WO2023093923A1

WO2023093923A1 - High-chrome steel resistant to creep at temperatures up to 650 °c

Info

Publication number: WO2023093923A1
Application number: PCT/CZ2022/050045
Authority: WO
Inventors: Zbyšek NOVÝ; Jaromír DLOUHÝ; Eva Chvostová
Original assignee: Comtes Fht A.S.
Priority date: 2022-04-27
Filing date: 2022-04-27
Publication date: 2023-06-01

Abstract

The high-chrome steel resistant to creep at temperatures up to 650 °C contains W in the range of more than 2 up to 4% wt., Si in the range of more than 0.1 up to 1% wt., Mn in the range of more than 0.1 up to 1% wt., Nb in the range of more than 0.05 up to 0.2% wt., C in the range of more than 0.10 up to 0,15 % wt., A1 not more than 0.05% wt., Cu in the range from 0.01 up to 2% wt., P in the range from 0.005 to 0.02% wt., S in the range from 0.001 to 0.01% wt., and Ti in the range from 0.005 to 0.1% wt. The Ti weight amount is more than four times higher than N weight amount. The steel is not nitrogen-alloyed and no nitrides and carbonitrides of MX type are present as in currently used steels. Owing to this, no phenomena of sudden loss of creep strength due to dissolution of such nitrides and precipitation of M2N Z-phase exist.

Description

High-chrome steel resistant to creep at temperatures up to 650°C

Field of the invention

The present invention relates to the field of steels resistant to creep at high temperatures, applicable in particular in high-temperature applications in thermal power plants. The steel is not nitrogen-alloyed and does not contain MX-type nitrides and carbonitrides.

Background of the invention

At present, thermal power plants represent one of significant sources of electric energy. The requirements for lower prices while reducing quantities of harmful emissions cause substantial pressure on improved efficiency of either modernized or newly developed thermal power plants. This can be achieved by improving of steam parameters, i.e., steam temperature and pressure at input to steam turbines. At present, standard steam parameters are about 565°C but use of steam over 610°C is considered. These ultra-super critical (USC) steam parameters, at which there is no difference between gaseous and liquid state of water, permit substantial increase of thermal efficiency of power plants while reducing consumption of fossil fuels. They contribute to reduction of harmful emissions thereby. The use of USC steam parameters in the thermal power plants is conditional on availability of suitable structural materials.

For materials operating in the material creep area, e.g., rotors and turbine vanes, boiler tubes, etc., the resistance to creep is the fundamental material property. Suitable properties are posed by so-called high-chrome steel with chrome content of at least 9 % wt. and with admixture of other elements - tungsten, cobalt, molybdenum, vanadium, or niobium. The chromium content provides for resistance to corrosion of the material, and the other elements provide resistance to creep. Tungsten and cobalt improve strengthening of solid solution. Molybdenum, vanadium and niobium participate in forming of fine carbides that bring about precipitation strengthening, and avoid softening processes upon thermal exposure of the material. These fine carbidic particles are of various types - MX, M2X (where M - metal, X - carbon or nitrogen). Carbides M23C6 are always present where M is predominantly chromium. These carbides are least stable under the required exposure temperatures. An important milestone in development of modern 9%Cr steels is P91 (or P92) steel. In this case, the level of RmT/io⁵h/6oo°c was almost doubled by optimized addition of vanadium, niobium, and nitrogen to 9%Cr-l%Mo steel. Such high level of creep strength may be achieved through suitable composition and distribution of carbides and carbonitrides within the structure. More coarse carbides of M23C6 type are usually present on the borders of martensitic crystals, and hinder their movement. The fine particles of the secondary MX phase effectively anchors dislocations inside sub-grains. However, this microstructure may not be regarded stable with respect to long-term exposure because fine MX particles dissolve due to precipitation of thermodynamically more stable modified M2N Z-phase. Thermodynamically, the Z-phase is the most stable of all carbides, carbonitrides, and nitrides in P91/P92 steels, and formation thereof may not be avoided but it may be postponed only. The Z-phase emerges in the form of coarse particles where high volume of MX particles perishes in favour of one Z-phase particle, and overall strengthening reduction of the material occurs thereby. This process showed to be non-suppressible and determines a limit for maximum applicable steel temperature alloyed by nitrogen to about 620°C. It presents an important microstructure degradation mechanism that causes substantial drop in the creep strength level. However, nitrides and carbonitrides are an important reinforcing factor to which P91/92 steels owe much of their creep strength (unless dissolution and precipitation of the Z-phase occurs).

Under COST 501 programme, E911 steel was developed with the guiding chemical composition (9-10)%Cr-l%Mo-l%W-0.25%V-0.05%Nb-0.05%N. The creep strength of said steel is almost RmT/io⁵h/6oo°c = 100 MPa. Of the steels developed under COST programme, the steel identified as B2 achieves the highest refractory level with the guiding chemical composition 9%Cr-1.5%Mo-0.25%V-0.05%Nb-0.01%B-0.02%N, where the predicted creep strength is RmT/io⁵h/6oo°c = 120 MPa. The creep tests obtained so far indicate that said steel could achieve the required value of RmT/io⁵h/T°c = 100 MPa at 625 °C. In view of good experience with molybdenum-alloyed steels, current European flow diagrams prefer molybdenum-alloyed steels over tungsten-alloyed ones.

Current trend of the refractory improving in individual steel types focuses on further improvement of substitution strengthening by tungsten additives, and in particular by boron, both for low-alloyed (T23, T24) as well as ferritic 9-12% Cr steels. In this regard, it is worth of mentioning a new Euro steel E911 or Japanese Nf616, disclosed e.g., in: R. Mishnev, N. Dudova, A. Fedoseeva, R. Kaibyshev: Microstructural aspects of superior creep resistance of a 10%Cr martensitic steel. The steel described herein contains 2 % wt. tungsten, 0.008 % wt. boron, and only 0.003 % wt. nitrogen. Owing to this composition, no carbonitrides are formed and therefore, no Z-phase is formed and no jump reduction of creep strength occurs. However, original optimistic assumptions of the refractory guarantee 600°C/180 MPa of the steel based on relatively short-term creep tests are currently being reduced to the level of 600°C/120- 140MPa.

Summary of the invention

The present invention discloses chemical composition of high-chrome chrome steel resistant to creep at elevated temperature. Here, the elevated temperature refers to temperature at least 600°C with upper limit defined up to 650°C. The steel contains 9-12% wt. chrome (Cr), 2-4% wt. cobalt (Co), 0.5-1% wt. molybdenum (Mo), max. 0.4% wt. vanadium (V), 0.005-0.015% wt. boron (B), max. 0.01% wt. nitrogen, more than 2 and max. 4% wt. tungsten (W), 0.1-1% wt. silicon (Si), more than 0.1 and max 1% wt. manganese (Mn), more than 0.05 and max 0.2% wt. niobium (Nb), more than 0.10 and max 0.15% wt. carbon (C), 0.005-0.1% wt. titanium (Ti), max 0.05% wt. aluminium (Al), 0.01-2% wt. copper (Cu), 0.005-0.02% wt. phosphorus (P), and 0.001-0.01% wt. sulphur (S).

The essential is that titanium (Ti) % wt. is more than four times higher than nitrogen (N) % wt. Owing to this, bonding of free nitrogen to titanium nitride TiN is achieved. Then, no nitrogen is available for forming of the deleterious Z-phase.

In a preferred embodiment, copper (Cu) content may exceed 0.5% wt. In this way, precipitation strengthening of matrix by copper precipitates is achieved. The precipitates may then operate also as nucleation sites for precipitation of Laves phase, and diffusion may be finer thereby. This contributes to strengthening of the material.

In another preferred embodiment, boron (B) content may be less than 0.01% wt. This achieves deceleration of carbide growth and Laves phase during creep, i.e., deceleration of degradation of mechanical properties.

The present invention relates to high-chrome steel with optimized chemical composition. The resulting microstructure is high-tempered martensite containing chrome carbides and carbides of other alloying elements of MC and M2C type. Owing to its composition, the steel is not nitrogen-alloyed and no nitrides and carbonitrides of MX type are present as in currently used P91/92-type steels. Owing to this, no phenomena of sudden loss of creep strength due to dissolution of such nitrides and precipitation of Z-phase M2N exist. Absence of MX-type nitrides and carbonitrides is replaced by strengthening of other reinforcing factors so that the creep strength of the new steel is comparable or better than P91/92-type steels. Therefore, the alloying strategy was therefore modified from P91/92-type steels:

The parameters of fundamental alloy elements and technical effect of their inclusion are in particular:

- tungsten content more than 2 and max 4% wt. for improved reinforcement of solid solution and improved dispersion reinforcement upon inevitable precipitation of Laves phase Fe2(W, Mo);

- cobalt content 2 to 4% wt. for improved reinforcement of solid solution and slowing down of phase transformations;

- boron content 0.005 to 0.015% wt. to slow down coarsening of carbides, in particular chrome carbides;

- titanium content 0.005 to 0.1% wt. for alloying to bind all free nitrogen from the steel to TiN, and hence, no nitrogen is available for forming of the Z-phase;

- copper content 0.01 to 2% wt. for forming of precipitates in ferritic matrix - the precipitates reinforce as well as form a dense network of potential nucleation sites for crystals of Laves phase, which may be owing to this expelled more finely, and may have an improved effect on dispersion reinforcement thereby.

The default microstructure of the steel being proposed is consistent with normal temper - high- tempered martensite, tempered to temperatures of about 50°C over maximum working temperature. Before use, the microstructure includes:

- martensitic crystals;

- particles of M23Ce-type carbides (containing in particular chrome), and M2C-type and MC-type (containing in particular further carbide-forming alloying elements - molybdenum, vanadium, niobium);

- copper precipitates.

The following changes in the microstructure may be expected when using at working temperature 600-650°C:

- precipitation of Laves phase Fe2(W, Mo) - depleting tungsten and molybdenum from matrix solid solution (loss of solution reinforcement), formation of dispersion reinforcement by Laves phase particles; - gradual coarsening of carbide particles of all types;

- gradual coarsening of copper precipitates.

The precipitation of Laves phase of said effects causes no sudden creep strength loss. Increased tungsten content compared to experimental steel Nf616 (Mishnev, R., et al.) allowed higher share of precipitated Laves phase. Increasing the share of Laves phase is usually regarded undesirable. There is a concern that very coarse particles of the Laves phase could be detrimental to material toughness. A combination of high boron content and potential copper content may, however, keep Laves phase in the form of finer particles both in the nucleation and growth stadia. Then, higher share of Laves phase is not a disadvantage; on the contrary, it helps in stabilizing the structure for long-term creep exposure. The other effects are just gradual coarsening in the beginning of present particles.

As far as the mechanical and creep properties are concerned, improving the creep strength compared to currently available steels of P91/P92-type will be decisive. Considering the expected application of the steel in power engineering industry, the creep strength needs to be extrapolated for time of at least 10⁵ h. It would be appropriate to increase the creep strength for this time up to 90MPa.

Description of drawings

The exemplary embodiment of the disclosed solution is described with reference to the drawings, in which

Figure 1 - is a chart comparing creep test results for individual steels (mutual lifecycle comparison) at 650°C, where vertical axis depicts stress [MPa] and horizontal axis depicts time until fracture [h], where results for melt No. T20-066 (without nitrogen, relatively low copper, and relatively high boron) are illustrated by squares, the results for melt described in Microstructural aspects of superior creep resistance of a 10%Cr martensitic steel (Mishnev, R, et al.) (identified as Nf616 steel) are illustrated by triangles, and the results for the P92 steel taken from ECCC data sheet are illustrated by circles;

Figure 2 - is a chart comparing creep test results for individual steels (mutual lifecycle comparison) at 650°C, where vertical axis depicts stress [MPa] and horizontal axis depicts time until fracture [h], where results for melt No. T20-069 (without nitrogen, relatively low boron, and relatively high copper) are illustrated by squares, the results for melt described in Microstructural aspects of superior creep resistance of a 10%Cr martensitic steel (Mishnev, R, et al.) (identified as Nf616 steel) are illustrated by triangles, and the results for the P92 steel taken from ECCC data sheet are illustrated by circles;

Figure 3 - is a chart comparing creep test results for individual steels (mutual lifecycle comparison) at 650°C, where vertical axis depicts stress [MPa] and horizontal axis depicts time until fracture [h], where results for melt No. T22-005 (without nitrogen, relatively high boron, and relatively high copper) are illustrated by squares, the results for melt described in Microstructural aspects of superior creep resistance of a 10%Cr martensitic steel (Mishnev, R, et al.) (identified as Nf616 steel) are illustrated by triangles, and the results for the P92 steel taken from ECCC data sheet are illustrated by circles.

The drawings obviously show that the melts No. T20-066, T20-069, and T22-005 according to the present invention have creep strength higher than the reference steels known from the prior art.

Examples of embodiments of the invention

Example 1

A test melt of high-chrome steel resistant to creep at elevated temperature was made. The melt is internally identified as melt No. T20-066. For chemical composition of the steel, refer to Table 1.

Table 1

Long-term creep strength tests were conducted on this melt. The test parameters are: temperature 650°C, stress 220, 200 and 160 MPa. As seen from Figure 1, the steel of said composition is more resistant than the reference steels known from prior art.

With test parameters 650°C and 160MPa, the fracture of the reference test body occurred after 220 hours. With the same test parameters, the test body made from steel identified as melt No. T20-066 showed fracture after 1,921 hours, i.e., 1,701 hours more. Example 2

A test melt of high-chrome steel resistant to creep at elevated temperature was made. The melt is internally identified as melt No. T20-069. For chemical composition of the steel, refer to Table 2.

Table 2

Long-term creep strength tests were conducted on this melt. The test parameters are: temperature 650°C, stress 220, 200, 180 and 160 MPa. As seen from Figure 2, the steel of said composition is more resistant than the reference steel known from prior art.

With test parameters 650°C and 160MPa, the fracture of the reference test body occurred after 220 hours. With the same test parameters, the test body made from steel identified as melt No. T20-069 showed fracture after 2,317 hours, i.e., 2,097 hours more.

Example 3

A test melt of high-chrome steel resistant to creep at elevated temperature was made. The melt is internally identified as melt No. T22-005. For chemical composition of the steel, refer to Table 3. Table 3

For now, only short-term creep strength tests were conducted on this melt. The test parameters are: temperature 650°C, stress 220 and 200 MPa. As seen from Figure 3, the steel of said composition is more resistant than the reference steel known from prior art. It is expected that this steel composition will combine advantages of the increased boron and copper content for long-term creep exposures, but only long-term creep tests will show.

Claims

1. A high-chrome steel resistant to creep at temperatures up to 650°C containing:

Cr in the range from 9 to 12% wt.,

Co in the range from 2 to 4% wt.,

Mo in the range from 0.5 to 1% wt.,

V in the amount of max 0.4% wt.,

B in the range from 0.005 to 0.015% wt.,

N in the amount of max 0.01% wt., and further containing W, Si, Mn, Nb, C, Ti, Al, Cu, and Fe characterized in that it contains W in the range from more than 2 up to 4% wt.; it contains Si the range from 0.1 to 1% wt.; it contains Mn in the range from more than 0.1 up to 1% wt.; it contains Nb in the range from more than 0.05 up to 0.2% wt.; it contains C in the range from more than 0.10 up to 0.15% wt.; it contains Ti in the range from 0.005 to 0.1% wt., wherein Ti weight amount is more than four times higher than of N weight amount, it contains Al in the amount of max 0.05% wt., it contains Cu the range from 0.01 to 2% wt. and it further contains

P in the range from 0.005 to 0.02% wt. and

S in the range from 0.001 to 0.01% wt.

2. The high-chrome steel resistant to creep at elevated temperature according to claim 1 characterized in that the Cu content exceeds 0.5% wt.

3. The high-chrome steel resistant to creep at elevated temperature according to claim 2 characterized in that the B content is lower than 0.01% wt.