Review of the Effect of Cold Work on Stress

Corrosion Cracking of Alloy 690 Rickard R. Shen

*

Department of Solid Mechanics, KTH (Royal Institute of Technology)

Stockholm, Sweden

Abstract – It is well known that Alloy 690 is highly resistant against primary water stress

corrosion cracking (SCC), yet not completely immune. Cold work, which is known to

accelerate SCC crack growth rates (CGRs), has been shown to occasionally enhance the CGR

in Alloy 690 base metal to alarmingly high levels. While Alloy 690 is not typically used in a

heavily cold worked condition, effective plastic strains of comparable level have been

estimated in Alloy 690 heat affected zone (HAZ). Much recent work on Alloy 690 has been

focused on build up data in order to clarify whether or not these high CGRs are of concern for

operating power plants.

How cold work affects SCC CGR is still unclear. Based on trends in experimental data, some

factors are believed to increase CGRs. The clearest trend is that of crack orientation. Aligning

the plane and direction of crack growth with those of cold deformation generally yields a

much higher CGR than other orientations. Inhomogeneous microstructure due to banding,

fractured second phase particles and grain boundary voids also seem to enhance crack growth.

The presence of these factors does however not guarantee high CGRs, and the lack of them

does not guarantee low CGRs.

Introduction Alloy 690 is used in various components of the primary cooling system of pressurized water

reactors (PWRs), e.g. control rod drive mechanism (CRDM) nozzles, pressurizer nozzles and

steam generator (SG) tubing. This alloy was introduced as a replacement material for Alloy

600, a similar nickel-base alloy with roughly half the chromium content that has proven to be

prone to failure by stress corrosion cracking (SCC) in primary water. The first case of such a

failure occurred in a SG tube in 1971 after only two years in service [1]. Components made of

Alloy 690 have been in service since 1989 and cracking has not been reported to date.

It is clear from operational experience that Alloy 690, and its weld consumables Alloys 52

and 152, vastly outperform their predecessors in terms of SCC resistance. Laboratory tests

have shown considerable improvement in crack initiation times [1-6] and also in crack growth

rates (CGRs) [7-14]. The Bettis laboratory has however showed that heavily cold worked

* E-mail address: rshen@kth.se

materials can exhibit crack growth rates (CGRs) that may rival those of Alloy 600 [15]. The

high rates observed by the Bettis laboratory have since been confirmed by other labs as well

[16-19].

Although Alloy 690 is not generally used in a heavily cold worked state, comparable plastic

strains may be present as a result of manufacturing, e.g. from welding, fitting or grinding. The

recent testing on cold worked materials has especially had Alloy 690 heat affected zone

(HAZ) in mind. Although there has been some CGR testing of Alloy 690 HAZ [17, 20], most

tests so far have been on cold rolled or cold forged materials. The relevancy of estimating

CGRs for Alloy 690 HAZ from data on cold worked base metal has been questioned though;

especially since weld induced strain occur at a significantly higher temperature interval than

typical cold work methods.

One difficulty to overcome in understanding the role of cold work is the shortage of

understanding in the mechanism(s) governing SCC. A variety of mechanisms have been

proposed, and many of them have been summarized [21-23]. However, due to the complexity

of SCC, the true mechanism has not yet been determined. The slip dissolution mechanism

[24-25] is currently the most accepted one. It is based on a stepwise passive film degeneration

followed by crack growth by anodic dissolution of the crack tip material, eventually

repassivating the crack front with new oxides. Other proposed mechanisms exist, e.g. ideas

based on surface diffusion [26], high pressure gas bubble linkage [27, 28], creep damage [29]

and vacancy transport [30].

While the true mechanism of SCC is uncertain, there are trends in the experimental data. This

paper summarizes the recent work on the effect of cold work on SCC of Alloy 690 and

describes possible ways that cold work may cause susceptibility.

Metallurgy and Microstructure

Alloy 690 is a nickel-based alloy with a face centered cubic (FCC) matrix. Commercial Alloy

690 is typically heat treated with a mill anneal (MA) at high temperature followed by a

thermal treatment (TT), resulting in a microstructure with plenty of intergranular and few

intragranular carbides. This microstructure is very similar to that of the predecessor Alloy

600, apart from the almost exclusive precipitation of chromium rich M23C6 type carbides,

while Alloy 600 typically forms mainly M7C3 carbides and only some of M23C6 type [5]. This

difference is an effect of the near doubled chromium content in Alloy 690 compared to its

predecessor Alloy 600, which alters the stability for the two carbide types. Special Metals'

compositions specified for Alloy 690 and the compositionally similar nickel-based Alloys 600

and X-750 are summarized in Table 1.

Table 1 - Special Metal’s composition specifications for the Alloys 690 and the related 600 and X-750 expressed in

percentage by weight [31-33].

Alloy Ni Cr Fe C Mn Si Ti Al Nb+Ta S Cu

690 Bal . 28.0-31.0 7.00-11.00 <0.04 <0.50 <0.50 - - - <0.015 <0.50

600 Bal . 14.0-17.0 6.00-10.00 <0.50 <1.00 <0.50 - - - <0.015 <0.50

X-750 Bal . 14.0-17.0 5.0-9.0 <0.50 <1.00 <0.50 2.25-2.75 0.40-1.00 0.70-1.20 <0.01 <0.50

Apart from the chromium based carbides, TiN inclusions are also present. These inclusions

are formed during casting and are often observed in linear streaks along the longitudinal

direction, as shown in Figure 1. The extent of such banding varies between manufacturers,

heats and forms. The inclusions themselves are typically present in a variety of sizes, ranging

from submicron up to around 10 µm. The larger of these are often found to contain an oxide

slag core. These TiN inclusions may act as nucleation sites for chromium carbides during TT,

and have therefore often been reported as carbonitrides or carbides [34]. Generally, such TiN

banding cannot be completely removed by heat treatments due to the high stability of the TiN

inclusions, although high degrees of hot working may be helpful to some extent by

mechanical blending. The TiN inclusion bands may have a local effect on grain size, although

overall grain size is mainly determined by carbon content and MA temperature [35]. The

effect is probably caused by grain boundary pinning during recrystallization, and can give rise

to streaks of smaller grains within the TiN bands.

Figure 1 - Optical dark field micrograph of a heavily banded Alloy 690 plate material, where S, L and T are the thickness,

longitudinal and transverse directions relative processing respectively. 5% nital etchant was used electrolytically at 4V.

Smaller grains are often observed within the orange TiN bands.

Effect of Strength

Increased strength is the most notorious effect of cold work on the material. It is well known

that high strength materials are typically more susceptible to SCC than comparable softer

materials. By this argument, yield strength, or indentation hardness, has often been reported

with SCC test results. Increased strength can of course be achieved by other hardening

mechanisms than from cold working. Strengthening by any of the mechanisms does however

change the material in one way or another from a reference state. The effects of strength itself

can therefore by difficult to isolate.

Cold work, which is a well known accelerant of CGR in Alloy 690 [15, 19], increases the

flow strength by increasing dislocation density, and thereby hampering further dislocation

movement. The increase in susceptibility can often be correlated to macroscopic strength.

Radiation hardening works in a similar manner by generating dislocation loops, and its effect

on SCC susceptibility in stainless steels seems to be the similar to that of cold work [36].

It can be argued that dislocations increase the free energy of the material, and in doing so

increase the chemical reactivity. The effect would be the most pronounced near the grain

boundaries, where both finite element simulations [37, 38] and electron backscatter diffraction

(EBSD) misorientation maps [14, 39, 40] tend to show the highest densities of geometrically

necessary dislocations. Rebak and Szklarska-Smialowska added that this increased reactivity

may promote formation of NiO, resulting in a less protective oxide film [21].

Formation of martensite however hardens stainless steels without increasing dislocation

density to the same levels as plastic straining or radiation hardening. Still SCC susceptibility

in tests seemed to correlate to material strength rather than the proportions of martensitic

hardening to hardening by cold work [41].

Alloy X-750 is essentially a precipitation hardened high strength version of Alloy 600 and

often shows lower SCC resistance than the latter [42-44]. Since Alloy X-750 only form M23C6

type carbides, like Alloy 690, in addition to its γ' and η particles, it is difficult to say whether

this is an effect of strength or if the effect is more of a chemical or microstructural nature. It

does nonetheless contribute to the trend.

High strength can also be achieved from grain size strengthening due a low temperature MA

[35]. It has been shown that the strength associated with low MA temperature correlates with

shortened SCC initiation times in Alloy 600 reverse U-bend (RUB) specimens [5, 6, 45], as

shown in Figure 2. Nonetheless, high MA temperature also allows complete dissolution of

intragranular carbides and some reprecipitation at the grain boundaries during cooling,

resulting in higher grain boundary carbide coverage, which has been shown to improve SCC

resistance in Alloy 600 [5, 6, 46]. The same trend has been shown for Alloy 690 in caustic

environment [3, 4]. Bruemmer and Henager proposed that the intergranular carbides act as

low energy dislocation sources, and that high grain boundary coverage would allow a more

homogeneous deformation. Furthermore, small grained materials have much greater grain

boundary area per unit volume, which promotes diffusion processes, e.g. hydrogen

permeability [48].

Figure 2 – SCC initiation times for Alloy 600 MA RUB specimen of various yield strengths prior bending, reproduced with

data from Studsvik [5, 6] and Westinghouse [45].

250 300 350 400 450 0

4000

8000

12000

16000

Yield Strength [MPa]

Initia

tion T

ime [

h]

Norring 2008

Stiller1996

Jacko 2005

Although the effect of high strength can be difficult to isolate, it is plausible that it, by itself,

increases SCC susceptibility when considering the trends from the variety of hardening

mechanisms. An increased strength would reduce the plastic zone radius ahead of a crack and

allow higher stresses near the crack tip. Higher effective stress promotes mechanisms based

on plasticity, while higher hydrostatic stress promotes mechanisms based on concentration of

vacancies and formation of voids and gas bubbles. Either way, higher strength allows higher

stresses in the crack tip region, which is uniquely undesirable. Quantifying the susceptibility

contributions by different hardening mechanisms however remains a challenge.

Effect of Cold Work Direction and Banding There is a considerable spread in CGR data, and trends can be hard to identify. By comparing

data for specific heats or tests, however, it seems clear that the highest CGRs appear when

aligning the crack plane with the plane of deformation, and the growth direction with

direction of rolling, i.e. the S-L orientation [15, 17, 19]. Orienting the crack in the same plane,

but in the transverse growth direction, i.e. the S-T orientation often results in CGRs of order

of magnitude lower. Orienting specimens in the T-L direction, with the crack plane normal to

the transverse direction, lowers the crack growth rate even further [19]. Despite the S-L

orientation being the most interesting, many experiments have been performed in the T-L

orientation because of the limited thickness of the materials. The same orientation CGR

dependence has been demonstrated on Alloy 600 [7].

The significant influence of cold work direction on CGRs suggests that strain hardening and

increased free energy cannot fully explain the increased susceptibility. The role of

microstructural inhomogeneities caused by banding may be a partial cause of the directional

dependence. Cold worked materials with microstructural banding are associated with elevated

CGRs, especially when aligning the cold work and crack growth directions with the bands

[15, 16, 49]. High CGRs have nonetheless also been observed in heats without obvious

banding.

An interesting case is the Argonne National Laboratory (ANL) plate with heat number

NX3297HK12, which has been tested by several labs in a round-robin. While its

microstructure was reported as banded by the General Electric (GE) labs [16], Serco has

tested a specimen of the same heat with no visible long range banding [49], although with a

duplex grain size. The micrographs of both authors are shown in Figure 3. Despite the

microstructural difference, both labs reported high CGRs from this heat in multiple tests.

Tests at Serco on another heat also revealed that heavy banding does not necessarily

guarantee high CGRs [49].

It can be speculated that the inhomogeneous microstructure results in inhomogeneous

straining and thus locally enhanced susceptibility. It can also be speculated that since the grain

boundaries neighboring a banded region tends to be aligned along the bands in an almost

straight line, banding results in locally reduced tortuosity together with better alignment for

mode I cracking. The CGR data on effects of banding and cold work, and especially their

combined effect, is still limited for Alloy 690. More work is needed before their effects can

truly be quantified.

Figure 3 – The ANL heat NX3297HK12 has shown high CGRs both when the microstructure has been reported as (a)

banded [16], and as (b) not banded, but still with a duplex grain size distribution [49].

Effect of Grain Boundary Damage

Inducing heavy cold work typically yields grain boundary damage in the form of fractured

intergranular carbides and/or TiN inclusions and formation of microvoids [13, 14, 17, 40]. It

is speculated that grain boundary damage may be a reason for the high observed CGRs in

heavily cold worked materials, e.g. by allowing the SCC crack to grow by bridging the

microcracks or by reducing the internal friction against grain boundary sliding.

It has been shown in recent experiments that the CGR appears to correlate with the amount of

grain boundary damage. Alloy 690 is commonly delivered after a high temperature MA and

TT. The material is then typically cold rolled or cold forged before used for manufacturing of

cold worked specimens. Several authors have tried solution annealing and quenching the

material before cold rolling to minimize the grain boundary carbide coverage. This procedure

was found to drastically reduce the amount of grain boundary damage caused by cold work.

In addition this procedure also lowered the CGRs by roughly an order of magnitude [13, 18,

19].

Morra et al. reported that welding dissolved the M23C6 carbides near the weld interface. They

also reported that there were some discrete metastable M3C type chromium carbides on the

grain boundaries, although the grain boundary coverage was low [50]. These results supports

the relevancy of testing in a solution annealed state for simulation of Alloy 690 HAZ, and

may be connected to why high CGRs for Alloy 690 HAZ have not been observed yet.

There are however some controversies. Cracked particles are not always found on the crack

surfaces of specimens with high CGRs, and a microscopy study by Bruemmer et al. of several

crack fronts with heavy grain boundary damage did not reveal cracks growing between the

preexisting microvoids or cracked particles [40]. Instead, some cases showed cracks following

a migrated boundary rather than the damaged ghost boundary area just a few microns away.

Furthermore, the SCC crack often appeared locally arrested at damaged carbides. Since the

carbide cracks are typically oriented perpendicular to the grain boundary, and thereby also the

SCC crack, it was believed that the damaged carbides blunted the crack tip in a similar way as

when intentionally drilling a hole along a crack front. Work by Peng et al. on the other hand

(a) (b)

shows that the crack may deviate slightly from the grain boundary to follow the matrix-

carbide interface [14].

Bruemmer et al. also showed that the CGR could be drastically reduced by a recovery heat

treatment of 1 hour at 700 °C after cold rolling, despite the permanent grain boundary

damage. The behavior after cold rolling to 31% thickness reduction and a recovery heat

treatment was very similar to that of a similar specimen cold rolled to only 17%. These two

specimens had similar hardness and misorientation maps, i.e. the main difference being the

remaining grain boundary damage, and also showed comparable CGRs. Furthermore,

intergranular carbides are known to assist local straining. It was therefore proposed that high

local strains near the grain boundaries may be the true source of increased CGRs and not the

grain boundary damage [40, 51]. More work is needed to validate or invalidate this

hypothesis.

Summary Alarming CGRs in Alloy 690 have been observed in laboratory tests in certain conditions,

where the highest rates are comparable with those found in Alloy 600. All specimen showing

such rates had high degrees of cold work induced prior testing and also had a crack plane

coinciding with the plane of deformation. Several factors are believed to contribute to

elevated CGRs in cold worked specimens:

High macroscopic material strength.

Banded microstructure.

Crack orientation relative deformation and banding.

Grain boundary damage as fractured second phase particles and cavitation.

Localized plastic strain near grain boundaries.

There is very limited data, and no standard procedure of quantifying localized grain boundary

strain yet. The rest of the properties have not guaranteed high CGRs, and the lack of them has

not guaranteed low CGRs. More work is needed in this field in order to gain proper

understanding of when high CGRs are expected and when they might be relevant in field.

The current CGR data measured directly on Alloy 690 HAZ is still rather limited. The

reported CGRs in such tests have generally been relatively low. These low rates and the

dissolved carbides near the weld interface, where the largest strains are expected, suggest that

solution annealing the sample material before cold working it may improve the applicability

of the test results for simulation of Alloy 690 HAZ.

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Appendix – Glossary

ANL Argonne National Laboratory

CGR crack growth rate

CRDM control rod drive mechanism

FCC face centered cubic

GE General Electric

HAZ heat affected zone

MA mill anneal

PWR pressurized water reactor

RUB reverse U-bend

SCC stress corrosion cracking

SG steam generator

TT thermal treatment