FIELD OF THE INVENTION
This invention relates to a method of developing erosion resistant hard layer on
13Cr-4Ni Stainless Steel components.
The invention further relates to the field of developing erosion resistant hard
layer by Plasma Nitro carburizing process for hydro turbine components. More
particularly, the invention relates to the case hardening of 13Cr-4Ni steels used
in hydro turbine components by plasma nitro carburising process leading to
increased surface hardness, toughness and excellent erosion resistance as well
as establishing the process on Pelton turbine needle.
BACKGROUND OF THE INVENTION
The underwater components of hydro turbines mostly made of 13Cr-4Ni steel
such as needle, seat ring, buckets of Pelton turbine and labyrinths, guide vanes,
top cover/bottom ring, runner of Francis turbine are severely affected due to silt
erosion, particularly in the hydro power plants that are situated in Indian
Himalayan region because of excessive silt present in the river during monsoon
period. Silt erosion, when combined with cavitation erosion phenomenon which is
also present in hydro turbine application, causes severe erosion and sometimes
results in catastrophic failures. Earlier research in this field has given various
solution to this problem out of which HVOF is the most effective one till today.
However, HVOF being the line of sight process has limitations of accessibility apart
from having poor cavitation erosion resistance. These problems can be eliminated
by adopting plasma nitro- carburizing as the process is not line of sight process and
complicated geometries can be easily hardened in one go. Moreover combined
presence of carbon and nitrogen on the surface enhances hardness without
compromising on the ductility.
Plasma ion nitro-carburising is a process by which the surface hardness of steels
can be increased by diffusion of carbon and nitrogen atoms at low pressures in
plasma atmosphere using pulsed power generators. The electrical field is
established in such a way that the workload is at the negative potential
(cathode) and the furnace wall is at ground potential (anode). The nitrogen
transfer is caused by the attraction of the positively charged nitrogen ions to the
cathode (work pieces) with the ionization and excitation processes taking place
in the glow discharge near the cathode's surface. The rate of nitrogen transfer
can be adjusted by diluting the nitrogen gas with hydrogen (above 75%). Plasma
nitro-carburizing is achieved by adding small amounts of acetylene, methane or
carbon dioxide gas to the nitrogen-hydrogen gas mixture to produce the
nitrogen-poor gamma prime (Y') phase (Fe4N), the nitrogen-rich epsilon (e)
phase (Fe2-3N) and carbon-containing epsilon (e) compound layer (Fe2-3CxNY).
The major advantages of Plasma Nitro-carburizing are:
• High hardness of the treated surface
• Improved control on case depth
• Ability to select the compound layer type to suit the required usage
• Good friction, wear, and fatigue properties
• Flexibility to nitride stainless steels, titanium alloys
• Possibility to lower nitriding temperature and to limit distortion
• Ability to automate the system which gives good reproducibility of results
• Shorter cycle time
• No environmental hazard
According to United States Patent Application 20050112399, materials and
processes are described that are especially suited for providing erosion resistance
to hydroelectric turbine components. These Erosion resistant coating
compositions include hard particles in a metal matrix such as nickel-based,
cobalt-based and iron-based matrices applied by a plating process for complex
geometry or hard to access component surfaces or by thermal spray processes
for line of sight applications.
Another method reported in United States Patent Application 20050112411, for
Erosion resistant coating processes and material improvements for line-of-sight
applications. The erosion resistant coating composition includes nanostructured
grains of tungsten carbide (WC) and/or submicron sized grains of WC embedded
into a cobalt chromium (CoCr) binder matrix. A high velocity air fuel thermal
spray process (HVAF) is used to create thick coatings in excess of about 500
microns with high percentages of primary carbide for longer life better erosion
resistant coatings. These materials and processes are especially suited for
hydroelectric turbine components.
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.
Another method reported in US Patent 4,793,871 adapted 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.
According to US Patent US7827902, A piston made of a nitride-forming base
alloy for a reciprocating internal combustion engine is disclosed. The nitration
layer is generated by converting the nitride-forming base alloy by one of plasma
nitration and plasma nitro-carburization in at least one of a nitrogen atmosphere
and a nitrogen-carbon atmosphere. Another method reported in the patent DE
10310213 for improving the surface quality of structural components by plasma
nitriding and/or nitrocarburizing with subsequent oxidation, whereby the particles
are given at least partially an abrasive surface post-treatment, where on the
structural components, the stability of which is undesirably decreased by this
treatment, an improvement layer is released, so that the original material
properties of the component is restored at the release sites.
However, no background art has been found for development of Plasma nitro
carburizing process for erosion resistant hard layer on hydro turbine
components.
OBJECTS OF THE INVENTION
An object of this invention to propose a process for enhancing erosion resistance
of 13Cr-4Ni steel used in hydro turbine components by plasma nitrocarburising
method.
Another object of this invention is to propose a method of applying plasma nitro
carburized hard layer upto a case depth of 150 microns on a hydro turbine
component in particular Pelton needle by maintaining uniform process
temperature during plasma nitro carburizing process.
A further object of the invention is to propose an improvement in surface
properties of hydro turbine component such as hardness, silt erosion resistance,
cavitation erosion resistance and abrasion resistance.
BRIEF DESCRIPTION OF THE INVENTION
This invention relates to a method of developing erosion resistant hard layer on
13 Cr-4Ni Stainless Steel components comprising:
Placing the component in plasma ion nitriding equipment for carrying out
plasma, nitro carburizing process in the presence of H2, N2 and C2H2 gases at a
temperature range of 575-585°C and pressure in the range of 1.5-2.5 mbar for
20-24 hrs to achieve hardness of about 1100-1400 HV upto a case depth of
about 150 microns.
Thus a plasma nitro carburizing process, as mentioned above is provided on a
Pelton needle for erosion resistant application for hydro turbine components. A
special fixture has been designed for measurement of the process temperature
on the component directly and to maintain it uniformly during the entire process
cycle. The complete plasma nitro carburizing process cycle consists of following
steps:
• Pre-condition: to check base vacuum (0.04mbar) and current leakage
• Pre-heating: upto 80°C temp for degassing.
• Pre-cleaning first cycle: sputtering with hydrogen at temp 350°C-400°C
• Pre-cleaning second cycle: Sputtering with hydrogen and argon at temp
450°C-500°C
• Nitriding: at above mentioned parameters using N2, H2 and C2H2 gases.
• Cooling in Nitrogen environment till room temperature
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Plasma Ion Nitriding system
Figure 2: Plasma Nirto carburizing on test sample in progress
Figure 3: A typical Plasma Nitro carburizing Cycle
Figure 4: Effect of process temperature on Hardness
Figure 5: Optical micrographs showing (a) Various zones of nitro carburized
layer,
(b) Case depth measurement (c) Indent at 1kg load showing no
cracks
Figure 6: Cavitation Test Results
Figure 7: Silt Erosion Test Results
Figure 8: Abrasion Test Results
Figure 9: Fixture for Thermocouple attachment on the needle
Figure 10: Thermocouple arrangement during loading of the needle
Figure 11: Plasma Nitro-carburizing process being carried out on Pelton
turbine needle
Figure 12: Pelton turbine needle before and after Plasma nitro-carburizing
DETAILED DESCRIPTION OF THE INVENTION
There is provided a method for producing erosion resistant hard layer on 13Cr-
4Ni stainless steel substrate used for hydro turbine component by plasma nitro
carburizing process. In the process of Plasma Nitro carburizing, experiments have
been carried out using plasma Ion-Nitriding system (Figure 1). The system has
been designed for carrying out pulsed plasma ion nitriding, nitro carburizing and
oxy-nitriding in vacuum chamber having hot wall design for jobs of size 0 800
mm x 1000 mm with load capacity upto 1500kg. The chamber has three
separate zone mounting stations with temperature control upto 900°C. Loading
of the jobs has to be done in such a way that surface area to mass ratio in the
chamber is uniform for establishing temperature uniformity during the process.
In the present invention, the plasma nitro carburizing is carried out using
optimized process parameters such as process temperature in the range of 575-
585°C, gas ratio N2:H2::1:3, acetylene (C2H2) ratio in the range of 1-3%,
process pressure at 2.5mbar and process time in the range of 20-24 hrs.r Before
loading into the chamber the samples have been ultrasonically cleaned using
acetone to get rid of oil, dirt, grease etc. The samples have been loaded in the
chamber in zone 1 and zone 2 keeping dummy samples having similar geometry
for thermocouple attachment in each zone. The plasma nitro carburizing has been
carried out in following steps:
• Pre-condition: to check base vacuum (0.04mbar) and current leakage
• Pre-heating: upto 80°C temp for degassing.
• Pre-cleaning first cycle: sputtering with hydrogen at temp 350°C-400°C
• Pre-cleaning second cycle: Sputtering with hydrogen and argon at temp
450°C-500°C
• Nitriding: at above mentioned parameters using N2, H2 and C2H2 gases.
• Cooling in Nitrogen environment till room temperature
The plasma is being monitored during the process and any disturbance in plasma is
controlled during the whole process especially during the nitriding cycle. The total
operation of plasma nitro carburizing lasts for 36 hours. Figure 2 shows the plasma
nitro carburizing process on a test sample and Figure 3 shows a typical plasma
nitro carburizing process cycle. After the Nitro carburizing cycle, samples are
unloaded and preserved separately for testing and characterization.
Micro hardness measurement across the nitro-carburized hard layer is carried out
using Vickers hardness tester at a load of 300grams. Figure 4 shows the effect of
process temperature on hardness. A very high hardness of around 1350 HV is
achieved against the base metal hardness of around 280 HV. A substantially
higher depth of diffusion zone of about 150 micorn is achieved. Figure 5 shows
the microstructure, the indentation measuring case depth and indent at 1kg load
for measuring toughness properties in the sample. The micrograph clearly shows
the three zone of nitro carburized layer: A very thin ~ 1 micron white layer
followed by ~50 micron compound layer and ~100 micron nitrogen rich diffused
layer. No cracks were found on the indent at 1kg load showing better toughness
property. The X-ray diffraction of plasma nitro carburized steel samples was
carried out by using Philips X-pert system (Philips, Netherlands). The X-ray
diffraction (XRD) patterns confirms the presence of both Fe3C, Fe3N (e-
compound layer) and CrN phases.
For the cavitation testing, Plasma Nitro carburizing has been carried out on
20x20x60mm 13-4 steel block and then button samples were prepared as per the
standard ASTM G-32 by wire cutting from these plasma nitro-carburized blocks.
These samples were tested for cavitation erosion for 9 hrs. Weight loss has been
measured after each 1 hr and results has been plotted as cumulative weight loss vs
time as shown in figure 6. Figure shows that plasma nitro carburized layer has
shown excellent improvement in cavitation erosion as compared to base material
(around 22 times). This is because of improved toughness of plasma nitro
carburized layer enable it to withstand against erosion due to micro jets formed
in cavitation phenomenon.
Silt erosion tests were carried out on plasma nitro carburized samples using a
water jet pressure impingement erosion setup. It consists of convergent nozzle
through which mineral sand of 180-250um size and hardness up to 1100 HV is
added. The slurry passes through a 4 mm dia. x 40 mm long tungsten carbide
throat, providing a jet velocity of 29.0 m/sec. Rectangular samples of size 60 x
50 x 6 mm are kept in front of a sand laden water jet at a fixed angle of 30° to
the impinging jet. The silt erosion resistance of all the samples was determined
by mass loss measurement and results have been compared in figure 7. Silt
erosion resistance was found to be around 2 times better than the base material.
Dry abrasion test has been carried out on plasma nitro carburized samples of size
75x25x6mm as per ASTM G-65 using rubber wheel test set up at a load of 37.5N.
250g of erodent (sand of size 180-250um) was used to get the abrasion pattern on
the samples. Samples were weighed to know the erosion loss and results have
been compared in figure 8. Abrasion resistance was found to be around 8 times
better than the base material.
Another important step of the invention is to establish the method for providing
plasma nitro carburizing hard layer on a Pelton needle for erosion resistant
application for hydro turbine components. A special fixture has been designed to
attach the thermocouple on the actual needle for precise temperature
measurement and control during the process. Figure 9 shows the design of the
fixture and the complete arrangement is shown in Figure 10. The hydro turbine
needle is plasma nitro carburized using following optimized parameters. Figure 11
shows the plasma nitro carburizing process being carried out and Figure 12 shows
the needle before and after the process. The optimized parameters are given as
below:
Process Parameters
Process Temperature; 580°C
Gas Ratio: 1:3(N2:H2)
Acetelyne %: 2% of total gas
Process Pressure: 2.5 mbar
Process duration: 20 hrs
Total duration: 36hrs (including preheating, pre-cleaning and cooling
cycle)
EXAMPLE
The plasma nitro carburizing is carried out using optimized process parameters
on Pelton turbine needle. A very high hardness of around 1350 HV upto a depth
of about 150 microns has been achieved in the plasma nitro carburized hard
layer as against the base metal hardness of around 280 HV. No cracks were
found on the indent at 1kg load showing better toughness property. The increase
in the hardness is due to the presence of Nitrides (Fe2-3N & CrN) and carbides
(Fe3C) phases. Abrasion resistance has increased around 8 times better than
base, silt erosion resistance increased around 2 times better than base and
cavitation erosion resistance increased around 22 times better than base
material.
Dry abrasion test has been carried out on plasma nitro carburized samples of size
75x25x6mm as per ASTM G-65 using rubber wheel test set up at a load of 37.5N.
250g of erodent (sand of size 180-250|Jm) was used to get the abrasion pattern on
the samples. Samples were weighed to know the erosion loss and results have
been compared in figure 8. Abrasion resistance was found to be around 8 times
better than the base material.
Process Parameters
Process Temperature: 580°C
Gas Ratio: 1:3(N2:H2)
Acetelyne %: 2% of total gas
Process Pressure: 2.5 mbar
Process duration: 20 hrs
Total duration: 36hrs (including preheating, pre-cleaning and cooling
cycle)
WE CLAIM
1. A method of developing erosion resistant hard layer on 13 Cr-4Ni Stainless
Steel components comprising:
Placing the component in plasma ion nitriding equipment for carrying out
plasma, nitro carburizing process in the presence of H2, N2 and C2H2 gases
at a temperature range of 575-585°C and pressure in the range of 1.5-2.5
mbar for 20-24 hrs to achieve hardness of about 1100-1400 HV upto a
case depth of about 150 microns.
2. The method as claimed in claim 1, wherein the said plasma nitro
carburizing process is provided on a Pelton needle to establish the process
for erosion resistant application of hydro turbine components.
3. The method as claimed in claim 1, wherein a specially designed fixture
has been used to attach thermocouple directly on the
component for precise measurement and control of the process
temperature and to maintain it uniformly during the process.
4. The method as claimed in claim 1, wherein the said gas ratio is N2:H2::1:3
and acetylene ration is in the range of 1 to 3%.
5. The method as claimed in claim 1, wherein the pressure is preferably
2.5m bar.
6. The method as claimed in claim 4, wherein the said acetylene is
preferably 2%.

ABSTRACT

A method of developing erosion resistant hard layer on 13 Cr-4Ni Stainless Steel
components comprising placing the turbine component in plasma ion nitriding
equipment for carrying out plasma, nitro carburizing process in the presence of
H2, N2 and C2H2 gases at a temperature range of 575-585°C and pressure in the
range of 1.5-2.5 mbar for 20-24 hrs to achieve hardness of about 1100-1400 HV
upto a case depth of about 150 microns.