# Mechanisms of Elevated Temperature Galling in Hardfacings

Samuel R. Rogers<EMAIL_ADDRESS>David Stewart  Paul Taplin  David Dye
Imperial College, South Kensington, London SW7 2AZ, UK  Rolls-Royce plc,
Raynesway, Derby DE21 7WA, UK

###### Abstract

The galling mechanism of Tristelle 5183, an Fe-based hardfacing alloy, was
investigated at elevated temperature. The test was performed using a bespoke
galling rig. Adhesive transfer and galling were found to occur, as a result of
shear at the adhesion boundary and the activation of an internal shear plane
within one of the tribosurfaces. During deformation, carbides were observed to
have fractured, as a result of the shear train they were exposed to and their
lack of ductility. In the case of niobium carbides, their fracture resulted in
the formation of voids, which were found to coalesce and led to cracking and
adhesive transfer. A tribologically affected zone (TAZ) was found to form,
which contained nanocrystalline austenite, as a result of the shear exerted
within $30\text{\,}\mathrm{\SIUnitSymbolMicro m}$ of the adhesion boundaries.
The galling of Tristelle 5183 initiated from the formation of an adhesive
boundary, followed by sub-surface shear in only one tribosurface, Following
further sub-surface shear, an internal shear plane is activated. internal
shear and shear at the adhesion boundary continues until fracture occur,
resulting in adhesive transfer.

##  1 Introduction

Hardfacing alloys have been used for many years in components where surface
degradation is of concern, particularly when coupled with extreme
environments; be it high wear, high temperature, corrosive or erosive
environments, or a combination of these. Of particular interest is the use of
hardfacings in valve seatings within the pressurised water reactor (PWR)
environment. This is a corrosive environment at
$300\text{\,}\mathrm{\SIUnitSymbolCelsius}$, and in which wear and galling
(severe adhesive wear) are of critical concern. The galling of components may
result in gross surface degradation and component seizure, and is
characterised by the plastic deformation and adhesion of contacting asperities
[1]. The mobilisation of Co debris from e.g. Stellite hardfacings is of
concern due to the $\gamma$-radiation fields that are subsequently produced.
As such, the reduction or replacement of cobalt alloys is necessary [2]. Fe-
based hardfacings are viewed as a potential replacement system, resulting in
increased interest in the galling mechanisms, and with a number of alloys
being designed and investigated [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15].
Galling has been found to occur most readily in systems which have limited
movement perpendicular to the sliding direction, such as in valves [16, 17].

Although work in the literature has investigated some of the mechanisms of
galling, these have been focused upon single phase materials [18, 19, 20, 21,
22, 23, 24], with little work investigating more complex microstructures, such
as duplex microstructures or microstructures that include carbides or other
hard particles [25, 26, 27]. As such, Tristelle was chosen to be investigated.
Tristelle 5183 is an Fe-hardfacing alloy developed from 316 stainless steel.
Tristelle 5183 has a similar matrix composition to 316 stainless steel, albeit
with the addition of silicon. This means that this investigation can be
focused upon the effect of carbides on galling, which, as with the Stellite
alloys, appears to result in the improved galling resistance seen for
Tristelle 5183 at room temperature when compared with 300-series stainless
steels [28, 29]. It has been reported that at elevated temperature (e.g.
$300\text{\,}\mathrm{\SIUnitSymbolCelsius}$) Tristelle 5183 shows less
prominent galling resistance, which has been suggested to occur as a result of
a decreased ability to form deformation induced martensite, similar to other
Fe-hardfacing alloys [28, 30, 29, 8].

Figure 1: Bespoke galling rig manufactured by Rolls-Royce plc. for testing
large disk and seat samples. (a) Diagram showing key rig components, (b) CAD
drawing showing full rig, including actuators (in yellow). From [31]. Figure
2: Phase map of a Tristelle 5183 weld deposit, created using an EBSD phase map
and Nb EDX map.

##  2 Experimental Methods

A bespoke rig was manufactured by Rolls-Royce, Figure 1. The disk and seat are
made of a 316 stainless steel substrate with a layer of Tristelle 5183 plasma
transferred arc (PTA) welded onto its surface and ground down to give an
appropriate tribosurface. An actuator was used to apply a normal force to the
disc, compressing it against the seat whilst a second actuator was used to
slide the seat in a linear manner. The test was stopped early due to an
acoustic indication of galling.

Since tests using this rig were conducted at elevated temperatures and
oxidation was to be avoided, an inert environment was required. This was
created by surrounding the disc with a skirt, and pumping nitrogen into the
rig. By applying a positive pressure to the rig, nitrogen can escape the
chamber, without air ingress. The test temperature for the test of Tristelle
5183 was approximately $300\text{\,}\mathrm{\SIUnitSymbolCelsius}$.

Scanning electron microscopy (SEM) was performed using Zeiss Sigma 300 FEG-
SEM, Zeiss Auriga Crossbeam in both secondary electron and backscattered
modes, and using a working distance of $7\text{\,}$–$10\text{\,}\mathrm{mm}$
and an accelerating voltage of $20\text{\,}\mathrm{kV}$. An FEI Quanta SEM was
used for electron backscattered diffraction (EBSD) and phase maps, using a
working distance of $15\text{\,}\mathrm{mm}$ and an accelerating voltage of
$20\text{\,}\mathrm{kV}$. An in-situ lift-out was produced using a Helios
FIB/SEM to create a transmission electron microscopy (TEM) foil. TEM imaging
and diffraction was produced using a JEOL 2100Plus microscope using an
accelerating voltage of $200\text{\,}\mathrm{kV}$.

X-ray diffraction was performed using a Bruker D2 Phaser machine, which uses a
Cu K-$\alpha$ source with a wavelength of
$1.5406\text{\,}\mathrm{\SIUnitSymbolAngstrom}$. An angular resolution of
$0.034\text{\,}\mathrm{\SIUnitSymbolDegree}$ and time per step of
$3680\text{\,}\mathrm{s}$ were used in this work111The high time per step is
on account of the detector within the Bruker D2 Phaser being able to detect
multiple angles at once, in this case 160, meaning that whilst the detector
was only stationary in each position for $23\text{\,}\mathrm{s}$, the total
time a detector was detecting a signal at any given angle was
$3680\text{\,}\mathrm{s}$.. Analysis was performed using HighScore Plus
software, alongside the ICDD’s PDF-2 database.

Table 1: Tristelle 5183 phase composition in wt.% and at.%, as measured using SEM-EDX. C and N are not reliably detectable in EDX and so are not included within the analysis. Phase | Composition / wt.%
---|---
Fe | Co | Ni | Si | Cr | Nb
Austenite | 61.9 | 2.0 | 12.3 | 3.5 | 20.1 | 0.17
Ferrite | 60.6 | 1.7 | 13.0 | 5.1 | 19.2 | 0.40
M7C3 | 33.5 | 2.8 | 7.5 | 1.6 | 53.8 | 0.83
MC | 4.2 | 0.2 | 1.1 | 1.0 | 4.5 | 89.0
Phase | Composition / at.%
Fe | Co | Ni | Si | Cr | Nb
Austenite | 59.5 | 1.8 | 11.2 | 6.7 | 20.7 | 0.10
Ferrite | 57.4 | 1.5 | 11.8 | 9.6 | 19.5 | 0.2
M7C3 | 32.0 | 2.6 | 6.7 | 3.0 | 55.3 | 0.48
MC | 6.4 | 0.2 | 1.6 | 2.9 | 7.4 | 81.5

##  3 Results

Prior to investigation of the gall scar, the bulk Tristelle 5183 weld deposit
was investigated using EBSD and SEM-EDX, Figure 2222Due to austenite and NbC
having the same space group, portions of the austenite were often misindexed
as NbC. As such, a composite image was created using the EBSD phase map which
used austenite, ferrite and Cr7C3, without NbC, and then overlayed with the Nb
EDX map, since Nb was only observed to be present in NbC precipitates.. As
expected, the primary matrix phase is austenite, having formed as dendrites
upon cooling. Interdendritic regions contain a eutectic of chromium carbide
and ferrite, whilst niobium carbide precipitates are seen to dispersed
throughout the microstructure. In the literature, Tristelle 5183 was observed
to contain both Cr7C3 and Cr23C6, however Cr23C6 was not observed in this
instance. Phase chemistries can be seen in Table 1. The austenite and ferrite
chemistries are observed to be very similar, likely as a result of the post-
weld solidification and cooling giving insufficient time for diffusion of
substitutional solutes to occur, in particular, the Ni and Cr. The niobium
carbide precipitates were approximately <$0.5\text{\,}$ –
$10\text{\,}\mathrm{\SIUnitSymbolMicro m}$ in size, and will have formed first
from the liquid, and are thus equiaxed. Chromium carbide precipitates formed
in the inter-dendritic regions during final solidification. This results in a
variety of sizes and morphologies of chromium carbides being observed.

Figure 3: X-ray diffraction pattern from a galled Tristelle 5183 surface which
was galled at approximately $300\text{\,}\mathrm{\SIUnitSymbolCelsius}$. Peaks
have been labelled, with the exception of minor Cr7C3 peaks and potential
$\alpha$’ contributions.

X-ray diffraction (XRD) of the galled Tristelle 5183 surface corroborates the
phase results found using EBSD, Figure 3. It is known that austenitic
stainless steels may form strain-induced martensite upon deformation[19], and
since some laths were observed within austenite grains approximately
$90\text{\,}\mathrm{\SIUnitSymbolMicro m}$ below the sample surface, it is
reasonable to suggest that martensite has formed. Thus the martensite
contributes to the ferrite peaks within the XRD pattern, on account of the
martensite having a low tetragonality. In addition, Cr23C6 is not observed in
the XRD pattern, which is in agreement with the EBSD phase data, Figure 2.

Figure 4: Tristelle 5183 (a) disk and (b) seat samples after testing at a
temperature of approximately $300\text{\,}\mathrm{\SIUnitSymbolCelsius}$,
however, the normal stress is unknown. Two regions of galling on the disk are
highlighted by red boxes. From [31].

Photographs of the disk and seat material can be seen in Figure 4, with two
primary areas of galling being visible on the disk, marked with red boxes.
These regions were sectioned and subsequently investigated.

Figure 5: White light interferometry images showing small sections of the
galled regions highlighted in Figure 4. (a) and (b) show the leading edge of
the galled surfaces, whilst (c) shows the full breadth of the galled surface.
A common colour scale is used for all images.

Portions of the galling scars shown in Figure 4 were investigated using white
light interferometry (WLI), Figure 5. Galling peaks are primarily seen to be
present from the leading and trailing edges of the samples, with little
galling seen in the middle of the tribosurfaces, Figure 5(c). In addition, the
galling observed on the trailing edge is considerably reduced in height
compared with that seen on the leading edge (maximum heights of
$60\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and
$255\text{\,}\mathrm{\SIUnitSymbolMicro m}$ respectively). Despite showing
clear galling peaks, galling troughs were not readily seen in these samples,
suggestive of adhesive transfer during galling.

Figure 6: A cross-section of a Tristelle 5183 galling peak, parallel to the
galling direction. (a) Low magnification, showing the stainless steel
substrate under the Tristelle 5183 weld deposit. (b) A higher magnification
image, showing a portion of the cast microstructure which has remained
undisturbed by the galling process. (c) A higher magnification of the
dendrites seen within the galling peak. Due to roughing of the sample after
testing, the galling peak has been partially removed from the disk surface.

A number of cross-sections parallel to the galling direction were
investigated, e.g. Figure 6(a). Due to post-test roughing, the galling peak in
Figure 6 has been partially separated from the bulk Tristelle 5183. The
stainless steel substrate can also be seen in Figure 6. Scratches appear
across the softer stainless steel substrate as a result of carbide pull-out
from Tristelle 5183 during the polishing process, Figure 6.

Whilst most of the galling peak shows significant material shearing and the
presence of a tribologically affected zone (TAZ), Figure 6(b), a portion of
the cast microstructure has remained undisturbed throughout the galling
process, observable by the dendrites and unbroken carbides, Figure 6(c). This
is of significance since this indicates that in some cases, material
immediately adjacent to the adhered region may not undergo shear. Since the
original surface is still visible it can be seen that the galling peak has
formed from material which has adhered from the opposing tribosurface (in this
case the seat). This is in agreement with the observations made using WLI,
Figure 5.

Figure 7: A cross-section of the galling peak of Tristelle 5183 shown in
Figure 6, viewing the TAZ in successively higher magnification. Red boxes
indicate the region imaged in the following sub-figure. Figure 8: A cross-
section of a galling peak of Tristelle 5183, viewing the TAZ and crack tip
between the adhered galling peak and original surface material. Niobium
carbides can be seen to have fractured and when sheared further have the
appearance of a stringer. Red box indicates the region imaged in the following
sub-figure. Figure 9: A cross-section of a small galling peak of Tristelle
5183 made entirely of the TAZ. Figure 10: Two cross-sections of Tristelle 5183
severed galling peaks: (a) & (d) viewed in BSE imaging mode and; (b) & (c)
with flow lines shown, and the highly sheared region shown in red. Flow lines
were traced from their respective BSE images images.

By viewing the crack tip separating the adhered peak seat material from the
disk material, it can be seen that the crack has grown at the boundary of two
regions within the TAZ with differing amounts of shear, Figure 7. The more
heavily sheared region is approximately $10\text{\,}\mathrm{\SIUnitSymbolMicro
m}$ deep, whilst the less heavily sheared region takes up the remaining
material of the galling peak. Although no discernible grain structure is
visible within either region, carbide fragments and precipitates show the
extent & direction of shear which are experienced within these regions. This
is similar to the way carbide stringers showed the shear direction in 316L
stainless steel [19].

Another galling peak examined was seen to contain a growing crack, Figure 8. A
number of carbides were observed to have fragmented and been drawn out by
shear, resulting in stringer-like formations. Whilst these formations are seen
for both chromium- and niobium carbides, voids were only observed to form in
the case of fractured niobium carbides. If sufficiently sheared, these voids
can coalesce to form a crack, as seen in Figure 8 with carbide fragments on
each side of the growing crack.

Across the galled regions, a number of smaller peaks can be seen, which are
made up of only the TAZ, Figure 9. In this case, there are a significant
number of voids which have been formed either from carbide pluck-out close to
the TAZ’s boundary with the underlying material, or within the TAZ, where
carbides were fractured.

The flow of material within a severed peak and at its adhesion boundary can be
seen in Figure 10. Using focussed ion beam milling, a TEM foil of the TAZ was
created and analysed, Figure 11. It can be seen that the TAZ contains chromium
and niobium carbides which have been significantly reduced in size during
galling and its accompanied deformation. In addition, a small isolated SiC
particle was observed. The matrix is shown to be nanocrystalline, as evidenced
by the almost complete diffraction rings with an aperture of
$200\text{\,}\mathrm{nm}$ and contains only austenite, Figure 11.

Figure 11: (a) - (e) STEM-EDX elemental maps from a TEM foil of Tristelle
5183, taken from a tribologically affected zone (TAZ). (f) An electron image
showing the area used to obtain a (g) selected area diffraction pattern,
indicated by a dashed red circle.

##  4 Discussion

Adhesion and galling is observed to occur between self-mated surfaces of
Tristelle 5183 at approximately $300\text{\,}\mathrm{\SIUnitSymbolCelsius}$.
As mentioned previously, adhesive transfer resulted in the formation of
galling peaks. Unlike in self-mated tests of 316L stainless steel and non-self
mated tests of 316L vs 304L stainless steels, when adhesion occurs, the
adhesion junction does not result in trough formation in each tribosurface,
Figures 6, 7 & 9. In order for galling to occur, one surface must therefore
preferentially deform.

Figure 12: The galling mechanism experienced by self-mated Tristelle 5183 at
approximately $300\text{\,}\mathrm{\SIUnitSymbolCelsius}$. (a) An adhesion
junction is formed between the two tribosurfaces. (b) Sub-surface shear occurs
in only a single tribosurface, and intermittently, as a result of low shear
strength regions along the adhesion junction. (c) Shearing continues,
resulting in a very heavily sheared region immediately below the adhesion
boundary. (d) An internal shear plane is activated. (e) Shear at the adhesion
boundary and internal shear boundary continue until (f) fracture occurs,
resulting in adhesive transfer.

The galling mechanism experienced by self-mated Tristelle 5183 is shown in
Figure 12. Initially, an adhesion junction is formed Figure 12(a), before
continued sample movement causes shearing within one of the tribosurface,
Figure 12(b). As was noted earlier, shear ‘shadows’ can occur. Shear ‘shadows’
may form due to a lack of adhesion in these locations, whilst adhesion and
shear occurs in adjacent regions. As shear continues, a heavily sheared region
may form immediately beneath the adhesion boundary, Figure 12(c). If the bulk
material is under sufficient shear, an internal shear boundary may form at an
angle to the macroscopic direction of movement, Figure 12(d). Eventually, as
tribosurface movement continues, Figure 12(e), the shear within the material
results in a crack forming and propagating and thus adhesive transfer occurs,
Figure 12(f).

This mechanism and the results seen for galled Tristelle 5183 are similar to
those seen by Dimaki et al. who modelled asperities with varying shear
strengths, attractive stresses and ductilities, [32]. Tristelle 5183 behaved
similarly to a material with a shear stress greater than its attractive stress
(whilst the attractive stress was not equal to zero), and a limited ductility.
Since the primarily austenitic matrix of Tristelle 5183 is based on 316
stainless steel, it is expected that the matrix has a propensity for adhesion
or attraction [19]. Similarly, due to the incorporation of carbides into its
matrix, Tristelle is known to have limited ductility. This was seen in Figure
8, where carbides were observed to have fractured and formed voids, resulting
in the formation and propagation of a crack through void coalescence.

In some cases, as in Figure 9, failure occurs at the boundary of the heavily
sheared region with the underlying sheared material. This could also have been
as a result of void coalescence, or may have occurred due to the change in
shear gradients at this point.

On this view of galling, the function of the ceramic phases in galling is
primarily to reduce the continuity and extent of the initial adhesion
junction, and the junction strength, as ceramics adhere less easily than
metals. The fragmentation of the carbides during the multi-length-scale shear
process occurring in the triboilogically affected zone (TAZ) and then during
the formation of the galling peaks and troughs presumably limits the shear
ductility of the material and promotes earlier, and easier, void formation and
hence, cracking and adhesive material transfer. However, even in a nominally
single phase material like 316 stainless steel that lacks the ceramic phases
in Tristelle 5183 or other hardfacings such as the Stellites [5], very similar
overall galling behaviour is observed.

##  5 Conclusion

Tristelle 5183 is an Fe-based hardfacing alloy containing niobium and chromium
carbides within a primarily austenitic matrix (although some ferrite is also
present).

Galling and adhesive transfer was found to occur in Tristelle 5183 that was
tested at approximately $300\text{\,}\mathrm{\SIUnitSymbolCelsius}$ on a
bespoke rig created to closely resemble an in-service valve. Shear at the
adhesion boundary on one tribosurface and the subsequent activation of an
internal shear plane within this tribosurface resulted in adhesive transfer
and galling. The ductility of Tristelle is limited by the incorporation of
carbides into the microstructure, which fracture upon sufficient shear,
resulting in stringer-like formations, and in the case of niobium carbides,
resulted in void formation, coalescence, internal fracture, and subsequent
adhesive transfer and galling occurring.

A tribologically affected zone (TAZ) is found to form during galling, which
results in the breakup of prior austenite grains into nancrystalline-sized
grains in addition to containing fragmented NbC and Cr7C3.

## Acknowledgements

We gratefully acknowledge support from Rolls-Royce plc, from EPSRC
(EP/N509486/1) and from the Royal Society (D Dye Industry Fellowship).

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