Mechanical Fatigue | Materials And Corrosion Control

Damage Description

Critical factors

·         Design: Fatigue cracks usually initiate on the surface at notches or stress raisers under cyclic loading. For this reason, design of a component is the most important factor in determining a component’s resistance to fatigue cracking. Several common surface features can lead to the initiation of fatigue cracks as they can act as stress concentrations.
Some of these common features are:

o   Mechanical notches (sharp corners or groves), tong marks

o   Weld joints, laws and/or mismatches

o   Tool markings

o   Grinding marks

o   Lips on drilled holes

o   Thread root notches

o   Corrosion

b.      Metallurgical Issues and Microstructure

·         For some materials such as titanium, carbon steel and low alloy steel, the number of cycles to fatigue fracture decreases with stress amplitude until an endurance limit reached. Below this stress endurance limit, fatigue cracking will not occur, regardless of the number of cycles.

·         For alloys with endurance limits, there is a correlation between Ultimate Tensile Strength (UTS) and the minimum stress amplitude necessary to initiate fatigue cracking.
The ratio of endurance limit over UTS is typically between 0.4 and 0.5. Materials like austenitic stainless steels and aluminum that do not have an endurance limit will have a fatigue limit defined by the number of cycles at a given stress amplitude.

·         Inclusions found in metal can have an accelerating effect on fatigue cracking. This is import when dealing with older, “dirty” steels or weldments, as these often have inclusions and discontinuities that can degrade fatigue resistance.

·         Heat treatment can have a significant effect on the toughness and hence fatigue resistance of a metal. In general, finer grained microstructures tend to perform better than coarse grained. Heat treatments such as quenching and tempering, can improve fatigue resistance of carbon and low alloy steels.

Affected Materials

Affected Units or Equipment

·         Workover equipment: coiled tubing, Fishing tools (over-shots, tubing/casing spears, milling tools, reverse circulating junk catchers, fishing magnets, fishing jars, wash over pipe)

Appearance or Morphology of Damage

·         Cracks nucleating from a surface stress concentration or defect will typically result in a single “clam shell” fingerprint.

·         Cracks resulting from cyclical overstress of a component without significant stress concentration will typically result in a fatigue failure with multiple points of nucleation and hence multiple “clam shell” fingerprints. These multiple nucleation sites are the result of microscopic yielding that occurs when the component is momentarily cycled above its yield strength.

Prevention/Mitigation

·         Select a metal with a design fatigue life sufficient for its intended cyclic service.

·         Allow for a generous radius along edges and corners.

·         Minimize grinding marks, nicks and gouges on the surface of components.

·         Insure good fit up and smooth transitions for welds. Minimize weld defects as these can accelerate fatigue cracking.

·         Remove any burrs or lips caused by machining.

·         Use low stress stamps and marking tools.

·         Select a proper acid inhibitor.

·         Ensure the corrosivity of the cleaning fluids

·         En sure complete recirculation after in-situ acid pickling

Corrosion Monitoring & Inspection Techniques

·         VT of small diameter piping to detect oscillation or other cyclical movement that could lead to cracking.

·         Vibration monitoring of rotating equipment to help detect shafts that may be out of balance.

·         In high cycle fatigue, crack initiation can be a majority of the fatigue life making detection difficult.

Inspection Frequency

KPIs

Competencies and Training

o   e-COE 101 Corrosion Basics

o   e- COE 701 Corrosion & Corrosion Prevention

o   PEW 407 Corrosion Technology

o   COE 104 Chemical Treatment For Producing Operations

Reference Resources (Standards/GIs/BPs)

·         ASTM STP1428, Thermo-mechanical Fatigue Behavior of Materials, American Society for Testing and Materials, West Conshohocken, PA.

·         ASTM MNL41, Corrosion in the Petrochemical Industry,
ASM International, Materials Park, OH, 1995.

·         Handling and Running Procedure for Chrome OCTG by Drilling & Workover Engineering Department, May 2003

·         Field Inspection Guidelines for Sumitomo Metal’s CRA Material- Saudi Aramco – Karan, Sumitomo Metal, 2008

·         Offshore Running and Handling Guidelines for Sumitomo Metal’s CRA Material, Saudi Aramco-Karan, 2008