FIELD OF INVENTION
The invention relates to a process for enhancing cavitation erosion resistance of
martensitic steel. More particularly, the invention relates to the modification of surface
microstructure by a combination of surface texturing and plasma ion nitrocarburising
process leading to increased surface hardness and excellent cavitation erosion
resistance in martensitic steels.
BACKGROUND OF THE INVENTION AND PRIOR ART
Plasma ion nitro-carburising is a process by which the surface hardness of steels can be
increased by diffusion of carbon and nitrogen atoms up to a case depth of 50-80 micron
under the surface. It is carried out in the ferritic region without any phase
transformation. The solubility of carbon and nitrogen in ferrite is small and most of the
carbon and nitrogen that enters the steel forms hard nitrides and
carbides. In typical gas nitro-carburising, the outer layer formed is called the "white
layer" which is hard and brittle. This layer has to be removed by grinding. However, in
plasma ion nitro-carburising this white layer can be avoided by a proper choice of
plasma parameters including temperature, and LPG, NH3 gases flow rates as well as the
applied and bias voltage. Plasma ion nitro-carburising of the steel samples was carried
out in a chamber under partial vacuum in which the white layer was completely

eliminated. These samples were evaluated for cavitation using the cavitation test
equipment as per ASTM G-32-03.
According to US Patent 5,030,064, in order to improve the resistance to erosion due to
cavitation, as well as erosion caused by mechanical action of the soil and sand particles,
a water contactable portion of the water turbine, in particular a portion of each moving
blade which tends to be eroded is formed of a stainless steel containing 0.07 to 0.2
wt% of C, not more than 2 wt% of Si, 7 to 15 wt% of Mn, 1 to 7% of Ni, 10 to 25% of
Cr, 0.1 to 3 wt% of W, and the balance substantially Fe and inevitably accompanying
impurities.
One method that refers to the absorption of nitrogen is reported in US Patent 4046601.
It is a method for internal strengthening of articles formed from deep drawing quality
low carbon steel strip in a nitriding solution at 570-580°C for a time sufficient to form
total nitrogen concentration adjacent the surfaces of the articles at least substantially
equal to the amount theoretically required to combine completely with the nitride-
forming elements in the steel.
Another method reported in US Patent 4,793,871 adopted to improving surface wear
qualities of metal components by gas nitriding or nitrocarburising includes the
preliminary step of heating the component to the nitriding temperature in an
atmosphere which is inert to the metal of the component.
One of the other methods to enhance the resistance to cavitation erosion of steel
surfaces is by laser hardening. The deficiency of the process for steel containing low

carbon content is its poor hardenability and hence has limited cavitation erosion
resistance.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process for enhancing cavitation
erosion resistance of martensitic steel by surface texturing and plasma ion
nitrocarburising which is capable of enhancing resistance to cavitation erosion of
martensitic steel due to lower cavitation induced volume loss.
Another object of the invention is to propose a process for enhancing cavitation erosion
resistance of martensitic steel by surface texturing and plasma ion nitrocarburising
which is capable of enhancing resistance to cavitation erosion of hydroturbine and
boiler feed pump components.
A further object of the invention is to propose a process for enhancing cavitation
erosion resistance of martensitic steel by surface texturing and plasma ion
nitrocarburising which is able to avoid the formation of hard and brittle outer layer
known as "white layer'.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - shows a comparative graph of cavitation erosion of various samples that
includes 'Plasma ion nitro-carburised' martensite steel sample for volume loss (mg)
against running hours.
Table-I - shows parameters used for plasma ion nitrocarburising.
Table-II- shows the hardness of Plasma ion nitro-carburising sample and other 13-4
steel samples.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The invention is based on the process of surface texturing coupled with the plasma ion
nitro-carburising of martensitic steel for enhancing its cavitation erosion resistance
manifold which is the novel concept. Surface texturing of the sample was carried out to
create roughness on the surfaces by using hard steel shots of mesh size 150. A
pressure of 3.50 kg/cm2 was maintained and the peening distance of the shots was also
controlled. A surface roughness of Ra 1.159 micron was achieved in textured as
compared to 0.509 micron on untextured sample. This texturing was carried out by
hard fine shots impinging on the martensitic steel substrate at sufficient pressure to
create controlled roughness close to the desired values. These surface textured samples
were than treated by plasma ion nitro-carburising. Plasma ion nitro-carburising is a

process by which the surface hardness of steels can be increased by diffusion of carbon
and nitrogen atoms up to a case depth of 50-80 micron under the surface. It is carried
out in the ferritic region without any phase transformation. The solubility of carbon and
nitrogen in ferrite is small and most of the carbon and nitrogen that enters the steel
forms hard nitrides and carbides. In typical gas nitro-carburising, the outer layer formed
is called the "white layer" which is hard and brittle. This layer has to be removed b/
grinding. However, in plasma ion nitro-carburising this white layer can be avoided by
selecting proper plasma parameters including temperature and gas flow rates as well as
the applied and bias voltage. Typical parameters used for plasma nitro-carburising are
given in the table below:

Testing:
Cavitation erosion resistance evaluation
The phenomena of cavitation generally occurs in hydro turbines, boiler feed pumps,
ship impellers, etc. and causes excessive damage to the materials in all these parts. In

the laboratory, cavitation erosion is evaluated as per ASTM G-32, according to which
the test equipment consists of a generator, generating high frequency-high voltage
signal which energizes the piezo-electric element in the converter via the transducer RF
cable to oscillate at 20 kHz with amplitude of +/- 50 micron. This mechanical vibration
is then transmitted to the horn made to Titanium alloy of matching impedance. The
sample to be tested is fitted onto the horn tip and securely tightened to ensure that
there is no loss of energy transfer from the horn tip to the samples. The sample also
oscillates at a frequency of 20 kHz thereby simulating the phenomena of cavitation on
the sample surface.
The mass loss is measured after carrying out cavitation tests on the sample at pre-
determined intervals. A precision balance was used for measuring the mass loss after
testing. The accuracy and repeatability of the test have been established on the
martensitic steel samples and the Nickel reference material. The extent of erosion
damage is measured in terms of volume loss and calculated from the cavitation mass
loss data and density of the material.
Cavitation erosion studies using volume loss was carried out on a number of samples
subjected to the surface treatments including surface textured and plasma ion nitro-
carburised (13-4 SPNC and X10 SPNC) shown as "P" in Fig.l, laser hardened (13-4 LH),
heat treated (13-4 HT), WC coated HVOF and untreated samples (13-4 AS) and (X10
AS). The cavitation erosion graph below shows that the "surface textured and plasma
ion nitro-carburised" steel samples (13-4 SPNC and X10 SPNC) are much superior in
performance than the rest.

Hardness
The hardness measured by Vicker's micro-macro hardness tester of the surface textured
and plasma ion-nitrocarburised samples are found to be much superior to that of
untreated as received 13-4 steel samples.

Advantages of the Process
1. With this process the surface textured and nitro-carburised martensitic steel
sample has a much higher hardness value (between 1100-1200 HV300) as
compared to the hardness achieved in untreated steel (~260 HV300) as well as in
laser hardened steel (~400 HV300).

2. The surface textured and nitro-carburised sample shows around eight fold
improvement in volume loss by cavitation indicating an eight fold increase in
cavitation erosion resistance as compared to the untreated martensitic steel.
3. The surface textured and nitro-carburised sample shows a more than six fold
increase in cavitation erosion resistance as compared to the laser treated
martensitic steel.
4. The surface textured and nitro-carburised sample shows a more than seven fold
increase in cavitation erosion resistance as compared to HVOF process coated
hard tungsten carbide coating.
5. The above process is applicable for the various components fabricated from
martensitic steel prone to cavitation erosion such as hydro turbine parts, boiler
fed pump impeller,and other related components and
6. The process can be adapted for other applications which have similarities to
cavitation erosion such as liquid jet impingement erosion.

WE CLAIM
1. A process for enhancing cavitation erosion resistance of martensitic steel by
surface texturing and plasma ion nitro-carburising comprising:
surface texturing the sample for creating controlled roughness on the surface;
treating the surface textured sample by plasma ion nitro-carburising for
increasing the surface hardness of steel;
characterized in that,
plasma ion nitro-carburising of the steel sample is carried out in a chamber
under partial vacuum by selecting proper plasma parameters including
temperature, gas flow rate and applied and bias voltage.
2. A process as claimed in claim 1, wherein the roughness on the surface is created
by hard steel shots of mesh size 150maintaining a pressure of 3.5 kg/cm2.
3. A process as claimed in claim 1 and 2, wherein the surface hardness of steels
can be increased by diffusion of carbon and nitrogen atoms up to 50-80 micron
depth under the surface.
4. A process as claimed in claim 1 to 3, wherein the surface textured roughness
achieved is Ra 1.159 micron and hardness value between 1100-1200 HV300-

A process for enhancing cavitation erosion resistance of martensitic steel by surface
texturing and plasma ion nitro-carburising consists of surface texturing the sample for
creating controlled roughness on the surface, treating the surface textured sample by
plasma ion nitro-carburising for increasing the surface hardness of steel. The plasma
ion nitro-carburising of the steel sample is carried out in a chamber under partial
vacuum by selecting proper plasma parameters including temperature, gas flow rate
and applied and bias voltage.