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Metallurgical aspects of rock bolt stress corrosion cracking
Materials Science and Engineering A 491 (2008) 8–18
Metallurgical aspects of rock bolt stress corrosion cracking Ernesto Villalba a , Andrej Atrens a,b,∗ a
b
School of Engineering, The University of Queensland, Brisbane, St. Lucia, Qld. 4072, Australia ¨ Swiss Federal Laboratories for Materials Science and Technology, EMPA, Dept 136, Uberlandstrasse 129, CH-8600 Dubendorf, Switzerland Received 8 October 2007; accepted 30 November 2007
Abstract This paper reports on the metallurgical influence on rock bolt stress corrosion cracking for a range of: (1) existing rock bolt steels and (2) commercial steels. Rock bolt steels 1355, MAC and MA840B displayed SCC when loaded at 0.019 MPa s−1 in the sulphate pH 2.1 solutions at the free corrosion potential. They had comparable threshold stresses and comparable stress corrosion crack velocities. Rock bolts steel 5152CW10D had the best SCC resistance of the rock bolt steels tested. Cold work increased the resistance of 5152 to SCC. The five commercial steels 1008, X65, X70, 4140 and 4145H were subjected to the linearly increasing stress test (LIST) in the dilute pH 2.1 sulphate solution at their free corrosion potential and at increasingly negative applied potential values to −1500 mV. The increasingly negative applied potential increases the aggressivity of SCC conditions because of increasing hydrogen liberated at the specimen surface. The steels 1008, X65, X70 and 4145H resisted SCC for all applied potentials including −1500 mV. © 2007 Elsevier B.V. All rights reserved. Keywords: Stress corrosion cracking; Rock bolt; Metallurgy
1. Introduction Rock bolts are a widely used method of strata control in mines worldwide [1–6]. The principle of rock bolting is illustrated in Fig. 1. Rock bolts provide reinforcement above mine openings such as roadways. The rock bolts are bonded into the rock strata keeping it together just like the steel in reinforced concrete. The rock bolts can be considered to provide a clamping action on the rock. For the strata to fail and collapse, the clamping action of the rock bolts has to be overcome. This requires considerably greater forces. The rock bolts thus maintain the load bearing capacity of the rock strata. The chemical bolt [1–4] provides a quick, easy and effective installation procedure, Fig. 2. An appropriate hole is drilled (to the appropriate diameter and depth), the chemical cartridge(s) and bolt is introduced into the hole, the bolt is rotated, and simultaneously pushed through the chemical cartridge(s) to mix together the resin and catalyst. After completion of the hold time (to allow curing of the chemical anchor) the bolt is tightened to the appropriate load. The load is distributed to the
∗
Corresponding author at: School of Engineering, The University of Queensland, Brisbane, St. Lucia, Qld. 4072, Australia. Fax: +61 733653888. E-mail address: [email protected] (A. Atrens). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.086
surrounding rock by means of the base plate. The chemical anchor fixes the bolt into the rock strata. Depending on the amount of the chemicals used, the anchor can be at the end of the bolt or it can extend along the length of the bolt. The amount of load on the bolt can be measured using strain gauged test bolts. Such experience allows estimation of the actual service load from the amount of plastic deformation of the support plate. Stress corrosion cracking (SCC) describes the failure mode where an environment induced crack slowly propagates through the material [7]. SCC requires a susceptible material, a suitable environment (an electrolyte solution) and sufficient stress. Crosky et al. [8–10] indicate that rock bolt SCC has been a problem in a significant number of Australian mines since about 1994. SCC is manifest as first a period of slow crack growth over a period of time until the crack reaches the critical size (typically 2–3 mm in depth). Subsequently there is a sudden fast fracture across the remaining section of the bolt. Most SCC is reported to occur in the un-encapsulated part of the rock bolt. However, some SCC cases have been reported in the resin-covered area. The fracture surfaces of rock bolts, made from 1335 steel, failed during service by stress corrosion cracking (SCC) have been described previously [3,5,11–13]. The microstructure of these rock bolts was pearlitic. Laboratory research [3–6,12,14]
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
Fig. 1. Rock bolts provide reinforcement above mine openings such as roadways.
on rock bolts with the same fine pearlitic microstructure has shown that service failures can be duplicated in the laboratory using the linearly increasing stress test (LIST) [15–21]. LIST testing of a tensile sample exposed to a sulphate pH 2.1 solutions at a rate of 0.019 MPa s−1 was shown to provide a good foundation for a test to reproduce service SCC. The resulting fractography of LIST samples were similar to those from service SCC. Rock bolts failed by SCC were subjected to a detailed examination of the fracture surfaces to understand the SCC fracture mechanism. The SCC fracture surfaces contained the following different surfaces: tearing topography surface (TTS), corrugated irregular surface (CIS) and micro-void coalescence (MVC). TTS is characterised by a ridge pattern independent of the pearlite microstructure, but having spacing only slightly coarser than the pearlite spacing. CIS is characterised as porous irregular corrugated surfaces joined by rough slopes. MVC found in the studied rock bolts are different to that found in samples failed in a pure ductile manner. The MVC observed in rock bolts is more flat and regular than the pure MVC, being attributed to hydrogen embrittling the ductile material near the crack tip.
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The interface between the different fracture surfaces revealed no evidence of a third mechanism involved in the transition between fracture mechanisms. The following SCC mechanism is consistent with the fracture surfaces [3–5]. Hydrogen diffuses into the material, reaching a critical concentration level. The thus embrittled material allows a crack to propagate through the brittle region. The crack is arrested once it propagates outside the brittle region. Once the new crack is formed, corrosion reactions start producing hydrogen that diffuses into the material once again. The present paper discusses the metallurgical influence on rock bolt stress corrosion cracking for a range of: (1) existing rock bolt steels and (2) commercial steels. 2. Experimental procedure 2.1. List SCC test The laboratory SCC test method, the linearly increasing stress test (LIST) [15–21], consists of applying a linearly increasing engineering stress to a specimen exposed to the environment of interest, either the standard sulphate pH 2.1 solution or the chloride pH 1.8 solution. The load on the specimen is increased linearly by means of a lever principle and a moving load on the right hand size of the lever. The lever is maintained horizontal via a linear actuator and servo-controller by means of a displacement signal from the end of the lever arm. One side of the lever beam is connected to the specimen while the other size has a mass of 14 kg, which moves away from the fulcrum. This movement increases the load on the specimen at 0.019 MPa s−1 for the standard test. The applied engineering stress is calculated from the position of the mass at any time and the original cross-section of the specimen. After LIST, the fracture surfaces were examined by scanning electron microscopy (SEM). 2.2. Solutions The standard sulphate pH 2.1 solution was made using reagent grade chemicals and distilled water to simulate what might be found in underground water samples at mines as follows: 1.6543 g H2 SO4 , 0.3285 g NaCl and 0.5959 g Na2 CO3 was dissolved to make 1000 ml solution. Experiments were carried out at the free corrosion potential or under potentiostatic control. The chloride pH 1.8 solution contained 1400 ppm chloride, 300 ppm sulphate and 100 ppm carbonate. This solution was made up as follows: 1.6543 g H2 SO4 , 4.6056 g NaCl and 0.5959 g Na2 CO3 was dissolved in distilled water to make up 1000 ml of solution. The pH of this solution was measured to be 1.8. 2.3. Existing rock bolt steels
Fig. 2. Principle of rock bolt installation.
The chemical composition, mechanical properties, and metallurgies of these steels are presented in Tables 1 and 2. Steel 5152 was cold worked 10% to produce 5152CW10D and 55% to produce 5151CW55.
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Table 1 Chemical composition of the commercial rock bolt steels (wt%) Grade 1355 MA840B MA810 MAC 10M30 5152
A B C AH D
C
Si
Mn
P
S
Ni
Cr
Mo
Al
V
0.54 0.37 0.36 0.25 0.29 0.54
0.26 1.05 1.00 0.36 0.24 0.10
1.63 1.46 1.40 1.32 0.72 0.90
0.017 0.013 0.021 0.016 0.015 0.015
0.027 0.009 0.013 0.027 0.026 0.025
0.09 0.01 0.01 0.07 0.14 0.35
0.08 0.02 0.02 0.05 0.11 0.90
0.03 0.01 0.01 0.01 0.03 0.10
0.004 0.004 0.005 0.005 – –
0.003 0.043 0.040 0.210 0.053 –
Table 2 Metallurgies and mechanical properties of the commercial rock bolt steels Grade 1355 MA840B MA810 MAC 10M30 5152CW10D
C–Mn Micro-alloyed Micro-alloyed Micro-alloyed Plain carbon Cr + Ni + 10% CW
D (m)
YS (MPa)
UTS (MPa)
Elongation (%)
Area reduction (%)
YS/UTS
CVN
75 65 43 – – –
622 635 689 – 400 745
954 873 838 – 670 890
18.0 22.2 21.4 – 22.0 12.0
37.9 50.3 52.2 – – –
0.65 0.73 0.82 – 0.60 0.84
6 18 29 – – –
D, grain size; YS, Yield strength; UTS, ultimate tensile strength; CVN, charpy V-notch impact energy.
2.4. Constant stress test (threshold stress determinations)
and whether there was SCC. If there was no SCC, it indicated that the stress was below the threshold stress for SCC.
The threshold stress was determined using a modified LIST. The mass was stopped at a predetermined position and the sample was held at a constant stress for 3 days in the sulphate pH 2.1 solution. If the sample did not fail at the end of the 3-day period, it was removed and cooled down to −197 ◦ C and them struck with a hammer to break it in two pieces. The fracture surface was observed with SEM to determine the failure mode
2.5. Commercial steels The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition of the steels is presented in Table 3; their mechanical properties and microstructures in Table 4. In
Table 3 Chemical composition (wt%) of the commercial steels and their critical potentials in the sulphate pH 2.1 solution below which SCC occurs Ecrit
Steel
Chemical composition (wt%) C
Si
Mn
P
S
0.03 0.05 0.08 0.19 0.53
<0.02 0.015 <0.02 0.22 0.25
0.22 0.32 0.77 0.78 0.80
0.024 0.011 0.009 0.011 0.008
0.017 0.012 0.010 0.029 0.016
Carbon plus manganese steels −250 X1340F
0.40
0.24
1.55
0.023
Alloys steels −250 −250 −800 −350 <−1500 −250
1355 XK5155S 5152 4140 4145H 4145V
0.54 0.54 0.54 0.39 0.45 0.37
0.26 0.29 0.10 0.26 0.20 0.29
1.63 0.91 0.90 0.85 1.10 0.76
Micro-alloyed steels −250 10M40 −650 10M30 −250 HSAC840 <−1500 X65 <−1500 X70
0.42 0.29 0.39 0.11 0.05
0.25 0.24 1.11 0.26 0.24
0.83 0.72 1.50 1.3 1.5
Carbon steels −250 −250 <−1500 −250 −250
1003 1004 1008 1019 MRB500
Ni
Cr
Mo
Al
V/Nb
0.02 0.008 0.02 0.07 0.058
<0.01 0.02 0.01 0.06 0.093
<0.01 0.001 <0.01 0.02 0.03
0.028 0.001 0.043 <0.005 0.003
<0.01 0.001 <0.01 <0.01 0.004
0.022
0.008
0.023
0.002
0.022
0.001
0.017 0.014 0.015 0.014 0.007 0.006
0.027 0.007 0.025 0.03 0.003 0.003
0.09 0.011 0.35 0.016 0.039 0.085
0.08 0.81 0.9 1.02 1.26 1.31
0.03 0.002 0.1 0.19 0.40 0.67
0.004 0.018 – 0.002 0.023 0.032
0.003 0.005 – 0.008 0.004 0.12
0.013 0.015 0.019 0.017 0.012
0.009 0.026 0.01 0.009 0.009
0.007 0.14 0.01 <0.01 0.01
0.022 0.11 0.027 <0.01 0.01
0.001 0.03 0.002 <0.01 0.15
0.001 0.001 0.001 0.026 0.029
0.12V 0.05V 0.04V 0.05Nb 0.06Nb
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Table 4 Mechanical properties and microstructures of the commercial steels Sample
Mechanical properties
Microstructure
YS (MPa)
UTS (MPa)
εf (%)
Carbon steel 1003 1004 1008 1019 MRB500
337 287 446 504 473
398 363 533 631 840
31 37 35 27 23
Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite and chromium carbide
Carbon + Mn steels X1340F
465
801
24
Ferrite–pearlite and chromium carbide
Alloy steels XK5155S 4140 4145H 4145V
586 725 749 808
948 962 902 944
22 15 22 20
Ferrite–pearlite and chromium carbide Ferrite–pearlite and chromium carbide Tempered martensite Tempered martensite
Micro-alloyed steels 10M40 X11M47 HSAC840 X65 X70
540 595 615 493 618
782 847 889 569 683
23 24 23 31 22
Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite
order to facilitate the discussion of metallurgical influences, Table 3 includes the commercial rock bolt steels 5152 and 10M30. All the steels are relatively straightforward except for 4145H, 4145V, X65 and X70. The grades 4145H and 4145V are alloy steels containing Cr and Mo. 4145V has slightly lower C and Mn contents, a somewhat higher Mo content and contains also the micro-alloying element V. Residual elements such as S and P are at low levels, and the heats are produced using clean steel practices. The steel grades are quenched and tempered to obtain a fine tempered martensite microstructure. X65 and X70 are steels produced for pipeline service where anodic stress corrosion cracking is a considerable issue [22,23]. These steels are micro-alloyed containing a small amount of Nb and attain their mechanical properties through a complex thermo-mechanical working sequence [24]. For X70, the slab was reheated to 1250 ◦ C in order to ensure complete solution of the micro-alloying carbonitrides (Ti, Nb, V)(C, N). Rough rolling was completed above about 1030 ◦ C with the metallurgical objective of achieving the finest possible austenite (␥) grain size. TiN particle control was used to inhibit the growth of the ␥ grains. The finish rolling was commenced below the ␥ non-recrystallization temperature. The prime intention was to accumulate rolling strain within the ␥ grains so that on subsequent a transformation, there were many ␣ nucleation sites and a very fine ␣ grain size resulted. The microstructure, as documented more fully in Refs. [25–29], consisted of a large amount of ␣ (with a grain size less than 5 m), some grain boundary (GB) carbides and few pearlite grains. Some ␣ grains had a high dislocation density. The GB carbides contained Fe, Mn and sometimes Si. They contained no Ti, Nb, V or N. The typical thickness of GB carbides was about 100 nm. This microstructure was consistent with the low C content of the X70 steel.
There were some fine precipitates inside the ␣ grains which were attributed to complex mixed Ti, Nb and V carbonitrides, designated as (Ti, Nb, V)(C, N). These fine precipitates were the only microstructural features associated with the micro-alloying additions: Ti, Nb and V. The (Ti, Nb, V)(C, N) precipitates within the ␣ grains and the high dislocation density within the ␣ grains reflect the thermo mechanical treatment described above, which were associated with the hot working sequence at temperatures at which the steel was austenitic. Apart from their influence as described above, they did not seem to have had an influence in the further development of the microstructure and particularly the development of the GBs. The following mechanisms were important in the final development of the microstructure, particularly the GBs. The composition of the steel was such that all the C (except for C associated with (Ti, Nb, V)(C, N)) was in solid solution in the ␥ at high temperatures during processing, and that there was more C than could be accommodated in solid solution in the ␣ on cooling to room temperature. The steel had a low C content and contained little pearlite. This meant that during cooling from the ␥, most of the ␥ transformed to ␣. 3. Results 3.1. Environmental influences A series of experiments were undertaken using steel 1355 to study the influence of the environment. The results have been displayed on a Pourbaix diagram in Fig. 3. Sulphate solutions are represented by circles and chloride solutions are represented by squares. A full symbol indicates SCC, whereas an empty circle or square means no SCC was detected. Symbols with a dash represent experiments performed with an applied potential.
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Fig. 4. Typical SCC in service is associated with SCC initiating at a rock bolt rib, SCC growing to a critical size, followed by fast fracture through the remaining section. Fig. 3. Conditions for rock bolt SCC superimposed on the Pourbaix diagram for Fe. Full symbols indicate SCC whereas there was no SCC under the conditions represented by the open symbols.
This data indicates that SCC was controlled by a combination of the applied potential and the pH. SCC only occurred for environmental conditions, which produced hydrogen on the sample surface, leading to hydrogen embrittlement and SCC. The fracture surface of a LIST sample failed by SCC displayed the same fracture regions as fracture surfaces of rock bolts failed in service by SCC [3–5], see Fig. 4. This series of experiments led to the standard sulphate pH 2.1 solution. 3.2. Commercial rock bolt steels Table 5 presents the threshold potential for different commercial rock bolt steels tested in the standard sulphate pH 2.1 solution. Steels 1355, MA840B and MAC all showed SCC in the standard sulphate pH 2.1 at the free corrosion potential. Steels 10M30 and 5152 were somewhat more resistant to SCC, they showed SCC for applied potentials of −800 mV and −1000 mV, respectively. Steel 5152CW10D can suffer from SCC (as shown by service failures), but the condition causing SCC in the laboratory was the most severe condition of all the studied commercial rock
Table 5 Threshold potential for rock bolt steels and commercial steels tested in the standard pH 2.1 solution Grade
1355 MA840B MAC 10M30 4145V 4140 5152 5152CW10D 5152CW55 1008 4145H X65 X70
pH 2.1
C–Mn eutectoid Micro-alloyed Micro-alloyed Plain carbon Tempered martensite Ferrite + pearlite Cr + Ni + 0% CW Cr + Ni + 10% CW Cr + Ni + 55% CW Ferrite and pearlite Tempered Martensite Micro-alloyed Micro-alloyed
A B C AH
D
No SCC
SCC
−250
−350 −350 −350 −800 −350 −700 −1000 −1200
−650 −350 −800 −1000 −1200 −1500 −1500 −1500 −1500
bolt steels, Table 5. 5152 steel suffered from SCC at a condition with less hydrogen production than those required for SCC in 5152CW10D steel, indicating that cold work helps increase the resistance to SCC of steel. Moreover 5152CW55 did not show SCC at an applied potential of −1200 mV.
Table 6 Determination of the SCC thresholds in the sulphate pH 2.1 solution Metallurgy
Stress not causing SCC (MPa)
Stress causing SCC (MPa)
Threshold stress (MPa)
1355 MAC MA840B 5152CW10D
770(NP) ; 861(NP) ; 885(NP) 700(P after 1.3 h) ; 800(P* after 50 h) 700(P after 27 h) ; 800(P* after 30 h) –
922 830 850 –
900 815 850 >960
“P” indicates pitting causing ductile failure after the specified period of load application. “NP” indicates NO pitting. “P*” indicates a sample failed by pitting and stress corrosion cracks were found.
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Table 7 Determination of SCC velocity for standard test Metallurgy
Sample
LIST test duration (s)
SCC crack size (mm)
Velocity (m/s)
1355 1355 MAC MA840B
LIST 24 LIST 23 LIST 70 LIST 69
53,398 54,000 44,280 50,400
1.3 1.1 1.0 1.0
2.4 × 10−8 2.1 × 10−8 2.2 × 10−8 1.9 × 10−8
Table 8 Mechanical data for the commercial steels tested in air and in the sulphate pH ∼ 2.1 solution at the free corrosion potential Sample
UTS air (MPa)
UTS sol (MPa)
εf air (%)
εf sol (%)
SCC presence
Carbon steel 1003 1004 1008 1019 MRB500
398 363 533 631 840
395 346 532 621 748
31 37 35 27 23
– 18 – 18 –
Yes, high Yes, high No Yes, high Yes, low
Carbon + Mn steels X1340F
801
771
24
9
Yes, high
Alloy steels XK5155S 4140 4145H 4145V
948 962 902 944
923 946 893 868
22 15 22 20
14 14 14 –
Yes, medium No No Yes, low
Micro-alloyed steels 10M40 X11M47 HSAC840 X65 X70
782 847 889 569 683
772 841 864 532 596
23 24 23 31 22
14 18 9 23 –
Yes, medium Yes, medium Yes, high No No
Fig. 5. Fractography for 1008 (a), (b) air (c) the sulphate pH 2.1 solution at the free corrosion potential and (d) the sulphate pH 2.1 solution at −1500 mV.
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3.3. Threshold stress
3.4. Commercial steels
The threshold stress of various commercial rock bolt steels was determined to be 900 MPa for 1355, and 815 and 850 MPa for MAC and MA840B steels. The high value of threshold stress suggests that SCC begins in rock bolts when they are sheared by moving the rock strata. Table 6 summarises the results of the testing program to determine the SCC thresholds for the various commercial rock bolt steels. A typical crack velocity was measured to be 2.5 × 10−8 m/s, indicating that there is little benefit for rock bolt to have steels with higher fracture toughness. Table 7 showed the determination of SCC velocity for the standard test, calculated from the crack size divided by the time for crack propagation. The time for crack propagation was taken to be the time to complete the experiment and assumed crack initiation and propagation as soon as the material was under any stress.
Table 8 presents the mechanical data, UTS and εf , for specimens tested in air and the sulphate pH 2.1 solution, and the occurrence or otherwise of stress corrosion cracking (SCC) for the steels tested in the sulphate pH ∼2.1 solution at the free corrosion potential. Although all samples in the LIST-test gave lower values of UTS, some of then were fairly close to the UTS value in air, such as the low carbon steel 1008, the alloy steels 4140 and 4145H and the micro-alloyed steels X65 and X70. Five steels did not show the presence of SCC as evaluated from their fracture surface: the low carbon steel 1008 (Fig. 5), the alloy steels 4140 and 4145H (Fig. 6) and the micro-alloyed steels X65 and X70 (Fig. 7). The five commercial steels 1008, X65, X70, 4140 and 4145H were subjected to LIST in the dilute pH 2.1 sulphate
Fig. 6. Fractography for 4145H (a), (b) air (c), (d) the sulphate pH 2.1 solution at the free corrosion potential and (e) the sulphate pH 2.1 solution at −1500 mV.
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Fig. 7. Fractography for X70 (a), (b) air (c), (d) sulphate pH 2.1 solution at the free corrosion potential and (e) sulphate pH 2.1 solution at −1500 mV.
solution at their free corrosion potential and at increasingly negative applied potential values (−700 mV, −1100 mV, −1300 mV and −1500 mV). The increasingly negative applied potential increases the aggressivity of SCC conditions because of increasing hydrogen liberated at the specimen surface. The UTS for all the samples tested was little influenced by the electrochemical potential, indicating that the combination of electrolyte and cathodic potential had a low effect in the mechanical properties of 1008, X65, X70, 4140 and 4145H. SEM fractography analysis revealed that there was no SCC for the steels 1008 (Fig. 5(d)), 4145H (Fig. 6(e)), X65 and X70 (Fig. 7(e)) tested in the pH 2.1 solution, even with an applied potential of −1500 mV. Steel sample 4140 did show SCC when tested at −700 mV in the pH 2.1 solution, Fig. 8.
4. Discussion 4.1. Mechanistic aspects The sulphate pH 2.1 solution produced SCC [3] in the LIST at an applied stress rate of 0.019 MPa s−1 . The fractures were macroscopically brittle in contrast to the ductile overload fracture measured in air and also in the distilled water. Detailed SEM examination [3,5,11–14] of the LIST samples tested in the sulphate pH 2.1 solution indicated a fractography similar to that of the service failures. There was a small region of SCC followed by fast brittle fracture. The SCC details were also similar to those of the service failures. Environmental conditions causing rock bolt SCC in the laboratory were exposure at the free corrosion potentia in the acid solution and hydrogen production. SCC conditions (pH and mV vs. SHE) have been determined for 1355, Fig. 3. A hydrogen
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Fig. 8. Fractography of 4140 tested in the sulphate pH 2.1 solution at −700 mV showing (a) a flat macroscopically brittle fracture surface containing (b) a corrugated irregular surface topography and (c) a brittle fast fracture.
embrittlement mechanism for the SCC was indicated by the particular restricted range of conditions for which SCC occurred in the laboratory. In particular, SCC only occurred in the laboratory for the restricted range of environmental conditions corresponding to acid conditions at the open circuit potential (∼pH 2.1 or more acid) or at negative applied electrochemical potentials corresponding to copious hydrogen evolution at the steel surface. This was consistent with reports from the USA indicating rock bolt failure due to the presence of H2 S in the mine atmosphere and with bacterial corrosion on the rock bolt surface during service producing acid conditions leading to SCC. The threshold stress was measured for 1355 (900 MPa), MAC (830 MPa) and MA840B Steel (850 MPa) Table 6, indicating that the loading causing SCC in service is due to a combination of the tensile load plus the bending load due to rock shear. This is consistent with the observation that rock bolts are typically bent after failure in service. This bending indicates a stress above the yield stress having been applied to cause the permanent deformation in bending, i.e. to cause permanent plastic deformation. The bending of the bolt has been attributed to shear in the rock strata. This leads to the issue of whether SCC could be prevented by a rock bolting strata design that prevented shear in the rock strata and thereby maintained the stress in the rock bolt below the threshold stress for SCC initiation. The threshold stress for MA840B steel was about 850 MPa and for MAC steel 830 MPa. These values were similar to that for the 1355 samples, but nevertheless lower in value. MA840B and MAC have poorer SCC properties than 1355, as shown by
extensive surface pits and secondary cracks. Although MA840B and MAC have failed by SCC the overall macroscopic appearance [4] was very similar to that of ductile overload. Only careful observation could distinguish the failure mode. For this reason, industry might perceive these bolts to be immune from SCC, believing that these bolts fail in a ductile manner. In reality, these test showed that these metallurgies fare worse than the 1355 metallurgy. 10M30 bolts were not believed to fail due to SCC in service, but laboratory tests show that fails can fail due to SCC but appear to have failed by ductile tearing. Environmental conditions required to cause SCC in 10M30 bolts are more acidic and/or at a more negative potential than for 1355, MA840B and MAC steels, but not as severe as those needed to cause SCC in 5152CW10D. 5152CW10D bolts can suffer from SCC (as shown by service failures), but conditions causing SCC in the laboratory were more severe than the free corrosion potential in the sulphate pH 2.1. 5152CW10D bolts required the harshest environmental conditions out of all studied metallurgies for them to fail due to SCC. 5152 steels suffered from SCC at conditions with less hydrogen production than those required to show SCC in 5152CW10D steels. This confirms that cold work helped to increase the resistance to SCC. Cold work of the 5152 steel by 10% and 55% showed an increased resistance of the steel to SCC. Cold working the material by 10% increased the SCC resistance by 200 mV versus SHE in the standard sulphate pH 2.1 solution. Cold working the material to 55% increased the SCC resistance by 400 mV
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
versus SHE. Cold work improved the SCC resistance of rock bolts. Fracture toughness √ values evaluated [4] for a 1355 rock bolt was about 50 MPa m. There is certainly scope for producing steels with higher values of fracture toughness, but the benefits in terms of increased SCC lifetimes seem to be small, particularly in view of the typical stress corrosion crack velocities, Table 7. 4.2. Influence of metallurgy In order to facilitate thinking about the influence of metallurgy, Table 3 includes the critical potential in the sulphate pH 2.1 solution below with there was SCC. Some noteworthy issues from Table 3 are as follows. The plain carbon steels 1003, 1004, 1008 and 1019 are all similar commercial steels with the only significant difference being the carbon content. It was therefore surprising that 1008 was much more resistant to SCC than the other plain carbon steels. The history of the 1008 steel was that it came from warm worked plate, and it is possible that a different dislocation structure was responsible. 5152 and 10M30 showed some resistance to SCC. Does this indicate an influence of alloying with Ni? For 5152, cold work increased SCC resistance as already noted. 4145H and 4145V were both tempered martensites; the slightly higher strength of 4145V correlates with a decrease in SCC resistance. The grades 4145H and 4145V are alloy steels containing Cr and Mo. 4145V has slightly lower C and Mn contents, a somewhat higher Mo content and contains also the micro-alloying element V. Residual elements such as S and P are at low levels, and the heats are produced using clean steel practices. The steel grades are quenched and tempered to obtain a fine tempered martensite microstructure. It is somewhat surprising that the results were different for steel grades 4145H and 4145V, since these steels are quite similar. Moreover, Mougin et al. [30,31] have shown that 4145V has a higher resistance to sour H2 S containing environments when testing according to the NACE Standard TM0177 [32]. These tests indicate that both steels show resistance to this environment with 4145V having the higher resistance. The difference revealed in our present study is attributed to the experimental methodology. This present program did not carry out a direct comparison of the two steels 4145H and 4145V. Rather first a survey was carried out in which the rock bolt stress corrosion cracking tendency was evaluated for 15 steels using our prior established testing methodology for evaluating rock bolt stress corrosion cracking. The survey test involves exposing the test specimen to the standard sulphate pH 2.1 solution, increasing the engineering stress at a linear rate until specimen fracture (i.e. a LIST linearly increasing stress test) and evaluating stress corrosion cracking tendency from the measured ultimate tensile stress and the fractography of the sample. In this survey test, 4145H had UTS (in the solution) slightly lower than the UTS in air, and a fracture appearance comparable to that for the test in air. The UTS of 4145V in the survey test was somewhat lower than in air, the specimen showed significant ductility, nev-
17
ertheless the fracture surface had significant differences to that tested in air and there were clear secondary cracks (although these cracks were relatively short, less than 0.1 mm in length). There was not sufficient time in the program to allow for an examination of these secondary cracks; although it would not be surprising from the fractography if these secondary cracks occurred at stresses close to the stress of final fracture in the LIST. Nevertheless, the existence of these secondary cracks in 4145V meant that 4145V was not included in the tests under more aggressive hydrogen conditions at more negative potentials, and since 4145V was not tested, it is not appropriate to make a comparison with 4145H. X65 and X70 showed high resistance to SCC. Can this be attributed to their fine grain size and the presence of Nb? Steels with similar metallurgies showed large differences in SCC susceptibility. This indicates that there are significant subtleties, probably associated with hydrogen mobility and trapping. 5. Conclusions • Conditions leading to SCC were associated with abundant hydrogen evolution (acid or at a negative potential), identifying hydrogen embrittlement as the likely mechanism. • 1355, MAC and MA840B rock bolt steels displayed SCC when loaded at 0.019 MPa s−1 in the sulphate pH 2.1 solutions at the free corrosion potential. They had comparable threshold stresses and comparable stress corrosion crack velocities. • 5152CW10D rock bolts steel had the best SCC resistance of the rock bolt steels tested. Cold work increased the resistance of 5152 to SCC. • The five commercial steels 1008, X65, X70, 4140 and 4145H were subjected to LIST in the dilute pH 2.1 sulphate solution at their free corrosion potential and at increasingly negative applied potential values to −1500 mV. The increasingly negative applied potential increases the aggressivity of SCC conditions because of increasing hydrogen liberated at the specimen surface. It was found that the four steels 1008, X65, X70 and 4145H resisted SCC for all applied potentials including −1500 mV. • Steels with similar metallurgies showed large differences in SCC susceptibility. This indicates that there are significant subtleties, probably associated with hydrogen mobility and trapping. Acknowledgement This work is supported by an ARC linking grant with Dywidag Systems International Pty Ltd. (DSI) Arnall Mining Product Division, One Steel, Smorgon Steel and Jenmar. References [1] ANI Arnall. Practical Guide to Rock Bolting, 1991, ANI ARNALL, 25 Pacific Highway Bennets Green, NSW 2290. [2] DYWIDAG-Systems International, DYWIDAG Threadbar Resin Anchored Bolts (2007) http://www.dywidag-systems.com/index.php.
18 [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12]
[13]
[14]
[15]
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18 E. Gamboa, A. Atrens, Eng. Fail. Anal. 10 (2003) 521–558. E. Gamboa, A. Atrens, Eng. Fail. Anal. 12 (2005) 201–235. E. Gamboa, A. Atrens, J. Mater. Sci. 38 (2003) 3813–3829. E. Villalba, A. Atrens, Eng. Fail. Anal. 15 (2008) 617–641. A. Atrens, Z.F. Wang, Mater. Forum 19 (1995) 9–34. A. Crosky, B. Smith, B. Hebblewhite, Pract. Fail. Anal. 3 (2) (2003) 41–49. Crosky A., Fabjanczyk M., et al., Final report—premature rock bolt failure, ACARP c 8008. Tech. Rep., Australian Coal Association Research Program (2002). A. Crosky, et al., International Conference and Exhibition on Failure Analysis (IFCA), Melbourne, Australia, 20–22 November, 2002. E. Gamboa, A. Atrens, Proceeding, 15th International Corrosion Congress, Granada, 2002, pp. 1–9 (paper 811). E. Gamboa, A. Atrens, in: N.R. Moody, A.W. Thompson, R.E. Ricker, G.S. Was, R.H. Jones (Eds.), Hydrogen Effects on materials Behaviour and Corrosion Deformation Interactions, TMS, 2003, pp. 641–647. E. Gamboa, A. Atrens, Fractography of SCC Features for Rock Bolts 2nd International Conference on Environmental Degradation of Engineering Materials, Event No 264 European Federation of Corrosion, June 29 to July 3rd (2003c) Bordeaux, France, ISBN 1-904350-70-0, paper SCC2-2-04b. E. Gamboa, A. Atrens, Laboratory testing of rock bolt Stress Corrosion Cracking, COAL 2003, 4th Underground Coal Operators Conference, University of Wollongong, 12–14 February (2003e) N Aziz and B Kinnimoth eds., publisher Aus IMM, pp. 132–153. A. Atrens, C.C. Brosnan, S. Ramamurthy, A. Oehlert, I.O. Smith, Measure. Sci. Technol. 4 (1993) 1281–1292.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
[32]
S. Ramamurthy, A. Atrens, Corros. Sci. 34 (1993) 1385–1402. Z.F. Wang, A. Atrens, Metall. Mater. Trans. 27A (1996) 2686–2691. J. Salmond, A. Atrens, Scripta Metall. Mater. 26 (1992) 1447–1450. A. Oehlert, A. Atrens, Mater. Forum 17 (1993) 415–429. A. Atrens, A. Oehlert, J. Mater. Sci. 33 (1998) 783–788. N. Winzer, A. Atrens, W. Dietzel, G. Song, K.U. Kainer, Mater. Sci. Eng. A 472 (2008) 97–106. J.Q. Wang, A. Atrens, Eng. Fail. Anal. 11 (2004) 3–18. J. Wang, A. Atrens, Corros. Sci. 45 (2003) 2199–2217. J.G. Williams, C.R. Killmore, F.J. Barbaro, J. Piper, L. Fletcher, Mater. Forum 20 (1996) 13. J. Wang, A. Atrens, J. Mater. Sci. 38 (2003) 323–330. A. Atrens, J.Q. Wang, K. Stiller, H.O. Andren, Corros. Sci. 48 (2006) 79– 92. J.Q. Wang, A. Atrens, D.R. Cousens, J. Mater. Sci. 34 (1999) 1711–1719. J.Q. Wang, A. Atrens, D.R. Cousens, N. Kinaev, J. Mater. Sci. 34 (1999) 1721–1728. J.Q. Wang, A. Atrens, D.R. Cousens, P.M. Kelly, C. Nockolds, S. Bulcock, Acta Mater. 46 (1998) 5677–5687. J. Mougin, B. Ghys, C. Reullon, C. Lemaitre, published in proceedings Eurocorr 2005, paper supplied by [email protected]. J. Mougin, M.S. Cayard, R.D. Kane, B. Ghys, C. Pichard, Sulfide stress cracking and corrosion fatigue of steels dedicated to bottom hole assembly components, Corrosion 2005, paper 05085, NACE, Houston TX. NACE International, Standard test method laboratory testing of metals for resistance to specific forms of environment cracking in H2 S environments, NACE TM0177-96, Item No 21212, NACE, Houston TX.
Metallurgical aspects of rock bolt stress corrosion cracking Ernesto Villalba a , Andrej Atrens a,b,∗ a
b
School of Engineering, The University of Queensland, Brisbane, St. Lucia, Qld. 4072, Australia ¨ Swiss Federal Laboratories for Materials Science and Technology, EMPA, Dept 136, Uberlandstrasse 129, CH-8600 Dubendorf, Switzerland Received 8 October 2007; accepted 30 November 2007
Abstract This paper reports on the metallurgical influence on rock bolt stress corrosion cracking for a range of: (1) existing rock bolt steels and (2) commercial steels. Rock bolt steels 1355, MAC and MA840B displayed SCC when loaded at 0.019 MPa s−1 in the sulphate pH 2.1 solutions at the free corrosion potential. They had comparable threshold stresses and comparable stress corrosion crack velocities. Rock bolts steel 5152CW10D had the best SCC resistance of the rock bolt steels tested. Cold work increased the resistance of 5152 to SCC. The five commercial steels 1008, X65, X70, 4140 and 4145H were subjected to the linearly increasing stress test (LIST) in the dilute pH 2.1 sulphate solution at their free corrosion potential and at increasingly negative applied potential values to −1500 mV. The increasingly negative applied potential increases the aggressivity of SCC conditions because of increasing hydrogen liberated at the specimen surface. The steels 1008, X65, X70 and 4145H resisted SCC for all applied potentials including −1500 mV. © 2007 Elsevier B.V. All rights reserved. Keywords: Stress corrosion cracking; Rock bolt; Metallurgy
1. Introduction Rock bolts are a widely used method of strata control in mines worldwide [1–6]. The principle of rock bolting is illustrated in Fig. 1. Rock bolts provide reinforcement above mine openings such as roadways. The rock bolts are bonded into the rock strata keeping it together just like the steel in reinforced concrete. The rock bolts can be considered to provide a clamping action on the rock. For the strata to fail and collapse, the clamping action of the rock bolts has to be overcome. This requires considerably greater forces. The rock bolts thus maintain the load bearing capacity of the rock strata. The chemical bolt [1–4] provides a quick, easy and effective installation procedure, Fig. 2. An appropriate hole is drilled (to the appropriate diameter and depth), the chemical cartridge(s) and bolt is introduced into the hole, the bolt is rotated, and simultaneously pushed through the chemical cartridge(s) to mix together the resin and catalyst. After completion of the hold time (to allow curing of the chemical anchor) the bolt is tightened to the appropriate load. The load is distributed to the
∗
Corresponding author at: School of Engineering, The University of Queensland, Brisbane, St. Lucia, Qld. 4072, Australia. Fax: +61 733653888. E-mail address: [email protected] (A. Atrens). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.086
surrounding rock by means of the base plate. The chemical anchor fixes the bolt into the rock strata. Depending on the amount of the chemicals used, the anchor can be at the end of the bolt or it can extend along the length of the bolt. The amount of load on the bolt can be measured using strain gauged test bolts. Such experience allows estimation of the actual service load from the amount of plastic deformation of the support plate. Stress corrosion cracking (SCC) describes the failure mode where an environment induced crack slowly propagates through the material [7]. SCC requires a susceptible material, a suitable environment (an electrolyte solution) and sufficient stress. Crosky et al. [8–10] indicate that rock bolt SCC has been a problem in a significant number of Australian mines since about 1994. SCC is manifest as first a period of slow crack growth over a period of time until the crack reaches the critical size (typically 2–3 mm in depth). Subsequently there is a sudden fast fracture across the remaining section of the bolt. Most SCC is reported to occur in the un-encapsulated part of the rock bolt. However, some SCC cases have been reported in the resin-covered area. The fracture surfaces of rock bolts, made from 1335 steel, failed during service by stress corrosion cracking (SCC) have been described previously [3,5,11–13]. The microstructure of these rock bolts was pearlitic. Laboratory research [3–6,12,14]
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
Fig. 1. Rock bolts provide reinforcement above mine openings such as roadways.
on rock bolts with the same fine pearlitic microstructure has shown that service failures can be duplicated in the laboratory using the linearly increasing stress test (LIST) [15–21]. LIST testing of a tensile sample exposed to a sulphate pH 2.1 solutions at a rate of 0.019 MPa s−1 was shown to provide a good foundation for a test to reproduce service SCC. The resulting fractography of LIST samples were similar to those from service SCC. Rock bolts failed by SCC were subjected to a detailed examination of the fracture surfaces to understand the SCC fracture mechanism. The SCC fracture surfaces contained the following different surfaces: tearing topography surface (TTS), corrugated irregular surface (CIS) and micro-void coalescence (MVC). TTS is characterised by a ridge pattern independent of the pearlite microstructure, but having spacing only slightly coarser than the pearlite spacing. CIS is characterised as porous irregular corrugated surfaces joined by rough slopes. MVC found in the studied rock bolts are different to that found in samples failed in a pure ductile manner. The MVC observed in rock bolts is more flat and regular than the pure MVC, being attributed to hydrogen embrittling the ductile material near the crack tip.
9
The interface between the different fracture surfaces revealed no evidence of a third mechanism involved in the transition between fracture mechanisms. The following SCC mechanism is consistent with the fracture surfaces [3–5]. Hydrogen diffuses into the material, reaching a critical concentration level. The thus embrittled material allows a crack to propagate through the brittle region. The crack is arrested once it propagates outside the brittle region. Once the new crack is formed, corrosion reactions start producing hydrogen that diffuses into the material once again. The present paper discusses the metallurgical influence on rock bolt stress corrosion cracking for a range of: (1) existing rock bolt steels and (2) commercial steels. 2. Experimental procedure 2.1. List SCC test The laboratory SCC test method, the linearly increasing stress test (LIST) [15–21], consists of applying a linearly increasing engineering stress to a specimen exposed to the environment of interest, either the standard sulphate pH 2.1 solution or the chloride pH 1.8 solution. The load on the specimen is increased linearly by means of a lever principle and a moving load on the right hand size of the lever. The lever is maintained horizontal via a linear actuator and servo-controller by means of a displacement signal from the end of the lever arm. One side of the lever beam is connected to the specimen while the other size has a mass of 14 kg, which moves away from the fulcrum. This movement increases the load on the specimen at 0.019 MPa s−1 for the standard test. The applied engineering stress is calculated from the position of the mass at any time and the original cross-section of the specimen. After LIST, the fracture surfaces were examined by scanning electron microscopy (SEM). 2.2. Solutions The standard sulphate pH 2.1 solution was made using reagent grade chemicals and distilled water to simulate what might be found in underground water samples at mines as follows: 1.6543 g H2 SO4 , 0.3285 g NaCl and 0.5959 g Na2 CO3 was dissolved to make 1000 ml solution. Experiments were carried out at the free corrosion potential or under potentiostatic control. The chloride pH 1.8 solution contained 1400 ppm chloride, 300 ppm sulphate and 100 ppm carbonate. This solution was made up as follows: 1.6543 g H2 SO4 , 4.6056 g NaCl and 0.5959 g Na2 CO3 was dissolved in distilled water to make up 1000 ml of solution. The pH of this solution was measured to be 1.8. 2.3. Existing rock bolt steels
Fig. 2. Principle of rock bolt installation.
The chemical composition, mechanical properties, and metallurgies of these steels are presented in Tables 1 and 2. Steel 5152 was cold worked 10% to produce 5152CW10D and 55% to produce 5151CW55.
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E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
Table 1 Chemical composition of the commercial rock bolt steels (wt%) Grade 1355 MA840B MA810 MAC 10M30 5152
A B C AH D
C
Si
Mn
P
S
Ni
Cr
Mo
Al
V
0.54 0.37 0.36 0.25 0.29 0.54
0.26 1.05 1.00 0.36 0.24 0.10
1.63 1.46 1.40 1.32 0.72 0.90
0.017 0.013 0.021 0.016 0.015 0.015
0.027 0.009 0.013 0.027 0.026 0.025
0.09 0.01 0.01 0.07 0.14 0.35
0.08 0.02 0.02 0.05 0.11 0.90
0.03 0.01 0.01 0.01 0.03 0.10
0.004 0.004 0.005 0.005 – –
0.003 0.043 0.040 0.210 0.053 –
Table 2 Metallurgies and mechanical properties of the commercial rock bolt steels Grade 1355 MA840B MA810 MAC 10M30 5152CW10D
C–Mn Micro-alloyed Micro-alloyed Micro-alloyed Plain carbon Cr + Ni + 10% CW
D (m)
YS (MPa)
UTS (MPa)
Elongation (%)
Area reduction (%)
YS/UTS
CVN
75 65 43 – – –
622 635 689 – 400 745
954 873 838 – 670 890
18.0 22.2 21.4 – 22.0 12.0
37.9 50.3 52.2 – – –
0.65 0.73 0.82 – 0.60 0.84
6 18 29 – – –
D, grain size; YS, Yield strength; UTS, ultimate tensile strength; CVN, charpy V-notch impact energy.
2.4. Constant stress test (threshold stress determinations)
and whether there was SCC. If there was no SCC, it indicated that the stress was below the threshold stress for SCC.
The threshold stress was determined using a modified LIST. The mass was stopped at a predetermined position and the sample was held at a constant stress for 3 days in the sulphate pH 2.1 solution. If the sample did not fail at the end of the 3-day period, it was removed and cooled down to −197 ◦ C and them struck with a hammer to break it in two pieces. The fracture surface was observed with SEM to determine the failure mode
2.5. Commercial steels The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition of the steels is presented in Table 3; their mechanical properties and microstructures in Table 4. In
Table 3 Chemical composition (wt%) of the commercial steels and their critical potentials in the sulphate pH 2.1 solution below which SCC occurs Ecrit
Steel
Chemical composition (wt%) C
Si
Mn
P
S
0.03 0.05 0.08 0.19 0.53
<0.02 0.015 <0.02 0.22 0.25
0.22 0.32 0.77 0.78 0.80
0.024 0.011 0.009 0.011 0.008
0.017 0.012 0.010 0.029 0.016
Carbon plus manganese steels −250 X1340F
0.40
0.24
1.55
0.023
Alloys steels −250 −250 −800 −350 <−1500 −250
1355 XK5155S 5152 4140 4145H 4145V
0.54 0.54 0.54 0.39 0.45 0.37
0.26 0.29 0.10 0.26 0.20 0.29
1.63 0.91 0.90 0.85 1.10 0.76
Micro-alloyed steels −250 10M40 −650 10M30 −250 HSAC840 <−1500 X65 <−1500 X70
0.42 0.29 0.39 0.11 0.05
0.25 0.24 1.11 0.26 0.24
0.83 0.72 1.50 1.3 1.5
Carbon steels −250 −250 <−1500 −250 −250
1003 1004 1008 1019 MRB500
Ni
Cr
Mo
Al
V/Nb
0.02 0.008 0.02 0.07 0.058
<0.01 0.02 0.01 0.06 0.093
<0.01 0.001 <0.01 0.02 0.03
0.028 0.001 0.043 <0.005 0.003
<0.01 0.001 <0.01 <0.01 0.004
0.022
0.008
0.023
0.002
0.022
0.001
0.017 0.014 0.015 0.014 0.007 0.006
0.027 0.007 0.025 0.03 0.003 0.003
0.09 0.011 0.35 0.016 0.039 0.085
0.08 0.81 0.9 1.02 1.26 1.31
0.03 0.002 0.1 0.19 0.40 0.67
0.004 0.018 – 0.002 0.023 0.032
0.003 0.005 – 0.008 0.004 0.12
0.013 0.015 0.019 0.017 0.012
0.009 0.026 0.01 0.009 0.009
0.007 0.14 0.01 <0.01 0.01
0.022 0.11 0.027 <0.01 0.01
0.001 0.03 0.002 <0.01 0.15
0.001 0.001 0.001 0.026 0.029
0.12V 0.05V 0.04V 0.05Nb 0.06Nb
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
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Table 4 Mechanical properties and microstructures of the commercial steels Sample
Mechanical properties
Microstructure
YS (MPa)
UTS (MPa)
εf (%)
Carbon steel 1003 1004 1008 1019 MRB500
337 287 446 504 473
398 363 533 631 840
31 37 35 27 23
Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite and chromium carbide
Carbon + Mn steels X1340F
465
801
24
Ferrite–pearlite and chromium carbide
Alloy steels XK5155S 4140 4145H 4145V
586 725 749 808
948 962 902 944
22 15 22 20
Ferrite–pearlite and chromium carbide Ferrite–pearlite and chromium carbide Tempered martensite Tempered martensite
Micro-alloyed steels 10M40 X11M47 HSAC840 X65 X70
540 595 615 493 618
782 847 889 569 683
23 24 23 31 22
Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite Ferrite–pearlite
order to facilitate the discussion of metallurgical influences, Table 3 includes the commercial rock bolt steels 5152 and 10M30. All the steels are relatively straightforward except for 4145H, 4145V, X65 and X70. The grades 4145H and 4145V are alloy steels containing Cr and Mo. 4145V has slightly lower C and Mn contents, a somewhat higher Mo content and contains also the micro-alloying element V. Residual elements such as S and P are at low levels, and the heats are produced using clean steel practices. The steel grades are quenched and tempered to obtain a fine tempered martensite microstructure. X65 and X70 are steels produced for pipeline service where anodic stress corrosion cracking is a considerable issue [22,23]. These steels are micro-alloyed containing a small amount of Nb and attain their mechanical properties through a complex thermo-mechanical working sequence [24]. For X70, the slab was reheated to 1250 ◦ C in order to ensure complete solution of the micro-alloying carbonitrides (Ti, Nb, V)(C, N). Rough rolling was completed above about 1030 ◦ C with the metallurgical objective of achieving the finest possible austenite (␥) grain size. TiN particle control was used to inhibit the growth of the ␥ grains. The finish rolling was commenced below the ␥ non-recrystallization temperature. The prime intention was to accumulate rolling strain within the ␥ grains so that on subsequent a transformation, there were many ␣ nucleation sites and a very fine ␣ grain size resulted. The microstructure, as documented more fully in Refs. [25–29], consisted of a large amount of ␣ (with a grain size less than 5 m), some grain boundary (GB) carbides and few pearlite grains. Some ␣ grains had a high dislocation density. The GB carbides contained Fe, Mn and sometimes Si. They contained no Ti, Nb, V or N. The typical thickness of GB carbides was about 100 nm. This microstructure was consistent with the low C content of the X70 steel.
There were some fine precipitates inside the ␣ grains which were attributed to complex mixed Ti, Nb and V carbonitrides, designated as (Ti, Nb, V)(C, N). These fine precipitates were the only microstructural features associated with the micro-alloying additions: Ti, Nb and V. The (Ti, Nb, V)(C, N) precipitates within the ␣ grains and the high dislocation density within the ␣ grains reflect the thermo mechanical treatment described above, which were associated with the hot working sequence at temperatures at which the steel was austenitic. Apart from their influence as described above, they did not seem to have had an influence in the further development of the microstructure and particularly the development of the GBs. The following mechanisms were important in the final development of the microstructure, particularly the GBs. The composition of the steel was such that all the C (except for C associated with (Ti, Nb, V)(C, N)) was in solid solution in the ␥ at high temperatures during processing, and that there was more C than could be accommodated in solid solution in the ␣ on cooling to room temperature. The steel had a low C content and contained little pearlite. This meant that during cooling from the ␥, most of the ␥ transformed to ␣. 3. Results 3.1. Environmental influences A series of experiments were undertaken using steel 1355 to study the influence of the environment. The results have been displayed on a Pourbaix diagram in Fig. 3. Sulphate solutions are represented by circles and chloride solutions are represented by squares. A full symbol indicates SCC, whereas an empty circle or square means no SCC was detected. Symbols with a dash represent experiments performed with an applied potential.
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E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
Fig. 4. Typical SCC in service is associated with SCC initiating at a rock bolt rib, SCC growing to a critical size, followed by fast fracture through the remaining section. Fig. 3. Conditions for rock bolt SCC superimposed on the Pourbaix diagram for Fe. Full symbols indicate SCC whereas there was no SCC under the conditions represented by the open symbols.
This data indicates that SCC was controlled by a combination of the applied potential and the pH. SCC only occurred for environmental conditions, which produced hydrogen on the sample surface, leading to hydrogen embrittlement and SCC. The fracture surface of a LIST sample failed by SCC displayed the same fracture regions as fracture surfaces of rock bolts failed in service by SCC [3–5], see Fig. 4. This series of experiments led to the standard sulphate pH 2.1 solution. 3.2. Commercial rock bolt steels Table 5 presents the threshold potential for different commercial rock bolt steels tested in the standard sulphate pH 2.1 solution. Steels 1355, MA840B and MAC all showed SCC in the standard sulphate pH 2.1 at the free corrosion potential. Steels 10M30 and 5152 were somewhat more resistant to SCC, they showed SCC for applied potentials of −800 mV and −1000 mV, respectively. Steel 5152CW10D can suffer from SCC (as shown by service failures), but the condition causing SCC in the laboratory was the most severe condition of all the studied commercial rock
Table 5 Threshold potential for rock bolt steels and commercial steels tested in the standard pH 2.1 solution Grade
1355 MA840B MAC 10M30 4145V 4140 5152 5152CW10D 5152CW55 1008 4145H X65 X70
pH 2.1
C–Mn eutectoid Micro-alloyed Micro-alloyed Plain carbon Tempered martensite Ferrite + pearlite Cr + Ni + 0% CW Cr + Ni + 10% CW Cr + Ni + 55% CW Ferrite and pearlite Tempered Martensite Micro-alloyed Micro-alloyed
A B C AH
D
No SCC
SCC
−250
−350 −350 −350 −800 −350 −700 −1000 −1200
−650 −350 −800 −1000 −1200 −1500 −1500 −1500 −1500
bolt steels, Table 5. 5152 steel suffered from SCC at a condition with less hydrogen production than those required for SCC in 5152CW10D steel, indicating that cold work helps increase the resistance to SCC of steel. Moreover 5152CW55 did not show SCC at an applied potential of −1200 mV.
Table 6 Determination of the SCC thresholds in the sulphate pH 2.1 solution Metallurgy
Stress not causing SCC (MPa)
Stress causing SCC (MPa)
Threshold stress (MPa)
1355 MAC MA840B 5152CW10D
770(NP) ; 861(NP) ; 885(NP) 700(P after 1.3 h) ; 800(P* after 50 h) 700(P after 27 h) ; 800(P* after 30 h) –
922 830 850 –
900 815 850 >960
“P” indicates pitting causing ductile failure after the specified period of load application. “NP” indicates NO pitting. “P*” indicates a sample failed by pitting and stress corrosion cracks were found.
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
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Table 7 Determination of SCC velocity for standard test Metallurgy
Sample
LIST test duration (s)
SCC crack size (mm)
Velocity (m/s)
1355 1355 MAC MA840B
LIST 24 LIST 23 LIST 70 LIST 69
53,398 54,000 44,280 50,400
1.3 1.1 1.0 1.0
2.4 × 10−8 2.1 × 10−8 2.2 × 10−8 1.9 × 10−8
Table 8 Mechanical data for the commercial steels tested in air and in the sulphate pH ∼ 2.1 solution at the free corrosion potential Sample
UTS air (MPa)
UTS sol (MPa)
εf air (%)
εf sol (%)
SCC presence
Carbon steel 1003 1004 1008 1019 MRB500
398 363 533 631 840
395 346 532 621 748
31 37 35 27 23
– 18 – 18 –
Yes, high Yes, high No Yes, high Yes, low
Carbon + Mn steels X1340F
801
771
24
9
Yes, high
Alloy steels XK5155S 4140 4145H 4145V
948 962 902 944
923 946 893 868
22 15 22 20
14 14 14 –
Yes, medium No No Yes, low
Micro-alloyed steels 10M40 X11M47 HSAC840 X65 X70
782 847 889 569 683
772 841 864 532 596
23 24 23 31 22
14 18 9 23 –
Yes, medium Yes, medium Yes, high No No
Fig. 5. Fractography for 1008 (a), (b) air (c) the sulphate pH 2.1 solution at the free corrosion potential and (d) the sulphate pH 2.1 solution at −1500 mV.
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3.3. Threshold stress
3.4. Commercial steels
The threshold stress of various commercial rock bolt steels was determined to be 900 MPa for 1355, and 815 and 850 MPa for MAC and MA840B steels. The high value of threshold stress suggests that SCC begins in rock bolts when they are sheared by moving the rock strata. Table 6 summarises the results of the testing program to determine the SCC thresholds for the various commercial rock bolt steels. A typical crack velocity was measured to be 2.5 × 10−8 m/s, indicating that there is little benefit for rock bolt to have steels with higher fracture toughness. Table 7 showed the determination of SCC velocity for the standard test, calculated from the crack size divided by the time for crack propagation. The time for crack propagation was taken to be the time to complete the experiment and assumed crack initiation and propagation as soon as the material was under any stress.
Table 8 presents the mechanical data, UTS and εf , for specimens tested in air and the sulphate pH 2.1 solution, and the occurrence or otherwise of stress corrosion cracking (SCC) for the steels tested in the sulphate pH ∼2.1 solution at the free corrosion potential. Although all samples in the LIST-test gave lower values of UTS, some of then were fairly close to the UTS value in air, such as the low carbon steel 1008, the alloy steels 4140 and 4145H and the micro-alloyed steels X65 and X70. Five steels did not show the presence of SCC as evaluated from their fracture surface: the low carbon steel 1008 (Fig. 5), the alloy steels 4140 and 4145H (Fig. 6) and the micro-alloyed steels X65 and X70 (Fig. 7). The five commercial steels 1008, X65, X70, 4140 and 4145H were subjected to LIST in the dilute pH 2.1 sulphate
Fig. 6. Fractography for 4145H (a), (b) air (c), (d) the sulphate pH 2.1 solution at the free corrosion potential and (e) the sulphate pH 2.1 solution at −1500 mV.
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
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Fig. 7. Fractography for X70 (a), (b) air (c), (d) sulphate pH 2.1 solution at the free corrosion potential and (e) sulphate pH 2.1 solution at −1500 mV.
solution at their free corrosion potential and at increasingly negative applied potential values (−700 mV, −1100 mV, −1300 mV and −1500 mV). The increasingly negative applied potential increases the aggressivity of SCC conditions because of increasing hydrogen liberated at the specimen surface. The UTS for all the samples tested was little influenced by the electrochemical potential, indicating that the combination of electrolyte and cathodic potential had a low effect in the mechanical properties of 1008, X65, X70, 4140 and 4145H. SEM fractography analysis revealed that there was no SCC for the steels 1008 (Fig. 5(d)), 4145H (Fig. 6(e)), X65 and X70 (Fig. 7(e)) tested in the pH 2.1 solution, even with an applied potential of −1500 mV. Steel sample 4140 did show SCC when tested at −700 mV in the pH 2.1 solution, Fig. 8.
4. Discussion 4.1. Mechanistic aspects The sulphate pH 2.1 solution produced SCC [3] in the LIST at an applied stress rate of 0.019 MPa s−1 . The fractures were macroscopically brittle in contrast to the ductile overload fracture measured in air and also in the distilled water. Detailed SEM examination [3,5,11–14] of the LIST samples tested in the sulphate pH 2.1 solution indicated a fractography similar to that of the service failures. There was a small region of SCC followed by fast brittle fracture. The SCC details were also similar to those of the service failures. Environmental conditions causing rock bolt SCC in the laboratory were exposure at the free corrosion potentia in the acid solution and hydrogen production. SCC conditions (pH and mV vs. SHE) have been determined for 1355, Fig. 3. A hydrogen
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E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18
Fig. 8. Fractography of 4140 tested in the sulphate pH 2.1 solution at −700 mV showing (a) a flat macroscopically brittle fracture surface containing (b) a corrugated irregular surface topography and (c) a brittle fast fracture.
embrittlement mechanism for the SCC was indicated by the particular restricted range of conditions for which SCC occurred in the laboratory. In particular, SCC only occurred in the laboratory for the restricted range of environmental conditions corresponding to acid conditions at the open circuit potential (∼pH 2.1 or more acid) or at negative applied electrochemical potentials corresponding to copious hydrogen evolution at the steel surface. This was consistent with reports from the USA indicating rock bolt failure due to the presence of H2 S in the mine atmosphere and with bacterial corrosion on the rock bolt surface during service producing acid conditions leading to SCC. The threshold stress was measured for 1355 (900 MPa), MAC (830 MPa) and MA840B Steel (850 MPa) Table 6, indicating that the loading causing SCC in service is due to a combination of the tensile load plus the bending load due to rock shear. This is consistent with the observation that rock bolts are typically bent after failure in service. This bending indicates a stress above the yield stress having been applied to cause the permanent deformation in bending, i.e. to cause permanent plastic deformation. The bending of the bolt has been attributed to shear in the rock strata. This leads to the issue of whether SCC could be prevented by a rock bolting strata design that prevented shear in the rock strata and thereby maintained the stress in the rock bolt below the threshold stress for SCC initiation. The threshold stress for MA840B steel was about 850 MPa and for MAC steel 830 MPa. These values were similar to that for the 1355 samples, but nevertheless lower in value. MA840B and MAC have poorer SCC properties than 1355, as shown by
extensive surface pits and secondary cracks. Although MA840B and MAC have failed by SCC the overall macroscopic appearance [4] was very similar to that of ductile overload. Only careful observation could distinguish the failure mode. For this reason, industry might perceive these bolts to be immune from SCC, believing that these bolts fail in a ductile manner. In reality, these test showed that these metallurgies fare worse than the 1355 metallurgy. 10M30 bolts were not believed to fail due to SCC in service, but laboratory tests show that fails can fail due to SCC but appear to have failed by ductile tearing. Environmental conditions required to cause SCC in 10M30 bolts are more acidic and/or at a more negative potential than for 1355, MA840B and MAC steels, but not as severe as those needed to cause SCC in 5152CW10D. 5152CW10D bolts can suffer from SCC (as shown by service failures), but conditions causing SCC in the laboratory were more severe than the free corrosion potential in the sulphate pH 2.1. 5152CW10D bolts required the harshest environmental conditions out of all studied metallurgies for them to fail due to SCC. 5152 steels suffered from SCC at conditions with less hydrogen production than those required to show SCC in 5152CW10D steels. This confirms that cold work helped to increase the resistance to SCC. Cold work of the 5152 steel by 10% and 55% showed an increased resistance of the steel to SCC. Cold working the material by 10% increased the SCC resistance by 200 mV versus SHE in the standard sulphate pH 2.1 solution. Cold working the material to 55% increased the SCC resistance by 400 mV
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versus SHE. Cold work improved the SCC resistance of rock bolts. Fracture toughness √ values evaluated [4] for a 1355 rock bolt was about 50 MPa m. There is certainly scope for producing steels with higher values of fracture toughness, but the benefits in terms of increased SCC lifetimes seem to be small, particularly in view of the typical stress corrosion crack velocities, Table 7. 4.2. Influence of metallurgy In order to facilitate thinking about the influence of metallurgy, Table 3 includes the critical potential in the sulphate pH 2.1 solution below with there was SCC. Some noteworthy issues from Table 3 are as follows. The plain carbon steels 1003, 1004, 1008 and 1019 are all similar commercial steels with the only significant difference being the carbon content. It was therefore surprising that 1008 was much more resistant to SCC than the other plain carbon steels. The history of the 1008 steel was that it came from warm worked plate, and it is possible that a different dislocation structure was responsible. 5152 and 10M30 showed some resistance to SCC. Does this indicate an influence of alloying with Ni? For 5152, cold work increased SCC resistance as already noted. 4145H and 4145V were both tempered martensites; the slightly higher strength of 4145V correlates with a decrease in SCC resistance. The grades 4145H and 4145V are alloy steels containing Cr and Mo. 4145V has slightly lower C and Mn contents, a somewhat higher Mo content and contains also the micro-alloying element V. Residual elements such as S and P are at low levels, and the heats are produced using clean steel practices. The steel grades are quenched and tempered to obtain a fine tempered martensite microstructure. It is somewhat surprising that the results were different for steel grades 4145H and 4145V, since these steels are quite similar. Moreover, Mougin et al. [30,31] have shown that 4145V has a higher resistance to sour H2 S containing environments when testing according to the NACE Standard TM0177 [32]. These tests indicate that both steels show resistance to this environment with 4145V having the higher resistance. The difference revealed in our present study is attributed to the experimental methodology. This present program did not carry out a direct comparison of the two steels 4145H and 4145V. Rather first a survey was carried out in which the rock bolt stress corrosion cracking tendency was evaluated for 15 steels using our prior established testing methodology for evaluating rock bolt stress corrosion cracking. The survey test involves exposing the test specimen to the standard sulphate pH 2.1 solution, increasing the engineering stress at a linear rate until specimen fracture (i.e. a LIST linearly increasing stress test) and evaluating stress corrosion cracking tendency from the measured ultimate tensile stress and the fractography of the sample. In this survey test, 4145H had UTS (in the solution) slightly lower than the UTS in air, and a fracture appearance comparable to that for the test in air. The UTS of 4145V in the survey test was somewhat lower than in air, the specimen showed significant ductility, nev-
17
ertheless the fracture surface had significant differences to that tested in air and there were clear secondary cracks (although these cracks were relatively short, less than 0.1 mm in length). There was not sufficient time in the program to allow for an examination of these secondary cracks; although it would not be surprising from the fractography if these secondary cracks occurred at stresses close to the stress of final fracture in the LIST. Nevertheless, the existence of these secondary cracks in 4145V meant that 4145V was not included in the tests under more aggressive hydrogen conditions at more negative potentials, and since 4145V was not tested, it is not appropriate to make a comparison with 4145H. X65 and X70 showed high resistance to SCC. Can this be attributed to their fine grain size and the presence of Nb? Steels with similar metallurgies showed large differences in SCC susceptibility. This indicates that there are significant subtleties, probably associated with hydrogen mobility and trapping. 5. Conclusions • Conditions leading to SCC were associated with abundant hydrogen evolution (acid or at a negative potential), identifying hydrogen embrittlement as the likely mechanism. • 1355, MAC and MA840B rock bolt steels displayed SCC when loaded at 0.019 MPa s−1 in the sulphate pH 2.1 solutions at the free corrosion potential. They had comparable threshold stresses and comparable stress corrosion crack velocities. • 5152CW10D rock bolts steel had the best SCC resistance of the rock bolt steels tested. Cold work increased the resistance of 5152 to SCC. • The five commercial steels 1008, X65, X70, 4140 and 4145H were subjected to LIST in the dilute pH 2.1 sulphate solution at their free corrosion potential and at increasingly negative applied potential values to −1500 mV. The increasingly negative applied potential increases the aggressivity of SCC conditions because of increasing hydrogen liberated at the specimen surface. It was found that the four steels 1008, X65, X70 and 4145H resisted SCC for all applied potentials including −1500 mV. • Steels with similar metallurgies showed large differences in SCC susceptibility. This indicates that there are significant subtleties, probably associated with hydrogen mobility and trapping. Acknowledgement This work is supported by an ARC linking grant with Dywidag Systems International Pty Ltd. (DSI) Arnall Mining Product Division, One Steel, Smorgon Steel and Jenmar. References [1] ANI Arnall. Practical Guide to Rock Bolting, 1991, ANI ARNALL, 25 Pacific Highway Bennets Green, NSW 2290. [2] DYWIDAG-Systems International, DYWIDAG Threadbar Resin Anchored Bolts (2007) http://www.dywidag-systems.com/index.php.
18 [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12]
[13]
[14]
[15]
E. Villalba, A. Atrens / Materials Science and Engineering A 491 (2008) 8–18 E. Gamboa, A. Atrens, Eng. Fail. Anal. 10 (2003) 521–558. E. Gamboa, A. Atrens, Eng. Fail. Anal. 12 (2005) 201–235. E. Gamboa, A. Atrens, J. Mater. Sci. 38 (2003) 3813–3829. E. Villalba, A. Atrens, Eng. Fail. Anal. 15 (2008) 617–641. A. Atrens, Z.F. Wang, Mater. Forum 19 (1995) 9–34. A. Crosky, B. Smith, B. Hebblewhite, Pract. Fail. Anal. 3 (2) (2003) 41–49. Crosky A., Fabjanczyk M., et al., Final report—premature rock bolt failure, ACARP c 8008. Tech. Rep., Australian Coal Association Research Program (2002). A. Crosky, et al., International Conference and Exhibition on Failure Analysis (IFCA), Melbourne, Australia, 20–22 November, 2002. E. Gamboa, A. Atrens, Proceeding, 15th International Corrosion Congress, Granada, 2002, pp. 1–9 (paper 811). E. Gamboa, A. Atrens, in: N.R. Moody, A.W. Thompson, R.E. Ricker, G.S. Was, R.H. Jones (Eds.), Hydrogen Effects on materials Behaviour and Corrosion Deformation Interactions, TMS, 2003, pp. 641–647. E. Gamboa, A. Atrens, Fractography of SCC Features for Rock Bolts 2nd International Conference on Environmental Degradation of Engineering Materials, Event No 264 European Federation of Corrosion, June 29 to July 3rd (2003c) Bordeaux, France, ISBN 1-904350-70-0, paper SCC2-2-04b. E. Gamboa, A. Atrens, Laboratory testing of rock bolt Stress Corrosion Cracking, COAL 2003, 4th Underground Coal Operators Conference, University of Wollongong, 12–14 February (2003e) N Aziz and B Kinnimoth eds., publisher Aus IMM, pp. 132–153. A. Atrens, C.C. Brosnan, S. Ramamurthy, A. Oehlert, I.O. Smith, Measure. Sci. Technol. 4 (1993) 1281–1292.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
[32]
S. Ramamurthy, A. Atrens, Corros. Sci. 34 (1993) 1385–1402. Z.F. Wang, A. Atrens, Metall. Mater. Trans. 27A (1996) 2686–2691. J. Salmond, A. Atrens, Scripta Metall. Mater. 26 (1992) 1447–1450. A. Oehlert, A. Atrens, Mater. Forum 17 (1993) 415–429. A. Atrens, A. Oehlert, J. Mater. Sci. 33 (1998) 783–788. N. Winzer, A. Atrens, W. Dietzel, G. Song, K.U. Kainer, Mater. Sci. Eng. A 472 (2008) 97–106. J.Q. Wang, A. Atrens, Eng. Fail. Anal. 11 (2004) 3–18. J. Wang, A. Atrens, Corros. Sci. 45 (2003) 2199–2217. J.G. Williams, C.R. Killmore, F.J. Barbaro, J. Piper, L. Fletcher, Mater. Forum 20 (1996) 13. J. Wang, A. Atrens, J. Mater. Sci. 38 (2003) 323–330. A. Atrens, J.Q. Wang, K. Stiller, H.O. Andren, Corros. Sci. 48 (2006) 79– 92. J.Q. Wang, A. Atrens, D.R. Cousens, J. Mater. Sci. 34 (1999) 1711–1719. J.Q. Wang, A. Atrens, D.R. Cousens, N. Kinaev, J. Mater. Sci. 34 (1999) 1721–1728. J.Q. Wang, A. Atrens, D.R. Cousens, P.M. Kelly, C. Nockolds, S. Bulcock, Acta Mater. 46 (1998) 5677–5687. J. Mougin, B. Ghys, C. Reullon, C. Lemaitre, published in proceedings Eurocorr 2005, paper supplied by [email protected]. J. Mougin, M.S. Cayard, R.D. Kane, B. Ghys, C. Pichard, Sulfide stress cracking and corrosion fatigue of steels dedicated to bottom hole assembly components, Corrosion 2005, paper 05085, NACE, Houston TX. NACE International, Standard test method laboratory testing of metals for resistance to specific forms of environment cracking in H2 S environments, NACE TM0177-96, Item No 21212, NACE, Houston TX.