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ML00150 - Dec.2018

Vibration Induced Fatigue in a SS Oil Piping

Micrography of the weld metal of the pipe to the fitting.

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of the of the to the .
M

Material

Duplex Stainless Steel
  • ASTM A 790 UNS S31803
  • ASTM A 182 grade F51
Ds

Description

After less than 5 years in operation, the vertical 14” duplex line of a (corrosive) oil processing has suffered at the main , associated to the margin of the weld between the pipe and the 2” branching. The system was in operation in a marine environment and the crack started from the external side, induced by vibration. The small defects in positions out of the margin of the weld showed not to be relevant in the fatigue life.

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Chemistry (% in weight)

Pipe analysis

0.033C; 0.41Si; 1.22Mn; 0.015P; 0.001S; 20.83Cr; 5.88Ni; 3.07Mo; 0.012Nb; 0.25Cu; 0.037W; 0.008Al; 0.08V.

Weld Metal

0.029C; 0.49Si; 1.29Mn; 0.010P; 0.001S; 21.20Cr; 8.50Ni; 3.04Mo; 0.006Nb; 0.06Cu; 0.01W; 0.007Al; 0.09V.

Associated Images
Vertical piping in a corrosive oil producing system of a FPSO. The line is a nominal diameter 14" sch 10S (pipe thickness about 5 mm) and the flanged branch line is a 2" nominal diameter welded to the main pipe using a weldolet (forged fitting). The fatigue induced cracking occurred longitudinal to the main pipe, originated at the margem of the weld interface with the pipe.
1481
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Aspect Before AnalysisDf - General View
Macrography of the section transverse to the ND 14" main pipe (that is only 5mm thick) and to the ND 2" branch that is a weldolet fitting welded to the main pipe and flange. This image is interesting because it shows clearly how thin is the pipe compared with the fitting thickness and the large amount of welding required to assure a full penetration weld in the weldolet.
220
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MacroDf - Transverse Section
Transverse section of the 14" pipe in the center of the 2" branch using a weldolet fitting welded to the main pipe. The internal side of the pipe is to the top of the figure and the through wall fatigue cracking is located at the margem of the weld. It is possible to see that it is a full penetration weld and the fitting thickness is several times larger than the pipe thickness.
174
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Electrolytically Etched MacroDf - Crack
Detail of the fatigue cracking, staring from the margin of the weld. The crack propagated though wall of the pipe (5 mm). The larger thickness of the weldolet requires a large amount of deposited weld metal, about 20 mm that is 4 times the pipe thickness. It is an interesting aspect to be considered by designers.
176
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Electrolytically Etched MacroDf - Fatigue Crack
After cutting the pipe and opening the crack, the fracture appearance shows a clean fracture, flat, starting from the margem of the weld of the pipe (right side of the figure), and propagating through wall into the pipe (aproximately 5 mm thickness). The external side of the pipe is to the right side of the figure (presenting the weld finishing) and the internal side of the pipe is to the left side of the figure.
176
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Fracture Surface AspectDf - Fracture
The same fracture appearance of the previous figure, looking at a higher magnification by scanning electron microscopy. The fracture is clean, flat, starting in the right side of the figure, at the margem of the weld and propagating into the pipe thickness. The external side of the pipe is to the right side of the figure and the internal side of the pipe is to the left side of the figure.
154
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Fractography by SEMDf - Fracture
Fracture apperance by scanning electron microscopy of the region where the fatigue crack started, from the piping outside . The fracture is flat, presents some smashed regions close to the surface (to the right side) and there are some thin secondary cracks.
140
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Fractography by SEMDf - Fatigue Fracture
Fracture detail by scanning electron microscopy of the region where the cracking started, from the piping outside . The fracture is flat and shows fatigue striation.
138
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Fractography by SEMDf - Fatigue Striation
Fracture apperance by scanning electron microscopy in the center of the thickness where the crack propagated through wall. The fracture is flat and the striation is not so clear at this magnification.
134
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Fractography by SEMDf - Fatigue Fracture
Fracture detail by scanning electron microscopy of the region cloin the center of the thockness. The fracture is flat and shows fatigue striation.
118
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Fractography by SEMDf - Fatigue Striation
Duplex stainless steel piping, in a region where the 14" ND pipe is only 5mm thick and the branch is a weldolet (forged fitting) welded to the pipe and to the flange, a lot more thicker than the pipe.
107
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Aspect Before AnalysisDf - General View
Micrography of a transverse section of the pipe, presenting a refined duplex structure. The ferrite is blue/red and the austenite is white/yellow.
95
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Duplex structure
Micrography of a transverse section of the pipe, presenting a refined duplex structure. The ferrite is blue/red and the austenite is white/yellow.
88
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Duplex structure
Micrographic detail of the ferritic/austenitic structure of the pipe, in a transverse section. The ferrite is blue/red and the austenite is white/yellow.
84
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Banded Structure
Micrography of a transverse section of the fititng, presenting a coarser structure. The ferrite is blue/red and the austenite is white/yellow.
77
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Coarse Structure
Micrography of a transverse section of the fititng, presenting a coarser structure. The ferrite is blue/red and the austenite is white/yellow.
81
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Duplex structure
Micrography of a transverse section of the fititng, presenting a coarser structure. The ferrite is red/orange and the austenite is white/yellow.
80
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - Nital Etched Micrography by OMDf - Duplex structure
Macrographic detail of the pipe to weldolet weld, and the through wall cracking in the pipe from the margin of the weld. The pipe thickness is only 5 mm (SCH 10S) but the large thickness of the fitting required several weld passes to create a full penetration weld.
84
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MacroDf - Welded Joint
Micrography of the heat affected zone of the pipe to the fitting, at the pipe position, where the weld is in angle to the pipe (welded at the pipe surface). Weld metal to the top.
85
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Heat Affected Zone
Micrography of the heat affected zone of the pipe to the fitting, at the pipe position. Base metal to the bottom of the figure and fusion line to the top.
87
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Heat Affected Zone
Micrography of the heat affected zone (HAZ) of the pipe. The amount of ferrite in the HAZ is a little higher than in the pipe base metal.
86
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Heat Affected Zone
Micrography of the heat affected zone of the pipe to the fitting, at the fitting position. The fitting is welded in the thickness. The fusion line is in the bottom of the figure.
74
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Heat Affected Zone
Micrography of the heat affected zone (HAZ) of the fitting, welded to the pipe. The forged fitting base metal is to the top of the figure and the fusion line is to the bottom.
71
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Heat Affected Zone
Micrography of the heat affected zone (HAZ) of the fitting. The amount of ferrite in the HAZ is a little higher than in the pipe base metal.
70
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Heat Affected Zone
Micrography of the weld metal of the pipe to the fitting. It is possible to see columnar grains and recristalized grains. In the same way, the yellow/white is the austenite and the blue/red is the ferite.
67
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Weld Metal
Micrography of the weld metal of the pipe to the fitting.
63
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Weld Metal
Micrographic detail of the weld metal of the pipe to the fitting. Yellow/white austenite and red/blue ferrite (delta).
59
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Weld Metal
Macrography of a transverse section of the welded joint, at the fatigue crack position in the pipe. It is very interesting to observe that the crack propagated from the margin of the weld, independently of the defects in the weld (porosity and lack of fusion), and without relationship with the unbalanced structure at the cap (with a large amount of ferrite). These characteristics are detailed in the next figures.
89
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MacroDf - Welded Joint
Large pore in the weld metal, that did not influence in the failure.
59
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Pore
Lack of fusion between passes in the weld metal. This defect is small, probably not detectable in any destructive test and not influencing the failure.
52
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Lack of fusion
Lack of fusion at the fusion line (between the weld metal and the base metal). This defect is small, probably not detectable by any destructive test and not affecting the fatigue life.
58
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Lack of fusion
Cap pass in the weld metal, presenting a large amount of delta ferrite (retained) due to a probable quick cool of the pool during welding (since this pass is too thin, perhaps a remelting). Although it is not good to present this microstructure, it did not affect the failure.
54
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Weld Metal
Micrography of the fatigue crack propagating in the pipe thickness. Fully flat and not influenced by any microstructure.
52
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Fatigue Crack
Micrography of the fatigue crack close to the internal wall, where (only) the final through wall rupture is associated to plastic deformation, as detailed in the next figure.
56
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Fatigue Crack
Micrographic detail of the final crack propagation through wall, with plastic deformation.
62
VIEW
ML00150 / Dec.2018
Vibration Induced Fatigue in a SS Oil Piping
M - Duplex Stainless SteelP - PipingAp - FPSOCd - Vibration Induced Fatigue
Cm - NaOH Electrolytic Etched MicroDf - Plastic Deformation