JP2024521893A - Alumina-forming austenitic-ferritic stainless steel alloys - Google Patents

Abstract

The present disclosure relates to an alumina-forming austenitic-ferritic stainless steel alloy. The present disclosure also relates to a method for producing an alumina-forming austenitic-ferritic stainless steel body. The present disclosure further relates to products and uses in the temperature range of 500-900°C.

Description

The present disclosure relates to austenitic-ferritic stainless steel alloys. More specifically, the present disclosure relates to alumina-forming austenitic-ferritic stainless steel alloys. The present disclosure also relates to methods of making alumina-forming austenitic-ferritic stainless steel objects, products that include the alumina-forming austenitic-ferritic stainless steel, and uses of the objects in certain environments.

The prior art discloses examples of duplex stainless steels containing austenite and ferrite phases that can form a protective alumina layer on their surface when exposed to oxygen-containing atmospheres at high temperatures. High contents of nickel, chromium and aluminum are typical for such duplex stainless steels.

Wang et al., "Effects of carbon and chromium on the solidification structure and properties of ferrite-austenitic duplex heat-resistant alloys", Science and Technology of advanced materials, Elsevier Science, vol. 2, no. 1, 30 July 2001, pp. 297-302, disclose ferrite-austenitic duplex alloys tested in air at 1250°C. However, a drawback of these alloys is that their oxidation resistance is not sufficient at temperatures between 500°C and 900°C. Furthermore, some of these alloys have been shown to be very brittle and therefore have too low ductility for conventional manufacturing routes.

Hyunmyung et al: "Development of alumina-forming duplex stainless steels as accident tolerant fuel cladding materials for light water reactors", Journal of Nuclear Materials, Elsevier Science, vol 507, 21 April 2018, pp. 1-14 discloses high aluminium content (>5 wt.%) duplex stainless steels that were tested for corrosion resistance at 1200°C steam and simulated pressurised water reactor (PWR) operating conditions. However, the oxidation resistance of the disclosed composition is not sufficient at temperatures between 500 and 900°C.

Therefore, there remains a need in the art for optimized austenitic-ferritic stainless steel alloys that enable objects comprising stainless steel to be manufactured using conventional manufacturing routes and that provide objects with excellent oxidation resistance when used in the temperature range of 500-900°C.

The present disclosure therefore provides an improved alumina-forming austenitic-ferritic stainless steel alloy composition having an optimized microstructure containing austenite and ferrite for use in the temperature range of 500-900°C.

An austenitic-ferritic stainless steel according to the present disclosure comprises a stainless steel having the following composition in weight percent:
Cr 11.0-16.0;
Ni 11.5-15.0;
A1 3.5-5.0;
C 0.01-0.15;
Nb 0.01-2.0;
Mn 0.01-3.5;
Si 0.01-0.8;
Cu 0-5.5;
Zr 0-0.3;
Mo+W 0-3.0;
The balance is Fe and inevitable impurities;
Austenitic ferritic stainless steels are characterized as having a microstructure containing greater than 15 volume percent and less than 45 volume percent ferrite, with the remainder being austenite.

In this disclosure, the ferrite content of austenitic ferritic stainless steels is in the range of more than 15% to less than 45% by volume, with the remainder being austenite, and this microstructure has been found to be critical for oxidation resistance properties in the temperature range of 500-900°C. Ferrite amounts less than 15% by volume have been shown to result in reduced oxidation resistance and mechanical strength. If the amount of ferrite is greater than 45% by volume, the stainless steels have been shown to have problems forming a protective oxide layer at temperatures above about 650°C.

The present disclosure further provides a method of producing an object comprising an austenitic-ferritic stainless steel composition having a microstructure as defined above or below. The method of producing the alloy is a conventional melt metallurgical production route, since it has surprisingly been found that the present stainless steels have sufficiently high hot ductility to make this possible.

The present disclosure further provides an object comprising an austenitic-ferritic stainless steel composition having a microstructure as defined above or below. The stainless steel of the present invention allows for the formation of an aluminum oxide layer on the object, thereby allowing the object to be used in atmospheres having a wide range of oxygen concentrations at temperatures ranging from 500 to 900°C.

The results of an oxidation test at 800° C. are disclosed. The results of an oxidation test at 900° C. are disclosed. A graph showing yield strength as a function of different heat treatments is disclosed.

The present disclosure relates to an alloy having the following composition in weight percent:
Cr 11.0-16.0;
Ni 11.5-15.0;
A1 3.5-5.0;
C 0.01-0.15;
Nb 0.01-2.0;
Mn 0.01-3.5;
Si 0.01-0.8;
Cu 0-5.5;
Zr 0-0.3;
Mo+W 0-3.0;
The balance is Fe and normally present impurities;
The austenitic ferritic stainless steel relates to an alumina-forming austenitic-ferritic stainless steel characterized by having a microstructure containing greater than 15 volume percent and less than 45 volume percent ferrite, with the remainder being austenite.

The alloying elements of the steel according to the present disclosure will now be described in more detail. The terms "weight percent" and "wt%" are used interchangeably. Also, the list of properties or contributions mentioned for specific elements should not be considered exhaustive.

The main function of Fe in austenitic and ferritic stainless steels is to balance the steel composition or alloying element composition of the object. The balance also includes unavoidable impurities, which will be described later.

Chromium (Cr) 11.0 to 16.0 wt%
Cr is an important element as it is a ferrite stabilizer, thus helping to maintain a proper microstructure with more than 15% by volume and less than 45% by volume ferrite, with the remainder being austenite.

Cr also promotes the formation of an alumina, i.e. an aluminum oxide, layer on the manufactured object through the so-called third element effect by the formation of chromium oxide in a transient oxidation phase. In particular, at temperatures in the range of 500-600°C, if the amount of chromium is insufficient, the formation of the alumina layer on the object can be impaired. Chromium is also an important element for improving corrosion resistance. Therefore, the minimum content of chromium in the present steel is 11.0 wt%.

According to one embodiment, the minimum chromium content is 12.0 wt%.

However, too much Cr leads to too high a ferrite content, which leads to reduced oxidation resistance, especially at high temperatures such as 800-900°C. Too high a Cr content also leads to the formation of secondary phases such as sigma phases, which cause embrittlement. Therefore, the maximum chromium content is 16.0 wt%, e.g. up to 15.5 wt%.

According to an embodiment, the Cr content is 11.0 to 16.0 wt%, for example 12.0 to 16.0 wt%, for example 12.0 to 15.5 wt%.

Nickel (Ni) 11.5 to 15.0 wt%
Ni is an important element because it is an austenite stabilizer and therefore helps to maintain the proper microstructure. Too little Ni runs the risk of too much ferrite phase content, which leads to a loss of oxidation resistance, especially at temperatures above 800°C. In addition, too little Ni also leads to a transformation from austenite to martensite at room temperature. Therefore, the minimum content of nickel is 11.5 wt%.

On the other hand, if the amount of Ni is too high, the amount of ferrite will be too low, resulting in poor mechanical properties such as low tensile strength.

Ni should be balanced with the amount of aluminum added, as it also binds aluminum as nickel aluminide, thereby inhibiting to some extent the formation of the alumina layer. However, nickel aluminide may provide improved mechanical properties in the manufactured object, such as increased hardness and improved yield strength. Nickel may be substituted to some extent with cobalt (Co). However, Ni is preferred, as Co is less preferred from an environmental point of view. Thus, the maximum Ni content is 15.0 wt%. According to an embodiment, the Ni content is 11.5-15.0 wt%, for example 12.0-14.5 wt%.

Aluminum (Al) 3.5 to 5.0 wt%
Al is also an important element in the steel since, when exposed to oxygen at high temperatures, Al forms a dense and thin aluminum oxide layer on the manufactured object, protecting the underlying surface from further oxidation. If the Al content is too low, the formation of a sufficiently thick protective alumina layer is limited or not formed when the steel is exposed to oxygen-containing atmospheres at high temperatures, such as 500-900°C. Furthermore, Al forms nickel aluminides together with Ni, thereby contributing to an increase in its hardness. The minimum content of aluminum is therefore 3.5 wt%.

Furthermore, A1 is a ferrite stabilizer and therefore helps maintain a proper microstructure. Too much A1 leads to too high a ferrite content, which reduces oxidation resistance, especially at temperatures such as 800-900°C. Therefore, the maximum content of alumina is 5.0 wt%. According to an embodiment, the content of A1 is 3.5-5.0 wt%, for example 3.7-4.9 wt%.

Carbon (C) 0.01 to 0.15 wt%
C forms carbides with some elements present in austenitic-ferritic stainless steels, thereby contributing to an increase in the hardness and strength of the steel (e.g. creep properties). In addition, C is also an austenite stabilizer. The minimum content of carbon is therefore 0.01 wt%, for example 0.03 wt%. Too high a carbon content o increases the risk of forming too many carbides, for example M23C6 and/or M7C3 carbides, which reduces the oxidation resistance. The maximum content of carbon is therefore 0.15 wt%, for example 0.13 wt%. According to an embodiment, the content of C is between 0.01 and 0.15 wt%, for example 0.03 and 0.13 wt%.

Niobium (Nb) 0.01 to 2.0 wt%
Nb is a ferrite stabilizer and therefore helps maintain the proper microstructure.

Furthermore, Nb forms niobium carbide together with C, thereby suppressing the excessive formation of chromium carbide, which may have a negative effect on the formation of the alumina layer. Therefore, the minimum content of Nb is 0.01 wt%. According to one embodiment, the minimum content of Nb is 0.05 wt%.

However, too much Nb will result in the formation of excessive amounts of niobium carbide, which will embrittle the steel. Therefore, the maximum content of niobium is 2.0 wt%, for example up to 1.50 wt%. Thus, according to an embodiment, the content of Nb is 0.01-2.0 wt%, for example 0.05-1.60 wt%, for example 0.05-1.60 wt%.

Manganese (Mn) 0.01 to 3.5 wt%
Manganese (Mn) is an austenite stabilizer and can replace nickel to some extent without compromising oxidation resistance. Therefore, the maximum content of Mn is 3.5 wt%, for example, up to 3.2 wt%. According to an embodiment, the content of Mn is 0.01-3.5 wt%, for example, 0.01-3.2 wt%, for example, 0.05-3.1 wt%.

Silicon (Si) 0.01 to 0.8 wt%
Si is added to improve oxidation resistance. Therefore, the minimum content of Si is 0.01 wt%. However, too much Si may increase the risk of sigma phase formation. Therefore, the maximum content of silicon is 0.8 wt%. According to an embodiment, the maximum content of Si is 0.7 wt%, for example 0.6 wt%. According to an embodiment, the content of Si is 0.01-0.8 wt%, for example 0.01-0.7 wt%, for example 0.01-0.6 wt%.

Copper (Cu) 0 to 5.5 wt%
Cu may be optionally added or may be considered as an impurity. If intentionally added and to obtain the desired effect, the minimum content is 0.5 wt%. Cu may have a positive effect on the formation of nickel aluminides, which may provide improved mechanical properties such as improved hardness and yield strength. However, too much Cu will result in an excessive amount of nickel aluminides, which will reduce the hot ductility properties during the manufacture of the object. For these reasons, the maximum content of Cu is 5.5 wt%, such as 5.3 wt%, for example 5.2 wt%. According to an embodiment, the content of Cu is 0-5.5 wt%. According to an embodiment, the content of Cu is 0-<0.5 wt% or 0.5-5.5 wt%.

Zirconium (Zr) 0 to 0.3 wt%
Zr may be added optionally or may be considered as an impurity. If intentionally added, the minimum content is 0.05 wt%. Zr forms zirconium carbonitrides with carbon and nitrogen, thereby inhibiting the formation of aluminum nitride and chromium carbide, and may inhibit the formation of the alumina layer. However, too much Zr reduces the hot ductility and makes the hot working of the steel difficult. For this reason, the maximum content of Zr is 0.3 wt%. According to an embodiment, the content of Zr is 0 to less than 0.05 wt% or 0.05 to 0.3 wt%.

Molybdenum (Mo) and/or tungsten (W) 0 to 3.0 wt%
Mo and W are considered as equivalent elements and may be added in some cases. Mo and/or W bind to carbon by forming corresponding carbides, thereby reducing the amount of chromium carbide formed. However, too much Mo and W may increase the risk of introducing intermetallic phases such as Laves and sigma phases. Therefore, for these reasons, the total content of W and Mo should be limited to a maximum of 3.0 wt.%.

Rare earth metals (REM) 0 to 0.1% by weight
REMs, such as La, Ce, Y, Pr and Sm, may be optionally added. These elements are strong sulfide formers, thereby cleaning the steel from sulfur (S) and thus improving hot ductility, and may be present in amounts up to 0.1 wt.%. Above this level, in combination with the Cu and Ni contents defined in this disclosure, REMs tend to have a negative effect on hot ductility.

In addition, hafnium (Hf), tantalum (Ta), and titanium (Ti) are considered to be functionally equivalent to the elements Zr and Nb and therefore may be present in the same amounts as specified for those elements and may partially or totally substitute for these elements.

Phosphorus (P) and sulfur (S) may be considered as normally present impurities in this context. Phosphorus P may be tolerated at low levels in the alloy. According to one embodiment, P is <60 ppm. Sulfur S may be tolerated at low levels in the alloy. According to one embodiment, S is <60 ppm. According to one embodiment, P+S is <60 ppm.

Nitrogen (N) should be considered a normally present impurity. According to one embodiment, the content of N<0.02 wt%.

Other impurities may also be present in the austenitic-ferritic stainless steels as defined above or below. Typically, such impurities are unavoidable due to the manufacturing process, for example due to the fact that they are present in the scrap metal that is melted to produce a melt having the composition of the steel.

Alternatively, even if such elements could be technically removed from the melt, they would not impair the functionality of the finished steel to the extent that the work required to remove them would be motivated from a technical or economic point of view. According to one embodiment, which can be combined with all other embodiments mentioned in this disclosure, the maximum content of said normally present impurities is not more than 0.5 wt.%.

Furthermore, the austenitic ferritic stainless steels defined above or below may contain the elements referred to herein in any of the ranges referred to herein. According to one embodiment, the austenitic ferritic stainless steels of the present invention consist of all of the elements referred to herein in any of the ranges referred to herein.

The present disclosure also relates to a method for producing an austenitic-ferritic stainless steel body using an austenitic-ferritic stainless steel composition as defined above or below, the method comprising the steps of:

a) Providing an alumina-forming austenitic-ferritic stainless steel melt having an alloying element composition as defined above or below.

b) Cooling the alumina-forming austenitic-ferritic stainless steel melt to a solid.

c) hot working the solid into a workpiece of a desired shape at a temperature of greater than 1000°C to 1300°C.

The hot working process must be carried out above 1000 ° C, as otherwise intermetallic formations would reduce ductility. According to an embodiment, the hot working temperature is above 1100 ° C. Above 1300 ° C, there is a risk of incipient melting which could cause cracks in the body.

According to one embodiment, the hot working process may be repeated several times to obtain a workpiece of the desired shape.

According to one embodiment, the hot working step may include forging or hot rolling.

d) heat treating the workpiece at a temperature in the range of 1050°C to 1200°C for approximately 2 to 120 minutes.

The time and temperature of the heat treatment step depend on the size and volume of the workpiece. However, to decompose the intermetallic phases, the temperature must be at least 1050°C, e.g. at least 1100°C. Also, the ferrite content increases, the amount of which depends on the heat treatment temperature, so to obtain the correct microstructure containing austenite and ferrite, the maximum temperature is 1200°C, e.g. 1170°C, e.g. 1150°C.

e) Rapidly cooling the heat treated workpiece to approximately room temperature.

Quenching can be accomplished by cooling the workpiece to about room temperature using an air, water, or oil bath.

According to one embodiment, an optional cold working step may be performed after at least one hot working step c) in order to obtain a workpiece of a predetermined shape with closer tolerances.

According to one embodiment, the quenching step e) may be followed by an optional ageing step, which is carried out at a temperature above 500°C, e.g. 650-850°C, for up to 240 hours, e.g. up to 100 hours, in order to obtain age hardening effects such as an increase in the yield strength of the final object. During the ageing step, no deleterious secondary phases such as sigma or Laves phases are formed. However, there may be a slight decrease in room temperature (RT) ductility after the ageing step.

Furthermore, a manufactured object of austenitic-ferritic stainless steel as defined above or below may comprise the austenitic-ferritic stainless steel alloying elements referred to herein in any of the ranges referred to herein. According to one embodiment, this object of austenitic-ferritic stainless steel consists of all alloying elements referred to herein in any of the ranges referred to herein.

The final object can be any shape, including but not limited to, tube, strip, sheet, or wire. The object has excellent oxidation resistance, good weldability, and also mechanical properties that allow it to be used in non-pressurized applications, including but not limited to, muffle tubes, recuperator tubes, and high temperature heat exchangers.

Thus, the present disclosure further relates to the use of an object comprising an alumina-forming austenitic-ferritic stainless steel as defined above or below in applications in which the object is exposed to temperatures in the range of 500-900° and low-oxygen atmospheres. Examples of such applications are muffle tubes, recuperator tubes and high-temperature heat exchangers.

This disclosure is further illustrated by the following non-limiting experiments.

Sample Preparation Fourteen alumina-forming austenitic-ferritic stainless steel melt heats were prepared having the compositions disclosed in Table 1. Heats marked with an "*" are comparative examples and therefore outside the scope of the present invention.

All melts except 068 were prepared by melting in an open atmosphere induction furnace, which was prepared by induction melting in a vacuum (VIM) and cooling the melt to a solid.

The solid was cast into a 9-inch ingot and then forged at a temperature of 1180-1280°C. The dimensions of the specimen were 50*120mm.

The forged ingot was then heat treated at 1100°C for 20 minutes and quenched in water to approximately room temperature.

Mechanical preparation of specimens by hot rolling from 50mm to 15mm.

After hot rolling, the samples were annealed at approximately 1130°C for 20 minutes.

The ferrite and austenite content of the annealed samples was measured according to ASTM E562 with 30 fields and a 100 point grid. An optical microscope was used for the measurements and the results can be seen in Table 1.

High Temperature Oxidation Specimens in the form of corrosion coupons (KO-5, 15 mm x 10 mm x 3 mm) were machined from different heat treated specimens, polished to 600 mesh, cleaned with ethanol/acetone and distilled water, and placed in a horizontal tube furnace and exposed to temperatures ranging from 500°C to 900°C for up to 500 hours under a well-controlled atmosphere simulating air containing 2.5% water vapor by volume.

Samples were removed from the furnace for weight measurements after 24 hours, 48 hours, 96 hours, 192 hours, and 500 hours. Gravimetric data sampling was performed on a Sartorius scale with 5-digit accuracy.

Figures 1a and 1b show the mass change (g/ ^m2 ) as a function of time at different temperatures. As can be seen from Figure 1b, the comparative samples (840, 841, 843) with high ferrite and low alumina content lose the ability to form a protective alumina layer as shown by the increasing mass change. The comparative sample (961) with high ferrite and high A1 content as well as the inventive sample perform well at 800°C.

Figure 1b shows that only the samples of the present invention have good oxidation properties at 900°C. Therefore, the combination of ferrite content and alumina content is very important to obtain excellent oxidation properties in these temperature ranges.

Mechanical Testing The yield strength of the inventive samples (953, 954958) in annealed and aged conditions was measured according to standard IS06892-1 and the results are shown in Figure 2. It can be clearly seen that the yield strength increases upon aging of the samples in the temperature range 650°C to 850°C. This strengthening mechanism is most likely due to the formation of nickel aluminides during aging.

It is therefore clear that the alumina-forming austenitic-ferritic stainless steel alloy of the present invention exhibits excellent oxidation resistance in the temperature range of 500-900°C and also has good mechanical properties.

Table 1 shows the heats of 14 alumina-forming austenitic-ferritic stainless steel melts that were prepared. Values are in wt%, the balance being inevitable impurities excluding Fe and ferrite, and values are in vol%. Heats marked with "*" are equivalent heats.

Claims (9)

1. An alumina-forming austenitic-ferritic stainless steel having the following composition in weight percent:
Cr 11.0-16.0;
Ni 11.5-15.0;
A1 3.5-5.0;
C 0.01-0.15;
Nb 0.01-2.0;
Mn 0.01-3.5;
Si 0.01-0.8;
Cu 0-5.5;
Zr 0-0.3;
Mo+W 0-3.0;
Optionally, one or more elements selected from the group consisting of rare earth metals (REM), up to a maximum level of 0.1 wt.%;
The balance is Fe and normally present impurities;
The austenitic ferritic stainless steel has a microstructure containing more than 15 volume percent and less than 45 volume percent ferrite, the remainder being austenite.
Austenitic-ferritic stainless steel. The austenitic-ferritic stainless steel alloy of claim 1, the composition of which includes 0.5-5.5 wt.% Cu. The austenitic-ferritic stainless steel alloy according to claim 1 or 2, the composition of which includes 0.05 to 0.3 wt.% Zr. The austenitic-ferritic stainless steel alloy according to any one of claims 1 to 3, the composition of which includes 0.05 to 1.6 wt% Nb. 1. A method for producing an austenitic-ferritic stainless steel article, comprising the steps of:
a) providing an alumina-forming austenitic-ferritic stainless steel melt having a composition according to any one of claims 1 to 4;
b) cooling the alumina-forming austenitic-ferritic stainless steel melt to a solid;
c) hot working the solid at a temperature above 1000°C to 1300°C into a workpiece of predetermined shape;
d) heat treating the workpiece at a temperature in the range of 1050° C. to 1200° C. for about 2 to 120 minutes;
e) quenching the heat treated workpiece to about room temperature. The method of claim 5, further comprising a cold working step after the hot working step c). The method according to claims 5 and 6, further comprising an aging step after the quenching step e). An object comprising an alumina-forming austenitic-ferritic stainless steel according to any one of claims 1 to 4 or produced according to any one of claims 1 to 7. The use of the article according to claim 8 in an application in which the product is exposed to temperatures in the range of 500-900°C.

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