SE1430130A1

SE1430130A1 - A steel for a lead cold reactor abstract

Info

Publication number: SE1430130A1
Application number: SE1430130A
Authority: SE
Inventors: Peter Szakalos; Jesper Ejenstam; Janne Wallenius
Original assignee: Blykalla Reaktorer Ab
Priority date: 2014-09-14
Filing date: 2014-09-14
Publication date: 2016-03-15
Also published as: EP3194633A1; WO2016039679A1; EP3194633A4; CA2960670C; CA2960670A1

Description

Nitrogen may be present in the steel in an amount of 5 0.06 % because N reacts With A1.

Molybdenum increases the high temperature mechanical properties and is a strong carbide forming element and also a strong ferrite former and may result in the formation of brittle Laves phase. The amount of molybdenum should be restricted to maximum 3 %, preferably to 2 % or less. If the alloy composition is prone to lave phase precipitation, the higher limit may be 2 %, 1.5 %, l %, 0.5 % or 0.1 %.

Niobium forms carbides, nitrides and carbo-nitrides and is beneficial for strength and creep resistance. In addition, Nb tends to improve the oxidation resistance and to fonn in?uence on the formation of intermetallic precipitates. Nb is therefore present in an amount of 0.1 - 3 %, preferably 0.6 - 1.2 %.

Tantalum forms carbides, nitrides and carbo-nitrides and is beneficial for strength and creep resistance.

In addition, Ta tends to improve the oxidation resistance and to form in?uence on the formation of interrnetallic precipitates. Ta is therefore present in an amount of 0.1 - 3 %, preferably 0.6 - 1.2 %.

Ti, Zr & Hf Reactive elements that promote formation of a protective alumina scale. Strong carbide formers and strong oxide particles formers, beneficial for high temperature mechanical properties When alloying With oxygen, so called ODS alloys.

The amount of Ti, Zr & Hf, individually, may be 0.0l-l %. lf alloyed With oxygen, the preferred amount is 0.5 - l % (ODS). If no oxygen is deliberately added, the amount may be < 0.5 %.

Yttrium Reactive elements that promote formation of a protective alumina scale. Strong carbide formers and strong oxide particles formers, beneficial for high temperature mechanical properties When alloying With oxygen, so called ODS alloys.

The amount of Y may be 0.05 - l %. If alloyed With oxygen, the preferred amount is 0.5 - l % (ODS). lf no oxygen is deliberately added, the amount may be E 0.5 %.

Silicon is beneficial for high temperature oxidation properties but is a strong ferrite former and should therefore be limited. The upper limit may be 2.0 %, 0.6 %, 0.55 %, 0.5 %, 0.45%, 0.4 % or 0.35 %.

Manganese Strong austenite stabilizer and may to some extent replace Ni. Mn also improves the mechanical properties to some extent. Mn is included in carbides as Well as oxides. Mn tends to promote secondary phases, such as sigma phase, Which may cause embrittlement. The Mn content should be limited to S 8 % for some alloy compositions, but preferably E 3 % for alloy compositions sensitive to sigma phase. The upper limit may be 3 %, 2.5 %, 2.0 %, l.5 %, 1 % or 0.5 %.

Copper is an optional element, Which has austenite stabilizing effects but it may form brittle phases, especially under irradiation. lt is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is normally limited to 3 %, preferably i 0.3 %. Most preferably, Cu is not deliberately added.

Cobalt The Co-content should be as low as possible in nuclear applications but for other application it is beneficial in stabilizing an austenitic structure and improves the strength at al temperatures. In compositions aimed for nuclear applications, the amount is preferably S 0.1 %. In compositions Where Co is deliberately added, the amount may be 5 5 %.

Vanadium forms carbides and carbonitrides of the type M(C,N) in the matrix of the steel.

However, if stronger carbide formers are present, than the V amount should be i 0.3 %. In other cases, the V amount may be 5 1 %.

Tungsten Increases the high temperature mechanical properties and is a strong carbide forming element and also a strong ferrite former and may result in the formation of brittle sigma phase. The amount of molybdenum should be restricted to maximum 3 %, preferably to 2 % or less. If the alloy composition is prone to lave phase precipitation, the higher limit may be 2 %, 1.5 %, 1 %, 0.5 % or 0.1 %.

Sulphur Sulphur should not deliberately be added, lowers the oxidation properties.

Boron Boron may act as a substitution to carbon, but is also a strong neutron absorber. Boron suppresses the nucleation of ferrite on austenitic grain boundaries. The amount of B may be 5 0.1 %, but preferably í 0.007 %.

Bi, Se, Ca, Mg These elements may be added to the steel in the claimed amounts in order to further improve the machinability, hot Workability and/or Weldability.

Oxygen In combination With oxygen active elements such as Y and REM in general, form small oxide particles, beneficial for high temperature mechanical properties, so called ODS-alloys. In the case of ODS alloying, the O amount may be 5 0.5 %, but preferably 0.05 - 0.15 %. In non- ODS alloys, O should not be deliberately added.

REM Irnproves the oxide scale properties and are beneficial for high temperature mechanical properties in combination With oxygen, so called ODS-alloys. (Rare Earth Metals) as used in this application embraces the elements With atomic numbers 21 and 57-71 because Yttrium is defined separately. The amount of REM may be i 0.3 %.

EXAMPLE In the present example three austenitic stainless steels are compared With the inventive steel.

The two steels 316 L and 15-15 Ti are commercial steels.

The AFA alloy and the inventive steel Were casted in a vacuum furnace, approximately 1 kg per batch. The alloys Were subsequently rolled into 8 X 1 mm strips in a total of 8 steps, With 5 min heat treatment at 1100 °C after each rolling step. Full compositional data for all alloys is presentenced in table 1.

Table 1 A110y Fe cr Ni A1 Mn Mo S1 Nb c Cu P Ti 316L(4404> Bai. 17 10 - 1 2 0.5 - 0.02 0.4 0.03 - 15-15T1 Bai. 15 15 - 1.8 1.2 0.5 - 0.09 - 0.01 0.5 (12R72) AFA20N1 Bai. 14 20 2.5 2 2.5 0.15 0.9 0.08 - - - Inventivesteel Bal. 14 14 2.5 2 2.5 0.15 0.9 0.08 - - - All alloys were cut into samples measuring 30 x 8 1n1n, with varying thicknesses deepening on initial shape. All samples Were polished to near mirror like surfaces using Struers abrasive SiC paper (final step #1200) and finally ultrasonically cleaned in ethanol for 10 minutes.

The experiment was conducted in a COSTA (COrrosion Test Stand for liquid metal Alloys) setup, constructed by Karlsruhe Institute of Technology (KIT). Samples were fitted into alumina crucibles using alumina holders and 1 mm molybdenum wire as support. Lead shots (2 mm) (Alfa Aesar, 99.95 % metal base) were poured in the crucibles until the samples were completely covered. All crucibles were subsequently placed on nickel trays and placed inside the sealed quartz tubes of the furnace. More information on the COSTA setup is presented in J. Nucl. Mater. 278(2000) 85-95.

The oxygen concentration in the liquid lead was controlled by means a gas mixture containing Ar, H2 and H20. The Hz/HzO ratio was set to 1.3, which corresponds to 1057 wt. %. A Zirox SGM5 oxygen analyzer was used to monitor the oxygen partial pressure at the systems gas outlet. Two corrosion tests, lasting 3,000 h and 8,700 h (1 year) respectively, were carried out at 550 °C.

Prior to evaluation, the samples were cleaned with ethanol, and dried with pressurized air.

Cross sections were prepared by mounting the samples in acrylic resin with Fe filler, followed by fine polishing.

The two chromia forming steels, 316L and 15- 15 Ti, were both attacked by the liquid lead although to various degrees. While a continuous dissolution front was seen for the 316L samples, only localized dissolution attacks were seen in 15-15 Ti. No obvious differences, with respect to dissolution depth, were seen for the exposure times for the two chromia forrners. However, for 15-15 Ti the frequency of localized dissolution attacks increase with exposure time, i.e. from 3000 h to 1 year. The lead penetration depth was roughly 100 - 300 um for 316L whereas the localized dissolution attacks on 15-15 Ti measured up to roughly 50 um. Selective dissolution of Ni, caused by exposure to liquid lead, is clearly seen When the attacked samples are examined by SEM-EDS. In addition, dissolution of an austenite stabilizing element, such as Ni, cause phase transformation from austenite (FCC) to ferrite (BCC).

The 20 Ni AFA alloy and the inventive steel are both alumina forming steels.

For the 20 Ni AFA alloy, localized dissolution attacks Were seen. Similar to the 15-15 Ti sample, the depth of the attacks measured up to about 20 um for both exposures times. The frequency of attacks Was again higher after 1 year exposure. In addition, localized internal oxidation Was noted. The oxide nodules, rich in Al, Fe and Cr, measured up to about 10 um in size and Were unevenly spread out in the metal/oxide interface. No significant difference in size or frequency of oxide nodules Was noted With increased exposure time.

The inventive steel Was the only alloy in the test that did not suffer from any dissolution attack, neither after 3,000 h nor after 1 year. HoWever, nodular internal oxidation Was found.

The size of the oxide nodules Were up to 10 um after 3,000 h, Whereas the largest ones measured about 25 um after 1 years exposure. As for the 20Ni AFA, the oxide nodules Were unevenly spread out in the metal/oxide interface. A thin protective oxide layer, measuring 10 to 100 nm, Was covering the sample surface.

In addition to differences in the alloys oxidation properties, differences in microstructure Were found. A part from the NbC present throughout the matrix, darker and lighter areas Was detected in the inventive steel. EBSD mapping revealed that the alloy Was not purely FCC phased, but rather two-phased (FCC and BCC). The fractions of FCC and BCC in the alloy bulk Were calculated to 83% and 17% respectively. The 20Ni AFA Was however essentially single phased (FCC). The results are summarized in table 2.

Table 2. Summary of corrosion and oxidation results in liquid lead. 3000 h 1 year Alloy P.O D.A. I.O. P.O. D.A. I.O. 3l6L 100-300 100-300 No um No No um No 15-15 Ti 20-50 20-50 Partially um No Partially um No 20Ni AFA 10-20 Yes (S10 10-20 Yes (S20 Partially um um) Partially um um) Inventive Yes (S100 Yes (S10 Yes (S20 steel nm) No um) Yes No um) P.O.- Protective oxide. D.A- Dissolution attack I.O.- Internal Oxidation.

It is evident from table 2 is the only alloy that formed a protective oxide cover and that did not suffer from any dissolution attack at any time. Accordingly, the claimed alloy is considered to have very attractive properties for use as structural components in Pb or LBE cooled reactors or in concentrated solar plants.

Claims

1. A steel for structural components used in contact With liquid lead in nuclear reactors consisting of in Weight % (Wt. %): Cr 8.0 - 15.0 Ni 10.0 - 16.0 Al 2.0 - 4.0 C 0.02 - 0.2 N S 0.06 Mo S 3.0 at least one of: Nb 0.1 - 3.0 Ta 0.1 - 3.0 Ti 0.01 - 1.0 Zr 0.01 - 1.0 Hf 0.01 - 1.0 Y 0.05 - 1.0 optionally Si S 2.0 Mn S 8.0 Cu S 4 Co S 5 V S 1 W S 3 B S 0.l Bi S 0.2 Se S 0.3 Ca S 0.01 Mg S 0.01 O 0.02-0.50 REM S 0.3 balance Fe apart from impurities, Wherein the content of REM does not include the amount of Y but only the amount of the elements having an atomic numbers 21 and 57-71.

2. A steel according to claim 1 containing in Weight % (Wt. %): Cr 9.5 - 14.5 Ni 10.0 - 15.0 Al 2.5 - 3.5 C 0.01- 0.15 Nb 0.6 - 1.5

3. A steel according to claim 1 fulfilling at least one of the following requirements (in wt. %): Cr 9.0 - 12.0 Ni 10.0 - 14.5 Al 2.3 - 3.7 C 0.02 - 0.1 N S 0.04 Si 0.1 - 1.0 Mn 2.0 - 4.0 Mo 0.5 - 2.8

4. A steel according to claim 1 fulfilling at least one of the following requirements (in wt. %): Cr 9.0 - 11.0 Ni 10.0 -14.5 Al 2.5 - 3.5 N S 0.03 Nb 0.6 - 1.2 Si 0.1 - 0.5 Mn 0.3 - 1.0 Mo 0.5- 1.5

5. A steel according to any of any of the preceding claims fulfilling at least one of the following requirements (in wt. %): Ti S 0.3 V S 0.3 Nb 0.8 -1.0

6. A steel according to any of claims 1 or 2 fulfilling at least one of the following requirements (in wt. %) Cr 9.5 - 13.0 Ni 10.0 - 14.0 Al 2.5 - 3.2 N S 0.03 C 0.02 - 0.09 Nb 0.7 - 1.1 Si 0.1 - 0.5 Mn 0.4 - 2.5 Mo 1.5 - 2.7

7. A steel according to any of the preceding claims fulfilling at least one of the following requirements (in wt. %): Ti V |/\|/\ 9.0 i-li-l 10. 11. 12. 13. 14. 15. Cu S l Co S 3 W S l B S 0.01 Bi S 0.02 Se S 0.03 Mg S 0.001 REM S 0.12 A steel according to any of the preceding claims fulfilling at least one of the following requirements (in wt. %): Cu S 0.3 Ti S 0.005 P S 0.025 S S 0.005 A steel according to any of the preceding claims fulfilling at least one of the following requirements (in wt. %): Ti V 0.05 0.05 I/\ I/\ A steel according to any of the preceding claims fulfilling at least one of the following requirements (in wt. %): Ti 0.01 - 1.0 Zr 0.01-1.0 Hf 0.01-1.0 Y 0.05 -1.0 and wherein the steel fulfil the requirement (in wt. %): O 0.02-0.50 A steel according to any of claims 1-9, wherein the steel contains Nb but has no deliberate addition of any of the elements Ta, Ti, Zr, Hf and Y. A steel according to any of the preceding claims, wherein the ratio Ni/CrJrAl is S 0.85, preferably larger than 0.95. Use of a steel as defined in any of claims 1-12 for a structural component in a lead or lead- bismuth alloy cooled nuclear reactor or in a concentrated solar power plant. Use of a steel as defined in claim 13, wherein the molten lead or lead-bismuth alloy has a temperature of S 600 °C and/or an oxygen content of at least l0'7 wt. %. Use of a steel as defined in claim 13 or 14, wherein the relative velocity between the molten lead or lead-bismuth alloy and the structural surface is less than 5 rn/s.