RU2585591C1 - Heat-resistant steel of martensitic class - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to heat-resistant chromium martensite steel used for manufacture of blades of turbine power plants. Steel contains components in following ratio, wt%: carbon 0.08-0.12, silicon less than 0.1, manganese 0.05-0.1, chromium 9.5-10.0, nickel not more than 0.2, tungsten 2.3-3.0, molybdenum 0.05-0.1, cobalt 2.5-3.5, vanadium 0.18-0.25, niobium 0.04-0.07, nitrogen no more than 0.003, boron 0.008-0.013, rhenium 0.1-0.2, sulphur no more than 0.006, phosphorus no more than 0.01, aluminium no more than 0.01, copper 0.05-0.3, titanium no more than 0.01, iron - balance.

EFFECT: higher long-term strength values, which allows using steel in heat engineering lift working temperature of thermal power plants to 630°C.

1 cl, 3 tbl, 1 ex

Description

The invention relates to the field of metallurgy, in particular to heat-resistant chrome steels of the martensitic class, containing 5-13% Cr, used in the energy industry for the manufacture of equipment for heat and gas turbines. The proposed steel can be used for the manufacture of blades of steam turbines of power plants with operating temperatures up to 630 ° C.

In the construction of the blades, corrosion-resistant and heat-resistant steels are used in accordance with GOST 18968-73, as well as metal alloys based on nickel. Steel grade 20X13 is used for blades operating at temperatures up to 440 ° C. At higher temperatures up to 540 ° C, 15X11MF steel is prescribed for the manufacture of blades. At temperatures up to 580 ° C, steel grade 15X12 VNMF is used. These steels belong to the martensitic-ferritic and martensitic classes. The chemical composition of these steels according to GOST 5632-72 is shown in Table 1.

Table 1 The chemical composition of steels of martensitic class 20X13, 15X11MF, 13X11H2 B2MF, 20X12 VNMF according to GOST 5632-72 Items Mass fraction of elements, mass. % 20X13 15X11MF 15X12 VNMF Carbon 0.160-0.250 0,120-0,190 0.12 - 0.18 Silicon no more than 0,800 no more than 0,500 no more than 0.4 Manganese no more than 0,800 no more than 0,700 0.5 - 0.9 Chromium 12,000-14,000 10,000-11,500 11 - 13 Nickel - - 0.4 - 0.8 Titanium - - no more than 0.2 Aluminum - - - Tungsten - - 0.7 - 1.1 Molybdenum - 0.600-0.800 0.5 - 0.7 Niobium - - - Vanadium - 0.250-0.400 0.15 - 0.3 Iron main main main Sulfur 0,025 0,025 no more than 0,025 Phosphorus 0,030 0,030 no more than 0,030 Copper - - no more than 0.3

The disadvantages of steels 20X13, 15X11MF, 15X12 VNMF are their low heat resistance at temperatures above 580 ° C, as well as limited weldability, which makes them impossible to use for the manufacture of turbine blades for operation at temperatures above 580 ° C. To increase the operating temperature of the turbine, it is necessary to produce blades from austenitic or heat-resistant nickel alloys. It should be noted that the use of blades made of martensitic steel steels containing 9-12% chromium has an advantage over austenitic steel steels and heat-resistant nickel alloys. Firstly, the rotors are also made of martensitic steel, therefore, the coefficient of thermal expansion will be close for the two materials and no structural changes will be required to compensate for thermal expansion. Secondly, in the literature there is little information about the experience of using austenitic and nickel steels for turbine blades of coal-fired power plants.

Closest to the proposed steel is the steel disclosed in patent RU2447184 (published on 04/10/2012]). Steel contains, mass. %:

carbon 0,080-0,120 silicon no more than 0,100 manganese 0,050-0,100 chromium 9,500-10,000 nickel no more than 0,200 tungsten 1,800-2,200 molybdenum 0.6-0.8 vanadium 0.180-0.250 niobium 0,040-0,070 nitrogen no more than 0,003 boron 0.008-0.01 cobalt 2.5-3.5 sulfur no more than 0,006 phosphorus no more than 0,010 aluminum no more than 0,010 copper no more than 0,010 titanium no more than 0,010 iron rest

The content of molybdenum and tungsten is defined as% W / 2 +% Mo <1.5. This steel has a high level of creep resistance up to a temperature of 630 ° C. This allows you to use it for the manufacture of turbine blades of power plants operating at 600-620 ° C.

These properties of steel are achieved due to the formation of a troostomartensitic structure during heat treatment. In the process of average tempering from martensite, most of the carbon in the form of carbides is released, and the processes of polygonization and recrystallization do not begin. A feature of alloying steel is a low nitrogen content. It is known that an increased nitrogen content causes the formation of vanadium VN nitrides, which with long-term creep transform into large particles of the Z phase of CrVN, which negatively affects the heat resistance of steel. The main disadvantage of this steel is that during creep, the Laves phase, Fe ₂ (W, Mo), is released. This process leads to depletion of the solid solution by tungsten and reduces solid solution hardening. Particles of the Laves phase coagulate very quickly, therefore, an increase in dispersion hardening due to their release does not compensate for a decrease in solid solution hardening.

The task of the invention is to eliminate the disadvantage of the prototype.

EFFECT: proposed steel has enhanced characteristics of long-term strength due to increased creep resistance and as a result is operable at a temperature of 630 ° C, which is 20-40 ° C higher compared to existing analogues.

The problem is solved by the proposed heat-resistant steel of the martensitic class, the composition of which is additionally introduced rhenium, the amount of molybdenum and tungsten is changed, in the following ratio of components, mass. %:

carbon 0,080-0,120 silicon no more than 0,100 manganese 0,050-0,100 chromium 9,500-10,000 nickel no more than 0,200 tungsten 2,300-3,000 molybdenum 0.05-0.1 vanadium 0.180-0.250 niobium 0,040-0,070 nitrogen no more than 0,003 boron 0.008-0.013 cobalt 2.5-3.5 rhenium 0.1-0.2 sulfur no more than 0,006 phosphorus no more than 0,010 aluminum no more than 0,010 copper 0,050-0,3 titanium no more than 0,010 iron rest

The composition of the proposed steel contains the following known features.

The carbon content in the amount of 0.08-0.12% increases the hardenability of steel, and also provides the formation of carbides of the type Me ₂₃ C ₆ . A carbon content of less than 0.08% does not provide the necessary level of short-term mechanical properties and long-term strength. An increase in carbon over 0.12% is impractical because degrades weldability of steel.

Molybdenum and tungsten in a total amount of 2.0-3.0% harden the solid solution, and also are part of carbides of the type Me ₂₃ C ₆ and impede their coagulation, which increases the heat-resistant properties of steel.

The content of vanadium in the amount of 0.18-0.25% and niobium to 0.07-0.1% provides hardening of the solid solution and the production of finer carbonitrides, which increases the long-term strength.

Cobalt in the amount of 2.5-3.5% increases solid solution hardening. As an austenite-forming element, cobalt inhibits the formation of delta ferrite. When the cobalt content is less than 2.5%, delta ferrite is formed. With an excess cobalt content of more than 3.5%, there is a decrease in the ductility of steel.

Titanium in an amount of not more than 0.01% contributes to the formation and stabilization of small MX carbonitrides enriched in vanadium and niobium. When the titanium content exceeds 0.01%, large carbonitrides are formed, which reduces the creep resistance.

Limiting the content of phosphorus to 0.01% and sulfur to 0.006% contributes to higher ductility characteristics of steel.

The introduction of boron in an amount of 0.008-0.013% increases the resistance to deformation during creep. Boron segregates along grain boundaries, mainly former austenitic, which suppresses grain-boundary slippage and thereby increases the time to failure. Boron in the proposed steel is part of carbides of the type Me ₂₃ C ₆ and reduces the rate of their coagulation at elevated temperatures, which increases the resistance to deformation during creep. In addition, boron increases stress corrosion resistance and eliminates the adverse effect of high vanadium content on scale resistance.

As deoxidizing agents, manganese in the amount of 0.05-0.1%, silicon in the amount of not more than 0.1%, nickel in the amount of not more than 0.2% and aluminum in the amount of not more than 0.01% were introduced into the composition of the steel. With a manganese content of more than 0.1% and silicon of more than 0.1%, the tendency to form delta ferrite increases, which adversely affects the toughness. Manganese also contributes to the release of carbides M ₂₃ C ₆ . Nickel improves hardenability of steel and toughness, inhibits the formation of delta ferrite. An increase in the nickel content of over 0.2% is impractical, since it reduces the long-term strength due to the acceleration of particle enlargement. When the aluminum content exceeds 0.01%, nitrides are formed, which reduce the long-term strength.

The proposed steel includes the following new, unknown from the prior art, features:

- rhenium in the amount of 0.1-0.2% is included in the composition of the steel, which provides solid-solution hardening by reducing the speed of all diffusion-controlled processes in steel and, accordingly, provides a decrease in the intensity of softening of steel under the influence of temperatures and stresses. Rhenium also reduces the rate of release of the Laves phase enriched in tungsten, Fe ₂ W, which provides an increase in creep resistance due to the preservation of most of the tungsten in the solid solution and, therefore, indicators of long-term strength. When rhenium is added in an amount of less than 0.05%, the effect of this element is negligible. When rhenium is added in an amount of more than 0.5%, the steel is hardened and the ductility and toughness characteristics are significantly reduced;

- the amount of molybdenum is changed to the minimum possible value of 0.05-0.10%, and the amount of tungsten is increased to 2.7-3.0%, provided that the molybdenum equivalent is kept within Mo _eq = Mo + 0.5W = 1.40 ÷ 1.60, which ensures the presence of tungsten in the solid solution during lengthy creep tests without the formation of the Laves phase and increases the heat resistance of steel;

- introduced copper in an amount of 0.2-0.3%, which prevents the formation of delta ferrite during high-temperature deformation, and also contributes to the formation of a finely dispersed Laves phase on copper clusters during creep, which increases the creep resistance of steel. The amount of copper added is determined from the balance of austenite and ferrite stabilizing elements. The copper content is limited to not more than 0.3%.

Implementation example

Alloys of various chemical compositions were cast both within the declared intervals and beyond (Table 2). Alloys were smelted in a vacuum induction furnace. As a charge, pure charge materials were used, which made it possible to obtain a low level of sulfur, phosphorus, and non-ferrous metals in the obtained materials. After stripping, the ingots were forged into billets in the form of bars of a 20 mm square cross section by free forging in the temperature range from 1200 ° C to 900 ° C. Then the hot-rolled bars were subjected to normalization at a temperature of 1050-1060 ° C and tempering at 750-770 ° C for 3 hours.

table 2 The chemical composition of the proposed steel according to examples 1-5 and steel prototype Steel C Si Mn Cr Ni Co Mo W V Nb N B Re Al S P Cu Example 1 0.11 0.06 0.1 10 0.20 3 0.1 3 0.2 0.04 0.003 0.008 0.18 0.01 0.006 0.008 0.30 Example 2 0.08 0,07 0.05 9.5 0.18 2.5 0.05 2,3 0.18 0.05 0.003 0.008 0.10 0.01 0.004 0.007 0.05 Example 3 0.12 0.06 0.1 10 0.15 3,5 0.1 3 0.25 0,07 0.003 0.01 0.20 0,03 0.006 0.005 0.30 Example 4 0.11 0.08 0.08 10 0.20 2.9 0.01 2 0.24 0.06 0.007 0.09 0.05 0.01 0.005 0.008 0.45 Example 5 0.11 0.05 0.08 9 0.10 2.7 0.5 3,7 0.19 0.05 0.003 0.012 0.29 0.015 0.006 0.007 0.30 Steel prototype 0.11 0.06 0.3 10 0.2 3 0.7 2.1 0.2 0.04 0.003 0.008 - 0.01 0.001 0.001 -

It should be noted that in the first three embodiments of the invention, the number of alloying elements is within the specified limits of the declared steel. However, in the last two examples deviations from the given chemical composition were allowed, namely, in example 4, the ratio of molybdenum and tungsten was violated, and the amount of rhenium was less than the lower acceptable limit of the rhenium content, and in example 5 the molybdenum equivalent value was overestimated (above 1.6) and rhenium is above the upper limit of rhenium.

Tests for long-term strength were carried out according to GOST 10145-62 (table 3). The tensile strength on the basis of 10 ⁵ hours was calculated using the Larsen-Miller parameter. Tests were conducted on the basis of up to 20,000 hours.

Table 3 Creep tests Long-term creep strength, σ for 10⁵ hour, MPa Example 1 Example 2 Example 3 Example 4 Example 5 Steel prototype T _test = 620 ° C 140 142 140 110 107 127 T _test = 650 ° C 110 115 105 85 80 99

As can be seen from table 3, the mechanical properties of the proposed steel alloyed in the specified allowable limits for the content of elements, namely, examples 1-3, is higher in comparison with the steel prototype. Long-term strength on the basis of 100,000 h of examples 1-3 at a temperature of 620 ° C is on average 140 ± 5 MPa, and at a temperature of 650 ° C - on average 110 ± 5 MPa, which exceeds the long-term strength of the prototype steel at both temperatures. However, when alloying steel not within the declared limits, the long-term strength decreases, which is associated with the low rhenium content in Example 4, which does not have a positive effect with this amount, as well as with a violation of the molybdenum equivalent - it is below 1.4, which reduces solid-solution hardening. In example 5, the amount of rhenium is overestimated, as well as an overestimated value of the molybdenum equivalent (above 1.6), which leads to hardening of the steel, on the one hand, and, on the other hand, to excessive precipitation of the Laves phase during creep, as a result of which ductility and toughness decrease, which leads to a drop in long-term strength.

As can be seen from table 3, when alloying steel within the specified limits, the long-term strength of the proposed steel is higher than that of the prototype, which allows it to be used for the manufacture of blades for steam turbines and other elements of power plants. The use of steel in the power industry will increase the operating temperature of thermal power plants to 630 ° C.

Claims (1)

Heat-resistant steel of martensitic class containing carbon, silicon, manganese, chromium, nickel, tungsten, molybdenum, vanadium, niobium, nitrogen, boron, cobalt, sulfur, phosphorus, aluminum, copper, titanium and iron, characterized in that it additionally contains rhenium in the following ratio of components, wt.%
carbon 0.08-0.12 silicon no more than 0.1 manganese 0.05-0.1 chromium 9.5-10.0 nickel no more than 0.2 tungsten 2.3-3.0 molybdenum 0.05-0.1 vanadium 0.18-0.25 niobium 0.04-0.07 nitrogen no more than 0,003 boron 0.008-0.013 cobalt 2.5-3.5 rhenium 0.1-0.2 sulfur no more than 0,006 phosphorus no more than 0,010 aluminum no more than 0,010 copper 0.05-0.3 titanium no more than 0,01 iron rest