In addition to solidification segregation, the local com­position of welds can change in-service via thermal exposure and via radiation-induced transmutation and radiation-induced segregation (RIS). Thermally induced embrittlement is most notable in low-alloy steels that are high in tramp elements, in locations where welding produces local compositional enrich­ment, and in higher nickel grades (e. g., A508 Gr4N), which are intrinsically more susceptible because ofthe cosegregation of nickel and phosphorous.79-82

RJS is a phenomenon in which irradiation-created defects cause spatial redistribution of alloying ele­ments as they diffuse to, and get trapped in, sinks

 

(AEcPNi/NiO ~ x0) 2

 

1 + b • exp

 

0.5

 

c

 

Effective R — T

 

[1]

 

• exp

 

ШШ ‘ r І g. fj’

A600heat affected zone

(a)

Far from A600 HAZ weld

EN82H GTA weld

Hardness indents

HAZ

Base

01 1378

6 018000

Figure 19 Comparison of Alloy 600 heat-affected zone and base metal structure, chemistry, and strain: (a) cross-section of stress corrosion sample showing the location of the cracking, (b) grain boundary chromium profiles for base metal (blue) and the HAZ (red), (c) qualitative strain map for the base metal, HAZ/weld interface, and (d) typical grain boundary microstructure for the HAZ (sparse M23C6 and M7C3) and base metal (extensive M7C3).

(e. g., voids and grain boundaries) (Chapter 1.18, Radiation-Induced Segregation). RIS can occur when a point defect flux interacts preferentially with a certain elemental species in the alloy, causing
that element to be enriched or depleted near the defect sinks. This preference can be driven kineti — cally (migration barriers) and/or thermodynamically (binding/ordering). RIS is segregation that occurs

1.0E-8

A600 HAZ

360 °C

1.0E-9

‘co E. (D 03

305 °C

1. 0E-10

л*.

о

‘ "O

(D

О

1.0E-11

CL

Figure 20 Illustration of the predicted crack growth rates for Alloy 600 HAZ material as a function of potential, stress intensity factor, and temperature.

Table 3 Fitted parameters for the Alloy 600 heat-affected zone stress corrosion crack growth rate data presented in Figure 20 In (A0) n m B X0 (mV) c (mV) Q(kJmoI -1) Best estimate 22.607 0.869 0a 3.604 -15.61 42.79 136.0 95% confidence ±3.729 ±0.349 — — ±20.71 ±19.25 ±18.2 insufficient data were available to determine this dependence.

in addition to thermal segregation. Like thermal seg­regation, this local change in composition can result in detrimental changes to mechanical and corrosion

83

properties.

RIS has been a concern in the nuclear industry for over 30 years and is considered one of the many factors that lead to irradiation-assisted stress corrosion cracking.6-9 This phenomenon was first predicted by Anthony84 and has been observed in a number of dif­ferent alloys and steels used in nuclear reactors.85-87 In both the iron — and nickel-based face-centered cubic Fe-Ni-Cr alloys, experimental RIS trends at grain boundaries are generally chromium depletion, nickel enrichment, and possible compensation through iron enrichment or depletion.85,86 In body-centered cubic ferritic-martensitic steels, both chromium enrichment and depletion have been reported.88,89 RIS in welds has not been extensively researched but the possibility ofgrain boundary depletion of chromium in corrosion-resistant
alloys that would act in addition to the chromium depletion that occurs during solidification segregation (e. g., Ni-Cr-Nb and Ni-Cr-Mo alloys) is of significant concern.

4.09.3.3 Microstructural Changes

In addition to the relatively short-time microstructural changes that can occur on-cooling or with postweld heat treatment (typically <10h) and induce PIC as discussed earlier, long-time microstructural changes can also occur. While most nuclear alloys have had sufficient vetting to preclude these concerns, recently developed high-alloy nickel-based filler metals raise the concern that topologically close-packed (TCP) phases (e. g., sigma) or long-range ordering could occur in Ni-Cr-Mo welds.10,50 The degradation of toughness by the formation of TCP phases is well established in the superalloy literature and the

formation of long-range order can lead to increased residual stresses and decreased resistance to EAC, most notably to hydrogen embrittlement.50,52,53,90