JPH01111839A - Austenitic alloy having high corrosion resistance in environment where hydrogen sulfide is present - Google Patents

Abstract

PURPOSE:To obain the material for an oil cell having excellent corrosion cracking resistance and corrosion resistance in the environment where H2S, CO2 and Cl<-> are present by using an austenitic alloy contg. specific ratios of Cr, Ni and Mo as the tube material for an oil cell. CONSTITUTION:As the material for an oil cell tube used in the manufacture of petroleum, natural gas, etc., the austenitic alloy having the compsn. contg., by weight, <0.03% C, 0.02-1.0% Si, 0.02-1.0% Mn, 19-26% Cr, 40-65% Ni, 5-20% Mo, 0.05-0.3% Nb, <0.03% P and <0.01% S, or furthermore contg. one or two kinds among <2% Cu, <1.5% Sn and <0.15% Sb with the balance Fe and satisfying the following two formulas among the contents of Cr, Ni and Mo: Cr+2Ni+1.5Mo>=135 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 <=250 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 ++2S
This invention relates to an austenitic alloy that has pitting corrosion resistance in an environment of 250° C. or lower in the presence of [02,C].

(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 and high pressure environment where 11°S exists. for example,
The technology disclosed in JP-A No. 57-20714.9 uses active ingredients (Ni, Cr, Mo,
The range of W) is set, and furthermore, Cu, (:o) is added 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).

In addition, 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 Cro to improve corrosion resistance, we have added 8 L, Nb, Ti, Ta, Lr, and V.
An alloy is disclosed in which the strength of the alloy is increased by precipitation hardening.

(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 11□S, CO2, CQ- exists,
The purpose of the present invention is to provide an austenitic alloy that has excellent stress corrosion cracking resistance and pitting corrosion resistance, and is compatible with the environmental temperature of 250° C. or lower in gas wells.

(Means for Solving the Problems) In the present invention, the basic components are configured to form a passive state and remain stable in the above environment. However, oil and gas well environments containing ordinary 1125 cause localized corrosion such as pitting corrosion and stress corrosion cracking because they contain C'l- 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, it is possible to effectively suppress the occurrence of stress corrosion cracking.

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 an alloy in a sour environment is lI
The oxidation-reduction potential of 25 is equivalent to 1125 partial pressure in the environment, but between diffuse pressure and several lθ atmospheres, 112S partial pressure,
Even if the temperature changes, Y-300 to -350mV vs.
SGE (Saturated Calomel El
ectrode, saturated grand electrode).

Therefore, the initial immersion potential determined by the environmental conditions (lr2S partial pressure, temperature) is -300mV vs. 11V VS SCE
If the alloy components are set to be more responsible, 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 total bending structure precipitate at the grain boundaries to form carbonitrides with Or and Mo, and Or and M at the grain boundaries.

form a deficient layer. For this reason, pitting corrosion occurs in areas deficient in Cr and Mo. (:r, In order to suppress the formation of a Mo-depleted layer, it is necessary to add a carbonitride-forming element that is stronger than (:r, Mo.) For this purpose, in the present invention, Nb is added. However, when a large amount of Nb is added, it forms a low melting point Nb-Fe-C compound, which causes the molten part to break during hot working, resulting in hot cracking. Therefore, the amount of Nb added is limited in order to suppress the 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 order to improve the corrosion resistance against concentrated acids, in the present invention, (:u, Sn, Sb
Addition of

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

That is, at 1 weight t, C: 0.03% or less, Si: 0
．． 02-1. OL Mn: 0.02-1.0%i,
Cr: less than 19-26%i, Ni: 40-6
5, Mo: 2.5 to less than 20'4, Nb: 0.
03 to less than 0.3, S: 0. O+96 or less, p
: o, o:+* or below, or as necessary, C
u: 296 or less, Sn: 0.15! below,
Sb: 0.15! l: An environment in which hydrogen sulfide exists, which is characterized by containing one or two of the following, other deoxidizing agents, unavoidable impurities, and the balance iron, and satisfying the conditions of each of the following formulas: It is an austenitic alloy with high pitting corrosion resistance.

! 1 In the environment where S exists, the environmental temperature Te is T
In the case of eC250℃, Cr+ 2Ni + 1.5Mo > 135Ni+
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 250°C is
It can be determined by finding the intersection of the initial immersion potential Ec and a straight line A showing the relationship between the pitting corrosion occurrence potential and the amount of alloying elements corresponding to each temperature.

Since the potential for pitting corrosion becomes more base as the environmental temperature rises,
If it contains the value of P corresponding to the intersection of A and Ec, that is, the amount of alloying elements, which gives 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 250°C, P = Cr+
If 2Ni + 1.5Mo 2135 is satisfied, pitting corrosion will not occur.

Figure 3 shows the difference between the pitting corrosion occurrence potential measured after aging the alloy of the present invention at 600°C x 60 ml and the pitting corrosion occurrence potential measured without aging. 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, the 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 80t, the product 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 (Cr+Mo) content and the necessary Ni addition amount 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 Ni addition amount 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 ((:r, Mo)-depleted layer, which becomes the starting point of pitting corrosion. In the present invention,
Either Nb, which forms carbides preferentially to C over Cr or Mo, is added, or excessive C forms other than Nb carbides.
(Cr, Mo) Since there is a possibility of forming carbides,
It is reduced during manufacturing to a limit of 0.03%.

Si: Si is necessary as a deoxidizing component, but 0.0
If it is less than 2'4, the effect is low. Also, if it exceeds 1%, the deoxidizing effect will be saturated, so the amount added should be 0.02! , and 1 or less.

Mn: Added as a deoxidizing agent. If it is less than 0.02!4, the effect will be low, and if 1'<H, the deoxidizing effect will be saturated;
．． 02% or more, and 1 or less.

Cr is one of the main components that forms a dimetal alloy passive film 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 996. Furthermore, when 2696 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 relative to the (Cr+Mo) content required to prevent the formation of δ-ferrite shown in FIG.
It does not change with respect to r content, therefore, excessive addition of Cr will simultaneously increase the amount of Ni added. Therefore, the Cr content was set to 19% or more and less than 26%.

Ni, Mo: In an environment where H2S exists, together with C, they form a passive film, imparting pitting corrosion resistance to the alloy.

The upper and lower limits of the Ni content change in accordance with the (Cr+Mo) content depending on the environmental temperature that establishes the relationship between the pitting corrosion occurrence potential and the P value shown in FIG. 2 and the relationship shown in FIG.

Furthermore, 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 250° C. or less, the lower limit of Mo that establishes the relationship between the pitting corrosion generation potential and the P value is 596. Furthermore, when Mo is added in an amount of 20 or more, the contribution to the P value decreases and the coefficient value of Mo becomes smaller than 1.5), and the effect on pitting corrosion resistance is saturated. For this reason, Mo content! i1 was set to be 5 or more and less than 20.

The lower limit of the availability that makes the P value of FIG. 2 hold is 40*. Furthermore, if Ni is added over 65*, the contribution to the P value decreases, and the value at which the effect of Ni addition is saturated is 65*. From this, the lower limit is 40t and the upper limit is less than 65%.

Nb: Figure 3 shows that due to the effect of Nb addition on the pitting corrosion potential, Nb,' (c + o, a N ) > 2 and the fourth
Due to the restriction based on the deterioration of hot workability shown in the figure, Nb
The upper limit of Tfft is 0. It's a seagull. Further, in the present invention, when less than 0.03% of Nb is added, the formation of carbonitrides is not observed in the C and N contents. For this reason,
The lower limit is 0.03! l);

Go, Sn, Sb: It was found that the corrosion resistance of the alloy of the present invention against concentrated acids was improved by adding one or two of these. However, the effect is
Cu: 2T, Sn: 0.15%, Sb:
Since the addition of 0.15% or more will result in saturation, the upper limit of each is set to Cu: 296. Sn: 0.15! k,
Sb: 0. +5! l;

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:Q, 01%. In addition, as a deoxidizing agent, Ai O,1! Below, Ca O, 0.3% or less, La O, 196 or less as necessary, Mg 0.1
! The following will be added to: Furthermore, the effect of Nb on forming carbonitrides is similar to that 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 30%, 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 is determined by measuring the pitting corrosion potential at various temperatures in a 2HNaCQ solution that is in equilibrium with 11□S gas at a partial pressure of 5 atm using a test piece taken from a product pipe after pipe production. Evaluated by. The present invention
Depending on the alloy composition, the pitting corrosion potential is -300mV vs.
The temperature at which SCE 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 provides high temperature! (This has made it possible to provide an alloy with excellent pitting corrosion resistance in a 2S environment, and will greatly contribute to the development of industry.)

[Brief explanation of the drawing]

Figure 1 is a conceptual diagram showing the relationship between corrosion forms occurring in alloys and environmental conditions, Figure 2 is a graph showing the influence Δ of environmental temperature and alloying element content on pitting corrosion potential (Vc), and Figure 3 is Graph showing the effect of Nb addition on the pitting 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 65% Mo: 5% or more, less than 20% Nb: 0.05% or more, less than 0.3% P: 0.03% or less S: 0.01% or less An austenitic alloy consisting of impurities and a balance of iron, and having high pitting corrosion resistance in an environment where hydrogen sulfide exists, which is characterized by satisfying the conditions of each formula below. Cr+2Ni+1.5Mo≧135 Ni+16≧1.5(Cr+Mo) (2) 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 65% Mo: 5% or more, less than 20% Nb: 0.05% or more, less than 0.3% P: 0.03% or less S: 0.01% or less, Cu 2% or less, Sn0 .15% or less, Sb0.
Adding one or two of 15% or less, and
An austenite alloy having high pitting corrosion resistance in an environment where hydrogen sulfide is present, which is characterized by comprising unavoidable impurities such as deoxidizers and the remainder iron, and satisfying the conditions of the following formulas. Cr+2Ni+1.5Mo≧135 Ni+16≧1.5(Cr+Mo)