10 inl seminar_high strength alloys pwr_ps.pdf

8/14/2019 10 INL Seminar_High strength alloys PWR_PS.pdf

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Selection and Performance of High

Strength Alloys for Bolts and SpringsP. M. Scott

INL Seminar on SCC in LWRs

Idaho Falls, Idaho, USA, March 19th 20th, 2013

INL 10


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Introduction

External bolting Low alloy steels RPV closure studs and nuts

SG and Pressurizer manholes bolting

Primary pump casing / Motor support assembly

Auxiliary valves body / Bonnet assembly and gland bolting

Internal bolting Stainless steels and Nickel base alloys

Valve stems

Valve and pump bolting

RPV internals and CRDM bolting

Springs

Note that some low alloy steel bolting has been replaced by

austenitic stainless steel (A 286 grade, resistant to concentrated boric

acid) where there is a risk of primary water leakage

INL SCC 2013, Peter Scott

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High Strength Nickel Base AlloysAlloy X-750

Heat Treatment : Solution annealing at 1093 C and age hardening () at 704-718C

Element C. Ni Cr TiNb +Ta

Fe AlYS(RT)

MPa

RCC-M M4104

ASTM B637

Gr 688 Type 3

70.014.0-17.0

2.25-2.75

0.70-1.20

5.0-9.0

0.40-1.00

655-900

Alloy 718

Heat Treatment : Solution annealing at 1000 to 1093 C and age hardening ( + )

at 720 and 620C

Element C. Mo. Ni Cr Ti

Nb +

Ta Fe Al

YS(RT)

MPa

ASTM B637 Gr670


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UPPER INTERNALS ASSEMBLY

SS 304 L

SS 304 L

NUT

X750

PINX

750

GUIDE TUBE and PIN ASSEMBLY

Guide tube pins allow accurate

positioning and fixing of the

control rod guide tubes

Alloy X-750 Guide Tube Pins5



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Alloy X-750 Guide Tube Pin Cracking

plaque

suprieure

de coeur

1erFilet

Cong

Zone

d'encastrement

des Branches

Flexibles

coupe

coupe

Facis de rupure IG

After Benhamou, 2004INL SCC 2013, Peter Scott

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Guide Tube Pins Design Requirements

During assembly Pre-stressing of the shank by torquing the nut to the required level

Deflection of the spring leaves to the required value

Limit misalignment between pin axis and upper core plate holes(Interference fit)

In service (PWR primary water at 325 C) Mechanical loading on the Guide tube due to cross flow in the upper

internals plenum

Thermal loading due to the difference in expansion coefficientsbetween Alloy X-750 and austenitic stainless steel

Neutron & gamma irradiation (2.5x1020n/cm2at 40 year EOL)Note that a detrimental effect of neutron irradiation on SCC resistancehas been observed for B contents >10ppm and doses >1019n/cm2,

probably due to 10B conversion into He (Mills et al, 1995)


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Finer Threads,

Reduced thread length

and subsequently

increased shank

length,

Slight Increase in

shank diameter,

Decrease in the height

of leaf bearing faces,

Nut with holes to

enhance circulation of

water.

Machining after all

heat Treatments,

Finer threads obtained

by cold rolling,

Machining of spring

leaves without plastic

deformation,

Shot peening of

critical areas,

Reduced nut torque

and leaf deflection.

Addition of Boron(25 ppm),

Water quenching afterannealing treatment.

DesignManufacturingMetallurgy

Good field experience to date with more than 100,000 hours (~ 14 Years) without cracking

Optimization of Resistance of Alloy X-750Guide Pins to SCC

Benhamou et al,2004


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AREVA NP PWR Fuel Element with Alloy 718

Hold-down Springs and Grid Springs


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Field Experience with Alloy 718

Generally excellent service experience in PWR primary waterSome in-service failures of springs by PWSCC due to a degradedsurface condition during fabrication (intergranular oxidation duringrolling)

Normal stress corrosion resistance was recovered by machining

off the layer affected by intergranular oxidation: Better quality control of protective atmospheres during rolling and

ageing

Yield stress should not be exceeded


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Intergranular Oxidation of Alloy 718 Springs

Affecting IGSCC Resistance

Before plastic deformation After plastic deformation

Poor control of furnace atmosphere during rolling resulted in intergranular oxidation of

a surface layer up to 200m thick, which severely degraded IGSCC resistance

SIMS analysis


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Alloy 625

Not generally used in power reactorsCan be used for reactor core and control rod components may

be cold worked for service at moderate temperatures

Hardening occurs when heated around 650C due to slow

precipitation of Ni3Nb " phase yield stress of ~700 MPa after

80 hours (Mills et al, 1995)

Exhibits relatively high strength, excellent general and stress

corrosion resistance in high temperature water (260 360C)

including neutron doses up to 4.4 x 1020n/cm2; cf. Alloy X-750

(Mills et al, 1995)Also being considered for advance reactor designs due to high

allowable design strength at elevated temperatures up to 760C


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Uses of Martensitic Stainless Steels

Valves stems16-4 and 17- 4PH due to their good resistance to pittingcorrosion (They have replaced plain chromiumstainless steels, particularly Type 410, affected bypitting when in contact with graphite based packing

gland materials)

Valves and pumps boltingAll grades

Control Rod Drive Mechanism (CRDM)components

Plain chromium steels Z12C13 (Type 410) andZ12CN13


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Thermal ageing decomposition of Fe-Cr solid solutions occurs when

Cr content (in-solution) > 10% Leads to precipitation hardening

Supplementary precipitation of phase (Cu) for 17- 4PH hardening

Thermal ageing kinetics:

Maximum embrittlement occurs after 2 years operation at 350C

No significant over-ageing Time / temperature equivalence (100 kJ / mol)

Reversible temper embrittlement:

Intergranular segregation of impurities at low tempering temperatures

(>400C)

Function of P, Sn, content and grain size

Suppressed by 1% Mo addition and reversed by heat treatment at

~600C

no generalized hardening but intergranular embrittlement

Martensitic Stainless Steels Thermal

Ageing Mechanisms


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Intergranular embrittlement Shift of CVN energy transition temperature

Hardening

Lowering of upper shelf and shift of transitiontemperature

Ageing reduces SCC resistance at high stress

levels

Use of 16-4 and 17-4PH materials should berestricted to low stress applications in the case of

continuous service above 250C

Effect of Thermal Ageing on Martensitic

Stainless Steel Properties


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Stem-gate-spacer ring assembly Detail of fractured surfaces of the valve

stem

Field Experience of Martensitic Stainless

Steels Example Catawaba 2


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Field Experience of Martensitic Stainless

Steels Example Catawaba 2

Failure of a 17-4PH valve stem detected in 1992

Tensile testing showed very low ductility in the fractured areas

Main reasons for damage:

Hardening and thermal embrittlement of 17-4PH at an operating

temperature above 316C for 40,000 h

Hydrogen embrittlement /Stress corrosion cracking

Similar events at Surry 1 and several other plants


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SCC of Martensitic Stainless Steels

Relation between hardness and SCC crack depth of as-tempered and as-aged specimens in BWR water (After Tsubota, et al, 1992).

Since the likely failure mechanism is hydrogen embrittlement, it is assumedthat the same susceptibility applies in PWR primary water conditions.


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C % Cr % Ni % Mo % Ti %YS at RT

(MPa)

Z6 CN 18-10(AISI 304) 0.05 18 10 - - 210

Z6 CND 17-12

AISI 3160.05 17 12 2.5 - 210

Z6 NCTDV 25-15

(SA 453 Gr660)0.05 15 25 1.2 2 590

Austenitic Stainless Steels

Typical chemical composition and mechanicalproperties :

Types 304 and 316 stainless steels with lower carbon contents (L grades)may also be used


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Uses of Austenitic Stainless Steels

Valves and pumps boltingall grades

Valve stems

cold worked Types 304 or 316 only

Reactor pressure vessel core internals boltingcold worked Type 316


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Manufacturing and Microstructure of Higher

Strength Austenitic Stainless Steels

Types 304 and 316 Electric arc or induction furnace steelmaking

Solution annealing between 1050 and 1150C / water quench

Microstructure : fcc solid solution

Cold working (stretching or drawing up to ~20% depending on

diameter)

SA 453 Grade 660 (A 286)

High quality steelmaking (VIM + VAR or ESR)

Annealing : 900C or 980C (preferred) / water or oil quench

Tempering : 725C 16 h / air cooling

Microstructure : fcc solid solution, precipitation hardened by

-Ni3(Ti, Al). Other possible phases: (Ni3Ti), G, Laves


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Field Experience of Precipitation Hardened

Austenitic Stainless Steel - A 286

Failures of SA 453 Grade 660 (A 286) bolting due to stresscorrosion cracking in primary water have been reported due

to excess preload or bad design (shank to head radius).

Use of SA 453 Grade 660 (A 286) in primary water

necessitates low stress concentration factors and accuratecontrol of preload (limited to 550 MPa to avoid stress

corrosion cracking).


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Low Alloy Steels for Bolting

Typical chemical compositions and mechanical properties

(RCC-M and ASME):

* depending on diameter

C % Cr % Ni % Mo % V %YS at RT

(MPa)

40 NCDV 7-3

(SA540 GrB24V Cl3) 0.4 0.8 1.8 0.5 0.07 900

40 NCD 7-3

(SA540 GrB24 Cl4)0.4 0.8 1.8 0.35 - 900

42 CDV 4

(SA193 B16)0.42 1.0 - 0.6 0.3 725*

42 CD 4

(SA193 B7)0.42 1.0 - 0.25 - 720*


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Uses of Low Alloy Steel Bolting

NiCrMoV grade: RPV closure studs and nuts

NiCrMo grade:

Pump casing-motor support assembly

CrMoV and CrMo grades: Manhole bolting

Auxiliary valves bolting


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M f t d Mi t t f L 26


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Manufacturing

NiCrMo or NiCrMoV grades : VIM + VAR or ESR

Heat treatment : 820-870C / water quench

590-630C 4h min/air cooling

Other materials

VIM or electric furnace steelmaking

Microstructure Tempered martensite

High tempering temperature to avoid hydrogen crackingsusceptibility

Intergranular segregation of impurities (P, Sn, Sb, As) mayoccur but fine (austenitic) grain size and Mo additions ensure lowsusceptibility to intergranular embrittlement

Manufacture and Microstructure of Low

Alloy Steel Bolting


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Variation of KIcof AISI 4340 Low Alloy Steel

with Test and Tempering Temperature


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Resistance to SCC/Hydrogen Embrittlement as

a Function of Yield Strength

The EPRI Materials Handbookcites LAS support bolt failures

when yield strengths are >150 ksi

(1050 MPa), equivalent to a

hardness of ~400HV.

The tempering temperature of590 to 630C used today gives a

minimum strength specification of

900 MPa.


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After NUREG CR 2467

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Summary High Strength Steels

Great care is needed with bolting design and manufacture

Field experience shows numerous possible degradation modes:

Steam cutting due to primary water leaks

Excessive hardness (>350HV) due to inappropriate initialheat treatment or thermal ageing or too much cold work

Loss of fracture toughness due to thermal ageing Stress corrosion cracking / hydrogen embrittlement

To avoid plant failures it is necessary to ensure:

Proper design (shank / head radius)

Materials compatible with environmental conditions Accurate control of preload

Permitted lubricants only


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10 inl seminar_high strength alloys pwr_ps.pdf

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