FIELD OF THE INVENTION
The present invention relates to high strength steel with superior resistance to hydrogen
induced cracking (HIC) and sulphide stress corrosion cracking (SSC) and a method of
producing such steel. More particularly, the present invention provides for selective copper
alloyed high strength steel with higher HIC and SSC resistance for pipe line application for
transporting crude oil or natural gas, adapted to avoid diffusion of hydrogen, which occurs
through autocatalytic regeneration of hydrogen ions from the adsorption film formed on
the steel surface and change the film characteristics to resist HIC and SSC failure when
exposed to sour gas environment. Importantly, the 0.26% Cu added API X65 steel
composition resulted in CSR, CTR & CLR of 0.2%, 0.7% and 2.0% respectively which are
substantially lower than the acceptable limit for Type I class material as per BS EN-10028-
3:2009 for sour gas application confirming HIC inhibition. Also, the Cu added steel
demonstrated resistance to SSC by achieving threshold stress (sth) for failure in the range
of 62-64% of yield stress.
BACKGROUND OF THE INVENTION
High strength line pipe steels used for transporting crude oil or natural gas, containing
hydrogen sulfide, are required to posses "sour-resistance" comprising hydrogen induced
cracking (HIC) resistance and resistance to stress corrosion crack (SCC) resistance, in
addition to adding to strength, toughness, and weldability. The phenomenon of hydrogen-
induced cracking (HIC) of steel is based on a process in which hydrogen ions generated by
corrosion reaction are adsorbed on the surface of steel, penetrate into steel as atomic
hydrogen, and diffuse and accumulate around non-metallic inclusions such as manganese
sulphide (MnS) and hard second phase of steel, thus triggering crack initiation by an
increase in internal pressure. Hence, research has been focused to control the morphology
and distribution of MnS inclusions in steel and also to reduce the sulphur (S) level to
<0.002 wt% in steel. However, reducing S to <0.002 wt% in industrial level is very
difficult task for steel makers. Diffusion of hydrogen occurs through autocatalytic
regeneration of hydrogen ions from the adsorption film formed on the steel when exposed
to sour gas environment. Altering the film will be viable route to control HIC. Therefore
the present work is an attempt to evolve an alloying addition, which can change the film
characteristics and resist for HIC and SSC failure.
HIC and SSC resistance of API X65 (having the composition of C: 0.08%, Mn:1.4%,
S:0.009%, P:0.01%, Si:0.31%, V:0.05%, Nb: 0.043%, Ti:0.018%) and API X70 steels
(having the composition of C: 0.09%, Mn:1.4%, S:0.009%, P:0.01%, Si:0.38%,
V:0.048%, 1Mb: 0.047%, Ti:0.022%) commercially produced from integrated steel plant
have been evaluated. Both API X65 and X70 steels were found to exhibit ferrite-pearlite
microstructures with an average grain size of 5-8 urn for API X70 and 10-15 urn for API
X65. These steels were found to conform to API 5LX specification in terms of yield
strength; API X65 exhibited an yield strength of 78,667 psi as against 83,101 psi for API
X70 steel.
The electrochemical corrosion behaviour of these steels was also investigated in aqueous
hydrogenated solution of 7N H2SO4 + 1 g/l thiocarbamide through Tafel polarization
experiments and electrochemical impedance spectroscopy (EIS). The API X65 and API X70
steels were found to exhibit moderately high corrosion rates of 128 and 93 mpy, with
charge transfer (or, polarization) resistances of the order of 110 and 132 O.cm2,
respectively. Electrochemical impedance spectra showed the inductive behaviour that
corresponds to a chemi-adsoption film (FeHS"ads) formed at the surface of the steel. This
complex FeHS'ads film decomposes/auto-catalytically regenerates the H2S at the surface of
the steel following the equations no. 1, 2 and 3 as given below.

The HIC performance of the above commercially available grades of steels was also
evaluated in sour gas (H2S) environment in accordance with NACE TM 0284 test method.
The tests revealed crack length ratio (CLR), crack thickness ratio (CTR) and crack
sensitivity ratio (CSR) values of 57.7%, 6.7% and 0.77% for API X65 steel, while the CLR,
CTR and CSR values for API X70 steel were determined to be 58%, 18.9% and 1.4%,
respectively. These CLR and CTR values were found to be substantially higher than the
acceptable limit stipulated for Type III class material for sour gas application as per British
Standard BS EN 10028-3:2009 (Type III class material should have CLR < 15%, CTR <
5% and CSR < 2%). Nevertheless, the more stringent CSR values for both steels were
found to be well within the specified limit under Type III class for sour gas application. In
SSC tests conducted as per NACE TM 0177 Method A in sour gas environment, the
threshold stress (sth) for failure was evaluated to be 51% of yield stress for API X65 steel,
while that for API X70 was estimated to be about 48% of its yield stress. Fractrographs
of fractured surfaces of SSC samples showed intergranular quasi cleavage type of
cracking, which is a typical characteristic of brittle fracture. The above stated inadequate
performance of the available grades of steels in hydrogenated and sour gas environments
has been attributed to high sulphur content (~0.01 wt %) of the steels.
US 20110253267 disclosed a high-strength steel pipe having a strength of API X65 grade
or higher consisting essentially of, by mass %, 0.02 to 0.08% of C, 0.01 to 0.5% of Si, 0.5
to 1.8% of Mn, 0.01% or less of P, 0.002% or less of S, 0.01 to 0.7% of Al, 0.005 to
0.04% of Ti, 0.05 to 0.50% Mo, at least one element selected from 0.005 to 0.05% of Nb
and 0.005 to 0.10% of V, and the balance being Fe, in which the volume percentage of the
ferritic phase is 90% or higher, and complex carbides containing Ti, Mo, and at least one
element selected from the group consisting of Nb and V are precipitated in the ferritic
phase. The high-strength steel pipe has excellent HIC resistance and good toughness of a
heat-affected zone, and can be manufactured stably at a low cost.
US 7959745 disclosed a high-strength steel pipe having a strength of API X65 grade or
higher consisting essentially of, by mass %, 0.02 to 0.08% of C, 0.01 to 0.5% of Si, 0.5 to
1.8% of Mn, 0.01% or less of P, 0.002% or less of S, 0.01 to 0.7% of Al, 0.005 to 0.04%
of Ti, 0.05 to 0.50% Mo, at least one element selected from 0.005 to 0.05% of Nb and
0.005 to 0.10% of V, and the balance being Fe, in which the volume percentage of the
ferritic phase is 90% or higher, and complex carbides containing Ti, Mo, and at least one
element selected from the group consisting of Nb and V are precipitated in the ferritic
phase. The high-strength steel pipe has excellent HIC resistance and good toughness of a
heat-affected zone, and can be manufactured stably at a low cost.
EP2407570 disclosed a thick steel plate and a UOE steel pipe having a high strength of at
least X60 grade and improved resistance to HIC have a chemical composition consisting
essentially of, in mass percent, C: 0.02 - 0.07%, Si: 0.05 - 0.50%, Mn: 1.1 - 1.6%, P: at
most 0.015%, S: at most 0.002%, Nb: 0.005 - 0.060%, Ti: 0.005 - 0.030%, Al: 0.005 -
0.06%, Ca: 0.0005 - 0.0060%, N: 0.0015 - 0.007%, at least one of Cu, Ni, Cr, and Mo in
a total amount of greater than 0.1% and less than 1.5%, and a remainder of Fe and
impurities, wherein the degrees of segregation of Nb and Ti are both at most 2.0, and the
ratio of (the degree of Nb segregation) / (the degree of Mn segregation) and the ratio of
(the degree of Ti segregation) / (the degree of Mn segregation) are both at least 1.0 and
at most 1.5.
JP-A 2006-63351 proposed a method of defining the size of the MnS inclusion to be the
initiation point of HIC and the hardness of the center segregation area.
Japanese Patent Publication No. 7-216500 proposed that the high strength steel should
be Ca treated to avoid MnS inclusion formation.
Japanese Patent Publication No. 54-110119 disclosed superior hydrogen induced cracking
resistance of high strength line pipe steel achieved by adding Ca or Ce in proper amounts
relative to the amount of S, and forming fine spherical inclusions to decrease stress
concentration instead of formation of needle-like MnS inclusions.
Japanese Patent Publication No. 61-227129 states about high strength steel developed
with superior sulphide stress corrosion cracking resistance behavior. This was achieved by
controlling or refining the microstructure with addition of micro alloying element. The SCC
resistance and HIC resistance are improved by ferritic microstructure and Mo orTi is added
to utilize precipitation strengthening by carbides.
Japanese Patent Publication No. 7-173536 disclosed a steel plate having a strength of API
X80 grade or higher, in which the shape of inclusions is controlled by adding Ca to a low-S
steel, center segregation is restrained by lower contents of C and Mn, and high strength is
provided by the addition of Cr, Mn and Ni and accelerated cooling.
Japanese Patent Publication No. 61-165207 disclosed a method of manufacture of steel
with excellent sour-resistant property was under taken by formation of island-like
martensite that functions as an origin of cracking in a center segregation region and hard
phases such as martensite or bainite that function as a propagation path of cracking is
restrained by a decrease in amount of segregation-prone elements (C, Mn, P, etc.),
soaking treatment at a stage of slab heating, accelerated cooling during transformation at
a stage of cooling, etc.
EP2392681 (Al) disclosed a thick-walled high-strength hot rolled steel sheet having
excellent hydrogen induced cracking resistance which is preferably used as a raw material
for a high-strength welded steel pipe of X65 grade or more and a method of
manufacturing the thick-walled high-strength hot rolled steel sheet are provided. To be
more specific, the composition of the thick-walled high-strength hot rolled steel sheet
contains by mass% 0.02 to 0.08% C, 0. 50 to 1.85% Mn, 0.03 to 0.10% Nb, 0.001 to
0.05% Ti, 0.0005% or less B in such a manner that (Ti+Nb/2)/C<4 is satisfied or also
contains one or two kinds or more of 0.010% or less Ca, 0.02% or less REM, and Fe and
unavoidable impurities as a balance. The steel sheet has the structure formed of a bainitic
ferrite phase or a bainite phase. Surface layer hardness is 230HV or less in terms of
Vickers hardness.
The above-described existing prior art for manufacturing alloy steel resistant to HIC and
SSC in sour gas media thus relate to one or more of following methodology:
(a) Controlling microstructure of the steel.
(b) Controlling Ca to S ratio in steel.
(c) Addition of some alloying elements like Ti and Mo.
(d) Morphology of MnS inclusion by adding Ca and Ce in steel.
(e) Control over center segregation through lowering of C and Mn content.
(f) Distribution of MnS inclusion in steel.
However, none of the above described prior arts attempted to control auto-catalytically
generation of H+ at the surface of steel and alter the surface reaction (avoiding the
formation of adsorption film) to restrict the hydrogen ingress into steel, thereby increasing
the corrosion resistance.
There has been therefore a need in the art to developing high strength steel with improved
resistance to hydrogen induced crack and sulphide stress corrosion crack so as to ensure
safe and reliable transportation of crude oil or natural gas containing hydrogen sulfide
through steel pipe line without failure due to HIC or SSC during operation in sour gas
environment. In order to explore the possibility of improving the HIC and SSC
performance of high strength steel, different alloy compositions with selective alloy
additions were studied by way of the present invention, particularly with relatively higher
sulphur content, for evaluating and achieving improved performance in respect of strength
as well as sour resistance directed to alter the surface reaction avoiding the formation of
adsorption film and restrict hydrogen diffusion in steel.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to providing high strength alloy
steel composition with improved HIC and SSC crack resistance for safe and reliable
transportation of crude oil and natural gas in sour environment and a method of producing
such steels.
A further object of the present invention is directed to providing high strength alloy steel
composition with improved HIC and SSC crack resistance which is produced by selective
Cu addition in API X65 grade steel even with higher S content for use in pipe line for
desired strength properties and resistance to sour environment.
A still further object of the present invention is directed to providing high strength alloy
steel composition with improved HIC and SSC crack resistance which would ensure crack
length ratio (CLR), crack thickness ratio (CTR) and crack sensitivity ratio (CSR) within
permissible limits as per the applicable standards.
A still further object of the present invention is directed to providing high strength alloy
steel with improved HIC and SSC crack resistance which would be produced following a
method comprising casting, soaking, rolling in first phase, resoaking, rolling in second
phase and air cooling, to ensure ferrite-pearlite microstructure with higher grain size.
Yet another object of the present invention is directed to providing high strength line alloy
with improved HIC and SSC crack resistance which would achieve significantly lower
corrosion rates and superior charge transfer resistances.
A still further object of the present invention is directed to providing high strength alloy
steel with improved HIC and SSC crack resistance such that the selective Cu alloyed steel
shows higher polarisation resistance and chemisorption film (FeHSads) which is responsible
for auto-catalytically generation of H+ at the surface is not formed on the surface favoring
attaining sour resistance.
A still further object of the present invention is directed to providing high strength line pipe
steel with improved HIC and SSC crack resistance such that in the selective Cu alloyed
steel, Cu alters the surface reaction (avoiding the formation of adsorption film) and
restricts the hydrogen ingress into steel, thereby increasing the corrosion resistance.
A still further object of the present invention is directed to providing high strength line pipe
steel with improved HIC and SSC crack resistance such that in the selective Cu alloyed
steel, Cu restricts the diffusion of H+ and alter the fracture morphology from intergranular
quasi cleavage failure to mixed mode type of failure.
A still further object of the present invention is directed to providing high strength line pipe
steel with improved HIC and SSC crack resistance such that threshold stresses of SSC
failure for Cu-added steels is substantially improved to be about 63% of yield strength.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to alloy steel composition
resistant to HIC and SSC in sour gas media comprising of:

A further aspect of the present invention is directed to said alloy steel composition
which is hydrogen Induced cracking resistant having CLR in the range of 2 to 6.5%,
CTR in the range of 0.7 to 2.5 % and CSR in the range of 0.5 to 0.9%.
A still further aspect of the present invention is directed to said alloy steel composition
preferably having Cu 0.26 and 0.36 by wt%.
A still further aspect of the present invention is directed to said alloy steel composition
having tensile properties comprising yield strength in the range of 65,300 to 68,000
psi, ultimate tensile strength in the range of 80,000 to 85,000 psi and elongation in
the range of 28 to 29%.
Yet another aspect of the present invention is directed to said alloy steel composition
having a ferrite-pearlite microstructure with grain size in the range of about 20 to
25um.
A further aspect of the present invention is directed to said alloy steel composition
which is sulphide stress corrosion cracking resistant having a threshold stress (sth) for
failure in the range of 62-64% of yield stress.
According to an important aspect of the present invention directed to said Alloy steel
composition wherein said level of Cu addition in the range (0.1 to 0.36 wt %)
selectively restricts the formation of adsorption film (FeHSads) and restricts the
hydrogen diffusion into steel, thereby increasing HIC and SSC resistance in sour
environment.
A further aspect of the present invention is directed to a process for manufacture of
alloy steel resistant to HIC and SSC in sour gas media as described above comprise of
obtaining the alloy steel selectively involving a steel composition having:

A still further aspect of the present invention is directed to a process for manufacture of
alloy steel resistant to HIC and SSC in sour gas media comprising the steps of:
(i) reheating the steels ingots/slabs and soaking in a reheating furnace to 1150
to 1250 °C for a period of 1 to 5 hours preferably about 3 hours;
(ii) hot-rolling to plate of 15 to 18 mm preferably about 16 mm with finish
rolling temperatures in the range of preferably about 830 °C; followed by ,
(iii) final rolling reduction and air cooling is done after hot rolling.
The various objects and advantages of the present invention are described in greater
details with reference to the following accompanying non limiting illustrative drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: illustrate the Microstructure of (a) API X65 and (b) API X70 steels showing
ferrite-pearlite structure of different grain size;
Figure 2: shows Electrochemical Tafel polarization plots of API X65 and API X70 steels in
hydrogenated sour gas environment (7N H2SO4 + 1 g/l thiocarbamide solution).
Figure 3: illustrate the Electrochemical Impedance spectroscopy spectra of API X 65 and
API X70 steels in hydrogenated sour gas environment (7N H2SO4 + 1 g/l thiocarbamide
solution) showing an inductive loop in low frequency region attributable to chemisorptions
film characteristics(a) Nyquist plots (b) Bode plots and (c) Equivalent circuit.
Figure 4: illustrates Typical stepwise microcracks observed after HIC test of (a) API X65
steel and (b) API X70 steel as per NACE TM 0284
Figure 5: illustrates Sulphide stress corrosion cracking (SSC) studies showing the
threshold stress (ath) for failure of API steels in sour gas environment as per NACE TM
0177 Method A.
Figure 6: illustrates Fractographs of fractured surface of (a) API X65 and (b) API X70
steels after SSC test showing intergranular quasi cleavage type of cracking confirming
brittle failure.
Figure 7: illustrates the Microstructure of Cu-added API X65 steels (a) 0.1% Cu-added API
X65 (b) 0.2% Cu-added API X65 and (c) 0.3% Cu-added API X65 showing ferrite-pearlite
structure of average grain size of about 20 to 25 urn.
Figure 8: illustrates the Electrochemical Tafel polarization plots of Cu-added API X 65
steels in hydrogenated sour gas environment (7N H2SO4 + 1 g/l thiocarbamide solution)
showing shift in corrosion potential towards noble potential and lowering corrosion current
with addition of Cu.
Figure 9: illustrates the Electrochemical Impedance spectroscopy spectra of Cu-added API
X 65 steels in hydrogenated sour gas environment (7N H2SO4 + 1 g/l thiocarbamide
solution) showing improvement in impedance of the passive film with addition of Cu (a)
Nyquist plot (b) Bode plots (c) Equivalent circuit of API X65 steel and (d) Equivalent circuit
of Cu-added API X65 steels.
Figure 10: illustrates the Microcracks observed after HIC test (a) 0.1% Cu-added API X65
steel (b) 0.2% Cu-added API X65 steel and (b) 0.3% Cu-added API X65 steel as per NACE
TM 0284 showing addition of cu has improved the HIC performance.
Figure 11: illustrates the Sulphide stress corrosion cracking (SSC) studies as per NACE TM
0177 Method A showing cu addition has increased the threshold stress (sth) for failure in
sour gas environment.
Figure 12: illustrates the Fractographs of fractured surface of Cu added API steels in SEM
(a) 0.1% Cu-added API X65 steel (b) 0.2% Cu-added API X65 steel and (c) 0.3% Cu-
added API X65 steel as per NACE TM 0284 showing mixed mode of failure (brittle and
ductile) with increasing Cu content.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention is directed to providing high strength line pipe steel with improved
HIC and SSC crack resistance. Importantly, the present invention is directed to providing a
selective alloying addition to conventional composition of high strength pipe line steel of
API X65 grade, which can change the characteristics of the chemi-adsoption film (FeHSads)
formed at the surface of steel and resist for HIC and SSC failure.
In order to evaluate the advantageous effect of the selective alloy addition on HIC and
SSC resistance, initial HIC and SSC resistance of API X65 and API X70 steels commercially
produced from integrated steel plant have been evaluated. The compositions of these
conventional grades are listed in accompanying Table-1. Both API X65 and X70 steels
were found to exhibit ferrite-pearlite microstructures with an average grain size of 5-8 urn
for API X70 and 10-15 um for API X65 as illustrated in accompanying Figure 1. The steels
were found to conform to API 5LX specification in terms of yield strength; API X65
exhibited an yield strength of 78,667 psi as against 83,101 psi for API X70 steel as
summarized in following Table-2.
The electrochemical corrosion behaviour of these steels was also investigated in aqueous
hydrogenated solution of 7ISI H2SO4 + 1 g/l thiocarbamide through Tafel polarization
experiments illustrated in accompanying Figure 2, and electrochemical impedance
spectroscopy (EIS). The API X65 and API X70 steels were found to exhibit moderately high
corrosion rates of 128 and 93 mpy, with charge transfer (or, polarization) resistances of
the order of 110 and 132 O.cm2, respectively as summarized in following Table-3 & 4.
Electrochemical impedance spectra for the above steel grades as illustrated in
accompanying Figure 3 showed the inductive behaviour corresponding to a chemi-
adsorption film (FeHSads) formed at the surface of the steel.
The HIC performance of the steels was evaluated in sour gas (H2S) environment in
accordance with NACE TM 0284 test method illustrated in accompanying Figure 4. The
tests revealed crack length ratio (CLR), crack thickness ratio (CTR) and crack sensitivity
ratio (CSR) values of 57.7%, 6.7% and 0.77% for API X65 steel, while the CLR, CTR and
CSR values for API X70 steel were determined to be 58%, 18.9% and 1.4%, respectively
as summarized in following Table-5. These CLR and CTR values were found to be
substantially higher than the acceptable limit stipulated for Type III class material for sour
gas application as per British Standard BS EN 10028-3:2009 (Type III class material
should have CLR < 15%, CTR < 5% and CSR < 2%). Nevertheless, the more stringent
CSR values for both steels were found to be well within the specified limit under Type III
class for sour gas application. In SSC tests conducted as per NACE TM 0177 Method A in
sour gas environment as illustrated in accompanying Figure 5, the threshold stress (sth)
for failure was evaluated to be 51% of yield stress for API X65 steel, while that for API X70
was estimated to be about 48% of its yield stress. Fractrographs of fractured surfaces of
SSC samples as illustrated in accompanying Figure 6 showed intergranular quasi cleavage
type of cracking, which is a typical characteristic of brittle fracture. This inadequate
performance in hydrogenated and sour gas environments has been attributed to high
sulphur content (~0.01 wt %) of the steels.
In order to explore the possibility of improving the HIC and SSC performance, laboratory
heat have been undertaken with controlled additions of copper (Cu) at levels of approx
0.1, 0.2 and 0.3 wt% Cu to API X65 steel with 0.01 wt% S. The advantageous aspects of
the present invention comprising high strength steel with HIC and SSC resistant properties
are illustrated as per the following Example I:
Example I:
According to an embodiment of the present invention, the heats of specified compositions
with approximately 0.1% Cu, 0.2% Cu and 0.3% Cu have been made in 100 kg air
induction furnace. The molten steel is cast into 100 mm square cross-sectioned 25 kg
ingots. Three ingots of different Cu compositions are obtained. The top and bottom end of
the ingots are cropped to exclude the pipe and other solidification defects. The ingots are
subsequently reheated and soaked in a furnace to around 1200 °C for 3 hours and then
hot-rolled in experimental rolling mill to 16 mm plate with finish rolling temperatures of
830 °C. The hot rolling is then carried out in two phases. In the first phase, 100 mm
square cross-sectioned ingots are hot-rolled to 30 mm plates using 5-pass draft sequence,
100-95-72-55-40-30 mm. In the second phase, 30 mm plates are further hot-rolled to 16
mm strips using 3-pass draft schedule, 30-28-24-16 mm, after re-soaking at 1200 °C for
30 min. The final rolling reduction achieved is about 33%. After rolling, the plates are
allowed to air cool. Following Table-6 gives the chemical composition of experimental
steels produced under laboratory conditions, Microstructure of Cu-added API X65 steel also
shows the similar ferrite-pearlite microstructure but with higher grain size of about 20 to
25pm as illustrated in accompanying Figure 7. The mechanical properties of the steels
are evaluated using Instron machine - Model 1195 and the results are tabulated in
following Table-7.
Tafel polarization experiments as illustrated in Figure 8 and electrochemical impedance
spectroscopy (EIS) are carried out in hydrogenated test environment of 7N H2SO4 + 1 g/l
thiocarbamide solution and results are tabulated in following Table -8. The 0.14, 0.26 and
0.36 wt% Cu-added steels exhibited significantly lower corrosion rates of 25, 16 and 14
mpy and superior charge transfer resistances of 480, 640 and 708 O.cm2, respectively.
It is clear that Cu-added API X65 steel showed higher polarisation resistance and corrosion
resistance than that of API X65. Interesting thing to note here is that, Cu-added API X65
steel did not exhibit any negative loop near the lower frequency region of Nyquist plot as
illustrated in Figure 9, which corresponds to absence of inductance behaviour. Therefore,
it is clear that chemisorption film (FeHSads) which is responsible for auto-catalytically
generation of H+ at the surface is not formed on the surface of Cu -added API X65 steel.
Hence, it is concluded that Cu alters the surface reaction (avoiding the formation of
adsorption film) and restricts the hydrogen ingress into steel, thereby increasing the
corrosion resistance.
Accompanying Figure 10 shows the optical micrographs of the HIC tested samples, from
which CLR, CSR and CTR are determined and tabulated in following Table-9. 0.14% Cu-
added API X65 showed few cracks, whereas with increasing Cu content in the steels, the
propensity of HIC cracks reduced substantially. From these studies, it is concluded that
the CLR, CTR and CSR of the 0.26 wt % Cu-added steel are significantly lower than the
level specified for sour gas application Type 1 class materials as per British standards BS-
EN 10028-3-2009. Hence, Cu additions have proved to be immensely beneficial for
improving HIC performance of API X65 steel for sour gas application by altering the
surface film and reducing the diffusion of hydrogen.
In order to substantiate the mechanism, cylindrical pin samples of both API X65 steel and
Cu-added steels of sixe 4 X 3mm are immersed in the NACE solution along with the HIC
sample. After the test duration of 96hr the samples are cleaned with alkaline solution and
analysed for hydrogen content using hydrogen analyzer. Following Table-10 shows the
total hydrogen content of the steels, before and after immersion in the NACE test solution
and also the diffusible hydrogen content derived. From the table it is inferred that Cu
addition has inhibited the hydrogen diffusion into the experimental steels.
The SSC experiments are carried out on Cu-added API-X65 steels for various percentages
of yield strength as per NACE TM 0177 Method A. The corresponding time to failure is
recorded as illustrated in accompanying Figure 11. The SSC test is carried out for a
maximum period of 720 h or 30 days. From Figure 11 it is clear that among the Cu-added
steels there is no significant difference in the SSC behaviour. The threshold stresses of
Cu-added API-X65 steels are almost found to be 63% of yield strength. For comparison,
the SSC data of API X65 steel is also given in Figure 11. It is clearly evident that Cu has
drastically improved the SSC performance of API X65 steel.
Accompanying Figure 12 shows the fractrographs of fractured surfaces of Cu-added API
X65 steels. From the figure, it is clear that 0.14% Cu-added API X65 steel shows
intergranular quasi cleavage failure, which corresponds to brittle failure. On increasing the
Cu content to 0.26%, the fracture morphology changes to a mixed mode of facture
comprising, river like patterns for brittle and dimples for ductile behavior. It has been
finally deduced that Cu restricts the diffusion of H+ and alter the fracture morphology from
intergranular quasi cleavage failure to mixed mode type of failure.
It is thus possible by way of the present invention to providing high strength alloy steel
composition with improved HIC and SSC crack resistance for pipe line application for
transporting crude oil or natural gas containing hydrogen sulfide. Importantly, the present
invention is directed to providing a selective alloying composition involving Cu addition
which can change the characteristics of the chemi-adsoption film (FeHSads) formed at the
surface of steel to inhibit hydrogen ingress and resist for HIC and SSC failure. Significantly
also, the 0.26% Cu added API X65 steel composition resulted in CSR, CTR & CLR of 0.2%,
0.7% and 2.0% respectively which are substantially lower than the acceptable limit for
Type I class material as per BS EN-10028-3:2009 for sour gas application confirming HIC
inhibition. Also, the Cu added steel demonstrated resistance to SSC by achieving threshold
stress (sth) for failure in the range of 62-64% of yield stress.
We Claim:
1. Alloy steel composition resistant to HIC and SSC in sour gas media comprising of :
C (0.06 to 0.08%) by wt.;
Mn (1.35 to 1.5%) by wt;
S (up to 0.01%) by wt;
P (up to 0.01%) by wt;
Si (0.25 to 0.45%) by wt;
V (0.02 to 0.035%) by wt;
Nb (0.4 to 0.5%) by wt;
Ti (0.003 to 0.008%) by wt; and
Cu (0.1 to 0.36%) by wt along with balance being Fe.
2. Alloy steel composition as claimed in claim 1 which is hydrogen Induced cracking
resistant having CLR in the range of 2 to 6.5%, CTR in the range of 0.7 to 2.5
% and CSR in the range of 0.5 to 0.9%.
3. Alloy steel composition as claimed in anyone of claims 1 or 2 preferably having Cu
0.26 and 0.36 by wt%.
4. Alloy steel composition as claimed in anyone of claims 1 to 3 having tensile
properties comprising yield strength in the range of 65,300 to 68,000 psi, ultimate
tensile strength in the range of 80,000 to 85,000 psi and elongation in the range
of 28 to 29%.
5. Alloy steel composition as claimed in anyone of claims 1 to 4 having a ferrite-
pearlite microstructure with grain size in the range of about 20 to 25µm.
6. Alloy steel composition as claimed in anyone of claims 1 to 5 which is sulphide
stress corrosion cracking resistant having a threshold stress (sth) for failure in the
range of 62-64% of yield stress.
7. Alloy steel composition as claimed in anyone of claims 1 to 6 wherein said level of
Cu addition in the range (0.1 to 0.36 wt %) selectively restricts the formation of
adsorption film (FeHSads) and restricts the hydrogen diffusion into steel, thereby
increasing HIC and SSC resistance in sour environment.
8. A process for manufacture of alloy steel resistant to HIC and SSC in sour gas media
as claimed in anyone of claims 1 to 7 comprising of :
obtaining the alloy steel selectively involving a steel composition having:
C (0.06 to 0.08%) by wt.;
Mn (1.35 to 1.5%) by wt;
S (up to 0.01%) by wt;
P (up to 0.01%) by wt;
Si (0.25 to 0.45%) by wt;
V (0.02 to 0.035%) by wt;
Nb (0.4 to 0.5%) by wt;
Ti (0.003 to 0.008%) by wt; and
Cu (0.1 to 0.36%) by wt along with balance being Fe.
9. A process for manufacture of alloy steel resistant to HIC and SSC in sour gas media
as claimed in claim 8 comprising the steps of:
(i) reheating the steels ingots/slabs and soaking in a reheating furnace to
1150 to 1250 °C for period of 1 to 5 hours preferably about 3 hours;
(ii) hot-rolling to plate of 15 to 18 mm preferably about 16 mm with finish
rolling temperatures in the range of preferably about 830 °C; followed
by,
(iii) final rolling reduction and air cooling is done after hot rolling.
10. Alloy steel composition resistant to HIC and SSC in sour gas media and a process
for its manufacture substantially as hereindescribed and illustrated with reference to the
accompanying examples and figures.

ABSTRACT

A high strength alloy steel with superior resistance to HIC and SSC and a method of
producing such steel is disclosed. More particularly, the present invention provides for
selectively copper alloyed high strength alloy steel composition with higher HIC and SSC
resistance in sour gas media, adapted to avoid diffusion of hydrogen, which occurs through
autocatalytic regeneration of hydrogen ions from the adsorption film formed on the steel
surface and change the film characteristics to resist HIC and SSC failure when exposed to
sour gas environment. Importantly, the 0.26% Cu added alloy steel composition resulted
in CSR, CTR & CLR of 0.2%, 0.7% and 2.0% respectively which are substantially lower
than the acceptable limit for Type I class material as per BS EN-10028-3:2009 for sour gas
application confirming HIC inhibition and resistance to SSC also by achieving threshold
stress (Oth) for failure in the range of 62-64% of yield stress.