AUSTENITIC STEEL ALLOY HAVING AN IMPROVED CORROSION RESISTANCE UNDER HIGH-TEMPERATURE LOADING AND METHOD FOR PRODUCING A TUBULAR BODY THEREFROM

Abstract

An austenitic steel alloy is provided having excellent corrosion resistance under high-temperature loading of more than 600° C. and up to 800° C., with the alloy having the following proposed chemical composition (in wt. %), consisting essentially of: C: 0.01 to 0.10; Si: max. 0.75; Mn: max. 2.00; P: max. 0.03; S: max. 0.03; Cr: 23 to 27; Ni: 17 to 23; Nb: 0.2 to 0.6; N: 0.15 to 0.35; the remainder being Fe and melting-related impurities. In a particular configuration a tubular body is made from this steel alloy, where an absorber pipe of a solar receiver of a solar power installation may be made from the tubular body. Still further, a solar receiver comprising this absorber pipe is provided, as well as a method for producing a tubular body from the steel alloy.

Claims

1. An austenitic steel alloy for operating temperatures of at least 600° C. to 800° C. substantially consisting of the following chemical composition in wt. %: C: 0.01 to 0.10; Si: max. 0.75; Mn: max. 2.00; P: max. 0.03; S: max. 0.03; Cr: 23 to 27; Ni: 17 to 23; Nb: 0.2 to 0.6; and N: 0.15 to 0.35; with the remainder being iron and melt-induced impurities.

2. The steel alloy as claimed in claim 1, having in wt. %: C: 0.04 to 0.10; Si: min. 0.1; Mn: min. 0.6; Cr: 23 to 25; Ni: min. 20; and N: 0.20 to 0.30.

3. The steel alloy as claimed in claim 2, having in wt. %: 0.3<Nb/(C+N)<3.8.

4. The steel alloy as claimed in claim 3, having in wt. %: 0.4<Nb/(C+N)<2.5.

5. A tubular body, said tubular body produced from a steel alloy as claimed in claim 1.

6. The tubular body as claimed in claim 5, wherein the tubular body is a seamless tube.

7. The tubular body as claimed in claim 5, wherein the steel alloy has in wt. % 0.4<Nb/(C+N)<2.5, and wherein the tubular body is a welded tube.

8. An absorber tube of a solar receiver of a solar power plant for transporting a liquid heating medium, wherein said absorber tube is produced from a tubular body as claimed in claim 5.

9. The absorber tube as claimed in claim 8, wherein the absorber tube has an outer surface, and wherein the absorber tube comprises a heat-absorbing coating applied to the outer surface.

10. The absorber tube as claimed in claim 9, wherein the coating is a lacquer application or a sol-gel coating.

11. A solar receiver comprising an absorber tube as claimed in claim 8.

12. A method for producing a tubular body from a steel alloy as claimed in claim 1, said method comprising: annealing a tubular body at annealing temperatures between 800° C. and 900° C. for an annealing time of 0.1 h to 24 h in an atmosphere containing oxygen and/or nitrogen in such a manner that a cover layer having a thickness of at least 2 μm is produced on the tubular body during the annealing.

13. The method as claimed in claim 12, wherein the annealing time comprises 2 to 4 h.

14. The method as claimed in claim 12, wherein the cover layer produced on the tubular body during the annealing has a thickness of at least at least 5 μm and at most 20 μm.

15. The absorber tube of claim 8, wherein the liquid heating medium comprises a molten salt.

16. The steel alloy as claimed in claim 2, having in wt. %: C: 0.05 to 0.08; and Ni: min. 21.

17. The steel alloy as claimed in claim 16, having in wt. %: 0.4<Nb/(C+N)<2.5.

18. The steel alloy as claimed in claim 1, having in wt. %: 0.3<Nb/(C+N)<3.8.

19. The steel alloy as claimed in claim 1, having in wt. %: 0.4<Nb/(C+N)<2.5.

20. The steel alloy as claimed in claim 2, having in wt. %: 0.4<Nb/(C+N)<2.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 is a graph of test results for comparative alloys and an alloy in accordance with the present invention; and

[0041] FIG. 2 is a graph of the thermal fatigue behavior of an alloy in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Table 1 below shows the chemical composition of the tested materials in wt. % (extracts):

TABLE-US-00001 Steel C Si Mn Al Cr Fe Ni Mo Ti Nb Co N 1 0.09 0.22 0.8 0.005 18.3 Bal. 9.07 0.3 0.009 0.3 0.092 0.05 (Comp.) 1 (Inv.) 0.059 0.36 1.1 nd 25.6 Bal. 21.2 nd nd 0.46 — 0.2 2 0.06 <0.3 <1 <0.025 27.1 Bal. 32.3 nd nd 0.8 — 0.02 (Comp.) 3 0.05 0.08 0.06 1.3 22.1 1.9 53.1 8.7 0.40 <0.02 11.9 <0.01 (Comp.) 4 0.18 <0.5 <0.5 2.1 24.9 9.3 Bal. — 0.13 — — (Comp.) Comp.: Comparative alloy Inv.: Inventive alloy nd: Not determined Bal.: Remainder

[0043] The technical demands placed on the austenitic steel alloy require a combination of alloy elements which, on the one hand, enables a low corrosion rate by reason of the formation of a cover layer under the operating conditions or by reason of conditioning and, on the other hand, enables the required mechanical properties, i.e. high resistance to thermal fatigue. From an economic point of view, the Ni content should be as low as possible.

[0044] These conditions are fully met with the alloy composition in accordance with the invention. While the comparative steels 2, 3 and 4 have a very high Ni content or an Ni-based alloy, the comparative steel 1 and the inventive steel 1 have significantly lower Ni contents. In relation to the required properties, the inventive steel 1 produces the best results from an economic point of view of a low Ni content. Corresponding sample sheets of the aforementioned alloys have been subjected to a 1000-hour corrosion test in a molten salt of KNO.sub.3-NaNO.sub.3 at temperatures of 700° C., 660° C., 640° C., 600° C. and 570° C. The test results for the tested comparative alloys 1 to 4 and the inventive alloy 1 (designated in FIG. 1 as Comp. 1, Comp. 2, Comp. 3, Comp. 4 and Inv. 1) are illustrated in the form of a bar chart for each alloy in FIG. 1. For each alloy 5 columns are illustrated which, from left to right, are allocated to the temperatures 700° C., 660° C., 640° C., 600° C. and 570° C. of the molten salt. A thickness of a corrosion layer which is formed during the corrosion test is plotted on a y-axis in μm, which is to be interpreted as a measure of the corrosion attack. The corrosion attack for the inventive steel 2 is comparatively moderate and optimum when measured against the lower alloy costs.

[0045] The thermal fatigue behavior of a sample consisting of the inventive steel 1 (Inv.1) is shown in a diagram as FIG. 2. A value Δε is plotted on a y-axis as the elongation of the sample between the start and end of the test in percent between 0.2 to 2.0 and on the x-axis a number of load cycles as the number of incipient crack cycles for a load drop criterion of 5% N.sub.A5 (LW) with values between 10.sup.2 to 10.sup.5. R.sub.ε=−1 indicates an alternating stress during the fatigue test. The inventive steel 1 (Inv. 1) shows excellent stability under alternating stress, which is almost independent of the temperature. This property is particularly important under operating conditions by reason of the relatively frequent temperature cycles of a solar power plant.