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ML00126 - Mar.2018

Hydrogen Fracture of Subsea Allen Bolts

Aspect of the fracture surface of the bolt, after cleaning with a plastic brush. Although there is a layer of oxides it is possible to see the intergranular morphology of the fracture, typical of a hydrogen embrittlement mechanism.

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Aspect of the surface of the , after cleaning with a plastic brush. Although there is a layer of oxides it is possible to see the intergranular morphology of the , typical of a mechanism.
M

Material

Low Alloy Medium Carbon QT
  • ISO 898-1 class 12.9
  • UNS G41300
  • AISI 4130
Ds

Description

The control pod was operating with 8 Allen in environment for 5 years and, during ROV inspection, it was identified that they were all ruptured, 2 of them were lost. The metallurgical analysis showed that all bolts presented a flat with integranular morphology due to , because this 12.9 strength class (with 40 ) is not compatible with subsea .

C

Chemistry (% in weight)

Bolt analysis

0.31C; 0.22Si; 0.69Mn; 0.013P; 0.012S; 0.90Cr; 0.01Ni; 0.16Mo.

Associated Images
Aspect of the fracture surface of the bolt, after cleaning with a plastic brush. Although there is a layer of oxides it is possible to see the intergranular morphology of the fracture, typical of a hydrogen embrittlement mechanism.
1154
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Fractography by SEMDf - Intergranular Fracture
Detailed aspect of the fracture surface of the bolt, evidencing the intergranular morphology of the fracture (although with an oxide scale), typical of a hydrogen embrittlement mechanism.
114
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Fractography by SEMDf - Intergranular Fracture
Aspect of the broken bolt, 5/8" Allen, socket head cap screw, originally 1 3/4" long, broken in subsea service under cathodic protection. The bolt presents a zinc coating.
116
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Aspect Before AnalysisDf - General View
Micrography of the longitudinal section of the bolt showing a martensitic structure, with segregation bands evidenced by the chemical etch.
99
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Banded Structure
Micrography of the longitudinal section of the bolt at the bottom of the thread fillet, showing the uniform martensitic structure, not strain hardened.
90
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Uniform Structure
Micrography of the longitudinal section of the bolt at the top of the thread fillet, showing a small circunferential cracking typically of a rolling process to manufacture the thread. The thread presented no evidences of plastic deformation suggesting that the heat treatment was applied later. It is important to mention that this cracking presented no influence in the fracture of the bolt.
101
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Rolled Thread
Micrography of the longitudinal section of the bolt at the bottom of the thread fillet, in the "as polished" condition, showing the zinc flakes coating layer, applied by a painting process that do not create metallurgical bonding with the substrate.
82
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - As polished micrographyDf - Coating Layer
Micrographic detail of the zinc flakes coating layer in the "as polished" condition of a longitudinal section of the bolt.
71
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - As polished micrographyDf - Coating Layer
Micrography of the longitudinal section of the bolt in the center of the thickness (ND 5/8") showing a uniform martensitic structure.
67
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Martensite
Micrographic detail of the martensitic structure, uniform in the longitudinal section of the bolt, in the center of the thickness (ND 5/8"). The hardness values of the bolts varied from 39 to 41 HRc.
63
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Martensite
Aspect of the fracture of a small Allen bolt, nominal diameter 5/8" 1 3/4" long, class 12.9, that was operating in subsea environment during 5 years. The fracture is rusted but the thread still presents the zinc coating.
81
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Aspect Before AnalysisDf - General View
Micrography of the longitudinal section of the bolt, at the fracture (top of the figure), starting from the bottom of the thread (to the left side of the figure). The presence of the coating at the bolt surface shows that the fracture occurred without corrosion. The rusted aspect of the fracture is associated to corrosion products after the rupture has occurred, also suggesting that the bolts were fractured and remained in service for some time after the failure occurred.
65
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - As polished micrographyDf - Fracture
Micrography, after NItal etching, of the longitudinal section of the bolt, at the fracture (top of the figure), starting from the bottom of the thread (to the left side of the figure). The presence of the coating at the bolt surface shows that the fracture occurred without corrosion.
56
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Fracture
Micrography of the longitudinal section of the bolt, in the center of the thickness, showing at the fracture surface (top of the figure), secondary cracks that are perpendicular to the fracture, associated to segregation bands.
55
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Secondary Cracks
Micrographic detail of the longitudinal section of the bolt, in the center of the thickness, showing the secondary cracks associated to segregation bands.
63
VIEW
ML00126 / Mar.2018
Hydrogen Fracture of Subsea Allen Bolts
M - Low Alloy Medium Carbon QTP - Coated ProductAp - SubseaCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Secondary Cracks