FIELD OF THE INVENTION:
[01] The present invention relates to a method of short term stress
relaxation testing of a low alloy steel grade using thermo mechanical
simulation.
BACKGROUND OF THE INVENTION:
[02] Background description includes information that may be useful in
understanding the present subject matter. It is not an admission that any
of the information provided herein is prior art or relevant to the presently
claimed subject matter, or that any publication specifically or implicitly
referenced is prior art.
[03] Creep strength deduced from the creep tests are basis for the high
temperature equipments used in power plants, petrochemical and other
applications. Generally, traditional creep test experiment in a typical stress
level runs for more than 10,000 hours. Many accelerated creep tests are
widely used to reduce the test duration by enhancing the test temperature
and stress level. A short term accelerated creep test that produces usable
creep data is desirable and has good value. It is important that the creep
rates obtained from short-term tests are derived from the current status of
the material and long term features of ageing, oxidation and etc. are not
considered.
[04] One such a test is stress relaxation test. Researchers have shown that
creep rates could be generated from the stress relaxation test. Literature
works shows that applying 0.8-2% plastic strain in stress relaxation tests
are in good agreement with conventional creep tests. In stress relaxation
test, total strain in the specimen is kept constant, but the total strain
consists of elastic part and plastic part. During the relaxation test, stress in

the specimen decreases because of the plastic strain increases as the
expense of the elastic strain while the total strain remains unchanged.
[05] The Cr-Mo steel pipes are widely used as super heater and re-heater
panels at high temperature upto 600°C in thermal power station. As per the
specification this steel should have minimum yield strength of 415 MPa and
a minimum UTS of 585 MPa. Typical YS of 500 MPa, UTS of 700 MPa, %El
of 20%, %RA of 70% and hardness of 195-265 HV are exhibited by the
commercial grade. Microstructure of this grade shows tempered martensite.
[06] The conventional creep tests are conducted in tensile creep test
machine and generally test runs for more than 10,000 hours. Alternatively,
a stress relaxation test at high temperature predicts the creep behaviour in
time of few hours to few days. This test is of type of accelerated creep test.
[07] A few prior arts are discussed in below:
[08] The cited invention (CN 101793644B), titled “Method for carrying out
stress relaxation test on Gleeble3800 hydraulic wedge unit in Jaw control
mode”, discloses a method for carrying out a stress relaxation test on a
Gleeble3800 hydraulic wedge unit in a Jaw control mode, which comprises
the following steps: welding a thermocouple onto a sample, adopting a K-
shaped thermocouple when the heating temperature of the sample is below
1200 deg C, and adopting an S-shaped thermocouple when the heating
temperature of the sample is above 1250 deg C; respectively spreading a
lubricant on a pressure head and the sample to reduce the friction force in
compression, and isolating the pressure head and the sample through a
tantalum sheet to prevent the pressure head and the sample to react in a
high temperature state; opening an air hammer, compacting the sample,
controlling the pressure to be about 100Kg, then opening a Gleeble 3800
thermal simulation tester and leftwards moving the pressure head to a
proper position; carrying out vacuum pumping, and charging Argon gas for
protection when the vacuum degree is up to 2.4*10 to 1tau; carrying out

compression by adopting the Jaw control mode; and after equipment
operation is finished, reserving a high temperature organization state by
adopting a direct quenching method, verifying precipitation and
recrystallization conditions, and finishing the stress relaxation test. The
invention has low cost and high precision and can be used for researching
the precipitation behavior of various strain induced precipitates.
[09] The cited invention (CN106568655A) titled “Method used for predicting
creep life of heat-resisting alloy” provides a method used for predicting the
creep life of heat-resisting alloy. According to the method, a creep life
predicting model is established on the foundation of Arrhenius law; tensile
creep life data at different stress at a certain temperature is predicted via
fitting of the creep life predicting model, model parameter values are
determined, a relationship formula of the creep life at a prediction
temperature with the stress is obtained, and creep life prediction is carried
out based on the relationship formula. In establishment of the creep life
predicting model, action of high temperature strength of alloy materials on
creep performance and influence factors of stress on creep deformation
mechanism are taken into consideration fully, so that obtained results are
more close to practical data, and creep life prediction accuracy is increased
obviously. It is shown by results of creep life prediction tests of a plurality
of materials that under experiment conditions, the prediction error of the
method is reduced by one magnitude order in comparing with Arrhenius law
and Larson-Miller method.
[010] The cited invention (KR1020030015500) discloses apparatus for
measuring creep strain using stress relaxation. The apparatus for measuring
creep strain using stress relaxation is provided to reduce the time required
for obtaining stress values appropriate for conditions and improve the
reliability of the measured values by obtaining the stress values with the

passage of time while maintaining the strained state of a test piece in
length.
[011] The cited invention (CN103743635) discloses creep deformation test
method and platform of full-dimension pipeline bend. The cited invention
relates to a creep deformation test method and platform of full-dimension
pipeline bend. The cited invention is suitable for researching creep
deformation mechanical behavior of non-standard sample with heavy
gauge. A loading which is more than 200 kN is realized by servo motor,
leading screw and guide rail slide block. A force transducer senses stress
relaxation, and controls running of the motor, thereby realizing quasi-steady
stress loading. A ceramic heater is used for heating and thermostatic control
of the pipeline bend. Data acquisition is completed by a high temperature
strain gauge, which is used for dynamic acquisition of strains at different
directions. The cited invention can be used for full-dimension high
temperature creep deformation test of high temperature part, and research
of creep deformation damage mechanism of high temperature part under
the state of multiaxial stress, thereby overcoming uncertainty of result
brought by using creep deformation experiment result of miniaturization
sample to evaluate a real member.
[012] Hence, there is a need of a method of short term stress relaxation
testing to overcome the above drawbacks.
OBJECTS OF THE INVENTION:
[013] In view of the foregoing limitations inherent in the state of the art, some
of the objects of the present disclosure, which at least one embodiment herein
satisfy, are listed herein below.
[014] The primary objective of the present invention is to conduct stress
relaxation testing of low alloy steel using reduced section specimen in the
thermo mechanical simulator.

[015] These and other objects and advantages of the present invention
will be apparent to those skilled in the art after a consideration of the
following detailed description taken in conjunction with the accompanying
drawings in which a preferred form of the present invention is illustrated.
SUMMARY OF THE INVENTION:
[016] The boiler materials are operating at high pressure (>100 bar) and
high temperature (>450°C) the material slowly degrades over years by
mechanism of creep. So these materials are subjected to creep test to find
the creep properties to understand the material behaviour at high
temperature. Hence, the present invention provides the solution to
overcome above problems.
[017] The present invention is directed to a method of short term stress
relaxation testing of low alloy steel, said method comprising the steps of:
selecting a section specimen with a predetermined size and length for
conducing the stress relaxation test; welding with the K-type thermocouple
at the centre of the specimen for controlling and measuring the test
temperature; heating the specimen in the load control mode to a pre-
determined temperature for a specific time duration with loading set to zero;
switching to L-strain control mode for applying a range of plastic strain to
the specimen for a specific duration of time; conducting a complete hot
tension test to estimate the elastic limit of the material using the same
specimen size; and plotting of stress versus time is generated for different
plastic strain values applied at a specific temperature and further stress
versus strain rate is plotted.
[018] In the embodiment of the present invention, said method is used for
conducting stress relaxation test of a low alloy specimen using thermo
mechanical simulator. A reduced section specimen having 6 mm diameter,
25 mm gauge length is selected, with threads at both ends is used for

conducting the test. A precise plastic strain of 0.8 to 1.7% is applied with
strain rate of 0.01% per sec using L- gauge mode in the simulator and
applied strain is allowed to relax at high test temperature for a duration of
4 hours. The plot of stress versus time in linear and log scale is drawn using
the data acquired from the test. And further stress versus strain rate in
linear/ log scale is drawn using the test data and using the applicable law,
creep property of the material is predicted.
[019] The preferred embodiment of the present invention is having other
features and advantages which are disclosed in the appended dependent
claims.
[020] Other objects, features, and advantages of the present disclosure will
become apparent from the following detailed description. It should be
understood, however, that the detailed description and the specific
examples, while indicating specific embodiments of the invention, are given
by way of illustration only, since various changes and modifications within
the spirit and scope of the invention will become apparent to those skilled
in the art from this detailed description.
[021] Various objects, features, aspects and advantages of the inventive
subject matter will become more apparent from the following detailed
description of preferred embodiments, along with the accompanying
drawing figures in which like numerals represent like components.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
[022] The illustrated embodiments of the subject matter will be best
understood by reference to the drawings, wherein like parts are designated
by like numerals throughout. The following description is intended only by
way of example, and simply illustrates certain selected embodiments of

apparatus that are consistent with the subject matter as claimed herein,
wherein:
[023] Figure 1a and 1b: illustrates a) reduced section specimen and b)
specimen fitted in the chamber, according to the present invention.
[024] Figure 2: illustrates plot of temperature versus time.
[025] Figure 3a and 3b: illustrates a) stress versus strain curve, and b)
stress versus time curve.
[026] Figure 4a and 4b: Figure 4a) illustrates stress versus strain curve at
600°C for total strain of 0.023 and 0.025, and fig 4b) illustrates stress
versus strain curve at 550°C for total strain of 0.028, 0.030, 0.032, 0.034
and 0.037.
[027] Figure 5a and 5b: Fig-5a illustrates plot of stress versus linear time
curve at 550°C for total strain of 0.028, 0.030, 0.032, 0.034 and 0.037,
and fig-5b) illustrates plot of stress versus log time curve at 550°C for
total strain of 0.028, 0.030, 0.032, 0.034 and 0.037.
[028] Figure 6a and 6b: fig 6a) illustrates plot of stress versus linear strain
rate curve at 550°C for total strain of 0.028, 0.030, 0.032, 0.034 and
0.037, and fig-6b) illustrates plot of stress versus log time curve at 550°C
for total strain of 0.028, 0.030, 0.032, 0.034 and 0.037.
[029] Figure 7a and 7b: fig 7a illustrates plot of stress versus linear time
curve at 600°C for total strain of 0.023 and 0.025, and fig-7b) illustrates
plot of stress versus log time curve at 600°C for total strain of 0.023 and
0.025.
[030] Figure 8a and 8b: fig 8a) illustrates plot of stress versus linear strain
rate curve at 600°C for total strain of 0.023 and 0.025, fig 8b) illustrates
plot of stress versus log strain rate at 600°C for total strain of 0.023 and
0.025.

[031] The figures depict embodiments of the disclosure for purposes of
illustration only. One skilled in the art will readily recognize from the
following description that alternative embodiments of the structures and
methods illustrated herein may be employed without departing from the
principles of the disclosure described herein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT:
[032] The detailed description of various exemplary embodiments of the
disclosure is described herein with reference to the accompanying drawings.
It should be noted that the embodiments are described herein in such details
as to clearly communicate the disclosure. However, the amount of details
provided herein is not intended to limit the anticipated variations of
embodiments; on the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the spirit and scope of the present
disclosure as defined by the appended claims.
[033] It is also to be understood that various arrangements may be devised
that, although not explicitly described or shown herein, embody the
principles of the present disclosure. Moreover, all statements herein reciting
principles, aspects, and embodiments of the present disclosure, as well as
specific examples, are intended to encompass equivalents thereof.
[034] The terminology used herein is for the purpose of describing particular
embodiments only and is not intended to be limiting of example
embodiments. As used herein, the singular forms “a”, “an”, and “the” are
intended to include the plural forms as well, unless the context clearly
indicates otherwise. It will be further understood that the terms “comprises”,
“comprising”, “includes”, “consisting” and/or “including” when used herein,
specify the presence of stated features, integers, steps, operations, elements
and/or components, but do not preclude the presence or addition of one or

more other features, integers, steps, operations, elements, components
and/or groups thereof.
[035] It should also be noted that in some alternative implementations, the
functions/acts noted may occur out of the order noted in the figures. For
example, two figures shown in succession may, in fact, be executed
concurrently or may sometimes be executed in the reverse order, depending
upon the functionality/acts involved.
[036] Unless otherwise defined, all terms (including technical and scientific
terms) used herein have the same meaning as commonly understood by one
of ordinary skill in the art to which example embodiments belong. It will be
further understood that terms, e.g., those defined in commonly used
dictionaries, should be interpreted as having a meaning that is consistent
with their meaning in the context of the relevant art and will not be
interpreted in an idealized or overly formal sense unless expressly so defined
herein.
[037] The present invention is directed to a method of short term stress
relaxation testing of low alloy steel, said method comprising the steps of:
selecting a section specimen with a predetermined size and length for
conducing the stress relaxation test; welding with the K-type thermocouple
at the centre of the specimen for controlling and measuring the test
temperature; heating the specimen in the load control mode to a pre-
determined/specific temperature for a specific time duration with loading
set to zero; switching to L-strain control mode for applying a range of plastic
strain to the specimen for a specific duration of time; conducting a complete
hot tension test to estimate the elastic limit of the material using the same
specimen size; and plotting of stress versus time is generated for different
plastic strain values applied at a specific temperature and further stress
versus strain rate is plotted.

[038] In the embodiment of the present invention, Fig 1a and 1b discloses
the method of conducting stress relaxation test using thermo mechanical
simulator. A reduced section specimen having 6 mm diameter, 25 mm
gauge length is selected with threads at both ends is used for conducting
the test, wherein the specimen is a low alloy steel. A precise plastic strain
of 0.8 to 1.7% is applied with strain rate of 0.01% per sec using L- gauge
mode in the simulator and applied strain is allowed to relax at high test
temperature for a duration of 4 hours. The plot of stress versus time in
linear and log scale is drawn using the data acquired from the test. And
further stress versus strain rate in linear/ log scale is drawn using the test
data and using the applicable law, creep property of the material is
predicted.
[039] The mid-section of reduced specimen having threads at both ends
are prepared to conduct the stress relaxation test. The specimen is fitted
on thermo mechanical simulator using stainless steel grips; gauge length
(span between the fixtures) of the specimen is 25 mm for all tests gripped
between stainless sample fixtures and diameter of the specimen is 6 mm as
shown in Figure 1a and 1b. Using the stainless fixture, it helps to minimize
the temperature difference along the span of specimen. The surface
temperature is measured by K-type thermocouples welded at the centre of
the sample. The specimen is heated in the load-control mode to
temperature of 550°C and 600°C in 2 min duration with loading set to zero
as shown in Figure 2. Once the temperature is reached, it hold for 5 min,
then the system is switched to L-strain control mode. In the strain control
mode, the L-strain is first zeroed, and then followed by applying a range of
0.8 to 1.7% plastic strain to the specimen. In order to estimate the elastic
limit and plastic strain level at a specific temperature, high temperature

tension is conducted and engineering stress versus strain/time curve are
plotted as shown in Figure-3a and figure 3b.
[040] In the embodiment of the present invention, five specimens are
subjected to a total strain value of 0.028, 0.030, 0.032, 0.034 and 0.037 at
the test temperature of 550°C as shown in Fig 4a and fig 4b. These five
total strain values correspond to the plastic strain of 0.8%, 1.0%, 1.2%,
1.4% and 1.7%. The strain rate of 0.01% per second was applied to
attained the specific strain. At 600°C, total strain value of 0.023 and 0.025
was applied and these values can be seen in terms of plastic strain as 0.8%
and 1.0% as shown in Figure 4a and figure 4b.
[041] Further during the 4 hour relaxation test, the temperature and strain
is held constant, while the stress is recorded at a suitable frequency.
Following testing, the recorded stress versus time data (s) are plotted. A
plot of logarithmic strain rate versus stress is drawn and based on the
Norton law, creep life is predicted. Typical chemical composition of this
grade is given in Table 1
[042] In the embodiment of the present invention, plot of stress versus
linear time/ logarithmic time curve is drawn at 550°C for total strain of
0.028, 0.030, 0.032, 0.034 and 0.037 from the acquired data as shown in
Figure-5a and Fig-5b. Plot of stress versus linear strain rate/ log strain rate
curve at 550°C for total strain of 0.028, 0.030, 0.032, 0.034 and 0.037 is
made using software as shown in Figure 6a and figure-6b. Plot of stress
versus linear time curve at 600°C for total strain of 0.023 and 0.025 as
shown in Figure-7a and figure 7b. Plot of stress versus linear/ log strain rate
curve at 600°C for total strain of 0.023 and 0.025 as shown in Figure-8a

and figure-8b. The chemical composition of low alloy steel is illustrated in
the Table-1.
Table 1. Chemical composition of low alloy steel.

TECHNICAL ADVANTAGE
[043] One of the advantages of the present invention is that, there is a
substantial savings time of the test.
[044] Another advantages of the present invention is that test data (total
strain value) are recorded at the temperature of 550°C and 600°C.
[045] Another advantages of the present invention is that a precise plastic
strain of 0.8 to 1.7% is applied with strain rate of 0.01% per sec using L-
gauge mode in the simulator and applied strain is allowed to relax at high
test temperature for a duration of 4 hours
[046] Thus, the present invention is having above advantages which are
reduced time taken for conducting the test method.
[047] Furthermore, each of the appended claims defines a separate
invention, which for infringement purposes is recognized as including
equivalents to the various elements or limitations specified in the claims.
Depending on the context, all references below to the “invention” may in
some cases refer to certain specific embodiments only. In other cases, it
will be recognized that references to the “invention” will refer to subject
matter recited in one or more, but not necessarily all, of the claims.
[048] Groupings of alternative elements or embodiments of the invention
disclosed herein are not to be construed as limitations. Each group member

can be referred to and claimed individually or in any combination with other
members of the group or other elements found herein. One or more
members of a group can be included in, or deleted from, a group for reasons
of convenience and/or patentability. When any such inclusion or deletion
occurs, the specification is herein deemed to contain the group as modified
thus fulfilling the written description of all groups used in the appended
claims.
[049] Furthermore, those skilled in the art can appreciate that the
terminology used herein is for the purpose of describing particular
embodiments only and is not intended to be limiting of the present
disclosure. It will be appreciated that several of the above-disclosed and
other features and functions, or alternatives thereof, may be combined into
other systems or applications. Various presently unforeseen or
unanticipated alternatives, modifications, variations, or improvements
therein may subsequently be made by those skilled in the art without
departing from the scope of the present disclosure as encompassed by the
following claims.
[050] The claims, as originally presented and as they may be amended,
encompass variations, alternatives, modifications, improvements,
equivalents, and substantial equivalents of the embodiments and teachings
disclosed herein, including those that are presently unforeseen or
unappreciated, and that, for example, may arise from applicants/patentees
and others.
[051] While the foregoing describes various embodiments of the present
disclosure, other and further embodiments of the present disclosure may
be devised without departing from the basic scope thereof. The scope of
the present disclosure is determined by the claims that follow. The present
disclosure is not limited to the described embodiments, versions or

examples, which are included to enable a person having ordinary skill in the
art to make and use the invention when combined with information and
knowledge available to the person having ordinary skill in the art.

We claim:
1. A method of short term stress relaxation testing of low alloy steel, said
method comprising the steps of:
a) selecting a section specimen with a predetermined size and length
for conducing the stress relaxation test;
b) welding with the K-type thermocouple at the centre of the specimen
for controlling and measuring the test temperature;
c) heating the specimen in the load control mode to a pre-determined
temperature for a specific time duration with loading set to zero;
d) switching to L-strain control mode for applying a range of plastic
strain to the specimen for a specific duration of time;
e) conducting a complete hot tension test to estimate the elastic limit
of the material using the same specimen size; and
f) plotting of stress versus time is generated for different plastic strain
values applied at a specific temperature and further stress versus
strain rate is plotted.

2. The method as claimed in claim 1, wherein the section specimen is
a low alloy steel, having diameter of 6 mm; gauge length of 25 mm.
3. The method as claimed in claim 1, wherein heating the specimen in
the load control mode to a pre-determined temperature either 550°C
and/or 600°C for a specific time duration.
4. The method as claimed in claim 1, wherein the temperature of the
specimen once reached, it holds for 5 minutes and switched to L-
strain control mode.

5. The method as claimed in claim 1, wherein in the strain control mode,
the L-strain is first zeroed, and then followed by applying a 0.8 to
1.7% plastic strain level at a specific temperature (550°C and/or
600°C) with strain rate of 0.01% per sec using L-gauge mode in the
simulator and applied strain is allowed to relax at high test
temperature for a duration of 4 hours.
6. The method as claimed in claim 1, wherein estimation of the creep
characteristics of material is achieved based on the plot of stress
versus linear / logarithmic strain rate and the method is further used
for estimating other critical grades of materials.