Fehleranalyse

Shell and tube heat exchanger

Was ist passiert:

Ausfall des Rohrbündelwärmetauschers

Analysis performed/conclusions drawn:
Chemical analysis of the material: In both cases the material is identified as AISI 347 (UNS S34700) according to ASTM A240
SEM-EDS microanalysis: SEM-EDS analysis conducted on the products present on the surface of the retaining plate indicated an operating environment rich in chlorides.
Metallographic analysis: base material – anomalous plastic deformation of the retaining plate
Metallographic inspection: analysis of the damage – Presence of ramified hairline cracks, typical of stress corrosion cracking (SCC)

Report on the cause of failure:
Failure caused by stress corrosion resulting from the simultaneous presence of anomalous internal stresses (high plastic deformation found in the plate) and a harsh working environment (products rich in chlorides).
Possible solutions
– Adequate storage
– Improved working environment
– Elimination of internal stresses (stress relieving or solubilisation)

What information do we have:
● Material: UNS S34700 (AISI 347)
● Type of damage: breakage of a retaining plate (6 mm thick)
● Additional information:
– 3 months in operation
– no heat exchanger leaks,
no external stresses

Worm screw failure

Cosa è successo:
Failure analysis di vite senza fine

What information do we have:
● Material: 44SMn28 steel
● Type of damage: brittle breakage of a screw thread

Analysis performed/conclusions drawn:
Chemical analysis of the material: Material steel identified as 44SMn28 according to EN 10087
Fractographic examination: Strongly oriented ductile fracture
Tensile test: Material complies with the customer’s technical specifications (44SMn28 cold drawn, untempered steel)
Metallographic tests:
● inclusional state – Presence of non-metallic MnS inclusions of considerable size oriented in the direction of mechanical processing
● microstructure of the material – Microstructure consisting of ferrite and perlite arranged in bands. Absence of a neutral hardening heat treatment
● Fracture profile without metallographic etching – «Linear» fracture in the same direction as the non-metallic inclusions
● Fracture profile with metallographic etching – Plastic deformation in the fracture zone (mechanical failure due to overload). The fracture appears to cross the ferritic/pearlitic bands
Report on the cause of failure:
Metallurgical factor: Ferritic/pearlitic band structure (absence of a neutral hardening treatment) and high level of inclusions (large and oriented inclusions).
– Excellent longitudinal resistance of the material but poor transversal resistance
Geometric factor: particular shape of the thread (high thread height, large pitch) with consequent orthogonal stresses to the mechanical machining direction of the component
– high flex stresses on the thread

Possible solutions
– Evaluate whether the geometry of the component (with consequent flex stresses on the thread) allows free-machining steels to be used.
– Carry out in any case a neutral hardening heat treatment (to provide a more homogeneous structure and enhanced material toughness).

Fractured luminaire

Cosa è successo:
Corpo illuminante fratturato

What information do we have:
● Material:
Die-cast aluminium alloy
● Type of damage:
failure in the vicinity of the screw fixing holes of the luminaire support

Analysis performed/conclusions drawn:
Chemical analysis of the material: Material identified as aluminium alloy EN AC-46500 AlSi9Cu3 (Fe) (Zn).
Fractographic examination: Brittle fracture with the presence of oriented facets
Metallographic tests:
● base material – Microstructure consisting of alpha phase surrounded by a eutectic and the presence of large needle-shaped precipitates
● fracture profile – Morphology of the failure profile typical of a fragile fracture, with no plastic deformation and evidence of the presence of needle-shaped platelets
Report on the cause of failure:
Excessive intrinsic fragility of the material the support is made of due to its metallurgical state (presence of coarse, needle-shaped crystals).
– the presence of these very fragile crystals constitutes a dangerous discontinuity in the metallic matrix. During production, this alloy requires a „modification“ treatment designed to reduce the size and to round off the silicon crystals in the matrix; in the case in question, this treatment doesn’t seem to have been completely successful.

Broken spring

Cosa è successo:
Molla fratturata

Analisi eseguite / cosa abbiamo capito:
Esami macrografici:
● Presenza di abrasioni in corrispondenza dell’intradosso della curvatura originate nel processo
di produzione
● Aspetto macrografico della frattura tipico di una rottura per fatica
● Linee di spiaggia chiaramente visibili.
● Presenza di una discontinuità centrale (ratchet mark).

Esame frattografico al SEM:
● Analisi della superficie di frattura a elevati ingrandimenti.
● Individuazione di due punti di innesco principali in corrispondenza dell’intradosso e di due
inneschi secondari all’estradosso.
Esame micrografico:
● Microstruttura fortemente incrudita e orientata nel senso di trafilatura, priva di anomalie
metallurgiche.
Relazione sulla causa del cedimento:
– Rottura innescata all’intradosso a causa della presenza di irregolarità superficiali dovute al
processo di fabbricazione.
– Sviluppo di due fronti di cricca che hanno generato la discontinuità centrale (ratchet mark)
con propagazione a fatica della rottura.
– La diminuzione della sezione resistente ha causato un innalzamento dello stato di tensione
con conseguente innesco di cricche anche all’estradosso.

Analysis performed/conclusions drawn:
Macrographic examinations:
● Presence of abrasions in correspondence of the intrados curve caused during the production process
● Macrographic appearance of the fracture typical of a failure caused by fatigue
● Clearly visible beach marks.
● Presence of a central discontinuity (ratchet mark).

SEM fractographic examination:
● Analysis of the fracture surface at high magnification.
● Identification of two main trigger points at the intrados and two secondary triggers on the extrados.
Micrographic examination:
● Strongly work hardened microstructure oriented in the direction of wire drawing, no metallurgical anomalies.
Report on the cause of failure:
– Breakage triggered in the intrados due to the presence of surface irregularities caused during the manufacturing process.
– Development of two crack fronts that generated the central discontinuity (ratchet mark) with fatigue propagation of the break.
– The decrease in the size of the resistant section has caused an increase in the state of tension with consequent triggering of cracks along the extrados as well.

What information do we have:
● Material: AISI 302 stainless steel
● Type of damage: failure at one of the curvatures
● Additional information: failure after 3700 km; component obtained by drawing and bending; presence of components with similar breaks

Corroded tank

Cosa è successo:
Serbatoio corroso

What information do we have:
● Material:
5xxx series wrought aluminium alloy
● Type of damage: Extensive corrosion of the metal underneath the installation sheet
● Additional information: Tank installed on a wheeled vehicle for industrial transport by means of an EPDM sheet

Analysis performed/conclusions drawn:
Visual and macrographic tests:
● Presence of violent corrosion in the form of pitting of variable shapes and extensions
● Presence of evident dark-coloured deposits caused by corrosion (in relief) and opaque white colouring (inside the pitting)

SEM observations and EDS microanalysis:
● Following chemical analysis, the material was confirmed to be EN AW-5754 aluminium alloy, as stated.
● The deposits inside the pits were found to be linked to aluminium sulphate Al 2 (SO 4) 3.
● At some sites within the pits, it could be seen that corrosion was not purely selective in nature.
Micrographic analysis:
● The corrosion presented as subsurface and undercutting pitting with development in all directions and with a depth that in some cases was found to be almost half the thickness of the wall.
● The microstructure of the base material was found to be a typical aluminium alloy of the 5000 series free from metallurgical anomalies.
● The observations confirmed that the corrosive attack was not selective in nature.

Report on the cause of failure:
– In polluted environments typically found of many industrial areas, the sulphur dioxide (SO2) present in the environment plays an important role in corrosion processes and gives rise to the formation of aluminium sulphate (a salt which is highly damaging for aluminium and which tends to increase the rate of corrosion). It’s also possible that the sheet (sulphur vulcanised EPDM) has released substances and compounds that have intensified the observed phenomenon.
– In addition, the presence of contact areas between the sheet and tank have favoured the corrosion mechanism known as Crevice Corrosion (corrosion in confined spaces), which has led to a severe and rapid degradation of the material, especially in curved areas (greater state of deformation).