EP3985139A1

EP3985139A1 - Low-chromium corrosion-resistant high-strength polycrystalline high-temperature alloy and preparation method therefor

Info

Publication number: EP3985139A1
Application number: EP20821820.6A
Authority: EP
Inventors: Jingbo Yan; Yuefeng Gu; Yong Yuan; Zheng Yang; Xinxing ZHANG
Original assignee: Xian Thermal Power Research Institute Co Ltd
Current assignee: Xian Thermal Power Research Institute Co Ltd
Priority date: 2019-06-14
Filing date: 2020-06-12
Publication date: 2022-04-20
Also published as: WO2020249113A1; EP3985139A4; CN110106398A; CN110106398B

Abstract

A low-chromium corrosion-resistant high-strength polycrystalline superalloy and a method for preparing the same, the superalloy comprising the following elements in percent by weight: from 15 to 18% chromium, from 15% to 20% cobalt, from 0.5% to 1.5% titanium, from 3.5% to 4.5% aluminum, from 5% to 8.5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, from 0.5% to 1.5% niobium, from 0.03% to 0.08% carbon, and balance being nickel; after the raw material is smelted, the ingot is subjected to homogenization, hot rolling, and finally heat treatment. The yield strengths of the alloy disclosed are higher than 850MPa and 550MPa at room temperature and at 850°C, respectively; and after the alloy is exposed to fireside corrosion (N₂-15% CO₂-3.5% O₂-0.1%SO₂) at 850°C for 500 hours, the weight change is less than 0.2mg/cm². Besides, the alloy has a superior structural stability during 850°C thermal exposure.

Description

FIELD

The present invention relates to the field of superalloy materials, and more particularly relate to a low-chromium corrosion-resistant high-strength polycrystalline superalloy and a method of preparing the same.

BACKGROUND

Ever-increasing demand in electricity consumption intensifies energy deficiency and environment pollution; therefore, it is pressing to develop an efficient, energy-conservative, and environment-friendly means of power generation. Since fossil-fired power generation has always been a leading power generation technology for a long time in China, it is believed that the most effective means to address the above problems is increase steam parameters of power units. Substantial practices reveal that service performance of the materials for critical components is a primary cause of restricting promotion of boiler unit steam parameters. A superheater/reheater tube, as one of critical components with severest service conditions in a fossil-fired boiler, poses a very stringent requirement on material service performance. The superheater/reheater withstands multiple impacts including high-temperature creep, thermal fatigue, oxidation, and high-temperature fireside corrosion, etc. With substantial increase of main steam parameters of the fossil-fired boiler, it is needed to develop a high-temperature alloy material that may satisfy operating performance requirements of a superheater/reheater tube of a high-parameter power unit in the industry of fossil-fired power generation.
The superheater/reheater as a component having severest service conditions in a fossil-fired boiler is very demanding on creep rupture strength and corrosion-resistant property of a candidate material. To meet the requirements on material properties of the superheater/ reheater in a high-parameter fossil-fired boiler, a variety of nickel-based wrought superalloy materials have been developed abroad, such as Inconel^®740H developed by Special Metals, Haynes^®282 developed by Haynes International, CCA 617 developed by Thyssenkrupp, Nimonic 263 developed by Rolls-Royce, FENIX700 developed by Hitachi, TOS1X developed by Toshiba, and LTESR700 developed by Mitsubishi. To ensure a superior creep rupture strength of an alloy, conventional candidate materials generally have a relatively low Al/ Ti ratio. Besides, the relatively high Cr content in the alloy also ensures its anti-oxidation and anti-corrosion properties. However, the ever-increasing steam parameters of fossil-fired generating units pose more harsh challenge to alloy properties. Al (Aluminum) is an important element promoting precipitation hardening in the alloy. A relatively high Al content facilitates increasing Ni₃Al volume fraction in the alloy, further conferring a superior strength performance to the alloy. Meanwhile, addition of the Al element also facilitates formation of Al₂O₃, which substantially promotes high-temperature anti-oxidation and anti-corrosion properties of the alloy. However, addition of the Al element also causes structural instability in the alloy; particularly, a relatively high Al content significantly affects the solidification structure of the alloy.

SUMMARY

An object of the present invention is to provide a low-chromium corrosion-resistant high-strength polycrystalline superalloy and a method of preparing the same, wherein by leveraging the characteristics of Al as an element for strengthening Ni₃Al formation in conjunction with its property of improving anti-corrosion property of the alloy, a critical Al content necessary for ensuring formation of Al₂O₃ in the oxidation/corrosion process of the alloy is added, and a range of Al content in the alloy is stringently controlled while ensuring structural stability of the alloy, so as to promote precipitation of a considerable amount of homogeneously dispersed, second-phase strengthened particles in the alloy to obtain a desired strength property.
To achieve the object, the present invention adopts the following technical solution:
A low-chromium corrosion-resistant high-strength polycrystalline superalloy, comprising the following elements in percent by weight: from 15 to 18% chromium, from 15% to 20% cobalt, from 0.5% to 1.5% titanium, from 3.5% to 4.5% aluminum, from 5% to 8.5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, from 0.5% to 1.5% niobium, from 0.03% to 0.08% carbon, and balance being nickel.
A method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy, comprising steps of:

1) preparing an alloy, wherein the alloy comprises the following elements in percent by weight: from 15% to 18% chromium, from 15% to 20% cobalt, from 0.5% to 1.5% titanium, from 3.5% to 4.5% aluminum, from 5% to 8.5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, from 0.5% to 1.5% niobium, from 0.03% to 0.08% carbon, and balance being nickel;
2) smelting: smelting the prepared alloy into an alloy mother liquor, refining the alloy mother liquor using an electroslag remelting process, followed by cooling, wherein a cooling rate is controlled not to exceed 15°C/min after the alloy mother liquid is solidified into an ingot and before the temperature reaches 900°C in the course of solidifying the alloy mother liquor into the ingot, and cooling at a cooling rate of over 10°C/min to room temperature after the temperature reaches 900°C in the course of solidifying the alloy mother liquor into the ingot;
3) homogenizing to obtain a superalloy ingot;
4) hot rolling: rolling the superalloy ingot with a total deformation ranging from 50% to 70%, deformation of each pass being controlled within a range from 15% to 25%, and a deformation temperature ranging from 1130°C to 1170°C;
5) heat treatment: maintaining the rolled alloy at a temperature ranging from 1110°C to 1130°C for 4 hours, followed by recrystallization; air cooling to room temperature and then maintaining a temperature ranging from 750°C to 770°C for a time ranging from 7 to 9 hours, followed by heating to a temperature ranging from 840°C to 870°C and maintaining the temperature for a time ranging from 1.5 to 2.5 hours, followed by air cooling to room temperature.

A further improvement of the present invention lies in that the smelting in step 2) is carried out in a vacuum melting furnace, wherein a vacuum degree in the vacuum melting furnace during smelting is not higher than 1.0×10^-4 MPa.
A further improvement of the present invention lies in that in step 2), before the temperature reaches 900°C in the course of solidification into the ingot, the cooling rate is controlled not to exceed 15°C/min, and the temperature is cooled to room temperature at a cooling rate exceeding 10°C/min after the temperature reaches 900°C in the course of solidification into the ingot.
A further improvement of the present invention lies in that in step 2), the time taken from starting solidification of the alloy mother liquor into the ingot till cooling to room temperature does not exceed 15 minutes.
A further improvement of the present invention lies in that step 3) specifically comprises: removing the ingot, followed by heating the ingot to a temperature ranging from 1030°C to 1070°C and maintaining the temperature for half an hour, and then continuously heating to a temperature ranging from 1170°C to 1200°C and maintaining the temperature in a heat treatment furnace for a time ranging from 20 to 24 hours, finally cooling to room temperature.
A further improvement of the present invention lies in that in step 3), a heating rate in the course of heating the ingot to the temperature ranging from 1030°C to 1070°C does not exceed 10°C/min, and the heating rate in the course of heating to the temperature ranging from 1170°C to 1200°C does not exceed 5°C/min.
A further improvement of the present invention lies in that in step 5), the temperature rises from the room temperature till the temperature ranging from 1110°C to 1130°C at a heating rate not exceeding 10°C/min, and rises from the room temperature till the temperature ranging from 750°C to 770°C at a heating rate not exceeding 10°C/min, followed by rising to the temperature ranging from 840°C to 870°C at a heating rate not exceeding 10°C/min.
Compared with conventional technologies, the present invention offers the following beneficial effects:
The present invention develops a novel superalloy with relatively high Al and Ti contents based on the alloy designing concept of precipitation hardening, wherein the relatively high Al and Cr contents in the alloy also ensure that the alloy has superior anti-oxidation and anti-corrosion properties.
The alloy prepared according to the method of the present invention has superior strength and corrosion-resistant properties, as well as a good structural stability. The alloy matrix is austenitic with an unordered face-centered structure, the average grain size of which is less than 100µm; the austenite grain boundary has carbides (NbC and Cr23C6) distributed in a discontinuous pattern, the total volume fraction of the carbides accounting for 5% to 20%; fine spheroidal Ni₃Al precipitation particles are homogeneously dispersed in the grain, the size of the precipitation particles being not greater than 50nm. The tensile yield strengths of the alloy at room temperature and at 850°C are higher than 850MPa and 550MPa, respectively; and after the alloy is exposed to fireside corrosion (N₂-15% CO₂-3.5% O₂-0.1%SO₂) at 850°C for 500 hours, the weight change is less than 0.2mg/cm². Besides, the alloy has a superior structural stability during 850°C thermal exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a microstructure of an alloy in a heat treated state according to a first example;
Fig. 2 shows a microstructure of the alloy in a thermal exposure state (850°C /1000h) according to the first example;
Fig. 3 shows a grain-boundary eutectic structure in a first comparative example;
Fig. 4 shows a microstructure of the alloy in a thermal exposure state (850°C /1000h) in a second comparative example.

DETAILED DESCRIPTION

Hereinafter, the present invention will be further illustrated with reference to the following embodiments.
A precipitation hardened alloy according to the present invention is a nickel-based superalloy material.
A low-chromium corrosion-resistant high-strength polycrystalline superalloy, comprising the following elements in percent by weight: from 15 to 18% chromium, from 15% to 20% cobalt, from 0.5% to 1.5% titanium, from 3.5% to 4.5% aluminum, from 5% to 8.5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, from 0.5% to 1.5% niobium, from 0.03% to 0.08% carbon, and balance being nickel.
A method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy, comprising steps of:

1) preparing an alloy, wherein the alloy comprises the following elements in percent by weight: from 15 to 18% chromium, from 15% to 20% cobalt, from 0.5% to 1.5% titanium, from 3.5% to 4.5% aluminum, from 5% to 8.5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, from 0.5% to 1.5% niobium, from 0.03% to 0.08% carbon, and balance being nickel;
2) smelting: smelting the prepared alloy into an alloy mother liquor, refining the alloy mother liquor using an electroslag remelting process, followed by cooling, wherein a cooling rate is controlled not to exceed 15°C/min after the alloy mother liquid is solidified into an ingot and before the ingot temperature reaches 900°C, and the ingot is cooled to room temperature at a cooling rate of over 10°C/min after the ingot temperature reaches 900°C;
3) homogenizing, comprising: removing the ingot, followed by heating the ingot to a temperature ranging from 1030°C to 1070°C and maintaining the temperature for half an hour, and then continuously heating to a temperature ranging from 1170°C to 1200°C and maintaining the temperature in a heat treatment furnace for a time ranging from 20 to 24 hours, finally cooling to room temperature to obtain a superalloy ingot;
4) hot rolling: rolling the ingot with a total deformation ranging from 50% to 70%, deformation of each pass being controlled within a range from 15% to 25%, and deformation temperature ranging from 1130°C to 1170°C;
5) heat treatment: maintaining the rolled alloy at the temperature ranging from 1110°C to 1130°C for 4 hours, followed by recrystallization; air cooling to room temperature and then maintaining a temperature ranging from 750°C to 770°C for a time ranging from 7 to 9 hours, followed by heating to a temperature ranging from 840°C to 870°C and maintaining the temperature for a time ranging from 1.5 to 2.5 hours, followed by air cooling to room temperature.

Example 1

A heat-resisting steel material in this example comprises the following elements in percent by weight: 17% chromium, 20% cobalt, 1.5% titanium, 4.0% aluminum, 7.0% tungsten, 0.5% silicon, 0.5% manganese, 1.0% niobium, 0.04% carbon, balance being nickel.
A method for preparing the heat-resisting steel material comprises steps of:

1) preparing raw material: comprising the following elements in percent by weight: 17% chromium, 20% cobalt, 1.5% titanium, 4.0% aluminum, 7.0% tungsten, 0.5% silicon, 0.5% manganese, 1.0% niobium, 0.04% carbon, balance being nickel;
2) smelting: placing a ceramic crucible and the prepared raw material simultaneously in a vacuum melting furnace, using a vacuum induction furnace with a vacuum degree not higher than 1.0 ×10^-4 Mpa to smelt the prepared alloy into an alloy mother liquor; pre-heating the ceramic crucible using an arc at a low power while the alloy mother liquor is being solidified. After the alloy is completely solidified into an ingot, the ingot is removed to the preheated ceramic crucible, so as to avoid cooling the alloy ingot at a too high cooling rate due to contact between the alloy ingot and the copper crucible.
3) homogenizing: removing the ingot, followed by heating the ingot till 1050°C at a rate of 10°C/min and maintaining the temperature for half an hour, and continuously heating to 1200°C at a rate of 5°C/min and maintaining the temperature in a heat treatment furnace for 24 hours, finally cooling to room temperature to obtain a superalloy ingot;
4) hot rolling: rolling the ingot with a total deformation ranging from 50% to 70%, deformation of each pass being controlled within a range from 15% to 25%, and the deformation temperature ranging from 1130°C to 1170°C;
5) heat treatment: heating the rolled alloy to 1120°C at a rate of 10°C/min and maintaining the temperature for 4 hours, followed by recrystallization; air cooling and maintaining the temperature at 760°C for 8 hours, followed by heating to 860°C and maintaining the temperature for 2 hours; afterwards, air cooling to room temperature.

The yield strengths of the alloy prepared according to Example 1 at room temperature and at 850°C are 913MPa and 590MPa, respectively; and the alloy has a weight change of 0.08mg/cm² after being exposed to fireside corrosion at 850°C for 500 hours.

Example 2

A heat-resisting steel material in this example comprises the following elements in percent by weight: 17% chromium, 20% cobalt, 1.0% titanium, 4.0% aluminum, 8.5% tungsten, 0.5% silicon, 0.5% manganese, 1.5% niobium, 0.04% carbon, balance being nickel.
A method for preparing the heat-resisting steel material comprises steps of:

1) preparing raw material: comprising the following elements in percent by weight: 17% chromium, 20% cobalt, 1.0% titanium, 4.0% aluminum, 8.5% tungsten, 0.5% silicon, 0.5% manganese, 1.5% niobium, 0.04% carbon, balance being nickel;
2) smelting: placing a ceramic crucible and the prepared raw material simultaneously in a vacuum melting furnace, using a vacuum induction furnace with a vacuum degree not higher than 1.0 ×10^-4 Mpa to smelt the prepared alloy into an alloy mother liquor; pre-heating the ceramic crucible using an arc at a low power while the alloy mother liquor is being solidified. After the alloy is completely solidified into an ingot, the ingot is removed to the preheated ceramic crucible, so as to avoid cooling the alloy ingot at a too high cooling rate due to contact between the alloy ingot and the copper crucible.
3) homogenizing: removing the ingot, followed by heating the ingot till 1050°C at a rate of 10°C/min and maintaining the temperature for half an hour, and continuously heating to 1200°C at a rate of 5°C/min and maintaining the temperature in a heat treatment furnace for 24 hours, finally cooling to room temperature to obtain a superalloy ingot;
4) hot rolling: rolling the ingot with a total deformation ranging from 50% to 70%, deformation of each pass being controlled within a range from 15% to 25%, and the deformation temperature ranging from 1130°C to 1170°C;
5) heat treatment: heating the rolled alloy to 1120°C at a rate of 10°C/min and maintaining the temperature for 4 hours, followed by recrystallization; air cooling and maintaining the temperature at 760°C for 8 hours, followed by heating to 860°C and maintaining the temperature for 2 hours; afterwards, air cooling to room temperature.

The yield strengths of the alloy prepared according to Example 2 at room temperature and at 850°C are 905MPa and 597MPa, respectively; and the alloy has a weight change of 0.07mg/cm² after being exposed to fireside corrosion at 850°C for 500 hours.

Comparative Example 1:

A heat-resisting steel material in this example comprises the following elements in percent by weight: 21% chromium, 20% cobalt, 6.0% aluminum, 7.0 tungsten, 0.5% silicon, 0.5% manganese, 0.04% carbon, balance being nickel.
A method for preparing the heat-resisting steel material comprises steps of:

1) preparing raw material: comprising the following elements in percent by weight: 21% chromium, 20% cobalt, 6.0% aluminum, 7.0% tungsten, 0.5% silicon, 0.5% manganese, 0.04% carbon, balance being nickel;
2) smelting: placing a ceramic crucible and the prepared raw material simultaneously in a vacuum melting furnace, using a vacuum induction furnace with a vacuum degree not higher than 1.0 ×10^-4 Mpa to smelt the prepared alloy into an alloy mother liquor; pre-heating the ceramic crucible using an arc at a low power while the alloy mother liquor is being solidified. After the alloy is completely solidified into an ingot, the ingot is removed to the preheated ceramic crucible, so as to avoid cooling the alloy ingot at a too high cooling rate due to contact between the alloy ingot and the copper crucible.
3) homogenizing: removing the ingot, followed by heating the ingot till 1050°C at a rate of 10°C/min and maintaining the temperature for half an hour, and continuously heating to 1200°C at a rate of 5°C/min and maintaining the temperature in a heat treatment furnace for 24 hours, finally cooling to room temperature to obtain a superalloy ingot;
4) hot rolling: rolling the ingot with a total deformation ranging from 50% to 70%, deformation of each pass being controlled within a range from 15% to 25%, and the deformation temperature ranging from 1130°C to 1170°C;
5) heat treatment: heating the rolled alloy to 1120°C at a rate of 10°C/min and maintaining the temperature for 4 hours, followed by recrystallization; air cooling and maintaining the temperature at 760°C for 8 hours, followed by heating to 860°C and maintaining the temperature for 2 hours; afterwards, air cooling to room temperature.

The yield strengths of the alloy prepared according to Comparative Example 1 at room temperature and at 850°C are 692MPa and 352MPa, respectively; and the alloy has a weight change of 0.08mg/cm² after being exposed to fireside corrosion at 850°C for 500 hours.

Comparative Example 2:

A heat-resisting steel material in this example comprises the following elements in percent by weight: 21% chromium, 20% cobalt, 2.0% titanium, 4.0% aluminum, 7.0% tungsten, 0.5% silicon, 0.5% manganese, 0.04% carbon, balance being nickel.
A method for preparing the heat-resisting steel material comprises steps of:

1) preparing raw material: comprising the following elements in percent by weight: 21% chromium, 20% cobalt, 2.0% titanium, 4.0% aluminum, 7.0% tungsten, 0.5% silicon, 0.5% manganese, 0.04% carbon, balance being nickel;
2) smelting: placing a ceramic crucible and the prepared raw material simultaneously in a vacuum melting furnace, using a vacuum induction furnace with a vacuum degree not higher than 1.0 ×10^-4 Mpa to smelt the prepared alloy into an alloy mother liquor; pre-heating the ceramic crucible using an arc at a low power while the alloy mother liquor is being solidified. After the alloy is completely solidified into an ingot, the ingot is removed to the preheated ceramic crucible, so as to avoid cooling the alloy ingot at a too high cooling rate due to contact between the alloy ingot and the copper crucible.
3) homogenizing: removing the ingot, followed by heating the ingot till 1050°C at a rate of 10°C/min and maintaining the temperature for half an hour, and continuously heating to 1200°C at a rate of 5°C/min and maintaining the temperature in a heat treatment furnace for 24 hours, finally cooling to room temperature to obtain a superalloy ingot;
4) hot rolling: rolling the ingot with a total deformation ranging from 50% to 70%, deformation of each pass being controlled within a range from 15% to 25%, and the deformation temperature ranging from 1130°C to 1170°C;
5) heat treatment: heating the rolled alloy to 1120°C at a rate of 10°C/min and maintaining the temperature for 4 hours, followed by recrystallization; air cooling and maintaining the temperature at 760°C for 8 hours, followed by heating to 860°C and maintaining the temperature for 2 hours; afterwards, air cooling to room temperature.

The yield strengths of the alloy prepared according to Comparative Example 2 at room temperature and at 850°C are 859MPa and567MPa, respectively; and the alloy has a weight change of 1.17mg/cm² after being exposed to fireside corrosion at 850°C for 500 hours.
Referring to Figs. 1, 2, 3, and 4, comparisons between the alloys resulting from Example 1 and the comparative examples may reveal that the alloy according to the present invention has a superior structural stability at 850°C without TCP phase precipitated during the high-temperature exposure period.

Example 3

1) preparing an alloy, wherein the alloy comprises the following elements in percent by weight: 15% chromium, 15% cobalt, 0.5% titanium, 3.5% aluminum, 5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, 0.5% niobium, 0.03% carbon, and balance being nickel;
2) smelting: smelting the prepared alloy into an alloy mother liquor, refining the alloy mother liquor using an electroslag remelting process, followed by cooling, wherein a cooling rate is controlled not to exceed 15°C/min after the alloy mother liquid is solidified into an ingot and before the ingot temperature reaches 900°C, and cooling to room temperature at a cooling rate of over 10°C/min after the ingot temperature reaches 900°C;
3) homogenizing: removing the ingot, followed by heating the ingot to 1030°C and maintaining the temperature for half an hour, and then continuously heating to 1170°C and maintaining the temperature in a heat treatment furnace for 23 hours, finally cooling to room temperature to obtain a superalloy ingot.
4) hot rolling: rolling the ingot with a total deformation of 50%, deformation of each pass being controlled to 15%, and deformation temperature being 1170°C;
5) heat treatment: maintaining the rolled alloy at a temperature of 1110°C for 4 hours, followed by recrystallization; air cooling to room temperature and then maintaining a temperature of 750°C for 9 hours, followed by heating to 840°C and maintaining the temperature for 2.5 hours, followed by air cooling to room temperature.

Example 4

1) preparing an alloy, wherein the alloy comprises the following elements in percent by weight: 18% chromium, 17% cobalt, 0.8% titanium, 4.5% aluminum, 6% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, 0.8% niobium, 0.08% carbon, and balance being nickel;
2) smelting: smelting the prepared alloy into an alloy mother liquor, refining the alloy mother liquor using an electroslag remelting process, followed by cooling, wherein a cooling rate is controlled not to exceed 15°C/min after the alloy mother liquid is solidified into an ingot and before the ingot temperature reaches 900°C, and cooling to room temperature at a cooling rate of over 10°C/min after the ingot temperature reaches 900°C;
3) homogenizing: removing the ingot, followed by heating the ingot to 1070°C and maintaining the temperature for half an hour, and then continuously heating to 1180°C and maintaining the temperature in a heat treatment furnace for 20 hours, finally cooling to room temperature to obtain a superalloy ingot;
4) hot rolling: rolling the ingot with a total deformation of 70%, deformation of each pass being controlled to 25%, and deformation temperature being 1130°C;
5) heat treatment: maintaining the rolled alloy at a temperature of 1130°C for 4 hours, followed by recrystallization; air cooling to room temperature and then maintaining a temperature of 770°C for 7 hours, followed by heating to 870°C and maintaining the temperature for 1.5 hours, followed by air cooling to room temperature.

The alloy prepared according to the present disclosure has a matrix of FCC (Face Centered Cubic) structure, the average grain size being about 30 to 70µm, and fine precipitation particles are homogeneously dispersed in the grain. The alloy has superior corrosion-resistant and strength properties, with yields at room temperature and at 850°C being no less than 850MPa and 550MPa, respectively. Besides, after being exposed to fireside corrosion at 850°C for 100 hours, the alloy has a weight increase of not more than 0.2mg/cm².

Claims

A low-chromium corrosion-resistant high-strength polycrystalline superalloy, comprising the following elements in percent by weight: from 15 to 18% chromium, from 15% to 20% cobalt, from 0.5% to 1.5% titanium, from 3.5% to 4.5% aluminum, from 5% to 8.5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, from 0.5% to 1.5% niobium, from 0.03% to 0.08% carbon, and balance being nickel.
A method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy, comprising steps of:
1) preparing an alloy, wherein the alloy comprises the following elements in percent by weight: from 15% to 18% chromium, from 15% to 20% cobalt, from 0.5% to 1.5% titanium, from 3.5% to 4.5% aluminum, from 5% to 8.5% tungsten, less than or equal to 0.5% silicon, less than or equal to 0.5% manganese, from 0.5% to 1.5% niobium, from 0.03% to 0.08% carbon, and balance being nickel;

2) smelting: smelting the prepared alloy into an alloy mother liquor, refining the alloy mother liquor using an electroslag remelting process, followed by cooling, wherein a cooling rate is controlled not to exceed 15°C/min after the alloy mother liquid is solidified into an ingot and before the temperature reaches 900°C in the course of solidifying the alloy mother liquor into the ingot, and cooling at a cooling rate of over 10°C/min to room temperature after the temperature reaches 900°C in the course of solidifying the alloy mother liquor into the ingot;

3) homogenizing to obtain a superalloy ingot;

4) hot rolling: rolling the superalloy ingot with a total deformation ranging from 50% to 70%, deformation of each pass being controlled within a range from 15% to 25%, and a deformation temperature ranging from 1130°C to 1170°C;

5) heat treatment: maintaining the rolled alloy at a temperature ranging from 1110°C to 1130°C for 4 hours, followed by recrystallization; air cooling to room temperature and then maintaining a temperature ranging from 750°C to 770°C for a time ranging from 7 to 9 hours, followed by heating to a temperature ranging from 840°C to 870°C and maintaining the temperature for a time ranging from 1.5 to 2.5 hours, followed by air cooling to room temperature.
The method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy according to claim 2, wherein the smelting in step 2) is carried out in a vacuum melting furnace, wherein a vacuum degree in the vacuum melting furnace during smelting is not higher than 1.0×10-4 MPa.
The method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy according to claim 2, wherein in step 2), before the temperature reaches 900°C in the course of solidification into the ingot, the cooling rate is controlled not to exceed 15°C/min, and the temperature is cooled to room temperature at a cooling rate exceeding 10°C/min after the temperature reaches 900°C in the course of solidification into the ingot.
The method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy according to claim 4, wherein in step 2), the time taken from starting solidification of the alloy mother liquor into the ingot till cooling to room temperature does not exceed 15 minutes.
The method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy according to claim 2, wherein step 3) specifically comprises: removing the ingot, followed by heating the ingot to a temperature ranging from 1030°C to 1070°C and maintaining the temperature for half an hour, and then continuously heating to a temperature ranging from 1170°C to 1200°C and maintaining the temperature in a heat treatment furnace for a time ranging from 20 to 24 hours, finally cooling to room temperature.
The method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy according to claim 6, wherein in step 3), a heating rate in the course of heating the ingot to the temperature ranging from 1030°C to 1070°C does not exceed 10°C/min, and the heating rate in the course of heating to the temperature ranging from 1170°C to 1200°C does not exceed 5°C/min.
The method for preparing a low-chromium corrosion-resistant high-strength polycrystalline superalloy according to claim 2, wherein in step 5), the temperature rises from the room temperature till the temperature ranging from 1110°C to 1130°C at a heating rate not exceeding 10°C/min, and rises from the room temperature till the temperature ranging from 750°C to 770°C at a heating rate not exceeding 10°C/min, followed by rising to the temperature ranging from 840°C to 870°C at a heating rate not exceeding 10°C/min.