A fractographic assessment of sulphide inclusion distributions and their influence in promoting environmentally assisted crack growth in ferritic pressure vessel steels

Int. J. Pres. Ves. & Piping 56 (1993) 149-181

A Fractographic Assessment of Sulphide Inclusion Distributions and their Influence in Promoting Environmentally Assisted Crack Growth in Ferritic Pressure Vessel Steels J. H . B u l l o c h Electricity Supply Board, Head Office, Dublin 2, Republic of Ireland (Received 11 April 1992; accepted 23 April 1992)

ABSTRACT The present study was aimed at fractographically assessing the extent of the sulphur concentration that is needed to initiate environmentally assisted crack (EAC) growth in fatigue crack growth tests on pressure vessel steels in a high temperature water environment. A relationship has been established between the extent of sulphide segregation within a cluster and the propensity towards the initiation of E A C growth. Also the E A C growth has been shown to be a function of g m a x at which the cluster occurred on the fatigue fracture surface. In terms of sulphur concentration, EAC growth occurred at sulphur concentrations of around 7000 ppm at K m a x values of 80 MPaVmm while at lower values (viz. 40MPaVmm) concentrations approaching 35000ppm were required to initiate E A C growth. Under an ideal sulphide distribution it was suggested that no EAC growth was possible. Finally the sulphur anion concentration levels for both E A C and non-EAC growth, assessed from the present fractographic evaluation, have exhibited good commonality with those predicted from a slip dissolution model which characterised fatigue phenomena in high temperature water environments.

1 INTRODUCTION During any major steelmaking process there can exist an appreciable a m o u n t of non-metallic materials which m a y generally originate from one of two basic sources: (a) the deoxidation process, or (b) the turbulence and erosion which cause furnace lining wear. In the former 149 Int. J. Pres. Ves. & Piping 0308-0161/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

150

J. H. Bulloch

case this results in inclusions such as silicates and aluminates being common, while non-metallic inclusions containing elements such as titanium, magnesium and potassium result from the latter source from the steelmaking process itself. The detrimental effects of sulphide non-metallic inclusions on the mechanical properties of steel products are now well established; when distributed with any steel matrix, they can influence the mechanical properties generally in three basic ways: ~-3 (a) (b) (c)

they provide an easy fracture path through which a crack can extend; they provide prime initiation sites for fracture; they reduce the cross-sectional area of load supporting material.

Such effects of non-metallic inclusions can be greatly amplified when segregation effects are considered because such effects can greatly increase the local non-metallic inclusion density. Basically two types of segregation can occur in cast steel ingots, viz. macrosegregation and microsegregation. A and V segregation, see Fig. 1, are two of the various forms of macrosegregation 4 and result from the movement of solute-enriched liquid along interdendr{tic channels

SHRINKAGE

t •

• O0 0 0 0 0 O0000•

•

:

t

i

i : •

A-SEGREGATION •

"." "

" " ° ••e~ -k

I

V-SEGREGATION

•

BANDS

CONE OF NEGATIVE SEGREGATION

Fig. 1.

Schematic illustration of A and V segregation in a cast steel ingot.

Fractographic assessment of sulphide inclusion distributions

151

within the mushy zone (see Fig. 2) of the solidifying steel ingot. Microsegregation or interdendritic segregation 5,6 of solute elements can also occur during the solidification process. Generally macrosegregation can be defined as the difference in composition between two points within the cast structure separated by several dendrite arm spacings, while microsegregation can be considered as the difference in composition between the dendrite arm and the adjacent interdendritic region. All types of segregation effects can be inherited, to varying extents, by the final wrought steel product where segregation significantly influences both chemical and mechanical properties. As well as influencing the normal mechanical properties, it has been established that the presence of sulphide non-metallic inclusions can adversely affect fatigue behaviour. 7,8 Parker 9 has also reported that manganese sulphide non-metallic inclusions can have a significant influence on the stress corrosion cracking characteristics of a wide range of commercial steels. Parker's report 9 also contained the very important statement that in an aggressive environment sulphide inclusions may play only a minor role, but in dilute or benign environments they can

TL

i

TEMPERATURE

(°C)

TS

I

i

I

I

i

il'l

Iv'

j,

Illl,ll°

SOLID

Fig. 2.

MUSHY ZONE

LIQUID

Schematic illustration of details at the solidification front in a cooling ingot. TL = temperature of pure liquid, Ts = solidfication temperature.

152

J. 11. Bulloch

take a major role inasmuch as through dissolution they can cause the environmentally assisted cracking (EAC) process to occur. Following the initial work of Kondo et al. ~° and Bamford 1~ it is well established that E A C can occur in reactor pressure vessel (RPV) steel subjected to light water reactor (LWR) coolant environments. Over the past few years a n u m b e r of research workers have shown that the presence of non-metallic sulphide inclusions on the fatigue fracture surface can cause E A C to occur in Pressurised Water Reactor (PWR) environments. This strong link between sulphides and E A C has resulted from detailed fractographic investigations which have established that EAC, during fatigue, is signalled by the appearance of a fan-shaped (cleavage-like) fracture mode that generally initiated from a group or cluster segregated region of manganese sulphide inclusions. ~2-~5 The present paper represents a realistic and practical attempt at quantitatively assessing the degree of sulphide segregation required to initiate the E A C growth process in R P V steels during fatigue testing in high temperature water environments. 2 EXPERIMENTAL CONSIDERATIONS The present assessment considers the fatigue fracture surface of low and R-ratio tests, R = 0.2, 0.7 and 0.8, in both air and high temperature water at 290°C. The steels pertinent at 25 °C to the ambient air tests were as follows: Steel

A533B (BSC) Ducol (BSC) A508 cl III (FRAM)

C

Si

0.19 0.13 0.16

0.21 0.25 0.25

Mn

S

1.25 0.013 1 - 2 6 0.014 1-38 0-008

P

0.017 0-009 0-013

Ni

Cr

0-68 0.13 0-81 0-66 0 " 7 7 0-06

Mo

0.49 0-30 0-52

The fatigue crack growth data and fractographic details have been reported elsewhere. ~6-1s The five steels used in the high temperature water fatigue tests conducted, at R-ratio values of 0-2, 0.7 and 0.85 at frequencies of 0.1 and 0-017 Hz, were as follows: Steel

A533B (M/F) A533B (BSC) A508 cl III (FRAM A) A508 cl III (FRAM B) Ducol (BSC)

C

Si

0-19 0-19 0.16 0.16 0.13

0.20 0-21 0.25 0-24 0.25

Mn

S

1.40 0.006 1 - 2 5 0-013 1.32 0.008 1.32 0-007 1.20 0.014

P

Ni

Cr

Mo

0.01 0.017 0.009 0.010 0.009

0.66 0.68 0-70 0.74 0-51

0-13 0.13 0.15 0.25 0-66

0.49 0.49 0.51 0.49 0.30

The relevant crack growth data and some fractographic details are reported in the literature. 15't9'2° Microstructural details of such steels are

Fractographic assessment of sulphide inclusion distributions

3~6

20KU

~3i300

(a)

i0~

153

~D~4~

(b)

Fig. 3. Typical microstructure of low alloy ferritic pressure vessel steels. (a) General view: microstructure is a mixture of acicular carbides and carbides. (b) Detailed view: note lath-like formations of acicular ferrite.

shown in Fig. 3. During the fractographic surveys of the high temperature water fatigue fracture surfaces, it was the goal to observe and characterise any group or cluster of sulphide inclusions that did or did not initiate localised E A C growth. In the present study 22 incidences of localised E A C were found where the initiating inclusion cluster was orientated in such a way that a reasonable assessment of the size, distribution and morphology of the sulphides within the cluster (or area of segregation) was possible. Nine other inclusion clusters which did not initiate E A C growth were also able to be assessed in this manner. It is the author's experience that only rarely does the advancing fatigue fracture surface encounter single manganese sulphide inclusions since it much prefers to seek out areas of segregation, i.e. clusters of sulphides. In high temperature pressurised water, however, manganese sulphide (MnS) inclusions are dissolved and only empty voids, within which the manganese sulphide inclusions once resided, are visible on the fatigue fracture surface. Hence in the case of the 27 incidences of assessable inclusion clusters the actual assessment will be the size, distribution and morphology of the voids or empty inclusion sites. In order to get an assessment of the size and distribution of the pre-existing MnS inclusions in each cluster, the Area Occupancy Ratio ( A O R ) of MnS inclusions is required ( A O R M n s is defined as the ratio of the area of MnS inclusions, AM,S, to the area of the void, Avoid, within which it resided, i.e. AMns/AvoiO). This parameter was measured by assessing numerous MnS inclusion clusters observed on the air fatigue fracture surfaces, because (a) the mechanical conditions, i.e. Kmax range, were the same as in the high temperature water tests, and (b) the MnS

154

J. H. Bulloch

inclusions remained undissolved on the fatigue fracture surfaces and, as such, an assessment of Avoid and AMnS was readily available. Both the e m p t y void size and the distribution from the high t e m p e r a t u r e water tests and the AORMns from the air fatigue tests were m e a s u r e d by an Image Analysis technique as follows.

1 Empty void assessment (high temperature water tests) In this case the characteristics of each void cluster were carefully traced on to clear acetate paper and the void sites coloured black. This is

(a)

(b) Fig. 4. (a) Fractographic details of cluster showing empty individual inclusion sites. (b) Empty void maps of inclusion cluster. Steel A533B (BSC), R = 0.7, PWR water, cluster no. A45.

Fractographic assessment of sulphide inclusion distributions

155

shown in Fig. 4. This black-and-white void site m a p was p h o t o g r a p h e d and the resultant 35 m m negative was used in the image analysis assessment. 2 AORMns (air tests) Again both a void and an inclusion m a p are carefully traced on to acetate paper; the inclusion is then coloured black and the void grey, see Fig. 5. The black, grey and white inclusion/void map is then p h o t o g r a p h e d and the 35 m m negative utilised for image analysis assessment. Both techniques accentuated the difference between void and matrix

(a)

(b)

Fig. 5. (a) Fractographic details of sulphide inclusions and associated void formation. (b) Inclusion and void map. Steel A533B (BSC), R = 0.7, air, 25 °C.

J. H. Bulloch

156

and void and non-metallic sulphide inclusion and led to accurate image analysis assessments which could never have been achieved from the original fractographs. The image to be analysed was obtained from 3 5 m m negative observed under an optical microscope. It is projected on the sensor array of a CCD video camera. For this particular image analysis a 256 line, 256 pixel matrix was selected. The remaining part of the frame is used to display a band of 64 grey levels and two sensors indicating the selected grey slice under observation.

3 EXPERIMENTAL RESULTS 3.1 The ideal MnS inclusion distribution In order to compare the MnS distribution of the segregated (clustered) areas with those for an ideal non-segregated distribution, an idea of the areas of influence of the bulk sulphur level on the volume fraction of MnS inclusions is needed. The results of a literature survey 6,21 2 4 are shown in Fig. 6 from which it can be seen that the area fraction of MnS in a general commercial steel can best be described by the following equation: A M n s = 0"053 (wt% S) while that of the pressure vessel s t e e l s 25,26 can be described by the relationship AM.S = 0.32 (% S) Both equations agree well with considerations based on simple stoichiometry (ignoring the small sulphur solubility in the solid state), with the densities of MnS and Fe taken as 3.99 and 7-87g/cm 3 respectively. Hence for a 0.01% S pressure vessel steel the volume fraction of manganese sulphide inclusions is 0.032. For the present steels tested in high temperature water the ideal (non-segregated) volume fraction of MnS, JMnS~C(idealhave ), the following values: Steel A533B (M/F) A533B ( B S C ) A508 cl III ( F R A M A ) A508 cl III ( F R A M B) Ducol ( B S C )

%S 0.006 0.013 0.008 0.007 0'014

f ~ s ~'~ 1.92 x 10 4

4.16 x 10 2-56 x 10 2 . 2 4 X ]0 4"48 X I0

4 4 4 z

Also in the present case the ideal volume fraction of MnS, JMnS,Cidealis

157

Fractographic assessment of sulphide inclusion distributions

0.002

LEGEND COMMERCIAL STRUCTURAL STEELS, REF 6, 21-24

REACTOR PRESSURE VESSEL STEELS, REF 25 PRESSURE VESSEL STEELS, PRESENT STUDY

0.O015

REACTOR PRESSURE VESSEL STEELS, REF 26

AREA FRACTION MnS AMnS AMnS= 0.053 (%S) O.OOl

J

•

(STRUCTURAL STEELS)

J O. 005

Ddl AMnS= 0.032 (~S) (PRESSURE VESSEL STEELS)

O~

0.O1

O.O2

0.03

% BULK SULPHUR LEVEL

Fig. 6. Relationship between area fraction manganese sulphide inclusions and bulk sulphur level for two different steel types.

taken as equal to the area fraction of MnS, Amns, as long as the sectioning plane, in this case the fatigue fracture surface, intersects the inclusion clusters at random, i.e.:

f

(ideal) MnS

d(ideal) =

,t J, M n S

The ideal MnS inclusion distribution is that which envisages that the individual MnS inclusions are evenly spaced throughout the steel matrix. The magnitude of this spacing obviously depends on the MnS inclusion size and volume fraction of sulphides, i.e. the bulk sulphur level in the steel.

158

J. H. Bulloch

3.2 Cluster (segregated) distribution of MnS inclusions

The results of the image analysis assessments of the individual empty voids were recorded and it was assessed that the average Area Occupancy Ratio, A O R , was 0.31, i.e.

AMnS -0.31 Avoid Using this value the size and distribution of the pre-existing MnS inclusions in each cluster can be derived assuming MnS inclusions are spherical in nature. The mean MnS inclusion size with the clusters varied from 7 to 49/~m in diameter while the average area fraction of MnS inclusions, AMns for each segregated cluster, denoted by a(seg) • JtMn S , varied between 0-0042 and 0.106. It was also evident that in instances where the inclusion cluster initiated E A C growth the mean sulphide inclusion diameter was nearly double that for clusters where no E A C growth occurred, viz. 21.8/xm compared to 12.2/zm. Sulphide size distributions within selected inclusion clusters are shown in Figs 7 and 8. Details of the nature of selected empty void maps of individual sulphide inclusion clusters are illustrated in Fig. 9. U p o n inspection it is clear that in some clusters type I (spherical) and type II (rod-like) manganese sulphide inclusions are present. 27 Type II sulphide inclusions are formed at low solidification temperatures in the melt and as such are consequently prone to segregation effects, i.e. clustering. In instances where the sulphide clusters initiated E A C growth the average cluster area was about 30% greater than the n o n - E A C initiating clusters while the average cluster volume was about double. Two instances where E A C growth was initiated from a sulphide cluster are shown in Figs 10 and 11. Figure 10 illustrates the initiation of a small localised region of micro-EAC growth from inclusion cluster A3 in steel A508 cl III B while Fig. 11 shows a large m a c r o - E A C rosette that initiated from an inordinately large inclusion cluster (number 31 in steel A533B M/F), see Fig. 9(c). A detailed fractographic view of the nature of E A C growth in these localised regions is shown in Fig. 12; note that E A C growth is typified by fiat cleavage-like fan-shaped facets. The surrounding fatigue crack surface was ductile striated-type growth and details of this are shown in Fig. 13. A parameter known as the localised sulphide concentration factor (LSCF) essentially defines the extent of segregation within a given sulphide inclusion cluster and can

159

Fractographic assessment of sulphide inclusion distributions CLUSTER NO 17A

0.2

~MnS=16.6

um

RELATIVE FREQUENCY

O.l

-'r 10

20 MnS DIAMETER (.,um)

DMnS= 17.8 ]Jm

I

30

CLUSTER NO 19

0.2

RELATIVE FREQUENCY -"----4t

0.1 b..~p

0

10

20

30

MnS DIAMETER ()um)

Fig. 7. Sulphide inclusion size for clusters found in A508 cl. III (FRAM) steel.

be e x p r e s s e d as the ratio of the actual or segregated sulphide area fraction A~gs ) to the ideal sulphide area fraction .a(ideal).MnS , viz.

A(seg) MnS

LSCF - A- ( i -d e a l ) MnS

T h e relationship b e t w e e n the L S C F and the m a x i m u m stress intensity, Kmax,o f the location of the cluster on the fatigue fracture surface is

160

J. H. Bulloch

DMnS= 18.2~m

0.2

CLUSTER No 120

RELATIVE FREQUENCY

0.1

, 20

-I

40

, 60

MnS DIAMETER (.~m) I~MnS: 49 ~m

0.2

CLUSTER No 31

--

RELATIVE FREQUENCY

0,1

i 40

80

M 120

MnS DIAMETER (./um)

Fig. 8. Sulphide inclusion size distribution for clusters found in A533B (M/F) steel. shown in Fig. 14. From this figure it can be seen that the EAC growth data showed a significant influence of Kma × at Kma x values below 80 MPaX/-mm. W i t h Kma× values that were below about 80 MPaV~m the LSCF value that initiated EAC growth dramatically increased with decreasing stress intensity, while with Km,x values that were above 80 MPaV~m the LSCF for E A C growth was almost insensitive to Kma× value. Essentially the data in Fig. 14 can be grouped into three distinct regions, viz. the

Fractographic assessment of sulphide inclusion distributions

161

(a)

•

.

.~',j

0

~,.-..,.,:~" ~ % .... ............

•

2

(b)

(c) Fig. 9 Empty void maps of selected sulphide inclusion clusters from high R-ratio PWR fatigue test specimens: (a) cluster no. 33, A533B (BSC) steel; (b) cluster no. 23, A533B (M/F) steel; (c) cluster no. 31, A533B (M/F) steel.

162

J. H. Bulloch

(a) Fig. 10

(b)

Initiation of localised micro-EAC growth region from inclusion cluster in A508 cl III steel: (a) general view; (b) detailed view.

X 6.7 X1.5

(a)

(b)

Fig. 11. Fractographic details of EAC growth region associated with cluster no. 31, A533B (M/F) steel; (a) macro details of a rosette of EAC growth (bright semi-elliptical region); (b) detailed view of EAC growth region.

Fig. 12.

Flat, cleavage-like failure facets typical of EAC growth.

Fractographic assessment of sulphide inclusion distributions

(a)

163

(b)

Fig. 13. Fractographic details of ductile striated fatigue crack growth: (a) general view of fissured areas (see arrow) at right angles to crack growth direction typical of ductile striated crack growth; (b) detailed view of individual striations; average spacing -~0-4/xm.

n o n - E A C growth region, the E A C growth region, and a critical interracial region within which exists some critical threshold relationship between LSCF value and the onset of E A C growth. The a m o u n t of free sulphur released during complete dissolution of the sulphide cluster can be assessed through knowing the density and the fraction of sulphur present in manganese sulphide, viz. 3.99 g/ml and 0-37 respectively; a plot of this against Kma x is shown in Fig. 15. From this figure it can be seen that (a) the results which exhibited E A C growth had free sulphur levels between 10 8 and 3 x 10 7 g, (b) the n o n - E A C growth data resided at about 10 -9, and (c) a distinct gap existed between the two data sets. Instead of considering the a m o u n t of free sulphur released from a sulphide cluster, it would be better and more meaningful to consider this point in terms of the free sulphur concentration that existed within each individual cluster enclave. To assess this free sulphur concentration it is assumed that (a) all sulphide inclusions are completely dissolved within a cluster, and (b) no free sulphur enters or leaves the cluster enclave, i.e. completely static liquid conditions apply. The volume of the cluster enclave is taken as the product of the fractographically assessed cluster area (Ac~us,er) and the average empty void diameter (Dvoid), see Fig. 16, i.e. Vduster = Aduster • Ovoid

The free sulphur concentration (in ppm) is taken as the ratio of the weight of free sulphur released by dissolution of sulphide inclusions from the cluster to the weight of high temperature water that resided

Fig. 14.

--

--

--

60

40

20

80

- -

o

40

°1

D

CRITZCAL THRESHOLD REGION

%

\

D

[]

~fa []

~

80

I

NON-EAC GROWTH REGION

{MPaV-m

MAXIMUM STRESS INTENSITY, Kmax

60

t

/

F 100

[] NON-EAC GROWTH

120

I

. . . .

Relationship between localised sulphide concentration factor and K .... for E A C and n o n - E A C growth conditions.

AIDEAL MnS

ASEG MnS

LOCALISED SULPHIDE CONCENTRAqIDN FACTOR 100

120

140

160

Fractographic assessment of sulphide inclusion distributions

165

10-6

10-7 FREE SULPHUR RELEASED FROMCLUSTER DUE TO DISSOLUTION OF MnS INCLUSIONS

(g)

10-8

O

0

0

10-9

LEGEND •

EAC GROWTH

[]

NON-EACGROWTH

I

I

100

120

I0-1C I 40

I

I

60

80

Kmax(MPaj'm)

Fig. 15. Free sulphur levels for EAC and non-EAC growth as a function of K .... at which cluster occurred on the fatigue fracture surface. within the cluster enclave: Free sulphur concentration (ppm) = Weight free sulphur in cluster

×10 6

Weight water in cluster The calculated values of the free sulphur concentration (ppm) for the individual clusters are plotted against Kma×and a plot of this p a r a m e t e r against the Kma×at which the cluster was located on the fatigue fracture

166

J. H. Bulloch AREA MEASUREDFRACTOGRAPHICALLY

_

~-

-

--_ _~--~--

MEANDIAMETEROF EMPTY VOID DVOID

SULPHIDE CLUSTER ENCLAVE Fig.

16.

Schematic illustration of interaction between advancing crack tip and sulphide inclusion cluster.

surface is given in Fig. 17. It is evident from this figure that, as in Fig. 14, below a Kma× level approaching 80MPaVmm the free sulphur concentration for E A C growth is dramatically increased with decreasing Kma× value, viz., taking the lower bound line at about 80 MPaVmm the free sulphur concentration is around 7000 p p m while at a Kmax value of 40 MPaVmm the value for E A C growth is of the order of 35 000 ppm. Above 80MPaV~m there is a slow decrease in the free sulphur concentration required to initiate E A C growth with increased Kmax level. The n o n - E A C growth data resided significantly below the E A C growth results, and it is evident that (a) at a K .... value of about 40 MPaVmm a free sulphur concentration of 2000 p p m is not enough to initiate E A C growth, and (b) the gap, the or extent of the critical interface region, between the two conditions (the n o n - E A C and E A C growth) is significantly reduced as the Kma× level is increased to about 80MPaX/-mm. Indeed at this particular location the free sulphur concentration value for E A C growth was about four times that at which n o n - E A C growth conditions were observed.

4 DISCUSSION It is quite evident from the foregoing section that non-metallic manganese sulphide inclusion segregation, which takes the shape of localised clusters of sulphide inclusions, can in some instances promote

Fractographic assessment of sulphide inclusion distributions

167

40,000

LEGEND •

EAC GROWTH

Q NON-EACGROWTH

\ 30,000 FREE SULPHUR CONCENTRATION WITHIN CLUSTER ENCLAVE

(ppm)

20,000

10,000 _

EAC GROWTH REGION

CRITICAL INTERFACIAL REGION

\

~••

~o

NON-EAC GROWTH REGION

40

60

I

I

I

80

i00

120

Kmax(MPa ,/"m)

Fig. 17.

Free sulphur concentration within inclusion cluster enclave as a function of

t h e g m a x level at which the cluster occurred on the fatigue fracture surface.

or initiate E A C growth during the fatigue testing of pressure vessel steels in high temperature water environments. A schematic of the development of a localised E A C growth facet is shown in Fig. 18. To get some pictorial idea of the extent of segregation, quoted as localised sulphide concentration factors, we can consider Figs 4 and 9. In Fig. 4, if an ideal sulphide distribution were prevalent, an area of seven times that shown in this figure would contain only one sulphide inclusion

168

J. H. Bulloch

FATIGUECRACKTIP

~

f

--\

~

A

f ~l ~

~

(a)

SULPHIDE INCLUSION CLUSTER

PARTA ILLY DISSOLVED SULPHIDE

ZERO I / . . . ~ - - - I r ' _ ~ / - ~ - v I-v--) FLUX

/ / 7 , ~

"..- A - - ~ /

DUT.RO

(b)

ZEROF INWARD

O

U

--T ~ W

L

//J/

~ A4 R

D

U ZERO ~

FLUX ~ _

(c)

~

v

~

~ ~

EAC GROWTH FACET

__

/

/

OUTWARD

(d)

Fig. 18. Schematic illustration of the development of EAC growth: (a) growing crack near inclusion cluster: (b) localised shear band causing ingress of high temperature water and initial dissolution of MnS; (c) complete MnS dissolution causing localised enriched sulphur species in inclusion enclave: (d) initiation and growth of EAC facet from inclusion cluster. 1 6 . 8 ~ m in diameter. Also if an ideal sulphide distribution were prevalent in Fig. 9 the following would be applicable: (a) in Fig. 9(a) one sulphide inclusion 2 3 . 7 / z m in diameter would occur in an area three times larger than is shown, (b) in Fig. 9(b) one inclusion 2.44 p~m in diameter would be present in the total area shown, and (c) only just over two sulphide inclusions 49 ~ m in diameter would be evident in the total area shown. The A r e a O c c u p a n c y Ratio, AORM.s, of the sulphide inclusions was assessed at 0.31 which was the average value taken over the Kmax range 40 to a b o u t 100 MPaV-mm. This averaged value does not reflect the real

Fractographic assessment of sulphide inclusion distributions

169

situation because at higher Kma x values more plasticity, and hence void growth around individual sulphide inclusions, would occur, causing the value of AORMns to be somewhat lower than 0.31. This being the situation, the trend illustrated in Fig. 14 should be even more accentuated because A~,gs) values, and hence LSCF levels, should be lower at high Kmax values. Figure 14 illustrates the existence of three discrete regions, viz. (a) an E A C growth region within which the LSCF is high enough that E A C growth is triggered or initiated locally at a sulphide inclusion cluster and the lower bound line for the E A C growth region can be described by the equation LSCF = 4.75 × 106 (K) 3 + 50 (b) a region where n o n - E A C growth was prevalent at LSCF values of 15 or less, and (c) an interfacial region between these two regions, within which resides a threshold condition or critical LSCF level, which signalled the onset of E A C growth for a particular Kma x value. At Kma x values below about 80 MPaV~m, E A C growth is d e p e n d e n t on both the extent of sulphide segregation within inclusion clusters and the Kmax level; this observation identified the E A C growth condition as some unique combination of a stress and environment driven process. At Kma× levels beyond 80 MPaVmm it would appear, however, that the E A C growth process was essentially insensitive to the stress intensity level at an environment condition which corresponds to a segregation (LSCF) value approaching 60. In the case of an ideal sulphide distribution, i.e. a LSCF value of unity, over the total Kmaxrange, it is evident from Fig. 14 that no E A C growth is possible under such circumstances, as the ideal case even resided well below the n o n - E A C growth cluster data. The extent of free sulphur released from the clusters, see Fig. 15, has been shown to vary in value depending on whether E A C growth occurred or not. This is, however, surprising because this particular parameter is size dependent, i.e. the bigger the cluster the greater the a m o u n t of free sulphur that can be released during the dissolution process; it indicated that, in general, clusters which show E A C growth are larger than those which do not show E A C effects. For comparison purposes the sulphur concentrations for (a) E A C growth, (b) n o n - E A C growth and (c) the ideal, non-segregated, sulphide distribution, are portrayed in Fig. 19 as a function of the Kmax value at which the cluster was located on the fatigue fracture surface. From this figure it is clearly evident that the sulphur concentration values for the ideal sulphide distribution situation in pressure vessel steels were about an order of magnitude and two orders of magnitude

J. H. Bulloch

170

EAC GROWTH DATA

gO • • ~

~ " 104

_

. / ------<. ,-

~.t..._ : ~

FREE SULPHUR CONCENTRATION WITHIN A CLUSTER ENCLAVE (ppm)

NON-EAC GROWTH DATA

O

[]

103

IDEAL SULPHIDE DISTRIBUTION DATA []

•

am

II-

•

•

m m

•

•

mum

102

•

•

n,

I-

--

I

I

40

60

....

I

80

•

I

100

n~ I

120

Kmax ( M P ~ )

Fig.1 9 .

Free

sulphur

concentration

within cluster enclave

for various

conditions.

lower than those for n o n - E A C growth and E A C growth respectively. This again strongly suggests that no E A C growth is remotely possible under situations where the sulphide inclusions are ideally distributed throughout the steel matrix, i.e. when A Mns _- - a0de,]) z IMn S and at LSCF = 1 no E A C growth is possible.

Fractographic assessment of sulphide inclusion distributions

171

The calculated data concerning the amount of flee sulphur and free sulphur concentration at the crack tip that was released as a result of complete sulphide dissolution in high temperature water environments during fatigue testing are portrayed in Figs 15 and 17. Two basic assumptions have been employed in calculating these data: (a) all the sulphur released from a cluster as a result of dissolution was available at the point of the initiation of E A C growth; and (b) the sulphide inclusions were pure manganese sulphides. The first assumption is not strictly correct because the crack tip anion content, i.e. sulphur species, is determined by the dynamic equilibrium between such factors as Fickian diffusion convection (density variations, mechanical or cyclic pumping and fluid or flow rates) and ionic migration. A number of workers have examined the flux of anion species, both inward and outward, within a crack 28'29 and it has been indicated that (a) the pumping action of the growing crack walls can influence the dissolved oxygen concentration at the crack tip, 3° and (b) the kinetics of Fickian diffusion of anions out of a crack tip enclave can be significantly affected by the mechanical fluid flow rate past the crack mouth. 31 The second assumption, viz. that the sulphides were all pure MnS inclusions, is also not strictly the case. Manganese sulphide, MnS, can accommodate considerable amounts of other transition elements, as well as calcium and magnesium, into substitutional solid solution forming sulphide of the Mn(Me)S type where Me represents one of the first period transition metals. Indeed Kiessling and Westman 32 have shown that small amounts of Ti, Ni, V and Co can be taken by the Mn(Me)S inclusion while as much as 60-70% of Cr and Fe can exist in the MnS lattice. Such Mn(Me)S inclusions are called 'active' sulphide inclusions in initiating corrosion effects 33 due to their polarisation and consequent strong contact adsorption on metal surfaces. The author has identified a significant amount of active inclusions from the present study in steels A508 cl III F R A M (A) and A508 cl III (B) and a small amount in the A533B M/F steel. However, because such active sulphur inclusions Mn(Me)S, from the first period of transition metals, contain an average sulphur content of 38.4% which is close to the sulphur content of pure MnS of 37%, the errors involved in this particular assumption are minimal. It has been reported 34 that sulphur-rich anions are present at the crack tip as a result of sulphide inclusion dissolution and that the average concentration at the crack tip enclave is probably 0.5-15 ppm; this concentration is, however, critically dependent on the amount of sulphide inclusions that are exposed, and subsequently dissolved, to the high temperature water environments.

172

J. H. Bulloch

Ford e t al. 35 and Ford and Andresen 36 have shown that the bare surface dissolution of alloy steels at potentials relevant to crack tip conditions is significantly influenced by the dissolved sulphur activity or concentration. Explanations for such a trend have been forwarded by Combrade e t al. 37 and Andresen. 3s However, irrespective of the correct mechanism of bare surface dissolution it is evident that dissolved sulphur atoms affect the oxidation current density. Indeed it has been shown that in the presence of molybdate anions the oxidation current density is greatly increased at dissolved sulphur concentrations in the range 1600-3500 ppm. 35 Ford and Combrade 3~ have reviewed the oxidation reaction rates that occur on the bare surface of carbon and low alloy steels in high temperature water environments; by utilising such data and invoking a slip dissolution/film rupture model, they have determined the environmentally assisted crack (EAC) propagation rates for a pressure vessel steel/water system at 290 °C. They determined upper and lower bound crack growth velocity lines which were coincident with 1600ppm and <0.5 p p m of sulphur at the crack tip respectively. These two sulphur concentrations represent two limiting conditions which span the conditions for the attainment of some critical concentration of sulphur anion at the crack tip, viz. the maximum critical sulphur crack tip concentration for E A C growth (the upper bound crack growth condition) was assessed at 1600ppm. When this maximum value of sulphur concentration for E A C growth is compared with that of the present study, see Fig. 17, it can be seen that a K .... value of 70-80MPaVmm, the critical sulphur concentration values for E A C growth (between 3000 and 8000ppm) were some two to five times higher than this maximum sulphur concentration. At this point it is pertinent to consider the Ford and Andresen slip dissolution model, 4° especially with respect to resultant predicted corrosion fatigue behaviour of low alloy steels exposed to high temperature aqueous environments. In low alloy steels anions such as sulphur are created by the dissolution of MnS inclusions which have been exposed to crack tip water conditions. The rate of creation of such anions is dependent upon the crack growth rate and a schematic of the creation of sulphur anions at the crack tip is shown in Fig. 18. Utilising this, together with certain assumptions regarding the rates of sulphur anion creation and dissolution kinetics of MnS, a relationship between the 'steady state' sulphur anion concentration at the crack tip enclave and the crack growth velocity was generated for various combinations of corrosion potential and steel bulk sulphur level; 4° this is illustrated in Fig. 20. This figure shows the unique relationships that exist between

173

Fractographic assessment of sulphide inclusion distributions i0 ~3

d

OTENT'AL-'OOVs.E .

/" Y /.

10-4

/ o,,/

10-5 CRACK GROWTH VELOCITY a

/

/

/

., -,oo,v,

(mm/sec) 10-6

10-7

I 10-2

I i0 -I

I i0 0

I i01

SULPHUR CONCENTRATION (ppm)

Fig. 20. Relationship between crack growth velocity and crack tip sulphur concentration as a function of potential and steel sulphur content.

crack velocity, crack tip sulphur anion concentration, corrosion potential and bulk sulphur level of low alloy steel in pressurised water at 290 °C. For example, for a 0.02% S steel in 8 p p m oxygenated water (i.e. corrosion potential-- +200mVsHE), Fig. 20 indicates that at a crack velocity of 10 -6 m m / s a sulphur anion concentration of about 10ppm could be maintained at the crack tip. However, lowering the crack velocity to about 10 Smm/s Fig. 20 shows that the dissolved sulphur concentration drops to around 0-1 ppm. Such concentrations as 10 ppm to cause upper bound E A C growth are much lower than those demonstrated by the present study. However, E A C events in this model are very different from those which occur when E A C growth is initiated

174

J. H. BuUoch MANGANESE SULHIDE .INCLUSIONS

W

} U

w~

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Fig. 21. Manganese sulphide dissolution model. Average thickness of metal supplying sulphide to crack tip environment = 1 + 6.

at a cluster of inclusions inasmuch as (a) the fatigue crack tip enclave encounters a mass of inclusions, see Fig. 21, and (b) the localised crack velocity across an exposed inclusion cluster is m u c h faster than the E A C growth velocities invoked in the model. Essentially, as a result of (a) the localised sulphur is dramatically increased to b e t w e e n 0.5% and 2% S (for E A C growth), see Fig. 22, and as a result of (b) the crack velocity is taken as the inclusion cluster diameter divided by the time for the crack to extend across the cluster; in this case the time of the tensile portion of the fatigue cycle, i.e. 30 s in a 0.0167 Hz test, was taken. This time was justified because the fatigue crack can easily extend across an inclusion cluster, which is essentially a group of both individual and connecting 'holes', thus offering little resistance to fracture. The conditions relating this localised sulphur concentration and e n h a n c e d crack velocity are shown in Fig. 23 for a corrosion potential of - 5 0 0 mVsHz which was relevant to the actual fatigue test conditions of the present study, i.e. - 5 - 1 0 x 10 -9 dissolved oxygen. It can be seen that if the MnS inclusions had been exposed at normal E A C growth velocities of 1-7-3.4 x 10 -5 m m / s (1-2 x 10 -3 m m / c y c l e in

175

Fractographic assessment of sulphide inclusion distributions

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a 0-0167 Hz test), crack tip sulphur concentrations of only 1-10 ppm could be maintained at the crack even for localised enhanced sulphur levels. However, as a result of the faster crack velocities prevalent in the vicinity of easily decohesed sulphide inclusion clusters it is evident that, at average crack velocity values, crack tip sulphur anion concentration levels of between 1600 and 9000 ppm could be maintained at the crack tip enclave, while values as high as 33 000ppm could be realised at maximum through-cluster crack velocity levels, see Fig. 23. Such sulphur anion concentrations, which were predicted by the Ford and Andresen model, of between 1600 and 9000 ppm exhibited good agreement with the present fractographic assessment and encompassed the three regions, viz. E A C growth region, non-EAC growth region and critical interfacial region, at intermediate Kmax levels, see Fig. 17; also model prediction values as high as 30 000 ppm sulphur were fractographically recorded from three clusters in the present study. Although the sulphur anion concentration values observed in the present study are at first inspection apparently very high, Combrade e t al. 41 have shown using a few simple assumptions that large sulphur

I 120

176

J. H. Bulloch 0.01%S

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MAXIMUM a THROUGHCLUSTER

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concentrations of the order of thousands of ppm of sulphur (up to 1 0 0 0 0 p p m in a 0.01% S steel) could be prevalent at a crack tip enclave. They further suggested that the size and orientation of manganese sulphide inclusions will be of primary importance in dictating the sulphur anion concentration. This particular assessment only considered a situation where a discrete isolated sulphide distribution was encountered by the extending crack, similar to Fig. 21. Hence if severe segregation or an inclusion cluster situation had been assessed

Fractographic assessment of sulphide inclusion distributions

177

the predicted sulphur concentrations would obviously have been greater. In the instances where non-EAC growth was observed it can be seen, from Fig. 22, that the localised sulphur level in the inclusion clusters varied from 0.1 to 0.2% S. Upon inspection of Fig. 23, it is evident that, at a crack velocity value of 1.6 x 10 -2 mm/s (which was the average value through the sulphide clusters where no EAC growth occurred), a crack tip sulphur concentration of between 160 and 450 ppm could be maintained for the given conditions. Again, remarkably, such levels predicted by the Ford and Andresen model show excellent agreement with the experimental values determined in the present study. The sulphur anion concentration values predicted by the Ford and Andresen model for EAC growth and non-EAC growth were 16009000ppm and 160-450ppm respectively. The relationship between oxidation current density and dissolved sulphur, which was established by a number of researchers, 35'39 is shown in Fig. 24 together with the above-predicted sulphur concentrations for EAC and non-EAC growth. From this figure it can be shown that the predicted EAC growth values reside at high (upper shelf) bare surface dissolution rates while the

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Relationship between bare surface dissolution rates and dissolved sulphur level.

178

J. H. Bulloch

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50

40 []

•

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AKEAC FOR

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Fig. 25. Comparison of reported AKEAc data for total EAC growth and the present localised EAC growth results as a function of bulk sulphur level. n o n - E A C growth values resided in the lower to mid-region of the transitory part of the trend in Fig. 24. Finally Atkinson and Forrest 42 have reported a relationship between the threshold AK level at which the onset of E A C occurred, AKEAc, and the bulk sulphur level in pressure vessel steels, and this is portrayed in Fig. 25. Also shown in this figure are the AK values of the clusters that exhibited E A C growth in the 0 . 0 1 7 H z frequency tests for the present study. From Fig. 25 it is evident that there is good agreement between the A K E A c values which signalled full E A C growth across a

Fractographic assessment of sulphide inclusion distributions

179

fatigue test specimen (25-50 mm thick) and the localised E A C growth of the present study that emanated from certain sulphide inclusion clusters.

5 CONCLUDING REMARKS It has been clearly demonstrated that localised sulphide segregation (sulphide clusters) can in some cases initiate E A C growth during the fatigue testing of pressure vessel steels in high temperature water. The extent of sulphide segregation was expressed by the localised sulphide concentration factor, LSCF, which was the ratio of the segregated sulphide area fraction, A~e~, to the ideal sulphide area fraction, A~d~~. At K m a x values below 80 MPaX/--mmthe LSCF value that initiated E A C growth dramatically increased with decreasing K . . . . while at K m a x values above 80 MPaV--mmthe LSCF was almost insensitive to gma×level. In terms of sulphur concentration the amount required to cause E A C growth was about 7000ppm; at lower K m a x levels approaching 40 M P a V ~ , however, the free sulphur concentration to initiate E A C growth increased to about 35 000 ppm. In general terms three separate conditions were demonstrated, viz. an E A C growth region, a non-EAC growth region, and a threshold interracial region, which existed between the first two regions, where a critical condition for E A C growth existed. The likelihood of the occurrence of E A C growth for an ideal, non-segregated, sulphide distribution has been shown to be remote. Finally the sulphur anion concentration values for E A C growth and non-EAC growth in the present study have been compared to those predicted by the Ford and Andresen slip dissolution model. 4° It has been shown that good commonality existed between the experimentally assessed and the model predicted values for both the E A C growth and non-EAC growth condition from sulphide inclusion clusters.

REFERENCES 1. Baker, T. J., Gove, K. B. & Charles, J. A., Metal Tech., 9 (1976) 183-91. 2. Simpson, I. D., Dyer, L. & McDonald, J. K., B H P Tech. Bull., 20 (1976) 30-7. 3. Bernard, G., Grambach, M. & Moliexe, F., Metal Tech., 8 (1975) 512-20.

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J. H. Bulloch

4. Moore, J. J. & Shah, N. A., Int. Metals Rev., 28 (1983) 338-56. 5. Flemings, M. C., Poirier, D. R., Barone, R. V. & Brody, H. D., J. Iron & Steel Inst., 195 (1970) 482-94. 6. Turkdogan, E. T. & Grange, R. A., J. Iron & Steel Inst., 195 (1970) 371-81. 7. Herbur, M. G., Abraham, K. P. & Prasad, Y. V. R. K., Int. J. Fatigue, 1 (1980) 331-58. 8. Wilson, A. D., Trans. ASME., J. Pres. Ves. Tech., II|, IV (1977) 459. 9. Parker, J. G., In Proc. Int. Symp. Sulphide Inclusions in Steels, New York, 1974, p. 403. 10. Kondo, T., Kikuyama, H., Shino, M. & Nagasaki, R., Corrosion Fatigue (Nace-2) 195 (1972) 539-47. 11. Bamford, W. H., The Influence of Environment on Fatigue, Institution of Mechanical Engineers, London, 1977, pp. 51-6. 12. Cullen, W. H., Provenzano, V., Tiorronen, K. J., Watson, H. E. & Loss, F. J., NRL Report No. NUREG/CR-0969, 4063, Naval Research Laboratories, Washington, DC, September 1979. 13. Torronen, K. J., Hanninen, H. & Cullen, W. H., In Proc. 1AEA Specialists' Meeting on Subcritical Crack Growth, Germany, 1981, p. 331. 14. Atkinson, J. P., Bulloch, J. H. & Forrest, J. E., In Proc. IAEA Specialists' Meeting on Subcritical Crack Growth, Sendai, Japan, 1985, Vols III & IV, p. 629. 15. Bulloch, J. H., Res. Mechanica, 18 (1986) 331. 16. Bulloch, J. H. & Buchanan, L. W., Corr. Sci., 24 (1984) 661. 17. Bulloch, J. H. & Buchanan, L. W., Res. Mechanica, 19 (1986) 227. 18. Achilles, R. D., Bulloch, J. H. & Bogie, K. D., In Proc. IAEA Specialists' Meeting on Subcritical Crack Growth, Sendai, Japan, Vols I and II, p. 379. 19. Bulloch, J. H., Accepted for publication in Int. J. Pres. Ves. & Piping (in press). 20. Bulloch, J. H. & Alexander, D., Int. J. Pres. Ves. & Piping, 24 (1986) 238. 21. Garrison, W. M., Scripta Metall., 20 (1986) 633. 22. Mesmacque, G. & Foot, J., 2753 23. Garrison, W. M., Metall. Trans., 17A (1986) 699. 24. Speich, G. R. & Spitzig, W. A., Metall. Trans., 13A (1982) 2239. 25. Van Der Sluys, W. A. & Emanuelson, R. H., In Proc. 3rd 1AEA Specialists' Meeting on Subcritical Crack Growth, Moscow, May 1990, Vol. 1, pp. 291-7. 26. Amzallag, C., Bernard, J. L. & Wagner, D., ibid., Vol. 1, pp. 91-103. 27. Fredriksson, H. & Hillert, M., Scand. J. Met., 2 (1973) 125-45. 28. Turnbull, A., Brit. Corr. J., 15 (1980) 162. 29. Turnbull, A., Corr. Sci., 23 (1983) 833. 30. Turnbull, A., In Proc. ICCGR Meeting, Warrington, UK, May 1984. 31. Anderson, P., In Proc. ICCGR Meeting, Schenectady, NY, October, 1983. 32. Kiessling, R. and Westman, C., JISI, 208 (1970) 699-700. 33. Kiessling, R. & Lange, N., Non Metallic Inclusions in Steels, The Metals Society, London, 1978. 34. Ford, F. P., In Proc. ICCGR Meeting, Schenectady, NY, October 1983. 35. Ford, F. P., Taylor, D. F., Andresen, P. L. & Ballinger, R. G., Corrosion Assisted Cracking of Stainless and Low Alloy Steels in L WR

Fractographic assessment of sulphide inclusion distributions

36. 37. 38. 39. 40. 41. 42.

181

Environments, EPRI Contract RP2006-6 Report NP5064M, February 1987. Ford, F. P. & Andresen, P. L., In Corrosion 89, Houston, TX, March 1989, Paper 498. Combrade, P., Focault, M. & Slama, G., In Proc. 3rd Int. Syrup. on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors, AIME, Traverse City, MI, August-September 1987, pp. 269-76. Andresen, P. L., ibid, pp. 301-12. Ford, F. P. & Combrade, P., In Proc. 2nd I A E A Specialists' Meeting on Subcritical Crack Growth, Sendai, Japan, May 1985, Vol. 2, pp. 231-68. Ford, F. P., Andresen, P. L., In Proc. 3rd I A E A Specialists' Meeting on Subcritical Crack Growth, Moscow, May 1990, Vol. 1, pp. 105-24. Combrade, P., Focault, M. & Slama, G., In Proc. 2nd I A E A Specialists' Meeting on Subcritical Crack Growth, Sendai, Japan, May 1985, Vol. 2, pp. 201-18. Atkinson, J. D. & Forrest, J. E., ibid., Vol. 2, pp. 153-78.