Metal treatment – Process of modifying or maintaining internal physical... – With measuring – testing – or sensing
Reexamination Certificate
1998-06-24
2001-03-06
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With measuring, testing, or sensing
Reexamination Certificate
active
06197131
ABSTRACT:
FIELD OF THE INVENTION
The object of the present invention relates to the field of analysis of the hydrogen stress corrosion and embrittlement resistance of a steel in the presence of an acid medium containing H
2
S. The present invention notably applies to the process of manufacturing flexible pipes comprising wires that form mechanical pressure or tensile strength armourings.
In the case of the manufacture of flexible pipes intended to work in the presence of H
2
S, the quality of the steels used for manufacturing armouring wires, as well as the mechanical and thermal treatments, must be so selected that these wires provide the mechanical strength that is indispensable during operation, while withstanding hydrogen corrosion and embrittlement that can be generated by the presence of H
2
S.
BACKGROUND OF THE INVENTION
Prior works have allowed to establish that there is a correlation between the resistance to corrosion in the presence of H
2
S and the hardness of the metal. It has been determined that carbon steels and/or low-alloy steels having a hardness lower than or equal to 22 HRC have a good corrosion strength in the presence of H
2
S. It has therefore been decided to characterize steel by its HRC hardness. The NACE/MR-0175 standard lays down that the carbon steels used in the petroleum field are considered to be compatible with H
2
S if they have a hardness below 22 HRC, the carbon contents being less than or equal to 0.38%. Considering the correspondence between the HRC hardness and the tensile strength Rm, this mode of characterization leads to select steels allowing to obtain products of strength Rm lower than about 800 MPa. It is clear that this can lead to non-optimized armouring wire dimensions. However, the NACE/MR-0175 standard provides for the case of low-alloy, hardened and tempered steels that do not meet the hardness requirement of less than 22 HRC. In this case, these steels must be capable of withstanding corrosion under stress &sgr; in an H
2
S medium (NACE/TM01-77 standard). A test performed on a representative sample will allow to check that steels in this state can be used for the manufacture of metal structures that must withstand the effects of stress corrosion in an H
2
S medium. The stress &sgr;
threshold
beyond which test TM01-77 is not satisfied also has to be determined. It needs to be known that a test TM01-77 (commonly referred to as SSCC for Sulfide Stress Corrosion Cracking) lasts for about 30 days, which is not comparable to a hardness type characterization test.
A faster method than steel characterization tests TM01-77 is therefore required.
SUMMARY OF THE INVENTION
The present invention thus relates to a method for controlling the hydrogen corrosion and embrittlement behaviour of steels in the presence of H
2
S wherein at least the following stages are carried out:
determining a characteristic value l
0
which is a function of the proportion of dislocations present in the lattice of a first steel sample treated under such conditions that, after treatment, the mechanical characteristics of the steel reach a maximum value and have a threshold stress &sgr;
threshold
such that &sgr;
threshold
=k.Rp
0.2
, k being less than or equal to 1, &sgr;
threshold
being defined hereafter,
determining a characteristic value l
1
which is a function of the proportion of dislocations present in the lattice of a second treated steel sample,
comparing l
1
and l
0
. If l
1
is less than or equal to l
0
, the treated steel of the second sample is said to be non sensitive to hydrogen corrosion and embrittlement in the presence of H
2
S according to the criterion defined by the value of k.
In the method, at least one of the first and second sample can be treated by quenching and tempering.
At least one of the first and second sample can be treated by work hardening.
The hardening treatment can lead to a mixed bainite/martensite structure or even to a mixed bainite/martensite/ferrite structure.
The hardening treatment can also lead to a mainly bainitic structure.
The second sample can be subjected to a different tempering treatment than the first sample.
The steel of the samples can be selected from a carbon steel and a low-alloy steel.
The steel of the second sample can have a different carbon content than the first sample, the nature and the composition of the other alloy elements being substantially identical.
The value of coefficient k can be about 0.9.
The characteristic value of the proportion of dislocations can be obtained by X-ray cristallography and correspond to the width at half height (l
½H
evaluated in angle degree) of the X-ray diffraction peak corresponding to a line K&agr; of the ferrite.
The value of l
0
can be about 0.4° for the quenched and tempered steels of the CD4 family.
The present invention can be advantageously applied for controlling the quality of the tempering treatment for a similar steel composition.
The method can also be applied for controlling the quality of a sample comprising a weld.
Other features and advantages of the present invention will be clear from reading the non limitative examples hereafter, notably illustrated by the accompanying figure which shows, for a given steel, the zones of non-sensitivity to SSCC (according to the TM01-77 standard) as a function of the threshold stress and of the hardness.
DETAILED DESCRIPTION
The examples hereafter have been dealt with for a family of various steels whose nature and composition are given in Table 1. This chromium-molybdenum family comprises steels of very similar alloy compositions in a carbon content range of between about 0.12% and 0.42%, which can reach 0.65% (AISI 4161).
TABLE 1
Grades
Chemical composition (%)
EURONORM
AFNOR
AISI
C
Si
Mn
S
P
Ni
Cr
Mo
12CrMo4KD
12CD4
4112
0.132
0.30
0.81
<0.003
<0.009
—
0.96
0.20
25CrMo4KD
25CD4
4125
0.255
0.21
0.63
<0.003
0.10
0.17
1.11
0.21
34CrMo4KD
35CD4
4135
0.335
0.28
0.73
<0.003
<0.009
0.11
1.03
0.18
42CrMo4KD
42CD4
4142
0.399
0.26
0.64
<0.003
0.009
—
0.98
0.22
The samples are hardened, after austenitization, so as to obtain a substantially 100% martensitic structure. The samples are subjected to a thermal tempering treatment with different temperature and time conditions. For each tempering condition pair (temperature-time), at least the Vickers hardness is measured with a load of 30 kg (HV30) and the conventional elastic limit at 0.2%: Rp
0.2
(MPa).
According to the TM01-77 test standard, the threshold stress &sgr;
threshold
below which the sample does not break after 720 h and above which it breaks is determined.
In the present case, the following threshold of non-sensitivity to SSCC is also taken: &sgr;
threshold
=0.9·Rp
0.2
, which means that the threshold stress according to the definition above is taken equal to 0.9·Rp
0.2
when this loading stress does not lead to more than two breakages in five tests during the TM01-77 test.
The results are shown in Tables 2, 3, 4 and 5.
TABLE 2
35CD4
RX
Mechanical tests
SCC test
Thermal
l
1/2H
Hardness
Rp
0.2%
&agr;
threshold
treatment file
(FWHM)
HV30
(MPa)
(MPa)
&agr;
threshold
/Rp
0.2%
Water quenching followed by tempering at
685° C.
8
min
0.4289
278
772
<540
<0.7
15
min
0.4101
275
755
603
0.8
30
min
0.3956
268
733
660
0.9
1
h
0.3587
256
695
625
0.9
8
h
0.2910
219
581
523
0.9
655° C.
0
h 30
0.4599
287
793
<555
<0.7
1
h
0.4213
278
<535
<0.7
2
h
0.3970
267
732
661
0.9
625° C.
1
h
0.4970
297
832
<499
<0.6
2
h
0.4621
289
806
<564
<0.7
4
h
0.4355
281
782
546
0.7
8
h 20
0.4019
268
738
664
0.9
TABLE 3
12CD4
RX
Mechanical tests
SCC tests
Thermal
l
1/2H
Hardness
Rp
0.2%
&agr;
threshold
treatment file
(FWHM)
HV30
(MPa)
(MPa)
&agr;
threshold
/Rp
0.2%
Round samples (diameter 7 mm) quenched in salt water tempered at
640° C.
1 h
0.4139
234
680
612
≦0.8
630
<0.9
2 h
0.3844
227
671
604
0.9
TABLE 4
25CD4
SCC test
RX
Mechanical tests
&agr;
thresh-
Thermal
l
1/2H
Hardness
Rp
0.2%
old
treatment file
(FWHM)
HV30
(MPa)
(MPa)
&agr;
threshold
/Rp
0.2%
Wat
Hourcade Martine
Lefrancois Patrice
Longaygue Xavier
Institute Francais du Petrole
Millen White Zelano & Branigan P.C.
Yee Deborah
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