American Welding Society - Primary Chromium Carbide Fraction …files.aws.org/wj/supplement/WJ_2015_01_s1.pdf · 2014. 12. 17. · When using a constant-voltage (CV) implementation

IntroductionIn Canada alone, wear is estimated

to cost $2.5 billion a year (Ref. 1). To cutdown on the cost of wear, hardfacing isemployed. Fe-Cr-C hardfacing is a sur-face treatment aimed at improving thesurface properties of metals, in which awelded cladding is deposited onto thesurface of a substrate to improve thepart’s resistance to wear. There are anumber of different types of materialsystems that are employed in surfacing

(Refs. 1, 2). This research focuses on Fe-Cr-C hardfacing.

Fe-Cr-C hardfacing is a surfacingtechnique applied to plain carbonsteel and is used in circumstanceswhere it may be undergoing wearcaused by abrasion, impact, erosion,and corrosion (Refs. 3–6). Fe-Cr-Chardfacing is used in a large numberof industries including mining, min-eral processing, cement production,and pulp and paper manufacturing(Refs. 7–9). In oil sands operations its

use includes crusher teeth, sizingscreens, and centrifugal pumps, wear-plate for truck bed liners, and slurrytransport pipelines (Refs. 6, 10–12).Fe-Cr-C hardfacing tends to be rela-tively inexpensive in comparison toother surfacing types such as tung-sten carbide-based and certain poly-mer liners, which lends them to beingused on the larger scale components(Ref. 12). This research examines Fe-Cr-C hardfacing produced with thesubmerged arc welding (SAW)process. The main reasons for usingSAW over other processes are its highproductivity and deposition rates(Ref. 13).

For the optimization of the pro-duction of Fe-Cr-C hardfacing, theuse of square-wave alternating cur-rent (AC) is attractive because of itsability to control dilution and de-crease heat input. The variables of ACwaves that can be controlled are thebalance, DC offset, and frequency.This research looks solely at the ef-fect of controlling the balance, whichhas been identified as the most im-portant waveform variable control-ling dilution (Ref. 14). The balance isgiven as a percent of time the arcspends in electrode positive polarity.When using a constant-voltage (CV)implementation of AC welding, thewire feed speed (WFS) and the volt-age are set and held constant whilethe machine varies the current (Ref.14).

WELDING RESEARCH

JANUARY 2015 / WELDING JOURNAL 1-s

SUPPLEMENT TO THE WELDING JOURNAL, JANUARY 2015Sponsored by the American Welding Society and the Welding Research Council

Primary Chromium Carbide Fraction Control with Variable Polarity SAW

Increasing the fraction of time spent welding in DCEN compared to DCEP can increase the amount of primary carbides

BY S. D. BORLE, I. LE GALL, AND P. F. MENDEZ

ABSTRACTUsing alternating current (AC) when welding chromium carbide hardfacing alloys

has a pronounced effect on the resulting welds. To examine exactly the effect of ACbalance (fraction of time in electrode positive) on FeCrC hardfacing, six differentsamples were made varying from 50 to 75% balance in 5% increments. The heatinput was found to increase from 3.82 to 4.30 kJ/mm and dilution along the centerline increased from 3.7 to 31.1%. The ultimate consequence of increasing the balancewas a decrease in the volume fraction of primary carbides from 21 to 3% and a decrease in average diameter of carbides from 30.3 to 21.8 mm with the increase in balance. The increase in the volume fraction of carbides also coincided withmicrostructures that had higher percentages of hypereutectic microstructures thatshould lead to more uniform wear throughout the height of the hardfacing. The increase in volume fraction of carbides as the balance decreases should also increasethe wear resistance. The use of AC waveform with balances near 50% gave microstructures expected to perform the best.

KEYWORDS • Hardfacing • Surfacing • Cladding • Submerged Arc • Chromium Carbide

S. D. BORLE was a MSc. student at the University of Alberta at the time of this research and is currently at Group Six Technologies. I. LE GALL was a visiting student at the University of Alberta from Université de Nantes. P. F. MENDEZ is a professor and director at the Canadian Center for Welding and Joining at the University of Alberta, Canada.

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Microstructure of SAWChromium Carbide Surfacing

The compositions of the Fe-Cr-Chardfacing tend to fall in the rangesof 8–35 wt-% Cr and 2–6 wt-% C(Refs. 3, 13, 15, 16). As shown in theidealized pseudo ternary in Fig. 1,this range of compositions encom-passes three distinct categories of mi-crostructure: hypoeutectic, eutectic,and hyper-eutectic. There is a largedifference between the microstruc-tures and the mechanical propertiesof the three different categories. Hy-pereutectic alloys’ wear resistance isnormally superior (Ref. 2), but corro-sion resistance and toughness areoften better in hypoeutectic alloys(Refs. 13, 17).

Microstructure of hypereutectic al-loys consists of primary M7C3 car-bides surrounded by a matrix ofeutectic austenite and eutectic M7C3carbides. The hexagonal M7C3 car-bides solidify first, as the composi-tion of the melt approaches theeutectic line and the temperature de-creases, the solidification changes toeutectic M7C3 carbides and austenite(Ref. 17). An example of the typical

microstructure is shown in Fig. 2. Thesize of the primary carbides increasesas the cooling rate decreases orchromium wt-% increases, while theaddition of carbon increases the vol-ume fraction of carbides while refin-ing them (Refs. 15, 18).

When the composition of the alloyfalls at or close to the eutectic line,M7C3 and austenite (g) solidify simul-taneously as a eutectic microstructure(Ref. 19). Figure 3 is an example of aeutectic microstructure in chromecarbide surfacing. The eutectic can ei-ther be a rosseta-like morphology, asshown in Fig. 3, or it can be fibrousbundles (Refs. 20, 21).

When the composition is in theaustenite region of the ternary, theresulting microstructure is hypoeu-tectic. In this case, the solidificationstarts as primary austenite dendritesand then as a eutectic of M7C3 andaustenite (Refs. 19, 22). The austen-ite dendrites are surrounded by theeutectic microstructure of M7C3 car-bides and austenite, as seen in Fig. 4.

Previous work has shown there canbe a large variance in the wear resist-ance within the different microstruc-tures and compositions of Fe-Cr-Chardfacing; however, it is generallyagreed that as composition variesfrom hypereutectic to eutectic to hy-poeutectic, the wear resistance de-creases (Refs. 2, 10–12, 17, 23–28). Asignificant part of the wear resistanceincrease going from hypoeutectic toeutectic to hypereutectic microstruc-tures is attributed to the increase inthe amount of carbides. In hypereu-tectic alloys, an increase in the sur-

face fraction of primary carbides in-creases wear resistance (Ref. 7).

An entirely hypereutectic mi-crostructure is wanted for the bestpossible abrasion resistance, but tra-ditional hypereutectic alloys can con-tain all three microstructures (Refs.2, 29, 30). Figure 5 is an example ofhow the microstructure of a SAW Fe-Cr-C hardfacing can be hypereutecticat the top, eutectic in the middle, andhypoeutectic close to the weld inter-face. This variation is representativeof Fe-Cr-C hardfacings in general, andit is not typically seen in cast struc-tures of similar compositions. Figure5 was taken from micrographs of anunetched sample taken using BSE im-aging.

This layering occurs because of dif-ferences in composition along theheight of the weld (Ref. 29). However,it is unclear between solidification,segregation, diffusion, lack of mixing,and diffusion what the main cause is.The issue with the layering is thewear resistance will not be uniformthroughout the service life of thehardfacing. Testing protocols addressthese variations by specifying a par-ticular depth at which wear testingmust be performed (e.g., 75% ofcladding depth). Current research isbeing performed in order to generatea better understanding of layering,which has very important practicalconsequences. This paper focuses onhow a balance in a square waveformaffects the amount of primary car-bides and the physical cause for this phenomenon.

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WELDING JOURNAL / JANUARY 2015, VOL. 942-s

Fig. 1 — Idealized liquidus surface of theFeCrC ternary phase diagram.

Fig. 2 — Hypereutectic microstructure showing large hexagonal M7C3 primary carbidessurrounded by a matrix of eutectic M7C3 and austenite.

Table 1 — Balance Settings Used in This Work

Sample Balance

A 50B 55C 60D 65E 70F 75


Experimental ProcedureWhen producing Fe-Cr-C hardfac-

ing using SAW, an alloying powderthat contains elements such aschromium, carbon, manganese, andmolybdenum is placed down in frontof the welding head. The powder usedfor the current research was based ona proprietary blend used to produceFe-Cr-C hardfacing industrially. Themain alloying elements werechromium and carbon with lesseramounts of manganese, silicon,

molybdenum, and boron added. Awelding gun attached to a weavingmechanism was used to producepasses of the desired width. Setupsused to produce Fe-Cr-C hardfacingsresemble the schematic shown in Fig.6. In the case of the current research,only a single weld bead was depositedand the powder was applied evenly byusing a wheel feeder rotating at a con-stant speed while traveling along thepath of the weld previous to actuallywelding. In these experiments, singleweld beads of 1¾ in. (44.5 mm) width

were made on a 9 ¥ 12-in. (228 ¥ 305-mm) A36 plate with a thickness of 5

⁄16

in. (7.94 mm). The sample plate wasclamped to the base plate using boltsgoing through 1⁄2-in.-thick plates on topof the sample plates with the boltsthreaded into the base plate to preventdistortion of the sample plate whenwelding.

Typical operating ranges for weldingof Fe-Cr-C hardfacing using SAW are30–40 V and 500–700 A. The other op-erating parameters such as wire feedspeed, alloy powder addition, and weaveare chosen to ensure proper melting ofthe powder, a good surface finish, andslag detachability. The basis of the weld-ing parameters were chosen to replicatean industrially produced product withthe balance being the only welding vari-able changing between the samples. Theflux used for these experiments was 1.5on the basicity index. The experimentalsettings were chosen to resemble an in-dustrially produced product and to ex-amine the possibility of process/productimprovement. The welding was donewith constant voltage, constant wirefeed speed, and constant translation ve-locity, so that the ratio of wire to pow-der was constant through all the welds.The offset was not adjusted, the fre-quency was kept constant at 45 Hz, andno preheat was applied to the basemetal. The samples and their respectivebalance settings are listed in Table 1.

The consumables used were an ag-glomerated basic flux along with a L-61 welding wire (EM12K). Aproprietary blend of powders was usedto produce a weld metal high inchromium and carbon. The powersource used was an AC/DC SAW ma-chine that allowed for the manipula-tion of AC waveforms along with dataacquisition through a built-in pro-gram. The waveform data collected bythe machine were then used to deter-mine the HI of the welds. HI was cal-culated by taking the average of theinstantaneous power throughoutwelding. Dilutions of the samples weremeasured using the formula shown inEquation 1 and with the areas de-scribed in Fig. 7 (Ref. 24).

Dilution(%) = DA/(DA + OA ) ¥ 100 (1)

DA is the area of dilution or theamount of the base metal that wasmelted, and OA is the area of the over-

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Fig. 3 — Eutectic microstructure of austenite and M7C3 .

Fig. 4 — Hypoeutectic microstructure of primary austenite dendrites surrounded by eutectic M7C3 and austenite.

Table 2 — Summary of Dilution and Heat Input for AC Balances between 50 and 75%

Sample Balance Dilution Center Dilution Side Heat Input(%) (%) (%) kJ/in. (kJ/mm)

A 50 3.7 34.0 97 (3.82)B 55 4.8 32.8 100 (3.96)C 60 19.7 47.5 101 (4.00)D 65 18.7 38.6 103(4.06)E 70 25.7 41.3 108 (4.23)F 75 31.1 41.4 109 (4.30)


lay that has been added.To calculate the dilution measure-

ments, samples were taken along thesagittal plane of the weld at the cen-terline and 15 mm off to the side, asshown in Fig. 8, to account for the lackof uniformity of the welds. The zig-zagline in Fig. 8 represents the weavingpattern in the deposition of a singlebead.

Metallographic samples for volumefraction measurements were madefrom along the longitudinal center ofthe welds, while dilution measure-ments were taken from samples takenfrom the transverse of the welds. Thesamples were hot mounted in Bake-lite® and then ground and polishedusing an automatic polisher with22.24 N force. For the first step, 60-grit rough grinding was carried out for5 min. After this, multiple other grind-ing passes down to 1200 grit were car-ried out at 3 min for each stage. Thesamples were then polished using a 1-mm diamond suspension for 5 min. Allof the micrographs with exception ofthose shown in Fig. 5 were etched for30 s using a mixture of 20 mL deion-ized water, 10 mL HCl, 30 mL HNO3,and 5 g FeCl3.

In order to get a good representa-

tion of the microstructure, a largenumber of photographs was taken ofsamples that were then photo mergedtogether to make large micrographsacross a large width and the entireheight of the weld. The total volumefraction of carbides was then meas-ured by using Photoshop® to select allof the light areas of the microstructurein the weld pool with the exception ofthe unfused powders, which were notincluded in the volume fraction analy-sis or size analysis. For the sampleswhere the primary carbides’ volumefraction and size were measured,smaller areas from the large micro-graph were examined. The primarycarbides in the smaller areas were thenselected and the total area of the car-bides measured using Photoshop. Theaverage size of the primary carbides inthe individual micrographs was thencalculated by dividing the area by thenumber of carbides selected. The aver-age size of primary carbides in thesample was calculated by taking thetotal number of all the carbides thathad been measured throughout thesmaller micrographs divided by thetotal area of all of those carbides thatwere measured. The diameter given isthe distance between two parallel sides

if the carbides are represented as regu-lar hexagonal shaped.

ResultsFigure 9 shows how the profiles

vary when cut transversely across thesamples. The concave weld interfaceobserved is a result of the weaving,which has a longer dwell time at theedges than at the center of the weld.These cross sections show that thepenetration is increasing throughoutthe samples as the balance increases.The actual dilution measurementswere taken from the longitudinal sec-tions and are tabulated in Table 2along with the heat input calculations.

Figure 10 shows that the trend forthe heat input and dilution is to in-crease as the balance increases. The ex-ception is at 60% balance, whichshowed exceptionally high dilutionamounts in the experiments per-formed. The sides are less sensitive tovariations of balance. One possiblereason for this observation is that thelarger depth of molten metal at thesides reduces the efficiency of cathodicheating on the weld bead.

The influence of balance in the mi-crostructural inhomogeneity across

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Fig. 6 — Schematic of the experimental SAW setup for the FeCrC hardfacing used in this work.

Fig. 5 — Microstructure of chromium carbide surfacing showinga microstructural variation of hypereutectic at the top, eutecticin the middle, and hypoeutectic at the bottom.

Fig. 7 — Schematic of transverse cross section of a chromiumcarbide surfacing highlighting the cladding area, the dilutionarea, and the base metal.


the depth of the hardfacing is illus-trated in Fig. 11. For all images, thetop corresponds to the free surface ofthe bead, and the bottom correspondsto the weld interface. For all balances,a hypereutectic structure is observedat the top, and often a hypoeutecticstructure near the weld interface. Thewhite round features correspond topowders that were not completelymolten, some of which reach a largesize, of the order of 1 mm (seen in the60% balance image). A steady decreasein the amount of hypereutectic mi-crostructure at the top was observedwith the increase in balance.

Figure 12 presents a higher magni-fication image of areas of hypereutec-tic composition from the crosssections in Fig. 11. Because of thesmall size covered by each image, con-clusions about primary carbide frac-tion were made by direct measurementover a number of different areas.

The measured volume fraction (VF)of hypereutectic microstructure, totalcarbide VF, VF of primary M7C3 car-bide, and the average primary M7C3size are given in Table 3 and Fig. 13.

Figure 13 illustrates graphically thetrends determined, showing how theamount of microstructure that is hy-pereutectic goes from under half to100% as the balance reaches 50%. Thevolume fraction and average primarycarbide size increases as the balancedecreases. The decrease in primarycarbide fraction is consistent with theincrease in dilution.

DiscussionThe steady decrease in primary car-

bide fraction with balance is consis-tent with the well-known effect ofincreased dilution with balance, cou-

pled with the understanding of thethermodynamics of the material sys-tem used. The increase in dilutionwith balance is due to the metalspending increasing amounts of timeas a nonthermionic cathode, with theassociate heat input on the surface(Refs. 32, 33). The reduction in theamount of primary carbides with a re-duction of Cr and C in the system hasbeen reported previously (Refs. 15, 18,22). Although no wear tests were per-formed for the samples prepared, it isto be expected that the increase in pri-mary carbides should increase thewear properties of the chromium car-bide surfacing (Refs. 11, 15). Anothereffect of decreasing the balance was toincrease the volume fraction of the hy-pereutectic microstructure, whichshould make the wear resistance moreuniform throughout the height of thecladding.

It is not clear at this time why forthe intermediate value of 60% balancethe dilution departed from the general

trend observed. Metallographic meas-urements of carbide fraction and sizedid not show an equivalent departurein the trends.

The decrease in size of primary car-bides with the reduction of balancewas unexpected, and opposite of whatwould be expected from welds withhigher heat input. Other experimentsperformed with similar alloy systemsshowed very little variation in carbidesize for a wide range of heat inputs.The effect observed here could be be-cause of an increase in the amounts ofMo and Si caused by the decrease inthe dilution, as both have been foundto reduce the rate of carbide precipita-tion, which could lead to the largercarbides (Refs. 29, 31).

This work did not test the effect ofoffset and frequency on the microstruc-ture. Based on previous studies (Ref.14), it is reasonable to expect that offsetwill be of secondary importance (with aslight decrease in fraction of primarycarbides with an offset biased toward

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Fig. 8 — Schematic of dilution measurement locations taken along the longitudinal of the weld hardfacing.

Fig. 9 — Cross sections of single bead FeCrC hardfacing for AC balance between 50and 75%.

Fig. 10 — Variation of dilution with AC balance.


DCEP). No effect is expected with fre-quency changes until frequencies are sohigh that the ramp-up and ramp-downtime is of duration comparable to thehold time. In this case, heat input wouldbe reduced, likely resulting in lower di-lution and higher carbide fraction. Suchhigh frequencies are not recommendedthough because of the difficulty in con-trolling the process accurately in thatrange.

Somewhat similar effects to theones reported here are also expectedby varying the contact tip-to-work-

piece distance (CTWD) while keepingthe same voltage and wire feed speed.Longer CTWD should have a compara-ble effect to decreased balance, result-ing in a higher carbide fraction.Adjusting CTWD involves mechanicaladjustments that are often undesirablein the large equipment typically usedin producing a Fe-Cr-C hardfacing.

Balances below 50% were also con-sidered informally in this work, butthe slag system used was too difficultto remove, and it was considered im-practical for industrial use.

Conclusion

This work assessed for the firsttime the effect of balance on AC wave-form in the SAW of Fe-Cr-C hardfac-ing. The practical implications of thefindings are very significant becauseFe-Cr-C hardfacing is the most com-monly used approach to wear protec-tion in ground engagement equipmentin mining, oil, gas, and logging indus-tries.

The amount of primary carbides in-creases steadily with a decrease in bal-ance. The cases studied indicated aprimary carbide fraction increase from3% for a 75% balance to 21% for a50% balance. This effect is directly re-lated to the decrease in dilution as thebalance decreases.

The size of primary carbides also in-creased with the reduction of balance,from a characteristic size of 21.8 mm for75% balance to 30.3 mm for a 50% bal-ance. This work also showed how thelayered structure of Fe-Cr-C hardfacingis present in a wide range of balancesand how it can be affected by alteringthe balance. Balances of 60% and belowresulted in pure hypereutectic mi-crostructures. A hypereutectic structurethroughout the hardfacing thickness isexpected to help make it reliable withgood wear resistance even at advancedstages of wear.

The authors want to acknowledgethe support of Wilkinson Steel andMetal, Lincoln Electric, and NSERC.

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Table 3 — Microstructural Characterization Measurements of Carbides

Balance (%) Total VF Carbide (%) VF Hypereutectic (%) VF Pimary M7C3 (%) Average Diameter (μm)50 45.8 100.0 21 30.355 42.7 100.0 20 26.760 32.8 100.0 18 24.965 37.8 84.8 14 23.070 31.4 75.6 9 22.775 21.0 40.3 3 21.8

Fig. 11 — Longitudinal sections of hardfacing with AC balances between 50 and 75%.

Fig. 12 — High magnification of hardfacing with AC balances between 50 and 75%. A —50%; B — 55%; C — 60%; D — 65%; E — 70%; F — 75%.

Acknowledgments

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Fig. 13 — Primary carbide volume fraction and average primary carbide size.


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American Welding Society - Primary Chromium Carbide Fraction …files.aws.org/wj/supplement/WJ_2015_01_s1.pdf · 2014. 12. 17. · When using a constant-voltage (CV) implementation