RU2426814C2 - Heat resistant steel for power engineering - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, nitrogen, niobium, tungsten, titanium, boron, aluminium, copper, iron and impurities at following ratio of components, wt %: carbon 0.005 - 0.02, silicon 0.30 - 0.50, manganese 0.40 - 0.60, chromium 8.00 - 9.00, nickel 0.40 - 0.60, molybdenum 0.40 - 0.60, tungsten 0.80 - 1.00, vanadium 0.20 - 0.30, niobium 0.04 - 0.06, nitrogen 0.04 - 0.06, titanium 0.01 - 0.03, aluminium 0.005 - 0.015, boron 0.001 - 0.002, copper 0.20 - 0.30, iron and impurities the rest. As impurities steel contains sulphur 0.001 - 0.01 wt %, phosphorus 0.001 - 0.015 wt %, and also tin, antimony and arsenic, also summary contents of phosphorus, antimony, tin and arsenic is not more, than 0.02 wt %.

EFFECT: increased durability and stable heat resistance at super-overcritical parameters of steam, which increases operational reliability and general resource of equipment operation.

2 cl, 3 tbl

Description

The invention relates to the field of metallurgy of heat-resistant steels of the martensitic class, containing iron with a different combination of alloying elements as a base, and is intended for use in power engineering in the manufacture of steam turbine elements with supercritical steam parameters.

In Russia and abroad, steels of the type 9Cr-1MoVNb and 9Cr-0.5Mo-1.8WVNb are used in the indicated field of technology [1, 2].

With the increase in steam parameters, in order to increase the efficiency of steam turbines, it became necessary to create materials for SSCP units with higher durability and greater stability of physicomechanical properties under prolonged exposure to pressure and elevated temperatures than the materials used.

Closest to the chemical composition and technical characteristics of the proposed steel is steel according to patent RU 2328547 C2 [3] (prototype) containing alloying components, wt.%:

carbon 0.10-0.18 silicon 0.05-0.10 manganese 0.10-0.70 chromium 9.50-11.00 nickel 0.00-0.70 molybdenum 1.00-2.00 vanadium 0.15-0.30 niobium 0.02-0.08 nitrogen 0.01-0.05 calcium 0.001-0.05 sulfur 0.002-0.012 phosphorus 0.002-0.012 iron rest

while the total content of sulfur and phosphorus does not exceed 0.020%.

This steel grade is recommended for use in the manufacture of rotors, shafts and other parts of high and medium pressure steam turbines, but the disadvantage of the prototype is the insufficient long-term strength and the lack of stability of the heat resistance characteristics in the conditions of long-term high-temperature operation.

The technical result of the present invention is the creation of heat-resistant steel with an increased level of long-term strength (not less than 120 MPa on the basis of 100,000 hours at a temperature of 600 ° C: σ ⁶⁰⁰ ₁₀ ⁵ ≥120 MPa) and the stability of the characteristics of heat resistance at super supercritical steam parameters, which ensures an increase in operational the reliability and overall service life of modern steam-powered equipment of thermal power units and power plants.

The technical result is achieved due to the fact that the composition of the known steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, niobium, nitrogen, calcium, sulfur, phosphorus and iron, additionally introduced tungsten, titanium, boron, aluminum and copper, and also the carbon and chromium content decreases and the silicon content increases, the content of antimony, arsenic, and tin impurities is controlled in the following ratio of components, wt.%:

carbon 0.005-0.02 silicon 0.30-0.50 manganese 0.40-0.60 chromium 8.00-9.00 nickel 0.40-0.60 molybdenum 0.40-0.60 tungsten 0.80-1.00 vanadium 0.20-0.30 niobium 0.04-0.06 nitrogen 0.04-0.06 titanium 0.01-0.03 aluminum 0.005-0.015 boron 0.001-0.002 copper 0.20-0.30 iron and impurities, including sulfur 0.001-0.01 and phosphorus 0.001-0.015 rest,

as impurities, steel additionally contains tin, antimony and arsenic when they are contained:

tin 0.001-0.005 antimony 0.001-0.005 arsenic 0.001-0.007

the following ratios must be observed:

a) chromium equivalent:

Cr _eq = Cr + 0.8Si + 2Mo + 1W + 4V + 2Nb + 1.7Al + 60B + 2Ti-2Ni-0.4Mn-0.6Cu-20N-20C≤9.5%;

b) molybdenum equivalent: Mo _equiv = Mo + 0.5W should not exceed 1.0%

c) the total ratio of impurities (P + Sn + Sb + As) should be no more than 0.020%;

d) 0.20 ((Ti + N) / (Nb + V) 0 0.25;

d) 0.650≤ (Ti + C + N) / (Nb + C + N) ≤0.910;

e) 0.060≤ (C + N) / (Mo + 0.5W) ≤0.070.

The ratio of these alloying elements and the accepted limitations of the total content of some of them are selected in such a way that steel, after appropriate special heat treatment, provides the required level of long-term strength and stability of the most important physical and mechanical properties that determine the material's working ability under operating conditions of the equipment. At the indicated content of alloying elements after heat treatment in steel, a hardening effect is achieved due to the release of nanosized (no more than 20-30 nm) particles that fix dislocations during steel operation and are highly stable when exposed to elevated temperatures and stresses.

In the inventive steel grade, in comparison with the prototype, the carbon content is reduced to the minimum achievable in practice when smelted in a vacuum induction furnace: not more than 0.02% instead of 0.10-0.18% in known steel. This makes it possible to suppress the release of unstable carbides and realize the action of nitrogen, forming nitrides and carbonitrides, which have greater thermodynamic stability and, therefore, have a more favorable effect on long-term strength than carbides. An increase in the carbon content above that indicated in the formula contributes to the rapid precipitation of carbides and their accelerated coagulation, especially along grain boundaries, to a decrease in the dispersion of precipitated phases, which leads to a decrease in long-term strength characteristics.

The nitrogen content was chosen so that it was sufficient for the formation of finely dispersed nitrides and at the same time did not contribute to the formation of the Z phase (complex nitride Cr (Nb, V) N), which rapidly coagulates at high temperatures and leads to a decrease in heat resistance during long-term operation .

Chrome is a ferrite-forming element. Chromium is the main element in heat-resistant martensitic and ferritic-martensitic steels, which provides martensitic hardenability and high resistance to corrosion and oxidation. As a rule, in steel of this class it is introduced in an amount of more than 8%. It strengthens the solid solution and provides the formation of chromium carbides, mainly, Cr ₂₃ C ₆ . However, chromium carbides are released mainly along grain boundaries and coagulate very quickly at high operating temperatures. In addition, chromium inhibits the release of more thermally stable finely dispersed carbides of molybdenum, vanadium and niobium, which coagulate more slowly and, therefore, make a greater contribution to hardening and provide more stable long-term strength characteristics [8]. As a result, the maximum chromium content should not exceed 9.00%. An increase in the chromium content above the specified limit negatively affects the long-term strength.

In this regard, the chromium content in the inventive steel grade was reduced compared to the prototype to 8.00-9.00%, which will reduce the emission of chromium carbides at the grain boundaries.

Molybdenum and tungsten are usually introduced into steel for parts of power equipment operating at high temperatures in order to increase long-term strength and ductility. This is due to their ability to delay diffusion processes, increase the temperature of recrystallization, and harden the basic solid solution.

Molybdenum additive is a means of increasing the heat resistance of steels containing elevated and high amounts of chromium. When the molybdenum content is up to 1.5%, it limits the coagulation of carbides. At a higher molybdenum content (above 1.5%), it participates in the formation of the carbide phase, which is both special molybdenum carbides of the type MeC, Me ₂ C, and carbides of the type Me ₃ C, Me ₇ C ₃ , Me ₂₃ C ₆ , the main component of which is chromium [4], as well as the intermetallic phases of Laves, prone to rapid coagulation. The presence of these phases negatively affects the heat resistance. In this regard, in the declared steel grade, the molybdenum content was limited to 0.40-0.60% instead of 1.00-2.00% in the prototype. In the declared steel grade, molybdenum was also partially replaced by tungsten in an amount of 0.80-1.00%, which contributes to increased heat resistance.

Tungsten promotes the formation of inclusions of the secondary (hardening) phase, which is either complex carbide with the participation of tungsten or tungsten-rich intermetallic compounds of the Laves phase type ((Fe, Cr) ₂ (Mo, W)). Entering complex carbides, such as Me ₂₃ C ₆ , tungsten delays their coagulation. However, the tungsten content above the specified limits (above 1.00%) can have a negative effect on creep resistance and reduce long-term strength during long-term operation due to the acceleration of the release and coagulation of the Laves phases.

When alloying steel with molybdenum and tungsten, it is necessary that the molybdenum equivalent of Mo _equiv = Mo + 0.5W does not exceed 1.0% (formula b)). With a molybdenum equivalent of 7.0%, long-term strength reaches its maximum. This is due to the fact that at higher values of the molybdenum equivalent in steel, a significant amount of intermetallic phases (Laves phases) is formed, which leads to an increase in long-term strength with relatively short tests, but then during longer tests the characteristics of long-term strength decrease due to the coagulation of these phases.

To obtain a fine-grained structure and high long-term strength, the optimal niobium content is ≈0.04-0.06% [5]. The introduction of niobium additives in the amount of 0.04-0.06% into the composition of heat-resistant steel promotes the formation of finely dispersed needle-shaped niobium carbonitrides located on the grain body, characterized by a high distribution density, a size of the order of several nanometers, and increased stability during high-temperature operation. This ensures that the required level of long-term strength is achieved. Improving the structural stability of steel, the formation during the tempering of a sufficient amount of finely dispersed niobium carbonitrides, stable over a wide temperature range, helps to ensure a high level of strength both after the main heat treatment and during operational heating.

A higher Nb content is undesirable due to its low solubility at austenitization temperatures of 1100-1150 ° C, as a result of which it will be present in steel with non-metallic inclusions, and not be released during tempering in the form of dispersed carbonitrides and nitrides. Also, the high niobium content degrades the processability of steel during hot processing.

In the inventive steel, the nickel content was narrowed to 0.4-0.6% instead of 0.0-0.7%. Nickel is an austenite-forming element, in connection with which an increase in the lower limit of nickel content to 0.4% will suppress the formation of δ-ferrite, which has a negative effect on the long-term strength of steel. An increase in the nickel content over established limits will lead to an increase in sensitivity to tempering (in the case of delayed cooling during tempering) and thermal brittleness (during prolonged operation at elevated temperatures), as a result, operational reliability can significantly decrease.

To suppress the formation of δ-ferrite, copper in an amount of 0.2-0.3% was also introduced into the composition of the inventive steel as an austenite-forming element.

To increase the long-term strength during operation at elevated temperatures and the possibility of implementing hardening with fine particles, titanium in an amount of 0.01-0.03% was additionally introduced into the declared steel grade. Titanium is an extremely strong carbide forming element. Even at relatively low titanium contents in steels, along with cementite, nanosized (20–30 nm) precipitates of TiC titanium carbide appear. Titanium carbide is highly stable and dissolves only at very high temperatures. At the same time, an increase in the titanium content above the specified limit leads to a decrease in the manufacturability of steels, making it difficult to conduct hot plastic processing, and complicating the smelting of complex alloys [8].

In the claimed steel grade, the silicon content was increased from 0.05-0.10% in the prototype to 0.30-0.50%. In the production of steel selected as a prototype, carbon was used for deoxidation; silicon is used as a deoxidizing agent in the declared steel grade due to the limitation of carbon content. In addition, alloying of heat-resistant high-chromium steel with silicon in an amount of 0.30-0.50% will make it possible to obtain more dense oxide protective films, thereby increasing the heat resistance.

To increase the scale resistance and grain refinement, aluminum in the amount of 0.005-0.015%, which is also a good deoxidizer and desulfurizer, was additionally introduced into the inventive steel grade. The aluminum content above the specified limits can lead to a decrease in heat resistance due to the formation of coarse Al-N aluminum nitrides at the grain boundaries and deterioration of metallurgical quality due to the formation of a significant amount of non-metallic inclusions. At the boundaries of aluminum nitrides, in turn, voids during creep are initiated, and the nitrogen content decreases, which can form vanadium, niobium, and titanium carbonitrides that are more thermodynamically stable than carbides and provide high creep resistance [9].

The inventive steel is additionally alloyed with boron in an amount of 0.001-0.002%. Additives of small amounts of boron contribute to a significant strengthening of grain boundaries and increase the long-term strength of high alloy steels for steam power plants operating at temperatures above 620 ° C [8].

To achieve maximum strength, steel should have a completely martensitic structure and not contain δ-ferrite after cooling in air, since it was established [7] that the presence of δ-ferrite reduces its strength, in addition, the degree of embrittlement of steel increases with an increase in the amount of ferrite prolonged exposure to elevated temperature. In order to predict the amount of δ ferrite formed in the structure of high-chromium steels, the concept of chromium equivalent was introduced. Formula a) expresses the chromium equivalent as a linear dependence on the content of various alloying elements. The coefficients for chemical elements are an estimate of their influence on the formation of δ-ferrite. The criterion for the absence of δ-ferrite is the value of the chromium equivalent, not exceeding 9.5% (formula a)).

With prolonged exposure to elevated operating temperatures from 500 to 630 ° C, segregation of impurity elements, such as P, Sn, Sb, and As, at grain boundaries is possible, which can lead to the appearance of areas of intergranular fracture in fractures of samples. In this case, there is a decrease in resistance to brittle fracture, an increase in the critical temperature of brittleness of steel. In this regard, it is necessary to limit the total content of these elements (P + Sn + Sb + As) to a value of not more than 0.020% (formula c)).

In addition to these ratios, the composition of the inventive steel must have additional restrictions expressed by formulas d) -f).

The limitation in formula d) is due to the fact that Nb and V can be released on the facets of the initially released titanium nitrides, which do not dissolve even at a temperature of 1200 ° C. This leads to a decrease in the volume of precipitates during tempering, increases the particle size and reduces the efficiency of their impact on grain size and strength.

The limitation by the formula d) provides the optimal ratio of titanium and niobium carbonitrides, which are the main hardeners.

The restriction according to formula e) establishes a ratio that allows hardening of the solid solution, the liberation of molybdenum and tungsten carbides and carbonitrides and limits the formation of Laves phases.

Three experimental-industrial swimming trunks weighing 100 kg each were carried out at Lasmet LLC with the participation of FSUE CRI CM Prometin. Metal was smelted in vacuum induction furnaces. The ingots were cast in a vacuum. The resulting metal was subjected to hot pressure treatment on industrial forging and rolling equipment.

From heat-treated material were made samples for static tension at a temperature of 20 ° C, impact and long-term strength.

The chemical composition of the investigated materials and the results of determining the mechanical and service properties are shown in tables 1 and 2.

The results of comparative tests of metal melts show some advantage of the steel of the claimed composition in mechanical properties and a significant advantage of the claimed steel in terms of performance.

Information sources

1. V.Yu. Skulsky, A.K. Tsaryuk. Problems of choosing weldable steel for high-temperature components of TPP power units (review). // Automatic welding. - 2004. - No. 3.

2. V.Yu. Skulsky, A.K. Tsaryuk. New heat-resistant steels for the manufacture of welded units of power units (review). // Automatic welding. - 2004. - No. 4. - p. 35-40.

3. Patent RU 2328547 C2.

4. V.F. Rezinsky, V.I. Gladstein, G.D. Avrutsky. The increase in the resource of long-running steam turbines. Chapter 2. Heat resistant steels for high temperature steam turbine components. // Moscow. - Publishing house MPEI. - 2007.

5. Hizume, Takeda, Ekota, Takano, Suzuki, Kinoshita, Koono, Tsuchiyama. New steel of the 12% Cr type for turbine rotors as applied to a steam temperature of 593 ° C // Theoretical Foundations of Engineering Calculations. - 1988, No. 3. - S. 55-66.

6. K.A. Lanskaya. Heat resistant steel. - "Metallurgy" - Moscow, 1969. - 246 p.

7. F.B. Pickering. Physical metallurgy and steel development. Per. from English Ed. Doctor of Philosophy G.V.Scheberdinsky // Moscow, "Metallurgy". - 1982. - 182 p.

8. P. B. Mikhailov-Mikheev. Metal gas turbines // GNTI Engineering literature. - 1958. - M.-L. - 352 p.

9. R. L. Bodnar and R. F. Capellini. Effects of residual elements in heavy forgings: past, present and future. // ASTM STP 979. - Philadelphia. - 1988. - p. 47-82.

Claims (2)

1. Heat-resistant steel for high-temperature elements of power equipment, containing carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, niobium, nitrogen, tungsten, titanium, boron, aluminum, copper, iron and impurities, characterized in that it contains components in the following ratio, wt.%:
carbon 0.005-0.02 silicon 0.30-0.50 manganese 0.40-0.60 chromium 8.00-9.00 nickel 0.40-0.60 molybdenum 0.40-0.60 tungsten 0.80-1.00 vanadium 0.20-0.30 niobium 0.04-0.06 nitrogen 0.04-0.06 titanium 0.01-0.03 aluminum 0.005-0.015 boron 0.001-0.002 copper 0.20-0.30 iron and impurities, including sulfur 0.001-0.01 and phosphorus 0.001-0.015 rest
the following ratios are observed:
chrome equivalent:
Cr _eq = Cr + 0.8Si + 2Mo + 1W + 4V + 2Nb + 1.7Al + 60B + 2Ti-2Ni-0.4Mn-0.6Сu-20N-20C≤9.5%,
molybdenum equivalent:
Mo _equiv = Mo + 0.5W should not exceed 1.0%,
0.20≤ (Ti + N) / (Nb + V) ≤0.25,
0.650≤ (Ti + C + N) / (Nb + C + N) ≤0.910,
0.060≤ (C + N) / (Mo + 0.5W) ≤0.070. 2. Steel according to claim 1, characterized in that it additionally contains tin, antimony and arsenic as impurities, while the total content of phosphorus, antimony, tin and arsenic is not more than 0.02 wt.%.