Tribological behaviour

1. PUNJAB ENGINEERING
COLLEGE
DEPARTMENT OF MATERIALS AND
METALLURGICAL ENGINEERING
LITERATURE REVIEW
TRIBOLOGICAL
BEHAVIOUR OF
NITRONIC - STEEL

2. Objective of Project
Part 01
Introduction
Part 02
Result & Discussion
Part 03
Conclusion
Part 04
References
Part 05
CONTENT

3. Objective of the Project
• To study cavitation erosion , solid particle erosion and sliding
erosion in hydro turbine blades and other products made of
nitronic-steel

4. The collision
between liquid
and solid
generate large
stresses which
can damage the
solid
Alloying with
nitrogen improves
the corrosion
resistance and
mechanical
properties of Steels
Nitrogen
alloyed
Steel
Cavitation
erosion
Solid
particle
erosion
Hydro
turbine
materials
The collision
between liquid and
solid generates
large stresses which
can damage the
solid and can cause
Cavitation erosion
Solid particle
erosion involves
impact of eroding
particles on a
surface
Underwater
components of
hydro turbines
operating in silt
Laden water suffer
from extensive
erosion
Key words

5. Introduction
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Erosion of Underwater parts is a serious problem in encountered
in hydropower plants.
Due to high amount of silt damage it becomes impossible to
run the unit for longer time thus increasing heavy loss of power
generation as well as replacement/repair cost.
Damage concerning water turbines is caused mainly by
Cavitation problem, silt erosion and material defect.
In presence of silt erosion, cavitation enhances the erosion
damages.

6. Introduction continued
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Erosion resistance increases with the increas ing hardness at
constant ductility.
When water bubbles collapse on a surface, the liquid adjacent
to the bubbles is at first accelerated and then suddenly
decelerated as it collides with the surface.
Transient pressures as high as 1.5 GPa are possible.
The solid particle erosion depends on the particle kinetic
energy, impact angle,
the particle shape, properties of target-material, and
envi ronmental conditions.

7. Introduction continued
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At lower impact angles the mode of erosion is cutting and
ploughing deformation.
During ploughing lip formation occurs due to deformation in the
localized regions near the surface of the target and lip is taken
out either by inertial-stress-induced tensile fracture or by
separation across adiabatic shear bands formed at the base of
the lip.
At high impact angles strain hardening and flaking/pitting of the
surface takes place.

9. Experimental
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The material used in this study is a nitrogen-alloyed
austenitic stainless steel. It was supplied by M/S Star Wire
(India) Ltd., Ballabhgarh, Haryana, India. The steel was
received in hot rolled condition. The chemical composi tion of
as-received (AR) steel is reported in Table 1.
The as-received steel was solutionized by heat treating at
1 000°C (HT-1) and 1 150°C (HT-2) for 3 hr followed by
water quenching. For comparison a 316L stainless steel
was also included in the present study.

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The steel samples
were mechanically polished using diamond suspension. A
surface roughness factor (Ra), 0.01 μm was obtained after
polishing. The polished surfaces were subsequently etched
using aquaregia (3HCl:1HNO3) for observation in a Leica
DMI5000M optical microscope. The grain size of the specimens
was measured using the line intercept method in the
optical microscope.

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Polished as well as etched samples were
subjected to microstructural characterization using a Zeiss
EVO18 scanning electron microscope (SEM) equipped with
energy dispersive analysis (EDS). X-ray diffraction studies
were carried out on polished samples in a Bruker AXS,
D8 Advance diffractometer using a CuKα radiation for
phase identification.

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Hardness measurements were carried
out on polished samples using a Vickers hardness testing
machine at a load 10 kg for 15 s. Tensile tests were carried
out as per ASTM E8M-09 ASTM22) standard at a strain
rate of 10 − 3 s − 1 using a computer controlled Hounsfield
H25K materials testing machine. Tensile toughness was
determined by calculating the area below the engineering
stress-strain curve between the YS and fracture stress.

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Tensile toughness measures the capacity of material to absorb
energy in the plastic range. Charpy impact tests were carried
out as per, ASTM E-23-0723) standard.
The cavitation erosion (CE) tests for the steel samples
were conducted using the vibratory test method, as per
ASTM standard G-32-10.24) A piezoelectric ultrasonic
transducer was used to produce oscillations at a frequency
of 20±0.5 kHz in distilled water.

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Water temperature was
maintained in the range of 25±2°C. The samples were
placed in distilled water next to the transducer. A distance
of 0.5 mm was maintained between the tip of the ultra sonic
probe (horn) and the specimen during the test. The
processor was stopped after every 3 h cycle. The specimen
was cleaned by acetone using ultrasonic cleaner and
subsequently weighed with an accuracy of 10 − 4 g.

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After weighing, surface roughness was measured by Mitutoyo
SJ-400
surface profilometer. The cavitation erosion test cycle was
repeated after every three hr up to 24 hr. Primary result
of CE test was cumulative weight loss which was further
converted into mean depth of erosion (MDE) using the following
formula:
MDE m ( ) µ
X W

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X A = 10 ∆
ρ
....................... (1)
Where, ∆W (mg) is the weight loss, ρ (g/cm3
) is the density of steel and A (cm2
) is the eroded area.
The solid particle erosion (SPE) was measured using an
air jet erosion tester, as per ASTM G76-10 standard.

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25) A
nozzle diameter of 3 mm was used. The test was performed
using alumina erodent powders having particle size range
between 53 and 75 μm. The parameters used in testing are
shown on Table 2. Polished samples were ultrasonically
cleaned in acetone, dried and weighed to an accuracy of
10 − 4 g using an electronic balance. Erosion test was
conducted for 3 min and samples were then weighed again to
determine the weight loss.

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The respective volume loss was
calculated for 3 min erosion testing. The ratio of this volume
loss to the weight of the eroding particles (i.e. testing
time × particle feed rate) causing the loss was then computed
as erosion rate in mm3
/g. Erosion testing was repeated
with each subsequent test of 3 min duration for nine cycles
when a steady erosion rate was obtained.

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The experiments
were conducted at 30°, 60° and 90° impingement angles at
ambient temperature for all the samples. The worn surface
morphology after cavitation as well as SPE tests was studied
using SEM.
3. Results and Discussion
X-ray diffraction results confirmed the steel to be austen itic.
Figures 1(a), 1(b) and 1(c) show the microstructures
of AR, HT-1 and HT-2heat treated steels, respectively.

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AR steel exhibits equiaxed grains. Some fine ( < 1 μm)
precipitates are observed along the grain boundaries. EDS

21. Results and Discussion
X-ray diffraction results confirmed the steel to be austenitic. Figures 1(a),
1(b) and 1(c) show the microstructures
of AR, HT-1 and HT-2heat treated steels, respectively.
AR steel exhibits equiaxed grains. Some fine ( < 1 μm)
precipitates are observed along the grain boundaries. EDSconfirmed these
precipitates to be carbides (Fig. 1(d)). These
are expected to be M7C3 and M23C6 types of carbides.26) The
volume fractions of these carbides are very small (~3% as
determined by Thermo-calc software) while no nitride pre cipitation is
expected. Annealing twins are observed in some
grains. The solutionising treatment also resulted in grain

23. coarsening.The resulting average grain size is 15 μm (fine
grain AR), 25 μm (medium grain HT-1) and 65 μm (coarse
grain HT-2), respectively
Table 3 shows the mechanical properties of AR, HT-1
and HT-2 steel. Figure 2 Engineering stress strain curves
of the same alloys. The heat treatment results in reduction
in hardness, yield strength (YS), ultimate tensile strength
(UTS) and strain hardening exponent. This is mainly due
to increase in the grain size. After the heat treatment, the
impact strength, tensile toughness and ductility show mar ginal
improvement. The mechanical properties of 316L

26. stainless steel were also determined and are listed in Table
3 for comparison.
The cumulative weight loss (CWL) due to cavitation
erosion is plotted as a function of time in Fig. 3(a), while
Fig. 3(b) shows mean depth of erosion (MDE) versus time
of erosion. Both CWL and MDE consistently increase with
time for the AR steel. The erosion rate increases gradually
up to 6 hr and then increases sharply for the HT-1 and HT-2
steel. The AR steel which had the finest grain size exhibited
the best erosion resistance. Thus the erosion rate seems to
increase with increase in grain size. Bregliozzi et al.21) have
reported that fine grains give rise to increase in the surface to

27. volume ratio of grain boundary, which provides a dominant
supporting action against cavitation. This was attributed to
a restriction in the movement of the dislocation at the grain
boundaries, which act as slip barriers. Coarse grains provide
less grain boundary area for impeding dislocation move ment. Thus
coarse grain size after the heat treatment may
cause an increase in the erosion rate. The surface roughness
Ra values after cavitation erosion are shown in Fig. 3(c). Ra
value of HT-1 and HT-2 steel increased sharply after 6 hr.
The erosion rate decreased after 15 hr (Fig. 3(b)). Ra value
of AR steel increased at a lower rate. The cavitation erosion
behaviour for the nitrogen steel is better than that observed
for the 316L stainless steel (Fig. 3)

29. Figures 4(a)–4(e) shows SEM images of eroded surfaces
after cavitation testing. Figure 4(a) shows that damage
mainly occurred at the austenite grain boundaries. The car bides are
located at the grain boundaries and the interface
regions between carbide and austenite serve as high stress
regions.27) These regions served as the preferential sites for
erosion. Preferential attack on the weakest phase of mate rial is a
characteristic of cavitation erosion.28,29) A few holes
are seen in the carbides region after erosion for 12 hr (Fig
4(b)). This is probably due to brittleness associated with the
carbides. Thus, the carbide-austenite interfaces are largely
attacked in comparison to austenite grains. After 24 hr of
cavitation erosion, wide and deep cavities are formed due to

31. removal of carbides and matrix (Figs. 4(c) and 4(f))
Figure 5 shows the morphology of coarse grain heat
treated steel after 12 hr of cavitation erosion. As can be
seen in Fig. 5(a) cavitation testing leads to the formation of
slip lines within the individual grains of the austenite that
end at the boundaries. The formation of waviness on the
surface corresponding to the presence of grain boundaries
and slip bands can be observed (Fig. 5(a)). At longer times
of exposure to cavitation, the number of slip lines increases.
The slip lines and grain boundaries act as preferential sites
of stress concentration. The continual impact of micro-jets
produces these defects and later, material removal starts
from these sites. Similar mechanism of material removal

33. has been reported for other austenitic steels.21) Figure 5(b)
shows material removal from annealing twins in the same
sample.
Figure 6 XRD trace of relative intensity vs 2θ showing the absence of
strain induced martensite in as received
nitrogen steel samples subjected to cavitation erosion and
solid particle erosion.
The XRD results show that strain induced martensite
does not form after cavitation erosion. This may be because
addition of nitrogen may prevent formation of strain induced
martensite.21) Similar results were obtained after particle
erosion. The role of strain induced martensite may be different in these

35. two modes of erosion. For cavitation erosion
the formation of strain induced martensite has been reported
to be detrimental.8) On the other hand it is reported that
strain induced martensite transformation may improve solid
particle erosion resistance by absorbing impact energy
Figures 7(a) and 7(b) show the cumulative weight loss
(CWL) curves in AR and heat treated steels at the impact
angles of 30° and 90°. At both the angles, CWL in heat
treated steels is almost same as that in the AR steel. Figure
7(c) gives the steady state of erosion rate at various angles
(30°, 60° and 90°). At all impact angles, erosion rate in
heat treated steel is slightly more than that of AR steel.

36. Particle erosion resistance of the nitrogen steel was found to
be better than that of 316L stainless steel (Fig. 7). Erosion
behaviour is affected by the impact angle with erosion rate
being higher at 90°. In general, the maximum erosion rate
appears at a low angle (30°) for ductile materials and at a
high angle (90°) for brittle materials.31) The higher erosion
rate observed at 30° in AR as well as heat treated steels
in the present study indicates ductile nature.32) Figure 7(d)
shows SEM micrograph of alumina particles used for solid
particle erosion.
Figure 7 Cumulative weight loss (CWL) at (a) 30°, (b)
90°, (c) steady state erosion rate of AR, HT-1, HT-2 and

38. 316L steels and (d) secondary electron SEM micrograph of
alumina particles.
The depth of the wear scars after erosion test at 30° and
90° is shown in Table 4. The depth of erosion is more for
samples tested at 90°. This is because the area of impact
reduces with increase in impact angle from 30° to 90°.
Thus variation in depth of wear is reverse of that for data
for material removal (Fig. 7). Weight loss during particle
erosion is a better measure of the erosion rate
Figures 8(a)–8(f) display the images of worn surfaces
obtained from AR and heat treated steels at 30° and 90°. The
erosion at 30° in AR and heat treated steels mainly occurred

39. due to ploughing. Shear process was observed by the presence of
ploughs with lip formation at sides. At impact angle
of 30°, the erosion loss occurred mainly due to shear cutting
of surface material. However, wider and deeper ploughs are
observed in heat treated steel (Figs. 8(c) and 8(e)) than that
in AR steel (Fig. 8(a)). The erosion occurred at 90° shows
rougher surface in comparison to 30°. The removal of the
material occurred by detachment. Some wear debris were
moved into the cavities produced by abrasive particles. The
same phenomenon was observed in the brittle fracture in
ceramics and other hard materials.33–35) The ploughing is
significantly less in the surfaces eroded at 90° in AR and
heat treated steels. Deeper cavities are observed in heat

41. treated steel (Figs. 8(d) and 8(f)) as compared to AR steel
(Fig. 8(b)). The erosion rate at any impact angle is slightly
higher for heat treated steels as compared to AR steel.
In solid particle erosion, the austenitic phase is considered to be a
beneficial constituent because of its plasticity
and toughness. The localized strength of austenitic phase
increases with work hardening of matrix resulting in improvement in the
erosion resistance.36) The target material deforms plastically due to the
repeated attack by hard
abrasive alumina particles. The tensile toughness also plays
important role in erosion resistance as the tough material
absorbs kinetic energy of impinging particles.37) The erosion
rate decreases with increasing toughness and ductility.

43. 31) In the present work there is no significant variation in ductility
and tensile toughness of the nitrogen steel with heat treat ment. During
erosion, the high strain hardening exponent
(n) of AR steel may lead to significant strain hardening and
higher erosion resistance in comparison to HT1, HT2 and
316L steel. The lower hardness, YS, UTS, tensile toughness
and strain hardening exponent of 316L results in a lower
resistance to erosion when compared with the nitrogen
steel samples. The grain size does not seem to significantly
affect the particle erosion behaviour in the nitrogen steel
tested. This is in agreement with the observation that grain
boundaries did not act as preferential sites for solid particle
erosion

44. Conclusions
The nitrogen alloyed austenitic stainless was subjected
to solutionizing heat treatment. This also resulted in an
increase in grain size from 15 to 65 μm. The influence of
grain size on cavitation erosion and particle erosion behav iour was
studied. The following are the major conclusions:
(1) The nitrogen alloyed austenitic stainless steel exhibits superior
resistance to cavitation erosion and particle erosion than 316L stainless
steel. This is related to the superior tensile toughness of the nitrogen steel
samples.
(2) The cavitation erosion damage started at grain boundaries. Because of
this the nitrogen alloyed austenitic

45. stainless steel with fine grain size exhibits superior resis tance to
cavitation erosion.
(3) No significant effect of grain size was observed on
particle erosion behaviour.
(4) No formation of strain induced martensite in the
nitrogen alloyed austenitic stainless steel during erosion
studies.

46. THANK YOU