Claims:We Claim:
1. Alloy steel resistant to hydrogen induced cracking(HIC) and sulphide stress cracking (SSC) in sour gas environment having composition comprising
C :0.06 to 0.08 wt%,
Mn :0.7 to 1.5 wt%,
S: up to 0.01 wt%,
P :up to 0.01 wt%,
Si :0.25 to 0.45 wt%,
V :0.03 to 0.055 wt%,
Nb:0.04 to 0.06 wt%,
Ti :0.005 to 0.015 wt%,
Mo: 0.1 to 0.3 wt% ,
Cu: 0.1 to 0.3 wt%, and balance being Fe.

2. Alloy steel as claimed in claim 1 having yield strength in the range of 500 MPa to 550 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa and elongation in the range of 30 to 40%.
3. Alloy steel as claimed in anyone of claims 1 or 2 having ferrite-pearlite microstructure with grain size in the range of about 8 to 12 µm
4. Alloy steel as claimed in anyone of claims 1 to 3 having hydrogen induced cracking resistance determined by CLR in the range of 1 to 4.5%, CTR in the range of 0.3 to 2.5 % and CSR in the range of 0.05 to 0.2% before pipe making conforming to Class I material as per BS EN 10028-3:2009; and CLR in the range of 1 to 6%, CTR in the range of 1.0 to 1.7 % and CSR in the range of 0.1 to 0.9% after pipe making conforming to Class II material as per BS EN 10028-3:2009.
5. Alloy steel as claimed in anyone of claims 1 to 4 having sulphide stress corrosion cracking resistance including threshold stress (sth) for failure in the range of 55-65% of yield stress before pipe making and the threshold stress (sth) for failure in the range of 47-61% of yield stress after pipe making.
6. Alloy steel as claimed in anyone of claims 1 to 5 having Stress corrosion cracking resistance determined by elongations of 10.2% and the UTS load of about 573 MPa and reduction in area of 0.40 to 0.65% and more dimples in the fracture surface featuring the behaviour of more ductile nature.
7. Alloy steel as claimed in anyone of claims 1 to 6 having corrosion resistance determined by:
(i) corrosion rate in the range of 10 to 15 mpy in hydrogenated test environment of 7N H2SO4 + 1g/l thiocarbamide solution and 15 to 20 mpy in actual NACE solution (without H2S purging) as determined by Tafel polarization test;
(ii) charge transfer resistance in the range of 500 to 800 O.cm2 in hydrogenated test environment of 7N H2SO4 + 1g/l thiocarbamide solution and 600 to to 800 O.cm2 in actual NACE solution (without H2S purging) as determined by electrochemical impedance spectroscopy test;
(iii) having a passive film with pitting potential/ break down potential of -400 to -300 mV.

8. A process for producing alloy steel resistant to HIC and SSC in sour gas environment as claimed in claims 1 to 7 comprising the step of providing alloy steel composition adapted for restricting the formation of adsorption film and hydrogen diffusion into the alloy steel produced therefrom by selective incorporation in the alloy steel composition Cu 0.1 to 0.3 wt% and Mo: 0.1 to 0.3 wt%, followed by subjecting the steel composition as above to produce steel slabs and obtaining therefrom the desired alloy steel.

9. A process as claimed in claim 8 wherein said alloy steel composition used comprises:
C :0.06 to 0.08 wt%,
Mn :0.7 to 1.5 wt%,
S: up to 0.01 wt%,
P :up to 0.01 wt%,
Si :0.25 to 0.45 wt%,
V :0.03 to 0.055 wt%,
Nb:0.04 to 0.06 wt%,
Ti :0.005 to 0.015 wt%,
Mo: 0.1 to 0.3 wt% ,
Cu: 0.1 to 0.3 wt%, and balance being Fe.

10. A process as claimed in anyone of claims 8 to 9 comprising the steps of
a. providing the steel composition including said Cu 0.1 to 0.3 wt% and Mo: 0.1 to 0.3 wt%, and producing therefrom the said alloy steel slabs through BOF-VAR-LHF_ARS-CC route ;
b. reheating the steel slabs and soaking in a reheating furnace to 1150 to 1250 oC for 3 hours;
c. rolling the reheated slabs at 12500C into 7.9 X 1120 mm hot rolled coil, with a finishing rolling temperature of 930–9500C and coiling temperature of 640oC;
d. optionally, hot-rolling the slabs to 8 mm plate with finishing rolling temperature of 900–9700C and coiling temperature of 620-670oC;
e. air cooling of coils after hot rolling.

Dated this the 16th day of February, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
, Description:FIELD OF THE INVENTION

The present invention relates to alloy steel composition resistant to hydrogen damage and stress corrosion cracking in sour gas environment and a process for manufacturing the same. More particularly, the present invention is directed to provide a steel grade with selective composition having excellent hydrogen induced cracking (HIC) resistance and sulphide stress corrosion crack (SSC) resistance, in addition to strength, toughness, and weldability to suit desired application in offshore oil drilling structures/platforms as well as in cross-country oil and gas pipelines.

BACKGROUND OF THE INVENTION

Microalloyed steels find wide application in offshore oil drilling structures/ platforms as well as in cross-country oil and gas pipelines. These steel required to posses "sour-resistance" against hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) resistance, in addition 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 particles 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 to sulphur (S) level <0.002 wt% in steel. For sour gas application Mo (~0.25%) was added to the steel and from recent laboratory studies it was found that Cu (0.2 to 0.3%) improves HIC resistant of the steel.
There is thus an impending requirement to evaluate material with respect to HIC and SSC. Also there is need to understand the effect of alloying addition, namely Cu and Mo with respect to hydrogen embrittlement and delayed failure. Therefore the present work was focused to evolve an alloying addition, which can change the film characteristics and resist for HIC and SSC failure.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide alloy steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment and a process for manufacturing the same.

A further object of the present invention is directed to provide a steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment involving a composition wherein Cu and Mo is selectively added to make the steel resistant with respect to hydrogen embrittlement and delayed failure.

A still further object of the present invention is directed to provide a steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment which would conform to API 5LX specification in terms of yield strength and impact toughness properties.

A still further object of the present invention is directed to provide a steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment so that its HIC and SSC resistance conform to NACE standard test method.

A still further object of the present invention is directed to provide a steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment wherein addition of Cu-Mo synergistically protects the steel with the formation of passive film and corrosion results in 7N H2SO4 + 1 g 1/l thiocarbamide shows better corrosion resistance of steel.

A still further object of the present invention is directed to provide a steel composition resistant to hydrogen induced cracking (HIC) and stress corrosion crack (SCC) in sour gas environment wherein Cu-Mo addition to steel increases the RA value, increases the % of elongation and increasing the UTS value suggesting Cu-Mo improves SCC susceptibility in stimulated NACE solution (7N H2SO4 + 1 g 1/l thiocarbamide).

A still further object of the present invention is directed to provide a steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment wherein synergistic effect of Cu and Mo restricts the diffusion of H+ diffusion and fracture morphology to mixed mode failure of more ductile nature.

A still further object of the present invention is directed to provide a steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment wherein Cu-Mo addition in the selective weight percent range would restrict the formation of adsorption film (FeHS-ads) and restrict the hydrogen diffusion into steel, thereby synergistically improves HIC and SSC resistance in sour environment.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide alloy steel resistant to hydrogen induced cracking(HIC) and sulphide stress cracking (SSC) in sour gas environment having composition comprising
C :0.06 to 0.08 wt%,
Mn :0.7 to 1.5 wt%,
S: up to 0.01 wt%,
P :up to 0.01 wt%,
Si :0.25 to 0.45 wt%,
V :0.03 to 0.055 wt%,
Nb:0.04 to 0.06 wt%,
Ti :0.005 to 0.015 wt%,
Mo: 0.1 to 0.3 wt% ,
Cu: 0.1 to 0.3 wt%, and balance being Fe.

A further aspect of the present invention is directed to said alloy steel having yield strength in the range of 500 MPa to 550 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa and elongation in the range of 30 to 40%.

Importantly also said alloy steel is having ferrite-pearlite microstructure with grain size in the range of about 8 to 12 µm.

A still further aspect of the present invention is directed to provide said alloy steel having hydrogen induced cracking resistance determined by CLR in the range of 1 to 4.5%, CTR in the range of 0.3 to 2.5 % and CSR in the range of 0.05 to 0.2% before pipe making conforming to Class I material as per BS EN 10028-3:2009; and CLR in the range of 1 to 6%, CTR in the range of 1.0 to 1.7 % and CSR in the range of 0.1 to 0.9% after pipe making conforming to Class II material as per BS EN 10028-3:2009.
A still further aspect of the present invention is directed to provide said alloy steel having sulphide stress corrosion cracking resistance including threshold stress (sth) for failure in the range of 55-65% of yield stress before pipe making and the threshold stress (sth) for failure in the range of 47-61% of yield stress after pipe making.

Another aspect of the present invention is directed to provide said alloy steel having stress corrosion cracking resistance with elongations of 10.2% and the UTS load of about 573 MPa and reduction in area of 0.40 to 0.65% and more dimples in the fracture surface featuring the behaviour of more ductile nature.

Yet another aspect of the present invention is directed to said alloy steel having corrosion resistance determined by:
(i) corrosion rate in the range of 10 to 15 mpy in hydrogenated test environment of 7N H2SO4 + 1g/l thiocarbamide solution and 15 to 20 mpy in actual NACE solution (without H2S purging) as determined by Tafel polarization test;
(ii) charge transfer resistance in the range of 500 to 800 O.cm2 in hydrogenated test environment of 7N H2SO4 + 1g/l thiocarbamide solution and 600 to to 800 O.cm2 in actual NACE solution (without H2S purging) as determined by electrochemical impedance spectroscopy test;
(iii) having a passive film with pitting potential/ break down potential of -400 to -300 mV in actual NACE solution (without H2S purging) .

A further aspect of the present invention is directed to a process for producing alloy steel resistant to HIC and SSC in sour gas environment comprising the step of providing alloy steel composition adapted for restricting the formation of adsorption film and hydrogen diffusion into the alloy steel produced therefrom by selective incorporation in the alloy steel composition Cu 0.1 to 0.3 wt% and Mo: 0.1 to 0.3 wt%, followed by subjecting the steel composition as above to produce steel slabs and obtaining therefrom the desired alloy steel.

A still further aspect of the present invention is directed to said process wherein said alloy steel composition used comprises:
C :0.06 to 0.08 wt%,
Mn :0.7 to 1.5 wt%,
S: up to 0.01 wt%,
P :up to 0.01 wt%,
Si :0.25 to 0.45 wt%,
V :0.03 to 0.055 wt%,
Nb:0.04 to 0.06 wt%,
Ti :0.005 to 0.015 wt%,
Mo: 0.1 to 0.3 wt% ,
Cu: 0.1 to 0.3 wt%, and balance being Fe.

A still further aspect of the present invention is directed to said process comprising the steps of
a. providing the steel composition including said Cu 0.1 to 0.3 wt% and Mo: 0.1 to 0.3 wt%, and producing therefrom the said alloy steel slabs through BOF-VAR-LHF_ARS-CC route ;
b. reheating the steel slabs and soaking in a reheating furnace to 1150 to 1250 oC for 3 hours;
c. rolling the reheated slabs at 12500C into 7.9 X 1120 mm hot rolled coil, with a finishing rolling temperature of 930–9500C and coiling temperature of 640oC;
d. optionally, hot-rolling the slabs to 8 mm plate with finishing rolling temperature of 900–9700C and coiling temperature of 620-670oC;
e. air cooling of coils after hot rolling.

The above and other objects and advantages are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows the Microstructure of (a) Cr added, (b) Cu-Cr added and (c) Cu-Mo added steels showing ferrite-pearlite structure.

Figure 2: shows Electro chemical Polarization Plots of steels in 7N H2SO4 + 1g thiobromaide solution.
Figure 3: shows Electro chemical Polarization Plots of steels in NACE solution (without purging of H2S gas).
Figure 4: shows Electro chemical Impedance Spectroscopy (EIS) studies (a) Nyquist Plots in 7N H2SO4 + 1g thiobromaide solution (stimulated NACE solutions) and (b) Nyquist Plots in NACE solution (without purging of H2S).
Figure 5: shows Equivalent circuits used for the above EIS plot (a) for Cr added steel (b) Cu-Cr added and Cu-Mo added steels.

Figure 6: shows typical stepwise microcracks observed after HIC test (a) Cr added steel (b) Cu-Cr added steel and (c) Cu-Mo added steel as per NACE TM 0284.

Figure 7: shows the Microstructure of cracks after HIC studies of (a) Cr added and (b) Cu-Mo added steel pipe as per NACE TM 0284

Figure 8: shows Sulphide stress corrosion cracking (SSC) studies of steels as per NACE TM O177 Method A

Figure 9: shows Sulphide stress corrosion cracking (SSC) studies of steel pipes samples as per NACE TM O177 Method A

Figure 10: shows Stress Corrosion Cracking studies of steels using Slow Strain Rate Tensile Testing (SSRT) machine in 7N H2SO4 + 1g thiobromaide solution.

Figure 11: shows SEM micrographs of the fracture surface of the steels after SCC studies (a) Cr added (b) Cu-Cr added and (c) Cu-Mo added steel.

DETAILED DECSRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide alloy steel composition resistant to hydrogen induced cracking (HIC) and sulphide stress corrosion crack (SSC) in sour gas environment and a process for manufacturing the same. More particularly, the invention provides a steel composition wherein Cu and Mo is selectively added to make the steel resistant with respect to hydrogen embrittlement and delayed failure.

The composition and properties of the target steel composition according to the present invention was achieved according to the Example I as follows:

Example I:
Three commercial heats were made with varying Cu (0.20 – 0.25 wt%), Cr ( 0.25 wt%) and Mo (0.15 – 0.20 wt%) content. The heats have been made through BOF-VAR-LHF_ARS-CC route. The slabs have been subsequently soaked at 12500C and rolled into 8 X 1120 mm hot rolled coil, with a finishing rolling temperature of 930–9500C and coiling temperature of 640oC. Low S and P content of <0.004 wt% and <0.01 wt% respectively, with desired chemistry have been achieved in all the heats. The chemical compositions of the steel produced are given in following Table-1.
Table 1: Chemical Composition of steels in wt%
Steels C Mn S P Si Al Nb V Ti Cu Cr Mo
Cr added Steel 0.06 0.98 0.0035 0.009 0.41 0.058 0.06 0.05 0.014 --- 0.26 ---
Cu-Cr added steel 0.10 1.33 0.004 0.014 0.24 0.031 0.045 0.044 0.011 0.23 0.26 ---
Cu-Mo added steel 0.065 0.90 0.004 0.01 0.43 0.06 0.052 0.05 0.013 0.21 --- 0.2

Test methods and results observed:
HIC and SSC resistance of three heats were evaluated as per NACE standard testing methods (NACE International, Houston, USA is the worldwide corrosion authority where NACE stands for National Association of Corrosion Engineers). All three steels were found to exhibit ferrite-pearlite microstructures with an average grain size of about 8 to 12 µm as illustrated in accompanying Figure 1. The steels were found to conform to API 5LX specification in terms of yield strength and impact toughness properties as presented in following Table-2.
Table 2 Mechanical Properties
Steels YS
Mpa UTS
Mpa % Elongation YS/UTS CIE, J
0° C -20° C
Cr added Steel 520 613 34.1 0.85 144 116
Cu-Cr added steel 530 630 35.4 0.84 243 ---
Cu-Mo added steel 528 628 38.2 0.84 154 120

The electrochemical corrosion studies (polarisation and electrochemical impedance spectroscopy [EIS]) were conducted in two media namely, 7N H2SO4 + 1 g 1/l thiocarbamide as shown in Figure 2 and NACE solution (without purging of H2S gas) as shown in Figure 3.

The corrosion results in 7N H2SO4 + 1 g 1/l thiocarbamide suggest that Cu-Mo addition has better corrosion resistance of steel under investigation (increase in ECorr Potential, decrease in corrosion current density (icorr) as presented in following Table-3.

Table 3 Electrochemical polarization parameter of steels in 7N H2SO4 + 1 g 1/l thiocarbamide solution
Steel Corrosion Current (A/cm2) Corrosion Rate (mpy) Film Capacitance (F/cm2) Charge transfer Resistance (O.cm2)
Cr added Steel 0.56 x10-4 130 5.178 x10-5 108
Cu-Cr added steel 1.26 x10-5 16 3.146 x10-5 716
Cu-Mo added steel 1.24 x10-5 15 2.986 x10-5 780

This was confirmed for the corrosion results obtained in actual NACE solution (without H2S purging), where Cu-Mo added steel shows passive film characteristics in the anodic region where as, in Cr added and Cr-Cu added steel only anodic dissolution is seen. This attributes confirm that addition of Cu-Mo synergistically protects the steel with the formation of passive film as illustrated in following Table 4. Further, it can also be commented that anodic and cathodic Tafel’s slope of Cr added steel is more or less equal in 7N H2SO4 + 1 g 1/l thiocarbamide environment (Figure 2). This behaviour might be due to absorption phenomenon of corrosion mechanism.

Table 4 Electrochemical polarization parameter of steels in NACE solution (without purging of H2S gas)
Steel Corrosion Current (A/cm2) Corrosion Rate (mpy) Film Capacitance (F/cm2) Charge transfer Resistance (O.cm2)
Cr added Steel 5.2 x10-5 45 4.666 x10-5 308
Cu-Cr added steel 0.26 x10-5 15 3.101 x10-5 736
Cu-Mo added steel 0.24 x10-5 16 2.996 x10-5 790

To understand the physical characteristics of the passive film, Electrochemical Impedance spectroscopy studies were carried out for all the steels under investigation both in actual NACE solution (without H2S purging) and stimulated sour gas environment (7N H2SO4 + 1 g 1/l thiocarbamide) as illustrated in accompanying Figure 4. A typical fitting result is shown in accompanying Figure 5 (a) and (b) where the EIS plots were measured on steel in both the environment. It is seen that the measured data and the fitted result matched very well. The impedance parameters obtained by circuit fitting are listed in Table-3 and 4. From the table is clear that Cu-Mo added steel showed a highest polarisation resistance than that of other steels under investigation. From the Nyquist plot it is clear that Cr containing steel showed a negative loop near the lower frequency, which corresponds to inductance behaviour. Inductive behaviour which appears at low frequency is due to adsorption phenomena at the surface of the electrode. The effect of H2S on the anodic reaction was likely caused by H2S chemisorbtion on the surface of the Cr containing steel. This complex FeHS-abs film will decompose/auto-catalytically (Eqn. 1, 2 and 3) regenerates the H2S at the surface of the steel.

Fe + H2S -› FeHS-ads + H+ (1)
FeHS-ads -› FeHS+ads +2e- (2)
FeHS+ads + H+ -› Fe2+ + H2S (3)

The HIC performance of the steels was evaluated in sour gas (H2S) environment in accordance with NACE TM 0284 test method. Accompanying Figure 6 shows the optical micrographs of the HIC tested samples. The crack length ratio (CLR), crack thickness ratio (CTR) and crack sensitivity ratio (CSR) values were determined and tabulated in the following Table-5. HIC resistance of Cr added steel found to be significantly marginal acceptable level specified for sour gas application [(Class III material should have CSR = 2 %, CLR = 15 % and CTR = 5% as per BS EN 10229]. However Cr-Cu and Cu-Mo added steel showed better HIC performance and within the limits of Class I specification required for sour gas application [Class I class material should have CSR = 0.5%, CLR = 5.0% and CTR = 1.5% as per BS EN 10229] (Table 5).
Table 5 Hydrogen induced cracking parameter of steels as per NACE TM 0284
Hot rolled coils (%) Acceptable limit for sour gas application [Class 1, Class 2 & Class 3] (%)
Cr added Steel Cu-Cr added steel Cu-Mo added steel
CSR 1.84 0.12 0.06 =0.5, = 1, =2
CTR 3.7 1.43 0.41 =1.5, = 3, =5
CLR 10.43 4.13 1.2 =5.0, = 10, =15

HIC behaviour of Cr added and Cu-Mo added steel was evaluated after pipe making. The HIC parameters for Cr added steel pipe found to significantly higher than that of the class III level specified for sour gas application and cannot be used for sour gas application. But Cu-Mo added steel pipe were within the limits of class II specification acceptable for sour gas application as illustrated in Figure 7 and data presented in following Table-6 confirming the resistance to HIC.

Table 6 Hydrogen induced cracking parameter of steel pipe as per NACE TM 0284
Cr added Steel pipe Cu-Mo added steel pipe Acceptable limit for sour gas application [Class 1, Class 2 & Class 3] (%)
CSR 2.7 0.8 =0.5, = 1, =2
CTR 5.3 1.64 =1.5, = 3, =5
CLR 16.04 5.8 =5.0, = 10, =15

In order to substantiate the mechanism, cylindrical pin samples of both steels of size 4 X 3mm were immersed in the NACE solution along with the HIC sample. After the test duration of 96h the samples were cleaned with alkaline solution and analyzed for hydrogen content using hydrogen analyzer. Table-7 shows the total hydrogen content of the steels, before and after immersion in the NACE test solution and also the diffusible hydrogen content. From the table it is inferred that Cu-Mo addition has inhibited the hydrogen diffusion into the experimental steels.

Table 7 Diffusible hydrogen content in steels in NACE Solution before and after HIC test

Steels Total hydrogen before immersion
in ppm Total hydrogen after immersion
in ppm Diffusible hydrogen
in ppm
Cr added Steel 5.6 11.2 5.6
Cu-Cr added steel 4.5 5.8 1.3
Cu-Mo added steel 4.4 5.6 1.2
Cr added Steel pipe 5.9 14.8 8.9
Cu-Mo added steel pipe 5.2 8.2 3.0

The SSC experiments were carried out on chosen steels for various percentages of yield strength as per NACE TM 0177 Method A. The corresponding time to failure was recorded as presented in accompanying Figure 8. The SSC test was carried out for maximum period of 720 h or 30 days. From Figure 8, the threshold stress was evaluated to be 51% and 65% of the yield stress for Cr and Cu-Mo added steel respectively. Cu-Mo added steel showed the highest threshold stress around 65% of the YS than that of other steels under investigation. SSC for pipe samples were also evaluated and it was observed that Cr added steel pipe showed poor in SSC resistance as compared to Cu-Mo added steel pipe are shown in accompanying Figure 9.

The SCC studies as illustrated in accompanying Figure 10 were also conducted to understand the qualitative nature of the failure. Load-Strain behavior of Cu-Mo steel showed elongations of 10.2% and the UTS load of about 573 MPa, Cr-Cu added steel showed 5.1% elongations, 572 MPa UTS and Cr added steel has showed lower elongation (3.1%) and UTS (503 MPa) as presented in following Table -8. Another important parameter to be noted in the table is that reduction in area (RA), which relates directly to the mode of fracture. Lesser the RA value more chance for brittle failure, where as higher the RA value more chance of ductile failure. It is clear from the figure that Cu-Mo addition to steel increases the RA value, increases the % of elongation suggesting Cu-Mo improves SCC susceptibility in stimulated NACE solution (7N H2SO4 + 1 g 1/l thiocarbamide)

Table 8 SCC Parameter of steels tested in in 7N H2SO4 + 1 g 1/l thiocarbamide solution using SSRT machine
Steels Yield strength
Mpa Ultimate tensile strength
Mpa % Elongation Reduction in area %
Cr added Steel 465 503 3.1 0.15
Cu-Cr added steel 499 572 5.1 0.35
Cu-Mo added steel 535 573 10.2 0.59

Fractographic studies were carried out after the SCC studies as illustrated in accompanying Figure 11. It is clear from the Figure 11 (a) that Cr added steel showed intergranular quasi cleavage of cracking, which are the characteristics of brittle fracture. This is because more hydrogen might have diffused into the surface causing higher stress. Higher strength steels are vulnerable to hydrogen embrittlement (HE), where interstitial hydrogen atoms can cause damage by forming molecular gas. It greatly lowers the ductility of the steel under applied loading. The mechanism of cracking can be explained using Troiano's theory of hydrogen embrittlement that suggests that accumulation of hydrogen followed by crack nucleation occurs at regions of high stress. This stress state occurs below the root of cracks initiate due to the critical combination of stress and hydrogen concentration.

Figure 11 (b and c) shows the fractrography micrographs of fractured surfaces of Cr-Cu and Cu-Mo added steels. From figure it is clear that on addition of Cu to steel the fracture morphology changes to mixed mode of facture (river like pattern for brittle and dimples for ductile). But addition of Mo along with Cu to steel has increase the dimple in the fractograph featuring the behaviour of more ductile nature. Since the facture behaviour corresponds to H+ ion concentration, as discussed earlier. Hence, from this study it confirms that synergetic effect of Cu and Mo restricts the diffusion of H+ diffusion and changes the fracture morphology from intergranular quasi cleavage failure to mixed mode failure (more of ductile failure).

From the above results it can be concluded that Cu-Mo synergistically improves both HIC and SSC behavior of steel in sour gas environment.

Accordingly, HIC and SSC resistant steel of present invention was developed following selective formulation and a comprehensive methodology for manufacturing alloy steel resistant to HIC and SSC in sour gas media as follows:

Steel Composition:
The Steel composition comprising of: C (0.06 to 0.08wt%), Mn (0.7 to 1.5wt%), S (up to 0.01wt%), P (up to 0.01wt%), Si (0.25 to 0.45wt%), V (0.03 to 0.055wt%), Nb (0.04 to 0.06wt%), Ti (0.005 to 0.015wt%), Mo (0.15 to 0.3wt%) and Cu (0.1 to 0.3wt%) and balance being Fe.

Processing steps:

Manufacturing method of claimed high strength HIC resistant steel, comprising following steps:
(i) The steels are produced through BOF-VAR-LHF_ARS-CC route
(ii) The steels slabs are reheated and soaked in a reheating furnace to 1150 to 1250 oC for 3 hours.
(iii) Reheated slab at 12500C is rolled into 7.9 X 1120 mm hot rolled coil, with a finishing rolling temperature of 930–9500C and coiling temperature of 640oC.
(iv) Hot-rolled to 8 mm plate with finishing rolling temperature of 900–9700C and coiling temperature of 620-670oC
(v) Air cooling is done after hot rolling.

The above-described methodology for manufacturing alloy resistant to HIC and SSC in sour gas media is very different from the existing prior art. Most of the prior art on the subject involve features related to following:

(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 etc.

Properties:
Properties of the Cu-Mo added Alloy steel according to the present invention produced by the above described process:
(i) Tensile properties: The claimed steel composition has a yield strength in the range of 500 MPa to 550 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa and elongation in the range of 30 to 40%

(ii) Microstructure: The claimed steel composition has a ferrite-pearlite microstructure with grain size in the range of about 8 to 12 µm

(iii) Hydrogen Induced cracking resistance:

(a) Before Pipe making
(i) The claimed steel composition before pipe making has CLR in the range of 1 to 4.5%, CTR in the range of 0.3 to 2.5 % and CSR in the range of 0.05 to 0.2%.
(ii) The CLR, CTR and CSR values for Cr-Cu and Cu-Mo steel added adequately meet the stipulations of Class I material as per BS EN 10028-3:2009 (Class I material should have CLR = 5%, CTR = 1.5% and CSR = 0.5%).

(b) After Pipe making
(i)The claimed steel composition after pipe making has CLR in the range of 1 to 6%, CTR in the range of 1.0 to 1.7 % and CSR in the range of 0.1 to 0.9%.
(ii)The CLR, CTR and CSR values for Cu-Mo added steel pipe adequately meet the stipulations of Class II material as per BS EN 10028-3:2009 (Class II material should have CLR = 10%, CTR = 3% and CSR = 1%).

(iv) Sulphide stress corrosion cracking resistance:

(a)Before Pipe making
(i)The claimed steel composition before pipe making has the threshold stress (sth) for failure in the range of 55-65% of yield stress, about 10 MPa higher than for Cr added steel.
(b)After Pipe making
(i)The claimed steel composition after pipe making has the threshold stress (sth) for failure in the range of 47-61% of yield stress, about 10 MPa higher than for Cr added steel.

(v) Stress corrosion cracking resistance:
a. The claimed steel composition has elongations of 10.2% and the UTS load of about 573 MPa and reduction in area of 0.40 to 0.65% and more dimples in the fracture surface featuring the behaviour of more ductile nature.

(vi) Corrosion resistance:
a. The claimed steel composition has corrosion rate in the range of 10 to 15 mpy in hydrogenated test environment of 7N H2SO4 + 1g/l thiocarbamide solution and 15 to 20 mpy in actual NACE solution (without H2S purging) as determined by Tafel polarization test.
b. The claimed steel composition has superior charge transfer resistance in the range of 500 to 800 O.cm2 in hydrogenated test environment of 7N H2SO4 + 1g/l thiocarbamide solution and 600 to to 800 O.cm2 in actual NACE solution (without H2S purging) as determined by electrochemical impedance spectroscopy test.
c. The claimed steel showed a passive film with pitting potential/ breadk down potential of -400 to -300 mV.

It is thus concluded based on test results that Cu-Mo addition in the range (0.1 to 0.30 wt %) has restricted the formation of adsorption film (FeHS-ads) and restricts the hydrogen diffusion into steel, thereby synergistically improves HIC and SSC resistance in sour environment.