Mechanisms for Boron Effect on Microstructure and Creep Strength of Ferritic Power Plant Steels

Transcription

1 Mechanisms for Boron Effect on Microstructure and Creep Strength of Ferritic Power Plant Steels - Boron Metallurgy of Ferritic Power Plant Steels - Fujio Abe, M Tabuchi and S Tsukamoto National Institute for Materials Science (NIMS), Japan Presented at the 8th International Charles Parsons Turbine Conference, 5 to 8 September 2011, University of Portsmouth, UK 1

2 Contents 1 Background and objectives 2 Creep strengthening mechanisms of 9Cr steel by boron 3 Additive strengthening due to boron and nitrogen 4 Suppression of degradation in creep strength in HAZ of welded joints by boron - creep strengthening mechanisms of HAZ - 5 Summary 2

3 Steam temperature ( o C ) Background 700 o C A-USC project in Japan ( ) Turbine manufacturers Japan Boiler manufacturers Materials company ( o C) 750 Ni-base alloy Institutes Universities 700 Austenitic steel In Japan USC A-USC aimed at 700 o C Ferritic steel Present max steam temp about 610 o C Dissimilar welded joint Year Austenitic steel or Ni base superalloy Type IV 9-12Cr steel 3

4 Objective: To make clear mechanisms for boron effect on microstructure and creep strength of 9Cr steel base metal and welded joints for boiler components in USC power plant at 4

5 Stress ( MPa ) Size of M 23 C 6 ( nm ) Boron content in M 23 C 6 ( at % ) Creep strengthening mechanisms of 9Cr steel by boron Effect of fine distribution of M 23 C 6 carbides along boundaries 8 Base steel 139 ppm B steel M 23 C 6 Boron in M 23 C 6 after 6aging Fe 2 W at, Aged for 10,300 h at 650 o o C h ppm B 9Cr-3W-3Co-02V-005Nb-008C steel with different boron but no nitrogen addition ppm N ppm N ppmB Time to rupture ( h ) 139ppmB 92ppmB 48ppmB M 23 C Fe 2 W Distance from prior austenite 1 m 160 grain boundary ( m ) On PAGB After aging 140 (, 10300h) 120 After tempering 100 Center of grain After aging (, 10300h) 80 After tempering Boron content ( mass% ) 5

6 Creep rate ( 1 / h ) Addition of boron retards onset of acceleration creep, which decreases minimum creep rate and increases creep life Cr-3W-3Co-02V-005Nb-008C, 80 MPa 0 ppm B 48ppm B 92ppm B ppmb 48 ppmb 92 ppmb 139 ppmb 139ppm B Time ( h ) Boron stabilizes fine distribution of M 23 C 6 near PAGB, which retards the onset of acceleration creep 6

7 The onset of acceleration creep is closely correlated with the onset of migration of lath and block boundaries log ( creep rate ) Climb motion of dislocations The transient creep : the movement and annihilation of dislocations e = rvb t r boron effect t r The acceleration creep : gradual loss of creep strength due to microstructure evolution s sg = 10Gb / l sg Lath boundary M 23 C 6 MX log ( time ) 9Cr steel with no boron lath or block boundaries 9Cr steel with boron lath or block boundaries M 23 C 6 M 23 C 6 7

8 Enrichment process of boron in M 23 C 6 carbides in the vicinity of PAGBs Mechanisms responsible for the reduction of coarsening rate of M 23 C 6 carbides by boron 8

9 Concentration of boron (ppm ) Concentration of boron ( % ) Concentration of boron Enrichment of boron in M 23 C 6 near PAGBs Normalizing 1100 o C Tempering Creep test 800 o C Enrichment of B in M 23 (CB) 6 Segregation of B around GB Creep test Temp Tempering Temp = 03nm d = 50 m C i = 100 ppm Grain boundary Normalizing Temp Within grain M 23 C 6 Boron C GB = C 0 exp ( B / RT ) r 0 B = 627 kj/mol L Karlsson et al (1988) C GB C C GB C i Temperature ( o C ) GB C 0 Distance GB 9

10 Mechanisms for reduction of coarsening rate of M 23 C 6 carbides near PAGBs by boron Volume diffusion controlled Ostwald ripening (M Y Wey, T Sakuma & T Nishizawa, 81) r 0 diffusion flux r r 3 - r 0 3 = k 3 t k 3 = 8 (a+b)σ V D M u M / 9aRT (u p M - u M ) 2 σ : Interfacial energy : ( 07 J / m 2 for cementite / ferrite) V : Molar volume of carbonitride D M : Volume diffusion coefficient of M atom u M : Concentration of M atom in matrix u p M : Concentration of M atom in carbonitride precipitate 10

11 Ostwald ripening in solid matrix requires accommodation of local volume change Dissolving Growing diffusive flux vacancies boron 1 As a small carbide goes into solution, carbon atoms take up interstitial sites in the matrix and vacancies are created near the carbide interface 2 Vacancies migrate through the matrix and arrive at a growing carbide, which accommodates local volume change 3 However, if boron atoms occupy vacancies in the vicinity of growing carbides, local volume change cannot be accommodated Reduction of coarsening rate 11

12 Stress Stress Further improvement of creep strength by the addition of both boron and nitrogen Further strengthening by a dispersion of fine MX nitrides 9Cr-3W-3Co-VNb steel Combination of boron effect and Combination MX nitride-strengthening of boron effect and MX nitride-strengthening What happens in the steel alloyed with 100 MPa both boron and nitrogen? MPa MPa - boron is a strong nitride forming element - Base steel Base steel 9Cr-3W-3Co-VNb steel Time to rupture Time to rupture 80 MPa Long-term stabilization Long-term by boron stabilization by 100,000 boron h 100,000 h 12

13 Time to rupture ( h ) Minimum creep rate ( 1 / h ) Boron ( mass % ) Boron ( ppm ) Influence of nitrogen on creep strength of 9Cr steel with 140 ppm boron 9Cr-3W-3Co-02V-005Nb-008C-0014B , 120 MPa 10 4 Nitrogen ( mass % ) At normalizing temperature : o C Nitrogen ( ppm ) ppm B BN Time to rupture Minimum creep rate ppm nitrogen Solid solution P122 P Nitrogen ( ppm ) Large BN 10-4 Small BN 1 No BN Nitrogen ( mass % ) The formation of boron nitrides consumes most of soluble boron, which degrades the creep strength 13

14 Creep strengthening mechanisms of HAZ in welded joints by boron The reason Boron why suppresses the creep strength Type IV fracture of Gr92 finegrained Formation of HAZ fine-grained is lower region is than that of base metal suppressed in HAZ of NIMS boron steel Type IV The reason why the addition of boron suppresses the Type-IV fracture WSRF < 075 P92 : 9Cr-05Mo-18W-VNb 14

15 Temperature Mechanisms for lower creep strength of HAZ than base metal of conventional 9 to 12Cr steels Softening? Grain refinement? Other possibilities? Simulated-HAZ specimens Simulated heating ( ) o C, 05 s Weld metal Coarsegrained HAZ Finegrained HAZ Lower creep strength Base metal 100 K s K s -1 PWHT 740 o C, 47 h Creep test Mechanical constrain effect Muti-axial stress condition tr < 10,000 h AC1 AC3 ( o C ) ( o C ) 90B Gr

16 Time to rupture ( h ) Gr92 Effect of different initial microstructure on creep strength of simulated-haz specimens A C3 heating + PWHT Fine grain, Poor GB M 23 C 6 Gr92N Coarse grain, Enough GB M 23 C 6 Gr92NN Fine grain, Enough GB M 23 C 6 PAGB of HAZ PAGB of HAZ 1 m 1 m 1 m , 110 MPa 9Cr-boron Gr92N Gr92NN Gr92 Normalizing + tempering Gr92N Normalizing (3 vol % retained austenite) 10 2 Gr92 Gr92 Ac1 Ac3 Gr92 Gr92N Gr92NN 90ppm boron-9cr Gr92NN Normalizing + sub-zero (liq N) (no retained austenite) Peak temperature ( o C ) Simulated heating + PWHT 16

17 Gr92 : poor GB M 23 C 6 & fine grains 9Cr-boron steel : enough GB M 23 C 6 & coarse grains Trace of PAGB in original microstructure New PAGB produced during thermal cycle Fine-grained Gr92 heating to 950 o C & PWHT very few precipitates are formed along new PAGBs lath-block subgrain structure is not clearly seen Boundary and sub-boundary hardening is significantly reduced PAGB Different microstructure 9Cr-boron steel heating to 950 o C & PWHT Lath & block Different transformation behavior during heating Boron effect 17

18 Amount of M 23 C 6 Grain size Production of new GBs by nucleation and growth of g phase Incomplete dissolution of M 23 C 6 during heating very few M 23 C 6 carbides along new GBs after PWHT reduction of boundary and sub-boundary hardening Gr92 Diffusive a / g transformation during heating to A C3 Type IV fracture Gr92 Fine grain Coarse grain heating Nucleation of g at GB Incomplete dissolution of M 23 C 6 Dissolution A C3 M 23 C 6 of M 23 C 6 Trace of PAGB in original microstructure M 23 C 6 re-formed during PWHT A C1 A C3 Peak temperature 18

19 9Cr-boron steel Martensitic reverse transformation of a to g during heating to A C3 heating A C3 Boron segregation shear Coarse g grains with high dislocation density high density dislo Direct observation of 9Cr-boron steel 9Cr-boron surface during steel heating s aa boron s aa Confocal scanning laser microscope Just above A C1 Just above A C3 Boron reduces GB energy and makes the boundaries less effective as heterogeneous nucleation sites cooling Boron retards recrystallization Un-recrystallized g transforms to martensite during cooling Retardation of diffusive a / g transformation 30 m 30 m after PWHT Lath-block microstructure The surface Martensitic relief must reverse be sign of the martensitic or displacive transformation transformation during heating 19

20 Stress ( MPa ) Boron ( mass % ) Boron ( ppm ) Stress ( MPa ) Stress ( MPa ) Stress ( MPa ) Suppression of Type IV fracture in welded joints by boron Type IV No Type IV (No grain refinement) Type IV (Grain refinement) NIMS 9Cr boron steel 10-2 Nitrogen ( ppm ) BN Cr-3W-3Co-VNb steel ppm B ppm N 60 Base metal Welded joints Time to rupture ( h ) ppm B ppm N 60 Base metal Welded joints Time to rupture ( h ) Solid solution o C Large BN Small BN No BN P P Nitrogen ( mass % ) Conventional steels Soluble boron is essential!! ppm B 15 ppm N Base metal Welded joints Time to rupture ( h ) ppm B ppm N 80 Base metal Welded joints Time to rupture ( h ) Stress ( MPa ) Stress ( MPa ) ppm B 11 ppm N Base metal Welded joints Time to rupture ( h ) P92 20 ppm B 500 ppm N Base metal Welded joints Time to rupture ( h ) No nitrogen addition Nitrogen addition Type IV fracture 20

21 Time to rupture ( h ) Stress ( MPa ) High strength 9Cr steel without Type IV fracture Minimum creep rate ( 1 / h ) Stress ( MPa ) (1) Suppression of Type IV Open : Base P92 (9Cr-05Mo-18WVNb) welded joint Time to rupture ( h ) (2) Creep strength of base metal tr Solid : Welded joint 9Cr-3W-3Co-VNb steel with 130ppm boron, 120 MPa 140ppm boron e min Nitrogen ( ppm ) P92 Base MARBN : martensitic 9Cr steel strengthened by boron and MX nitrides 9Cr-3W-2Co-02V-005Nb steel with 160 ppm B and 85 ppm N Improvement of creep strength NIMS 9Cr steel base metal NIMS 9Cr steel welded joints P92 base metal P92 welded joints Creep rupture time ( h ) NIMS 9%Cr steel 2 No degradation in welded joints Conventional steel P92 Degradation One of candidates for main steam pipe in A-USC project in Japan Type IV 21

22 Production of thick-walled MARBN pipe MARBN (9Cr-28W-3Co-02V-005Nb-008C-0008N-0014B) MARBN : Martensitic 9Cr steel strengthened by boron and MX nitrides 3 ton ingot Welding by automatic TIG 470 mm outer diameter 65 mm thickness 1300 mm length 22

23 Summary (1) Enriched soluble boron near PAGBs is essential for the stabilization of fine distributions of M 23 C 6 carbides near PAGBs in both base metal and HAZ of welded joints This suppresses the reduction of GB hardening during creep at, which suppresses the degradation in creep strength at long times (2) GB segregation of boron reduces GB energy and retards the diffusive a / g transformation in HAZ during heating of welding process The resultant microstructure after PWHT is substantially the same as the original one before thermal cycle This suppresses the Type IV fracture in welded joints (3) 9Cr-3W-3Co-02V-005Nb-008C steel with about 140 ppm boron and about 80 ppm nitrogen (MARBN) exhibits not only much higher creep rupture strength of base metal than Gr92 but also no Type IV fracture in welded joints at 23