The Development of Hardfacing Alloys for Wear and Corrosion Resistance
Ravi Menon, Ph. D, F. AWS
VP-General Manager
Richard Cook, Senior Product Line Manager
Stoody- An ESAB Company
5557 Nashville Road
Bowling Green, KY 42101
USA
Abstract
Hardfacing technology has progressed significantly in the past twenty years from the days where the primary
applications were in the mining and construction areas. With the rapid advancements in cored wire manufacturing
technology, composite alloy systems are being created that can combat wear and corrosion situations in a
multitude of applications. Some of these include slurry transportation in the oil sands, steel mill rolling
components, and severe erosive-corrosive conditions in oil and refinery process equipment. The presentation will
cover ferrous and non-ferrous consumables developed for many of these applications.
The application of martensitic stainless steel hardfacing on continuous caster rolls has evolved over the past
twenty five years from basic alloys of the 420 type to modified complex alloys that are stabilized with niobium,
vanadium and titanium. Some of the recent alloys utilize nitrogen as an alloying element in lieu of carbon, thus
improving the corrosion performance. These modified alloys have shown significant improvement in performance
when compared to the traditional stainless steels.
There is a significant amount of hardfacing overlaying used for material handling in the Oil Sands industry in
Canada. One of the key applications is the overlaying of slurry piping with a chromium carbide overlay (CCO).
Over the years, the base chromium carbide overlay that is typically of a 5C-25Cr composition has been modified
with micro-alloying elements such as boron to improve the wear as well as impact resistance. The addition of
secondary carbide formers such as Nb and V also significantly improves the slurry jet erosion performance.
Although these overlays provide excellent abrasion resistance, their corrosion resistance is relatively poor. In this
paper, the development of modified chromium carbide overlays that provide a combination of wear and corrosion
resistance is described. Wear tests conducted include the ASTM G65 test and slurry jet tests used in the oil sands.
Corrosion tests include ASTM G48 Pitting corrosion test and corrosion-erosion tests developed to uniquely define
the Oil Sands environment.
Another area of development described is the improvement of wear resistance of stainless and nickel overlays.
Although these overlays possess excellent corrosion and high temperature properties, their abrasion resistance is
relatively poor. The addition of titanium and tungsten carbides significantly improves the wear performance and
opens up the opportunity to have a combination of high corrosion resistance with improved wear resistance.
Potential applications include the replacement of stainless and nickel based piping with an overlaid carbon steel
pipe which has significantly better combination of wear and corrosion properties at a significantly lower cost.
Introduction
Cored wires continue to provide a unique route in the development of specialty alloys for hardfacing. Most
hardfacing alloys have relatively low ductility and cannot be produced as monolithic continuous wires without
incurring significant processing cost. The construction of the most common grades of these wires has been
described earlier publications (ref 1,2) . Cored wires also enable the production of small lots of special
composition when compared to melted lots of solid wires where the minimum order quantities may be
significantly higher.
The largest share of hardfacing welding consumables is held by the iron-based alloys. They are the most
economical alloys to produce. The wire sheath material is typically a low carbon steel and the core materials can
constitute of simple metal powders or fairly complex mixtures of matrix-carbide components. Within the ironbased family, the two common groups of hardfacing wires are comprised of martensitic/tool steels and the
chromium carbide type. Martensitic stainless steel wires are used extensively in the cladding of continuous caster
rolls whereas tool steel wires are used for operations in the hot and cold mills. The chromium carbide
compositions range from the hypoeutectic to the hypereutectic side of the Fe-Cr-C alloy system. In general, the
hypereutectic alloys (typical composition 5C-25Cr) have significantly better low stress wear resistance as
measured using the ASTM G65 Procedure A test (G65 test) when compared to hypoeutectic alloys (typical
composition 2C-23Cr). In general, comparisons of most iron-based alloys can be made using weight loss in the
test as a criterion. However, when comparing the wear resistance of materials from significantly different alloy
systems (such as iron, nickel and cobalt based alloys), volume loss should be used as a criterion.
Martensitic Stainless Steels
Martensitic stainless steels are very commonly used to overlay rolls used in continuous casting (CC). The first
generation of these used the basic 420 alloy type. Although the performance of this overlay far exceeded that of
non-cladded rolls, the 420 overlay is susceptible to thermal fatigue cracking (also referred to as “fire cracking”).
CC roll overlays also experience pitting corrosion which is a precursor to interbead cracking and general
corrosion. Significant interbead cracking and corrosion can lead to excessive roll wear and “chip-outs” as shown
in Fig. 1.
Typical compositions of CC roll overlays are shown in Table 1. The first generation Alloy 420 overlays were
replaced with Alloys 440 and 423 which significantly better performance. These second generation alloys are
significantly lower in carbon and are balanced to have a ferrite content of 5-10 FN in the deposit. In the third
generation of CC roll overlay wires (Alloy 423N), the carbon content is further reduced to below 0.10% and the
hardness is recovered through the addition of nitrogen as an alloying element. The addition of nitrogen also
improves the pitting corrosion resistance of the deposit.
Table 1. Typical Compositions of 420, 440, 423, and 423N CC Roll Overlays
Alloy
420
440
423
C
0.18
0.05
0.12
Cr
12.0
13.0
12.5
Ni
-4.5
2.5
Mo
0.03
1.0
1.0
N
-0.03
0.03
Stabilizers
--Nb, V
HRC
48
35
45
423N
0.05
12.5
4.0
1.7
0.10
V, W
43
Property
Wear Resistance
Corrosion Resistance
Wear Resistance / Thermal
Fatigue Resistance
Wear and Corrosion Resistance
Thermal fatigue tests were conducted to evaluate the performance of these overlays in a rapid heating and cooling
environments similar to what caster rolls experience in service. In the experimental set up shown in Fig. 2, an
overlay block is subjected to heating with a propane torch followed by a rapid water quench. The samples are
examined after going through a thousand cycles of heating and cooling. Fig. 3 shows the surface condition of the
samples after the test. Alloy 420 shows a significant level of cracking followed by Alloys 423 and 423N. Alloy
423N shows the lowest sensitivity with virtually no cracks appearing on the surface after the 1000 cycle test.
The superior corrosion resistance of Alloy 423N was confirmed with the conduct of two tests. The first shown in
Fig. 4, compares the response to a sensitization thermal treatment of Alloys 423 and 423N. In this condition,
Alloy 423 shows a high sensitivity to grain boundary attack when compared to Alloy 423N. In another corrosion
test (ASTM G48 Practice A), the results of which are shown in Fig. 5, Alloy 423N shows 60% better performance
than Alloy 423 with regards to weight loss measurements in the test. In actual service conditions, Alloy 423N
overlays have been reported to last in excess of two million tons of steel cast.
Modified Chromium Carbide Wires
The most common iron based chromium carbides wires used for hardfacing are shown in Table 2. The lowest
alloy of the 2C-23Cr type has relatively good ductility but poor abrasion resistance. The 5C-25Cr type is the most
commonly used grade is generally multi-passable and is also used to build-up components such as coal pulverizor
rolls and cement grinding rolls even up to thicknesses as high as 1-2” (25-50 mm). These wires find extensive
application in most hardfacing applications where there is moderate to severe abrasion and low to moderate
impact. They are of relatively low cost and are typically available in diameters ranging from 1/8” (3.2 mm) to
0.035” (0.9 mm). However, for many special applications that require a greater degree of wear resistance, these
compositions are modified. Traditionally, the complex carbides containing Mo, W, Nb and V can provide better
wear resistance; however, these deposits tend to be brittle. Moreover, wire costs escalate significantly with the
addition of the alloying elements such as Mo, V and W. A micro-alloyed version, 5C-25Cr-M, has been very
successfully used in critical applications where a combination of wear and toughness are required. This type of
wire finds significant use in the cladding of pipe used for slurry transport in the oil sands. Many of these
applications require a minimum degree of wear resistance as measured with the ASTM G65 test on the as-cladded
surface as well as at a depth 75% below the surface. In addition to this requirement, no underbead cracking is
permitted. Wires of the 5C-25Cr-M type have been used very successfully in this application. For more severe
abrasion applications, a modified complex version, 5C-25Cr-CLX (5C-25Cr-Nb-V) has been developed. These
deposits provide an enhanced degree of wear resistance due to the presence of secondary carbides of Nb and V. In
contrast to the conventional complex carbide deposits of the 6C-19Cr-5Mo-5Nb-2W-2V type, this alloy is leaner
in the secondary carbide formers. This results in the wire being multi-passable and the deposits having better
ductility. The leaner composition also results in a more economical wire. G65 test comparisons of the complex
carbide deposits to other chromium carbides are shown in Fig. 6a. The complex carbide deposits also perform
well in pin-on-disc tests that are designed to simulate high stress abrasion (Fig. 6b). This alloy therefore is well
suited for grinding applications such as in coal and cement mills. In these applications, the 5C-25Cr-CLX deposits
have been reported to give about 25% better life than the 5C-25Cr-M type overlays. In slurry-jet testing (Fig. 6c)
specific to the application required for slurry pipe, the complex carbide deposit performs better than the
microalloyed version. This makes it as a strong candidate for slurry pipe applications that involve severe wear
such as the extrados area of elbows. Fig. 7 shows the microstructures of the chromium carbide deposits. Fig. 7a
shows the eutectic carbide structure of the 2C-23Cr. Fig. 7c shows the refined carbide microstructure of the 5C25Cr-M deposit when compared to that of the 5C-25Cr deposit shown in Fig. 7b. The microstructure of the 5C25Cr-CLX deposit shown in Fig. 7d consists of precipitates of Nb and V carbides located in the matrix between
the primary chromium carbides.
In summary, modified complex carbide wires of the 5C-25Cr-CLX type possess optimal properties for a wide
variety of wear environments that include low stress and high stress abrasion as well as slurry jet erosion.
Table 2. Typical Iron Based Chromium Carbide Hardfacing Alloys
a
Designation/Nominal
Composition
2C-23Cr
5C-25Cr
Typical Hardness
(HRC)
38
47
Microalloyed, 5C-25Cr-M
62
Modified Complex, 5C25Cr-CLX
65
Microstructure
Austenite-Eutectic Carbide
Austenite-Primary Chromium
Carbide
Austenite-Primary Chromium
Carbide
Austenite-Primary Chromium
Carbide-Complex Nb, V Carbides
Maximum Thickness
in. (mm)
2 (50)a
2 (50)a
2 (50)a
2 (50)a
With use of proper base materials and welding procedures.
Corrosion Resistance of Chromium Carbide Deposits
As the key attribute expected from iron based chromium carbide overlays is abrasion resistance, their corrosion
resistance has not been well documented. It has become apparent from field experiences that the corrosion
behavior of chromium carbide deposits may have a larger role to play than has been anticipated in the past. In an
effort to benchmark the corrosion behavior of conventional chromium carbide deposits, a dedicated study was
conducted on those alloys that are typically used in oil sands applications. In this study, two layer deposits of the
alloys described in the earlier section were tested for corrosion performance using the ASTM G48 Method A
Ferric Chloride Pitting Corrosion test. Epoxy mounted samples of the deposits were exposed to 10% ferric
chloride solution at room temperature (22.50 C) and the weight loss (in gms) measured at the end of an exposure
of 72 hours. Weight loss data were then normalized against the area of the deposits exposed to the test solution.
The results shown in Fig. 8 indicate that the lowest carbon deposit (2C-23Cr) has the best corrosion resistance.
The most common grade (5C-25Cr) had the poorest corrosion performance. Addition of alloying elements in
microalloyed and complex grades significantly improved the corrosion performance. However, the corrosion
performance did not match the 2C-23Cr type. In contrast, the abrasion resistance of the 2C-23Cr deposit was
significantly lower when compared to all the 5C-25Cr alloys (see Fig. 6a) and will not meet the criteria
established by most end-user companies for pipe ID cladding.
SEM/EDAX analysis was conducted on selected samples to check the difference in elemental composition
between the carbide phase and the matrix phase. In Fig. 9a, this analysis is shown for the 5C-25Cr deposit. It is
clear that a significant amount of chromium is partitioned to the hexagonal Cr7 C3 carbide leading to chromium
depletion in the matrix to below 12%. This renders the matrix incapable of developing a passive film that provides
corrosion resistance. In contrast, the analysis for the 2C-23Cr deposit in shown in Fig. 9b. Here the matrix is
significantly more enriched in chromium that results in better corrosion resistance. The formation of extensive
primary carbides in the higher carbon alloys leads to chromium depletion in the matrix which has a deleterious
effect on corrosion performance.
Development of Modified Chromium Carbide Deposits to Optimize Wear and Corrosion
Resistance
The goal of this program was to develop a hardfacing alloy that at minimum would have the abrasion resistance of
the 5C-25Cr-M deposit but also would have corrosion resistance of the 2C-23Cr deposit. A series of wires was
made with varying degrees of primary elements such as carbon and chromium and secondary elements such as
molybdenum, niobium and other micro alloying elements. An optimal combination of wear and corrosion
resistance was obtained by lowering the carbon content and through a judicious injection of secondary carbide
formers and elements. These alloys have been developed in the StoodCor™ 136 family of hardfacing wires
(patent pending). The properties of this hardfacing wire are shown against some of the more conventional alloys
in Figs. 10a and 10b. The new alloy has wear properties better than conventional overlays (Fig. 10a) and
corrosion properties significantly better than conventional 5C-25Cr deposits and almost equal to that of the lower
carbon 2C-23Cr deposits (Fig. 10b).
This product is currently has been under field trials in the Oil Sands and has been reported to double the life of
components when compared to conventional chromium carbide deposits.
Stainless Steel Based Hardfacing Wires for Corrosion and Wear
In this group of alloys, the concept of utilizing a second phase such as NbC and TiC to provide wear resistance
has been extended to the stainless steel alloy system. These wires (Table 3), labeled 316-Nb/TiC and 420Nb/TiC,
deposit a microstructure that comprises a matrix of the 316 or 420 compositions with dispersed precipitates of
NbC/TiC. This results in a significant improvement in the wear performance of the deposit when compared to
stainless steel without any degradation in their corrosion resistance. Fig. 11 shows the G65 test data for the
316TiC and the 420TiC deposits in comparison to austenitic AISI 304 and martensitic AISI 410 wrought base
materials. The results indicate a more than a three-fold improvement in low stress abrasion resistance of the
overlay deposits. In testing methodology newly developed at the National Research Council of Canada, the
synergistic effects of wear and corrosion were evaluated. In Fig. 12, the results of these tests indicate the superior
erosion corrosion performance of the deposits over the base AISI 316 stainless steel. The test separates out the
influence of erosion and corrosion. Thus, although the results indicate a significant improvement in the erosion
behavior of the TiC bearing deposits, there is no degradation in the corrosion resistance. The microstructures of
the deposits are shown in Fig. 13. They are very similar for the austenitic as well as the martensitic deposits
showing a fine dispersion of sub-5 micron TiC precipitates in their respective matrices.
These new wires offer the exciting potential of obtaining superior wear performance from stainless steels without
a loss in their corrosion resistance. Applications where they are being field tested include slurry pipe, and
cladding applications for boiler tubes.
Table 3. Stainless Steel Based Hardfacing Alloys for Corrosion-Wear Applications
Designation
316 Nb/TiCa
Bulk Hardness (HRC)
21
420 Nb/TiCa
a
48
Microstructures
NbC and TiC in an austenitic (316)
stainless steel matrix.
NbC and TiC in a martensitic (420)
stainless steel matrix
Patent Pending
Nickel Based-Tungsten Carbide Wires
Additional development for improved wear resistance in extreme applications has led to the development of
alloys having nickel-silicon-boron matrix systems with dispersed tungsten cast carbides (WC / W2C) or
macrocrystalline carbides (WC). Spherical sintered carbides with high matrix hardnesses have recently been
introduced for extreme wear resistance and to develop low friction coefficients. The various grades of carbides are
shown in Fig. 14. These tubular constructed alloy wires are made up of a nickel sheath with a blend of tungsten
carbide particles in the core. Smaller diameters wires, 1.6mm to 2.0mm will have 40 - 45% tungsten carbide by
weight and the larger diameters wires 2.4 - 2.8mm typically have 55 -65%. These self-fluxing alloys produce
microstructures as shown in Fig. 15. Slurry jet erosion tests show a significant improvement in performance (see
Fig. 16) when compared to iron based chromium carbide deposits.
Critical Factors: Dissolution and Dilution
During welding, dissolution of tungsten carbides lowers wear resistance. The WC phase dissociates at 2800°C and
precipitates into softer secondary carbides. The W2C phase is detrimental because it decomposes first in the
molten pool, starting as low as 1300°C. Carbon has a greater affinity for elements such as iron and chromium
causing the tungsten carbide to decompose.
Dilution from the base metal affects overlay composition. Base metal and the hardfacing matrix tend to mix at the
weld interface, modifying the intended overlay chemistry. This is typically caused by higher amperage and lower
travel speed resulting in increased heat input and penetration, leading to the higher dilution rates.
The other critical factor is the matrix system in which high hardness is desirable for low stress abrasion and
sliding erosion resistance. However, high hardness is detrimental when coating toughness is needed to prevent
excessive cracking and resistance to impact and abrasion. Hard precipitates like borides and carbides reduce
toughness and induce micro cracking. This requires a balance between the alloying elements and the application
process to produce the desired deposit. The ideal Ni-Si-B matrix hardness is around 40-50 Rockwell C. The
higher the carbide content, the better is the abrasion resistance resulting in a reduction of the composite hardfacing
toughness. Another variable is the carbide size. The carbide grain size ideally should be smaller than the abrasive
particle to avoid preferential wear of the matrix.
Summary
In many of the hardfacing applications encountered today, the wear environment is complex involving both wear
and corrosion. Typically, alloys designed for enhanced wear resistance do not have good corrosion resistance and
vice versa. The cored wire approach enables the development of complex alloys that can be designed to combat
the synergic effects of these factors. With judicious control of the matrix and secondary alloy compositions, these
properties can be optimized. Nickel based tungsten carbide systems offer overlays that have wear properties to
combat extreme wear conditions such as those encountered in oil and gas drilling operations.
References
1. R. Menon, “New Development in Hardfacing Alloys”, Welding Journal, February 1996, pp. 43-49.
2. R. Menon, “Recent Advances in Cored Wires for Hardfacing”, Welding Journal, November 2002, pp.53-58
Fig.1 Roll Wear experienced in a continuous casting operation
Fig.2 Thermal Fatigue Testing Apparatus
a)
b)
c)
Fig. 3 Thermal Fatigue Test Samples, a) 420, b) 423, c) 423N
a)
b)
Fig. 4 Sensitized (500oC for 24 hours) samples of a) alloy 423N and b) Alloy 423 exposed to Lepera’s
etchant (1% Na2S 2O5 in water + 4% picric acid in ethanol in equal amounts)
Corrosion Weight Loss (g/cc)
ASTM G-48 Method A Corrosion Test Weight Loss-72 Hour
6% Ferric Chloride at room temperature
0.18
0.15
0.16
0.14
0.12
0.1
0.08
0.061
0.06
0.04
0.02
0
ThermaClad 423N
ThermaClad 423
Fig. 5 ASTM G48 tests comparing the pitting corrosion resistance of Alloys 423N and 423
G65 Mass Loss (gms)
2.5
2
1.5
G65-Surface
G65-at 75%Depth
1
0.5
0
5C25Cr
2C23Cr
5C25CrM
5C25CrCLX
6a)
0.12
Relative Wear Rate
0.1
0.08
0.06
0.04
0.02
0
5C-25Cr
6 b)
5C-25Cr-M
5C-25Cr-CLX
8
7
Volume Loss (cu.mm)
6
5
SJ90
4
SJ45
SJ20
3
2
1
0
6C-28Cr
5C-25Cr-M
5C-25Cr-CLX
6 c)
Fig. 6 Comparison of the wear performance of modified complex carbide deposit (5C-25Cr-CLX) with
other chromium carbide deposits, a) low stress abrasion, ASTM G65 (Procedure A) Tests, b) pin-on-disc
high stress abrasion tests and c) slurry-jet tests at impingement angles of 90, 45 and 20 degrees.
a)
c)
b)
d)
Figure 7. Microstructures of Iron-Based Chromium Carbide Deposits, a) 2C-23Cr, b) 5C-25Cr, c) 5C25Cr-M, d) 5C-25Cr-CLX
G48 Mass Loss (gms/sq cm)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2C-23Cr
5C-25Cr
5C-25Cr-M
5C-25CrCLX
Fig. 8 ASTM G48 Method A Pitting Corrosion Tests of Chromium Carbide deposits at a test temperature
of 22.50 C for 72 hours
5C-25Cr
Image
Location
C
Si
Cr
Mn
Fe
Zr
(6-1)
Carbide
2.32
0.19
53.47
0.71
43.11
0.20
(6-2)
Matrix
0.00
3.10
9.59
1.24
85.91
0.16
(6-3)
Combination
3.80
2.68
16.54
1.34
75.45
0.19
a) SEM Analysis 5C-25Cr Deposit
Image
Location
C
Si
Ti
Mn
Cr
Fe
Ni
Zr
Mo
(2-1)
Austenite
0.00
2.11
(2-2)
Matrix
0.27
2.06
18.34
75.67
3.50
0.14
0.25
1.39
32.71
60.49
2.30
0.05
0.74
b) SEM Analysis 2C-23Cr Deposit
Fig. 9 SEM/EDAX Analysis of a) 5C-25Cr and b) 2C-23Cr Deposits
2.5
G65 Wt Loss (gms)
2
1.5
G65-Surface
G65-at 75%Depth
1
0.5
0
2C-23Cr
5C-25Cr
StoodCor
136
a)
G48 Mass Loss (gms/sq cm)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2C-23Cr
5C-25Cr
StoodCor 136
b)
Fig. 10 Wear and corrosion properties of StoodCor 136 vs. Conventional Hardfacing Alloys a) ASTM G65
Abrasion Tests and b) ASTM G48 Pitting Corrosion Tests
3.5
3
Wt Loss (gms)
2.5
2
1.5
1
0.5
0
AISI 304
316TiC
AISI 410
420TiC
Fig. 11 ASTM G65 (Procedure A) low stress abrasion tests results comparing TiC modified hardfacing
deposits to base metals
0.600
Total E-C rate
Total corrosion rate
Erosion only
0.500
Synergy
E-C rate (mg/h.cm2)
0.400
0.300
0.200
0.100
0.000
AISI 316L SS
316TiC
420TiC
Fig. 12 Erosion-Corrosion tests comparing 316TiC and 420TiC hardfacing deposits to AISI 316L
a)
b)
Fig. 13 Microstructures of TiC containing stainless hardfacing deposits, a) 316TiC, b) 420TiC
Figure 2
Cast and Crushed
Tungsten Carbide
WC/W2C
Eutectoid product
2200-2600 HV
Most Affordable
Macrocrystalline
Tungsten Carbide
WC
Reaction product
1800-2200HV
Thermal Stability
Good Wetting
Spherical Fused
Tungsten Carbide
WC 1-X
Refined product
2700-3300-HV
Highest Hardness
Friction friendly
Fig. 14 Grades of carbides utilized with a Ni-Si-B matrix in cored wires
Fig. 15 Typical microstructure of cast tungsten carbide particles in a Ni-Si-B matrix.
Fig. 16 Slurry jet test results comparing nickel based tungsten carbide deposit Ni-WC to iron
based chromide carbide deposits