FIELD OF INVENTION
This invention relates to a process for short-term creep testing for finding out the
deformation mechanism for new boiler materials subjected to simulated boiler
condition.
BACKGROUND OF THE INVENTION
Steam pipes which are exposed to high-temperature and internal-pressure
loading, are prone to creep damage. Material properties including creep data of
in-service or ex-service steels are critical input parameters for accurate life
assessment of steam pipes. Material properties are usually developed on virgin
materials, which have not experienced in-service operation. Usually creep tests,
take long duration e.g 10,000hrs.
In recent years, the subject of remaining life prediction of boiler components has
drawn considerable attention in the power generation industry. The interest in
the issue of remaining life prediction stems from the necessity to avoid costly
forced outages, from the need to extend the component life beyond the original
design life for economic reasons, and also from safety considerations. In power
Generation systems, a variety of structural components, e.g. Steam pipes,
turbine rotors and casings and super heater headers, typically operate at high
temperatures, where creep deformation and rupture are an important
consideration. Thus, a life prediction methodology, which accounts for creep, is
needed for these components.

Further, because of the long-term, high-temperature operation, material
properties of an in-service (or ex-service) steel may be significantly different
from those of corresponding virgin steel. Thus, it is extremely important to
develop material properties of in-service and/or ex-service steels for life
assessment of steam pipes.
Potentially, failure in steam pipes can result from high-temperature crack
propagation associated with pre-existing fabrication inclusions or defects. In
particular, seam-welded steam pipes are prone to cracking along the fusion line
area where inclusions or weld defects could be present. The effects of inclusion
content on creep properties of ex-service material, steam pipe weldments needs
to be investigated.
Recently, a creep crack growth life prediction methodology based on the concept
of time-dependent fracture mechanism (TDFM) has been developed for Elevated-
temperature structural components. Because of High-temperature exposure and
internal-pressure loading, steam pipes are prone to creep damage. Thus,
material properties including creep data of in-service or ex-service steels are
critical input parameters for accurate life assessment of steam pipe systems.
Material properties have been developed on virgin materials, which did not
experience in-service operation. However, this known method does not take into
account the material behavior of in-service or ex-service steam pipe steels which
are critically important to determine the deformation mechanism of boiler
materials.

US Patent no.:6606910Bl discloses a method and apparatus for assessment
creep damage of metal material by Time of Flight Diffraction (TOFD),
Technique. A damage evaluation method and apparatus capable of determining
whether an internal crack of a metal material is developed by the creep damage
by an aged determination or caused in the material process, and which is
capable of evaluating the remaining service life of a metal component. The
method for evaluating a flaw by transmitting probe for transmitting the ultrasonic
waves and receiving probe for receiving ultrasonic waves and analyzing the
diffracted waves to find out the distribution of voids.
US Patent no.: 6935552, describes a high precision method for evaluating creep
damage of high tension heat resistant steel in high temperature exposing
components which comprises a working curve or working map prepared in
advance by looking for the relation of grain sizes to creep damage extents at
every level of loaded stress on the basis of variation of the particle size
behaviour corresponding to creep damage progress of crystal grains having
crystal orientation difference of about 2 degrees or more, preferably 3 degrees or
more at an elevated part.
US patent no.: 6, 801,871 B2, provides a method in which creep damage for
each component which is used at a high in service temperature for a long period,
is approximated by a relational expression containing the Larson-Miller
parameter. The degree of creep damage is estimated by an approximation
expression obtained by determining constants for each component. The creep
damage degree is subjected to Weibull Statistical analyses to estimate the creep
damage degree probabilistically. Therefore probabilistically estimated creep

damage degree and thermal fatigue and damage degree allows the life of each
component subjected to a high in-service temperature to be assessed precisely
and quickly.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention is to propose a system to
conduct a short term creep testing of boiler materials and weidments.
Another object of the present invention is to propose a method of estimating the
creep deformation mechanism of boiler materials including life expectancy of the
component.
A still another object of the present invention is to propose a method of
estimating the creep deformation mechanism of boiler materials including life
expectancy of the component, in which creep specimen adapted being small in
size and prepared by NDT method.
Yet another object of the present invention is to propose a method of estimating
the creep deformation mechanism of boiler materials including life expectancy of
the component, in which creep specimen is prepared from in- service component
are prepared.

A further object of the present invention is to propose a method of estimating
the creep deformation mechanism of boiler materials including life expectancy of
the component, in which creep testing can be conducted during a substantially
shorter time.
A still further object of the present invention is to propose a method of
estimating the creep deformation mechanism of boiler materials including life
expectancy of the component, which allows short term creep test in all
specimens from weld metal, Heat affected Zone including the base metal.
SUMMARY OF THE INVENTION
For accurate life assessment, the indentation creep data of all boiler material at
simulated boiler condition, temperature and load are important parameters.
Conventional creep tests take years together to assess the creep life of the
components. This invention proposes a short -term creep testing for finding out
the deformation mechanism for new boiler components in simulated boiler
condition, i.e. with different load (different stress) and at different temperature.
Indentation creep testing can be conducted on not only base material but also
on weld metals. The mechanism of creep damage can be assessed, accordingly
the remaining life is predicted.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l. shows a system for short-term creep testing of boiler materials and
weldments according to the invention.

Fig.2. shows an indentation creep with cylindrical indenter.
Fig.3. graphically represents an Indentation creep analysis of three different
boiler material, each material testing, results being plotted based on depth of
indentation in microns versus Time in seconds.
DETAILED DESCRIPTION OF THE INVENTION
The Creep Testing System as shown in Fig.1 comprises a lever arm/Pull rod(1); a
support rod/ Frame(4); a cooling device with a cooling water inlet(2), a cooling
water outlet (15), and a cooling oil (15); a specimen cage(6), accommodating a
specimen(9) and a specimen holder(10); at least one indenter(8); and a LVDT(4)
including signal conditioners. The indenter (8) is of cylindrical, ball or conical
shape and made of Tungsten Carbide. The diameter of the indenter (8) ranges
from 0.5mm to 2.0mm is supported by an indenter support(7).
The creep testing system is provided with an in-built furnace(5) with operating
temperatures ranging from 400 to 800 deg.C. The temperature of Tubular
furnace could go up1000 deg.C The furnace (5) accommodates the sample
holder(10), and is made of double wall high quality SS body. The furnace (5) is
covered with the glass wool to avoid heat radiation. The furnace (5) is provided
with a control device with a programmable digital temperature indicator. The
system is covered with a brass plate(3).

Loads ranging from 500g to 2500g can be applied on to the specimen.
The system further comprises a vacuum chamber (13) made of SS material, and
mountable on a movable rectangular plate (11) which can be moved to any
height in a support platform (16) by using balancing weight. A provision is made
for connecting the vacuum chamber (13) to a vacuum pump. A vacuum of 10-
6Torr is maintained with an effective cooling oil (14). Copper tubes are provided
around the outer surface of the vacuum chamber (13) for cooling and vacuum
sealing.
Specimen preparation
A specimen (9) with flat surface, perpendicularly adjustable to the direction of
the indentation is selected. The specimen (9) is bordered by two opposite plane
parallel surfaces, and having thickness of 4mm, length of 55mm or height of
55mm. Specimen of circular or square or rectangular shape can also used with
out any thickness variation. In case of any weld samples of specimen size
without any bur, a smooth surfaced and even without polishing can be used.
TESTING
The furnace (5) is lifted up in order to view the indenter (8) and an area of the
specimen (9), both the indenter (8) and the specimen (9) are placed, and
ensured that the specimen (9) is in slight contact with the indenter (8) by lifting
the base of the specimen (9). The power wires of the furnace (5) are
reconnected and kept intact to the connector. Then the vacuum chamber (13) is

closed to the top plate (11) of the system slowly. The inner parts of the chamber
(13) are checked so as not to touch the furnace components. The controller unit
is switched on and the required temperature is set. The vacuum is set on and
then the heater is switched on. After reaching the vacuum and set temperature,
a data acquisition unit is switched on and a dead weight is loaded on the back of
lever arm (1). The LVDT (4) is set to zero and the display is seen. Acquisition of
data is commenced and an on line graph of Time vs. Depth of Indentation is
produced.
A sampling interval can be selected at 5 seconds when the indenter material is
Tungsten Carbide. And the Indenter with a diameter range of 0.5mm-2.0mm is
used. The Indenter type can be cylindrical and an operating temperature range
from 400-800 deg.C. is set on. A load between 500Kg. -2500Kg. is applied. The
Chromel-Alumel thermocouple is used to sense the inside temperature of the
cylinder. The vacuum chamber (5) is made of Stainless steel. The LVDT (4)
constitutes a Deflection Sensor.
The Creep testing system of the invention is connectable to a Data processor.
The system provides indentation depth in microns vs. Time in seconds. For every
five seconds the Data processor provides an average of the increasing
indentation depth value. The graph indicating indentation depth vs. Time is
initially obtained.
The strain rate value can be calculated by the formula for all the indentation
depth values as under:


Strain rate vs time in seconds is then plotted for the tests conducted.
The minimum strain rate is calculated for every individual testing.
D) Log strain rate is calculated for minimum strain rate.
E) For every testing, Temperature corresponding to minimum strain rate is
converted to absolute temperature, (Kelvin scale) and then 1/T is calculated
F) Then a graph is plotted between Log Strain rate vs 1\T.
G) Activation energy calculation: O
From the graph of Log Strain Rate vs. 1\T,
the slope(X) that is directly proportional to activation energy. X ~Q
i.e. X= -Q/R
The slope is indicated as -Q/R where
R is universal gas constant, R = 8.314*10 -3.

On substituting the X value and R, the value of Q in thousands can be calculated
and then converted into Kilo joules, and the slope usually comes in negative
value, and according to standard practice, the negative sign is usually eliminated.
The unit of Q is KJ/Mol. Activation energy reveals the type of deformation
mechanism of the material tested. From the Deformation map, the deformation
mechanism identified.
Nomenclature:
d: Indentation depth in microns
t: Time in seconds
T: Temperature in Kelvin
R: Universal gas constant, = 8.314*10 -3.
Q: Activation energy, KJ/Mol.
X: slope, i.e -Q/R

WE CLAIM
1. A system for short-term creep testing of boiler materials and weldments,
the system being connectable to a data processor, the system comprising;
- a cage (6) to accommodate a specimen holder (10) holding a
specimen (9);
- atleast one indenter (8) supported by an indenter support (7)
touchingly disposed in respect of the specimen (9);
- a lever arm (1) for placing a dead-weight thereon for loading;
- an in-built furnace (5) capable of accommodating the sample
holder (10);
- a deflection sensor (4) with signal conditioner connectable to a
display unit via a data acquisition unit;
- a control device having a programmable temperature indicator
attachable to the furnace (5); and
- a vacuum chamber (13) mountable on a movable rectangular plate
(11) such that the chamber (13)can be lifted via a support platform
(16), the vacuum and the set temperature when attained the dead-
weight being applied preceded by setting the furnace (5) on, the

data acquisition unit acquires data on the depth of indentation of
the specimen (9) at predefined intervals , the acquired data
transmitted to the data processor for determining the data
representing deformation mechanism of the boiler materials.
2. A process for short term creep testing of boiler materials comprising the
steps of;
- preparing a specimen;
- mounting the specimen in a creep testing system, as claimed in claim 1;
- evacuating the chamber to a pre-determined level of vacuum;
- heating the chamber to a pre-determined temperature;
- continuously applying load on the specimen through the indenter;
- recording the depth of indentation at predefined interval;
- plotting a depth of indentation Vs. time graph; and
- computation of activation energy, which represents the type of
deformation mechanism of the boiler materials.

3. The process as claimed in claim 2, wherein determination of creep
properties is achieved within a shorter period for example, 8 hours.
4. The process as claimed in claim 2, wherein determination of creep
properties of boiler materials is carried-out at simulated boiler conditions.
5. The process as claimed in claim 2, wherein determination of creep
properties can be achieved for base metal including weldments.
6. The process as claimed in claim 2, wherein the specimen size is about, 55
mm x 55 mm x 4 mm.
7. The process as claimed in claim 1, wherein it enables determination of
deformation rates of different materials at different temperatures under a
constant creep load.
8. A system for short-term creep testing of boiler materials and weldments,
as substantially described and illustrated herein with reference to the
accompanying drawings.
9. A process for short term creep testing of boiler materials, as substantially
described and illustrated herein with reference to the accompanying
drawings.

This invention relates to a system for short-term creep testing of boiler materials
and weldments, the system being connectable to a data processor, the system
comprising; a cage (6) to accommodate a specimen holder (10) holding a
specimen (9); atleast one indenter (8) supported by an indenter support (7)
touchingly disposed in respect of the specimen (9); a lever arm (1) for placing a
dead-weight thereon for loading; an in-built furnace (5) capable of
accommodating the sample holder (10); a deflection sensor (4) with signal
conditioner connectable to a display unit via a data acquisition unit; a control
device having a programmable temperature indicator attachable to the furnace
(5); and a vacuum chamber (13) mountable on a movable rectangular plate (11)
such that the chamber (13)can be lifted via a support platform (16), the vacuum
and the set temperature when attained the dead-weight being applied preceded
by setting the furnace (5) on, the data acquisition unit acquires data on the
depth of indentation of the specimen (9) at predefined intervals, the acquired
data transmitted to the data processor for determining the data representing
deformation mechanism of the boiler materials.