ES2973764T3 - New austenitic alloy - Google Patents

Description

DESCRIPTION

New austenitic alloy

Technical field

The present disclosure relates to an austenitic alloy having a high content of Ni, Mo and Cr which, after solidification, will have a low content of intermetallic phases (less than 0.3%). The present disclosure also relates to the use of the austenitic alloy in different products and to a method for manufacturing said alloy.

Background

Nickel-based alloys are used in many corrosive applications where the corrosion resistance and microstructure stability of current stainless steels are insufficient. However, there are problems associated with these alloys, as they are prone to form microsegregations during the solidification process and therefore form unwanted intermetallic phases. These, in turn, will lead to poor ductility and poor corrosion properties. The content of intermetallic phases can be reduced by using certain manufacturing methods, such as remelting and soaking, but these methods are very expensive. Although US 4400211, US 4400210 and JP57-207150 describe nickel-based alloys and their use in the manufacture of high strength deep well casings having resistance to stress corrosion cracking, there is a need for an alloy based on nickel that has a low content of intermetallic phases and that can be manufactured by conventional metallurgical methods.

Summary

Therefore, one aspect of the present invention is to solve or at least reduce the problems mentioned above. Therefore, the present invention provides an austenitic alloy that, in % by weight, consists of: C < 0.03;

If < 1.0;

Mn < 1.5;

S < 0.03;

P < 0.03;

Chr 25.5 to 32.0;

Nor 42.0 to 52.0;

Mo 6.0 to 9.0;

N 0.07-0.11;

Cu < 0.4;

rest, Faith and inevitable impurities;

optionally one or more of Al, Mg, Ca, Ce and B < 1.0;

characterized in that the austenitic alloy meets the following condition:

E<ní>> 1.864 * ECr - 19.92

where

ECr = [wt% Cr]+[wt% Mo]+1.5*[wt% Si],

E<ní>= [% by weight of Ni]+30*[% by weight of C]+30*[% by weight of N]+0.5*[% by weight of Mn]+0.5*[ % by weight of Cu], where the CPT is greater than 88 °C (ASTM G150 standard with MgCl23 M as electrolyte).

The austenitic alloy as defined above or hereinafter will have good corrosion resistance and high ductility since the austenitic alloy will comprise less than 0.3% intermetallic phases after solidification, which means that there will be less intermetallic phases present in the austenitic alloy. Intermetallic phases have a negative impact on any of the processes carried out after solidification.

The present disclosure also relates to an object comprising the austenitic alloy as defined above or hereinafter. Examples of an object, but not limited to them, are a tube, a pipe, a bar, a rod, a thick-walled hollow tube, a billet, a square section blank, a strip, a wire, a plate and a sheet.

Furthermore, the present disclosure also provides a method of manufacturing an austenitic alloy that, in wt%, consists of:

C < 0.03;

If < 1.0;

Mn < 1.5;

S < 0.03;

P < 0.03;

Chr 25.5 to 32.0;

Nor 42.0 to 52.0;

Mo 6.0 to 9.0;

N 0.07-0.11;

Cu < 0.4;

optionally one or more of Al, Mg, Ca, Ce and B < 1.0

rest, Faith and inevitable impurities;

wherein the austenitic alloy will have a content of intermetallic phases of less than 0.3% after solidification, wherein said method comprises the steps of:

• provide a melt;

• analyze the melt to obtain the % by weight of the elements contained in it;

• insert the values obtained from the analysis into the following equation:

E<ní>> 1.864 * Ecr - 19.92

where

Ecr = [wt% Cr]+[wt% Mo]+1.5*[wt% Si], and

E<ní>= [% by weight of Ni]+30*[% by weight of C]+30*[% by weight of N]+0.5*[% by weight of Mn]+0.5*[ % by weight of Cu],

• optionally add one or more alloying elements to the austenitic alloy until the equation is satisfied;

• solidify the austenitic alloy.

By integrating the aforementioned stages into conventional metallurgical manufacturing processes, the final object obtained will have a low content of intermetallic phases, such as < 0.3%.

Brief description of the Figures

Figure 1 describes a DeLong diagram where the X and Y axes show the equivalent of Cr (Ecr) and the equivalent of Ni (E<ní>). The unfilled squares in the figure are the batches with less than 0.3% of intermetallic phases in interdendritic areas after solidification, that is, alloys that meet the conditions of the present invention;

Figure 2A shows a light optical microscope (LOM) photograph of the cast structure and intermetallic phases of an austenitic alloy (sample 3) outside the scope of the present invention; Figure 2B shows an LOM photograph of the cast structure and intermetallic phases of an austenitic alloy (sample 2) as defined above or hereinafter;

Figure 3A shows an LOM photograph of the cast structure and intermetallic phases of an austenitic alloy (sample 4) outside the scope of the present invention;

Figure 3B shows an LOM photograph of the cast structure and intermetallic phases of an austenitic alloy (sample 7) as defined above or hereinafter.

Detailed description

The present disclosure relates to an austenitic alloy consisting of the following elements in weight %: C < 0.03;

If < 1.0;

Mn < 1.5;

S < 0.03;

P < 0.03;

Chr 25.5 to 32.0;

Nor 42.0 to 52.0;

Mo 6.0 to 9.0;

N 0.07-0.11;

Cu < 0.4;

optionally one or more of Al, Mg, Ca, Ce and B < 1.0

rest, Faith and inevitable impurities;

and that satisfies the following equation:

E<ní>> 1.864 * ECr - 19.92

where

ECr = [wt% Cr]+[wt% Mo]+1.5*[wt% Si], and

E<ní>= [% by weight of Ni]+30*[% by weight of C]+30*[% by weight of N]+0.5*[% by weight of Mn]+0.5*[ % by weight of Cu], and where the CPT is greater than 88°C (ASTM G150 standard with MgCl23 M as electrolyte).

The austenitic alloy of the present disclosure will have a low fraction (amount) of intermetallic phases (less than 0.3%) formed in the interdendritic areas during the solidification process. The fraction is calculated by dividing the volume of intermetallic phases in the interdendritic zones by the total volume of the material. Examples of intermetallic phases are the sigma phase, the Laves phases, and the chi phase.

Solidification is a phase transformation where an alloy will transform from a liquid phase to a solid crystalline structure phase. The solidification process begins with the formation of dendrites and during the solidification process microsegregation will occur. Microsegregation is an uneven distribution of alloy elements between solidified dendrites that will promote the formation of unwanted intermetallic phases. The area between the dendrites is called the interdendritic area. Typical solidification processes are, but are not limited to, casting, such as ingot casting, continuous casting and remelting.

The austenitic alloy as defined above or hereinafter will have, due to the low content of intermetallic phases in the intermetallic areas, good corrosion resistance and very good ductility. Therefore, the austenitic alloy will be very suitable for use in applications where high corrosion resistance is necessary, such as in the oil and gas industry, petrochemical industry and chemical industry. Furthermore, the austenitic alloy as defined above or hereinafter also meets the condition of having a critical pitting corrosion temperature (CPT) greater than 88 °C.

The present disclosure also relates to an object comprising the austenitic alloy as defined above or hereinafter. Examples of an object, but not limited to them, are a tube, a bar, a pipe, a rod, a thick-walled hollow tube, a billet, a square section blank, a strip, a wire, a plate and a sheet. Other examples include production piping and heat exchanger piping.

Hereinafter, the alloying elements of the austenitic alloy as defined above or hereinafter are discussed with respect to their contribution to the properties of the alloy. It should be noted that alloying elements could also contribute to other properties of the austenitic alloy, even if they are not mentioned here. The figures provided in this document are given in % by weight:

Carbon (C): < 0.03% by weight

C is an impurity contained in austenitic alloys. When the C content exceeds 0.03 wt%, the corrosion resistance is reduced due to the precipitation of chromium carbide at the grain boundaries. Therefore, the C content is <0.03% by weight, such as <0.02% by weight.

Silicon (Si): < 1.0% by weight

Si is an element that can be added for deoxidation. However, Si will promote the precipitation of intermetallic phases, such as the sigma phase; therefore, the Si is contained in a content of <1.0% by weight, such as <0.5% by weight, such as <0.3% by weight. According to one embodiment, the lower limit of Si is 0.01% by weight. Manganese (Mn): < 1.5% by weight

Mn is often used to bind sulfur forming MnS and thus increase the hot ductility of the austenitic alloy. Mn will also improve the strain hardening of the austenitic alloy during cold working. However, too high a Mn content will reduce the strength of the austenitic alloy. Accordingly, the Mn content is set to <1.5% by weight, such as <1.2% by weight. According to one embodiment, the lower limit of Mn is less than 0.01% by weight.

Phosphorus (P): < 0.03% by weight

P is an impurity contained in the austenitic alloy and is well known to have a negative effect on hot workability and hot crack resistance. Accordingly, the P content is <0.03% by weight, such as <0.02% by weight.

Sulfur (S): < 0.03% by weight

S is an impurity contained in the austenitic alloy and will deteriorate hot workability. Accordingly, the allowed content of S is <0.03% by weight, such as <0.02% by weight.

Copper (Cu): < 0.4% by weight

Cu can reduce the corrosion rate in sulfuric acid. However, Cu will reduce hot workability, therefore, the maximum Cu content is <0.4% by weight, such as <0.25% by weight. According to one embodiment, the lower limit of Cu is 0.01% by weight.

Nickel (Ni): 42.0 to 52.0% by weight

Ni is a stabilizing element of austenite. Additionally, Ni will also contribute to resistance to stress corrosion cracking in both chloride and hydrogen sulfide environments. Therefore, a Ni content of 42.0% by weight or more is required. However, higher Ni content will decrease the solubility of N, therefore, the maximum Ni content is 52.0 wt%. According to one embodiment of the present austenitic alloy, the Ni content is 43.0 to 51.0% by weight, such as 44.0 to 51.0% by weight.

Chromium (Cr): 25.5 to 32.0% by weight

Cr is an alloying element that will improve resistance to pitting corrosion. In addition, the addition of Cr will increase the solubility of N. When the Cr content is less than 25.5% by weight, the effect of Cr is not enough for corrosion resistance, and when the Cr content exceeds 32 .0% by weight secondary phases will form, such as nitrides and intermetallic phases, which will negatively affect corrosion resistance. Therefore, the Cr content is 25.5 to 32.0% by weight.

Molybdenum (Mo): 6.0 to 9.0% by weight

Mo is an alloying element that is effective in stabilizing the passive film formed on the surface of the austenitic alloy. Furthermore, Mo is effective in improving pitting corrosion. When the Mo content is less than 6.0 wt%, the resistance to pitting corrosion in harsh environments is not high enough, and when the Mo content is greater than 9.0 wt%, the workability in hot it deteriorates. Accordingly, the Mo content is 6.0 to 9.0% by weight, such as 6.1 to 9.0% by weight, such as 6.4 to 9.0% by weight, such as from 6.4 to 8.0% by weight.

Nitrogen (N): 0.07 to 0.11% by weight

N is an effective alloying element for increasing the strength of austenitic alloy through solution hardening and is also beneficial for improving the stability of the structure. The addition of N will also improve strain hardening during cold working. To have these effects in the present alloy, the N content has to be greater than 0.07% by weight. However, when the N content is greater than 0.11 wt%, then the flow stress will be too high for efficient hot working and the resistance against pitting corrosion will be reduced. Therefore, the N content is 0.07 to 0.11% by weight. The austenitic alloy as defined above or below may optionally comprise one or more of the following elements Al, Mg, Ca, Ce and B. These elements may be added during the manufacturing process to improve e.g. e.g., deoxidation, corrosion resistance, hot ductility or machinability. However, as is known in the art, the addition of these elements and their amount will depend on what alloying elements are present in the alloy and what effects are desired. Therefore, if added, the total content of these elements is <1.0% by weight, such as <0.5% by weight.

The austenitic alloy consists of all the alloying elements mentioned above or hereinafter in the ranges mentioned above or hereinafter.

The term "impurities", as referred to herein, means substances that will contaminate the austenitic alloy when produced industrially due to raw materials, such as ores and scrap, and various other factors in the production process and is permitted that contaminate within ranges that do not adversely affect the properties of the austenitic alloy as defined above or hereinafter. Examples of alloying elements that are considered impurities are Co and Sn. In the present disclosure, carbide-forming elements, such as Nb and W, are considered impurities and/or trace elements and, if present, are only present in very low concentrations, meaning that they will not form carbides and therefore Therefore, they will not have an impact on the final properties of the austenitic alloy.

The present disclosure also provides a method of manufacturing an austenitic alloy having a composition consisting of the following elements in weight %:

C < 0.03;

If < 1.0;

Mn < 1.5;

S < 0.03;

P < 0.03;

Chr 25.5 to 32.0;

Nor 42.0 to 52.0;

Mo 6.0 to 9.0;

N 0.07-0.11;

Cu < 0.4;

optionally one or more of Al, Mg, Ca, Ce and B < 1.0;

rest, Faith and inevitable impurities;

wherein the austenitic alloy will have a content of intermetallic phases of less than 0.3% after solidification, wherein said method comprises the steps of:

• provide a melt;

• analyze the melt to obtain the % by weight of the elements contained in it;

• insert the values obtained from the analysis into the following equation:

E<ní>> 1.864 * ECr - 19.92

where

ECr = [wt% Cr]+[wt% Mo]+1.5*[wt% Si], and

E<ní>= [% by weight of Ni]+30*[% by weight of C]+30*[% by weight of N]+0.5*[% by weight of Mn]+0.5*[ % by weight of Cu]

• optionally add one or more alloying elements to the austenitic alloy until the equation is satisfied;

• solidify the melt.

The inventors, through extensive research, have surprisingly found that by integrating this method into conventional metallurgical manufacturing processes, the object obtained from it will have a low content of intermetallic phases after solidification, which will have a positive impact on the result of other metallurgical processes used.

According to an embodiment of the present method as defined hereinbefore or hereinafter, the equation may also be used when designing the austenitic alloy, that is, before the austenitic alloy is cast.

Melt analysis can be performed using, for example, X-ray fluorescence spectrometry, spark discharge optical emission spectrometry, combustion analysis, extraction analysis, and inductively coupled plasma optical emission spectrometry. The element content obtained from the analysis is then inserted into the equation. If the condition (equation) is not met, alloying elements are added until the equation is met. When the additional alloying elements have been added, the melt can be analyzed again and these steps can be repeated several times until the equation (condition) is met.

According to yet another embodiment of the present method, samples of the austenitic alloy can optionally be taken after solidifying to measure and verify the intermetallic phases.

According to an embodiment of the method defined above or hereinafter, the solidification method is casting.

After the solidification step, the method may comprise conventional metal fabrication steps such as hot working and/or cold working. The method may optionally comprise heat treatment and/or aging steps. Examples of hot work processes are hot rolling, forging and extrusion. Examples of cold working processes are pilgrim rolling, drawing, and cold rolling. Examples of heat treatment processes are soaking and annealing, such as solution annealing or quench annealing. Examples of objects, but not limited to them, that can be obtained by the method defined above or hereinafter are a tube, a pipe, a bar, a rod, a thick-walled hollow tube, a billet, a section blank square, a strip, a wire, a plate and a sheet.

The present disclosure is further illustrated by the following non-limiting examples.

Examples

Example 1

The alloys in Table 1 were manufactured by casting in a 270 kg HF induction furnace and subsequently converted into ingots by casting in a 22.86 cm (9 in.) mold. After casting and solidification, the molds were removed and the ingots were cooled in water. The compositions of the experimental batches, the Cr and Ni equivalents and the fraction of intermetallic phases in interdendritic areas are given in Tables 1 and 2.

Samples were cut from the top of the ingots and metallographically prepared and etched with the Beraha 9b etchant. This engraving showed the dendritic structure and intermetallic phases. Microstructure studies were performed using a light optical microscope (LOM) (Nikon light optical microscope) to investigate the intermetallic phases. The percentage (%) of intermetallic phases in interdendritic areas was measured using an inset grid of 10x10 lines at a magnification of 200 times and then counting the number of intersections in the grid that collided with the intermetallic phases in the interdendritic area and divided between the total number of intersections. A total of 10 fields randomly placed on the metallographic sample were measured to decide the fraction of intermetallic phases in the interdendritic area.

Two typical examples of the microstructure, one with intermetallic phases in interdendritic areas, see Figure 2A, and one without them, see Figure 2B. The composition of the intermetallic phases was measured by EDS (energy dispersive x-ray spectroscopy) on a FEG-SEM (Field Emission Gun Scanning Electron Microscope), Zeiss ZIGMA VP. Figure 2A (sample 3), which is outside the present invention, shows, as can be seen, that it has intermetallic phases in an amount greater than 0.3% after solidification. Figure 2B (sample 2), which is according to the present invention, shows, as can be seen, that it has no intermetallic phases after solidification. Additional examples are shown in Figures 3A to 3B, where Figure 3A (sample 4) is outside the scope of the present invention while Figure 3B (sample 7) is within the scope of the present invention and has no intermetallic phases. .

The Cr and Ni equivalents of the batches are represented in Figure 1, which shows a DeLong diagram where the X and Y axes are the equivalents of Cr (Ecr) and Ni (E<n>¡). The unfilled squares in the figure are the batches with less than 0.3% intermetallic phases in interdendritic areas after solidification, that is, alloys that meet the condition of the present invention.

Table 2. Fraction of intermetallic phases (IP) measured for each sample. Samples marked with a are within the present invention.

Example 2

Three samples of the cold-rolled, solution-annealed material were obtained and tested for pitting corrosion according to ASTM G150 with MgCl<2>3 M as the electrolyte. The average pitting corrosion temperature (CPT) values for each series are shown in the following table.

Table 3. CPT of each sample. Samples marked with an "*" are within the present invention.

Claims (11)

1. An austenitic alloy consisting of the following elements in % by weight: C < 0.03; If < 1.0; Mn < 1.5; S < 0.03; P < 0.03; Chr 25.5 to 32.0; Nor 42.0 to 52.0; Mo 6.0 to 9.0; N 0.07-0.11; Cu < 0.4; optionally one or more of Al, Mg, Ca, Ce and B < 1.0; rest, Faith and inevitable impurities; characterized in that the austenitic alloy meets the following condition: E<ní>> 1.864 * Ecr - 19.92 where Ecr = [wt% Cr]+[wt% Mo]+1.5*[wt% Si], E<ní>= [% by weight of Ni]+30*[% by weight of C]+30*[% by weight of N]+0.5*[% by weight of Mn]+0.5*[ % by weight of Cu], where the CPT is greater than 88 °C (ASTM G150 standard with 3 M MgCL as electrolyte). 2. The austenitic alloy according to claim 1, wherein the Cu content is <0.25% by weight. 3. The austenitic alloy according to any of claims 1 to 2, wherein the Mn content is <1.2% by weight. 4. The austenitic alloy according to any of claims 1 to 3, wherein the Si content is <0.5% by weight. 5. The austenitic alloy according to any one of claims 1 to 4, wherein the Mo content is between 6.1 and 9.0% by weight. 6. The austenitic alloy according to any one of claims 1 to 5, wherein the Ni content is between 43.0 and 51.0% by weight. 7. A method for manufacturing an austenitic alloy according to any one of claims 1 to 6; austenitic alloy that will have an intermetallic content of less than 0.3% after solidification, where said method includes the steps of: - provide a melt; - analyze the melt to obtain the % by weight of the elements contained therein; - insert the obtained values of the melted mass into the following equation: E<ní>> 1.864 * ECr - 19.92 where Ecr = [wt% Cr]+[wt% Mo]+1.5*[wt% Si], and E<ní>= [% by weight of Ni]+30*[% by weight of C]+30*[% by weight of N]+0.5*[% by weight of Mn]+0.5*[ % by weight of Cu] - optionally add an alloying element to the melt until the equation is satisfied; - solidify the melt. 8. The procedure according to claim 7, wherein the solidification method is casting. 9. The process according to claim 7 or 8, wherein said process also comprises at least one hot working step. 10. The process according to any of claims 7 to 9, wherein said process also comprises at least one cold working step. 11. The method according to any of claims 7 to 10, wherein said method also comprises at least one heat treatment step.