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.
Fig. 1: 24″ High pressures steam line actual disposition and welidng features
Methodology
Design and Fabrication Data:
- Welding detail
- Welding process
- Pre and Post welding thermal treatments
- Materials (properties, storage…)
- Inspections and NDT
Operation Data
- Previous observations and Evidences
- Failures record and details (critical area pictures, crack pictures…)
- Macrographs, metallography, hardness tests, traction tests, chemical composition.
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
- 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
Observations and Evidence
- Laboratory Tests
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
1.2 LOCATION AND SIZE OF CRACKS
- CIRCUMF. between 100º-270º
- LENGTH. 250mm-400mm
- Crack on melting area
- buttering Inconel 625 – P91
1.3 INSPECTIOSN AND NDT
✔ Full cracks VT and superficial cracks (PT-NDE)
✔ Internal part of the pipe in crack location ⇒ inadequate filling
1.4 FRACTOGRAPHY, STEROSCOPIC EXAMINATION AND CHEMICAL COMPOSITION
✔ Sreroscopy ⇒ oxide deposits ⇒ CAPING PASS
✔ Fractography ⇒oxide fragments on internal and external surfaces
✔ Chemical composition ⇒acording to materials specifications requirements
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
1.6 HARDNESS TEST
✔ Identification of high hardness points in HAZ areas of P91 and INCONEL, coincident with the location of cracks
1.7 WELDING PROCESS AND HEATING TREATMENTS
✔ Warm-up 200ºC
✔ Welding GTAW
✔ Welding 5g Weld / 2G buttering
✔ PWHT in buttering ⇒ no PWHT in welding
1.8 OPERATION CONDITIONS AND SUPPORT SYSTEMS
✔ Support Systems and stress Calculation with no anomalies detected
✔ Insulation temperatures are higher in leakages area
2. Physical characterization
2.1 Detail Level 1- Pipe & Weld Analysis under Operation Conditions
2.2 Detail Level 2- Welding process simulation
2.3 Failure Causes
3. Solution. Corrective Measures
3.1 Installation of Spool Pipe section between P91 and SS347H
3.2 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
3.3 Modification of welding pass sequence
ABOUT THE PROJECT
Solution proposed has solved a critical and recurring problem which involved a high cost in terms of plant availability.
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.
