Failure Analysis Case Studies

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Summaries Of Failure Analysis Projects Performed At Smith-Emery Company

1.

CRANE WIRE ROPE FAILS AFTER 10 MONTH SERVICE

As-Received Failed Wire Rope

Broken Wires In Valleys

A 0.5-in diameter wire rope of 6 x 37 Right Regular Lay (RRL) construction from a crane failed after 10 months service. The hoist design was typically 4/2 reeving with a rated capacity of 20, 000lbs. The rope away from the failure exhibited many broken wires in the valleys. The Inner Wire Rope Core (IWRC) although generously lubricated was significantly fragmented suggestive of high tensile loads. Extensive visual, microscopical, and scanning electron microscopy indicated failure was not by overload. The failure had been progressive that had been facilitated by a combination of bending and vibration fatigue of the wire rope with occasional high tensile loads.

‘Z’ Type Wire Fracture

Flat Wire Fracture

Ductile Wire Fracture

SEM IMAGES OF INDIVIDUAL WIRE FRACTURE SURFACES

2.

RUPTURE AND SEVERE CORROSION OF BURIED CAST IRON SEWER PIPE

As-Received Cast Iron Sewer Pipe

Microstructure At Pipe Wall Mag.: X200

A section of nominally 6.5-in OD x 0.25-in wall thickness cast iron sewer pipe reportedly disintegrated unexpectedly during modification to attach a clean-out line. The asreceived pipe was significantly brittle. Microscopical and Scanning electron microscopy determined the failure mode to be brittle fracture of the pipe segments facilitated by graphitic corrosion that had occurred from both the ID and OD surface. Graphitic corrosion is the corrosion of gray iron in which the iron matrix is selectively leached away, leaving a porous mass of graphite. The graphite in gray iron is cathodic to iron and remains behind.

3.

FAILURE OF STAINLESS STEEL HEAT EXCHANGER TUBE

Failed Stainless Steel Heat Exchanger Tube

Microstructure Of Crack Mag.: X200

The subject stainless steel tube failed after a relatively short service life. The heat exchanger tube is submerged in chemically treated water. The OD surface is also characterized by randomly distributed pits. Microscopically a section of the tube wall displays transgranular branched cracking that originated from the OD surface. Energy Dispersive Spectroscopy of the tube surface and the crack deposit found substantial amounts of chlorine. The failure mode was evaluated as chlorine induced Stress Corrosion Cracking (SCC). The root cause was inadequacy of the water treatment.

4.

PIN-HOLE LEAK IN WATER PIPING CAUSES SUBSTANTIAL HOME FLOODING

Water Pipe Section Showing Pin-Hone (arrowed)

Poor Up-Lift At Soldered Elbow Insert

Close-Up Of Pin-Hole At Elbow

Good Soldered Elbow Insert

A leaked copper water pipe caused significant flooding and water damage to a 9-year-old house. Subsequent metallurgical evaluation of the leaked copper pipe revealed an almost innocuous failure mechanism, and root cause for the failure. Leakage was from a pin-hole in the soldered elbow section. The cut end of the elbow inserted into the straight section of the copper pipe is not reamed as shown. This results in an uplift of the cut surface that disturbs the smooth flow of the water. This disturbance in the water flow can lead to small swirls (turbulence) of water that gradually impinge on the tube wall progressively thinning the tube wall. Eventually the wall is perforated to cause a pin-hole and subsequent leakage. In inserted soldered joints the cut pipe diameters should always be reamed to produce a good-fit and minimize any potential water turbulence and hence the propensity to pin-holes.

5.

BOILER TUBE FAILURE

Failed Waterwall Tube From A Gas Fired Water-Tube Boiler

Schematic Of Water-Tube Boiler

Tube rupture is typically thin-lip ‘Fishmouth’ failure. The tube wall at failure has thinned down to a knife-edge. No liquid-level marks were observed in the tube ID suggesting non-separation of steamwater. The bore surface was also relatively clean without any fireside deposits. The tube OD surface microstructure in line with but away from the rupture exhibited a normal steel microstructure. The microstructure of the tube wall at failure exhibited severe overheating and subsequent quenching by escaping steam. Failure is characteristic of overheating failure. Root cause was identified as flameimpingement.

5.

FAILURE OF GALVANIZED TENSIONING CLAMPS

The subject tensioning units experienced random failures within months of installation and often without any appreciable in-service loads. The weight of a hanging galvanized link-chain was often sufficient to cause failure. The units were heat-treated (quenched and tempered) and then galvanized. Fracture was frequently thru the threaded portion of the units.

Fractured Tensioning Unit

Photomicrograph Of Branched Cracks Emanating From Galvanized Thread Root. X 200

Intergranular crack surface

Galvanized Layer

SEM Image Of Fractured Surface Showing Galvanized Layer & Crack Surface. X370

Pre-existing cracks (aligned transverse to the longitudinal axis of the threaded portion) were observed microscopically emanating from the galvanized thread roots of units that had not fractured. On the fracture surface of failed units, a small region of intergranular fracture was evident on the periphery. These are pre-existing crack surfaces. Cracks that are sharp and aligned perpendicular to applied stress axis act as severe stress concentrators promoting failures at very low-stresses. Hydrogen embrittlement was eliminated by systematic visual, microscopical and scanning electron microscopy. Failure was brittle facilitated by the pre-existing cracks. Deficient heat-treatment was identified as the root cause.

6.

‘WINDOW’ TYPE FAILURE OF BOILER TUBE DUE TO HYDROGEN DAMAGE

Piece of Boiler Tube that was detached from the tube

The tube carries very hot chemically treated water for generation of superheated steam to drive a turbine. The small protrusions on the tube surface effectively increase the surface area for efficient heat transfer. Failure occurred by ejection of a small irregular shaped piece of the tube wall. The tube wall fracture lips are thick-walled, that is they have not thinned. The microstructure of the tube at fracture and away from it is similar without any evidence of overheating. Hydrogen is produced by a chemical reaction that corrodes the internal surface of tube. At very high temperatures, the nascent hydrogen reacts with the carbon in the steel to form Methane. The escape of methane thru the steel at high temperatures causes grain boundary separation in the steel, resulting in embrittlement. Hydrogen embrittlement was determined as the failure mode, and deficiencies in the water treatment as the root cause.

The tube carries very hot water for generation of superheated steam to drive a turbine. Hydrogen is produced by a chemical reaction that corrodes the internal surface of tu