine theminimum preheat temperature (AnnexXI) that uses three levels of diffusiblehydrogen. In Annex XI, category H1 iscalled an “extra low hydrogen” at lessthan 5 mL/100g. Category H2 islabeled as “low hydrogen” at less than10 mL/100g. The third category, H3, isa hydrogen level that is not controlled.Although category H2 is labeled “lowhydrogen,” this does not define low

Table 2. Optional Hydrogen Designators

Diffusible Hydrogen,mL/100g

H8 8

H4 4

H2 2

Hydrogen does not typically influence the

impact toughness, ultimate tensile

strength or yieldstrength results

To simply state “use low hydrogen”

is not enough

Welding Innovation Vol. XVII, No. 1, 2000

hydrogen electrode as less than 10mL/100g. The actual diffusible hydro-gen value can also be used to calcu-late the minimum preheat temperaturewith this method instead of using theH1, H2 and H3 categories.

The Fracture Control Plan of the AWSBridge Welding Code7 (AWS D1.5-95)is another fine example of hydrogencontrol. This code requires the follow-ing for welding Fracture CriticalMembers:• H16, H8 or H4, when the minimum

specified yield strength is 50 ksi orless.

• H8 or H4, when the minimum speci-fied yield strength is greater the 50ksi.

Furthermore, SMAW electrodes canbe used for tack welding without pre-heat if the electrode has an H4 desig-nator, according to AWS D1.5.

Other agencies such as the UnitedStates Military8 and the AmericanBureau of Shipping9 also set limits onthe diffusible hydrogen levels. Bothuse limits of 15, 10 and 5 mL/100g,and the military specification has astricter limit of 2 mL/100g for someapplications. Today, a logarithmic sys-tem (i.e., H16, H8, H4, and H2) is pre-ferred in the United States.

Other IssuesUsing an H8, or even an H4, electrodewith controlled diffusible hydrogenalone provides no guarantee of elimi-nating problems related to hydrogenduring or after welding. In addition tothe electrode, several other factors caninfluence the diffusible hydrogen leveland the potential for cracking. Theseshould be considered as well.

• base metal surface condition (conta-mination from oils, grease, dirt,moisture, acid, rust and other hydro-gen containing materials canincrease hydrogen levels);

• relative atmospheric humidity (humidconditions generally lead to higherhydrogen levels);

• welding shielding gas (higher mois-ture content results in higher hydro-gen levels);

• consumable storage conditions(improper or prolonged storage canlead to higher hydrogen levels);

• welding procedures (electrical stick-out, arc voltage, wire feed speedand other parameters can influencediffusible hydrogen).

Conclusions1.A “low hydrogen electrode” refers

only to a SMAW electrode that has acoating moisture of less than 0.6%.

2.The maximum diffusible hydrogenlevel associated with low hydrogenSMAW electrodes has been a pointof confusion because SMAW elec-trodes with low hydrogen coatingsare not tied to any specific hydrogenlevel.

3.“Low hydrogen” should not be speci-fied in order to achieve specificimpact properties. If notch tough-ness is required, then it should belisted separately from the hydrogenlimits (if any).

4.Job specifications should be writtenclearly and precisely regarding theuse of “low hydrogen.” The intent ofthe specifier should be listed in sucha way that the contractor will under-stand what is required.

5. If a contractor has any questionsregarding in the intent of the engi-neer, or if the specifications are notclear, the contractor should seekclarification before welding. Forexample, if “use low hydrogen elec-

trodes only” is listed on the contract,then the contractor may want to ask:“Is only SMAW allowed, or can otherprocesses also be used?”

6.Supplemental hydrogen designators(e.g., H8 and H4) are the preferredway to define a specific level of dif-fusible hydrogen in the weld depositand should be used when needed.

7.Finally, there are applications wherelow hydrogen electrodes are notrequired or where non-low hydrogenSMAW electrodes, like E6010, arepreferred. Therefore, utilizing theblanket statement “use low hydro-gen” should be avoided.

References1 Robert O’Con. “Welding with Low HydrogenElectrodes: A Look at the Past with Tips forToday.” Practical Welding Today. March/April2000, pp. 33-35.

2 American Welding Society. Standard WeldingTerms and Definitions. (ANSI/AWS A3.0-94),1994.

3 American Welding Society. Specification forCarbon Steel Electrodes for Shielded Metal ArcWelding. (ANSI/AWS A5.1-91), 1994.

4 American Welding Society. Standard Methodsfor Determination of Diffusible HydrogenContent of Martensitic, Bainitic, and FerriticSteel Weld Metal Produced by Arc Welding.(AWS A4.3-93), 1993, p. 16.

5 American Welding Society. Structural WeldingCode – Steel. (AWS D1.1:2000), 2000.

6 Federal Emergency Management Agency.Interim Guidelines: Evaluation, Repair,Modification and Design of Welded SteelMoment Frame Structures. (FEMA 267), August 1995, p. 8-11.

7 American Welding Society. Bridge WeldingCode. (AWS D1.5-95), 1995.

8 United States Military. Military Specification –Electrodes – Welding, Flux Cored, OrdinaryStrength and Low Alloy Steel, (MIL-E-24403/1D), November 14, 1985.

9 American Bureau of Shipping. RuleRequirements for Materials and Welding, Part 2, 1997.

Job specs should be written clearly

and precisely regarding the use of “low hydrogen”