Austenitic alloy

Abstract

An austenitic alloy comprising (in weight %): C: 0.01-0.05 Si: 0.05-0.80 Mn: 1.5-2 Cr: 26-34.5 Ni: 30-35 Mo: 3-4 Cu: 0.5-1.5 N: 0.05-0.15 V: 0.15 the balance being Fe and unavoidable impurities, characterized in that 40% Ni+100*% N50.

Claims

1. An austenitic alloy comprising in weight %: C: 0.01-0.015 Si: 0.40-0.52 Mn: 1.7 Cr: 27 Ni: 33 Mo: 3.4 Cu: 0.9-1.0 N: 0.09-0.11 V: 0.07-0.09 the balance being Fe and unavoidable impurities, wherein the weight % of N and Ni is balanced to fulfill the requirement 40<% Ni+100*% N<50, and wherein the alloy has a microstructure that includes a MX phase.

2. The austenitic alloy according to claim 1, wherein the weight % of N and Ni is balanced to fulfill the requirement 40<% Ni+100*% N<45.

3. The austenitic alloy according to claim 1, wherein a creep test at 600 C. and a stress of 160 MPa exhibits a rupture elongation of 71 to 90% and a time to rupture of at least 100,000 hours, and wherein a creep test at 650 C. and a stress of 95 to 105 MPa exhibits a rupture elongation of 31 to 70% and a time to rupture of at least 95,000 hours.

4. The austenitic alloy according to claim 1, wherein the time to rupture for the creep test at 600 C. and stress of 160 MPa is from 100,000 hours to about 117,500 hours, and wherein the time to rupture for the creep test at 650 C. and stress of 95 to 105 MPa is from 95,000 hours to about 188,000 hours.

5. A component for a combustion plant, the component comprising the austenitic alloy according to claim 1.

6. The component for the combustion plant according to claim 5, wherein said component is a reheater or an evaporator.

7. An austenitic alloy comprising in weight %: C: 0.009-0.014 Si: 0.40-0.52 Mn: 1.70-1.73 Cr: 27.11-27.29 Ni: 33.21-33.45 Mo: 3.42-3.44 Cu: 0.95-0.98 N: 0.093-0.11 V: 0.07-0.09 the balance being Fe and unavoidable impurities, wherein the weight % of N and Ni is balanced to fulfill the requirement 40% Ni+100*% N45, and wherein the alloy has a microstructure that includes a MX phase.

8. The austenitic alloy according to claim 7, wherein a creep test at 600 C. and a stress of 160 MPa exhibits a rupture elongation of 71 to 90% and a time to rupture of at least 100,000 hours, and wherein a creep test at 650 C. and a stress of 95 to 105 MPa exhibits a rupture elongation of 31 to 70% and a time to rupture of at least 95,000 hours.

9. A component for a combustion plant, the component comprising the austenitic alloy according to claim 7.

10. The component for the combustion plant according to claim 9, wherein said component is a reheater or an evaporator.

11. An austenitic alloy comprising in weight %: C: 0.01-0.05 Si: 0.05-0.80 Mn: 1.5-2 Cr: 26-29 Ni: 33 Mo: 3.4 Cu: 0.5-1.5 N: 0.05-1.5 V: 0.07-0.09 the balance being Fe and unavoidable impurities, wherein the weight % of N and Ni is balanced to fulfill the requirement 40<% Ni+100*% N<50, and wherein the alloy has a microstructure that includes a MX phase.

12. The austenitic alloy according to claim 11, wherein Cr is present in an amount of 27 weight %.

13. The austenitic alloy according to claim 11, wherein the weight % of N and Ni is balanced to fulfill the requirement 40<% Ni+100*% N<45.

14. The austenitic alloy according to claim 13, wherein Cr is present in an amount of 27 weight %.

15. The austenitic alloy according to claim 11, wherein a creep test at 600 C. and a stress of 160 MPa exhibits a rupture elongation of 71 to 90% and a time to rupture from about 100,000 hours to about 117,500 hours, and wherein a creep test at 650 C. and a stress of 95 to 105 MP a exhibits a rupture elongation of 31 to 70% and a time to rupture of from about 95,000 hours to about 188,000 hours.

16. A component for a combustion plant, the component comprising the austenitic alloy according to claim 11.

17. The component for the combustion plant according to claim 16, wherein said component is a reheater or an evaporator.

18. An austenitic alloy comprising in weight %: C: 0.01-0.05 Si: 0.05-0.80 Mn: 1.5-2 Cr: 26-29 Ni: 30-35 Mo: 3-3.4 Cu: 0.5-1.5 N: 0.05-0.15 V: 0.07-0.09 the balance being Fe and unavoidable impurities, wherein the weight % of N and Ni is balanced to fulfill the requirement 40<% Ni+100*% N<50, and wherein the alloy has a microstructure that includes a MX phase.

19. The austenitic alloy according to claim 18, wherein Ni is present in an amount of 33-35 weight %.

20. The austenitic alloy according to claim 18, wherein the weight % of N and Ni is balanced to fulfill the requirement 40<% Ni+100*% N<45.

21. An austenitic alloy comprising in weight %: C: 0.01-0.05 Si: 0.05-0.80 Mn: 1.5-2 Cr: 26-29 Ni: 30-35 Mo: 3-3.4 Cu: 0.5-1.5 N: 0.05-0.15 V: 0.07-0.09 the balance being Fe and unavoidable impurities, wherein the weight % of N and Ni is balanced to fulfill the requirement 40% Ni+100*% N45, and wherein the alloy has a microstructure that includes a MX phase.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1: A table over alloy compositions

(2) FIG. 2: A diagram showing results from creep tests at 600 C. of inventive alloys and comparative alloys.

(3) FIG. 3: A diagram showing results from creep tests at 650 C. of inventive alloys and comparative alloys.

EXAMPLE

(4) Following the inventive alloy will be described with reference to a concrete example.

(5) Ten steel heats were prepared by conventional steel making methods. The composition of respective steel heat is shown in table 1. The conventional metallurgical process according to which the heats were prepared was as follows: Melting by AOD methodhot rollingextrudingcold pilgring (cold deformation)-solution annealingwater quenching. The hollow bar material after the hot extruding was then cold pilgred with a cold deformation between 40 to 80%, followed by a solution annealing at a temperature between 1050 to 1180 C. depending on the dimension. The following table shows the details.

(6) TABLE-US-00001 Cold deformation Alloy Heat (%) Annealing Cooling 1 763554 40-80 1050-1180 C./5-25 water minutes quenching 2 462269 40-80 1050-1180 C./5-25 water minutes quenching 3 477353 40-80 1050-1180 C./5-25 water minutes quenching 4 469837 40-80 1050-1180 C./5-25 water minutes quenching 5 471988 40-80 1050-1180 C./5-25 water minutes quenching 6 469718 40-80 1050-1180 C./5-25 water minutes quenching 7 477217 40-80 1050-1180 C./5-25 water minutes quenching 8 477203 40-80 1050-1180 C./5-25 water minutes quenching 9 460335 40-80 1050-1180 C./5-25 water minutes quenching 10 463024 40-80 1050-1180 C./5-25 water minutes quenching

(7) Alloys 1, 7-9 are comparative samples and contain relatively low concentrations of nitrogen. Alloys 2, 3 and 10 are comparative samples and contain comparatively high nitrogen concentrations. Alloys 4-6 are inventive samples which fulfil the requirement 40% Ni+100*% N50. Alloys 1 and 10 are low in silicon content.

(8) Test samples of each steel heat were prepared. The samples were subjected to creep testing in order to determine their creep properties. Creep testing was performed at two different temperatures: 600 C. and 650 C., by applying a constant stress on each sample and determining the time to rupture and rupture elongation of each sample. Rupture elongation is the length increase until rupture expressed as percentage of nominal length for each sample. The applied stress equals the creep rupture strength of the alloy. The creep rupture strength is defined as the stress which, at a given temperature, will cause a material to rupture in a given time.

(9) The creep tests were performed according to conventional testing methods and conventional mathematic models were used for extrapolating the results.

(10) FIG. 2 shows the creep strength at 600 C. for inventive alloys 4-6 in comparison to the creep strengths of comparative alloys 1, 7 and 9. FIG. 3 shows the creep strength at 650 C. for inventive alloys 4-6 in comparison to comparative alloys 1, 8, 9. From FIGS. 1 and 2 it is clear that the inventive alloys, for a given creep stress, shows a longer time to rupture than the comparative alloys.

(11) Some other results from the creep testing are shown in tables 2 and 3.

(12) TABLE-US-00002 TABLE 2 Creep testing at 600 C. Time to Rupture rupture Stress elongation Alloy Heat (hours) (MPa) (%) 1 763554 32621 150 55 2 462269 49738 170 71 3 477353 50986 170 72 4 469837 117561 160 71 5 471988 67644 160 79 6 469718 102321 160 90 7 477217 104958 150 38 8 477203 105889 150 46 9 460335 85940 140 63 10 463024 7629 165 65

(13) Table 2 shows the time to rupture and the creep strength or applied stress of each alloy at 600 C. Table 2 further shows the rupture elongation i.e. the length increase until rupture expressed as percentage of nominal length for each sample.

(14) From the test results it can be concluded that the inventive alloys 4-6 shows the highest time to rupture when the magnitude of the creep strength i.e. applied stress is taken into consideration. Alloy 4 shows a peak value of 117561 hours at an applied stress of 160 MPa. Alloys 4-6 further show very high rupture elongation.

(15) The high results on time to rupture in alloys 4-6 are believed to depend on a synergistic effect of addition of both nitrogen and nickel. Addition of nitrogen increases the time to rupture by interstitial solution strengthening and also by precipitation strengthening by the formation of carbonitrides. The dense small carbonitrides that are precipitated in the material effectively block dislocation movement through the grains of the alloy material and hence increase the resistance to deformation. Addition of nickel, and also nitrogen, suppresses the formation of intermetallic phase, such as sigma phase, that affects the ductility negatively and hence improves the ductility of the material. The improved ductility reduces stress concentration, crack initiation and crack propagation. The synergistic effect of these properties results in a very high creep strength.

(16) High ductility, which is expressed as rupture elongation in tables 2 and 3, is further advantageous when the material is used in steam boilers since it allows for high thermoplastic expansion and contraction of the material during start and shutdown of the boiler. Thus, the material can be subjected to cyclic heating and cooling without cracking.

(17) The comparative alloys 1-3, 9 and 10 have comparatively high rupture elongation, see for example comparative alloys 2 and 3 which exhibit a rupture elongation of 71% and 72% respectively. However, theses alloys exhibit a shorter time to rupture, than the inventive alloys. It is believed that the shorter time to rupture in alloys 1-3, 9 and 10 is due to the fact that these alloys contain relatively small amounts of nitrogen. The low nitrogen content results in that fewer carbonitrides are precipitated in these materials than in the inventive alloys. Since alloys 1-3, 9 and 10 comprise few carbonitrides, dislocations can move more easily through these materials. This causes in turn a higher strain rate in the material, i.e. the material deforms faster.

(18) Comparative alloys 7 and 8 exhibits rather high creep resistance, expressed as longer time to rupture at a given applied stress. However, it should be noted that the longer time to rupture for these alloys was determined at a lower stress, i.e. 150 MPa, than the inventive alloys which were evaluated at a stress of 160 MPa. Hence, the time to rupture of the comparative alloys 7 and 8 is lower than the time to rupture of the inventive alloys 4 and 6. The low time to rupture of alloys 7 and 8 is believed to be caused by brittleness induced by intermetallic phase precipitates. As is shown in table 2, alloys 7 and 8 have a rupture elongation of merely 38% and 46% respectively.

(19) Table 3 shows the result of creep testing at some applied loads at a temperature of 650 C.

(20) TABLE-US-00003 TABLE 3 Creep testing at 650 C. Time to Rupture rupture Stress elongation Alloy Heat (h) (MPa) (%) 1 763554 32621 95 45 4 469837 116711 95 70 5 471988 106165 95 52 6 469718 95883 105 45 6 469718 188609 95 31 8 477203 32665 120 62 9 460335 44168 105 50

(21) Table 3 shows that inventive alloys 4-6 have better creep properties expressed as time to rupture, creep strength and rupture elongation than the comparative alloys. The ductility for all alloys, i.e. the rupture elongation is lower at 650 C. in comparison to the ductility at 600 C. The reduction in ductility is caused by the fact that more precipitations are formed at higher temperatures and by faster grain growth at higher temperature.