WO2014008881A1

WO2014008881A1 - Austenitic steel alloy having excellent creep strength and resistance to oxidation and corrosion at elevated use temperatures

Info

Publication number: WO2014008881A1
Application number: PCT/DE2013/000369
Authority: WO
Inventors: Juliane Mentz; Michael Spiegel; Joachim Konrad; Patrick SCHRAVEN
Original assignee: Salzgitter Mannesmann Stainless Tubes GmbH
Priority date: 2012-07-13
Filing date: 2013-06-27
Publication date: 2014-01-16
Also published as: JP2015528057A; DE102012014068B3; KR20150023935A; US20150203944A1; BR112015000274A2; UA113659C2; EP2872664A1; CN104718306A

Abstract

The invention relates to an austenitic steel alloy having excellent creep strength and resistance to oxidation and corrosion at elevated use temperatures up to approximately 750°C with a specific chemical composition, and to a seamless or welded steel tube, sheet steel or workpiece or tool steel produced by forging or casting having excellent creep strength and resistance to oxidation and corrosion, in particular at use temperatures above 620°C, produced from such a steel alloy.

Description

Austenitic steel alloy with excellent creep rupture strength and oxidation and corrosion resistance at elevated service temperatures

description

The invention relates to an austenitic steel alloy with excellent

Creep rupture strength and oxidation and corrosion resistance at elevated use temperatures according to claim 1 and workpieces produced from the steel alloy according to claim 8.

In particular, the invention relates to a heat-resistant austenitic material for the production of pipes, sheets or as a forging material z. For example, for seamless superheater pipes in highly efficient new-generation power plants suitable for steam temperatures up to 750 ° C. The material requirements for these

Conditions are sufficient creep strength combined with good oxidation resistance in water vapor and corrosion resistance

Presence of flue gases and ashes.

To reduce the C0 ₂ emissions at power plants and to increase the efficiency of the steam boilers are subjected to ever higher steam temperatures and pressures. To improve the efficiency in the

Power generation in power plants is therefore increasingly the requirement to increase the steam temperature up to 700 ° C and above and also the vapor pressure in the boiler.

In particular, attempts have been made in recent years to reduce the

Steam temperature, which was previously about 600 ° C, to 650 ° C or more and further to 700 ° C or more to increase.

The specific requirements in the upper temperature levels to the

Heat exchanger tubes at these high operating temperatures are sufficient creep strength, especially in combination with high oxidation resistance in water vapor and corrosion resistance in the presence of flue gas and ashes.

High temperature materials with high creep rupture strength and corrosion resistance for the application z. In power plants are generally based either on ferritic, ferritic / martensitic or austenitic iron-based alloys or nickel-base alloys.

Chromium-rich ferritic steel is significantly less expensive compared to austenitic steel and, moreover, has a higher coefficient of thermal conductivity and a lower thermal expansion coefficient. Also owns

chromium-rich ferritic steel also has a high oxidation resistance, which is advantageous for hot steam use z. B. in heaters or boilers.

However, if high high temperature strength, i. H. Creep rupture strength is required at the same time high oxidation and corrosion resistance, only austenitic steels or nickel-based alloys come into question.

Since nickel-base alloys are very expensive compared to austenitic steels, the market demands materials made of austenitic iron-based alloys, in particular for pipes or pipelines which offer the required creep and corrosion properties even at high operating temperatures of up to about 750 ° C.

For example, creep strengths of 10 ⁵ hours at 700 ° C for a load of 100 MPa must be achieved without breakage.

The known materials, which are available up to about 620 ° C or 650 ° C application temperature are ferritic / martensitic steels with Cr contents of z. B. 8 to 15%. These materials usually have other expensive alloying additions or are also not suitable for use in temperature ranges above 620 ° C.

Austenitic steels for use in steam boilers with steam temperatures up to 700 ° C and above are z. B. from DE 60 2004 002 492 T2. In this steel, the creep resistance is achieved in particular by an addition of titanium and oxygen within the specified limits. A disadvantage of this steel, however, is the still insufficient oxidation resistance in water vapor and lack of resistance to flue gas corrosion at these high levels

Operating temperatures.

The object of the invention is to provide an alloy for an austenitic steel, which satisfies the stated requirements with respect to creep rupture strength and oxidation and corrosion resistance even at operating temperatures up to about 750 ° C and above. Another object is, from this steel alloy workpieces such. B. seamless or welded tubes, sheets, forgings and castings or

Tool steels to provide.

The first object is achieved with the features of claim 1. Advantageous developments are the subject of dependent claims. invention

Workpieces are provided by claim 8.

According to the teaching of the invention, a steel alloy with the following chemical composition (in% by weight) is proposed:

0.02 <C <0.15%

0, 1 <Si <2.0%

25 <Cr <> 33%

22 <Ni <38%

1 s. Mo <6%

0.4 <Nb <1, 5%

B <0.0120%

0.01 <N <0.2%

Mn <2%

Co <5%

W <2%

AI <0.05%

Cu <5%

Ti <0.5%

Ta <0.5%

V £ 0.5%

P <0.05%

S <, 0.05%

Remaining iron with impurities due to melting as well as optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and / or Y up to a total of 1%.

The austenitic high temperature alloy according to the present invention has excellent creep properties as well as good oxidation resistance in water vapor and corrosion resistance in flue gas. The alloy concept differs fundamentally from the known ones

Alloy concepts.

The reinforcement of the austenitic matrix against dislocation creep occurs in known austenitic materials up to temperatures of 650 ° C sufficiently by M23C6 on the grain boundaries and finely divided carbides and nitrides

Grain boundaries and inside the grain. At higher temperatures, no sufficient creep properties are ensured.

In experiments it was recognized that for the strength failure under

Creep conditions of known austenitic materials at elevated temperatures up to about 750 ° C weakening of the grain boundaries by the local excretion of a sigma phase and associated with a dissolution of the stabilizing carbides is essential. In addition, the coarsening of sigma phase excreted in the grain causes rapid after initially good creep behavior

Loss of strength.

As essential to the invention was recognized that an improvement of

Creep strength and the oxidation and corrosion resistance at elevated temperatures can only be achieved by avoiding the above-described effects of grain boundary weakening and coarsening of the excreted in the grain sigma phase.

The present alloy therefore makes essential use of the finely divided sigma phase precipitated in the grain as reinforcing component in combination with the components M ₂ 3C ₆ and further finely divided carbides, carbonitrides and nitrides, especially of niobium, which have precipitated in the grain and on the grain boundaries, in order to increase the creep rupture strength.

The additional excretion of the sigma phase inside the grains with simultaneous suppression of these precipitates on the grain boundaries and a stabilization against coarsening of all types of precipitation leads to an excellent creep resistance up to 750 ° C.

Experimental investigations showed that according to the o. G. Analysis after solution annealing z. At 1200 ° C / 15 min. with water quenching one

Austenitic matrix microstructure with primary niobium carbonitrides (Nb (C, N)) is formed. After heat treatment at 700 ° C or 740 ° C for 4000 h or im

Creep test formed finely divided sigma phase precipitates in the grain, as well as small carbonitride precipitates of the MX type, where M is essentially niobium is. Coarsening of the sigma phase was not observed until test times of nearly 20,000 h. FIG. 1 schematically shows the microstructure of the alloy according to the invention after annealing or creep rupture test.

This combination of properties is achieved according to the invention by a specifically coordinated alloying of chromium, molybdenum and silicon as well as carbon, niobium and nitrogen within the limits described. In Table 1 are the

examined materials. The non-inventive steels investigated for comparative purposes are marked with an "X".

Table 1: Composition of the investigated alloys (% by weight)

In order to ensure the strength-increasing effect and the stability of finely divided in the grain sigma phase, a sufficient amount of finely divided sigma phase must be able to excrete fast enough at operating temperature. According to the invention, it is therefore provided that the sum amount of molybdenum, chromium and silicon should be at least 29% by weight.

To ensure sufficient stability and effectiveness of the other precipitated phases Nb (C, N) and M ₂₃ C ₆ , it has been found that the ratio in weight% Nb / (N + C) plays a significant role. Experiments showed that there is sufficient stability at elevated operating temperatures when the ratio is between 1, 5 and 10. An advantageous development of the invention provides for a further improved

Corrosion resistance at high temperatures before setting the minimum chromium content to 26% and the minimum nickel content to 25%. By increasing the lower limit of the chromium content, a higher chromium content is achieved in the austenitic matrix, which significantly influences the oxidation and corrosion properties. The upper limit of the chromium content is lowered to 30% for the limitation of the sigma phase content. The adjustment of the nickel content is carried out to stabilize the austenitic structure. The upper limit can be lowered here to 35%, which results in a further improvement of the corrosion properties in sulfur-containing flue gases and under sulphate-containing deposits.

Optimal properties with regard to creep rupture strength and corrosion are to be set with further limited element limits. The contents of molybdenum are limited to 2 - 5% and silicon to 0, 1 - 1% with regard to an optimal amount and distribution of the sigma phase. In addition, the limitation of Nb (0.4-1%), N (0.05-0.12%) and C (0.05-0.12%) has a positive effect on the amount of niobium carbonitride at high temperature ( Grain boundary pinning) on the one hand and the amount and distribution of M23C6 and other carbides, carbonitrides and nitrides at operating temperature on the other. The limitation of the upper limits also has a positive effect on reducing the tendency to segregation and on the processability of the steel.

Detailed descriptions of the alloy concept

Carbon: The carbon content is an integral part of the

Alloy concept and serves to increase the creep strength and yield strength by precipitation of carbides. However, a higher carbon content reduces weldability. For this reason, the upper limit is set at 0.15% by weight and the lower limit is set at 0.02% by weight.

Silicon: Silicon is needed to increase corrosion resistance and kinetically accelerate sigma phase excretion. A content of at least 0.1% by weight has proved to be advantageous. The weldability is adversely affected by silicon, in addition, silicon stabilizes the Laves phase, which sets by precipitation chromium, so that an upper limit of 2 wt.% Should not be exceeded. Manganese: Manganese is a cheap element that is the austenitic matrix of the

Alloy stabilized. In addition, manganese slows down in the oxidation

Water vapor the chromium loss of the alloy by evaporation volatile

Chromium oxides in water vapor due to the formation of ternary Mn-Cr oxides. Of the

Manganese content, on the other hand, is kept low to avoid accelerated oxidation in water vapor and flue gas. In addition, increased manganese content degrades creep resistance. A content of max. 2.0% by weight is not considered harmful.

Chromium: The oxidation resistance in water vapor but especially the

Resistance to flue gas corrosion is achieved by a chromium content greater than 25 wt.%. Chromium is also necessary for the formation of carbides M ₂ 3C ₆ and for the separation of finely divided sigma phase. Since chromium is set by the precipitates, a content of at least 25% by weight is required in order to maintain the matrix concentration necessary for corrosion resistance. In conjunction with molybdenum within the specified limits, the dissolution of reinforcing M ₂ 3C ₆ carbides at the grain boundary in favor of brittle sigma phase is additionally prevented. At high chromium contents, however, the occurrence of δ-ferrite and, consequently, coarse-grained sigma phase is to be expected more frequently. The maximum chromium content is therefore limited to 33% by weight.

Nickel: Nickel is a necessary element for maintaining the austenitic structure and the associated strength benefits, such as creep resistance. In combination with chromium, nickel has a clearly positive effect on the resistance to steam oxidation. The

Resistance in sulfur-containing flue gases is rather negatively influenced by high contents of nickel, so that at most 38% by weight of nickel should be added. The lower limit should not be less than 22% by weight, since due to the high chromium and molybdenum content, the austenitic matrix should be stabilized against δ-ferrite.

Molybdenum: The alloying of molybdenum is done to increase the creep strength by solid solution hardening. In addition, a not too high content of molybdenum promotes resistance to chloride-containing gases and ashes. Molybdenum stabilizes the sigma phase in addition to M ₂₃ C ₆ and should therefore not fall below a minimum content of 1% by weight. According to the invention, a molybdenum content of up to 6% by weight in combination with chromium and boron hinders the dissolution of reinforcing M ₂ 3C ₆ carbides at the grain boundary in favor of a brittle sigma phase. At the same time molybdenum promotes the excretion of finely divided sigma phase in the grain to increase the

Creep resistance. Higher contents of molybdenum than 6% by weight cause the formation of too high a content of sigma phase and are furthermore to be avoided because of the tendency of molybdenum to segregate.

Tungsten: Tungsten can be alloyed as an optional element and causes accelerated oxidation in water vapor and corrosion under ash covers. Therefore, the proportion should not exceed 2% by weight. At the same time tungsten causes an increase in the creep rupture strength by solid solution hardening and

Elimination formation, so that, depending on requirements, a corresponding

Zulegierung of tungsten can take place.

Niobium: The precipitation of hardening niobium carbides, niobium carbonitrides and

Niobium nitrides in the grain leads to a significant increase in creep rupture strength at application temperatures. In addition, niobium acts through the use of

Grain boundary pinnings by precipitating Nb (C, N) on the grain boundaries are positive for the formation of a homogeneous microstructure under production conditions. Higher levels of niobium. lead, however, to segregations and diminished

Hot forming and weldability. The upper limit of 1.5% by weight should therefore not be exceeded. At least about 0.4 weight percent is required to effectively precipitate carbides and nitrides. For an effective precipitate size, the Nb, N and C content must be exactly matched as described above.

Titanium, tantalum, vanadium: Even precipitates involving titanium, tantalum and / or vanadium can lead to a significant increase in zeistandfestigkeit. To avoid accelerated oxidation or sulfur corrosion, however, the upper limit is set to 0.5% by weight.

Boron: The optional addition of boron increases the creep rupture strength by a

Reduction of the coarsening tendency and additional chemical stabilization of M ₂ 3C ₆ particles. In addition, it increases the stability of grain boundaries against creep damage and increases the ductility. Boron prevents coarsening of the sigma phase by interfacial segregation and its separation at the grain boundaries. The lower limit for the effectiveness of boron is about 0.0010 wt.%. High boron content makes welding difficult, which is why the upper limit is 0.0120% by weight. Nitrogen: Nitrogen increases the creep rupture strength by precipitation of nitrides and must therefore be as described above depending on the carbon and

Niobium content be alloyed; Nitrogen also stabilizes the austenitic matrix. The lower limit for nitrogen is therefore set at 0.01% by weight. A high nitrogen content causes reduced toughness and ductility and reduces hot workability. Therefore, an upper limit of 0.2% by weight is set.

Cobalt: Optional addition of Cobalt will increase the

Solid-solution hardening and thus the creep strength achieved. Replacement of the nickel in favor of the cobalt is sufficient for stabilizing the

austenitic matrix also conceivable. At the same time, the desired

Microstructure remain, so an upper limit of 5 wt.% Is set.

Copper: Copper can optionally be alloyed and used as a further hardening mechanism for the creep rupture strength (precipitation of a Cu phase). Higher levels of copper reduce processability to give an upper limit of 5 wt%.

Rare Earths and Reactive Elements: The optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and / or Y can be used to adjust specific properties such as. B. increased thermal shock resistance in levels of up to 1 wt.% Take place.

Figure 2 shows the excellent creep behavior at different

Operating temperatures of steels according to the invention in comparison to known steels, while Figures 3 and 4 represent the time-stretching behavior on the basis of strain rates at 740 and 700 ° C.

Although the steel alloy according to the invention advantageously z. For example

Heat exchanger tubes can be used in the power plant area, but their use is not limited thereto. In addition to the production of pipes that can be seamlessly extruded, hot and cold rolled or welded, this is

Steel alloy also for the production of sheet metal, cast iron, forged,

Centrifugal castings or for tools for mechanical processing

(Tool steels) can be used, with their field of application via pressure vessels, Boilers, turbines, nuclear power plants or the chemical apparatus construction, that extends to all areas with corresponding requirements at elevated temperature.

Even if the steel alloy according to the invention is particularly advantageous because of the excellent creep strength, corrosion and oxidation properties up to temperatures of 750 ° C or above, the use of this steel, for example, even at temperatures above 600 ° C advantageous if it is more on the Strength of the material arrives.

Claims

claims

1. Austenitic steel alloy with excellent creep rupture strength and oxidation and corrosion resistance at elevated use temperatures up to about 750 ° C with the following chemical composition (in% by weight):

C 0.02-0.15%

Si 0.1 -2.0%

Cr 25 - 33%

Ni 22-38%

Mo 1 - 6%

Nb 0.4-1.5%

B <0.0120%

N 0.01 - 0.2%

Mn <2%

Co <5%

W <2%

AI <0.05%

Cu <5%

Ti <0.5%

Ta <0.5%

V <0.5%

P <0.05%

S <0.05%

2. Steel alloy according to claim 1

characterized,

the steel has the following composition (in% by weight):

Cr 26 - 30%

Ni 25-35%

B <0.010%

3. Steel alloy according to claim 1

characterized,

the steel has the following composition (in% by weight):

C 0.05-0.12%

Si 0.1 - 1%

Cr 27 - 30%

, Ni 25 - 35%

Mo 2 - 5%

Nb 0.4 - 1, 0%

B max. 0.0090%

N 0.05-0.12%

4. Steel alloy according to one of claims 1 to 3

characterized,

the minimum content of boron is 0.0010% by weight.

5. Steel alloy according to one of claims 1 to 4

characterized,

that the following alloy contents are used to stabilize the sigma phase:

Wt.% Mo + wt.% Cr + wt.% Si> 29

6. Steel alloy according to one of claims 1 to 5

characterized,

that the following alloy contents are maintained in order to form a sufficient amount of Nb (C, N) with simultaneous stability of M ₂ 3C ₆ :

1, 5 <wt.% Nb / (wt.% N + wt.% C) <10

7. Steel alloy according to one of claims 1 to 6

characterized,

that the structure of the steel under operating conditions at operating temperature precipitated phases of M ₂ 3C ₆ and other carbides, carbonitrides and nitrides on the grain boundaries and excreted sigma phase and carbides,

Carbonitrides and nitrides in the grain has.

8. Seamless or welded steel tube, sheet steel or by forging or casting produced workpiece or tool steel with excellent creep strength and oxidation and corrosion resistance especially at use temperatures above 620 ° C, made of a steel alloy according to at least one of claims 1 to 7.