1 MSc. Graduate Student, Canadian Centre for Welding and Joining, University of Alberta 2 Assistant Professor, Canadian Centre for Welding and Joining, University of Alberta 3 Associate Professor, Weldco/Industry Chair in Welding and Joining, Director - Canadian Centre for Welding and Joining, University of Alberta

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Depositing Ni-WC wear resistant coatings with hot-wire assisted

GTAW

S. D. Guest1, A. P. Gerlich2, P. F. Mendez3

Abstract

With the oil sands holding a large portion of the natural resources industry in Alberta, the

protection of mining and transportation equipment operating in this erosive and corrosive

environment has become a paramount issue. A wide variety of overlay materials exist but it has

been determine that a nickel matrix reinforced with tungsten carbide particles performs the best

in this demanding environment. Currently, these overlays are deposited by Plasma Transferred

Arc Welding (PTAW) with the nickel and tungsten carbide powders blown into the arc. The

formation of tubular wire consumables using a nickel sheath and tungsten carbide powder

contained within has provided industry with a promising alternative method to depositing wear

resistant nickel tungsten carbide overlays for oil sands and mining operations.

Tungsten carbides are very susceptible to dissolution as a result of high weld pool

temperature, forcing industry to seek out low temperature welding processes. Hot-wire assisted

gas tungsten arc welding (GTAW) holds promise for deposition of these wear protection

overlays as it exhibits a colder weld pool. Hot-wire involves resistively heating a consumable

wire to or near its melting point just before insertion into the weld pool. In this manner, the arc

does not have to provide the energy necessary to melt the consumable. This results in a lower

heat input weld with a colder weld pool, providing a favourable alternative to PTAW for the

deposition of nickel tungsten carbide overlays. Current research at the Canadian Center for

Welding and Joining has indicated that overlays produced using hot-wire assisted GTAW is of

comparable quality and productivity as PTAW deposits. Deposits of 45-48 volume fractions of

tungsten carbide have been observed at a deposition rate of 7.3 pounds per hour. Further

refinement of this process may yield improved wear resistance and productivity using lower

capital cost equipment than PTAW, allowing smaller fabrication shops to produce wear resistant

products.

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Background

Oil Sands and Wear

The scope of the mining operations and the demand of the Oil Sands in northern Alberta

have created a multi-billion dollar industry. The oil sands deposit is the second largest in the

world and currently produces more than half of Canada’s crude oil exports (DOE, 2010). With

the demand for oil increasing throughout the world, the Alberta Oil Sands mining and

transportation operations will have to expand. One of the obstacles faced by the Oil Sands

industry is that they contain large portions of quartz sand (Flores et al, 2009). During mining and

hydrotransport operations, the immense weight of the oil sands and slurries causes the quartz

sand to erode and corrode equipment. The wear and subsequent premature repair or

replacement of equipment increases the downtime during mining and hydrotransport sequences

and higher production costs. The cost associated with the downtime and repair or eroded

equipment results in hundreds of millions of dollars budgeted and spent on an annual basis by

each of the oil sands producers (Anderson et al, 2004).

Nickel Tungsten Carbide Overlays

Nickel tungsten carbide (Ni-WC) overlays are used on critical components such as

crusher teeth on digging wheels, bucket shovel teeth, and cyclofeeder sieves as Ni-WC

overlays are deemed necessary to combat the erosive/corrosive effects of the oil sands. Ni-WC

overlays are comprised of a nickel matrix hardened with combinations of chromium, boron, and

silicon additions and reinforced with either monocrystalline WC or eutectoid W2C/WC carbides.

The addition of silicon and boron to the primary nickel matrix (approximately 350 HV) forms hard

borides (Ni3B) and silicides (Ni3Si) with an approximate hardness of 500-860 HV (Liyanage,

2010). Although the addition of chromium to the nickel matrix will form hard chromium carbides,

it drastically increases the dissolution of tungsten carbides during welding. This can be

attributed to chromium having a higher affinity for carbon than tungsten at elevated

temperatures, promoting the tungsten carbides to dissolve. In consumables containing high

chromium levels of approximately 14 wt%, a blocky Cr-Ni-Si-W-C phase surrounds the carbides

as the carbides had been partially dissolved from the carbon hungry chromium additions.

Although this blocky phase had high hardness (960-1600 HV), it exhibited an extremely brittle

behaviour as loads greater than 50 grams caused the phase to crack during Vickers hardness

testing. The addition of chromium will therefore increase the bulk matrix hardness but will be

very detrimental to the spalling and wear resistance of the overlay as the Cr-Ni-Si-W-C matrix

phase will chip easily and increase the likelihood of the carbides being pulled out of the matrix.

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High levels of chromium will increase tungsten carbide dissolution, increasing the distance

between carbides and exposing the matrix to the abrasive medium, increasing the wear rate of

the overlay (Liyanage, 2010).

Tungsten carbides exhibit a high hardness and a low thermal expansion, making them

ideal for use as a wear resistant particle in a nickel matrix for oil sands applications. The

downside to using tungsten carbides is the low heat of formation (-40 kJ/mol), which during the

welding temperature cycle, promotes the dissolution of the carbides in the weld pool (Gassman,

1996). Monocrystalline WC carbides are formed with carbon content higher than 6.1 wt% and

have an approximate hardness of 2200 HV and melting point of 2870 oC (Gauthier, 1995).

Eutectic W2C/WC carbides are formed with a carbon content range of 3.7-4.3 wt% and have a

hardness of 3100 HV and melts at 2785 oC (Gubish et al, 2005). The spacing of the eutectic

lamellae is controlled by the cooling rate during manufacturing. Although the eutectoid carbides

exhibit higher hardness and impact resistance compared to the monocrystalline WC particles,

W2C is less stable at elevated temperatures compared to the WC. This can cause preferential

dissolution of the W2C while leaving the WC intact, producing a jagged or saw-tooth edge to the

carbide, seen in Figure 1.

Figure 1: Preferential dissolution of W2C in welded overlay, leaving jagged or saw-toothed carbide edges. Consumable: Castolin Eutectic D0*11.

Current Ni-WC wear resistant overlays are produced using Plasma Transferred Arc

Welding (PTAW) using powder consumables. The powder consumable is comprised of a

NiCrBSi matrix powder mixed with a predetermined volume fraction of monocrystalline WC or

eutectic W2C/WC powders. These powders are then blown into the sides or rear of the PTAW

torch in order to avoid directly exposing the carbides to the heat of the arc in an attempt to

reduce carbide dissolution. Careful control of welding parameters with a deposition rate of 25-30

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grams per minute will result in very low carbide dissolution and produce the desired volume

fraction of carbides (approximately 50%) (Liyanage, 2010).

Hot-Wire Assist Technology

Hot-wire assisted technology involves the stable addition of a resistively heated

consumable to the weld pool as exemplified in Figure 2. Using Joule heating the wire is heated

very close to its melting point; reducing the energy needed to be supplied by the GTAW arc to

melt the consumable. This in effect reduces the weld pool temperature, decreasing dissolution

of tungsten carbides and lowering dilution to the baseplate. The unique nature of having the

deposition rate decoupled from the penetration and arc energy, the GTAW process lends itself

to depositing temperature sensitive tungsten carbides.

Figure 2: Hot-wire assisted GTAW setup with tubular Ni-WC consumables.

Figure 3: Top view of hot-wire assisted GTAW.

Experimental Approach

The study outlined in this paper was conducted to determine the feasibility of hot-wire

assisted GTAW for deposition of nickel tungsten carbide wear resistant overlays using tubular

wire consumables. The study includes: operation of the hot-wire assisted GTAW setup to

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produce a high quality overlay, analysis of five tubular consumables, and microstructural

analysis of deposited weld metal.

Hot-Wire Assisted GTAW

The hot-wire power supply is a Bab-Hitachi K.K. Power Assist unit from BHIC, Japan,

and capable of 18 m/min wire feed speed and 230A output. The Ni-WC wires pass through a

contact tip before entering a ceramic guide tube which serves to guide the wire into the weld

pool and increase the length of wire available for resistance Joule heating. The use of a 40 mm

long ceramic tube plus 30 mm of un-guided wire provides 70 mm of wire extension for

resistance heating. In order to accurately position the insertion of the Ni-WC wire into the weld

pool, X-Y movement dovetail slides accurately move the wire heating assembly to the desired

position. During testing, wire insertion angles of 60o, 45o, and 30o relative to the base plate were

attempted. It was found that an angle of 45o and 60o brought the top-side of the Ni-WC wire too

close to the GTAW arc. This resulted in premature melting of the top-side of the wire from the

heat of the arc, causing the tungsten carbide powder to fall out before entering the weld pool.

Inserting the wire at an angle of 30o moved the top-side of the wire away from the arc,

eliminating the premature melting. Our colleagues at Hiroshima University in Japan arrived at

the same conclusion and a picture of this effect can be seen in Figure 4 (Nakashin, 2010).

Figure 4: Hot-wire GTAW welding with Ni-WC wire entering at 60o. Proximity of the top-side of the wire to

the arc leads to premature melting and exposing carbides to the arc (Nakashin, 2010).

The Bab-Hitachi hot-wire power supply utilizes a touch screen interface in which

programs can be stored with current, wire feed speed settings, and the hot-wire start settings.

The wire starting settings include initial current and wire feed speed settings and the time at

which the unit operates on the starting settings before reverting back to the selected program.

The Bab-Hitachi hot-wire unit and was run in electrode positive polarity and utilizes a pulsed

current output, a sample of which can be seen in Figure 5. Pulsing the wire current allows for

higher current outputs without increasing the likelihood of arc formation between the wire and

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the weld pool. Due to the magnetic field surrounding the electrified hot-wire, the arc is blown

away from the wire during the current pulse. Figure 6 depicts the arc blow occurring at the 100

Hz pulsing frequency of the hot-wire power supply.

Figure 5: Current and voltage output from Bab-Hitachi hot-wire power supply during welding of 0.045" mild steel wire at 6 m/min wire feed speed and 250A GTAW current.

Figure 6: Effect of hot-wire pulse on arc movement. Picture a) denotes no pulse while b) shows arc blown away from wire during current pulse.

A Miller Dynasty 700 GTAW/SMAW power source was attached to a Weldcraft WP-27B

20” machine GTAW torch operating in electrode negative polarity. We used a 1/8” (3.2 mm) 2%

thoriated tungsten electrodes and commercially pure welding grade argon shielding gas. The

torch and hot-wire assembly is kept stationary while the plate was moved underneath the arc,

making high speed videography possible.

Tubular Ni-WC Consumables

We utilized five 1.6 mm (1/16”) tubular nickel tungsten carbide wires from HC Starck,

Sulzer, Castolin-Eutectic, and Stoody. The Sulzer Ni-WC wires contain WC powders while the

HC Starck and Castolin Eutectic consumables contain eutectic carbides. The Stoody

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Hot-Wire Current and Voltage Data Acquisition Output

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consumables contained both WC and W2C/WC eutectoid carbides mixed in the powder. HC

Starck provided us with two variations of their Ni-WC wires. The HC Starck CTC wire contains

standard eutectic W2C/WC carbides while the HC Starck MTC wire contains eutectic W2C/WC

carbides surrounded with a thin layer of monocrystalline WC. The WC coating resists dissolution

of the carbide in the weld pool, protecting the harder and more impact resistant but temperature

sensitive eutectic carbide from dissolving during welding of the MTC wire, seen in Figure 7.

Figure 7: HC Starck MTC carbide and nickel matrix microstructure.

In order to gain an understanding of the welding properties of each individual wire, tests

were conducted to determine the pure nickel sheath area, internal area holding the carbides,

and the initial volume fraction of tungsten carbide contained within the wire prior to welding. In

order to determine the nickel sheath and internal area, pieces of the wire were cross sectioned

and polished before microscopy and computer area measurements with Adobe Photoshop CS5.

Cross sections of the Ni-WC wires can be seen in Figure 8.

Figure 8: Cross sections of 1.6 mm Ni-WC wires tested: a) Castolin Eutectic D0*11, b) Sulzer WokaDUR, c) HC Starck CTC, d) HC Starck MTC, e) Stoody 160D.

The initial carbide volume fraction measurements were calculated by measuring the

linear mass of the powders and the internal area of the wire in the manner shown below. In

order to determine the linear mass of the nickel sheath and the powders, measured lengths of

wire were pried apart and the powders emptied out and weighed separately.

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The initial wire carbide volume fraction calculations assumed that all the powder

contained in the wires were entirely made up of monocrystalline WC or eutectic W 2C/WC

carbides. This approximation is accurate as the wire contains mostly carbide powders which are

heavier than the lighter boron, silicon, and chromium powders that manufacturers add to alloy

the nickel matrix. The density of wire powder was assumed to be that of either WC or W2C

based on the high fraction of carbides in the powder on the type of carbide found in the

overlays. A summary of the wire areas, linear masses, and initial wire carbide volume fractions

are shown in Table 1 for all the wires tested.

Table 1: Summary of Ni-WC consumable measurements.

Castolin Eutectic D0*11

Sulzer WokaDUR

HC Starck CTC HC Starck MTC Stoody 160D

Total Wire Area(mm2) 1.905 1.946 1.967 2.036 2.002

Nickel Sheath 0.949 ± 0.094 1.030 ± 0.007 0.691 ± 0.025 0.769 ± 0.014 1.056 ± 0.030

Internal 0.963 ± 0.124 0.916 ± 0.019 1.297 ± 0.051 1.267 ± 0.033 0.946 ± 0.030

Wire Linear Mass(g/m) 16.8 16.9 18.2 18.3 17.3

Nickel Sheath 8.7 8.8 6.5 6.9 9.4

Powder 8.1 8.1 11.8 11.4 7.9

Initial Wire Carbide Volume Fraction

49% 56% 53% 53% 49%

Microstructural Analysis

To characterize the difference between various consumable manufacturers and the

viability of hot-wire assisted GTAW system for depositing wear resistant Ni-WC overlays,

various tests were carried out. Electron probe microanalysis (EPMA) is conducted on a stringer

bead of HC Starck MTC, indicated in Figure 9, to determine the amount of carbide dissolution

as a result of the welding temperature cycles. EPMA can provide pinpoint micrometer stepped

elemental concentration gradient map of various elements found in the overlay. Analysis of the

EPMA results indicated that the carbides experienced no bulk carbide dissolution from welding

as there is no tungsten detected in the surrounding nickel matrix.

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Figure 9: EPMA analysis of HC Starck MTC wire, showing no carbide dissolution at the bottom of the weld bead. This can be seen by comparing the back scattered electron image, a), and the tungsten distribution mapping, b).

Further analysis of the HC Starck MTC microstructure using Auger electron

spectroscopy (AES) provided deeper insight into the chemical composition and constituents of

the stringer bead, seen in Figure 10. AES analysis exhibits very high resolution and pinpoint

chemical composition analysis as the technique probes only the very surface of the sample,

eliminating scatter caused by other more bulk sample measurement techniques. It should be

noted that AES has decreasing accuracy with lighter atomic elements such as boron, carbon,

and silicon. The AES analysis revealed sub-micron eutectic lamellae structure, indicated by

points 3 and 4, consisting of Ni-S and Ni-B phases. The finely spaced lamellae is likely formed

during the fast cooling rates of hot-wire assisted GTAW as the heat input can be lowered

compared to traditional GTAW and Ni-WC GMAW overlays. The larger white eutectic region,

indicated by point 1, showed that the nickel matrix contained some tungsten, which is most

likely due to carbide dissolution during welding.

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Figure 10: AES-SEM micrograph of HC Starck MTC.

Table 2: AES chemical composition analysis results of HC Starck MTC from Figure 9.

A common measure of the potential abrasion performance and quality of the overlay is

the hardness of the matrix and carbides. In general, a harder matrix provides greater low stress

wear resistance while a softer matrix provides more impact resistance. The WC and W2C/WC

carbides used in the different consumable manufacturers also exhibited various hardness

values. It was observed that the eutectic W2C/WC carbides had higher fracture resistance as

the WC carbides found in the Sulzer WokaDUR overlay cracked during Vickers microhardness

testing. This cracking phenomenon can be visually identified in Figure 11.

Atomic %

Point C W Ni Fe Si B

1 2.9 48.8 25.5 22.8 0 0

2 5 3.4 63.8 27.8 0 0

3 26.3 42 17.5 12.4 1.9 0

4 8.8 2.9 51.2 17.2 0 19.8

5 7.8 2.4 53.9 16.1 0 19.7

6 6.7 3.5 65.6 24.3 0 0

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Figure 11: Vickers microhardness testing for a) Sulzer WokaDUR with cracks extending from 30 gram load indent, b) cracks extending from Castolin Eutectic D0*11 with 1 kg load, and c) HC Starck MTC carbide without cracking at 30 gram load.

Results and Discussion

In order to test the various consumables and the validity of the hot-wire assisted GTAW

process to deposit the temperature sensitive Ni-WC overlays, stringer beads were overlaid onto

3/8” mild steel plate which was restrained with clamps to minimize any distortion. GTAW and

hot-wire parameters were chosen based on the ability to even addition of the Ni-WC wires to the

weld pool while keeping the arc current as low as possible to retain maximum carbide volume

fractions. The stringer beads of all five Ni-WC wires were cross-sectioned and the carbide

volume fractions were measured at various points throughout the bead, seen in Figures 12-16.

The stringer beads were made at 3 m/min wire feed speed, 0.5 m/min travel speed, and

between 200-220A GTAW current with a 3 mm arc length.

Figure 12: Castolin Eutectic D0*11 stringer bead with 8-9% carbide volume fraction.

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Figure 13: Sulzer WokaDUR stringer bead with 30-32% carbide volume fraction.

Figure 14: HC Starck CTC stringer bead with 45-49% carbide volume fraction.

Figure 15: HC Starck MTC stringer bead with 48% carbide volume fraction.

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Figure 16: Stoody 160D stringer bead with 20-25% carbide volume fraction.

Table 3: Vickers microhardness of matrix and carbides of the 5 Ni-WC wires tested. Hardness Castolin Eutectic D0*11 Sulzer WokaDUR HC Starck CTC HC Starck MTC Stoody 160D

Matrix 508 469 630 636 457

Carbide 1926 1408 2068 2357 1183 & 1860*

*: hardness measurements for WC and W2C/WC carbides respectively.

Examining the weld cross-sections seen in Figures 12-16, the retained carbide volume

fractions in the overlays varied considerably between consumable manufacturers. Overall, the

HC Starck CTC and MTC Ni-WC wires produced the highest volume fraction of carbides, only

losing ~4% of the carbides during welding. The Ni-WC wires from Castolin Eutectic, Sulzer, and

Stoody retained far fewer tungsten carbides. This may be due to the wires containing small

carbides which have a decreased tendency to survive the welding heat cycles due to their large

surface area to volume ratio. These Ni-WC wires may also contain more alloying powders and

hence occupies room that could be filled with tungsten carbides compared to the HC Starck

consumables. The HC Starck consumables also had the highest matrix and carbide hardness

compared to the other manufacturers, seen in Table 3. The overlay properties from the HC

Starck consumables in terms of carbide volume fraction, nickel matrix and carbide hardness

values were very similar to the best performing Ni-WC PTAW overlays conducted by Liyanage

(2010). The deposition rate of the HC Starck overlays was 55 g/min (7.3 lbs/hr) compared to 25

g/min (3.3 lbs/hr) for equivalent quality overlays produced by Liyanage (2010). The hot-wire

assisted GTAW process for deposition of Ni-WC is therefore viable as it can produce high

quality overlays comparable to PTAW but more than doubling the deposition rate. It is important

to note that this deposition rate and carbide volume fraction measured were completed at initial

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testing stages and not at higher productivity levels. The good performance and high quality

overlays from the hot-wire assisted GTAW welding process can be attributed to the resistive

heating of the Ni-WC wire, creating a colder weld pool and increasing the survivability of

carbides following welding.

One of the limiting factors for using hot-wire assisted GTAW for deposition of Ni-WC

overlays is the growth of contamination on the GTAW electrode. Research into the

contamination indicated that the contamination is a dendritic structure comprised entirely of

tungsten and carbon is solution, seen in Figure 17 and Table 4. High speed videography was

conducted to determine how the electrode was being contaminated. It was found that tungsten

carbides are ejected from the weld pool and contact the electrode during welding.

Figure 17: AES analysis HC Starck MTC contaminated GTAW electrode.

Table 4: AES chemical analysis of HC Starck MTC contaminated GTAW electrode.

Summary and Conclusions

1) The hot-wire assisted GTAW process can deposit high volume fraction overlays (48%) with

low carbide dissolution at a deposition rate of 7.3 lbs/hr during initial testing stages.

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2) The Ni-WC consumable manufacturer directly controls the retained carbide volume fraction,

and the matrix and carbide hardness. The HC Starck MTC Ni-WC consumable produced

the highest carbide volume fraction and the hardest carbides and nickel matrix.

3) Eutectic W2C/WC carbides exhibit higher fracture resistance and hardness than the more

thermally stable and widely used monocrystalline WC carbides.

4) GTAW electrode contamination results from tungsten carbides leaving the weld pool and

striking the electrode. Further research may mitigate this problem.

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Hardfacing Welding Using Pulse Heated Hot Wire GTAW – Workshop #3