Investigation of stress corrosion cracking of low-alloy steel in water

En,vkwin~

Froctun

Mechunks

Vol. 16. No. I, pp. I15-127. 19X?

wI3-7944/82/010115-13$03.00/0 Pergamon Press Ltd

Printed in Great Britain

INVESTIGATION OF STRESS CORROSION CRACKING OF LOW-ALLOY STEEL IN WATER WU-YANG Department

of Metal

CHU,

Physics,

CHI-MEI

Beijing

HSIAO

University

and SHI-QUN

Ll

of Iron and Steel Technology.

China

Abstract-For four low-alloy steels with a wide range of tensile strength, the dynamical processes of the nucleation and propagation of stress corrosion cracking (XC) in water with various polarization conditions and in a 0.1 N K:Cr:OT solution were traced with an optical microscope. The results show that if the tensile strength of the steel is higher than a critical value that is different in different polarization conditions and K, Kr\~r, the plastic zone in front of a loaded crack tip is enlarged with time, i.e. the delayed plastic deformation occurs in all the environments used. The nucleation and propagation of SCR will proceed when this delayed plastic deformation develops to a critical condition. Neither anodic and cathodic polarization nor the inhibitor can change the feature of the delayed plasticity and the nucleation and propagation of SCC in water. In all the environments used, K IXC is increased and da/df is decreased with the decrease in the strength of the steel. Krscc is increased and da/df is decreased vvith the anodic polarization and the addition of the inhibitor, but the cathodic polarization has the opposite effect.

INTRODUCTION of stress corrosion cracking (SCC) of low-alloy stell in water has been thoroughly investigated. But there are still some contentions [ l-31. Using the polished modified WOL type constant defection specimen, we have successfully traced the dynamical process of the nucleation and propagation of SCC of ultra-high strength steel in water[4]. In the present work, the nucleation and propagation of SCC in water were investigated with this metallographic shadowing technique for four low-alloy steels with a wide range of tensile strength. The influence of an odic polarization, cathodic polarization and the inhibitor on the process of the nucleation and propagation of SCC has been investigated. The KIscc and da/dt of all the four steels in various environments were measured. The influence of various heat treatments on the K rscc and daldt has been studied. Based upon these results, a new mechanism of SCC in aqueous solution has been proposed. THE

MECHANISM

EXPERIMENTALPROCEDURES The chemical compositions and strength of the steels are listed in Table 1. All the specimens were of the modified WOL type whose thickness is 20 mm. Owing to the decrease in K, with the increase in the crack length of the constant deflection specimen, the K,scc and daldt can be measured with one specimen[5]. In order to trace the dynamical processes of the nucleation and propagation of SCC with an optical microscope, the polished precrack specimens were loaded and then put into various aqueous solutions with their polished surfaces remaining in air. Because of the capillary effect, the crack tip of a polished surface was still filled with water, and the nucleation and propagation of SCC could be followed with time. Copper was cathode in the condition of anodic polarization and magnesium was anode in the condition of cathodic polarization. The polarization voltage was 1.0 V. The inhibitor used was 0.1 N KzCrz07 aqueous solution. The fracture surfaces of SCC in various environments were investigated with SEM. Table

I. Composition

of the steels (Wt. %)

Steel

C

Si

Mn

S

P

Cr

Ni

MO

30CrMnSiNi: 30CrMnSi 40CrNiMo ZG-18 Cast

0.29 0.30 0.38 0.24

I.1 I 1.18 0.22 0.65

I.15 0.95 0.64 0.87

0.005 0.006 0.007 0.005

0.028 0.019 0.012 0.018

I .07 0.91 0.69 1.27

1.57 1.59 -

1.19 0.54

EFM Vol 16. No I-H

II5

WU-YANG

116

CHU

et al.

EXPERIMENTAL RESULTS The KIscc and da/dt in various heat treatments are listed in Table 2 for four low-alloy steels. The variation of the Klsci with the strength and microstructures of the steel 3OCrMnSiNi~is shown in Fig. I from which it can be seen that the K Isc‘(.of tempering bainite is higher than those of temperated martensite and bainite that was not tempered for the same strength level. Table 2 shows that the KIscc of sample No. 5 which has a larger grain size (ASTM No.5)is higher than that of sample No. 1 whose grain size is ASTM No. 9. The variations of the average Klsc.t, and dajdt of all the four steels in various environments Table

NO

Steel

3OCrMnSiN~~

4 ! 6 I 8 9

10 II I?

30CrMnSi

40CrNiMo ZG-18

Cast

13 14 I5 I6 1: 1% 19 20 21 22

2. The KI\(

(

and da/dt

Heat Treatment

OQ., 200°C OQ., 270°C OQ., 350°C OQ., 500°C OQ.. 200°C 28S”C ISO. 360°C ISO. 310°C ISO. 310°C ISO.. ISO., ISO.. ISO., ISO.. ISO.. ISO.. 00.. OQ., OQ.. OQ.. OQ., OQ., 200°C

Temp. Temp. Temp. Temp. Temp.

400°C Temp. 450°C Temp. SOo”C Temp. 550°C Temp. 600°C Temp. 650°C Temp. 700°C Temp. 400°C Temp. 49s”C Temp. 200°C Temp. 450°C Temp. 500°C Temp. 550°C Temp. ISO.

of four low-alloy

T.S. IMPa)

Kiycc IMPam’? 0.C A.P.

1760 1750 1670 l?&l 1740 1610 1350 l%O

16.0 18.7 37.5 20.0 17.8 23. I

I560 1440 1300 1230 10% 980 900 1420 i 270 IRIO 1400 1320 1180 1580

23.4 31.6 55.2 80.6 no** no nc, 38. I 89.2 is.1 45.5 61.0 no 22.4

I LX

1X.6

steels

C.P.

80

‘-

30.4 --

32. I XL:!

IS.5 14.7

I.0 7.’ 7.‘) 6.0 1.X

-

0.7

-

M

66.7 no no 44.3 11.0 22.0 36.6 54.0 44.3

no no 17.1 69.0 no -

? \ i,

of the &SK

-

0.4 0.1 no no no 0.9 0.1 1 I.6 I.5 0.5 no

no no no

4.3 _0.9 no

O.C. = open circuit;

1 Quenched and temper ’ Isothermal 310°C IS0 and temp

l

2000

1500 TS, of different

structures

MPo Nith the strength

C.l’

-

---~

IO00

Fig. I. Variation

x.0 21.0 ‘60.0

--

OQ. = 900°C oil quenched; OQ?’ = 1200°C oil quenched; no ** = SCR did not occur: polarization: C.P. = cathodic polarization; da/dt is an average value. IO@ --

diz/dr
of 3OCrMnSiNil

steel.

n,ns no no _I.7 35.6 2.5 1.4 0.3

A.P. = anodic

Stress corrosion

cracking

of low-alloy

117

steel in water

with the tensile strength of the steels are shown in Figs. 2 and 3 respectively. As expected, the K,scc is decreased and the daldt is increased with the increase in the strength of the steels for various environments. Cathodic polarization decreases the Krscc and increases the da/dt, anodic polarization increases the K rscc and decreases the da/dt for the same tensile strength of steels. The inhibitor of 0.1 N KzCrz07 aqueous solution increased the KIscc for steel 40CrNiMo(No. 18) from 15.1 to 27.2 MPadm and decreased the da/dt from 11.6 x IO-’ to 1.7 X IO-’ mm/min. When the tensile strength of the steels was lower than a critical value (e.g. 1090 MPa for 30CrMnSiNi? and 1180 MPa for 40CrNiMo), XC in water would not occur. The cathodic polarization could decrease the critical value, e.g. from 1090 to 980 MPa for 30CrMnSiNL (2) The nucleation and propagation of SCC in water The process of the nucleation and propagation of SCC in water with anodic polarization is shown in Fig. 4 for ultra-high strength steel (No. 8). There was a small plastic zone in front of the loaded crack tip (Fig. 4.1) that did not change if it was put in air after being loaded. But when it was put into water with its surface remaining in air, the size of the plastic zone was continuously enlarged with the time, i.e. delayed plasticity was generated (Fig. 4.2). After the closure of the delayed plastic zone, a discontinuous stress corrosion cracking was nucleated at its tip B (Fig. 4.3). After 34 hr, there was a second delayed plastic zone in front of crack B and the plastic deformation of the plastic zone continuously enlarged. In the dark field metallograph, the slip lines inside the plastic zone appear as light ripple lines, but the cracks appear as a brown black narrow line. These processes were repeated (Figs. 454.7). These discontinuous cracks grew and joined each other during the progress of the delayed plasticity (Fig. 4.7). The process of the delayed plasticity and SCC in open circuit and cathodic polarization are shown in Figs. 5 and 6 respectively for the same sample (No. 8). When the strength of steels and K, were greater than a critical value, the same sequences of the delayed plasticity and cracking were also observed for other ultra-high strength steels, such as 30CrMnSiNi? (Nos. 1, 2, 3, 5 and 6), 40CrNiMo (No. 18) and ZG-18 cast steel (No. 22).

1000

1 Anodlc polartzatlon * Cathodic polamatlon 0 K, Cr, 0, solution ?

’ Open clrcult ’ Anodlc polarization lCathod~c polarlzatlon 3 K,Cr,O, solution

_.-___ loo0

1200

_

I400 TS,

I600

I

l8cYJ

IL

1000

MPo

Fig. 2. Fig. 2. Variation

of the KISC

, I200

1400

T %

MPa

I600

leoo

Fig. 3.

c of

four low-alloy

steels in various

environments

with the tensile

strength

of

the steels. Fig. 3. Variation

of the daldt

(average

value) of four low-alloy steels in varions tensile strength of the steels.

environments

with

the

118

WU-YANG

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et ai.

For lower strength steels, the progress of the delayed plasticity and SCC were somewhat different as shown in Fig. 7 for 40CrNiMo (No. 19) in open circuit condition. There was an initial plastic zone in front of the crack tip after loading (Fig. 7.1). The size of the plastic zone was continuously enlarged with time (Fig. 7.2), i.e. delayed plasticity was generated. When this delayed plasticity developed into a critical condition, SCC occurred and propagated along the path near the border of the delayed pIasti~~elastic region (Fig. 7.3). These processes were repeated and the crack tips formed at different times were marked with ,A, B, C and D respectively in Fig. 7. The process of the delayed plasticity and SCC in anodic and cathodic polarization are shown in Figs. 8 and 9 respectively for the same sample (No. 19). The same sequences of the delayed plasticity and SCC were also obseri
The fractographies of SCC of the ultra-high strength steel (No. 8) in water with different polarization conditions are shown in Fig. 14. All are typical intergranular fractures. As the strength of the steel was decreased, the mode of fracture was changed from intergranular to quasi-cleavage (Fig. 14). DISCUSSION The experiments show that if the tensile strength of the steel is higher than a critical value :. the delayed plasticity will which is different in different polarziation conditions and K, > K ,\‘c, occur in all the environments used. It was discovered that SCC was preceded by delayed

Stress corrosioncracking of low-alloy steel in water

Fig. 4. Nucleation and propagation of SCC of ultra-high strength in water with anodic polarization (No. 8, t in hr) dark field x 25.

Fig. 5. Nucleation and propagation of SCC of ultra-high strength in water (open circuit, No. 8,), dark field x 25.

119

WU-YANG CHU et al.

6. Nucleation and propagation of SCC of ultra-high strength in water with cathodic polarization (NO. t in hr) x 25.

Fig. 7. Nucleation and propagation of XC of lower-strength steel in water (open circuit, No. 19, t in h0. dark field X25.

Stress corrosion cracking of low-alloy steel ir. water

Fig. 8. Nucleation and propagation of SCC of lower strength steel in water with anodic polarization (No. 19, 1 in hr) x 25.

Fig. 9. Nucleation

and propagation of SCC of lower strength steel in water with cathodic polarization (No. 19, I in hr) x 25.

121

122

WU-YANG CHU et al.

Fig. II Fig. 10. Nucleation and propagation of SCC of ultra-high strength in 0.1 N KZCr207solution (No. 18).dark field x 25. Fig. 11.The processes of HIDP and HIDC for ultra-high strength steel specimen after charging (No. 8) X25.

Fig. 12. The processes of HIDP and HIDC for lower strength steel specimen after charging (No. 19)x 25

Stress corrosion cracking of low-alloy steel in water

I

3

2

Fig. 13. Relation between HIDP and HIDC (No. 8, precharged specimen).

I...-~---

--

(b)

(a)

(4 Fig. 14. Fractographs of SCC of ultra-high strength steel in water (No. 8) (a) open circuit, (b) anodic polarization, (c) cathodic polarization x 1000.

Fig. 15. Fractograph of SCC of lower strength steel in water (No. 12)x 1000.

123

Stress corrosion cracking of low-alloy steel in water

125

plasticity, as shown in Figs. 5-11. In other words, this delayed plasticity is the necessary and sufficient condition for SCC. Because the feature of the delayed plasticity and the relation between this delayed plasticity and SCC in water are the same as the feature of HIDP and the relation between HIDP and HIDC (comparing Figs. 4 and 7 with Figs. 11 and 12), the delayed plasticity observed in stress corrosion in water is also caused by hydrogen. In aqueous solution, the occluded cell study of a crack tip indicated that the PH value of the solution within the crack tip remains as 3.7-3.9, even if the PH value of the bulk solution varies from 2 to 10[6]. This result provides the necessary condition for the occurrence of the hydrogen-evolution type of cathodic reaction during corrosion, even for anodic polarization[6]. Our recent results to be published showed that if a stress gradient existed, e.g. in a bending or pre-crack specimen and the tensile strength of the steel exceeded a critical value, hydrogen could apparently reduce the local yield strength (see Figs. 16 and 17) though there were arguments in whether hydrogen can reduce the yield strength of a smooth tensile specimen (3.7). We believe that the mechanism of SCC in water is as follows: the hydrogen atom released during cathodic reaction of corrosion tends to diffuse to and enrich in the front region of a crack tip under a multiaxial stress gradient; when the effective concentration of hydrogen reaches a critical value, the effective yield strength in the local region of a crack tip can be reduced considerably; consequently, hydrogen induced delayed plasticity and SCC can occur under the action of a lower KZ value. The delayed plastic deformation extended along 0 = + (~r/4) for ultra-high strength steels (Fig. 5). and then rotating the coordinated axes 0 = (n/4), the shear stress T in the slip direction in front of a crack tip can be calculated, i.e.

r = :(S, - 6,) sin 20 + T,, cos 20 = 0.328 &.

It is our view that when the shearing stresses in a great distance in front of the crack tip are all equal to the effective shear strength TT, first hydrogen induced delayed plasticity and then

r

PH 0

0

0

0

02 /,

5

20

50

80

mA/cmZ

Fig. 16. Variation of ratios of yield strength of the charged and uncharged bending specimens with charging condition.

126

W&YANG

CHU et al

Steel 30CrMnSIN1,

T.S.iMPol 1760 I 1620 ’ 1440’ 1352’ 780 “1 450 01

0

0

3

0

02

/r Fig. 17. Variation

5

I

20

50

80

m&/cm’

of T?'/T
SCC will occur, i.e. r: = 0.328 e;,

K,sc,c = DrT.

Our recent results to be published indicated that TTTis decreased with the increase in hydrogen amount entering into a specimen and/or tensile strength. These new findings can be used to explain the influence of the strength and environments on the Krsrt and dtr/di shown in Figs. 3 and 4. The primary cause of the effect of environments on K rscc can be attributed to the sufficiency of the supply of hydrogen. The cathodic polarization in water promotes the cathodic evolution of hydrogen, the Krscc is thereby decreased. The anodic polarization retards the evolution of hydrogen, therefore K ,sfC is increased. The inhibiting effect of Crz07 in 0.1 N KZCr207 solution is due to the fact that the concentration of H” is decreased by the following reaction: Cr?O, = f 14H’i6e---+7H,O+2Cr”. It is interesting to point out that when the tensile strengths are the same, the KIscc: of 40CrNiMo and that of 30CrMnSi appear to be higher than those of 30CrMnSiNi: (Table 2). but this effect cannot overshadow the effects of the strength and the amount of hydrogen. CONCLUSIONS (1) if the strength of the steel is higher than a critical value which is different for different polarization conditions and Kr > Krscc, the delayed plasticity can certainly occur in all the aqueous solutions used, and then the nucleation and propagation of SCC will follow. Thus delayed plastic deformation induced by hydrogen is the necessary and sufficient condition for SCC of low-alloy steels in aqueous solutions. (2) For ultra-high strength steels, after the closure of the delayed plastic zones, SW’s are nucleated at their tips. These discontinuous cracks will grow and join each other with the progress of the delayed plasticity.

Stress corrosion

cracking

of low-alloy

127

steel in water

(3) Neither anodic and cathodic polarization nor the inhibitor can change the feature of the delayed plasticity and the relation between this delayed plasticity and XC in water and aqueous solution. (4) The Klscc are increased and the daldt decreased with the decrease in the strength of the steel in all the environments used. The anodic polarization in water or the addition of K2Cr207 to water will increase the KIscc and decrease the da/dt. On the other hand, the cathodic polarization in water will decrease Kfscc and increase the daldt. 4ckno,~,led,~ements-Thank~

are due to Mr. Tian-hua

Liu and Mr. Jyh-yih

Li for part of the experimental

work

REFERENCES of the mechanism of stress corrosion cracking in high strength steels. III Y. A. Marichev and I. L. Rosenfely, Investigation Corrosion 32. 423-429 (1976). PI J. C. Scully, The role of hydrogen in stress corrosion cracking. On effect of hydrogen on behaviour of materials (Edited by A. W. Thompson and 1. M. Bernstein) pp. 129-149. AIME, New York (1976). cracking (Hydrogen “Embrittlement”) Met. Trans. 3, 427-451 [31 C. D. Beachem, A new mode1 for hydrogen-assisted (1972).

[41 W. Y. Chu. C. M. Hsiao and S. Q. Li. Hydrogen

induced

delayed

plasticity

and cracking.

Scripfa

Mel.

13, 1063-1069

(1979). and S. Q. Li. A new engineering fracture toughness parameter KMX (p). Scriprcl Met. 13. 1057-1062 (1979). conditions at the tip of an advancing stress corroGon 161 J. .4. Smith, M. H. Peterson and B. F. Brown, Electrochemical crack in AISI 4340 steel. Corrosion 26. 539-542 (1970). of steels. Ann. Rec. Mot. Sri. 8, 539-542 (1978). I71 R. A. Oriani. Hydrogen embrittlement

[51 W. Y. Chu. C. M. HGao

(Receioed

3 March

1981: receiced for puhlicc~tion I5 April 1981)