CRYOGENIC PRESSURE VESSELS FORMED FROM LOW-CARBON, HIGH-STRENGTH 9% NICKEL STEELS

Abstract

A cryogenic pressure vessel of formed of an ASTM A553 Type 1 cryogenic steel alloy including in wt. %: C: 0.01-0.04; Mn: up to 2.0; P: up to 0.02; S: up to 0.15; Si: up to 1.0; Ni: 7-11; Cr: up to 1.0; Mo: up to 0.75; V: up to 0.2; Nb: up to 0.1; Al: up to 0.1; and N: up to 0.01. The steel alloy may have an ultimate tensile strength of at least 900 MPa, a total elongation of at least 20%; a microstructure including between 5 and 20 area % reverted austenite and the remainder tempered martensite; a transverse Charpy impact energy of at least 27 J at −196° C.; and a lateral expansion of at least 0.381 mm at −196° C.

Claims

1-19. (canceled)

20: A cryogenic pressure vessel formed from a cryogenic steel alloy, the alloy comprising in wt. %: C: 0.01-0.06; Mn: up to 2.0; P: up to 0.02; S: up to 0.15; Si: up to 1.0; Ni: 7-11; Cr: up to 1.0; Mo: up to 0.75; V: up to 0.2; Nb: up to 0.1; Al: up to 0.1; and N: up to 0.01; the alloy having an ultimate tensile strength of at least 900 MPa, a total elongation of at least 20%; a microstructure of between 5 and 20 area % reverted austenite and a remainder of tempered martensite; a transverse Charpy impact energy of at least 27 J at −196° C.; and a lateral expansion of at least 0.381 mm at −196° C.

21: The cryogenic pressure vessel as recited in claim 20 wherein the alloy comprises C between 0.04-0.06 wt. %.

22: The cryogenic pressure vessel as recited in claim 21 wherein the alloy comprises Mn between 0.5-0.7 wt. %.

23: The cryogenic pressure vessel as recited in claim 22 wherein the alloy comprises Si between 0.2-0.4 wt. %

24: The cryogenic pressure vessel as recited in claim 23 wherein the alloy comprises Ni between 7.5-9.5 wt. %

25: The cryogenic pressure vessel as recited in claim 24 wherein the alloy comprises Cr between 0.25-0.5 wt. %

26: The cryogenic pressure vessel as recited in claim 25 wherein the alloy comprises Mo between 0.5-0.7 wt. %

27: The cryogenic pressure vessel as recited in claim 26 wherein the alloy comprises P: up to 0.006 wt. %; S: up to 0.002 wt. %; V: up to 0.1 wt. %; Nb: up to 0.05 wt. %; Al: up to 0.06 wt. %; and N: up to 0.008 wt. %.

28: The cryogenic pressure vessel as recited in claim 20 wherein the alloy has undergone a heat treatment comprising: austenitizing at a temperature of between 750-1000° C. for 10 minutes to 3 hours; quenching to room temperature; lamellarizing at a temperature of between 600-725° C. for 10 minutes to 3 hours; cooling to room temperature in air; tempering at a temperature of between 500-620° C. for 10 minutes to 3 hours; and cooling to room temperature in air.

29: The cryogenic pressure vessel as recited in claim 28 wherein the austenitizing is at a temperature of between 800-950° C. for 30 to 60 minutes; the lamellarizing is at a temperature of between 625 and 700° C. for 30 to 60 minutes; and the tempering is at a temperature of between 550 and 610° C. for 30 to 60 minutes.

30: The cryogenic pressure vessel as recited in claim 29 wherein said austenitizing is at a temperature of between 820-900° C. for 30 to 60 minutes; said lamellarizing is at a temperature of between 650 and 675° C. for 30 to 60 minutes; and said tempering is at a temperature of 575 and 600° C. for 30 to 60 minutes.

31: The cryogenic pressure vessel as recited in claim 20 wherein the alloy has a microstructure of between 8 and 15 area % reverted austenite and the remainder tempered martensite.

32: The cryogenic pressure vessel as recited in claim 20 wherein the alloy has a microstructure of between 13 and 15 area % reverted austenite and the remainder tempered martensite.

33: The cryogenic pressure vessel as recited in claim 20 wherein the alloy has a lateral expansion of at least 1.0 mm at −196° C.

34: The cryogenic pressure vessel as recited in claim 33 wherein the alloy has a lateral expansion of at least 1.5 mm at −196° C.

35: The cryogenic pressure vessel as recited in claim 34 wherein the alloy has a lateral expansion of at least 2.0 mm at −196° C.

36: The cryogenic pressure vessel as recited in claim 20 wherein the alloy has a transverse Charpy impact energy of at least 50 J at −196° C.

37: The cryogenic pressure vessel as recited in claim 36 wherein the alloy has a transverse Charpy impact energy of at least 100 J at −196° C.

38: The cryogenic pressure vessel as recited in claim 37 wherein the alloy has a transverse Charpy impact energy of at least 150 J at −196° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is an SEM micrograph of an alloy useful for forming the inventive cryogenic pressure vessel of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] ASTM A553 Type I steel, commonly known as “9% Ni Steel,” has requirements of a transverse Charpy absorbed energy of at least 27 J at −196° C., a lateral expansion of at least 0.381 mm at −196° C., and a total elongation of at least 20%. The A553 alloy must also have an ultimate tensile strength of at least 690 MPa. The inventive cryogenic pressure id formed from an alloy that meets all the mechanical requirements of the A553 alloy and has an ultimate tensile strength of at least 900 MPa.

[0014] The steel is heat treated by austenitizing/quenching, lamellarizing, and tempering. The resulting microstructure consists of predominantly martensite with significant volume fraction of reverted austenite plus carbides. The Charpy impact energy absorption of plates of the steel alloy useful in forming the inventive cryogenic pressure vessel are comparable with historic production values of ASTM A553 Type I despite the new steel exhibiting greater than 30 percent higher design strength. The broad compositional ranges of the cryogenic steel alloy used in the production of the inventive cryogenic pressure vessel are given in Table 1.

TABLE-US-00001 TABLE 1 C Mn P S Si Ni Cr Mo V Nb Al N Min 0.01 0 0 0 0 7 0 0 0 0 0 0 Max 0.06 2 0.02 0.15 1 11 1 0.75 0.2 0.1 0.1 0.01

[0015] More preferred compositional ranges are given in Table 2

TABLE-US-00002 TABLE 2 C Mn P S Si Ni Cr Mo V Nb Al N Min 0.04 0.5 0 0 0.2 7.5 0.25 0.5 0 0 0 0 Max 0.06 0.7 0.006 0.002 0.4 9.5 0.5 0.7 0.1 0.05 0.06 0.008

[0016] Plates 13 mm thick were formed of an alloy composition in Table 3.

TABLE-US-00003 TABLE 3 C Mn P S Si Ni Cr Mo V Nb Al N 0.044 0.59 <0.005 <0.002 0.26 8.9 0.45 0.65 0.08 0.013 0.028 <0.005

[0017] The plates were austenitized at 843° C. for 15 minutes and immediately water quenched. The broad austenitization temperature range for the alloys of the present invention is between 750-1000° C., more preferably between 800-950° C. and most preferably between 820-900° C. The broad austenitization time range is between 10 minutes and 3 hours, most preferably the austenitization time is 30 to 60 minutes.

[0018] After austenitization and quenching the plates were “lamellarized and tempered.” This is a two-step tempering process where the plate is lamellarized for a fixed time and temperature, air cooled to room temperature, and subsequently tempered for a fixed time and temperature and again air cooled to room temperature. The plates were lamellarized at 660° C. for 50 minutes. The broad lamellarization temperature range for the alloys of the present invention is between 600-725° C., more preferably between 625 and 700° C., and most preferably between 650 and 675° C. The broad lamellarization time range is between 10 minutes and 3 hours, most preferably the lamellarization time range is between 30 to 60 minutes.

[0019] The plates were tempered at 590° C. for 25 minutes. The broad tempering temperature range for the alloys of the present invention is between 500-620° C., more preferably between 550 and 610° C., and most preferably between 575 and 600° C. The broad tempering time range is between 10 minutes and 3 hours, most preferably the tempering time range is between 30 to 60 minutes.

[0020] FIG. 1 is an SEM micrograph of an alloy used to form the present inventive cryogenic pressure vessel. The micrograph shows the microstructure thereof after austenitizing/quenching, lamellarizing, and tempering. The microstructure of the alloys is tempered martensite with interlath austenite containing lamellae. The presence of retained austenite was confirmed by x-ray diffraction. The percentage of retained austenite as well as the ultimate tensile strength (UTS) in MPa, yield strength (YS) in MPa, and total elongation % for samples of the inventive alloy are shown in Table 4. The broadest range of retained austenite in the alloys used to form the inventive cryogenic pressure vessel is between 5 and 20%, more preferably between 8 and 15%, most preferably between 13 and 15%.

TABLE-US-00004 TABLE 4 Sample # YS (MPa) UTS (MPa) Tot. El. (%) % Retained Austenite 1 729 964 23.4 14.6 2 757 973 25.3 12.9 3 871 989 25.0 14.7

[0021] The tensile results in Table 4 demonstrate that the desired minimum tensile strength requirement of 900 MPa was achieved for all tested specimens. Table 4 also demonstrates that the minimum tensile elongation of 20% specified by the ASTM A553 requirements was achieved for all specimens.

[0022] Table 5 presents results for transverse Charpy impact energy at −196° C. in Joules and the lateral expansion at −196° C. in mm for samples of the alloy used to form the inventive cryogenic pressure vessel. Clearly the alloy easily meets/exceeds the ATM A553 requirements. Thus, the alloy used to form the inventive cryogenic pressure vessel has a lateral expansion of at least 0.381 mm at −196° C., preferably at least 1.0 mm, more preferably at least 1.5 mm and most preferably at least 2.0 mm. Also, the alloy used to form the inventive cryogenic pressure vessel has a transverse Charpy impact energy of at least 27 J at −196° C., preferably at least 50 J, more preferably at least 100 J and most preferably at least 150 J.

TABLE-US-00005 TABLE 5 Test # TCVN @ −196 C (J) Lat. Exp. @ −196 C (mm) 1 151 1.73 2 170 2.39 3 159 1.45 4 181 2.01 5 166 2.08