RU2688017C1 - Method of thermomechanical treatment of heat-resistant steel of martensitic class - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to metallurgy, namely to thermomechanical treatment of heat-resistant chromium steel of martensite class, used for production of elements of boilers and steam lines, and also steam turbines of power plants with operating steam temperature up to 650 °C. To increase long-term durability, method includes homogenisation at 1,150 °C with holding for 16 hours and cooling in air to room temperature, forging at temperature of 1,150 °C with degree of deformation of 20 % and heating of workpiece between passes to 1,150 °C, cooling in air to room temperature. Heating to temperature of 1,050 °C, holding at said temperature for 2 hours, cooling in furnace to 900 °C with further holding for 1 hour, deformation by forging at temperature 900 °C with degree of deformation of 80 % with workpiece heating up to 900 °C between passes, isothermal annealing at deformation temperature for 3 hours with subsequent cooling in air to room temperature, tempering at 770 °C is cured for 3 hours and cooled in air to room temperature.EFFECT: increased index of long-term strength.1 cl, 2 tbl, 2 dwg

Description

The invention relates to the field of metallurgy, in particular to the processing of heat-resistant chromium steels of martensitic class, containing 5-13% Cr, intended for the manufacture of elements of boilers and steam pipes, as well as steam turbine blades of power plants with operating temperatures up to 650 ° C.

To date, new chemical compositions of heat-resistant high-chromium steels of martensitic class of the type 10Kh9K3V2MFBR, 10Kh9K3V3MFBR, 10Kh10K3V2MFBR, 02H9K3V2MFBR, and others have been proposed in Russia as materials for the production of power plants. The application temperature of new steels can reach 600 ° С with long-term operation and 620 ° С with short-term operation. As a result, the data became capable of providing a high level of heat resistance at supercritical steam parameters of 550–620 ° C, 20–25 MPa, however, they are limited in use for supercritical steam parameters of 620–650 ° C, 25–30 MPa. A further increase in the heat resistance of steels of the martensitic class can only be achieved through a combination of improving the alloying base with the use of thermomechanical processing.

At present, the traditional heat treatment of heat-resistant steels of the martensitic class is an exposure in the austenitic region at 1000-1200 ° C, followed by air cooling and tempering at temperatures of 720-800 ° C with air cooling. Various variations of heat treatment in the framework of the above scheme are applied to martensitic steel with chemical compositions of the type (wt.%): Carbon 0.01-0.2, silicon not more than 0.2, manganese 0.01-0.6, chromium 9 , 0-13.0, nickel not more than 0.2, tungsten 0.5-2, molybdenum 0.5-1.0, cobalt 0.1-5.0, vanadium 0.18-0.25, niobium 0 , 03-0.1, nitrogen 0.04-0.1, boron 0.0005-0.005, sulfur not more than 0.01, phosphorus not more than 0.01, aluminum not more than 0.02, copper not more than 0.05 iron - the rest. In the process of aging at temperatures of 1000 - 1200 ° C, almost complete dissolution of particles of secondary phases such as M ₂₃ C ₆ carbides and MX carbonitrides present in steels occurs, which has a significant effect on the size of the original austenitic grain. With further cooling in air or in water, martensitic transformation occurs in steels, as a result of which the structure of packet martensite is formed. Subsequent tempering at temperatures of 720–800 ° C leads to the precipitation of particles of M ₂₃ C ₆ carbides (50–170 nm in size) and MX carbonitrides (14–30 nm in size) [K. Maruyama, K. Sawada, J. Koike Strengthening mechanisms of creep resistant tempered martensitic steel // ISIJ Int. - 2001. - Vol.41. - P. 641-653]. The stability of the creep tempering structure formed after tempering of troostite is determined by the volume fraction, distribution and particle size of the secondary phases. Particles of carbides M ₂₃ C ₆ , located along the boundaries of the original austenitic grains and martensitic rails, restrain the migration of boundaries, stabilizing the lath structure of the temporal troostite. The dispersed particles of MX carbonitrides uniformly distributed inside the matrix hinder the reorganization of free dislocations into more stable configurations. As a result, the borders of the laths of trostro martensite retain their structure in the creep process at elevated temperatures. The combination of the dislocation structure of temporal troostite with nanoparticles of secondary phases provides unique heat-resistant characteristics of new martensitic steels compared to bainitic or ferritic steels of type 08X13, 12X13, 15Х11МФ, 15Х12ВНМФ, which are currently used as materials for steam turbines of power plants. The combination of alloying and heat treatment made it possible to operate new steels at supercritical temperatures. The use of thermomechanical processing for steels with a chemical composition of the type 10H9M1FBR, 10H9V1M1FBR and 10H9V2MFBR, the Russian analogues of the steels P91, P911 and P92 used abroad, leads to an increase in creep resistance of these steels. Thus, in sources US 6899773B2 and US 7470336B2, to increase heat resistance, a scheme of thermomechanical processing is proposed, consisting in heating in the austenitic region to temperatures above 1000 ° C with an exposure of more than 2 hours, hot deformation at temperatures above 1000 ° C with a degree of deformation of at least 20% , air cooling, in the source US 6966955B2 - heating to temperatures of 1100-1250 ° C and holding at these temperatures for 1-3 hours, hot deformation with decreasing temperature in the temperature range 1040-780 ° C, the degree of deformation more than 40%, subsequent cooling at a rate of 1 ° C / min to a temperature of 10 ° C, in source US 6162307В2 - heating in the austenitic region to a temperature of 1150 ° C, hot deformation in the temperature range of 850-1100 ° C, cooling at a rate of 10 ° C / h to 1500 ° C / hour, recrystallization annealing at 700 ° C for 5 h, homogenization in the temperature range 920-1050 ° C for 10-180 min (cooled in air, in oil or in water) and tempering at 700 ° C for 30-120 minutes

Hot or warm deformation, included in the process of thermomechanical processing, examples of which are given in the above sources, was carried out with the aim of ensuring a high density of dislocations as nucleation sites for the fine dispersion of MX carbonitrides.

The known method of thermomechanical processing of heat-resistant steel of martensitic class P91 (10H9M1FBR), described in the patent US 7520942B2 (published 04/21/2009) includes:

1) heating and aging in the austenitic region in the temperature range 1000-1400 ° C for 1-5 hours to form 100% austenite with a certain IAP size;

2) cooling to temperatures of 500-1000 ° C, warm deformation in this temperature range (rolling, forging, extrusion, etc.) with a degree of deformation of not more than 20% for the formation of dislocations in the microstructure, acting as heterogeneous nucleation sites for separating the dispersion of small particles MX carbonitrides, enriched with vanadium, niobium and / or tantalum, with preform heating between passes;

3) after deformation, annealing in the temperature range 500-1000 ° C up to 10 hours, necessary for the growth of particles to a certain size;

4) cooling to room temperature in air or in water to ensure martensitic or ferritic transformation;

5) additional, but not mandatory, tempering in the temperature range of 500-850 ° C to increase the toughness and ductility of steel.

As noted in the said patent, the microstructure of the modified P91 (10X9M1FBR) steel, after thermomechanical treatment according to the method described in the patent, was significantly different from the microstructure formed during the traditional thermomechanical processing: the average size of MX carbonitrides was reduced by 4 times 3 times.

At the same time, along with the significant grinding of carbonitrides MX, the size of carbides M ₂₃ C ₆ along the boundaries of the original austenitic grains, which are separated from the solid solution in the annealing process in the temperature range of 600-900 ° C after the operation of warm deformation has increased significantly, which is a significant drawback of way, because leads to lower creep resistance of steels. In [R. Mishnev, N. Dudova, A. Fedoseeva, R. Kaibyshev. Microstructural aspects of superior creep resistance of a 10% Cr martensitic steel // Materials Science and Engineering A. –2016. - Vol. 678. - pp. 178–189] it is noted that M ₂₃ C ₆ carbides make a greater contribution to the microstructural stability of temporal troostite under creep conditions located along the martensite rails boundaries and preventing their migration and transformation into subgrain boundaries rather than MX carbonitrides.

Moreover, in the method according to patent US 7520942B2 (published 04/21/2009), there is a significant methodological error: the heating and holding temperatures in the austenitic region in traditional heat treatment and modified thermomechanical processing are significantly different from each other, which affects the interrelated structural parameters: the proportion precipitated MX carbonitrides and the size of the original austenitic grains. Both of these parameters have a significant effect on mechanical properties in creep. In this regard, it is not possible to specify the exact cause responsible for the increase in mechanical properties - deformation during the normalization stage or elevated normalization temperature.

The main disadvantage of this method according to patent US 7520942B2 is that it does not apply to steels with optimized chemical compositions, namely 10-11% Cr steels, which already demonstrate high creep resistance only due to optimally selected alloying elements, for example, according to the patent 2655496 dated 05.28.2018.

The task of the invention is to improve the mechanical properties of steel with the chemical composition described in the patent RU 2655496 from 28.05.2018.

The technical result is an increase in long-term creep strength due to the formation of dispersion of fine particles of M23C6 carbides and MX carbonitrides of about 50 nm and 10 nm, respectively, in steel containing alloying elements, in the following ratio of components, mass. %:

carbon 0.08 - 0.12

silicon is not more than 0.1

manganese less than 0.05

chromium from 10.5 to 12.0

nickel not more than 0.1

tungsten 1.5-2.5

molybdenum 0.4-1.0

cobalt 3.0-3.5

vanadium 0.18-0.25

niobium not more than 0.07

nitrogen not more than 0,003

boron 0,008-0,013

copper 0,6-0,8

sulfur is not more than 0.01

phosphorus not more than 0.01

aluminum not more than 0.01

titanium to less than 0.01

iron else.

As a result, the efficiency of this steel increases at a temperature of up to 650 ° C by 7-14%.

The proposed method of thermomechanical treatment of heat-resistant steel of martensitic class contains the following features:

1) Homogenization is carried out by heating to a temperature of 1150 ° C with aging for 16 hours and subsequent cooling in air, which makes it possible to equalize the chemical composition of the steel over the workpiece, to evenly distribute the alloying elements, to eliminate chemical and structural segregations;

2) Then, a comprehensive forging is carried out at a temperature of 1150 ° C with a degree of deformation of 20%, while between the passes the billet is heated to a temperature of 1150 ° C, cooling after forging is carried out in air, as a result, equilibrium of the structure and the fraction of boundaries as the nuclei of the origin of M ₂₃ carbides increase C ₆ ;

3) Next, carry out normalization by heating in the austenitic region to a temperature of 1050 ° C with a holding at the specified temperature for 2 hours to dissolve excess phases and form 100% austenite;

4) Cooling is carried out in a furnace to a temperature of 900 ° C, followed by exposure at the same temperature for 1 hour;

5) Deformation by rolling or forging is carried out at a temperature of 900 ° C to a degree of deformation of 80% with mandatory heating to a specified temperature between passes in order to form a larger number of dislocations as the origin of MX carbonitrides and high density M ₂₃ C ₆ carbides boundaries.

6) After deformation, isothermal annealing is carried out at a deformation temperature for 3 hours to nucleate MX carbonitride particles at the sites of nucleation and growth of M ₂₃ C ₆ carbides prepared during deformation to a size of no more than 50 nm.

7) Air cooled to room temperature to form a martensite structure due to martensitic transformation;

8) Then, tempering is carried out at a temperature of 770 ° C with an exposure for 3 hours, air cooling, which provides for the removal of internal stresses, formation of tempering troostite structure with small particles of secondary phases, stabilizing the non-equilibrium dislocation structure, as well as low-angle and high-angle boundaries of the original austenitic grains, blocks, packages and martensitic rails.

The achievement of the stated technical result is confirmed by the images of the microstructure of steel, where:

- in FIG. 1a shows an image of the structure of steel after traditional heat treatment, obtained using a JEM JEOL-2100 transmission electron microscope (hereinafter referred to as TEM), equipped with an INCA energy-dispersive attachment, on a thin foil;

- in FIG. 1b shows the image of the steel structure after traditional heat treatment, obtained using PEM, equipped with the INCA energy-dispersive prefix, on a carbon replica;

- in FIG. 2b shows the image of the structure of the steel after the proposed heat treatment, obtained using PEM, equipped with the INCA energy-dispersive attachment, on a thin foil;

- in FIG. 2d shows the image of the steel structure after the proposed heat treatment, obtained using PEM, equipped with the INCA energy-dispersive attachment, on a thin foil.

As can be seen in the figures, the proposed method of thermomechanical treatment leads to the formation of a temporal troostite structure, the size of the original austenitic grain in which reaches 50-60 microns with martensitic slats with an average size of less than 200 nm, whose boundaries are stabilized by fine particles of M ₂₃ C ₆ carbides 50 nm. The high density of dislocations formed both during the deformation process and during the martensitic transformation ensures the formation of a dispersion of fine MX carbonitrides with a size of less than 10 nm and a high density of about 10 ²² m ^-2 (Fig 2c and 2 g) This structure formed in the above steel, significantly different from the structure (figa and 1b), formed in the process of traditional heat treatment, which consists in heating in the austenitic region to a temperature of 1050 ° C, exposure for 1 h and cooling in air, followed by release in the temperature range of 750-770 ° C for 3 hours. Namely, the average size of MX carbonitrides was reduced by 4 times, and the density of these particles increased by 3 times, which led to the grinding of martensitic slats by 2 times. In addition, together with a significant grinding of MX carbonitrides, the size of carbides M ₂₃ C ₆ has significantly decreased by 2 times. Thus, the increase in long-term creep strength of heat-resistant steel to 650 ° C as a result of the proposed thermomechanical processing is provided by additional dispersion hardening from grinding particles of secondary phases and substructural hardening by reducing the size of martensitic slats.

Examples of implementation.

Example 1

An alloy was cast, the chemical composition of which is described in patent RU 2655496 of May 28, 2018 (Table 1). The alloy was smelted in a vacuum induction furnace. Pure charge materials were used as the charge, which allowed us to obtain a low level of sulfur, phosphorus and non-ferrous metals in the obtained material.

Table 1. The chemical composition of cast steel (in wt.% Fe - base)

Samples of cast steel were treated in three different ways: the first is the traditional heat treatment, consisting in normalization at a temperature of 1050 ° C, air cooling, followed by tempering at a temperature of 770 ° C for 3 hours, air cooling; the second is according to the method described in the patent US7520942B2, the third according to the proposed invention.

After homogenization by heating to a temperature of 1150 ° C with a holding for 16 hours and then cooling in air, comprehensive forging is carried out at a temperature of 1150 ° C with a degree of deformation of 20%, while between the passes the workpiece is heated to a temperature of 1150 ° C spend on the air. Normalization is carried out by heating in the austenitic region to a temperature of 1050 ° C with a holding at the specified temperature for 2 hours, followed by cooling in a furnace to a temperature of 900 ° C and then holding at the specified temperature for 1 hour. The deformation is carried out by rolling or forging at a temperature of 900 ° C to a degree of deformation of 80% with mandatory heating to the specified temperature between passes. After deformation, isothermal annealing is carried out at a deformation temperature for 3 hours and cooled in air to room temperature. Then spend the holidays at a temperature of 770 ° C with a holding time for 3 hours and cooling in air.

Example 2

Tests for long-term strength were carried out according to the standard GOST 10145-62 The test results are shown in Table 2, from which it can be seen that the mechanical properties of the steel subjected to thermomechanical processing by the present method are higher than those of the steel subjected to traditional heat treatment by 7-14% , and the mechanical properties of the cast steel subjected to thermomechanical processing according to the method described in the patent US7520942B2 (published 04/21/2009) are even lower than after traditional heat treatment, which indicates the possibility of using this method to a new generation of steel with an optimized chemical composition.

Table 2. Tests for long-term strength of steel subjected to various methods of thermomechanical treatment (TMT) temperatures of 620 ° C and 650 ° C

= 112±6 МПа, то предел длительной прочности стали, подвергнутой термомеханической обработке по предлагаемому способу, составляет

= 120±3 МПа. Пределы длительной прочности были получены с использованием параметра Ларсена-Миллера на основе испытаний, проведенных в течение 2×10³ ч. If the limit of long-term strength of steel subjected to traditional heat treatment at 650 ° С is

= 112 ± 6 MPa, the limit of the long-term strength of steel subjected to thermomechanical processing by the proposed method is

= 120 ± 3 MPa. Limits of long-term strength were obtained using the Larsen-Miller parameter based on tests carried out for 2 × 10 ³ h.

Thus, the above examples prove that the task to develop a method of thermomechanical treatment of martensitic high-temperature steel with enhanced heat-resistant properties has been achieved thanks to the achievement of the stated technical result — formation of a dispersion of small particles of M ₂₃ C ₆ carbides and MX carbonitrides of about 50 nm and 10 nm in size respectively. As can be seen from table 2, the properties of steel, subjected to thermomechanical processing according to the claimed method, improve its performance, which makes it possible to use it for the manufacture of steam turbine elements for power plants with a working steam temperature up to 650 ° C.

Claims (3)

The method of thermomechanical processing of heat-resistant steel of martensitic class, including obtaining a casting of steel containing alloying elements, wt.%: Carbon             0.08–0.12 Silicon          no more than 0.1 Manganese            less than 0.05 Chromium            from 10.5 to 12.0 Nickel          no more than 0.1 Tungsten               1.5-2.5 Molybdenum               0.4-1.0 Cobalt               3.0-3.5 Vanadium             0.18-0.25 Niobium         no more than 0.07 Nitrogen             no more than 0,003 Boron                0,008-0,013 Copper                    0.6-0.8 Sulfur              no more than 0.01 Phosphorus         no more than 0.01 Aluminum         no more than 0.01 Titanium         to less than 0.01 Iron             rest homogenization of the casting at a temperature of 1150 ° С with exposure for 16 h and air cooling to room temperature, all-round forging at a temperature of 1150 ° С with a degree of deformation of 20% and with heating of the workpiece between passes to a temperature of 1150 ° С temperature, then heating in the austenitic region to a temperature of 1050 ° C with a holding for 2 hours, cooling in a furnace to a temperature of 900 ° C, followed by holding for 1 hour, deformation by forging at a temperature of 900 ° C with a degree of deformation of 80% billet and between passes up to 900 ° C, annealing at 900 ° C for 3 hours, air cooling to room temperature and tempering at 770 ° C for 3 hours, followed by air cooling to room temperature.

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