much as possible, the significant abrasive and corrosive wear
that occurs along the inner diameter (ID) of oilsands slurry
pipelines. As a quick synopsis, during transport of oilsand
slurry, the most significant wear tends to occur in the
extrados or ID of where flow direction changes (elbows are
especially prone), as well as along the bottom ID of straight
pipe sections where the quartz bearing oil slurry scours the
surface under sometimes intense pressure.
Quite a number of factors can come into play that
impact the choice of hardfacing overlay wire to be used for
cladding repair, including location of piping in the process
string, nature of the corrosive media, and obviously cost; a
quantitative analysis based not just on the alloy formulation
itself, but on the desired deposit and weldability
characteristics, and the anticipated extension of the
serviceable life of the pipe. Metal cored wires (those wires
containing a powdered metal and carbon fill within a metal
sheath), are a primary electrode choice with iron-based
alloys making up the lion’s share of wires utilised today.
The foundation of cost-effective pipe ID
cladding
Iron-based alloys are popular because these consumables
are economical to produce and can be formulated with
various powdered fills and special metal sheaths to address
most slurry cladding applications. Of these iron-based
alloys, chromium carbide wires are the most commonly
produced and utilised in cladding applications. While
other complex carbides (those containing molybdenum,
tungsten, niobium, and vanadium, for example) can provide
better wear resistance, there can be tradeoffs to consider,
including a tendency to be more brittle. With the addition
of more complex carbides in the wire fill and possibly
even the use of specialty metal strip sheathings (nickel-
based tungsten carbide wires are a prime example), it can
be expected that costs will rise. Despite the increased
consumable cost, these complex carbides have their place,
especially in critical applications where a combination
of wear resistance and toughness are required. In fact,
despite the economic downturn, the demand for these
more expensive alloys for oil and gas upstream applications
remains high; especially in pipe and elbow cladding
applications.
The most common iron-based chromium carbide
wires, however, remain the 5C-25Cr type, which have
been optimised to result in superior weldability in pipe
ID overlay applications in the flat position, as well as in
elbows in the horizontal position. With excellent tie-in and
flat bead profile, 5C-25Cr cladding deposits offer adequate
wear resistance and a relatively low cost, and a range
of wire diameters. For applications requiring a minimum
degree of wear resistance on the as-cladded surface, and
at a 75% depth below the surface with no under bead
cracking permitted, 5C-25Cr-M type alloys have been used
successfully. For even more severe abrasion applications,
complex carbides or a modified 5C-25Cr-CLX (5C-25Cr-Nb-V)
can provide enhanced wear resistance thanks to secondary
niobium and vanadium carbides.
ID cladding maintenance
Limiting the effects of hydrocarbon
contamination during reapplication
In essence, iron-based chromium carbide wires provide a
formulaic foundation for alloy development geared toward
oilsands slurry pipe cladding applications on the larger scale.
Affordable, these alloy products are versatile enough to
allow for formulaic modifications as necessary to address
dynamic real-world applications and wear conditions
Figure 1.
The weld deposit on the lower section of pipe elbow,
made using conventional chromium carbide, shows gross
porosity as a result of hydrocarbon contamination of the base
metal. The upper two layer deposit of Stoody PR2009 indicates
the effectiveness of the special chromium carbide formulation
to minimise the effects of embedded contamination.
Figure 2.
A completed two layer deposit of chromium carbide.
In this case, Stoody PR2009 on elbow ID.
122
World Pipelines
/
JUNE 2015
