RU2404281C1 - Heat-resistant steel for power equipment - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: steel contains the following component ratio, wt %: carbon 0.015 - 0.020, chrome 11.0 - 12.0, nickel 0.05 - 0.15, manganese 0.4 - 0.6, sulphur 0.005 - 0.010, phosphorus 0.005 - 0.015, molybdenum 0.4 - 0.6, nitrogen 0.04 - 0.06, silicone 0.2 - 0.3, niobium 0.04 - 0.06, cobalt 2.5 - 3.0, tungsten 0.4 - 0.6, vanadium 0.12 - 0.20, boron 0.001 - 0.002, titanium 0.03 - 0.05, aluminium 0.010 - 0.015, antimony 0.001 - 0.005, stannum 0.001 - 0.005, arsenic 0.001 - 0.007, and the rest is iron. The following conditions are met between components: Moequiv=(Mo + 0.5W) ëñ 0.85%, 3.35 ëñ (Nb + V + Ti) /(C + N) ëñ 3.85, 14.5 ëñ (Cr + V + Nb)/(Mo + 0.5W) ëñ 17.0, Co/(Mo + V) ëñ 4.6, (P+Sn+Sb+As)ëñ0.025%. ^ EFFECT: steel has high level and stability of heat-resistance characteristics at supercritical steam parametres, which provides improvement of operating reliability and total service life of modern steam power equipment of thermal power units and power stations. ^ 3 tbl

Description

The invention relates to the field of metallurgy, in particular to the selection of the composition of heat-resistant steel, which can be used for the manufacture of rotor blades, rotors and other parts of steam turbines operating at super supercritical parameters of steam.

Known steels used in the specified field of technology in Russia and abroad 15X1M1F, EP 450, 12Cr-WCo-NiVNb; 12Cr-WCo-VNb [1, 2]. It should be noted that the known materials have a complex of sufficiently high mechanical and operational properties.

However, the known steels do not provide the required level of long-term strength and stability of the basic physical, mechanical and service characteristics, which reduces the operability and industrial safety of heat-exchange equipment operating under conditions of long-term operation and exposure to steam of high parameters.

Closest to the chemical composition and technical characteristics of the proposed steel is steel according to the patent RU 2272852 C1 [3], which contains alloying elements in the following ratio, wt.%:

carbon 0.11-0.20 nitrogen 0.005-0.05 manganese 0.1-0.3 chromium 9.0-12.0 nickel 0-0.7 molybdenum 0.9-1.6 tungsten 0-2.0 vanadium 0.15-0.30 niobium 0.02-0.06 boron 0-0.02 silicon 0.03-0.1 tin 0-0.006 antimony 0-0.005 arsenic 0-0.007 sulfur <0.015 phosphorus <0.020 iron rest

the following ratios must be observed:

0.12≤C + N≤0.22;

0.9≤Mo + 0.5W≤2.0;

0.15≤V + Nb≤0.3;

Cr _eq = Cr + 6Si + 4Mo + 1.5W + llV + 5Nb-40C-2Mn-4Ni-30N≥6.5;

P≤ [0.12-5 (Sb + Sn) -As] / 10;

10S≤Mn≤20S.

The disadvantages of the known steel are the low level and insufficient stability of the characteristics of heat resistance in conditions of long-term high-temperature operation of 10 ⁵ hours (σ ⁶⁰⁰ ₁₀ ⁵ = 100 MPa).

The technical result of the present invention is the creation of heat-resistant steel with a high level and stability of the characteristics of heat resistance at super supercritical steam parameters, which improves the operational 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 additional introduction of the known steel (containing carbon, nitrogen, manganese, chromium, nickel, molybdenum, tungsten, vanadium, niobium, boron, silicon, tin, antimony, arsenic, sulfur, phosphorus and iron) titanium, cobalt , aluminum, as well as a significant reduction in carbon content, in the following ratio of components, wt.%:

carbon 0.015-0.020 chromium 11.0-12.0 nickel 0.05-0.15 manganese 0.4-0.6 sulfur 0.005-0.010 phosphorus 0.005-0.015 molybdenum 0.4-0.6 nitrogen 0.04-0.06 silicon 0.2-0.3 niobium 0.04-0.06 cobalt 2.5-3.0 tungsten 0.4-0.6 vanadium 0.12-0.20 boron 0.001-0.002 titanium 0.03-0.05 aluminum 0.010-0.015 antimony 0.001-0.005 tin 0.001-0.005 arsenic 0.001-0.007 iron rest

the following ratios must be observed:

Molybdenum equivalent: Mo _equiv = (Mo + 0.5W) ≤0.85%

the total ratio of impurity elements (P + Sn + Sb + As) should not exceed 0.025%.

The ratio of these alloying elements and the accepted limitations of the total content of some of them are selected so that the steel after appropriate heat treatment provides the required level of physical and mechanical properties and long-term strength. The heat treatment of the inventive steel is austenitization at a temperature of 1100-1150 ° C and tempering at a temperature of 750-780 ° C for 10 hours. After austenitization and tempering, the steel structure is tempered martensite with a high dislocation density. The inventive steel implements the mechanism of nanoscale self-regulation of the structure under long-term operation, which consists in fixing dislocations with nanoscale precipitates (no more than 20-30 nm in size), which are also highly stable when exposed to elevated temperatures and high voltages.

The minimum carbon content (up to 0.02%) during smelting in a vacuum induction furnace allows one to suppress the release of unstable carbides and activate the action of nitrogen, forming nitrides and carbonitrides, which are more thermodynamically stable and therefore have a more favorable effect on long-term strength than carbides.

An increase in the content of carbon and chromium above that indicated in the claims contributes to the precipitation of carbides of the type M ₂₃ C ₆ and their accelerated coagulation along grain boundaries, a decrease in the dispersion of the precipitated phases, and depletion of the solid solution, which leads to a decrease in the characteristics of short-term and long-term strength.

The nitrogen content was chosen so that it was sufficient for the formation of finely divided nitrides and, at the same time, did not lead to the formation of the Z phase, which quickly coagulates at high temperatures and leads to a decrease in heat resistance during long-term operation.

To increase heat resistance and improve long-term strength, as well as to suppress the formation of δ-ferrite, 2.5-3.0% cobalt was introduced into the inventive steel, and the manganese content was increased for this purpose in comparison with the known steel.

Steel has a lower molybdenum content compared with the known steel to suppress the release of Laves phases, which, like the Z phase, quickly coagulate at high temperatures, which leads to a decrease in heat resistance characteristics.

Due to the fact that modern equipment should be operated at temperatures up to 620 ° С, the following ratio of molybdenum and tungsten is provided in its composition to increase the heat resistance of the claimed material: Mo _equiv = (% Mo + 0.5% W) ≤0.85%, This ratio suppresses the intense isolation and coagulation of the Laves phases and the hardening of the matrix solid solution by tungsten and molybdenum is maintained for a long period of operation.

The limits of nickel content in the inventive composition are narrowed to 0.05-0.15% to increase the tempering resistance and reduce the sensitivity of steel to temper brittleness. An increase in the nickel content over the set limit will lead to an increase in sensitivity to tempering and thermal brittleness, as a result of which operational reliability can be significantly reduced. The critical temperature of brittleness, low values of which are a criterion of high resistance to brittle fracture, at significant nickel contents can increase not only as a result of prolonged operation at elevated temperature, but also during slow cooling from tempering temperature [4].

To increase the long-term strength during operation at supercritical temperatures and the possibility of realizing hardening with fine particles, titanium in the amount of 0.03-0.05%, which is an extremely strong carbide former, was introduced into the declared steel grade. Titanium carbide (release size 20-30 nm) is highly stable and passes into the solid solution only at very high temperatures. Increasing the titanium content above the specified limit is impractical, because can lead to a deterioration in the manufacturability of steel, making it difficult to conduct hot plastic processing, and complicating the smelting of complex alloys.

To increase the scale resistance, grinding grain in the inventive steel grade, aluminum was additionally introduced in an amount of 0.010-0.015%, which is also a good deoxidizer and desulfurizer. The aluminum content above the specified limits can lead to a decrease in heat resistance, because coarse Al-N particles are formed at the grain boundaries. At the boundaries of these particles, in turn, voids during creep are initiated, and the nitrogen content is also reduced, which can form vanadium carbonitrides and other elements that are more thermodynamically stable than carbides and provide high creep resistance [7].

To achieve maximum strength, the steel must be completely martensitic after cooling in air, any δ-ferrite content reduces its strength: with an increase in the amount of δ-ferrite, the embrittlement of steel increases with prolonged exposure to elevated temperature [5]. In order to predict the amount of δ-ferrite formed in high-chromium steels, the concept of the chromium equivalent was introduced, which is exactly consistent with the equivalent amount of Cr in the Fe-Cr binary system. The formula expresses the chromium equivalent as a linear relationship of various alloying elements through regression analysis. The coefficients for the elements are an estimate of their influence on the γ-region, which is associated with the formation of δ-ferrite. The criterion for the absence of δ-ferrite is the value of the chromium equivalent not exceeding 10%: Cr _equiv = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb-40C-2Mn-4Ni-2Co-30N≤10% [6].

With prolonged exposure to elevated operating temperatures up to 620 ° C, segregation of impurity elements such as Sb, P, Sn, and As at grain boundaries is possible, which leads 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, a restriction was introduced on the content of impurity elements: Σ (P + Sn + Sb + As) ≤0.025%.

At Lasmet LLC, with the participation of FSUE Central Research Institute KM Prometey, 3 pilot melts weighing 100 kg each were performed. Metal was smelted in vacuum induction furnaces. The ingots were cast in a vacuum. The resulting metal was subjected to pressure treatment on industrial forging and rolling equipment.

Static tensile, impact, and long-term strength samples were made from heat-treated material.

The chemical composition of the investigated materials and the results of determining the mechanical and service properties are given in tables 1-3.

The results of comparative tests show the advantage of steel of the claimed composition in terms of mechanical properties and a significant advantage of the claimed composition in terms of performance, which provides increased operational reliability and overall service life of modern and promising steam-powered equipment of thermal power units and power plants, in particular for equipment operating at super supercritical steam parameters.

Table 1 The chemical composition of the claimed and known steel grades Steel Conventional heat number The content of elements in wt.% Carbon Silicon Manganese Chromium Nickel Molybdenum Vanadium Niobium Cobalt Sulfur Phosphorus proposed one 0.017 0.25 0.5 11.0 0.10 0.50 0.16 0.05 2.5 0.007 0.005 2 0.015 0.30 0.6 12.0 0.15 0.60 0.12 0.06 3.0 0.005 0.010 3 0.020 0.20 0.4 11.5 0.05 0.40 0.20 0.04 2.8 0.010 0.015 famous four 0.150 0.07 0.2 10.0 0.35 1.30 0.23 0.04 - 0.011 0.010 Continuation of table 1 The chemical composition of the claimed and known steel grades Steel Conventional heat number The content of elements in wt.% Tungsten Nitrogen Aluminum Titanium Boron Antimony Arsenic Tin proposed one 0.5 0.05 0.015 0.04 0.0010 0.003 0.007 0.005 2 0.4 0.04 0.012 0.03 0.0020 0.005 0.001 0.003 3 0.6 0.06 0.012 0.05 0.0015 0.001 0.004 0.001 famous four 1.0 0.02 - 0.01 0.003 0.005 0.003 Continuation of table 1 The chemical composition of the claimed and known steel grades Steel Conventional heat number The content of elements in wt.% Moe _equ ^*

proposed one 0.75 3.73 14.95 3.79 0.020 2 0.80 3.81 15.23 4.17 0.019 3 0.70 3.63 16.77 4.58 0.021 famous four 1.80 1.59 5.71 - 0.021

table 2 The mechanical properties of the claimed and known steel grades Steel Conventional heat number Mechanical properties at test temperature + 20 ° С σ _in , MPa σ _0.2 , MPa δ,% Ψ,% no less proposed one 750 630 17 61 2 800 698 eighteen 63 3 860 745 twenty 70 famous four 730 600 fifteen 55

Table 3 The test results for long-term strength of the claimed and known steel Steel Conventional heat number The tensile strength on the basis of 10 ⁵ hours at a temperature of 600 ° C

, МПа

MPa proposed one 114 2 113 3 115 famous four one hundred

Note:

1) the results of mechanical tests are averaged over three samples per point;

2) mechanical properties are presented after tempering;

3) the long-term strength is determined on the basis of the results of accelerated tests at higher temperatures.

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.

3. Patent RU 2272852 C1.

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

5. F.B. Pickering. Physical metallurgy and steel development. Translation from English. Edited by Ph.D. G.V. Shcheberdinsky // Moscow "Metallurgy". - 1982. - 182 p.

6. S.H. Ryu and Jin Yu. A New Equation for the Cr Equivalent in 9 to 12 Pct Cr Steels. // Metallurgical and Materials Transactions A. - vol. 29A, June 1998, p. 1573 - 1578.

7. 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 (1)

Heat-resistant steel for power equipment containing carbon, nitrogen, manganese, chromium, nickel, molybdenum, tungsten, vanadium, niobium, boron, silicon, tin, antimony, arsenic, sulfur, phosphorus, characterized in that it additionally contains titanium, cobalt, aluminum with a limited content of carbon, nickel, tungsten and molybdenum in the following ratio of components, wt.%:
carbon 0.015-0.020 chromium 11.0-12.0 nickel 0.05-0.15 manganese 0.4-0.6 sulfur 0.005-0.010 phosphorus 0.005-0.015 molybdenum 0.4-0.6 nitrogen 0.04-0.06 silicon 0.2-0.3 niobium 0.04-0.06 cobalt 2.5-3.0 tungsten 0.4-0.6 vanadium 0.12-0.20 boron 0.001-0.002 titanium 0.03-0.05 aluminum 0.010-0.015 antimony 0.001-0.005 tin 0.001-0.005 arsenic 0.001-0.007 iron rest,
subject to the following ratios:
molybdenum equivalent: Mo _EKB = (Mo + 0.5) ≤0.85%,

total ratio of impurity elements (P + Sn + Sb + As) ≤0.025%.