HOT ROLLED PLATE OR FORGING OF AN AUSTENITIC STEEL

Abstract

A hot rolled plate or forging of an austenitic steel not susceptible to relaxation cracking is provided. The hot rolled plate or austenitic steel includes a composition having in percentages by weight: 0.019%≤C≤0.030%, 0.5%≤Mn≤2%, 0.1%≤Si≤0.75%, Al≤0.25%, 18%≤Cr≤25%, 14%≤Ni≤17%, 1.5%≤Mo≤3%, 0.001%≤B<0.008%, 0.25%≤V≤0.35%, 0.23%≤N≤0.27%, the balance being iron and unavoidable impurities, Ni(eq.)≥1.11 Cr(eq.)−8.24, Cr(eq)=Cr+Mo+1.5Si+5V+3Al+0.02, Ni(eq)=Ni+30C+x(N−0.045)+0.87; x=30 for N≤0.2, x=22 for 0.2<N≤0.25, x=20 for 0.25<N≤0.35.

Claims

1. A hot rolled plate or forging of an austenitic steel resistant to relaxation cracking, with composition comprising, in percentages by weight: 0.019%≤C≤0.030% 0.5%≤Mn≤3% 0.1%≤Si≤0.75% Al≤0.25% 18%≤Cr≤25% 14%≤Ni≤17% 1.5%≤Mo≤3% 0.001%≤B≤0.008% 0.25%≤V≤0.35% 0.23%≤N≤0.27%, the balance being iron and unavoidable impurities, and wherein: Ni(eq.)≥1.11 Cr(eq.)−8.24, wherein: Cr(eq)=Cr+Mo+1.5Si+5V+3Al+0.02 Ni(eq)=Ni+30C+x(N−0.045)+0.87 wherein: x=22 for 0.23%<N≤0.25% x=20 for 0.25%<N≤0.27%.

2. A hot rolled plate or forging of an austenitic steel resistant to relaxation cracking, with composition comprising, in percentages by weight: 0.019%≤C≤0.030% 0.5%≤Mn≤3% 0.1%≤Si≤0.75% Al≤0.25% 18%≤Cr≤25% 12%≤Ni≤20% 1.5%≤Mo≤3% 0.001%≤B≤0.008% 0.25%≤V≤0.35% 0.23%≤N≤0.27%, the balance being iron and unavoidable impurities, and wherein: Ni(eq.)≥1.11 Cr(eq.)−8.24, wherein: Cr(eq)=Cr+Mo+1.5Si+5V+3Al+0.02 Ni(eq)=Ni+30C+x(N−0.045)+0.87 wherein: x=22 for 0.23%≤N≤0.25% x=20 for 0.25%≤N≤0.27%; and a fully austenitic structure.

3. The hot rolled plate or forging with composition according to claim 1, wherein the hot rolled plate or forging is resistant to stress relaxation cracking at temperatures from 500 to 900° C.

4. The hot rolled plate or forging with composition according to claim 1, wherein the hot rolled plate or forging is resistant to stress relaxation cracking at temperatures from 550 to 750° C.

5. The hot rolled plate or forging with composition according to claim 1, wherein an elongation is higher than 30% at the temperature of 750° C.

6. The hot rolled plate or forging with composition according to claim 1, wherein a creep rupture lifetime under 36 MPa at 750° C. is higher than 0.5×10.sup.5 h.

7. Reactor vessels, forgings or pipelines comprising: the hot rolled plate or forging as recited in claim 1.

8. The hot rolled plate or forging of the steel according to claim 1, which has a fully austenitic structure.

9. The hot rolled plate or forging of the steel according to claim 1, wherein the hot rolled plate has a thickness from 5 to 100 mm.

10. The hot rolled plate or forging of the steel according to claim 1, wherein the steel has a Charpy V fracture energy greater than 100 Joules.

11. The hot rolled plate or forging with composition according to claim 2, wherein the hot rolled plate or forging is resistant to stress relaxation cracking at temperatures from 500 to 900° C.

12. The hot rolled plate or forging with composition according to claim 2, wherein the hot rolled plate or forging is resistant to stress relaxation cracking at temperatures from 550 to 750° C.

13. The hot rolled plate or forging with composition according to claim 2, wherein an elongation is higher than 30% at the temperature of 750° C.

14. The hot rolled plate or forging with composition according to claim 2, wherein a creep rupture lifetime under 36 MPa at 750° C. is higher than 0.5×10.sup.5 h.

15. Reactor vessels, forgings or pipelines comprising: the hot rolled plate or forging as recited in claim 2.

16. The hot rolled plate or forging of the steel according to claim 2, wherein 14%≤Ni≤17%.

17. The hot rolled plate or forging of the steel according to claim 2, wherein the hot rolled plate has a thickness from 5 to 100 mm.

18. The hot rolled plate or forging of the steel according to claim 2, wherein the steel has a Charpy V fracture energy greater than 100 Joules.

19. A hot rolled plate or forging of an austenitic steel resistant to relaxation cracking, with composition comprising, in percentages by weight: 0.019%≤C≤0.030% 0.5%≤Mn≤3% 0.1%≤Si≤0.75% Al≤0.25% 18%≤Cr≤25% 14%≤Ni≤17% 1.5%≤Mo≤3% 0.001%≤B≤0.008% 0.25%≤V≤0.35% 0.23%≤N≤0.27%, the balance being iron and unavoidable impurities, and wherein: Ni(eq.)≥1.11 Cr(eq.)−8.24, wherein: Cr(eq)=Cr+Mo+1.5Si+5V+3Al+0.02 Ni(eq)=Ni+30C+x(N−0.045)+0.87 wherein: x=22 for 0.23%≤N≤0.25% x=20 for 0.25%<N≤0.27%; and the hot rolled plate or forging being resistant to stress relaxation cracking at temperatures from 550 to 750° C.

20. The hot rolled plate or forging with composition according to claim 19, wherein a creep rupture lifetime under 36 MPa at 750° C. is higher than 0.5×10.sup.5 h.

21. The hot rolled plate or forging with composition according to claim 1, wherein the composition comprises 18%≤Cr≤21%.

22. The hot rolled plate or forging with composition according to claim 21, wherein the composition comprises 19%≤Cr≤21%.

Description

DETAILED DESCRIPTION

[0038] The following examples are presented as an illustration of the present invention. It should be understood, however, that the invention is not limited to the particular details in these examples.

EXAMPLE

[0039] Steel compositions were elaborated, of which elements are indicated on table 1 with their compositions in weight %. Compositions of steels A and B correspond to the invention. Ingots were cast, pre-forged under the form of flat products and hot-rolled down to plates with thicknesses ranging from 15 to 40 mm. The plates were solution-annealed at 1100° C. and water-quenched.

[0040] Steels with references C to I are comparative steels.

TABLE-US-00001 TABLE 1 Steel chemical compositions (% weight). Underlined values: non conform to the invention Other Steel C Mn Si Al Cr Ni Mo B V N elements Reference (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) Invention A 0.023 1.44 0.26 0.005 19.87 14.61 2.49 0.0034 0.31 0.23 S: 0.0017 P: 0.014 O: 0.007 B 0.019 1.49 0.51 0.010 20.1 14.82 2.51 0.004 0.30 0.26 S: 0.001 P: 0.003 O: 0.004 Reference C 0.02 1.51 0.49 0.005 20 11.9 — 0.0033 — 0.276 S: 0.001 P: 0.004 O: 0.006 D 0.02 1.5 0.52 0.29 20 13.3 2.54 0.004 — 0.25 S: 0.002 P: 0.005 O: 0.004 E 0.072 1.47 0.50 0.005 20.1 12.1 2.52 0.0044 — 0.267 S: 0.001 P: 0.002 O: 0.006 F 0.022 1.50 0.51 0.005 25.9 17.3 — 0.0037 — 0.351 S: 0.002 P: 0.003 O: 0.006 G 0.06 1.04 0.53 0.23 20.6 31.3 0.16 0.0013 0.065 0.015 S: 0.005 P: 0.012 O: 0.001 H 0.016 1.71 0.38 0.015 17.0 12.86 2.26 0.004 0.049 0.12 S: 0.005 P: 0.020 O: 0.002 I 0.064 1.71 0.39 0.030 18.2 10.5 — — — 0.05 S: 0.005 P: 0.035 O: 0.002

[0041] The following tests were performed: [0042] Tensile tests at 750 and 850° C. in order to determine the tensile strength (TS) and the total elongation (A) An elongation higher than 30% is desired in order to attest a good ductility at high temperature. [0043] Charpy V tests were performed in the following condition: After thermal treatment (ageing) at 650° C. for 1000 hours, the plates were cooled down to ambient temperature and tested in such condition. Specimens were machined in the plates and tested at 20° C. on a Charpy V pendulum. A Charpy V fracture energy greater than 100 Joules is desired in order to ensure satisfactory toughness. This criterion is severe since the ageing usually corresponds to a marked toughness drop for this kind of materials. [0044] Isocreep tests were performed for determining rupture lifetime at 750° C. under a stress level of 36 MPa, and at 850° C. under a stress level of 16 MPa. A creep rupture lifetime superior or equal to 0.5×10.sup.5 h at 750° C. under 36 MPa is desired [0045] Total scale thickness after 3000 h at 750° C. was measured on some specimens, indicating the level of resistance to the oxidation at high temperatures. [0046] Results of tensile, creep and Charpy V tests, scale thickness, are indicated in table 2.

TABLE-US-00002 TABLE 2 Results obtained on steels compositions of table 1 Charpy Lifetime Lifetime Scale energy at at thickness after 750° C. 850° C. after 1000 h creep creep 750° C.- TS at TS at A at at stress stress 3000 h 750° C. 850° C. 750° C. 650° C. 36 MPa 16 MPa (micro- Alloy (MPa) (MPa) (%) (Joules) (10.sup.5 h) (x10.sup.5 h) meters Invention A n.d n.d 50 133 n.d n.d 50 B 407 269 40 130 1 0.8 n.d Reference C 300 175 30 126 0.1 0.02 n.d E 370 275 50  41 1 0.7 n.d F 350 220 35  61 0.3 0.1 n.d G 270 150 40 182 1 1 150 H 275 nd 65 166 0.25 n.d. n.d [0047] The susceptibility to relaxation cracking was evaluated by the following procedure: After three-point bending of at ambient temperature, full-thickness specimens were submitted to a constant strain at temperature ranging from 500 to 900° C. during 150 hours. Load variation was recorded and an eventual damage by relaxation cracking was assessed by examining polished cross sections of the specimens. Some of them showed no damage or very minor cavities: these were classified as Non-Susceptible (“NS”). On the other hand, specimens with micro- or macro-cracks and cavities reveal a susceptibility (“S”) to SRC. For the purpose of use in industrial conditions, a non-susceptibility in the range of 550 to 900° C., and particularly in the range of 550-750° C., is desired. Results of the SRC tests are indicated in table 3.

TABLE-US-00003 TABLE 3 Results of stress relaxation cracking tests at different temperatures on steel compositions of Table 1. Alloy 500° C. 550° C. 600° C. 650° C. 700° C. 750° C. 800° C. 850° C. 900° C. Invention A NS NS NS NS NS NS NS NS NS B NS NS NS NS NS NS NS NS NS Reference C n.d n.d S S S n.d n.d n.d n.d E n.d n.d S S S n.d n.d n.d n.d F n.d n.d S S S n.d n.d n.d n.d G NS NS S S S NS NS NS NS H NS NS NS NS NS NS NS NS NS I NS NS S NS NS NS NS NS NS S = Susceptible to SRC NS = Non Susceptible to SRC n.d.: Not determined

[0048] From the results above, steels according to the invention display a particular combination of properties: non susceptibility to relaxation cracking on the temperature range 500-900° C., excellent creep resistance, high ductility in a large range of temperatures. These steels display also good toughness at ambient temperature after a holding at high temperature, and limited scale thickness.

[0049] The susceptibility to hot cracking in welding for the steels according to the invention was also assessed by the following test : the surface of the plates was melted with Gas Tungsten Arc Welding with heat inputs ranging from 4.5 up to 10.3 kJ/cm and travelling speeds ranging from 5.7 up to 24.3 cm/mn. In all cases, no cracks were detected in the remelted material and in the Heat Affected Zones. Thus, the compositions according to the invention display good resistance to hot cracking.

[0050] By comparison, the results obtained on the reference steels are as follows:

[0051] Alloy C which is a reference steel without molybdenum and vanadium, is extremely susceptible to stress relaxation cracking since macro-cracks initiate even after a relaxation time of 75 h. Furthermore, the elongation at 750° C. is not satisfactory.

[0052] Alloy D does not contain vanadium and has an excessive aluminum content, thus leading to insufficient ductility at elevated temperature.

[0053] Alloy E has excessive carbon content and does not contain vanadium. As a consequence, precipitation of carbonitrides, coarse sigma-phase and M.sub.23C.sub.6 carbides occur, which cause Charpy energy reduction after 1000 h at 650° C. Furthermore, this alloy was susceptible to SRC, particularly at temperatures around 650° C.

[0054] Alloy F has excessive chromium content, but no molybdenum and no vanadium. As a consequence, intermetallic phases form and reduce Charpy toughness, and on the other hand this alloy is very susceptible to SRC.

[0055] Alloy G has excessive contents in carbon and nickel but insufficient contents in molybdenum, vanadium and nitrogen. Consequently, after treatments at 600-700° C., alloy G shows damage with SRC since macro-cracks appear.

Even if the alloy H is not susceptible to SRC, its lifetime at 750° C. is less than the desired value of 0.5×10.sup.5 h, due to its low contents in vanadium and nitrogen.

[0056] In accordance with its inadequate contents in carbon, nickel, molybdenum, boron, vanadium, nitrogen, alloy I is susceptible to SRC at 600° C.

[0057] The steels according to the invention are used with profit for the fabrication of installations such as reactor vessels, forgings and pipelines operating at temperatures above 550° C.