CAUSTIC CRACKING SUSCEPTIBILITY OF SAE A 516 Gr. 70 STEEL IH ALKALINE SULPHIDE

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CAUSTIC CRACKING SUSCEPTIBILITY OF SAE A 516 Gr. 70 STEEL
IH ALKALINE SULPHIDE SOLUTIONS
by
@ SHANKAR RAMAKRISHNA
B. Tech., Banaras Hindu University, 1981
A THESIS IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE
in
THE FACULTY OF GRADUATE STUDIES
DEPARTMENT OF METALLURGICAL ENGINEERING
We accept this thesis as conforming
to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA
January 1984
® Shankar Ramakrishna, 1984
In presenting this thesis in partial fulfilment of the
requirements for an advanced degree at the University
of British Columbia, I agree that the Library shall make
it freely available for reference and study. I further
agree that permission for extensive copying of this thesis
for scholarly purposes may be granted by the head of my
department or by his or her representatives. It is
understood that copying or publication of this thesis
for financial gain shall not be allowed without my written
permission.
Department of /Vyl ^^M^r^cs>J C^^-^^s^^^y
The University of British Columbia
1956 Main Mall
Vancouver, Canada
V6T 1Y3
Date / ° - ( - t^&g.
ABSTRACT
The stress corrosion cracking (SCC) of A516 Gr. 70 steel was
investigated in three solution composed of 3.35 m NaOH, 2.5 m NaOH +
0.42 m Na2S, and 3.35 m NaOH + 0.02 m Na2S. The electrochemical
potential for maximum susceptibility was assessed by the slow strain
rate testing technique (SSRT), and was found to reside in the active
passive transition zone in each solution (-1 Vggg in the 3.35 m NaOH
solution, and -0.88 Vg^g in the solution containing sulphide ions).
Some secondary cracking was visible at potentials corresponding to the
passive zone in the 3.35 m NaOH + 0.42 m Na2S solution, indicating that
the material was mildly susceptible to stress corrosion cracking at
anodic protection potentials.
Since most industrial failures have occurred, in the vicinity
of welds, a series of tests with a weld incorporated in SSRT specimen
was conducted to ascertain whether or not changes in microstructure
affected stress corrosion susceptibility. The fusion zone of a single
pass weld was found to be most susceptible to cracking in alkaline
sulphide solutions, at potentials corresponding to the active-passive
transition.
The fracture mechanics technique, utilizing fatigue precracked
to study the effects of stress intensity, electrochemical potentials,
microstructure, and heat treatment, on crack propogation rates in the
3.35 m NaOH + 0.42 m Na2S solutions. Both stress intensity dependent
(regions I and III) and stress intensity independent (region II)
- iii -
cracking propogation behavior was observed. Region II crack velocities
VgcE»
of the order of 4 x 10~10 m/sec were observed at -0.88 ana" 2 x
VgQg.
10~*° m/sec at -0.75 No significant change in region II crack
velocity was observed when the base material was subjected to a
simulated stress relief anneal (650°C for 1 hr.) and tested at -0.88
VgQ.
The region II crack velocity through a material with a
E
dendritic microstructure (fusion zone of a weld) was found to be
approximately 1 x 10~*° m/sec.
A mechanism for failure due to coalescence of cracks and not
due to the penetration of a single crack through the wall, has been
suggested. The applicability of anodic protection in prolonging the
service life of digesters has been examined.
Although no experiments were conducted to determine the
mechanism of crack propogation, hydrogen embrittlement has been ruled
out as a possible mechanism contributing to failure.
The results obtained are expected to find applications in the
pulp and paper industry.
- iv -
TABLE OF CONTENTS
Page
ABSTRACT ii
TABLE OF CONTENTS iv
LIST OF TABLES vi
LIST OF FIGURES v ii
LIST OF SYMBOLS AND ABBREVIATIONS x
ACKNOWLEDGEMENT x ii
CHAPTER 1 - INTRODUCTION 1
1.1 Stress Corrosion Cracking .. 1
1.2 Origin of the Present Work 2
1.3 Theories of Stress Corrosion Cracking 5
1.4 Structure and Properties of Welds 10
1.5 Effect of Heat Treatment on SCC Susceptibility.... 14
1.6 SCC Testing Techniques 14
CHAPTER 2 - EXPERIMENTAL 21
2.1 Scope of the present work 21
2.2 Specimen 23
2.3 Equipment and Apparatus 26
2.4. Experimental Procedures.... 29
2.4.1 Show Strain Rate Test Experiments 29
2.4.2 Fracture Mechanics Experiments 33
2.4.3 Arodic Polorizaion curves 35
CHAPTER 3 - RESULTS 37
3.1 Anodic Polarization Tests 37
3.2 Slow Strain Rate Testing Technique 41
3.2.1. 2.5m NaOH + 0.42 m NallS. 41
3.2.2 3.35m NaOH 43
3.2.3 3.35 m NaOH + 0.42 m Na S 43
2
3.2.3.1. Base Material 43
3.2.3.2. Welded Samples 47
- v -
Page
3.3 Fracture Mechanics Results 55
3.3.1 General comments 55
3.3.2 Effect of Stress Intensity 56
3.3.3 Effect of Potential 56
3.3.4 Effect of Microstructure 59
3.3.5 Effect of Stress-Relief Anneal 60
3.4 Fractography 60
3.4.1 General Comments 60
3.4.2 Effect of Stress Intensity and Potential... 62
3.4.3 Effect of Stress Relief Anneal 74
3.4.4 Effect of Microstructure 74
CHAPTER 4 - DISCUSSION 79
4.1 Slow Strain Rate Testing 79
4.2 Cracking in Digesters 80
4.3 Analysis of Partial Surface Cracks 81
4.3.1 General Coments 81
4.3.2 Leak before Catostrophic failure 82
4.3.3 Catastrophic failure before leak 84
4.4 Effect of residual stresses 86
4.5 Anodic Protection of digesters 87
CHAPTER 5 - CONCLUSIONS 89
REFERENCES 91
- vi -
LIST OF TABLES
page
Table
1. Yield Strength and Chemical Composition
of steels 22
2. SSRT Test Results in 3.35m NaOH 44
3. SSRT Test results in semulated White Liquor 48
4. Secondary Cracking in the Base Material
in the 3.35m NaOH + .42m Na£S environment 49
5. Secondary Cracking in the Welded Specimen in the
3.35m NaOH + 0.42M Na2S environment 54
6. Critical coalsced crack data 85
- vii -
LIST OF FIGURES
Page
Figure 1 A typical continuous Kraft digester 3
Figure 2 Location of different welds in a continuous
digester 11
Figure 3 Stress-strain curves for specimen tested in
SCC susceptible and inert environments using
SSRT, where A and A are the areas
Q gcc
under the respective curves 17
Figure 4 Schematic of a typical log V - Kj curve,
showing regions I, II and III 20
Figure 5 SSRT specimen geometry 24
Figure 6 DCB specimen geometry 25
Figure 7 DCB specimen incorporating the fusion zone of
a weld in the crack plane 27
Figure 8 SSRT test cell 29
Figure 9 Calibration curve for DCB specimen 31
Figure 10 Fracture mechanics test cell 34
Figure 11 Anodic polarization test cell..... 36
Figure 12 Anodic polarization curve of A516 Gr 70 steel
in 2.5 m NaOH + 0.42 m Na S at 92°C 38
2
Figure 13 Anodic polarization curves of A516 Gr 70
steel in 3.35 m NaOH and 3.35 m NaOH +0.42 m
Na S solutions at 92°C 39
2
Figure 14 Anodic polarization curves of A516 Gr 70
steel in 3.35 m NaOH at 92°C with varying
Fe"1-1" ion concentrations 40
Figure 15 Effect of potential upon reduction in area
in 2.5 m NaOH + 0.42 m NaS superimposed upon
2
the anodic polarization curve obtained in the
same solution 42
Figure 16 Effect of potential upon reduction in area
for 3.35 m NaOH superimposed upon the anodic
polarization curve obtained in the same
solution 45
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Page
Figure 17 Secondary cracking at -0.98 Vg^g in a
3.35mNaOH environment at 92°C 46
Figure 18 Effect of potential upon reduction in area in
3.35 m NaOH + 0.42 m Na2S superimposed upon
the anodic polarization curve obtained in the
same solution 50
Figure 19 Secondary cracking at -0.88 V in a 3.35m
SCE
NaOH + 0.42 m Na S environment 51
2
Figure 20 Secondary cracking at -0.750 VgQg in a
3.35m NaOH + 0.42 m Na S environment 52
2
Figure 21 Oxide scale on a SSRT specimen at -0.88
Vg in a 3.35 m NaOH + 0.42 m Na S
CE 2
environment 53
Figure 22 Effect of stress intensity on crack growth in
a 3.35 m NaOH + 0.42 m Na2S environment at
-°'88 V S C E 57
Figure 23 Effect of potential on crack growth in a 3.35
m NaOH + 0.42 m Na2S environment 58
Figure 24 Effect of microstructure and heat-treatment
on crack propagation in a 3.35 m NaOH + 0.42m
Na2S environment 61
Figure 25 Fatigue surface, on the DCB specimen, showing
evidence of etching 63
Figure 26 Brittle overload failure zone, showing a
transgranular mode of failure 64
Figure 27 Variation of fractography with stress
intensity in a 3.35 m NaOH + 0.42 m Na S
2
environment at -0.750 VgQ 65
E
Figure 28 Stereographic photographs of crack front at
40 MPa/m, -0.75 V.„_ 67
Figure 29 Transgranular secondary cracks at 40 MPa/m,
"°-75 V S C E 68
Figure 30 Variation of fractography with stress
intensity in a 3.35 m NaOH + 0.42 m Na2S
environment at -0.88 Vg 69
CE
- ix -
Page
Figure 31 Intergranular crack path at 40 MPa/m, -0.88
Figure 32 Stereographic photographs of crack front at
40 MPa/m, -0.88 V___, 72
Figure 33 Intergranular secondary cracks at 40 MPa/a,
-°'88 V S C E 73
Figure 34 SCC crack front in the heat treated specimen
at 40 MPa/m, -0.88 V„„_, 75
' SCE
Figure 35 Intergranular secondary cracks in heattreated
specimen at 40 MPa/m, -0.88 V___ 76
SCE
Figure 36 Fractograph of DCB specimen incorporating a
weld, indicating the crack front and the
overload region 77
Figure 37 SCC crack front in the DCB specimen
incorporating a weld 78
Figure 38 Proposed mode of propogation of flaws in the
digester wall 83