Influence of material, environment and strain rate on environmentally

toughness ductile-to-brittle transition, i.e., the ‘Master Curve’. Work as of late was focusing on determination of WST model parameters, which resulted in computational demonstration of both the (i) statistical size effect and (ii) temperature dependence to be in line with the experimentally determined Master Curve.

• The experimental programme (fracture mechanical and tensile testing) and the test specimens for the fracture resistance measurements on different materials were in part completed. The numerical modelling and computations for analysing specimen’s measuring were reported as a MSc(Tech) thesis.

• Analysis of extensive data-base of VVER-440 surveillance data was performed applying recently developed, advanced non-linear methods. Non-linear analyses of the surveillance data were completed and reported as a conference paper. Participation to the IGRDM meeting was carried out.

• A detailed test matrix on ATOM Probe and PA characterisation of irradiated materials was agreed with Tohoky University. Samples representing weld 501 material in eight different IAIA-conditions and ten different model alloys in irradiated conditions were prepared by VTT with EDM and transported to Japan.

2.6.6 Influence of material, environment and strain rate on environmentally

nuclear material in LWR environments. In 2008 the work focused on SSSRT using cold-deformed, non-sensitised stainless steel and oxidising environment

2. Characterisation of deformation mechanisms and their influence on EAC in austenitic nuclear materials, where the work in 2008 focussed on detailed investigations on localisation of deformation in specimens from earlier SSSRT using sensitised Type304 stainless steel using EBSD and TEM. The influence of dynamic strain ageing on austenitic nuclear materials was continued on nickel-based materials as well as austenitic stainless steels.

3. Characterisation of irradiated stainless steel, where the work in 2008 consisted of FEGSTEM investigations of three cold-worked irradiated stainless steels from LWR plant internals and one non-cold-worked stainless steel material.

4. Influence of the strain rate and environment on fracture toughness properties of austenitic materials, where the work in 2008 consisted of finalising the Master thesis and fracture toughness measurements in hydrogenated environments using nickel based weld materials Inconel 52, 82, 152 and 182.

5.

Participation in international co-operation, which in 2008 consisted of participation in international conferences and working groups and participating in international projects, especially the O

ECD/NEA SCAP project (stress corrosion cracking and cable ageing)

and in the CIR-project (Co-operative IASCC Research)

6.

A new task, report archiving was started in 2008, and the work in 2008 focused on

summarising lists on existing reports and preparing an archiving strategy.

Deliverables in 2008

1. Super slow strain testing of austenitic nuclear materials in LWR environments

Super slow strain testing in simulated BWR environment was initiated in the bellow loading testing equipment using non-sensitised Type 316L stainless steel, with four different levels of cold-deformation, i.e., 8, 15, 20 and 28%. The test failed due to loss of pressure. One test set using cold-deformed Type 316L material was performed in a step motor driven autoclave, and the test was successfully interrupted after 5% plastic strain. SSSR-testing as a whole is delayed compared to original plans due to the technical difficulties encountered. The specimens from the SSSRT using cold-deformed non-sensitised stainless steel will be characterised in 2009.

2. Characterisation of deformation

Electron back-scattered diffraction, EBSD, is a very versatile method for characterising crystalline materials. EBSD maps can be formed in numerous ways to reveal desired properties. Intra-grain mis-orientation mapping is one method, and has been calibrated for quantitative measurement of local plastic strain in 2007 within this project. The results obtained in 2008 show a non-uniform distribution of plastic strain, with higher plastic strains in the vicinity of grain boundaries than inside the grains, Figure 1. Further observations include local non-uniform distributions of plastic strain at the surfaces and an influence of local changes in the grain size on the localisation of plastic deformation. The non- homogeneous microstructure of materials thus results in non-homogenous distribution of local plastic strain, which in turn affects the crack initiation and growth.

In addition to characterisation of SSSRT specimens, the local strain distribution in a welded

pipe made of nuclear grade Type 304 austenitic stainless steel was characterised using EBSD,

and the results were compared to the results of residual stress measurements made on the

same pipe. The highest degrees of plastic strain (10 – 20%) were measured in the heat

affected zone (HAZ) close to the root area of the weld, Figure 2, with the deformed zone

extending 5-7 mm from the fusion line to the base material.

=50 µm; BC; Step=0.3 µm; Grid606x441

a) b)

50 µm

Figure 1. Pattern quality map (a) showing the microstructure of a crack tip area in a solution annealed and sensitised Type 304 stainless steel after SSSR-testing in simulated BWR environment and the local mis-orientation map in the same area (b). Blue, through green to red colour correspond to smallest to highest degree of mis-orientation, respectively, i.e., from lowest to highest amount of plastic strain.

a) b)

Figure 2. Pattern quality map (a) showing the microstructure of the fusion line area at the weld root, and local mis-orientation map from the same area of the investigated weld.

1000µm

The role of deformation in EAC has also been investigated by continued studies on dynamic strain ageing properties of austenitic nuclear materials using internal friction and tensile tests.

A doctoral thesis work is ongoing on the subject at TKK and the DEFSPEED-project funds a part of this work. Based on the tensile tests using different strain rates, DSA-maps have been constructed for austenitic stainless steels of Type 304 and 316 and for nickel-based materials Alloy 600 and 690. Serrated yielding starts at a lower temperature in Alloys 600 and 690 (observed between 150 and 600°C) compared to Type 316NG austenitic stainless steel (observed above ~200°C.) However, DSA-behaviour as manifested by serrated yielding is observed at LWR-relevant temperatures in all of the above mentioned materials at strain rates below 10-4 s-1. An example of a DSA-map is shown in Figure 3. The activation energies for the onset of DSA have been determined for both Alloy 600 and 690, and it has been observed to be about the same both, at ~1.65 eV. The results indicate that plastic deformation causes redistribution of interstitials (e.g. nitrogen and carbon), which can affect localisation of plastic deformation.

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

-8 -7 -6 -5 -4 -3

Alloy 600 Alloy 690

log(dε/dt)

1000/T, K ^-1

800700 600 500 400 300 200 100

H ~ 3.76 eV H ~ 1.65 eV

H ~ 1.65 eV

10^-8 10^-7 10^-6 10^-5 10^-4 10^-3

dε/dt, s-1

TEMPERATURE, ^oC

H ~ 363 kJ/mol H ~ 159 kJ/mol

H ~ 159 kJ/mol

Figure 3. DSA-map for Alloys 600 and 690, where serrated yielding is observed at temperature between 150 and 600 ° C at strain rates <10

^-4

s

^-1

.

The deformation microstructures of Type 316 stainless steel specimens tested inside and outside the DSA regime were examined by transmission electron microscopy, [17]. Though post-mortem TEM cannot directly observe solute interactions with dislocations, the observed long-range planarity in the dislocation structures at 400°C, and short-range planarity at 288°C, in conjunction with the serrations observed in the tensile curves, gives indirect evidence that the mechanism of DSA is operating in the material. DSA affects the deformation behaviour of the material by restricting cross-slip and therefore promoting strain localisation to the principle slip planes. The work has demonstrated that DSA is present in commercial Type 316 SS, and occurs at temperatures relevant to nuclear power plant operation.

3. Characterisation of irradiated stainless steels

The characterisation work on irradiated stainless steels were continued in 2008 on three cold-

worked irradiated stainless steels from LWR plant internals and one non-cold-worked stainless

steel material irradiated to a dose of about 2 dpa. The presence of a high density of dislocation

loops was confirmed in the materials, a consequence of the fact that the density rapidly

saturates at about 1 dpa. No evidence of void features was observed. Compositional analyses indicated only minor depletion of chromium at the grain boundaries, Figure 4.

Figure 4. Radiation induced depletion of Cr at the grain boundaries of the irradiated materials of this project, compared to literature data.

4. Influence of the strain rate and environment on fracture toughness properties of austenitic materials

Recent international research has indicated a clear influence of stain rate and environment on fracture resistance and tearing modulus in the case of nickel-based weld materials. Literature data on this Low Temperature Crack Propagation (LTCP) show a remarkable influence of hydrogen containing environment on fracture toughness properties of e.g. Inconel 182. The most susceptible temperature, according to the literature, is 55°C. No open literature data is available on fracture toughness of welded joints, but only on pure weld metals. The work within the DEFSPEED-project was started in 2007 as a Master of Science work, which was completed in April 2008. The work is continued with a broader test matrix. The influence of environment and strain rate is measured using sub-size specimens prepared from dissimilar metal welds (DMW), where the weld material is Inconel 52 and 182 and from pure weld metals made of Inconel 52, 152, 82 and 182. A pneumatic servo-controlled loading device and potential drop technique to measure the crack growth continuously during the test is used. Tests are mainly performed at 55°C in hydrogenated water with 100 cc H

2

/kg H

₂

O and loading rates of either 0.1 or 6.7 mm/h. The selected hydrogen content for the fracture toughness tests is higher (100 cc H

2

/kg H

2

O) than the typical hydrogen content of PWR primary water (~30 cc/kg H

2

O), i.e., the selected environment for the fracture toughness tests is accelerated and will obviously result in overly conservative results compared to real PWR environments. However, the main objective is to compare and evaluate the behaviour of different weld metals and to compare pure weld metals to dissimilar metal welds.

The results reveal a decrease in the fracture toughness of Ni-based weld materials in hydrogenated (100 cc H

2

/kg H

2

O) water at 55°C at low strain rate. The decrease is more remarkable in pure weld metals than in the specimens from dissimilar metal welds. Concerning the welded joint specimens, the decrease in fracture toughness value J

_IC

was remarkable for Alloy 182, from >260 kJ/m

²

in air to as low as 40 kJ/m

²

in hydrogenated water. The fracture toughness of specimens cut from the DMW made using Alloy 52 filler material showed no influence of hydrogenated water, and the toughness was so high (>300 kJ/m2), that valid J

IC

values were not obtained. However, low J

_IC

values, down to 46 kJ/m

²

, were measured on pure

Alloy 52 weld metal specimens, Figure 55. Also the J

IC

values of pure Alloy 182 specimens were lower than those from DMW specimens, but the difference was smaller (the lowest J

_IC

- value was 40 kJ/m

²

in DMW specimen and 31 kJ/m

²

in a pure weld metal specimen). A summary of selected results obtained so far are presented in Figure 6. The reasons for the difference between welded joints and pure weld metals will be further investigated. One reason may be the dilution of nickel in the weld metal during welding.

Alloy 52

0 50 100 150 200 250 300

0 0,5 1 1,5

∆a [mm]

J [kJ/m

2

]

2 Alloy 52 (TU2) Alloy 52 Environment:

Water, T = 55 °C 100 cm³ H2/kg H2O 200 ppm H₃BO₃ 2.1 ppm LiOH

Figure 5. Fracture toughness, i.e., J-R curves for pure weld metal specimens made of Alloy 52 and from Alloy 52 dissimilar metal welds (TU2) in hydrogenated water at 55 ° C using a loading rate of 0.1 mm/h.

Fracture toughness

0 50 100 150 200 250 300

Alloy 182 1 J (kJ/m2 )

Alloy 182, DMW, TV, air Alloy 182, DMW, TV, 100cc H2 Alloy 182, DMW, NO, 100 cc H2 Alloy 182, WM, 100 cc H2 Alloy 52, DMW, TU2, air Alloy 52, DMW, TU2, 100 cc H2 Alloy 52, WM, 100 cc H2 Alloy 82, WM, 100 cc H2

Alloy 52 a

i r

a i r

Alloy 82

DMW env. WM env.

DMW

env. WM env. WM env.

Figure 6. Fracture toughness, JIC or JQ, values obtained in air and hydrogenated water at 55 ° C (“env.”) using a loading rate of 0.1 mm/h. WM denotes pure weld metal specimens and DMW specimens from welded dissimilar metal welds.

5. International co-operation and education

International co-operation is an important part of the DEFSPEED project, both for bringing

the latest knowledge to Finland, and to educate experts. In 2008 project members participated in

an work shop on initiation, in an EPRI MRP Alloy 690 expert panel meeting, in international

projects, especially the O

ECD/NEA SCAP project (stress corrosion cracking and cable ageing),

the CIR-project (Co-operative IASCC Research) and in the Halden IASCC project. The project manager is steering committee member of the ICG-EAC (International co-operative group on environmentally assisted cracking) and in the scientific board for the 13th Symposium on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors.

Professor Roger Staehle visited VTT in September and gave three public lectures on steam generator corrosion related topics for more than 20 attendees.

6. Report archiving

The new task, report archiving, started in 2008 by collecting information on types, amounts

and diaries for nuclear materials reports, and continued with prosing a strategy for digital

report archiving.

No documento SAFIR2010 Annual Report 2008 - VTT project pages server (páginas 69-76)