JPH01111838A - Austenitic alloy having high corrosion resistance in environment where hydrogen sulfide is present - Google Patents

Abstract

PURPOSE:To obtain the material for an oil cell tube having excellent stress corrosion cracking resistance and corrosion resistance in an environment where H2S, CO2 and Cl<-> are present. CONSTITUTION:As the material for a tube of an oil cell used in the manufacture if petroleum, natural gas, etc., the austenitic alloy contg., by weight, <0.03% C, 0.02-1.0% Si, 0.02-1.0% Mn, 19-26% Cr, 40-55% Ni, 3.5-9% Mo, 0.03-0.3% Nb, <0.03% P, <0.01% S, or furthermore contg. one or two kinds among <2% Cu, <1.5% Sn and <0.15% Sb and the balance Fe and having the compsn. satisfying the following formula among the contents of Cr, Ni and Mo: 135>Cr+2Ni+1.5Mo>=120 and Ni+16>=1.5(Cr+Mo) is used. The tube for an oil cell having excellent corrosion resistance and stress corrosion cracking resistance at the temp. of <=200 deg.C in the environment of an oil cell and gaseous cell where H2S, CO2 and Cl<-> are present can be manufactured.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an austenite alloy for oil well pipes used in the production of oil and natural gas, or line pipes used to transport these. Person in charge, especially 11□S
The present invention relates to an austenitic alloy having pitting corrosion resistance in an environment of 200°C or lower in which , CO□, and ci- are present.

(Conventional technology) Some oil and gas wells that produce oil or natural gas include
There are many wells called sour oil wells or sour gas wells that produce oil or natural gas mixed with hydrogen sulfide. It is known that a Ni-Cr-Mo-Fe austenitic alloy containing Ni has high corrosion resistance in a high-temperature, high-pressure environment where II 25 exists. for example,
The technology disclosed in Japanese Patent Application Laid-Open No. 57-207149 is
In order to impart stress corrosion cracking resistance in accordance with the temperature conditions of the usage environment, active ingredients (Ni, Cr, Mo, W
), and furthermore, O is added to Cu to improve corrosion resistance, and N and Nb are added.

An alloy is disclosed in which the strength is increased by precipitation hardening by adding (1).

Further, the technique disclosed in JP-A-57-203739, similar to the technique disclosed in JP-A-57-207149, provides stress corrosion cracking resistance.
In addition to setting the range of active ingredients (Ni, Cr, Mo, W), and adding Cu and Go to improve corrosion resistance,
An alloy is disclosed in which the strength is increased by precipitation hardening by adding Nb, Ti, Ta, Zr, and V.

(Problems to be Solved by the Invention) The environmental conditions of oil and gas wells where 11□S exists are extremely harsh environments in which metal materials are used. The environments of these wells exhibit various temperatures and hydrogen sulfide partial pressures, and it is necessary to use oil country tubular goods with high corrosion resistance in accordance with each environment.

On the other hand, in order to impart corrosion resistance to OCTG, it is necessary to add various alloys, but using an alloy containing a larger amount of alloying components than the required amount required by the environment may cause damage to OCTG. Increase cost and reduce practicality.

The present invention provides an oil well in which lhs, CO2, (:R- exists,
The purpose of the present invention is to provide an austenitic alloy with excellent stress corrosion cracking resistance and pitting corrosion resistance that can withstand temperatures of 200°C or lower in gas well environments.

(Means for Solving the Problems) The present invention configures the basic components so that they can form a passive state in the above environment. However, the oil well/gas well environment containing ordinary 11□5 causes local corrosion such as pitting corrosion and stress corrosion cracking because it contains Ci- ions. In order for the alloy to be used in the above 11□S environment, it is necessary to suppress the occurrence of stress corrosion cracking, but the research results of the present inventors have shown that the critical environmental conditions that cause local corrosion in the alloy are It was found that stress corrosion cracking occurs starting from pitting corrosion. That is, as shown in the conceptual diagram of FIG. 1, the first form of corrosion that occurs in an environment where local corrosion occurs is pitting corrosion. Therefore, by suppressing the occurrence of pitting corrosion, the occurrence of stress corrosion cracking can be effectively suppressed.

The condition for pitting corrosion to occur is that the pitting potential of the alloy is less noble than the natural potential determined by the environmental conditions, that is, the initial immersion potential that the alloy exhibits when placed in the environment.

Typically, the initial immersion potential of the alloy in a sour environment is 11
The oxidation-reduction potential of 2S is equal to 1125 partial pressure in the environment, but between diffuse pressure and several tens of atmospheres,
Even if the temperature changes, f-300 to -350mV vs.
SCE (Saturated Ca1oIIIel
Electrode is saturated [large electrode].

Therefore, the initial immersion potential determined by the environmental conditions (1125 partial pressure, temperature) is -300 mV vs SCE,
When the alloy is placed in each environmental condition, if the alloy components are set so that the potential for pitting corrosion of the alloy in that environment is greater than -300 mV vs. E, pitting corrosion will not occur.

Next, the alloy contains C and N as inevitable impurities. C and N in an amount corresponding to the C content and N content that can be dissolved in the metal structure precipitate at the grain boundaries, form carbonitrides with Cr and Mo, and form (:r, M) at the grain boundaries.

form a deficient layer. For this reason, pitting corrosion occurs in areas deficient in Cr and Mo. In order to suppress the formation of a Cr and Mo depletion layer, it is necessary to add a carbonitride-forming element stronger than Cr and Mo. For this purpose, Nb is added in the present invention. However, when a large amount of Nb is added, it forms a Nb-Fe-C-based low melting point compound, which causes hot cracking to occur due to rupture of the molten portion during hot working. Therefore, the amount of Nb added is limited in order to suppress deterioration of hot workability.

Furthermore, in an environment where 112S exists, a fine ferrite structure (
δ-ferrite) has relatively low pitting corrosion resistance and becomes the starting point of pitting corrosion. Therefore, it is necessary to suppress the formation of δ-ferrite.

Furthermore, in order to smooth the production of oil and the like, oil country tubular goods are sometimes subjected to a process called acidizing, in which concentrated acid is injected to dissolve a portion of the oil layer. Therefore, in the present invention, Cu, Sn, and Sb are added to improve the corrosion resistance against concentrated acids.

Based on the knowledge described above, the gist of the present invention is as follows.

That is, weight t, C: 0.03 $ or less, Si:
0°02~1.0%, Mn: 0.02~1.0%
%Cr: 19~26!426! Ni: 40-55%
Mo: 3.5 to less than 99%, Nb: 0.03
~0.3! k, S: 0.01% or less, P: 0
．． 03!6 or less, or as required for this, Cu 2
! ! Below, Sn 0.15% or less, Sb 0.15
% or less, other deoxidizing agents, unavoidable impurities, and the balance iron, and satisfying the conditions of each formula below, in an environment where hydrogen sulfide exists. It is an austenitic alloy with high pitting corrosion resistance.

11 In an environment where S exists, the environmental temperature Te is T
e (at 200°C, 135 > Cr+ 2Ni + 1.5Mo > 12
ONi+ 16) 1.5 (Cr+ Mo) The details of the present invention will be explained below.

(Function) FIG. 2 shows the relationship between the pitting corrosion occurrence potential, Vc, and alloying elements.

In FIG. 2, Ec is the initial immersion potential of the alloy, and if the potential for pitting corrosion is greater than Ec, pitting corrosion will not occur. Therefore, the amount of alloy required at temperatures below 200°C is
It can be determined by finding the intersection of the initial immersion potential Ec and a straight line A indicating the relationship between the pitting corrosion occurrence potential and the amount of alloying elements corresponding to a temperature of 200° C. or lower.

Since the potential for pitting corrosion becomes more base as the environmental temperature rises,
If the alloy contains the value of P corresponding to the intersection of A and Ec, that is, the amount of alloying elements that results in a higher value of P, pitting corrosion will not occur at an environmental temperature lower than that environmental temperature.

Therefore, when the environmental temperature is below 200°C, P = Cr+
If 2Ni + 1.5Mon120 is satisfied, pitting corrosion will not occur.

Figure 3 shows the difference ΔVc between the pitting corrosion potential measured after the alloy of the present invention was aged at 600°C x 60 ml and the pitting corrosion potential measured without aging = pitting corrosion of the annealed alloy. It shows the relationship between the pitting corrosion occurrence potential of the alloy after aging and Nb/(C+0.8 N).

From the results shown in Figure 3, the value of the potential for pitting corrosion is usually 10 mV.
(±5 mV) vs. SCE exists, so if Nb/(C+0.8N) is 2 or more, deterioration of pitting corrosion resistance can be suppressed by adding Nb.

Next, FIG. 4 shows the relationship between Nb content and deterioration of hot workability when Nb is added. When producing oil country tubular goods by hot extrusion, rolling, etc., if the cross-sectional area of area in the Grieple test exceeds 80%, it has the hot workability necessary for pipe production. Therefore, from Fig. 4, it can be seen that Nb
If the content does not exceed 0.3*, hot workability will not deteriorate.

FIG. 5 shows the relationship between the ((:r+Mo) content and the necessary amount of Ni added in order to suppress the deterioration of pitting corrosion resistance due to the precipitation of fine δ-ferrite.

From FIG. 5, the relationship between the amount of Ni added and the (Cr+Mo) content is: Ni+16) 1.5 (Cr+Mo).

Next, the reasons for limiting each component are shown below.

C: Precipitates carbides at grain boundaries and forms a (Cr, Mo)-depleted layer, which becomes the starting point for pitting corrosion. In the present invention, C
r, N forms carbides preferentially with respect to C over Mo;
b is added, but the excess C causes other than Nb carbide (
Cr, Mo) may form carbides, so 0
．． 03!6 is the limit and will be reduced during manufacturing.

Si: Si is necessary as a deoxidizing component, but 0.0
If it is less than 2%, the effect is low. Moreover, since the deoxidizing effect becomes saturated if it exceeds 1%, the addition amount is set to be 0.02% or more and 1 or less.

Mn: Added as a deoxidizing agent. If it is less than 0.02%, the effect is low, and if it is 1%M, the deoxidizing effect is saturated, so 0.0
2% or more and 17 or less.

Cr: One of the main components that forms a passive film on the alloy and imparts pitting corrosion resistance. In the P value shown in Figure 2, the lower limit of the Cr content that establishes the relationship between the pitting corrosion occurrence potential and P is 1.
It is 9 trees. Furthermore, when 26% or more is added, the contribution of Cr to the P value decreases and the coefficient value of Cr becomes smaller than 1), and the effect on pitting corrosion resistance is saturated. On the other hand, the Ni content with respect to the (Cr+Mo) content required to prevent the formation of δ-ferrite shown in FIG. 5 does not change with respect to the Cr content. This will result in an increase in For this reason,
C: The content of I was 99% or more and less than 26.

Ni, Mo: In an environment where It□S exists, together with C, it forms a passive film and imparts pitting corrosion resistance to the alloy.The upper and lower limits of the Ni content are determined by the pitting corrosion generation potential and P
The value changes depending on the environmental temperature that establishes the value relationship and the relationship shown in FIG. 5, depending on the (Cr+Mo) content.

Further, the lower limit and upper limit of Mo are determined depending on the environmental temperature. Therefore, the amounts of Ni and Mo are limited depending on the environmental temperature.

In the P value shown in FIG. 2 when the environmental temperature is 200° C. or less, the lower limit of Mo that establishes the relationship between the pitting corrosion generation potential and the P value is 3.5 t. Furthermore, if more than 9t is added, the contribution to the P value decreases and M
The coefficient value of o becomes smaller than 1.5), the effect on pitting corrosion resistance is saturated. Therefore, the Mo content is 3.5!6
Above, it was set as less than 99%.

The lower limit of Ni content that satisfies the P value shown in Figure 2 is 40%.
It is. Furthermore, if 55 or more wood is added, the contribution to the P value decreases, so the lower limit of Ni was set at 40% and the upper limit was set at less than 55 wood.

Nb: Figure 3 shows that due to the effect of Nb addition on the pitting corrosion potential, Nb/(C+0.8N) > 2.Also, due to the restriction based on the deterioration of hot workability shown in Figure 4, the Nb content The upper limit of is 0.3t. Furthermore, in the present invention,
When less than 0.03% of Nb is added, no formation of carbonitrides is observed in the C and N contents. Therefore, the lower limit is 0
．． 03%.

Cu, Sn, Sb: It was found that the corrosion resistance of the alloy of the present invention to concentrated acids was improved by adding one or two of these. However, the effect is
(:u: 2%, Sn: 0.15!l;, S
b: If 0.15% or more is added, it will become saturated, so the respective upper limits are set to Cu: 2%, Sn: 0. +5! k
, Sb: 0.1! d;

In addition, P and S as unavoidable impurities each degrade pitting corrosion resistance, so the upper limit is set to P: 0.0.
3%, S: 0.0+wood. In addition, as a deoxidizing agent, 8L 0.1% or less, Ca 0.03* or less,
La 0. if necessary. lJ or less, Mg 0. I'1
The following is added: Nb further forms carbonitrides.
The effect of Ti.

Although Zr and V can be substituted, they are not included in the present invention because they cause problems such as formation of giant inclusions and deterioration of the material.

The alloy of the present invention is required to have strength when used as oil country tubular goods. Normally, methods such as work hardening, precipitation hardening, and solid solution hardening are used to impart strength, but in the present invention, strength is imparted by work hardening. Excessive work hardening deteriorates pitting corrosion resistance, but pitting corrosion resistance does not deteriorate by cold working up to 3H, so in the present invention, cold working of 30 or less is performed after final annealing.

(Example) Table 1 shows examples. The alloy tube of the example was manufactured by a process of vacuum melting, hot extrusion, solution formation, and cold working, which is an example of a manufacturing method.

Hot workability was evaluated by the Gripple test, and test pieces were worked from ingots. Pitting corrosion resistance was measured using a test piece taken from a product pipe after pipe making, and was found to be in equilibrium with 1125 gas at a partial pressure of 5 atm (20!). It was evaluated by measuring the pitting potential at various temperatures in k NaCl solution. The present invention has a pitting corrosion potential of 13Q mV depending on the alloy components.
The temperature at which vs SCH is reached is determined, the target temperature is shown in the α column, and the evaluation result is shown in the X column.

Furthermore, hot workability is evaluated using the cross-sectional area of reduction in the Grieple test, and the target value is 80t, which is the lowest limit value for all alloy groups, and the evaluation results are shown in column Y.

(Effects of the Invention) The present invention makes it possible to provide an alloy having excellent pitting corrosion resistance in a high-temperature 1125 environment, and thus greatly contributes to the development of industry.

[Brief explanation of the drawing]

Figure 1 is a conceptual diagram showing the relationship between the corrosion forms that occur in alloys and environmental conditions, Figure 2 is a graph showing the influence of environmental temperature and alloying element content on pitting corrosion potential (Vc), and Figure 3 is a graph showing the effect of pitting corrosion occurrence potential (Vc) on the amount of alloying elements. Graph showing the effect of Nb addition on the corrosion potential, 4th
The figure is a graph showing the effect of Nb on the aperture value at 1200°C, and FIG. 5 is a graph showing the effect of alloying elements on the occurrence of pitting corrosion starting from δ-ferrite.

Claims (2)

[Claims] (1) In weight% C: 0.03% or less Si: 0.02% or more, 1.0% or less Mn: 0.02% or more, 1.0% or less Cr: 19% or more, less than 26% Ni: 40% or more, less than 55% Mo: 3.5% or more, less than 9% Nb: 0.03% or more, less than 0.3% P: 0.03% or less S: 0.01% or less Other, deoxidizer An austenite alloy having high pitting corrosion resistance in an environment where hydrogen sulfide exists, which is characterized by comprising unavoidable impurities such as and the balance iron, and satisfying the conditions of each formula below. 135>Cr+2Ni+1.5Mo≧120 Ni+16≧1.5(Cr+Mo) (2) In weight% C: 0.03% or less Si: 0.02% or more, 1.0% or less Mn: 0.02% or more, 1.0% or less Cr: 19% or more, less than 26% Ni: 40% or more, less than 55% Mo: 3.5% or more, less than 99% Nb: 0.03% or more, less than 0.3% P: 0.03% or less S: 0.01% or less, and Cu 2% or less , Sn0.15% or less, Sb0.
Adding one or two of 15% or less, others,
An austenitic alloy comprising unavoidable impurities such as deoxidizers and the balance iron, and having high pitting corrosion resistance in an environment where hydrogen sulfide is present, which satisfies the conditions of the following formulas. 135>Cr+2Ni+1.5Mo≧120 Ni+16≧1.5(Cr+Mo)