FIELD OF INVENTION
The present invention relates to a process of ascertaining the effects of ageing
time on area reduction of hot rolled high carbon steel wire rods. More
particularly, the invention relates to a process to measure area reduction of hot
rolled high carbon wire rods due to the effects of reverse ageing.
BACKGROUND OF INVENTION
Drawn Pearlitic steel wires are of common use as a structure materials because
of their very high strength [1]. However some authors have already presented
that the higher strength could be achieved during strain ageing of heavily cold
drawn pearlitic steels at low temperature several years ago [2]. It is now well
established that such ageing effects in steel are due to the presence of interstitial
solutes, usually carbon and nitrogen, which generally diffuse to dislocations and
restrict their movements [3-4]. Moreover, due to the higher solubility of nitrogen
than carbon, the former is predominantly responsible for strain ageing, although
solute carbon can contribute to this phenomenon at higher temperatures [4-5].
Thus it is generally recognized that solute nitrogen is detrimental to the cold
formability properties of steel [6-7]. In order to prevent both static and strain
aging, one known approach is to add a carbonitride former such Al, B, Nb, Ti and
V [8-13]
High carbon steel wire rod, hereinafter referred to as wire rods, constitutes semi
finished hot rolled carbon steel products with a circular cross section of various
diameters (ranging from 5 mm to 13 mm or higher) in the form of coiled wire,
with a variable chemical composition depending upon the application of rods.
The high carbon wire rods are known for their very high strength and excellent
drawability and these are used in ropes for suspension bridges, armour cables,
cables for ropeways, hoisting cables in cranes, reinforcements in concrete. The
"As Rolled" high carbon wire rods show a very low ductility and thus cannot be
immediately drawn into wire. They are kept for some period (normallyl5-20
days) in the plant, in stocks, at room temperature during which they undergo
"Reverse Aging" which leads to substantial recovery of their ductility.
Because of this intervention period between the rolling of rods and drawing of
wires, the productivity rate gets slowed down. This type of aging process is
different from other ageing processes. Here, strain is not involved as in the case
of strain ageing, in particular no precipitation occurs as the material is not heated
to higher temperature followed by quenching as done in respect of quench
ageing or age hardening. The problem with reverse aging in high carbon wire
rod is a worldwide problem in steel industries. There is at present no solution
available to this problem. When hot rolled wire rods are tested just after rolling
in wire rod mill, they show a very low ductility and hence they can not be
subjected to drawing operation. To solve this problem, steel industries all over
the world typically hold the wire rods in stocks for at least 15 days where upon
by natural ageing their ductility substantially improves for example, in the form
of reduction in area taking place more than 35% . The strength remains almost
unaltered in this 'reverse ageing' phenomenon. However,such a solution gives
rise to a considerable inventory problem. Since the ductility improves over time
in room temperature,the process is generally called reverse ageing. In case of
strain ageing, the normal phenomena occur, where the strength improves but
the ductility drops over time in room temperature as well as in elevated
temperature.
Prior art is silent or having different speculative findings on the causes leading to
the phenomenon of reverse aging that occurs in high carbon wire rod. Some
literature observe that residual stress cause such problem [14] and others
attribute the cause to the rolling process itself. But none of the observations can
be technically acceptable, as the wire rods are cooled after stelmore cooling,
and the heat dissipation takes long time. If there remains any residual stress, the
same shall relax because of the slow cooling. Similarly if there is any defect in
the rolling process resulting in such phenomenon, it should also take place in
other
diameters wire rod or low carbon wire rod which does not indeed takes place in
those conditions. One investigation has found a relationship between pressure of
hydrogen with low ductility [15]. Sometimes it is also speculated that martensite
forms upon cooling of the wire rod and tempering and rearrangement of
dislocations takes place over time [16]. It is also speculated that some retained
austenite forms upon cooling which slowly transforms to bainite over time and
thus it gives improved ductility. ,
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a process to measure area
reduction of hot rolled high carbon wire rods.
Another object of the invention is to propose a process to measure area
reduction of hot rolled high carbon wire rods, which is enabled to further
determine the effect of ageing time on area reduction of the wire rods.
A still another object of the invention is to propose a process to measure area
reduction of hot rolled high carbon wire rods, which is capable to establish that it
is the hydrogen content in the steel which causes reverse ageing.
A further object of the invention is to propose a process to measure area
reduction of hot rolled high carbon wire rods, which provides the findings that
the reduction of area increases with ageing time at room temperature.
A still further object of the invention is to propose a process to measure area
reduction of hot rolled high carbon wire rods, which establishes the technical
feature that the ultimate tensile strength remains unaltered inspite of area
reduction of the hot rolled high carbon steel wire rods.
BRIEF DESCRIPTION OF THE ACCOMAPNYING DRAWINGS
Fig. 1- shows the microstructure of 5.5mm F wire rod
Fig. 2- shows the microstructure of 8.0mm F wire rod
Fig. 3- shows the microstructure of 12 mm F wire rod
Fig. 4- shows the microstructure of 17mm F wire rod
Fig. 5- a graphical representation of interlamellar spacing versus diameter of wire
rod.
Fig. 6- a graphical representation of the colony size versus the size of the rolled
wire rod.
Figure 7- shows a X-ray diffraction pattern of 5.5mm wire rod
Fig. 8- A composite X-ray profile showing the peaks generated by cementitle and
ferrite phase of various diameters of wire rods.
Fig. 9- shows a dilatometric curve for 8mmF wire rod subjected to l°C/sec
heating and cooling cycle in the step of gleeble simulation step.
Fig. 10- shows the percentage of area redction against the ageing time (days) for
5.5mm F wire rod.
Fig. 11- shows the percentage of area reduction against the ageing time (days)
for 8.00mm F wire rod.
Fig. 12- shows the relationship between the ultimate tensile strength and the
ageing time (days) for 5.5mm and 8.00mm F wire rods.
Fig. 13- shows the relationship between the hydrogen content and the ageing
time for 5.5mm, 8.00mm and 17.00mm F wire rods.
SUMMARY OF INVENTION
Accordingly, there is provided a process to measure area reduction of hot rolled
high carbon wire rods due to the effect of reverse ageing, the process
comprising the steps of providing immediately after hot rolling a plurality of wire
rod samples of different diameters having one of a hypereutectoid composition
determined through optical emission spectroscopy, and a pre-stressed concrete
wire rod composition; measuring the hydrogen content of the samples via a DH-
103 hydrogen determinator preparing said samples by adapting a known
metallographic procedure including etching in nital solution, the samples being
subjected to field emission gun scanning electron microscopy (FEGSEM) to
generate the respective micrographs followed by determining interlamellar
spacing including colony size by adapting the linear intercept technique applied
on a plurality of images which provide average values to be plotted against the
size of a finished wire rod, determining the ageing time on mechanical properties
of the samples drawn immediate after hot rolling by adapting a universal testing
machine which provide data on ultimate tensile strength (UTS) and reduction in
area in the samples at ambient temperature followed by collecting identical data
after heating the samples at about 100°C for 85 to 95 minutes, the latter
producing data on the effect of temperature; exposing the prepared samples to
an x-ray source generated by application of a cu-target in a slow scan speed and
acquiring data representing intensity, and Bragg angle (?) for defraction
including identification of the peaks generated by reflecting planes based on the
acquired data; acquiring the data relating to the phase transformation
characteristics including existence or otherwise of martensitic transformation due
to effect of heating and cooling rates where the Gleeble simulation technique
being applied; and comparing the measured data on the reduction of area
change against the ageing time including the data respecting to hydrogen
content against aging time for the plurality of samples, which provide that the
hydrogen contents against ageing time for different diameter of samples
upwardly varies, that the effect of hydrogen content causes the reverse ageing in
hot rolled high carbon wire rod, that the reduction of area increases with ageing
time at room temperature, and that the ultimate tensile strength remains
unaltered.
DETAIL DESCRIPTION OF THE INVENTION
Wire rods of 5.5 and 8 and 17 mm diameters were selected for developing the
invention, and the sample pieces were collected from wire rod mill just after hot
rolling. The compositions of the sample pieces are shown in Tables 1 and 2. The
composition of the steel was determined using optical emission spectroscopy.
The samples have hypereutectoid composition. Chromium is used for
strengthening purpose. It is known that Cr, Ni, V, Mo etc. form M3C, M6C, M23C6
[8-13] compounds in ferrite lamellae improves the strength. Ni and V, Ti, Nb are
also used for strengthening purposes. Nitrogen and carbon is found to cause
strain ageing [3-4]. Such alloying elements reduce the effect of strain aging by
scavenging nitrogen and carbon and they form carbonitrides and carbides and
eventually strengthen ferrite lamellae and improve the ductility and strength.
This steel is known as low relaxation pre-stressed concrete (LRPC) wire rod.
These are used for concrete structures for reinforcements, armour cables, crane
hoist, rope ways cables etc. However, Table 2 shows the pre-stressed concrete
(PC) wire rod composition with very little alloying elements. These are mainly
used for reinforcements. There is distinct difference between the materials
especially in terms of chemistry. Cr and other elements are in trace quantities in
PC material. However, the objective for these two types is to show that the alloy
elements especially Cr Ni, V do not play any significant role on ageing
phenomena since both type of materials show 'reverse ageing' phenomena.
After Selection and Collection of the sample pieces, their hydrogen content
immediately after hot rolling measured with the help of a DH-103 Hydrogen
determinator. The DH-103 determinator includes a resistance furnace, a thermal
conductivity cell to measure the hydrogen gas and a microprocessor with related
electronics. The sample is weighed on a suitable balance. The weight is entered
as input and the sample is placed at the mouth of reaction tube of resistance
furnace. The sample is heated to 1100°C. Hydrogen released by heating the
sample is swept out the furnace by the nitrogen carrier gas to the thermal
conductivity cell. The thermal conductivity cell has the ability to detect the
difference in the thermal conductivity of gases. The cell consists of two pairs of
matched filaments used in four legs of Wheatstone bridge. The "reference"
filaments are located in carrier gas stream at a constant pressure, flow, and
temperature environment. The "measure" filaments are also maintained in a
constant pressure, flow, and temperature environment, but the gas composition
changes as the measured gas comes through. The bridge is balanced with
nitrogen flowing in both the measure and reference chamber. The introduction of
hydrogen will cause the temperature of measure filament to decrease because
hydrogen has a higher thermal conductivity than nitrogen. The bridge becomes
unbalanced and an output is received to preamp which results in
positive reading. The amount of hydrogen determines the magnitude of the
reading. The hydrogen content is measured in ppm. The accuracy of the
determinator is ±0.1 ppm.
Simultaneously, the wire rods of various diameters have been sectioned and
ground and polished using the conventional metallographic procedures and
etched in nital solution. The samples have been examined in field emission gun
scanning electron microscopy (FEGSEM) at an operating voltage of 20kV. Figures
1 through 4 show the micrographs which interalia shows that the structure is
fully pearlitic with the spacing and colony size varying with the size of the
finished wire rod. After determining the Interlamellar spacing and colony size by
using the linear intercept technique, the average values were determined and
plotted against the size of the finished wire rod. Figure 5 shows the interlamellar
spacing against the wire rod size. It is evident from the graph that as the size
decreases, the interlamellar spacing decreases, and figure 6 shows that as the
size of wire rod decreases the colony size decreases. Although the rolling is
completed at 842°C but subsequent cooling due to stelmore, causes the
refinement of the microstructures. The higher reduction combined with faster
cooling in wire rod refines the pearlitic nucleation and growth.
Mechanical Properties of the as-received wire rods just after hot rolling and
cooling were tested in known universal tension testing machine. The reduction in
area and UTS (Ultimate tensile strength) were measured at ambient temperature
and graphs were generated. The samples were also aged in 100°C for 90
minutes and tested simultaneously to determine the effect of temperature and
ageing time on the mechanical properties of the wire rods.
The polished samples of various sizes of wire rods were exposed to X-ray
generated using a Cu target in a very slow scan speed. The monochromatic Cu
Ka radiation was used. The fluorence correction was also used. X'PERT Philips
diffractometer was used for the X-Ray study. The intensity versus 20 were
generated and plotted. Here 0 is the Bragg angle for diffraction. The peaks
generated by reflecting planes were identified form data tables using software.
An example X-Ray profile for 5.5 mm diameter including a composite plot for the
three sizes is generated.
To estimate the effect of heating rate and cooling rate on the phase
transformation behavior, the known gleeble 1500 simulations were conducted.
The simulator also can simultaneously be used for loading as well as
temperature change. However a simple heating and cooling at a specified rate
has been used in this
invention to ascertain the phase transformation characteristics. The samples
selected were 7 cm in length and the thermocouples were spot welded at the
center and a dilatometer was placed right at the center. Transformation
temperature on heating and cooling were estimated from the graphs of dilation
versus temperature curves. The objective here was to evaluate if there is any
martensitic transformation occurring during cooling and hence the Acl and Ac3
and Arl and Ar 3 were estimated. Prior art provide that in wire rods some
retained austenite may exists and this may transform to martensite or bainite
over time and eventually this can give rise to better mechanical properties.
However, such speculations are non-existing as would be evident from the
further description.
Figure 7 shows the profile of X-ray obtained from 5.5 mm diameter wire rod. The
profile obtained does not show any martensites. Generally when martensite
forms it gives rise to peak doublet but in the profile only ferrite and cementite
peaks are observed. Figure 8 shows the composite profile of X-Ray for all the
three sizes of wire rod. In all cases, the peaks remain the same except in 17 mm
diameter wire rod the first cementite peak is not observed and the reason is that
the intensity is increased and the peak is suppressed due to lower intensity.
Although the intensity of the peaks are increased but martensitic peak doublets
are not observed in any of the three profiles. It should be pointed out that during
stelmore cooling, the cooling rate of the order of 10°C/s is obtained but due to
mass effect the larger diameter wire rod undergoes slower cooling in stelmore
cooling. This absence of retained austenite or martensite is supported with
gleeble simulation carried-out by the inventors.
Figure 9 shows the measured Dilatometric curve of the 8.0 mm PC wire rod for a
heating and cooling rate of 1°C . The curve is shifted along y axis to coincide
with each other in the austenite region. Similar behavior is observed for other
heating and cooling rate, but they exhibit some deviation along both the
temperature and the length scales. The shift of the dilatometric curve along the
length scale is ascribed to the difference in magnitude of the non-isotropic effect
[17]. The transformation temperature for pearlite to austenite on heating is
found to be 705°C (Acl) and 742°C (Ac3) while cooling the transformation
temperature is 632°C (Arl) and 657°C (Ar3). The transformation on heating and
cooling is different due to the effect of superheating and supercooling which is a
known phenomenon. Similar curves were generated for 10°C/s and 40°C/s
heating and cooling rate. Tables 3 and 4 show the complete list of Acl and Ac3
and Arl and Ar3 for other heating and cooling rate. The transformation is quite
normal and martensitic transformation or retained austenite formation is not
observed. After hot rolling of the wire rod, the wire rod undergoes stelmore
cooling and the cooling rate is of the order of 10°C /s and the lay head
temperature is around 842°C and after which the wire rod is subjected to
stelmore cooling and this cooling is continued until the wire rod temperature is
around 600°C. Finally the wire rod undergoes normal cooling in air and this
makes the temperature to reach 300°C and then goes for handling and storage.
Considerable amount of time is passed during this stage and thus the residual
stress if remains, gets eliminated. Sometimes the residual stress generation is
speculated for the anamoly in ageing behavior which can be ruled out since the
temperature is moderately high enough and it stays in the heated state for quite
some time. Hence the residual stress is relaxed due to moderate temperature.
Figures 10 and 11 show the reduction of area change against the ageing time in
days. It can be seen that the reduction of area reaches a plateau state after 4
days for 5.5 mm wire rod whereas for 8 mm, the plateau is reached after 8 days.
It is widely observed that as the diameter is increased it takes a longer time for
reduction of area to recover for high value. The reduction of area for 5.5 mm
increases from 15.5 to 32% after ageing for 4-5 days. When the wire rod is
subjected to 100°C ageing for 90 minutes, it reached 35 percent immediately
after aging and it does not change appreciably. Similarly for 8 mm wire rod, the
plateau is reached after 8 days of ageing in ambient temperature. When the
samples are aged at 100°C for 90 minutes; the RA is increased to more than 30
percent and does not change appreciably over time. This time increase is higher
for higher diameter rods. Thus the transformation of any phase is ruled out and
this increase is not due to any precipitation or dislocation locking or unlocking by
interstitials since it happens at room temperature. Figure 12 shows the UTS as a
function of ageing time. Since the UTS does not change with time appreciably
hence strain aging theory or phase transformation theory can not be applied in
this case. The prior art pointed out that when strain ageing or phase
transformation occurs, the change will be reflected in the UTS value. However,
the present invention does not show any such change. Thus the possible cause
for such effect is due to the hydrogen and this is illustrated when the hydrogen
content is determined as a function of time.
Figure 13 shows the hydrogen content against aging time for the three sizes of
wire rod. It can be seen that the hydrogen content at zero days is the highest for
17 mm and correspondingly, it is the lowest for 5.5 mm diameter wire rod. The
hydrogen content drops rapidly and it levels off after few days of ageing
depending on the wire rod diameter. The hydrogen content of 17mm dia after
12 days of aging levels off whereas for 5.5 and 8 mm, it levels off after 5-6 days
of aging. Larger the diameter of the wire rod, higher is the hydrogen elimination
time. Thus the problem of reverse aging is due to the hydrogen in wire rod.
Aging the wire rod expels the hydrogen and the reduction of area is recovered
immediately after ageing. The effect of hydrogen on ductility was studied by
Costa et al [18] and they postulated that residual hydrogen can bring down the
ductility as much as 82 % when the samples are charged with Hydrogen. Similar
observation was also made by fang et al [15] where they used hydrogen analysis
and Cartography to establish the fart that hydrogen cause the transformation
from cleavage to ductile fracture. The hydrogen content in their study was of the
order of 0.6 ppm in zero ageing time and it drops to 0.2 ppm after 10-14 days.
Thus, it is established that the reverse aging is due to Hydrogen in steel. Now
hydrogen can be picked up during the steel making stage due to moistures and
such moistures can come from wet lime addition or any moist ferroalloy addition.
It has been pointed out hereinbefore that when hydrogen transforms to
molecular state it generates pressure as much as 100000 atmosphere and this
high pressure generates high hydrostatic stress and can interact with the tensile
stress causing abrupt cleavage type of fracture and such fracture is seen in the
first few days when wire rods were tested. It is also known that hydrogen
reduces the cohesive strength of the steel. This is due to the fact that Hydrogen
atom is very small in size and it can diffuse to the inter-phases or even change
atomic distance and thus the cohesive strength gets altered causing cleavage
fracture.
Atomic hydrogen H diffuses inside the material and recombines as molecular
hydrogen, H2, at specific sites. This H2 can develop high pressure at these sites.
To estimate the pressure developed by Hydrogen, the reaction [19] has been
considered:
For this reaction the equilibrium constant is considered as:

where pH=partial pressure of hydrogen.
Now since (pH) is proportional to [H], where [H] is the concentration of atomic
hydrogen so,
As per Fast [20],

1) At 1540°C i.e at 1815K, when iron is in molten state,

at this temp. [H]= 3 x 10-3 so,

2) At room temp i.e at 300K when Kh2= 1020 and [H] = 6.2 x 10-8

Thus, the pressure developed by transformation of hydrogen from atomic to
molecular state is around 100000 atmospheres at room temperature. This
intense pressure causes the failure in brittle manner when the tensile stress is
coupled with it.
The same calculation when carried out for nitrogen, such a high pressure is not
generated. For example, for nitrogen if the equilibrium concentration at room
temperature is around 1.4 x 10-5 and the pressure that is generated, is around 1
atmosphere. Thus nitrogen does not cause the ageing problem in wire rod. The
influence of nitrogen on embrittlement is due to formation of iron nitride.
Based on the results and discussions, it is established by using the dilatometry
and x-ray results that there is no retained austenite that can transform to
martensite or bainite since the data show normal transformation temperature on
heating and cooling. Also, no direct martensite formation is observed in the x-ray
data and thus the low ductility is not due to these factors. The profiles obtained
show that there is no martensite or retained austenite in the steel. The reverse
aging is due to the presence of excess hydrogen in steel after rolling operation.
The UTS of various wire rods does not change and the reduction area is the
lowest just after hot rolling. The hydrogen content changes with time, and in
zero aging time, the hydrogen content is the highest and it drops drastically in
the first few days and levels off after many days. Thus this excess hydrogen
causes low ductility and the ductility is recovered due to the elimination of
hydrogen from the steels either at ambient temperature or at a higher
temperature of ageing. The reasons for low ductility are two folds. One is due to
the fact that hydrogen reduces the cohesive strength of the steel and the
transformation from atomic state to molecular state generates intense pressure
which when coupled with tensile stress cause cleavage or mixed type of fracture
of the steels depending on the ageing time. Nitrogen can not cause such
problem since the pressure generated by atomic to molecular state is around one
atmosphere. High carbon wire rod is prone to such problems and this is due to
the fact that excess hydrogen hides in lamellae interphase and colony
boundaries. Since pearlite occupies the space fully, this provides excess sites for
hydrogen atoms. The alloy chemistry such as Cr, V, Al etc does not play any role
since the PC grade which has very low alloy content and LRPC grade which has
higher alloy content both respond to reverse aging.
6.0. References
1. S. Tagushira, K. Sakai, T. Furuhara, T. Maki, ISIJ International, Vol. 40
(2000), p. 1149.
2. J. Languillaume, G. Kapelski, B. Baudelet, Mater. Lett., Vol. 33 (1997),
p. 241.
3. ID. Baird, Metall. Rev., Vol. 16, 1971, pp. 1-18
4. W.C. Leslie, The Physical Metallurgy of Steel, McGraw-Hill, New York,
N.Y, 1983, pp.68-109
5. D.V. Wilson and B. Russel, Acta Metall., Vol. 8, 1960, pp 36-45
6. I.D. Mclvor, Ironmaking and Steelmaking, vol. 16, No. 1, 1989, pp. 55-
62
7. W.B. Morrison, Ironmaking and Steelmaking, Vol. 16, No2, 1989, pp.
123-128
8. D.T. Llewellyn, Ironmaking and Steelmaking, Vol. 20, No. 1, 1993, pp.
35-41
9. C. Weidig et al, Wire Journal International, Vol. 28, No. 1, 1995, pp. 82-
85
10. F.B. Pickering, B. Garbarz, Mater. Sci. Tech., Vol. 5 (1989), p. 227
11. G.L Dunlop, C.J. Carlsson, G. Frimodig, Met. Trans., Vol. 9A (1978), p.
261
12. F.A. Khalid and D.V. Edmonds, Scripta Metall, Vol. 30 (1994), P. 1251
13. D.T. Gawne, Mat. Sci & Tech, Vol. 1, August 1985, pp. 583-592.
14. D. Roy, MS thesis, Calcutta University, 1996.
15. H. Xianjun, C. Shaohui, F. Feng, J. I. Jian, Private communication.
16. R. Bhattacharya, G. Jha, Internal Report, TATA Steel R&D, 2005.
17. D.W. Suh, C.S. Oh, H.N. Han, S.J. Kim, Acta Mater., 55, 2007, pp.
2659-2669
18. J.E. Costa, A.W. Thompson, Met. Trans A, Vol. 13A, July 1982, pp 1315.
19. D. Seferian, The Metallurgy of Welding, Chapman and Hall, London,
1962.
20. J.D. Fast, Philips Research Reports, 1950, 5, 37.
We Claim:
1. A process to measure area reduction of hot rolled high carbon wire rods due
to the effect of reverse ageing, the process comprising the steps of:
- providing immediately after hot rolling a plurality of wire rod samples
of different diameters having one of a hypereutectoid composition
determined through optical emission spectroscopy, and a pre-stressed
concrete wire rod composition;
- measuring the hydrogen content of the samples via a DH-103
hydrogen determinator;
- preparing said samples by adapting a known metallographic procedure
including etching in nital solution, the samples being subjected to field
emission gun scanning electron microscopy (FEGSEM) to generate the
respective micrographs followed by determining interlamellar spacing
including colony size by adapting the linear intercept technique applied
on a plurality of images which provide average values to be plotted
against the size of a finished wire rod;
- determining the ageing time on mechanical properties of the samples
drawn immediate after hot rolling by adapting a universal testing
machine which provide data on ultimate tensile strength (UTS) and
reduction in area in the samples at ambient temperature followed by
collecting identical data after heating the samples at about 100°C for
85 to 95 minutes the latter producing data on the effect of
temperature;
- exposing the prepared samples to an x-ray source generated by
application of a cu-target in a slow scan speed and acquiring data
representing intensity, and Bragg angle (?) for defraction including
identification of the peaks generated by reflecting planes based on the
acquired data;
- acquiring the data relating to the phase transformation characteristics
including existence or otherwise of martensitic transformation due to
effect of heating and cooling rates where the Gleeble simulation
technique being applied; and
- comparing the measured data on the reduction of area change against
the ageing time including the data respecting to hydrogen content
against aging time for the plurality of samples, which provide that the
hydrogen contents against ageing time for different diameter of the
samples upwardly varies, that the effect of hydrogen content causes the
reverse ageing in hot rolled high carbon wire rod, that the reduction of
area increases with ageing time at room temperature, and that the
ultimate tensile strength remain unaltered.
2. A process to measure area reduction of hot rolled high carbon wire rods due
to the effect of reverse ageing, as substantially described and illustrated herein
with reference to the accompanying drawings.

Effects of ageing time on area reduction of hot rolled high carbon steel wire rods
were studied. Tensile testing and X-ray study of as rolled wire rods were carried
out. Gleeble simulation and hydrogen content determination were also
conducted. The results show that the reduction of area increases with ageing
time at room temperature and the UTS remain unchanged which are contrary to
normal ageing or strain ageing. In normal ageing, the ductility drops and the
yield strength increases. In this study, the gleeble simulation and x-ray data
support that the transformation from pearlite to austenite is normal and there is
no evidence of retained austenite or martensitic transformation in the steel. The
hydrogen content drops as the time passes. The drop is rapid in first few days
and this drop increases the ductility in rolled high carbon wire rod. Hydrogen
reduces the cohesive strength and the pressure generated due to transformation
of atomic hydrogen to molecular state combines with tensile stress and causes
cleavage or mixed type of fracture.