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Method for the quantitative assessment of transverse weldmetal hydrogen cracking

M. Lawa,⁎, D. Nolanb, R. Holdstockb

aANSTO (Australian Nuclear Science and Technology Organisation), AustraliabUniversity of Wollongong, Australia

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +61 2 97179102;E-mail address: mlx@ansto.gov.au (M. Law

1044-5803/$ – see front matter © 2007 Elsevidoi:10.1016/j.matchar.2007.08.015

A B S T R A C T

Article history:Received 30 July 2007Received in revised form3 August 2007Accepted 7 August 2007

Diffusible hydrogen introduced during the welding process can drastically reduce thetoughness and ductility of welds in steel structures. In welds experiencing high levels oflongitudinal residual stress (such as upper passes in multipass thick sections), transverseweld metal cracking can occur. While there are methods available for assessingsusceptibility of heat affected zones in the parent material to hydrogen cracking, thereare few that are applicable to the deposited weld metal, and none that are quantitative inrelation to transverse cracking. Therefore, the current work was undertaken to develop atest that isolates and quantitatively assesses the effects of diffusible hydrogen on transversecracking in the weld metal. The test method was based on four point bending of as-deposited weldmetal with controlled levels of diffusible hydrogen. It is proposed that such atest configuration allows quantitative assessment of the relationship between appliedstress/strain, hydrogen content, microstructural characteristics and the fracture behaviour.There were two variations on the test method; the first involved rectangular specimensmachined from bead on plate welds providing a geometrically regular specimen with lowvariability in test results, and the second involved an as-welded bead on plate specimen (un-machined) which inherently has a higher level of variability due to greater irregularity in thespecimen geometry, but which has the potential for industrial application due to the muchsimplified sample preparation and handling required. The method shows a high level ofreproducibility and shows great potential for future application for both assessment ofsusceptibility of weld metals to hydrogen embrittlement, as well as more fundamentalstudies of the physico-chemical mechanisms underlying hydrogen embrittlementphenomena.

© 2007 Elsevier Inc. All rights reserved.

Keywords:Hydrogen embrittlementWeldingTestingFinite element modelling

1. Introduction

Hydrogenmay degradematerial properties due to interactionsbetween hydrogen atoms, microstructure and the local stress.Welding is a major problem area as hydrogen decomposesduring the welding process, producing hydrogen ions that areeasily absorbed into the molten weld pool. As the weld poolsolidifies themonatomic diffusible hydrogen is retained in theweld metal, diffusing through the weld metal and heat af-

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fected zones (HAZ), and thereby interacting with variousmicrostructural features, such as vacancies, dislocations,voids, grain boundaries and inclusions. It is this highlydiffusible hydrogen that is responsible for hydrogen crackingassociated with welding processes. This hydrogen cracking isreferred to as hydrogen-assisted cold cracking (HACC), and itmay occur in the HAZ or the weldmetal (WM). In modern highstrength steels, HACC typically occurs in the weld metalbecause these steels gain their strength from controlled

.

Fig. 2 –Drawing of loading conditions and geometricalparameters in the 4-point bend test.

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thermo-mechanical processing, while weld metals haveadditional alloying elements added to achieve the requiredstrength, and are thereforemore susceptible. The risk of HACCoccurring in theweldmetal is also higher because of its as-castmicrostructure and structural heterogeneity [1]. Additionally,structures made from higher strength steels experience highstresses and high strength welds experience higher residualstresses; both of which will increase the risks of HACC.

While the precise physico-chemical mechanism(s) forHACC remain a subject of some debate, there is long-standingagreement that the fracture processes responsible for bothHAZ and WM cracking are dependent on the microstructureand strength of the steel, the stress intensity factor (crackdriving force/fracture toughness), and the concentration ofhydrogen at the crack tip [2]. While there are industrystandards and guidelines for the avoidance of HACC in theHAZ, this is not the case for the weld metal. In this case, in-dustry and academia rely on a handful of qualitative methodsin order to test the resistance of weld metal to hydrogencracking [3]. Reliable testing methods are required to betterunderstand and predict the occurrence of HACC in WM.

Quantitative testing for weld metal hydrogen crackingmust incorporate variables such as stress, varying hydrogenconcentration within the weld metal, microstructure andincubation time. HACC depends upon transport of diffusiblehydrogen to the crack tip and fracture initiation can thereforebe regarded as time dependent and strain rate sensitive. It isknown that lower strain rates are more effective in producinghydrogen-related cracking behaviour [4,5] as a result of therequirement for hydrogen to diffuse through the material,and to then accumulate at the site of crack initiation orpropagation.

Most available WM HACC tests incorporate time to failureas an indication of susceptibility. However, a controlledmeasure of stress is more difficult to achieve, and currentlyonly a crude measure of restraint is used, such as for theGapped Bead on Plate and Rigid Restraint Cracking Tests.

2. Test Methods

The test methods developed in the current work were basedon four point bending (see Figs. 1 and 2) of as-deposited weldmetal with controlled levels of diffusible hydrogen. It isproposed that such a test configuration allows quantitativeassessment of the relationship between applied stress/strain,hydrogen content, microstructural characteristics and the

Fig. 1 – Image of the 4-point bend test applied to anas-deposited weld metal specimen.

fracture behaviour. This test configuration was selected inorder to concentrate the loading in the weld metal in isolationfrom the weld HAZ. The test reduces stresses on the parts ofthe weldment closer to the neutral axis, avoiding a prematuretest endpoint caused by HAZ cracking. It also provides asubstantial volume of weld metal exposed to the appliedstress/strain, thereby enabling the study of crack initiation atexisting microstructural features or defects. During this test, 2rollers on the load span push and deform the specimen. Theload span is subject to a uniform bendingmoment and surfacestresses. Below the elastic limit, stress and strain varythroughout the depth of the specimen with the support sidein tension and the load side in compression. After yieldingoccurs only the strains vary uniformly.

No artificial stress raisers, such as notches or pre-cracks,were introduced into the test specimens since the aim was toproduce crack initiation influenced only by the existingmicrostructure, applied load and level of hydrogen. Theabove approach also enables a number of possibilities inrelation to experimental design and mechanistic studies. Forexample, it is possible to subject the specimen to a typicalstrain controlled tensile test where properties such as yieldstrength and hardening behaviour may be determined.Further, it is possible to perform a much slower strain ratetest which often leads to failure at a lower strain than the first

Fig. 3 –Macrograph showing a typical cross-section of themachined weld bead (see dotted line) used during 4-pointbend testing.

Fig. 4 –Schematic showing as-deposited bead on plate specimens. Bead height 5 mm, bead width 10 mm, plate height 15 mm.

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test type. The third method is to hold the material at aparticular strain, and determine the time to failure, if itoccurs. Each of these testmethods can reveal different aspectsof the complex hydrogen diffusion, trapping, and failuremechanisms.

Two specimen types were developed in the work. The firstwas developed to provide the most accurate and reproducibletestmethod; however, it required significant preparation. Thisis referred to as the machined specimen. Three rectangularbars measuring 10 mm×14 mm×300 mm were clampedtogether in a welding jig to provide a surface 42 mm widelong to weld upon. A single 150-mm long pass of weld metalwas deposited on the centre bar of the 42 mm wide surface.This resulted in all three bars being fused together by the weldbead. They were then quenched in ice water and thenimmediately placed in liquid nitrogen. Once the welded barswere cooled to ∼77 K, they were removed from the liquidnitrogen and the outside bars were broken off. Cryogenicconditions were required for machining and storage becauseof the high mobility of hydrogen in weld metal at ambienttemperatures. The cross-sectional geometry of the bendspecimens is illustrated by the dotted line in Fig. 3.

Fig. 5 –Graph showing relative stresses in a weld bead compar

With such a rectangular specimen the following equationscan be used to calculate the maximum stress and maximumstrain in the outer fibers, respectively.

rmax ¼ 3⁎ðS� LÞ⁎P2⁎w⁎h2

emax ¼ w⁎h⁎d3S2 � 4L2

where S is the specimen span, L is the load span, P is the totalload on the 2 load rollers, w is the specimen width, h is thespecimen height, and d is the midpoint deflection. Theseequations ignore plastic straining.

While the machined specimen provides very good repeat-ability and accuracy, it was felt that a test should bedeveloped which does not require machining that could beused as an industry test method. Testing was performed on aweld bead on plate sample with no machining carried out.This was referred to as the un-machined specimen. However,in order to define the stresses and strains in this it isnecessary to analyse the cross-section behaviour. To definethe stresses from a test on a specimen with an irregular

ed to the plate (plate thickness 15 mm, bead width 10 mm).

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cross-section, the second moment of area and the resultingstresses in the complete weld bead on plate were calculated. Theweld bead was assumed to approximate a half-ellipse (Fig. 4), andthe moments of inertia and centroid positions of the base plateand the weld half-ellipse were calculated.

From this configuration, the centroid, neutral axis, andmoment of inertia of the composite bead on plate can becalculated. From this and the imposed bending moment, thestress and strain at the top of the weld bead can be calculated.The details of this are shown in Appendix A. According to thismodel, as the plate width increases, the second moment ofinertia of the plate also increases, and the composite centroidheight is reduced. The distance from the centroid to the weldbead and plate upper surface both increase, reducing the ratioof the maximum stresses in the weld relative to those in theplate at the adjacent surface (Fig. 5). The weld bead height ismore significant that the plate width in this relationship.

The shape of the weld bead causes differences even in anominally rectangular specimen. The specimen in Fig. 2 wasinitially treated as a rectangle, but considering it as a rectanglewith a superimposed elliptical weld bead of 0.75 mm heightgivesmaximum stresses in the weld beadwhich are 6% higherthan would be normally calculated.

Fig. 6 –Section along midplane of bead on plate specimen showinstress between load rollers.

Finite element modelling was also performed on the beadon plate specimen under 4-point bending. The model(presented in Figs. 6 and 7) assumed the same Young'smodulus for the parent metal and weld, though modelsincorporating different values showed similar results. Thesemodels compared well with analytical results. Divergencefrom the analytical results is due to bending across the platedue to the greater stiffness of the bead and plate in thecentre (Fig. 7).

3. Experimental

The bead on plate specimens were produced using high-strength ferritic weld metal, deposited on quenched andtempered martensitic base material by flux cored arcwelding (FCAW). The nominal properties and compositionsof both parent and weld metal are presented in Table 1.Different levels of diffusible hydrogen were achieved byadding 2% H2 and 5% H2 to the prescribed CO2 shielding gas.There was no noticeable difference in the mean voltage ormean amperage values when 2% H2 or 5% H2 was added tothe shielding gas.

g maximum stress at top of weld bead, and zone of constant

Fig. 7 –Cross-section showing strain distribution in bead on plate 4-point bend specimen. Deformation exaggerated×8 showsthe in-plane bending.

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3.1. Testing

The specimenswere removed from the liquid nitrogen and de-iced inwater for three and a half-minutes prior to 4-point bendtesting. The edge with the weld bead was positioned along thesupport span (facing away from the load rollers) and the testwas initiated at a strain rate of 2 mm/min (a strain of 1.3 E-5strain perminute) at the cross-head until fracture or extensivedeformation occurred. Care was taken to ensure that thepositioning of all the test specimens was similar. Thepositioning of the rollers was identical for each specimenwith respect to its centre and weld direction. The load span

Table 1 – Flux cored arc welding process details

Consumable Yield strength=720 MPaCigweld-Tensi-cor110TXP

Tensile strength=800 MPa

Parent steel Yield strength=690 MPaBisalloy 80 Tensile strength=830 MPaProcess details 28–30 V, 280–310 A, welding speed of 210 mm/min

heat input ∼1.5 kJ/mm

measured 40 mm wide and the support span measured120 mm wide. Three or more specimens were tested for eachcondition.

4. Results

The machined specimens gave regular and reproducibleresults which are discussed below. The un-machined speci-mens had more variable results, but because they differed inspecimen setup and strain rate the results are not reported.The level of hydrogen in the shielding gas did not appear tohave a significant effect on the yield strength (Table 2, Fig. 8).

Table 2 – Results of changes in shielding gas hydrogenconcentration

Level of hydrogen inshielding gas (%)

Level of hydrogen inweld metal (ml/100 g)

Failurestrain (%)

0 2.5 122 6.5 7.55 15 2

Fig. 8 –Graph showing typical stress versus strain curves for the bend tests.

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The 0.2% offset yield strength was calculated from the 4-pointbend test (which is not a valid tensile test, but providesrepeatable results) as 903MPa, 890MPa and 900MPa for 0%, 2%and 5% H2 conditions, respectively. This level of variation isconsidered to be within the experimental error. Work hard-ening occurred with the 5% H2 condition; this occurs after thepoint where yield strength has been determined, indicated bythe arrow in Fig. 8. Themeasured levels of diffusible hydrogenand failure strains are presented in Table 2.

5. Discussions and Conclusions

This work presents two new methods of mechanical testingthat can be used to quantify the effect of hydrogen on themechanical properties of weld metal under varying stressesand hydrogen concentrations; these are referred to here as themachined specimen and the un-machined specimen. The fourpoint bend test was chosen for two reasons; first, to place theweld bead at the position of maximum stress so the test wasnot prematurely concluded due to parent or HAZ cracking, andsecond, to expose the maximum volume of weld metal to theimposed maximum stress. Both the machined specimen andthe un-machined specimen were able to demonstrate hydro-gen-assisted cold cracking (HACC).

The machined sample, subjected to 4-point bending,proved to be a reliable method for gaining reproducibleresults; this method is well suited to research. The un-machined sample may provide an industry test method forassessing HACC. Results for the un-machined sampleprovided similar results as the machined samples; however,the un-machined tests had a higher variability, but offer thepotential for less preparation and handling. For the un-machined specimen, the narrower samples increased thestresses in the weld bead compared to the stresses in themachined specimen plate.

The addition of 2% H2 resulted in a significant reduction instrain to failure, from N0.12 to ∼0.07. The addition of 5% H2 tothe shielding gas resulted in severe embrittlement, with strainto failure in the region of 0.02.

The effects of welding residual stresses were not measuredor accounted for in this work.

Appendix A

Method of calculation of stresses in bead on plate specimenDisplacement of load point=dLet: −X=(S−L) /2 Y=d /2Rectangle propertiesMoment of inertia Ir=wh^3/12Centroid Cr=h /2Area Ar=whHalf-ellipse propertiesMoment of inertia Ie=πab^3/8Centroid Ce=4b /3πArea Ae=πab /4Composite bead on plate propertiesTotal area At=Ar+AeHeight of centroid for composite Cc=(Ae(Ce+h)+ArCr)/AtMoment of Inertia for composite Ic=Ir+Ae(h−Cc)^2+IeRadius of curvature of specimen R=sqrt ((−x /y ⁎ (L /2−x)+

y)^2+ (L /2)^2)Test resultsBending moment in test M=P (S−L) /4Distance from neutral axis to bead y=(b+h−Cc)Maximum stress in weld bead surface σ=My / IcMaximum strain ε=y /R where R is the radius of curvature

of the specimen

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[2] Yurioka N, Suzuki H. Hydrogen assisted cracking in C-Mnand low alloy steel weldments. Int Mater Rev April1990;25:217–49.

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[3] Davidson JL. Hydrogen induced cracking of low carbon–lowalloy steel weldments. Met Forum 1995;19:35–51.

[4] WuXQ, Kim IS. Effects of strain rate and temperature on tensilebehavior of hydrogen-charged SA508C1.3 pressure vessel steel.Mater Sci Eng A (Switzerland) May 15 2003;348(1–2):309–18.

[5] Okahana J, Kuramoto S, KannoM. Analysis of hydrogen relatedto embrittlement of an Al–Zn–Mg–Cu alloy. Internationalconference on hydrogen effects on materials behavior andcorrosion deformation interactions; Moran, WY; USA; 22–26Sept. 2002; 2003. p. 909–17.