FRACTOGRAPHY

David M. ChristieSenior Failure AnalystIMR Test Labs

Focus of this Presentation: Examples of fracture:

cast, wrought, and powder metals Overload

ductile and brittle Fatigue Stress corrosion cracking Hydrogen embrittlement Liquid metal embrittlement Cleaning techniques

CAST METALS Fractures tend to be more difficult to

interpret than in wrought material Non-uniform microstructure and

chemistry Section-size dependent properties Fracture along second phase

particles Generally rougher fracture surfaces

than wrought material

Type 383 Aluminum die cast

Aluminum die cast Cast-in letter

Aluminum die cast fracture surface

Fatigue striations are often difficult to find in castings, compared with wrought material

Fatigue origins are also difficult to determine, because the fracture surfaces tend to be rougher than in wrought material

Fatigue Striations die cast aluminum

Ductile overload in some casting alloys will occur primarily through or at the boundaries of secondary phases

Only small ligatures of matrix will show ductile dimples

Aluminum die cast overload fracture

Fatigue features and origins are difficult to determine, compared to wrought material

Ductile fracture will often occur primarily at boundaries of secondary phases

From this Example:

Cast aluminum impact wrench housing

What are the small planar regions at the fracture edge?

Note sharp boundary

Fracture ridges and beachmarks = multiple origin fatigue.... Low nominal load or high nominal load?

Overload zone Where are the dimples?

Overload zone fracture primarily through second

phase particles

Ductile Iron but is this a ductile fracture?

Microstructure shows carbides

Ductile iron - overload

-Nodules loose in matrix- Many sites of lost nodules

- Ductile dimples in matrix ligatures

Ductile dimples

Nodules loose in matrix, dimples evident

Ductile Iron Differential Case

Ductile Iron Fatigue fracture

Ductile iron Fatigue - Nodules are tight in matrix, fatigue goes through nodules

Ductile iron fatigue fracture

Ductile iron Fatigue striations

Ductile Iron fatigue striations in matrix

Fatigue of cast alloys with multiple phases

Fatigue will usually go through secondary phases, rather than around them: Graphite nodules of ductile iron Eutectic silicon of aluminum alloys

GRAY CAST IRON OVERLOAD

Fracture mostly along graphite flake boundaries

Matrix ligatures show ductile dimples and/ or evidence of microstructure (pearlite).

GRAY CAST IRON OVERLOAD

Gray Iron Backscatter SEM image, shows graphite flakes well

GRAY CAST IRON OVERLOAD

GRAY CAST IRON OVERLOAD

White Cast Iron Brittle Fracture

White Cast Iron Brittle Fracture

Brittle Fracture - White Cast Iron. Note cleavage facets and fracture at carbide boundaries

Cast Magnesium AZ 91C Mg

Cast Magnesium Fracture

Cast Magnesium - AZ 91C Mg

Alloy 319 Cast Aluminum Commercial Juicer

Cast Aluminum 319 Al

Cast Aluminum 319 Al

Stainless Steel Pump Impeller (CG-8M)

Stainless steel pump impeller failed after 14 months in service

One of six vanes fractured off

Pump Impeller

Vane Radius is highest stress location

Fracture surface generally rough

Fatigue beachmarks present

Fatigue features at high magnification

Dendritic region close to origin

Manufacturing Related CG-8M

Pump Impeller Conclusions Fatigue began at a small weld repair

crack Weld repair to fill in casting void was in

a critical location Recommended revising procedure to

prohibit welding at the leading vane radius, and to include dye penetrant check of welds

Observations of Fatiguein cast metals

Due to rougher surfaces and poorly developed fracture ridges, the locations of fatigue surfaces and origins can be difficult.

The locations of fatigue origins can sometimes be determined from striation direction and curvature.

Frequently fatigue direction will change from grain to grain in cast material, as the crack follows the weakest crystallographic plane. This complicates things!

Mn Bronze Alloy C863 Automotive Transmission Fork

Failed very early in vehicle life Low stress part Mature part, no significant history of

failures

Planar Fracture Regions

Discolored Regions = Shrinkage porosity

Overload Region not discolored

Cross section of Shift Fork

Shift Fork Conclusions Gas and shrinkage porosity occupied

50% of cross section Resulting reduction in load-bearing

cross section increased stress intensity Result was fatigue initiation at inside

radius of fork

Mn Bronze (C670) adjusting nut

Adjusting Nut Conclusions Mn Bronze (C670) adjusting nut from

offshore oil rig was exposed to salt water, mud, hydrogen sulfide, diesel fuel environment

Nut is held in constant tension and exposed to radial vibration, mated to 316 stainless

Nut failed by intergranular stress corrosion cracking (IGSCC)

Recommended alloy change to (SCC resistant) cast nickel (K-500)

Chrome-plated Leaded Brass Flush Valve (C857)

Chrome-plated leaded yellow brass flush valves developed leaks after six months of service

Small Cracks resulted in leaking

Fracture Surface was discolored

Tip: When cleaning fracture surfaces of

leaded material, avoid the use of Alconox detergent, as it can remove the lead!

TIP: When the main fracture surface is

heavily corroded or damaged, look near the edges of the fracture (at the crack tip)

There is often less damage in this location, and the fracture mode is probably consistent with the rest of the fracture surface

Fracture at Crack Tip

Tip: Fractures produced in the laboratory

can aid in your interpretation of the field fractures To confirm fracture mode and compare to

the field fracture. To determine if the material has been

embrittled. To test response of freshly exposed

material to different environments, cleaning techniques.

Material was not embrittled

Branched transgranular cracking -indicative of SCC

Brass Flush Valve Conclusions Failure was due to transgranular stress

corrosion cracking No specific corrosive agent was

determined Alloy contained 35% zinc, which makes

it a susceptible alloy Not a highly stressed part, suspected

residual casting stresses Recommended stress relief of castings,

or material change to aluminum bronze

Cast Nickel Pump Impeller (Cast Super-Duplex Stainless Steel -

Jessup 700)

After nine months of pumping a low pH (1.5 2.0) slurry of 50% wet phosphoric acid, one of four vanes fractured from an impeller

Pump Impeller

One Vane Fractured

Tip: Fatigue fractures in cast material will

often change direction with each grain, depending on crystallographic planes

Faceted fracture surfaces are often fatigue fractures

SEM can aid in determining fracture mode

Damage near fracture origins dont panic.

Remember look at crack tip!

Tip: When fracture surfaces are corroded or

damaged, look for secondary cracks Opening secondary cracks will reveal

fresher fractures, with more detail

Near Secondary Crack Origin

Cast JS700 Pump Conclusions

Corrosion fatigue had occurred, with all four vanes showing cracks

Alloy was appropriate, met specification Recommended checking chemistry of

pumpage, checking residual stress of cast impellers, checking balance of impellers

Corrosion Fatigue depends on environment and stress intensity

Cast Bismuth Bronze Wear Rings

Cast Bismuth Bronze (lead-free C89320) wear rings failed prematurely in a pump

Bronze Wear Ring Conclusions

Metallographic section showed intergranular cracking

Microstructure indicated the parts had run dry and overheated

Molten bismuth had embrittled the part, resulting in fracture by Liquid Metal Embrittlement (LME)

Powder Metals Green crack vs. Sinter Bond fracture Ductile overload Fatigue

Green Cracks PM parts are pressed and ejected

Green at this point The stresses of pressing and/or ejection

can result in cracks at this stage Sintering to produce diffusion bonding

between particles will not bridge the gap created by a crack

The result: a green crack

Powder Metals Green crack

Powder Metals Green crack

Powder Metals Green crack

Green crack of a steam-treated part

Powder metal - Overload

Powder metal - Overload

Powder Metal Overload The percentage of sinterbond fracture is

directly related to the part density The higher the density, the greater the

percentage of sinterbond fracture

Powder metal - Fatigue

Powder metal - Fatigue

Powder metal - Fatigue

Powder Metal Fatigue Fatigue does not seek the weakest

path, as overload fracture does The result is that generally a larger

percentage of the fracture surface is actual fracture, as opposed to void area

The patches of fatigue fracture are generally larger than overload sinterbond fracture

Powder metal Ferrite core fractureby thermal shock

Powder metal Ferrite core fracture by thermal shock

WROUGHT METALS OVERLOAD

Ductile Brittle

FATIGUE CORROSION CRACKING

OVERLOAD AXIAL TENSION

OVERLOAD - SHEAR

LONGITUDINAL OVERLOAD FRACTURE DIMPLE NUCLEATION AT MnS INCLUSIONS

Brittle Fracture Cleavage Fracture below DBTT

Same Steel Ductile Fracture above DBTT

Brittle Intergranular fracture in hardened case of carburized steel this is the expected overload morphology

Type 303 Stainless steel Fatigue

Use of topographic backscatter mode in the SEM can show steps or ridges in the fracture,

indicative of multiple fatigue origins.

Type 303 Stainless steel Fatigue MnS inclusions evident on fracture surface

Type 303 Stainless steel Overload

Wrought Aluminum - Fatigue

Wrought Aluminum - Fatigue

Wrought Aluminum - Fatigue

Wrought Aluminum - well-developed striations

Wrought Aluminum - Overload

Steel Shaft Rotating Bending Fatigue

Steel Shaft Rotating Bending Fatigue

Steel Shaft Rotating Bending Fatigue

Steel Shaft Rotating Bending Fatigue

Steel Shaft Rotating Bending Fatigue

Steel Shaft Rotating Bending Fatigue

Stainless Steel Bellows Fatigue Fracture

Stainless Steel Bellows Fatigue Fracture

Tip: Fatigue striation curvature indicates direction of crack propagation

Brass Intergranular Stress Corrosion Cracking

Crack tip Field crack to left, overload at right

Brass Laboratory overload fracture

Hydrogen Embrittlement

Hydrogen embrittlement

Hydrogen Embrittlement ductile ornamentation of grain boundaries

Hydrogen Embrittlement ductile ornamentation of grains, gaping grain boundaries

Hydrogen Embrittlement patches of ductile fracture

Use of the SEM for Fractography Always examine fracture surface optically

before SEM examination Scan first at high refresh rate, high probe

current Once the critical areas are established, take

photographs, adjusting SEM conditions Consider the use of Backscatter and

Secondary modes Consider the use of topographic modes in

both Backscatter and Secondary

Optimizing the SEM for Fractography Photographs

Working distance (WD) should be minimized (e.g. 10 15 mm)

Accelerating voltage should be 10 Kevor less

Probe current (spot size) should be low (~100 picoamps)

SEM not optimized: 31 mm WD, 30 Kev, 1.8 nanoamps

SEM not optimized: 31 mm WD, 30 Kev, 100 picoamps

SEM optimized: 15 mm WD, 10 Kev, 100 picoamps

SEM Low Magnification Techniques

BEI composition mode BEI topographic mode SEI mode SEI mode with reverse voltage bias

Backscatter composition mode

Backscatter topographic mode

Secondary mode

Secondary mode, negative bias

Cleaning Fracture Surfaces Photodocument As-received condition Clean starting with least aggressive method Use step-wise approach and examine at each

step It is often not necessary to remove all oxides

or contamination from the fracture and attempting to do so may damage the surface

If in doubt, submit a polished metallographic mount of your material to the proposed cleaning method, examine for etching or other damage

Cleaning Fracture Surfaces The use of an alkaline detergent

(Alconox) has proven most useful. Mix 160 g to one gallon of DI water Can be used at room temperature or

heated to 100 degrees F Ultrasonic for up to 15 30 minutes in

five minute increments, with examinations after each five minutes.

Cleaning Fracture Surfaces Before Alconox

Cleaning Fracture Surfaces After Alconox

SUMMARY Examples of cast, wrought, and powder

metals were reviewed Overload ductile and brittle Fatigue Stress corrosion cracking Hydrogen embrittlement Liquid metal embrittlement Cleaning techniques were presented