FIELD OF THE INVENTION
The present invention relates to a method for optimization of thermo-mechanical
process parameters to reduce relaxation loss in high carbon steel product.
BACKGROUND OF THE INVENTION
Steel wires or strands used for pre-stressing applications need a low value of
stress relaxation to avoid the loss of load during actual application. Hence, stress
relaxation loss is one of the important properties guaranteed along with
mechanical properties. Stress relaxation needs to be controlled to as low as
possible during actual applications in bridges, sea link, railways and big building
constructions for better durability.
Various factors such as chemistry and properties of input materials, solute
content and residual stress may affect the stress relaxation in steel. During wire
drawing, a high amount of strain is applied, which increases the residual stress
and at the same time, a part of cementite dissolves causing an increase in
dissolved carbon in the matrix. Hence, the material in this condition results in a
very high stress relaxation loss making it unsuitable for pre-stressing
applications. To minimize the residual stress and fix the solutes in lattice sites for

low relaxation losses, a thermo-mechanical treatment is necessary to be applied
to wires or strands. It is known that simultaneous stretching and heating
operation reduces the relaxation loss but the optimum process parameters to
achieve this along with the high strength and high elongation are yet
established.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for optimization of
thermo-mechanical process parameters to reduce relaxation loss in high carbon
steel product.
Another object of the invention is to propose a method for optimization of
thermo-mechanical process parameters to reduce relaxation loss in high carbon
steel product, in which a thermo-mechanical simulation is first performed to
optimize the process parameters.
A further object of the invention is to propose a method for optimization of
thermo-mechanical process parameters to reduce relaxation loss in high carbon
steel product, in which a thermo-mechanical treatment is provided on the high
carbon steel based on the optimized parameters.

SUMMARY OF THE INVENTION
Chemistry of the material used for developing the invention.
High carbon steel (C - 0.8%) wires of 4.25 mm (cold drawn) successively passed
from 8.0 mm input wire rod, is taken for experimentation. Various alloying
additions (Mn, Si, Cr and V) were made to this steel. High carbon steel is
required to achieve a desired strength and fully pearlitic structure, which is
appropriate for wire drawing. Mn and Si are added for solid solution
strengthening enhancing the strength and toughness of the material. Chromium
is effective in ensuring cementite stabilization and improving corrosion
properties. Microalloying such as V will be beneficial in fixing carbon and nitrogen
and to achieve a low amount of solute atoms in finally processed conditions,
which is beneficial in improving the relaxation properties of LRPC wires and
strands.
To achieve a low value of stress relaxation to avoid the loss of load during
prolonged use, a suitable thermo-mechanical treatment to cold drawn wires or
strands is applied in production line. According to the invention, a thermo-

mechanical simulation is performed using Gleeble-1500 to optimize the process
parameters. Based on the simulated data, the cold drawn high carbon steel wires
are provided with a thermo-mechanical treatment (heating / tensile stretching).
Percentage stretch and temperature is varied during the trial phase and then the
wires are evaluated for stress relaxation losses at 0.7 UTS for 1 h using Instron
tensile machine. Results showed variations in strength, elongation and stress
relaxation values. Best combination of properties (high strength (>1900 MPa),
high elongation (>4.0 %) and low relaxation (<0.7 %) were obtained in the
temperature range of 325-350°C and a stretch level of 40-45%.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - a graphical representation of the effect of stress relieving temperatures
over a time period.
Fig. 1(a) - shows yield strength (YS, MPa) of samples as function of stress
relieving temperature.

Fig, 1(b) - shows UTS, MPa function of stress relieving temperatures.
Fig. 1(c) - shows elongation (%) as function of stress relieving temperature.
Fig. 2 - shows increase in yield strength as a function of stress relieving
temperature.
Fig, 3 - shows stress relaxation as a function of stress-relieving temperature.
Fig. 4 - shows stress relaxation as a function of % strength at stress-relieving
temperature of 350°C.
BRIEF DESCRITPION OF THE INVENTION:
Initial experiments were conducted to investigate the effect of stress relieving
temperatures on the stress relaxation without any stretch. The temperature was
varied from 200 to 400°C at a interval of 50°C. The heat-treatment was
conducted on Gleeble-1500 by heating the wire samples to various temperatures
at a fast heating rate of 40 to 80°C/s and then after keeping at this temperature
for 10 sec followed by quenching the samples in normal water. To see the effect
of stretch with simultaneous heating on stress relaxation, the samples were also
given a stretch with stress level of 35, 40 and 45 % of ultimate tensile stress of
cold drawn wire during heating. A typical cycles obtained from Gleeble is shown
in Fig. 1. Samples were tested for tensile properties (yield strength, ultimate

tensile strength and elongation) and stress relaxation for a maximum period of 1
hour at nearly constant stress level.
Stress relaxation measurements:
For stress relaxation measurements, the wire samples in different heat-treated
conditions were stressed to around 0.7 UTS on an Instron Tensile machine and
then crosshead speed was made zero to observe the drop in stress at a fixed
strain. The cross head speed during loading and gauge length were kept
constant as 1 mm/min and 40 mm respectively for all the experiments. These
experiments were conducted as a air-conditioned room of 25°C to avoid any
effect of variation of temperature on the results. The drop in stress as a function
of time was noted. To eliminate the modulus effect of machine and material, the
initial stress drop for one minute was deducted from final stress after one hour.
Analysis of results
Tensile properties of the heat-treated samples without any stretch and with
stretch are compared in Fig. 2. Fig. 2(a) shows the effect of heating temperature

on yield strength. It increases with the increase in temperature, reaches a
maximum at in the range of 250-300°C and then decreases. Yield strength
reaches a maximum in the temperature range of 250-300°C. The rise is
attributed to strain aging phenomenon caused by solute carbon and nitrogen
present in cold drawn stage. At this temperature, the maximum increase in yield
strength was estimated to be around 15 % (Fig.3). The fall after that is
attributed to relieve of residual stresses. Ultimate tensile strength also follows
the same trend. It reaches a peak at similar temperature. Samples with and
without stretch showed similar results except at a lower temperature of 200°C,
where yield strength and UTS were lower in case of without stretch. The
elongation decreased at the low temperatures up to 300°C due to strain aging.
As shown in Fig.2(c), it is very low around 1 % and suddenly increases at 325°C
and reaches about 4 %. The temperature corresponding to a change from low
elongation to high elongation is very important since, the steel with such a low
elongation is not at all suitable for actual application and may lead to a
catastrophic failure.

Stress relaxation results are shown in Fig. 4. Initially, it was found to high and
decreases with the rise in temperature and reaches a minimum at 300°C and
then increases up to 400°C. Similar trend i.e. a minimum at 300°C was noted for
both with and without stretch condition. Although the relaxation loss at 300°C is
minimum and the strength is high, the elongation at this temperature is very
poor, which will not be suitable for actual application. At 325°C, both strength
and elongation are good along with a low value of stress relaxation can be
achieved. Effect of amount of stretch at 350°C is shown in Fig.5. The stress
relaxation was found to be minimum at a stretch level of 40%.
As per the old concept, the increase in temperature of stress relieving or stretch
decreases the stress relaxation but the present result shows that the relaxation
becomes minimum at a particular temperature and stretch. Any rise in
temperature or stretch again increases the stress relaxation values.
From the present result, the following process parameters were found suitable to
achieve the best combination of properties. Temperature: 325-350°C. total time:
10-15 sec prior to quenching, tensile stretch: 40-45% of input cold drawn wire.

Best combination of properties achieved are as follows.
Ultimate tensile strength > 1900 MPa
Elongation > 4.0 %
Stress relaxation < 0.70 % (at a stress of 70% of UTS, Gauge length of 40 mm.
1 hr)
Based on these results, the temperature in plant was reduced by 20°C at the
same stretch level, which resulted in an significant improvement in stress
relaxation properties as shown below:


WE CLAIM:
1. A method for optimization of thermo-mechanical process parameters to
reduce relaxation loss in high carbon steel product, comprising the steps of:
providing a sample of cold drawn high carbon steel (C~ 0.8 %)
wire alloyed with mn, si, cr, and v;
providing a thermo-mechanical treatment on the sample to obtain a
typical cycle on stress-relaxation temperature corresponding to
coding time;
testing the sample in respect of tensile properties and stress
relaxation parameter;
measuring the stress relaxation parameter of the sample under
different heat-treated conditions and having stressed around 0.7
UTS.
determining the optimum process parameters from a comparison of
the measured parameters, and
correspondingly adjusting the plant temperature.

2. The method as claimed in claim 1, wherein the optimum process
parameters in respect of the given sample and under test condition constitute a
combination of temperature about 325-350°, total time between 10-15 sec prior
to quenching, and tensile strength in the range of 40-45% of the sample.
3. The method as claimed in claim 1 or 2, wherein the optimum mechanical
properties achievable under the optimized process parameters and in respect of
the given sample constitute the combination UTS> 1900MPa, elongation> 4%,
stress relaxation < 0.70%.
4. A method for optimization of thermo-mechanical process parameters to
reduce relaxation loss in high carbon steel product, substantially as herein
described and as illustrated in the accompanying drawings.

A method for optimization of thermo-mechanical process parameters to reduce
relaxation loss in high carbon steel product, comprising the steps of: providing a
sample of cold drawn high carbon steel (C~ 0.8 %) wire alloyed with mn, si, cr,
and v; providing a thermo-mechanical treatment on the sample to obtain a
typical cycle on stress-relaxation temperature corresponding to coding time;
testing the sample in respect of tensile properties and stress relaxation
parameter; measuring the stress relaxation parameter of the sample under
different heat-treated conditions and having stressed around 0.7 UTS,
determining the optimum process parameters from a comparison of the
measured parameters, and correspondingly adjusting the plant temperature.