Engineering Failure Analysis 38 (2014) 49–57

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Engineering Failure Analysis

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Failure analysis on a fractured 34CrMo4 steel high pressurecylinder filled with a mixture of inert gases

1350-6307/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.engfailanal.2014.01.004

⇑ Corresponding author. Tel.: +39 0620943237; fax: +39 062052358.E-mail address: a.alvino@inail.it (A. Alvino).

Antonello Alvino ⇑, Alessandra Antonini, Daniela LegaINAIL – Settore Ricerca, Certificazione e Verifica, Dipartimento Tecnologie di Sicurezza, Via del Torraccio di Torrenova 7, 00133 Roma, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 October 2013Received in revised form 25 November 2013Accepted 13 January 2014Available online 22 January 2014

Keywords:Failure analysisStress corrosion crackingInert gasGas cylinderLow alloy steel

After 10 years of service, one high-pressure cylinder made by 34CrMo4 low alloy steel frac-tured catastrophically, causing extensive damages. The cylinder was filled with a commer-cially available mixture of nitrogen, argon and carbon dioxide. A failure analysis has beencarried out in order to identify the cause of rupture. The examination of cross sectionsspecimens has revealed several branched cracks originating at cylinder inner wall sideand propagating across the bulk. The observation of fracture surfaces supported thehypothesis of an environmental attack that affected the cylinder inner surface. No evi-dences of other damage form have been found: the chemical analysis, the microstructureexamination and the mechanical tests performed provided results in full compliance withthe technical specifications of the alloy. On the basis of such considerations, it can beassumed that the cylinder fractured by stress corrosion cracking. The exposition to aCO/CO2 containing atmosphere in presence of undesired traces of moisture promoted thelocal damage, as long as the applied tensile stress triggered cracks formation and propaga-tion. Then, a progressive reduction of the effective toughness occurred, leading to a sudden,catastrophic, overload rupture.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

34CrMo4 is a widespread chromium–molybdenum steel showing good mechanical properties. Its balanced chemicalcomposition and a proper heat treatment provide a typical fine tempered structure. This low alloy carbon steel is widely usedin gas cylinders manufacture, since it ensures good performances at high pressures, in terms of high strength, resistance andductility [1]. Nevertheless, this material may be sensitive to stress corrosion cracking (scc), a serious problem for high pres-sure vessels, because it causes unexpected catastrophic ruptures. The damage starts typically with crack initiation at selectedsites (stage 1 propagation); then a steady-state (or stage 2) crack propagation follows and, finally, a stage 3 propagation or anoverload fracture of the remaining section occurs [2]. Aqueous chlorides, hydrogen sulphide, hydroxides, nitrates, ammoniaand carbonate solutions are the most common environmental factors that promote cracks formation and propagation on lowalloy steels [3], but stress corrosion can also occur in presence of other chemical substances, though less frequently. Carbondioxide may cause cracking if water and CO are simultaneously present, even in trace concentrations [4–9]. Some ruptureshave been described in CO2–CO–H2O environment [4,6,10] and in particular a Mn–steel cylinder containing a CO–CO2

mixture was reported to explode in 1970 [7,11].Carbon dioxide is successfully employed, in mixture with non toxic non-combustible (inert) gases such as nitrogen and

argon, in gaseous fire suppression applications for sensitive electrical and electronic instruments and equipments.

Table 1Batch specifications of the examined cylinders.

External diameter (mm) Total length (mm) Wall thickness (mm) Volume (l) Material Heat treatment

267 1735 6.40 80.0 34CrMo4 Quenched and tempered

Table 2Results of chemical analysis performed on fractured and reference cylinder, compared to the nominal composition fixed by the standard in force when thecylinders were built.

C (%) Mn P S Cr Mo Si

EN 10083-1:1998 0.30–0.37 0.60–0.90 60.035 60.035 0.90–1.20 0.15–0.30 60.40Reference 0.34 0.85 0.012 0.0006 1.13 0.22 0.24Fractured 0.31 0.85 0.012 0.0007 1.12 0.22 0.23

Table 3Tensile strength test results for fractured and reference cylinder. Rs = yield strength; Rm = ultimate tensile strength.

Rs (N/mm2) Rm (N/mm2) Elongation (%)

Nominal values according to UNI EN 10083-1:1998 800 1000–1200 14.0Reference 934 1004 15.0

942 1013 15.0Fractured 931 1005 15.0

935 1007 16.5

Table 4Brinell hardness test results for fractured and reference cylinder.

HB hardness Reference

337 337 337 345337 345 345 345337 337 337 337

Mean 337 340 340 342

Fractured

329 337 345 337 337 321 329 337345 337 337 345 337 321 329 337337 345 337 337 337 321 329 337

Mean 337 340 340 340 337 321 329 337

Table 5Charpy impact test results for fractured and reference cylinder; according to the italian law in force when the cylinders were built, minimum required valuesare 40 J/cm2 for each specimen and the mean value of all the measures must be higher than 50 J/cm2.

Impact test values (J/cm2) Mean (J/cm2)

Reference 120 113 119 118 118 115 117Fractured 87 86 88 93 84 87 88

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After 10 years of service, one high-pressure cylinder filled with a commercially available mixture of nitrogen, argon andcarbon dioxide fractured catastrophically into three fragments, causing extensive damages. This paper reports the detailedfailure analysis carried out in order to identify the nature of the accident and the cause of the rupture.

2. Materials and methods

All the measures and experiments carried out on the fractured cylinder have been performed also on a non-fractured ves-sel from the same batch production, having the identical service life of the exploded one. The batch technical specificationsare reported in Table 1. The fractured cylinder was storing INERGEN, a compressed mixture of nitrogen, argon and carbondioxide (52%N2, 40%Ar, 8%CO2). The total pressure was 200 bar.

Fig. 1. The three fragments from the fractured cylinder (a, b and c respectively) and the reference intact cylinder.

A. Alvino et al. / Engineering Failure Analysis 38 (2014) 49–57 51

Chemical analysis was carried out using a Thermo ARL 3460 optical emission analyzer according to ASTM E 415-08 stan-dard [12]. For macro observations a Leica M125 stereomicroscope was used. Metallographic analyses were performed usingoptical microscope (LOM) and scanning electron microscope (SEM). Metallographic specimens were taken from each frag-ment to examine bulk material and fracture surface. The cutting was performed by a precision saw (Buehler Isomet4000). Specimens were mounted in an automatic mounting press (Buehler Simplimet 1000) and carefully polished usingan automatic grinder–polisher system (Buehler Phoenix Beta) using a series of grit papers and finished by diamond polishingdown to 1 lm. Afterwards, polished areas were etched with a ferric chloride aqueous solution, according to ASTM E 407-07standard [13] and examined first under a Nikon Eclipse ME600 optical microscope. Some samples were examined inun-etched conditions too. Then, SEM analysis was carried out with a Fegsem Zeiss Ultra Plus instrument.

Destructive tensile tests, Brinell hardness tests (HB2.5/187.5/20) and kV Charpy tests were conducted on specimensmachined from both the fractured and non-fractured cylinder. Specimens from the exploded cylinder were cut away fromthe fractures. Tensile tests were carried out in a W+B 60 ton capacity test machine, at room temperature, according toUNI EN ISO 6892-1:2009 standard [14]; hardness tests were carried out on a Wolpert durometer according to UNI EN ISO6506-1:2006 standard [15] and Charpy tests were performed using a 300 J impact test machine Zwick/Roll according toUNI EN 10095-1:1992 standard [16].

3. Experimental

3.1. Chemical analysis

The results of chemical analysis, reported in Table 2, are in full agreement with the nominal chemical composition fixedby the standard in force when the cylinders were built [17]. Moreover, no significant differences between the reference andexploded cylinder material have been found.

3.2. Mechanical tests

The results of tensile tests, reported in Table 3, satisfy the requirements of the standard in force when the cylinders werebuilt [17] and show no significant differences between the reference and fractured material. Brinell hardness and kV Charpytest results, reported in Tables 4 and 5 respectively, are in agreement with values fixed by the Italian law in force when thecylinders were built.

a

b c

d e

zone 2

zone 1

zone 1

zone 2

2 mm

3 mm

zone 1

zone 2

zone 1

zone 2

zone 1

zone 2

2 mm

Fig. 2. Macrographs of a longitudinal fracture evidencing the two zones with different fracture orientation and appearance: (a) cross section; (b) overview;(c) macro of the fracture surface.

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3.3. Visual and macro examination

The visual and macro examination of fractured cylinder showed that it was fractured into three fragments, classified as ‘‘A‘‘B’’ and ‘‘C’’ respectively. The fragments are shown in Fig. 1, as well as the reference cylinder. The fractures propagatedmainly along two different directions: circumferentially (i.e. perpendicular to the cylinder axis) and longitudinally (parallelto the cylinder axis). No appreciable thickness reductions have been observed.

The appearance and characteristics of fracture surfaces are remarkably related to the fracture orientation. Generally, cir-cumferential fractures appear shiny and propagate along the entire thickness of the cylinder following a 45� oriented plane.

Longitudinal fractures have, instead, a more complex aspect, as evidenced by the macrographs reported in Fig. 2: two dis-tinct zones can be easily detected, moving from the internal wall side to the external surface. In the first zone (named zone 1in Fig. 2a) the fracture is oriented orthogonally with respect to the cylinder axis. The material is dark coloured and seems tobe affected by an environmental attack (Fig. 2b–e); many micro- and macro-cracks have been detected. In the second zone(named zone 2 in Fig. 2a) the fracture is oriented at approximately 45� in the wall thickness and appears smooth and plain.Plastic deformation is almost absent in both zone 1 and zone 2.

Fig. 3. The damaged area on the inner surface of the fractured cylinder: (a) and (b) fragment A, overview of branched macrocracks propagating fromfracture surfaces; (c) and (d) fragment A, macrographs evidencing cracks and metal damage; (e) fragment B, overview of the dark coloured zone (redrectangle) with cracks; (f) fragment B, macrograph showing the darker zone near the longitudinal fracture (on the right). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. The fine tempered microstructure of 34CrMo4 steel from the fractured (a) and reference cylinder (b) respectively. LOM micrographs of cross sections,etching: FeCl3 solution.

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Fig. 5. Branched cracks and microcracks originating from the cylinder inner wall side on fragment A and B. LOM micrographs of cylinder cross sections; un-etched specimens.

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The inner wall side of fragment A shows clear evidences of branched macro-cracks originating from fracture surfaces(Fig. 3a–d): this is by far the most damaged area observed on the cylinder: the metal is severely cracked and appears opaque,due to the presence of corrosion products.

The inner wall side of fragment B shows a dark coloured zone of approximately 100 mm width, spanning from the lon-gitudinal fracture and covering the entire length of the fragment. Some cracks are visible (Fig. 3e–f), though the damage doesnot seems to be as massive as observed for fragment A.

No evidences of cracks or damage have been found on internal surface of fragment C.

3.4. Microscopy

Samples taken from both the fractured and the reference cylinder show an almost identical microstructure. A typical finetempered structure is observable, as consequence of a proper quenching and tempering heat treatment (Fig. 4). No metal-lurgical defects have been found; non-metallic inclusion test, performed according to UNI EN 10247 [18] standard revealedno significant indications.

Some specimens cut from cross sections of fragment A and B show severe cracking at the inner surface (Fig. 5). Thesecracks, easily observable by LOM measures, are often branched and propagate in the bulk material for 20–30% of thickness.SEM investigations detected a large number of microcracks, as well as an extensive damage suffered by the metal at crackssurfaces (Fig. 6). Furthermore, corrosion products, such as iron and manganese oxides, have been detected by EDX analysis(Fig. 7) on the attacked zones and into the cracks.

4. Discussion

The results of chemical analysis and mechanical tests, carried out for both the reference and the exploded cylinder, showthat the measured values are in full compliance with the technical specifications. Microstructural examination also shows

Fig. 6. Branched cracks and microcracks originating from the cylinder inner wall side on fragment A and B. Scanning electron micrographs of cylinder crosssections; etching: FeCl3 solution; (a) and (b) overview of cracks; (c)–(e) crack tips at different magnifications; (f)–(h) micrographs evidencing the severedamage suffered by the metal at cracks surfaces.

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good consistence with the standard requirements of such alloy, which exact an homogeneous fine tempered structure. Theresults of the inclusions analysis are satisfactory, since no indications have been detected.

Some metallographic samples taken from the exploded cylinder show, however, clear evidences of branched cracks thatoriginate at the inner surface and propagate across the bulk, suggesting that an environmental attack, such as stress corro-sion cracking, may have affected the material. Stress corrosion damage, in fact, is generally characterized by extensivebranched cracks that propagate orthogonally with respect to the applied stress [19].

This hypothesis is also supported by the results of the observations of fracture surfaces: its well known, in fact, that a sccfracture surface shows easy detectable crack initiation sites, a slow crack propagation zone and a final rupture by tensileoverload; the area of slow crack growth may usually be stained or discoloured with respect to the area of final fracture[20]. These typical features have been detected on the fractures which propagated longitudinally along the cylinder: if ob-served in the cylinder cross section, they show a bi-layer aspect (Fig. 2). This appearance reflects the corrosion on the mate-rial in terms of alteration of its chemical, physical and mechanical properties: starting from the inner wall side, in fact, thefirst zone encountered shows clear evidences of an environmental attack, i.e. cracks, porosities and corrosion products; then

Fig. 7. SEM image and EDX element mapping on the crack surface from a metallographic specimen cut from fragment A; the relative abundance of eachelement is shown as colour intensity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)

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a second layer follows, appearing as a surface that suffered a brittle overload fracture, caused by a fatal reduction of the cyl-inder toughness.

The fracture origin can be reasonably located in the upper part of the cylinder (fragment A) – where the damage is ex-tremely severe and a large number of cracks is visible even at the naked eye (Fig. 3a–d) – along the main longitudinal frac-ture surface. The attack started on the cylinder inner wall; only a restricted area was affected. The crack tips advanced in thebulk material until a catastrophic overload rupture suddenly occurred.

The aggressive environment that triggered stress corrosion cracking must be found in the gaseous CO2–N2–Ar mixturestored in the cylinder. Nitrogen and argon are not aggressive chemicals able to promote scc. Carbon dioxide, however,may cause stress corrosion cracking in low alloy steels, but only if traces of CO and H2O (even few ppm) are present, too[4–11]. The quality grade of CO2 stored into the fractured cylinder was not high enough to exclude the presence of CO asimpurity; moreover, an undesired water presence into the cylinder could be likely hypothesized, as moisture traces acciden-tally introduced during filling and/or storage operations.

The mechanism is proposed to be local dissolution of iron due to the carbonic acid formed between water and carbondioxide, with general corrosion being inhibited by carbon monoxide [10], leading first to the formation of branched cracksand then to a fatal rupture.

This supposition has been corroborated by the evidence that no other forms of damage have been detected in the materialof both the exploded and the intact reference cylinder.

5. Conclusions

Several branched cracks have been detected on samples taken from the fractured cylinder: they initiated at the inner wallside and then propagated radially into the bulk, suggesting that the steel suffered by stress corrosion cracking. The observa-tion of longitudinal fracture surfaces supported the hypothesis of an environmental attack that affected the cylinder’s innerwall.

No evidence of other damage has been found: the chemical analysis of the alloy and the mechanical tests performed onboth exploded and reference cylinder gave results that are in full compliance with the technical specifications. The micro-structure examination also indicated that the steel has been subjected to a proper heat treatment, since a typical finetempered structure has been clearly identified in all specimens.

On the basis of such considerations, it can be assumed that the examined gas cylinder fractured by stress corrosion crack-ing. The damage initiated in the upper part of the cylinder, on the inner wall side; cracks then propagated deeply into the

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bulk, as well as along the inner surface, leading to a progressive reduction of the effective cylinder toughness, until a sudden,catastrophic, overload rupture occurred.

References

[1] ISO 11114-1 Transportable gas cylinders – compatibility of cylinder and valve materials with gas contents – Part 1; Metallic materials.[2] Jones RH, Ricker RE. Mechanisms of stress corrosion cracking in stress corrosion cracking – material performance and evaluation. In: Russel H. Jones,

editor. Stress Corrosion Cracking – Material Performance and Evaluation. ASM International. ISBN 0-87170-441-2.[3] Ciaraldi SW. Stress corrosion cracking of carbon and low-alloy steels (yield strengths less than 1241 MPa) in stress corrosion cracking – material

performance and evaluation. In: Russel H. Jones, editor. Stress Corrosion Cracking – Material Performance and Evaluation. ASM International. ISBN 0-87170-441-2.

[4] Brown A, Harrison JT, Wilkins R. Trans-granular stress corrosion cracking (s.c.c.) of ferritic steels. Corros Sci 1970;10:547–8.[5] Brown A, Harrison JT, Wilkins R. Electrochemical investigation of stress corrosion cracking of plain carbon steel in the CO2–H2O system. In:

International conference on stress corrosion cracking and hydrogen embrittlement of iron base alloys. France: Unieux-Firminy; 1973. p. 680.[6] Kowaka M, Nagata S. Transgranular stress corrosion cracking of mild steels and low alloy steels in the H2O–CO–CO2 system. Corrosion 1968;24:427–8.[7] Kowaka M, Nagata S. Stress corrosion cracking of mild and low alloy steels in CO–CO2–H2O environments. Corrosion 1976;32:395–400.[8] Traversa E, Calderon T. Electrochemical investigation on carbon steel behavior in CO–CO2–H2O environment for the interpretation of the scc

mechanism. Werkst Korros 1991;42:35–40.[9] EIGA IGC doc 95/12/E. Avoidance of failure of CO and CO/CO2 mixtures cylinders.

[10] Naito K, Hashimoto T, Kihara T. Stress corrosion cracking of mild steel in the gas treating plant. J Jpn Pet Inst 1971;14:672–8.[11] Report of the safety of high pressure gas association Japan. 1978; No 73:8.[12] ASTM E415-08. Standard test method for atomic emission vacuum spectrometric analysis of carbon and low-alloy steel.[13] ASTM E 407-07. Standard practice for microetching metals and alloys.[14] UNI EN ISO 6892-1:2009. Materiali metallici – Prova di trazione – Parte 1: Metodo di prova a temperatura ambiente.[15] UNI EN ISO 6506-1:2006. Materiali metallici – Prova di durezza brinell – Parte 1: Metodo di prova.[16] UNI EN 10045-1:1992. Materiali metallici. Prova di resilienza su provetta Charpy. Metodo di prova.[17] UNI EN 10083-1:1998. Acciai da bonifica – Condizioni tecniche di fornitura degli acciai speciali.[18] UNI EN 10247:2007. Micrographic examination of the non-metallic inclusion content of steels using standard pictures.[19] ASM handbook. Failure analysis and prevention. vol. 11. p. 1740.[20] ASM handbook. Failure analysis and prevention. vol. 11. p. 1767–8.