The Use of Protective Weld Overlays in Oil Sands Mining

Gary Fisher1, Tonya Wolfe

1, Matthew Yarmuch

1, Adrian Gerlich

2 & Patricio Mendez

2

1 Alberta Innovates – Technology Futures, Edmonton, AB

2 University of Alberta, Edmonton, AB

Alberta’s oil sands are the second largest reserve of oil in the world, after Saudi Arabia

[1], with an estimated 170 billion barrels recoverable using current technologies [2]. The

oil sands are processed primarily by surface mining; a schematic of the open pit process

is shown in Figure 1. Oil sand deposits are removed via truck-and-shovel operations and

transported to a crusher/conveyor system where the sands are sized. The processed sands

are mixed with water and other additives to form slurry, which is piped to the extraction

plant for subsequent processing to extract the bitumen. Approximately two tonnes of oil

sands ore has to be processed to produce one barrel of synthetic crude. The sands in the

ore are mainly composed of quartz sand, silt, clay, water and bitumen [3]. Quartz

particles typically comprise of between 80 to 95% of the total solids, and are generally

less than 150 µm in size with a semi-angular shape. This scale of processing has

produced unique challenges as the hard particles within the oil sands are responsible for

severe wear of equipment and components.

In the Fort McMurray, Alberta region, the four commercial oil sands mining and

processing operations encounter high maintenance costs and significant production losses

due to material degradation through wear and corrosion. These costs include both the

replacement of the equipment and the associated labour and lost production. In 2011, the

annual operating budget for Syncrude Canada Ltd. was estimated at $1.3 billion [4].

Operating costs across the four commercial mine sites can be estimated at approximately

$5 billion. A major portion of this budget is due to wear damage to machinery and

equipment [5].

The main wear mechanisms encountered when extracting the oil sands are abrasion,

impact and erosive wear. Low stress abrasion is prominent where sands are sliding over

a surface with a relatively low contact force (e.g. a hopper wall) [6]. High stress

abrasion (and gouging) tends to result in higher levels of material loss as abrasives are

driven under high load into the contacting surfaces of equipment (e.g. bucket teeth) [7].

Rocks and boulders present in deposits cause impact-related damage to equipment; the

degree of which is somewhat governed by climatic conditions. During winter months the

sand and bitumen can consolidate into larger lumps, increasing the level of impact

damage [8]. The addition of water in the hydrotransport operations has introduced wear-

corrosion issues. The main corrosive species present are chloride-containing compounds

in the sands and dissolved oxygen in the water [9]. Increased wear rates of separating

screens, slurry pumps and tailings lines have been observed and attributed to the

synergistic effects of wear and corrosion producing an amplified rate of material removal

[10].

There has been a wide adoption of wear resistant weld overlays in an effort to combat

these repair and maintenance issues. By employing weld overlays it is possible to retain

the desired properties of the bulk material used to produce a component while vastly

improving the protective qualities of surfaces in contact with the wear- and corrosive-

media. The two main categories of weld overlays used for wear protection are chromium

carbide-based alloys and tungsten carbide metal matrix composites.

The compositions of chromium carbide-based hardfacing alloys are essentially based on

high chromium cast irons. Typically, the grades of alloy will contain in the range of 25

to 32 w% chromium and up to 5% of carbon. Their microstructures tend to contain large

amounts of M7C3 carbides, where M can represent metals such as chromium, iron or

manganese (Figure 2) [11]. The carbides are contained within an iron-based austenitic or

martensitic matrix, and have the microstructure of acicular, hollow rods. These are

aligned parallel to the heat flow direction of the weld deposition, i.e. roughly

perpendicular to the surface of the substrate material [12]. The M7C3 carbides have a

reported hardness of approximately 1700 Hv, whilst the matrix of the alloys tends to be

softer (approximately 350 Hv) [13].

Chromium carbide-based overlays (CCOs) are usually applied using arc welding

processes [14]. These can be either manual, or for larger components automated

processes are used. Compared to other wear-resistant materials (tungsten carbide-based

overlays and certain polymer liners), CCOs are relatively inexpensive. As such, they are

utilised for larger scale, high volume applications such as wear-plate for truck beds and

piping in hydrotransport applications [15]. The overlays are generally deposited in two

passes, with the second pass having a larger thickness. This is to compensate for the

dilution of the hardfacing alloy with the base material, and allows the second pass to

nucleate and grow the large, acicular carbides necessary for wear resistance. The

overlays tend to contain transverse cracking resulting from the contraction of the weld

pool upon cooling (i.e., often called ‘relief’ or ‘check’ cracking).

CCOs are commonly employed to protect against abrasive wear in wet or dry

environments. There is a general consensus that having a high proportion of M7C3

carbides in the overlay is beneficial to protect against abrasion. However, there is a lack

of available information correlating the effects of carbide size or populations with the

performance in specific, abrasive oil sands mining applications. CCOs are generally less

suitable for protection from impact wear. If the impact loading is high enough to produce

plastic deformation, the brittle nature of the carbide-rich alloy can act in tandem with the

pre-existing transverse cracks in the hardfacing, resulting in the spallation of the overlay.

To extend the service life of production-critical components it has been found necessary

to use tungsten carbide-based metal matrix composites (MMCs) [16]. These tend to be

deposited by plasma transferred arc welding (PTAW); however for specific components

or in-field repairs gas metal arc welding (GMAW) techniques are employed.

The MMCs contain distributions of hard tungsten carbide particles suspended in a tough,

ductile metal matrix alloy (Figure 3). PTAW is the most common method for depositing

MMC overlays. The PTAW process involves an arc being established between a non-

consumable tungsten electrode and the work piece, typically with argon gas shielding and

powder feeding through the welding torch (Figure 4). This process can produce

relatively thick deposits (between 3 to 6 mm in a single pass), with low dilution rates

while maintaining the integrity of the composite materials [17].

Commercial MMCs usually combine tungsten carbide particles with a Ni-based matrix

alloy. The carbide particles can be either angular or spherical, with the angular particles

being sub-divided between monocrystalline tungsten carbide (WC) and eutectic

(WC/W2C) (Figure 5). The heat produced by the PTAW deposition can promote the

dissolution of the tungsten carbide particles into the metal matrix. This dissolution effect

can embrittle and reduce the overall corrosion and wear resistance performance of the

MMC. WC is more chemically stable than W2C and less prone to this effect [18]. WC-

bearing MMCs are more widely used in the oil sands industry as they allow for a wider

window of production welding parameters and improved material consistency compared

to WC/W2C particles, since Spherical carbides generally have a eutectic WC/W2C

composition, making them more prone to dissolution. However, due to the fine lamellar

structure of WC/W2C, these tend to be harder than WC carbides, with measured hardness

in excess of 2,200 Hv (while WC has a range of hardness from approximately 1,100 to

2,100 Hv [19]).

Commercial MMC overlay powders tend to contain a blend of between 60 to 65 w%

tungsten carbide combined with a Ni-based matrix alloy. The Ni-alloys are based on

NiBSi or NiCrBSi, and contain typically 2 to 5w% B and Si to suppress the melting point

of the alloy to around 1050ºC, and act as self-fluxing agents. The matrix microstructure

typically consists of primary nickel dendrites, with interdendritic eutectics composed of

Ni+Ni3B or Ni+Ni3Si. These boride and silicide phases are harder than the Ni phase,

however the hardness of the tungsten carbide is far superior [20].

The addition of up to 15wt% Cr to the alloy can augment the hard precipitate phases in

the matrix due to the formation of Cr-borides and Cr-carbides, however this may not

necessarily improve the wear or corrosion resistance of the MMC overlay. The issue is

that the presence of Cr will accelerate the dissolution of tungsten carbide particles [21].

This not only reduces the volume fraction of primary tungsten carbide, but may also

promote other secondary carbides to precipitate in the matrix during cooling. These

secondary carbides are often very fine in size/shape, and may be too brittle to provide

significant improvements in wear resistance.

The PTAW process is generally limited to shop production environments, and is

impractical for most field welding applications since the gravity fed powder requires

deposition in the flat position. In-field repair welding tends to be conducted using

techniques such as GMAW using tungsten carbide-based wire consumables. However,

due to the increased WC dissolution caused by the high droplet temperatures during

deposition and limitations in the composition of the consumables, the performance of

overlays deposited by GMAW are currently significantly lower than those deposited by

PTAW [22].

The extreme wear-conditions in oil sands processing has led to a wide adoption of

tungsten carbide-MMC overlays deposited by PTAW, especially for production-critical

components. To be economically feasible, PTAW overlays should at least double the

service life of a component. Field experience of overlays used to protect equipment such

as bucket teeth, hydrotransport screens and crushers report improved service lives of up

to 600%, with the associated economic savings [23].

Research and development is on-going in an effort to build on the performance exhibited

by current, commercial composite weld overlays. Key areas of work include the design

of MMCs with improved corrosion resistance and the tailoring of overlay compositions to

combat specific types of wear. Research is also being conducted on developing processes

(such as advanced complex-waveform modes of GMAW and GTAW welding) for the

effective deposition of MMC materials for the demanding wear and corrosions

application encountered in Oil Sands operations.

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Figure 1: Schematic diagram of the open pit process used for oil sands mining.

Figure 2: Micrograph of a chromium carbide-based overlay, etched, showing the acicular

carbides in an iron-based matrix. Magnification x50.

Figure 3: Micrograph of a tungsten carbide-based MMC overlay, showing the tungsten

carbide particles distributed in a nickel-based matrix alloy. Magnification x50.

Figure 4: Photograph showing PTA welding apparatus

Figure 5: SEM Micrograph of a typical angular tungsten carbide powder. Magnification

x100.