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ROOT CAUSE ANALYSIS AND CORRECTIVE MEASURES OF
DISSIMILAR WELD JOINT FAILURE IN A HIGH PRESSURE STEAM
PIPE
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PROBLEM DEFINITION


During the operation of a petrochemical plant (located in Middle East), recurring
appearance of cracks in dissimilar metal welding joint of a critic, high pressure steam
pipe has been detected (2010-2014).
Cracks appear periodically over the same area, which is the butt weld joining two pieces
made of different materials A-335-P91 and SS347H. Weld material is INCONEL625.
24” diameter and great thickness due to severe operating conditions: tª=530ºc, p=110 bar
None of the reparations conducted so far has been effective to stop recurring breakage
after 6-12 months.
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ROOT CAUSE ANALYSIS AND CORRECTIVE MEASURES OF DISSIMILAR WELD JOINT FAILURE IN A HIGH PRESSURE STEAM PIPE | CADE Engineered Technologies
(https://cadeengineering.com)
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Fig. 1: 24″ High pressure steam pipe actual disposition and welding features
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ROOT CAUSE ANALYSIS AND CORRECTIVE MEASURES OF DISSIMILAR WELD JOINT FAILURE IN A HIGH PRESSURE STEAM PIPE | CADE Engineered Technologies
(https://cadeengineering.com)
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Fig. 2: Failures sequence
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METHODOLOGY
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Design and Fabrication Data:
Welding detail
Welding process
Pre and Post welding thermal treatments
Materials (properties, storage…)
Inspections and NDT
Operation Data
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Previous observations and Evidences
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Failures record and details (critical area pictures, crack pictures…)
Macrographs, metallography, hardness tests, traction tests, chemical composition.
(https://cadeengineering.com)
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Regular Operation
Pipe and welding on operation conditions
Welding process
Residual stresses
Best Mechanical and Process Engineering Practices
Simulation: FEA
International Codes (ASME, FFS)
Physical Modelling (understanding how it works)
Tests and Hypothesis Modelling
Corrective Measures Modelling
2.1: DETAIL LEVEL 1
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Piping and welding analysis under operation conditions
2.2: DETAIL LEVEL 2
Welding process simulation
2.3. DETAIL LEVEL 3
Tests to obtaining residual stresses
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Observations and Evidence
Laboratory Tests
Root Cause Analysis and Determination of Failure Cause
Specific proposal of corrective measures
Modification of welding detail + Pipe spool
PWH
Modification of welding process and pass sequences
Modification of welding filler material
1. PROBLEM AWARENESS
1.1 Failure Identification

⇒ FAILING EVERY 6-12 MONTHS
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ROOT CAUSE ANALYSIS AND CORRECTIVE MEASURES OF DISSIMILAR WELD JOINT FAILURE IN A HIGH PRESSURE STEAM PIPE | CADE Engineered Technologies
(https://cadeengineering.com)
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1.2 Location and Size Craks
CIRCUMF. between 100º-270º
LENGTH. 250mm-400mm
Crack on melting area
buttering Inconel 625 – P91
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1.3 Inspections and NDT
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Full cracks VT and superficial cracks (PT-NDE)
Internal part of the pipe in crack location ⇒ inadequate filling
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1.4 Fractography, Steroscopic Examination and Chemical
Composition
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Sreroscopy ⇒ oxide deposits ⇒ CAPING PASS
Fractography ⇒oxide fragments on internal and external surfaces
Chemical composition ⇒acording to materials specifications requirements
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Chemical composition ⇒acording to materials specifications requirements
(https://cadeengineering.com)
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1.5 Metalography
Tempered martensite structure in P91, HAZ
Dendritic austenitic structure in INCONEL and Buttering
Tempered martensitic structure with ferrite in crack location area
Presence of nucleation of voids and spheroidal graphite
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1.6 Hardness Test
Identification of high hardness points in HAZ areas of P91 and INCONEL, coincident with the
location of cracks
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(https://cadeengineering.com)
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1.7 Welding Process And Heating Treatments
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Warm-up 200ºC
Welding GTAW
Welding 5g Weld / 2G buttering
PWHT in buttering ⇒ no PWHT in welding
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1.8 Operation Conditions And Support Systems
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Support Systems and stress Calculation with no anomalies detected
Insulation temperatures are higher in leakages area
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(https://cadeengineering.com)
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2. PHYSICAL CHARACTERIZATION
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2.1 Detail Level 1- Pipe & Weld Analysis under Operation
Conditions
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2.2 Detail Level 2- Welding Process Simulation
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Transient Thermal Analysis (Análisis Térmico Transitorio)
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(https://cadeengineering.com)
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2.3 High Pressure Steam Pipe Failure Causes


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SOLUTION: CORRECTIVE MEASURES
(https://cadeengineering.com)
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Installation of Spool Pipe section between P91 and
SS347H
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


Definition of new PWHT
Optimal PWHT treatment on weld area is defined
⇒ Decreasing residual stresses, decreasing material hardness, improving
toughness
Spacial care with reached temperatures (do not exceed 760ºC)
⇒ Fragilization issues
Verification of residual stresses by means of FEA, and physical tests

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Modification of Welding Pass Sequence
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ABOUT THE PROJECT


Solution proposed has solved a critical and recurring problem which involved a high
cost in terms of plant availability.
Root cause analysis and project execution was subjected to hard constraints of
time, and had to be solved over a 2-months period (remaining time until
Turnaround).
Solid and consistent methodology thanks to simulation.
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3D POINT CLOUD GENERATION TO MODIFY EXISTING SYSTEMS
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