FIELD OF INVENTION
The present invention relates to a method for metallurgical evaluation of indentation
crept samples. More particularly, the invention relates to the assessment of the creep
deformation level by metallurgical evaluation of the samples which are specimens of
new boiler materials subjected to simulated boiler conditions and creep tested at
different load and temperature.
BACKGROUND OF THE INVENTION AND PRIOR ARTS
In recent years, the subject of remaining life prediction has drawn considerable
attention in the power generation industry. The interest in the issue of remaining life
prediction seems 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.

Steam pipes are generally used at elevated temperature operation conditions. Because
of high temperature exposure and internal pressure loading steam pipes are prone to
creep damage. Thus the material property including creep data of in-service and ex-
service steels are critical input parameters for life assessment of steel pipes. Material
properties were usually developed on virgin materials, which have not experienced in-
service operation. Material properties were usually performed to characterize the
material behaviour of in-service or ex-service steam pipe steels. Thus it is extremely
important to develop material properties of in-service/or ex-service steels for life
assessment of steels pipes.
High temperature failure analysis
Creep occurs under load at high temperature. Boilers, gas turbine engines, and oven
are some of the systems that have components that experience creep. An
understanding of high temperature materials behaviour is beneficial in evaluating
failures in these types of systems. Failures involving creep are usually easy to identify
due to the deformation that occurs. Failures may appear ductile or brittle. Cracking may
be either transgranular or intergranular. While creep testing is done at constant
temperature and constant load actual components may experience damage at various
temperatures and loading conditions.
Indentation creep
Requirements for the applicability of the indentation creep technique are determined by
the characteristic of the indentation measurement. This test requires only one flat
surface of the sample, which has to be adjusted perpendicularly to the direction of the
indentation. The specimens should be bordered by two opposite plane parallel surfaces.
For choosing the appropriate dimensions of the sample, the size of the deformation
zone and the diameter of the indenter have to be considered.
PRIOR ART
1. Method for measuring damage to structural components
Abstract
A method is provided for measuring crack growth within a material utilizing reversing
d.c. potential measurements across a preformed crack. Preferably, the material is
representative of structural components of interest and the material is located within
the aggressive environment of such components. The measured values are plotted
versus distance to obtain intercept values. These intercept values correspond to the
depth of the crack at the time the measured values were obtained.
U.S. Patent number: 4677855
Issue date July 7,1987
Assignees Electric Power Research Institute, Inc.
2. Method of and apparatus for estimating remaining service life of material
Abstract
A method and an apparatus for estimating the remaining service life of an object by
measuring the physical quantity of a sample and which are applicable to a nuclear
reactor. A plurality of model samples are experimentally prepared by preliminarily
disposing a material having substantially the same composition as the object to be
subjected to the estimation in an environment substantially the same as the object and
measuring the physical quantities of the model samples in relation with exposure times.
From this experiment, a relationship between the exposure time and the physical
quantity under such an environment is obtained. Further, a critical exposure time which
will cause an unstable fracture of a material of an actual sample, which is made of a
material substantially the same as the object and placed in substantially the same
environment as the object, is preliminarily obtained from the relationship between the
exposure time and the physical quantity for the model samples.
Patent Number: 5307385
Issue date April 26,1994
Assignees Hitachi Ltd.
3. Field indentation microprobe for structural integrity evaluation
Abstract
Apparatus and methods for the in-field measurement of mechanical and physical
properties of metallic structures. The apparatus is a Field Indentation Microprobe (FIM)
using an indenter that is caused to contact and indent the structure using cyclically
applied and released successive increasing loads at the same location. The load and
penetration depth data during both cyclic loading and unloading are used to determine
the flow properties and fracture toughness of the structure. An X-Y driven testing head
supports a load cell which is connected to an indenter holder and the indenter. A
displacement transducer is carried by the load cell to measure the depth of penetration
of the indenter into the structure, and one or more ultrasonic transducers determine the
physical phenomena such as crack size, material pile-up around indentation, and
residual stress presence and orientation. A polishing tool is provided to prepare the
surface of the structure prior to indentation.
Patent number: 4852397
Issue date August 1,1989
Inventor Fahmy M. Haggag

4. Title: INDENTING RHEOMETER
United States Patent 3805598
Abstract:
Apparatus is disclosed for dynamically determining the rheological properties of a
sample. The apparatus includes a table for supporting the sample, a loaded indenter for
deforming the sample, and a recording monitor. The apparatus continuously determines
and records deformation versus time as the loaded penetrator indents the surface of
the sample, as well as determining and recording recovery of the sample upon removal
of the load from the penetrator. The monitoring means includes an electrical transducer
connected to the penetrator. The transducer is arranged so that it is independent from
the applied loading weight, thereby permitting a large load to be applied while
permitting a small and sensitive transducer to be used.
Application Number: 05/204871
5. Title: High temperature ultra-high vacuum infrared window seal
United States Patent 4448000
Abstract:
A window seal apparatus capable of transmitting infrared radiation for use under high
temperature ultra-high vacuum conditions. The apparatus includes a window clamped
between an annular rim of a clamp flange and an annular rim of a sealing flange. The
rim of the sealing flange includes an annular sealing knob machined therein having an
annular indentation in its upper surface. An annular lead gasket is located between the

sealing knob and one side of the window. An annular lead gasket having roughened
surfaces is positioned adjacent to the other side of the window and a teflon gasket is
positioned adjacent to the annular rim of the clamp flange. A constant force means is
provided for pushing the clamp flange toward the window and the sealing flange
thereby producing a seal there between. The window is preferably formed from alkali
halide material.
6. Title: Thermal expansion driven indentation stress-strain system
United States Patent 5133210
Abstract:
Disclosed is a method and apparatus of indentation testing of variable types of
specimens which includes mounting an indenter member onto a thermally expandable
member, mounting the specimen to be tested on a pedestal, loading the specimen with
the indenter member by thermally expanding the thermally expandable member and
measuring applied force to and displacement of the specimen so as to determine the
hardness or other material properties of the specimen. The indenter member is
thermally insulated from the thermally expandable member prior to thermally
expanding the thermally expandable member. The thermal expansion driven
indentation system in the present invention allows for controlled application of precise
and continuous reproducible loads to produce stress-strain plots. Continuous
indentation testing with this system has yielded results which correlate well to Rockwell
fixed load hardness tests on standard test blocks. Further, efforts in drawing

relationships with tensile results have shown support for the use of continuous
indentation testing for the generation of local material properties.
7. Title: Creep resistant composite alloys
Unites States Patent: 5223347
Abstract:
A fabrication method of strengthening metallic alloys by composite technology has been
developed by mixing steel shots or aggregates with conventional alloys, thus preventing
cold flow or creep. Preventing creep is advantageous in thermal plugs which must
withstand fluid pressure without leakage until subjected to dangerous temperatures
such as caused by fire. The matrix alloy primarily consists of some or all of copper,
magnesium, bismuth, tin, lead, cadmium and indium and the particle material is
preferably iron or steel. New alloys exhibit a higher strength against a hydrostatic gas
pressure than that of conventional matrix phase containing no reinforcing particles,
while maintaining the melting temperature of new alloys in the same range of
conventional unreinforced matrix alloy. The mixing of steel particles with the matrix is
achieved by employing a flux such as ammonium chloride. Other reinforcible matrix
alloys include tin-based, lead-based, copper-based, zinc-based, cadmium-based,
indium-based, bismuth-based, magnesium-based, and aluminium-based alloys used for
dynamic and structural parts requiring strength and creep resistance. Ferroaluminium
shots are comprised primarily of iron and aluminium and they are light, relatively
nonreactive with zinc, and bondable to aforementioned matrix alloys by using inorganic
acid-based fluxes of zinc chloride, ammonium chloride, a mixture of chlorides, or a

mixture of chlorides and fluorides. Other fluxes such as organic acid-based chemicals
work as a cleaning agent when they can clean surface oxides of both the matrix alloy
and reinforcing shots. Materials for alternative reinforcement include conventional steel
or iron shots coated with sodium nitrite, ferroaluminium shots coated with sodium
nitrite, nickel, copper, their base alloys refractory metals, copper or nickel-coated
metals, copper or nickel-coated plastics, and copper or nickel coated ceramics.
OBJECTS OF THE INVENTION
Therefore it is an object of the invention to propose a method for metallurgical
evaluation of indentation crept samples which is capable of developing metallurgical
properties of in-service and/or ex-service steel for life assessment of steam pipes.
Another object of the invention is to propose a method for metallurgical evaluation of
indentation crept samples which can ascertain comparative creep behaviour of the
different boiler grade materials and their weldments with their microstructure.
A further object of the invention is to propose a method for metallurgical evaluation of
indentation crept samples which is capable of finding the deformation rate of the two
materials at different temperatures under a constant creep load and correlate with their
microstructure.

A still further object of the invention is to propose a method for metallurgical evaluation
of indentation crept samples which is capable of evaluating the metallurgical properties
of indentation crept surfaces of base metals and weldments including Heat Affected
Zone (HAZ).
A still another object of the invention is to propose a method for metallurgical
evaluation of indentation crept samples which is able to make a metallurgical evaluation
for assessing material for creep properties of small volume specimens.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - shows a schematic diagram of the specimen for indentation test
Fig.2 - shows a specimen holder
Fig.3 - shows the indentation crept sample with the sample holder.
Fig.4 - shows the diagram of the specimen of the indentation
Fig.5 - shows the specimen mounted on a araldite base
Fig.6 - shows the position of the sample in the mounting
Fig.7 - shows the specimen after grind
Fig.8 - shows the area of creep region, metallurgically evaluated
Fig.9 - shows the crept surface of the specimen
Fig.10 - shows a graph between Depth of indentation in microns and time in seconds
Fig.11- shows a graph between strain rate and time in seconds

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Main object of the present invention is to metallurgically evaluate a short term creep
(Indentation) tested boiler materials. Because of the long term, high-temperature
operation, material properties of in-service (or ex-service) steel may be significantly
different from those of corresponding virgin steel.
Thus, it is extremely important to develop metallurgical properties of in-service and/or
ex-service steels for life assessment of steam pipes.
Specimen preparation:
The short-term creep testing equipment required the sample with both sides flat
surface and of uniform thickness. The sample was placed inside the furnace
perpendicular to the indenter. The application of the method depends on the applied
load. At low load values the indentation depth is in the micron regime. The standard
procedure of testing was that, placing the specimen inside the furnace with the indenter
perpendicular to the surface. Then close the furnace with the vacuum cylinder and
switch on the vacuum system. Then ensure the cooling water supply is ON and set the
specified temperature required in the controller and switch ON the furnace. After the
required temperature is achieved, the load is applied.

Creep Testing System
The sampling interval is 5 seconds with the indenter material as Tungsten Carbide and
Indenter diameter range of 0.5mm-2.0mm. The indenter type is cylindrical and the
operating temperature ranges from 400-800 deg.C. The load ranges from 500Kg -
2500Kg. The Chromel-Alumel thermocouple is used to sense the inside temperature of
the cylinder. The vacuum chamber used of stainless steel and the Linear Velocity
Deflection Transducer(LVDT) is Deflection Sensor. The Creep Testing System is
connected 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.
Initially obtained the graph indicating indentation depth vs. time
The strain rate value is calculated by the formula for all the indentation depth values
Strain rate = Indentation depth in microns
Time in seconds
Strain rate vs time in seconds was plotted for tests conducted.
Thereby minimum strain rate is calculated for every individual testing.
A) Log strain rate is calculated for minimum strain rate
B) For every testing, temperature corresponding to minimum strain rate is converted to
absolute temperature, Kelvin scale and then 1/T is calculated
C) Then a graph is plotted between Log Strain rate vs 1/T
D) Activation energy calculation:Q
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 was calculated and then
converted into Kilo joules, and the slope usually comes in negative value by 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, it was clearly indicated that the deformation mechanism is identified.
Metallurgical evaluation
The samples taken after the indentation test were cleaned in acetone by Ultrasonic
cleaning process. Then the macro images of the sample were taken using Stereo
Microscope. The width of the creep indentation was measured and compared with the
indenter width. The macrostructures of the samples revealed the crept surface. Then
the samples were mounted on the araldite base in its cross section to view the indented
spot. Then the samples were grinded with the emery sheets of 80 X to reach the
indented spot using the grinding machine. Then the samples were polished using the
emery sheets from 120X, 200X, 400X and 800X till polished surface was obtained. The
polished crept specimen was etched with suitable etchant to get microstructure with
clear grain boundaries. Then the etched specimens were examined under the Scanning
Electron Microscope. Creep regions were revealed from the SEM examination.
Microscopy of crept specimen (both base and weldment) showed high density of
cavities mostly of carbides. Micro hardness of the specimen also showed softening at
the failure spot.
Carbides at the crept area were found to have coarsened and spheroidised. The HAZ
samples were showing higher secondary creep rate and lowest microhardness. Carbides
undergo significant coarsening and alloy enrichment leading to loss of matrix strength
and shorter rupture times.
In particular seam-welded steel pipes are prone to cracking along the fusion lines where
the inclusions and the weld defects can be present.
The hardness and strength of 2.25CrlMo steels with lower the inclusion content is
lower. Steel weldments demonstrated compared creep deformations rates in the fusion
area regardless of inclusion content.
In the fusion line area, the material with a low inclusion content exhibited slower creep
crack propagation rate properties.
Brief Description of the Testing Details
Specimen
The sample was rectangular in its cross section 10mm X 10mm and of uniform
thickness 3mm.

Methodology for Indentation Creep Evaluation
The conical /cylindrical indenter made from Tungsten Carbide material was used. The
operating temperature ranges from 400°C- 800°C. The applied load ranges from 0.5 Kg
to 2.5 Kg. The Chromel-Alumel thermocouple is used to sense the inside temperature of
the cylinder. The vacuum chamber is made of stainless steel. The samples taken after
the indentation test were cleaned in acetone by Ultrasonic cleaning process. Then the
macro images of the sample were taken using Stereo Microscope. The width of the
creep indentation was measured and compared with the indenter width. The
macrostructures of the samples revealed the crept surface. Then the samples were
mounted on the araldite base in its cross section to view the indented spot. Then the
samples were grinded with the emery sheets of 80 grits to reach the indented spot
using the grinding machine. Then the samples were polished using the emery sheets
from 120X, 200X, 400X and 800X grits till polished surface was obtained. The polished
crept specimen was etched with suitable etchant to get microstructure with clear grain
boundaries. Then the etched specimens were examined under optical microscope and
the Scanning Electron Microscope. Creep regions were revealed from the SEM
examination. The level of damage can be assessed.
The method which is involved in indentation creep analysis is finally described with the
help of the following steps:

WE CLAIM
1. A method for metallurgical evaluation of indentation crept samples comprising:
preparing the specimen to be subjected to indentation creep testing by cleaning
in acetone;
keeping the specimen perpendicular to the tungsten carbide indenter placed in
the indentation testing machine;
setting the testing temperature;
creating vacuum inside the chamber;
applying constant load;
adapting LVDT to measure the indentation depth;
connecting data acquisition system to PC to record the LVDT reading for
particular time interval;
calculating creep exponent and activation energy from the recorded data;
characterised in that,
the indentation tested sample is immersed in acetone to make ultrasonic
cleaning to prepare the specimen for macro examination when the crept sample
is examined under stereo microscope, mounted on a araldite base in its cross
section, grinded up to the indented region in belt grinder, polished with emery
sheets, etched with suitable etchant wherein the said specimen after micro
preparation is examined under Scanning Electron Microscope to reveal the creep
regions and to assess the level of damage.
2. A method as claimed in claim 1, wherein the sample tested is of size 10mm X
10mm X 4mm and thickness 3mm.
3. A method as claimed in claim 1, wherein the range of testing temperature is
400°C - 800°C.
4. A method as claimed in claim 1, wherein the constant load applied is in the range
of 0.5 kg to 2 kg.
5. A method as claimed in claim 1, wherein the samples are grinded with emery
sheets of 80 grits to reach the indented spot adapting the grinding machine.
6. A method as claimed in claim 1, wherein the samples are polished with emery
sheets of 120, 200, 400 and 800 grits.
7. A method as claimed in claim 1, wherein the crept specimen is etched with
suitable enchant to get microstructure with clear grain boundaries.
8. A method as claimed in claim 7, wherein the etched specimen is examined under
the scanning electron microscope to reveal the creep regions and to ascertain
the level of damage.

A method for metallurgical evaluation of indentation crept samples is developed for
evaluating the metallurgical behaviour of crept surface of new boiler materials. The
Indentation Creep Testing is performed in the temperature range of 400-800 deg. C.
with the constant loading conditions being followed during all tests so as to access the
minimum creep rate and deformation behaviour of the base metal and weldments. With
the available relation between the time and depth of indentation, strain rate is
calculated to evaluate the creep behaviour of the base metal and weldment. Activation
energy is also determined to predict the deformation behaviour. The mechanism is to
be found that deformation followed power law creep. Then the specimens are taken for
metallurgical evaluation. Initially the specimen is washed with acetone and its structure
is seen under Stereo Microscope. Width of the indentation developed during indentation
creep testing is calculated. Then the specimens are cleaned with ultrasonic cleaner. The
samples are such seen in Scanning Electron Microscope. The crept surface is analyzed.
Then the same sample is mounted vertically with araldite. Then the mounted sample is
polished (the cross section of the indentation) until reaching the indent portion very
carefully. The microstructures of crept sample are analyzed by optical microscope and
Scanning Electron Microscope (SEM).